Decarbonizing the pulp and paper industry: a critical and systematic review of sociotechnical developments and policy options

and paper processes and products, thus making the pulp and paper industry more environmentally sustainable. This study reviews 466 studies to answer the following questions: what are the main determinants of energy and carbon emissions emerging from the pulp and paper industry? What are the benefits of this industry adopting low-carbon manufacturing processes, and what barriers will need to be tackled to enable such adoption? Using a sociotechnical lens, we answer these questions, identify barriers for the pulp and paper industry ’ s decarbonization, and present promising avenues for future research.


Introduction
Paper has shaped society for centuries and is considered one of humanity's most important inventions. The origins of paper trace back to China at the beginning of the first millennium, about 105 AD [1]. The art of papermaking then diffused across Asia and North Africa and was later introduced in Europe by the Arabs around 1150 AD [2]. Later, when Johann Gutenberg invented the first printing press in 1453 AD, paper was adopted as an information medium. Despite its rising popularity, the use of the product remained relatively small-scale at the global level until it became industrialized during the 19th century [3]. Today, paper is a significant component of our daily lives [4], involving uses as disparate as newspapers and artwork, diapers, cardboard, shoes, coffee filters, and even the liquid crystal displays for televisions and monitors.
In essence, paper comprises a meshwork of cellulosic fibres extracted from vegetative biomass by mechanical or chemical means [5] and is built upon four components: recycled fibres, non-fibrous components (additives and minerals), chemical and mechanical wood pulp and water [6]. Paper products are rigid and strong, and although they have a low elongation to break, they also have a relatively high tensile strength [7].
Due to its low price, specific structural properties, nontoxicity, biocompatibility and lightweight, paper is favoured over other options, such as silicon, polyimide and terephthalate (PET), when suited to use [8]. The pulp and paper industry (PPI) often manufactures short-lived products compared to the solid-wood products segment [7]. However, given its heterogeneity, products emerging from the PPI are an important necessity in different aspects of our lives, whether for sanitary and hygiene reasons [9], as banknotes to enable economic transactions [10], as instruments to spread knowledge [11], or in the form of banners to display at events [12]. More recently, the PPI is exploring other business routes by producing second-generation biofuels as a substitute for fossil fuels [13].
The PPI is diverse concerning the size of its companies, extending from small and medium-sized enterprises (SME) to large corporations [14]. International trade occurs in all the different product categories within this industry, including semi-finished products and raw and recycled materials [15]. The PPI is also characterized by its mature markets, high capital investments, and price volatility [16,17]. As an established industry, the PPI produces over 400 million tons (Mt) of paper per year [18] and is expected to reach 900 Mt by 2050, with the largest share of growth taking place in Asia [19].
Although the PPI presents itself as bio-based and sustainable, its processes are energy and capital intensive [20]. For instance, Li and colleagues estimate that energy for electricity and steam generates between 20 and 30% of total operating costs [21]. The PPI is the fourth largest industrial energy user because chemical pulping, black liquor evaporation, and drying require enormous amounts of energy [22]. In addition, this industry has traditionally been under scrutiny due to its heavy dependence on water, air and water pollution, intensive use of natural resources, forest degradation and deforestation and waste generation; such issues have been extensively studied in the literature [23][24][25][26][27][28][29][30][31][32][33][34][35] and are discussed in this review. The environmental impacts of the PPI become more apparent considering that this industry generates 1.3% of global greenhouse gas (GHG) emissions (nearly 2% of industrial across its lifecycle [36]. Further, more than 400 million tons of waste paper are generated each year, representing around half of all municipal solid waste (MSW) [37].
Despite these negative effects, the PPI has managed to be a reference point in reducing emissions and fuel switching [35]. For instance, from 1973 to 1990, the PPI cut its emissions by 85% [38]. Others have noted that despite an increase of 23% in paper and paperboard production in the last 20 years, the industry's energy use has increased only 1% due to the high share of biomass as feedstock and the use of onsite process by-products [39]. Although debatable, The Confederation of European Paper Industries (CEPI) considers that in the European Union (EU), the PPI is the biggest industrial user and producer of renewable energy, where more than 55% of the primary annual energy consumption is biobased [40], which is CO 2 neutral [41]. This dynamic has positioned the PPI as the world's largest woody biomass utilization system and among the largest user and producers of biomaterials and bioenergy [42]. Regardless of its progress in fuel switching and mitigating emissions, the PPI, at least in the EU, still has a long way to go if it wants to meet its target of 80% CO 2 reduction relative to 1990 levels by 2050. Reaching this point translates into mitigating 60 Mt CO 2 in 1990 to 12 Mt CO2 by 2050, with the 60 Mt CO 2 derived from 40 Mt emerging from direct emissions, 15 Mt of indirect emissions and 5 Mt from transport emissions [43].
In this systematic review, we use a "sociotechnical" approach to study a critical issue linked with the future of the PPI: achieving significant decarbonization or even net-zero production. This study asks five key questions: What options exist to minimize the climate effects of the PPI manufacturing processes, and therefore, what makes the full life cycle of pulp and paper more sustainable? What are the key elements of energy and carbon emissions from the PPI? What technical innovations have been identified to produce low carbon pulp and paper products? What benefits will result from more carbon-friendly PPI manufacturing processes and what barriers will need to be tackled? Our study presents the findings of a critical and comprehensive systematic review of 466 documents to answer these questions. It employs a sociotechnical approach [44,45] that examines the PPI manufacturing processes and different pulp and paper uses while providing options for the industry's decarbonization.
The review proceeds as follows. Section 2 offers a comprehensive background on PPI production processes and market dynamics. In Section 3, we discuss the research design and Section 4 presents the PPI energy use and carbon emissions as well as other environmental problems stemming from this industry. Section 5 presents approaches to decarbonize the PPI and Section 6 identifies current challenges and barriers to decarbonizing the PPI. Section 7 presents three promising avenues for future research and Section 8 concludes.

Processes in the PPI
For its satisfactory operation, the PPI depends primarily on wood and wood residues that need to have high amounts of cellulose to satisfy the lignocellulosic biomass criteria [5]. The PPI also operates using recovered paper, which results from waste management activities [46]. During pulp and paper production, papermaking and pulping are the most important phases, both of which require intensive use of resources and energy inputs [47]. Regarding pulping, wood fibres are separated mechanically or chemically [48]. In the mechanical pulping process, cellulose fibres are chiefly separated through mechanical procedures, often transforming 95% of the wood into pulp [49]. However, since lignin causes that mechanical pulp to turn yellow, it is mainly used for products with a short lifespan (e.g. magazine paper and newsprints) [50,51].
Chemical pulping separates cellulose fibres from lignin and other wood components and delivers higher quality paper, with stronger and brighter properties but with a lower yield since the lignin is dissolved [51,52]. Chemical pulp is often employed for different types of boards, fine papers and sack paper products. The majority of pulp production in the EU relies on chemical pulping processes [49]. That is, 72.7% of the total output, 26.2 Mt, and a market share of over 68% [53]. Globally, CEPI estimates that kraft pulping represents more than 90% of pulp production [54].
Regarding paper production, it fundamentally consists of five steps. First, the pulp is mixed with additives and water in the stock preparation and is deinked, cleaned and refined to acquire the correct properties. In the wire section, water is removed through gravitational forces and vacuumed. After the wire section, the wet paper passes through the press section, where more water is removed mechanically. At this stage, the dry solid content should be between 33 and 55% depending on the press section design and paper grade [55]. During the fourth stage, the water that remains is thermally removed in the pre-drying section, and conditional to the end product specifications, paper is further treated with a sizing step (e.g. starch, glue, or coating). Finally, if there is a second drying stage, a small quantity of moisture, between 5 and 9%, remains in the paper even after drying [55]. Fig. 1 illustrates, in detail, the papermaking process. e Swedish exports of PPI products in 2018 constituted an export value of US$16.59 billion [72] and contributed to 5% of the Swedish GDP [73]. In the UK, the PPI has a turnover of US$15.43 billion per annum [74], while in India, this accounts for circa US$8.0 billion [75]. The Chilean forestry and forest products sector represents up to 3% of the national GDP and more than 7% of the country's export revenues [76]. Meanwhile, the PPI accounts for 0.24 and 0.89% of the national GDP of Taiwan and Austria, respectively [77] and generates revenues of more than US$1.30 billion for the former [22]. In Indonesia, the PPI represents about 6.7% of the GDP's manufacturing component and contributes with about US$3.8 billion to the national economy [78]. Finally, in Mexico, the total market value of the PPI industry accounts for more than US$11.5 billion annually, which equates to 6.3% of the Table 1 Paper grades and their attributes. Source authors, compiled from [57].

Paper grade Characteristics and attributes
Bond paper This paper is characterized by its durability for repeated filling and handling, a degree of stiffness, resistance to the spreading and penetration of the ink, cleanliness and bright colour.
The main uses of Bond paper are: announcements, advertising pieces, letterhead stationery, insurance policies, legal documents, and certificates. Book paper This type of paper contains various amounts of sizing, fillers, and dyes. It also requires multiple combinations of chemical wood pulp; groundwood, semichemical, and de-inked wastepaper are also used for lower-priced grades. Bristol This type of paper often refers to stiff, heavy paper thicknesses ranging from 0.15 mm (0.006 inch) upward. Grades from this Bristol paper are made from multiple combinations of chemical wood pulp. These grades are made from various combinations of chemical wood pulp. The use of bristols has gained relevance for the punch cards used in tabulating and sorting machines. Groundwood and newsprint papers These are converting and printing grades containing different amounts of groundwood pulp and small percentages of chemical wood pulp for durability and strength. Groundwood papers are known for having a high degree of opacity and a uniform formation. They lack high whiteness and often turn yellow after long ageing or when exposed to light. These papers are often bulky and receptive to printing ink. This sort of paper could be used for magazines, directories, catalogues, paperbound books, and general commercial printing. In the EU, the newsprint paper sector represented about 4.9% of the total paper production. Kraft wrapping A heavy stock of this paper is employed for paper bags. It is made of wood pulp, in unbleached conditions, made from softwoods, often pine. It stands out for having impressive tensile and tearing strength. This paper also retards wetting when exposed to water. Paperboards These products often are 0.30 mm (0.012 inches) or thicker and are composed of fibrous materials on paper machines. It is typically made from wood pulp, waste paper, straw, or combining these materials. In the EU, the paperboard (including other packaging papers) sector represented about 20.9% of the total paper production.
There are three main types of paperboard: -Boxboards: are used for food trays, food boards, paper boxes, and plates.
-Container boards: employed for corrugated and solid fibre shipping containers.
-Paperboards specialities: Include products such as electrical pressboards, binders boards, and building boards.

Sanitary paper
This sort of paper includes facial tissues, toilet tissues, napkins and toweling. The grades are made of bleached kraft pulps and sulfite with relatively little refining of the stock to preserve it soft. Due to the bulky texture and softness of these papers, they are relatively weak. In the EU, the sanitary paper sector represented about 9.3% of the total paper production. manufacturing GDP [79]. Since 2013, China has dominated global paper production with 104.4 Mt, representing around 25% of worldwide output, with production expected to increase to about 180 Mt by 2050 [80,81]. In China, too, the average annual growth rate of the PPI has been much higher at up to 12%, with the number of paper mills increasing at a rate of 4.3% annually, generating $86.4 billion in gross output [82]. Although only having just 1% of the world's forests, Sweden represents more than 10% of the global forest industry, which consists of forestry and logging, wood product manufacturing, and paper manufacturing, and is the world's second-largest exporter of PPI products [83]. The Indian paper industry occupies third place in global pulp and paper production, accounting for 3% [84].

Research design and conceptual approach
To investigate the decarbonization of the PPI, we employed a systematic searching protocol with a critical review method and the guiding theoretical view of sociotechnical systems [44,45,[85][86][87][88].

Critical and systematic review approach
We classify our review as both critical and systematic since a "critical review" seeks to demonstrate that the researchers have broadly scoured the literature and critically evaluated its quality [89]. Therefore, this approach goes beyond just revising the literature to interpreting it and making evaluative statements on the possible research gaps and quality of evidence [45]. To do so, it presents, synthesizes and analyses a variety of material from various sources. A critical review offers the possibility to "take stock" and assess valuable information across multiple bodies of evidence associated with a particular topic or research question. It offers both a "launchpad" for conceptual novelty and an empirical testing ground to judge the strength of evidence.
Assuming that a weakness of critical reviews is that they do not  always prove the systematic nature of more rigorous approaches to reviewing, this review is also "systematic" [90,91]. Specifically, this technique provides the following benefits: • It avoids opportunistic evidence, • Provides a focused investigation, • Allows replicability through documented study inclusion, • It discriminates between sound and unsound studies. In turn, it provides an assessment of methodological quality and, • It increases transparency, which decreases bias in results and subjectivity.
Furthermore, systematic reviews minimize unintentional bias (excessive self-citations or those of friends and colleagues, e.g., "citation clubs") while encouraging diversity. For these reasons, studies have called for greater use of systematic reviews in the domains of environment and energy, climate change and energy social science [92][93][94].

Searching protocol and analytical parameters
For this review, we employed three distinct classes of search terms, as Fig. 4 shows. We executed each permutation of these search terms across 12 separate databases or repositories, resulting in 1080 search strings. Performing this method allowed us to access the most pertinent state-of-the-art research related to our topic. In this space, we also note that although we did not directly include Scopus and Web of Science as part of our databases, they are covered by our searches of databases such as Google Scholar and ScienceDirect also captures most of what is on Scopus. We nevertheless encourage researchers to include them in future research since these are also prominent databases with qualitycontrolled journals. Our search terms for String 1 and 2 were selected based on previous studies conducted by the authoring team (see for example [44,45,85,86,95,96]). Whereas the search terms for String 3 (topical area) were selected after examining relevant abstracts and articles for alternative subject phrases and words. Once identified, we tested and refined the search terms after reviewing the search results. Table 3 displays our results. While the overall searches delivered more than 9.1 million documents, this number fell to a sample of 782  relevant studies after screening titles, keywords and abstracts. After further detailed screening for topical relevance (they had to address the topic of climate change mitigation and/or decarbonization), originality (we adjusted the results to remove duplicates), and recency (documents had to be published from 2000 onward), this number dropped to 466 documents. We reference most of these studies throughout the review.

The analytical frame of sociotechnical systems
To help to structure our results from this body of 466 studies, we used the conceptual analytical frame of sociotechnical systems [97,98]. As Fig. 4 illustrates, this framework considers the PPI industry as far more than just a collection of physical products such as paper, cardboard or tissues. Instead, this approach views the entire social and technical systems involved in making, distributing, and using paper and pulp products. As a consequence, this approach includes not only the instruments used to manufacture paper and how products are shipped to stores but also a sociotechnical frame that entails issues pertaining to local and regional regulations and paper manufacturing and waste. Sociotechnical systems thinking helps "open up" the black box of technologies to show their social and nontechnical context [98,99]. The approach helps refute belief in technological determinism; thus, they suggest that no technological system is truly self-sustaining, and that there is hope in changing the course of technology if one alters its other nontechnical dimensions. By identifying the different actors and interests involved in technology, they suggest who activists should approach and deal with. Similarly, such efforts help re-politicize the usually technical discourses of security, economics, and politics, showing that it is neither objective nor neutral. By identifying the relational aspects between people, artifacts, and knowledge, they also help show that there is no one person or institution masterfully manipulating the course of military technology. Instead, it is a complex amalgam of political, social, economic, and technical interests. The approach lastly suggests that any barriers to changing the system will fall into diverse dimensions cutting across technology, economics, politics, behaviour, and aesthetics.
To offer a slightly different view of the PPI system, Fig. 5 organizes different elements to include resource extraction of raw and natural materials, the intersection of social organizations, policy frameworks, means to mitigate emissions, legislation, new business models and markets and environmental impacts. The sociotechnical system for the PPI therefore incorporates dimensions such as, but not limited to, the adjacent industries and social wellbeing.
Though not all documents in our study approached the PPI through a sociotechnical lens, we use it in this review to structure our analysis.

Raw and natural materials
A large part of the forest industry is managed by private firms, mainly by the pulp and paper industries. This dynamic has led to controversies and debates. On the one hand, some posit that it is exploitative, often negligent of local communities' rights and environmentally undesirable [100,101]. On the other hand, there are arguments that the forests industry has benefited regional and national economies by building public infrastructure (e.g. schools, medical support and roads), generating rural employment, and is positively perceived by the public due to the increasing acquisition of green and social credentials for external evaluations and certifications [102]. Nonetheless, negative aspects of the PPI are significant as the PPI is a major consumer of natural resources, including wood, water and energy.
According to FAO, the PPI is the largest virgin wood user [103], it accounts for 13-15% of total wood consumption and employs between 33 and 40% of all industrial wood traded globally [104]. Moreover, Cheung and Pachisia indicate that 35% of harvested trees are used for manufacturing paper [105]. Worldwide, about 130 billion m 2 of forests are razed, of which 40% are employed for paper production. In the EU alone, around 423 million m 3 of woody mass are removed from forests from which the PPI utilizes about 108 million m 3 [106], and it is expected that the demand for woody mass will increase to 160 million m 3 by 2030 [107]. From these, the major consumer is newsprint, printing, and writing paper [108].
Indeed, there is a clear worrying trend in terms of deforestation. For example, from 2010 to 2015, it was reported that annual forest loss oscillated at 7.6 million hectares versus a yearly gain of 4.3 million hectares per year [109]. This means that there was an annual net loss of forests' area of 3.3 million hectares. Perhaps results more worrying is that the global loss of intact forest landscapes has tripled between 2003 and 2013-16. Potapov and colleagues indicate that 37% of the loss is driven by timber harvesting, including papermaking purposes [110]. Among the major impacts emerging from deforestation is loss of natural habitat, leading to biodiversity loss and species extinction, soil erosion, ecosystem disruptions, and increased global warming since around 15% of GHG emissions result from forests' degradation [1,111].

Emissions emerging from pulp and paper processing and manufacturing
Pulp and paper products have a short life cycle, and most of the carbon from papermaking ends in the atmosphere within a year. After the carbon is released either by fire to clear land or from the decomposition of roots, slashes, and leaves, there is a continuous period of prolonged emissions as branches and coarse roots decay [112]. Manufacturing processes in this industry are energy intensive and require large amounts of different energy sources (gas, steam, and electricity) during all of their production stages [113]. In the EU, 93% of the total energy consumption by the PPI is from heat power, and the remaining 7% is from electricity [71]. Nevertheless, energy use varies by product type and whether the PPI manufacturing processes are separated or integrated. For instance, energy demand is lower in integrated pulp and paper manufacturing because there is no need to dry the pulp, and it also provides better opportunities for heat integration [16]. Regardless of being considered an efficient technology, kraft mills represent 73% of European PPI emissions [114]. This percentage makes more sense given that kraft pulping consumes around 4.4 GJ of steam Fig. 6. Traditional stand-alone pulp mill with closed chemical loop. Source, adapted from [121]. and about 406 kWh e per ton of pulp [115]. While chemical pulp mills require less external energy than mechanical pulps, these mills require about 2.2 tonnes of wood to manufacture one tonne of bleached kraft paper [116].
Since kraft mills manufacture most products from the PPI, they are responsible for 90% of CO 2 biogenic emissions and 80% of all emissions emerging from the PPI [117]. According to Onarheim and colleagues [118], CO 2 emissions from modern Kraft pulp mills emerge chiefly from three major sources: the lime kiln, multi-fuel boiler, the recovery boiler, and the power generator boiler, with Fig. 6 displaying a traditional stand-alone pulp mill. The paper and pulping production often entail heat processes of less than 200 • C, steam generators in boilers operating at about 500 • C, and lime kilns requiring 1000 • C or higher [16]. Pulp and paper production is divided into papermaking and pulping. The first represents 94% of the thermal energy consumption and 47.2% of electricity consumption [119]. The latter accounts for 4.5% of the thermal energy consumption and 41.7% of the electricity consumption [120].
The PPI is among the top five most energy-intensive industries globally and is positioned as the fourth-largest industrial energy user [122], growing energy use on average 0.3% annually during 2000-18 [123]. It accounts for 6% of global industrial energy consumption and 2% of direct industrial CO 2 emissions [124], with the Chinese PPI alone contributing to 26%-29% of the total PPI CO 2 emissions [125]. This rough amount of 2% puts paper on par with other emissions-heavy industrial or commercial sectors such as aviation [126], marine shipping [127], and internet datacentre energy use [128,129], which are also responsible for about 2% of direct global carbon emissions.
In fact, Kuparinen and colleagues estimate that the CO 2 production of the 15 largest bleached and unbleached sulphate pulp manufacturer countries accounts for more than 300 Mt [130]. Research estimates that each tonne of paper manufactured produces 0.6 tonnes of fossil CO 2 [131] and consumes between 5 and 17 GJ of process heat [15]. Another study indicates that, on average, one metric ton of paper, regardless of the paper type, generates 951 kg CO 2 -eq GHG emissions cradle to gate. Nevertheless, this might vary by country. For example, China holds the highest GHG emissions in papermaking, whilst Scandinavia and Brazil have the lowest [47]. Indirect emissions emerging from the PPI are also a concern since these account for 12.3 Mton CO 2 from purchased electricity and 5 Mton CO 2 from transport. Globally, emissions resulting from the PPI are higher than those of Denmark, Finland, Norway, and Sweden combined [132]. To put things in perspective, paper cups alone could emit over 7.5 Mt CO 2eq , which is comparable to Cyprus' annual carbon footprint [133].
Boilers are the main source of emissions within the PPI, representing 85% SO 2 , 65% NOX, 45% of the filterable PM emissions, and about 95% of the industry's GHG emissions [134]. In Sweden, emissions emerging from the recovery boiler are often considered biogenic as many mills employ bio-oil instead of fossil-based oil in their limekilns [135]. However, in the USA, on average, boilers released about 57 million metric tons of CO 2 -equivalent GHG emissions, chiefly nonbiogenic CO 2 , nd 113 million metric tons of biogenic CO 2 emissions resulting from biomass combustion [136].
As an industry, the PPI holds the highest carbon intensity (measured as t CO 2e per £k GVA), with energy accounting for about 16% of the industry's costs on average but as high as 30% depending on the country [66,137,138]. For instance, in the US, the PPI consumes more than $7 billion worth of purchased electricity and fuels per year [139]. From these, about $2.8 billion of this was for buying electricity, and the rest (approximately $4.2) was to purchase fuels [140]. The magnitude of energy use emerging from this industry is well illustrated in China's case. In 2013, the total energy consumption of the Chinese PPI was 41.53 Mtce, which accounts for more than the total energy consumption of countries like Hungary, Bulgaria, Denmark, Portugal, Belarus, Greece and Finland in the same year [81]. At the EU level, PPI consumes about 1325.5 PJ, representing about 11.5% of final industrial energy consumption and 5% of total energy use [141] with an energy mix composed as follows: biomass 57.7%, natural gas 34.7%, coal 3.9%, fuel oil 1.7% and other types of fuel 2% [142]. The energy mix is similar to onsite by-products in the US, accounting for about 55% of energy consumption [143]. In most cases, black liquor (19 GJ/t pulp) and hog fuel Table 4 The magnitude of energy use and climate impacts emerging from the PPI. Source, authors compiled from [1,14,35,39,47,74,77,[80][81][82]122,131,[147][148][149][150][151][152][153][154][155][156][157][158][159][160][161][162][163][164][165][166]: Country Energy use and climate impacts

Sweden
The PPI is the largest industrial energy user in Sweden, consuming around 21 TWh of electricity yearly. This is equivalent to 15% of all electricity consumption and 52% of Sweden's total industrial energy use. Emissions emerging from 50 paper and pulp mills in 2015 were 21 MtCO2, where 97% were of biogenic origin and 3% from the combustion of fossil fuels. However, it is worth noting that from 1973 to 2019, the PPI decreased its CO 2 emissions by about 80%. Finland The PPI represents nearly 50% of the annual industrial energy use and around 23-24% of the total energy demand. In previous years, the PPI used more than 25% of the total electricity consumption in Finland.

UK
The PPI represents 6% of GHG emissions from UK industries and 7% of the total heat consumption. In previous years, this industry emitted 2.4 million tonnes/year of CO 2 , with a further 0.9 million tonnes/year of CO2 from electricity consumption.

Netherlands
The PPI's final heat consumption accounted for 11.9 PJ, while electricity consumption was 4.7 PJ in 2015. GHG emissions from this industry are attributed as follows: . Paper and board products: total production capacity of 2,870,000 mt that resulted in 1,054,000 mt CO 2 eq. . Graphic paper mills: total production capacity of 520 kt resulted in 294,000 mt CO 2 . Graphic paper mills using recovered paper: when their total production capacity of 280,000 mt resulted in 19 kt CO 2 Germany The PPI accounts for around 9% of industrial energy demand and about 2.5% of all energy-related GHG emissions. PPI energy costs surpassed 13% of total production costs in 2014.

Austria
The PPI consumes circa 20 TWh of final energy and accounts for 7% of Austria's industrial CO 2 emissions. That makes this industry the largest final energy consumer in the country. Brazil This industry accounts for about 38% of final energy consumption in Brazil. The PPI increased its energy consumption share from 6.8% in 1979 to 11.8% in 2010. This translated into an increase of electricity consumption that ranged from 16 (0.7-3 GJ/t pulp) are generated onsite as main by-products [144], with research indicating that this approach could mitigate 12 million tonnes of CO 2 by 2050 [142]. A traditional paper machine consumes 140 TJ of electrical energy yearly, where 36% is consumed for paper production and mass transportation, 32% is consumed to overcome friction, and 32% is consumed for other losses [113]. Overall, in terms of electricity, in 2016, the European PPI amounted to 96 TWh [145]. The multi-cylinder dryer is the traditional approach to drying all kinds of paper (e.g. printing papers, cardboard newsprint and printing papers), and it is estimated that about 90% of the paper produced is dried using this technology. This approach represents the most energyintensive phase within the papermaking process, accounting for 70% of fossil fuel use and 50% of the energy consumed [38] due to the high energy requirements for vaporization [39]. In fact, in Sweden, the drying process accounts for approximately 20% of the country's total industrial energy use [146].
To show the impacts emerging from the PPI, Table 4, below compiles key facts to illustrate the magnitude that this industry has on energy use and emissions at the country level.
The PPI is one of the principal producers of self-generated electricity in the manufacturing sector and therefore produces enough energy to supply most of its needs. Since the forest industry is both a biomass consumer and a large producer, biomass's role within PPI production processes is paramount. Unlike most industrial sectors, the PPI generates energy sources as byproducts, and more than 50% of its energy needs are provided through biomass residues [55]. In consequence, the use of biomass lowers CO 2 intensity. Nevertheless, mills must still purchase natural gas, coal, fuel oil, and electricity to meet their energy needs [167]. Although the International Energy Agency (IEA) suggests that PPI products could, theoretically, be manufactured without generating CO 2 emissions [116], this idea applies mainly to Nordic countries where virgin wood represents the core raw material for pulping processes. In this region, biomass share is so high that fossil fuel consumption is nearly zero [53]. Moreover, mills are now producing heat and power for external use. For instance, in Finland, waste liquor from the forests industry represented about 12% (46 TWh) of the total national energy consumption in 2018 [53]. However, others claim that the Nordic PPI, even though it relies largely on renewable energy, is not as carbon neutral as it is often perceived [152].
In total, biogenic emissions emerging from the PPI were about 68 Mt in 2016 [16]. In the US alone, the PPI emits circa 150 million metric tons of CO 2 each year, of which 77% are biogenic [168]. The role of by-products is so relevant in the PPI that pulp mills are the largest consumers of wooden biomass, is among the largest emitters of bio-based GHG emissions and for each tonne of pulp production, an additional 19 GJ of black liquor is generated as a by-product [169]. The IEA estimates that around 2.5 tonnes of (primarily biogenic) CO 2 are generated per dry ton of pulp [170]. The combustion of black liquor in the recovery boiler creates the majority of the CO 2 emissions, about 74%, the fuel boiler accounts for 14%, and the calcination in the limekiln represents 12% [171].

PPI transportation and biomass use
The PPI consumes large quantities of forest biomass, including bark, forest logging residues, and black liquor. These resources can be further employed to produce electricity, heat, and biofuels [50]. Biomass, therefore, not only provides the raw material for the PPI but also contributes to 90% of the total fuel mix and facilitates internal electricity generation equivalent to 25% of the industry's electricity demand [172]. The PPI, in consequence, could use biomass instead of fossil fuels to increase the profitability of its traditional core business and improve its energy efficiency [173]. Depending on strategic choices, this process enables the further development of biorefinery options and novel technological pathways, a point we revisit in Sections 5 and 7.2.
In the EU, the share of biomass in this industry accounted for 38% in 2017 of the final energy consumption [71]. Considering the biomass employed in generating electricity and heat sold to third parties, the share of biomass in this respect increased to 59% in 2016 [173]. In the EU, biomass incorporation in the PPI varies from 0% to 89% in terms of final energy consumption, with the most representative case being Sweden [173]. In Norway, this represents about 24% only for heat production [174], while in Finland, more than 70% of the renewable energy mix comes from the forest industry [53]. In Finland, too, from the 38 TWh produced for district heating, forests chips and wood residue accounted for 36% of the fuel [175]. This may explain why regardless of an increase of 23% in PPI products, the Finish PPI's energy use only increased by 1% [39]. Hildingsson et al. [176], argue that this transition started during the oil crisis in the 1970s due to increased energy prices and government policies (e.g. renewable energy certificates and carbon taxes), which forced the PPI to seek new energy sources. The energy required for transportation is also a vital element that contributes to the emissions of this industry. Large amounts of fossil fuels are needed to collect and transport wood from forests to pulp mills and later to the manufacturing site [29]. Others note that paper mills are often located far from agricultural lands, which leads to high costs and GHG emissions resulting from transport [177]. One study indicates that transport accounts for 22 MJ per tonne of manufactured paper [178], while CEPI estimates that CO 2 emissions resulting from transportation could amount to 5 Mt CO2 per year [142]. Another important issue emerging from transport is described by Bataille and colleagues [179]. They argue that with the significantly increased consumption of PPI products from China, Latin America, and Africa, the emissions related to transport have increased considerably. For instance, in Finland, the contribution of transportation to climate impacts accounted for 8% of the total GHG emissions. However, if the pulp came from South America, climate impacts emerging from transportation could increase up to 27%. The authors note two issues arising from this dynamic. First, with the increase in offshore pulp, emissions from transoceanic freight transport will continue to increase. Second, presently no country is taking responsibility for international freight transport. Therefore, although countries like Sweden and Finland argue that their emissions have decreased, the global GHG emissions are actually increasing [180].

Waste and landfilling
The PPI is the third-largest water consumer after the metal and chemical industries [181], with some stating that "papermaking is essentially a massive dehydration operation [182]." Given the PPI's substantive use of water, large volumes of wastewater result from its operations. For instance, researchers estimate that one ton of product results in 60-300 m 3 of wastewater [183] or more than 47,000-80000 gallons of wastewater [184]. Under these circumstances, it may be unsurprising that the PPI contributes to about 40% of industrial wastewater globally [185], where close to 90% of pollutants originate from the cooking phase in the Kraft pulping process (i.e., black liquor) [186]. In China, the impacts of the PPI are huge since it contributes to 18.9% of industrial wastewater discharge [187].
Within the PPI, sludge is chiefly generated through biological, mechanical, or chemical methods in the primary and secondary wastewater treatment processes and accounts for 60% of the total cost of wastewater treatment [188]. Pulp and paper sludge (PPS) is a residual organic substance formed from rejected cellulose fibres and ash as a solid waste stream. A paper mill will, on average, generate up to 50 kg (dry weight) of primary PPS for each tonne of paper manufactured, which is equivalent to about 83 kg wet weight of PPS [189]. In Europe, the PPI generates more than 11 million tonnes of paper sludge waste per annum [190], while in Taiwan and Sweden, on average, around 250,000 tons of PPS are produced yearly [191,192]. Meanwhile, the USA and Japan have an annual PPS production of 5 million tonnes, while the UK and China produced 2 and 12 million tonnes, respectively [193].
Since PPS could increase by 48%-86% in the next 50 years, there is an urgency to find sustainable approaches for its disposal [191]. Current practices for waste management include incineration, burning and landfilling, all three leading to loss of valuable bio-energy resources, soil degradation, groundwater contamination and increased risk of methane and other GHG emissions [194,195]. Such effects come with high environmental and social costs [177]. Due to the large volumes of production and consumption resulting from the PPI, the industry is also responsible for large amounts of waste, generating between 25 and 40% of the annual municipal solid waste (MSW) worldwide [196]. It is estimated that for every tonne of paper manufactured, about 0.4 tonnes of waste is generated [185]. For instance, in the United States alone, from over the 250 tons of MSW generated yearly, around 72 million tons are attributable to wastepaper [197], while in Brazil, wastepaper accounts for 13.1% of the total MSW [198].
Paper products, including corrugated cardboard packaging, are the most common solid waste resulting from the foodservice industry [199]. For instance, in the UK alone, about 2.5 billion paper cups are consumed annually, producing around 30,000 mt of coffee cup waste each year. This means that around 7 million paper cups are used and disposed of daily [133]. On a global scale, numbers become more distressing. In mainland China and the USA, more than 10 and 50 billion paper cups, respectively, are consumed annually [200], while worldwide, the global annual paper cup consumption is between 250 and 300 billion [201]. Another study shows that due to the increasing demand for packaging materials, total world paper and cardboard production is estimated to reach 700-900 million mt by 2050 [202]. To put things in perspective, only in the EU, over 87 million mt of packaging waste were generated in 2016, from which 35.4 million mt were cardboard and paper [203].
Despite policies to reduce waste disposal and landfilling (mainly through energy recovery from different waste streams and fibre recycling), efforts are falling short. In turn, paper is thus landfilled or stored on-site, generating high costs for the companies. For example, in EU countries, storage costs between €15/tonne and €70/tonne for nonhazardous solid waste [196]. Moreover, when the paper is landfilled, it releases up to 398 ml of methane per dry gram, contributing to three-quarters of the total climate change emissions from the paper life cycle [178]. Others incinerate PPI waste. However, this entails high costs and creates environmental risks due to the compounds released into the atmosphere [204], the large quantities of acid, or the infiltration of organic materials into the soil resulting from anaerobic digestion [205].

Technologies and approaches to decarbonize the PPI
Although the global output from pulp and paper increased by more than 25% between 2000 and 2018, the sector's global energy use increased only by around 6% [123]. This dynamic indicates a decoupling of energy use from production. For instance, in most EU countries, over the last 50 years, fuel consumption in the PPI has progressively shifted towards biomass and natural gas along with heavy investments in combined heat and power (CHP) generation. This has resulted in phasing out most fossil fuel use, and the use of coal has substantially declined [54,206]. In turn, the PPI reduced total (direct and indirect) CO 2 emissions by 43% per tonne of product from 1990 to 2017 [207]. The reduction in emissions, however, is not limited to the EU. For instance, between 2000 and 2010, the Australian PPI decreased its emissions by 20% [208] and the Canadian PPI mitigated its emissions by 40% during the same period [171]. China, between 1990 and 2010, achieved a 30% improvement in terms of energy efficiency per tonne of paper produced [209], and the primary energy intensity of pulp production dropped from 16.1 to 13.2 GJ/t in only five years (2005-2010) [210]. Meanwhile, the Indian PPI reduced its energy intensity by 42% between 2003 and 2014 [157], while the UK cut its PPI emissions from 1990 to 2016 by 64%. The UK PPI emissions fell from 6.6 million to 2.3 million tonnes during this period [74]. Fig. 7, below, shows the evolution of environmental impacts emerging from the European PPI during the 1991-2019 period. This Figure also helps capturing how from an overall perspective, the environmental impacts caused by this industry decreased over the past decades despite the increase in the production market.
The literature attributes the reductions to a number of factors such as improvements in energy efficiency, increasing costs of fossil fuel due to market dynamics as well as taxes, increasing utilization of bioenergy and fuel switching, decreasing manufacturing activity (i.e., newsprint production), increasing use of recycling of paper, better-training of staff and favourable regulations concerning the use of low-carbon fuels [59,212]. Reductions in emissions, however, are not limited to CO 2 . For instance, US pulp and paper mills from 1990 to 2005 managed to mitigate SO 2 emissions by 60%, and NO x emissions decreased by approximately 15% [213]. More recently, from 2010 to 2012, SO 2 emissions decreased by 6.4%, while NO X emissions reduced by 26.4% over the same period [136].
Perhaps, the Swedish PPI is the most iconic case in terms of emissions mitigation. From 1973 to 2006, the Swedish PPI achieved an 80% CO 2 reduction in emissions [214]. The largest share of reduction of CO 2 emissions occurred between 1973 and 1990. During this period, the PPI reduce its carbon emissions from 8 million to 1.8 million tons. Bergquist and Söderholm [215] attribute this massive achievement to the oil crisis in 1973 and the increase in oil prices; these two factors influenced the Swedish PPI to reduce its oil dependency. For instance, in 1973, oil accounted for 43% of the total external energy use, and by 1984, the share of oil dropped to 16%. In 2011, oil accounted for only 5% of the total external energy. Moreover, in-between this period (1973 and 1984), the share of energy generated from onsite bioenergy increased from 55 to 72%, and later, by 2011, the share of bioenergy reached 79% [172]. As we will discuss in the following sections, there are different approaches to abate emissions from the PPI, including energy efficiency improvements, increased use of self-generated biomass, cogeneration, and increased rates of recovered and recycled paper. We, however, raise awareness that mitigating emissions from the PPI is not limited to the technologies but could also include social approaches, such as modifying users' preferences, corporate image and regulations [130].
Sticking to our sociotechnical framework, this section presents different technological innovations and practices that could help to decarbonize the PPI, with Fig. 8 displaying an overview of such approaches. We then describe 41 technological advancements and processes (see Table 6) to help transition the PPI towards a low-carbon future. We note that a successful transition towards the decarbonization of the PPI will depend on several aspects, ranging from the countries' geography to their political and economic environment; such elements should always be considered in low-carbon policies design [216].

Decarbonization options for raw and natural materials
Products emerging from the forest industry (i.e. paper) continue to store carbon even when the product is finished. It is only if the paper is left to decay, landfilled or burned that it releases CO 2 again. Recent research suggests that photosynthetic activity and vegetation growth are increasing globally [217], resulting in more terrestrial biomass carbon sink exists, which contributes to mitigating the growth rate of atmospheric CO 2 emissions. In the last decades, forests have absorbed about 30% (2,000,000,000 tons) of carbon each year of annual global anthropogenic CO 2 emissionsclose to the same amount as oceans absorb [218]. The US Environmental Protection Agency estimates that in 2016, wood products and forests stored and sequestered 10% of all CO 2 emitted by the US [219]. The United Nations Economic Commission for Europe (UNECE) indicates that the forest biomass in the EU comprises 9.8 billion tons of carbon, which is equivalent to nearly one-seventh of the CO 2 emissions from the EU. The same report adds that forests in the UNECE region account for about 40% of all carbon contained in forests [220]. Benefits are not only translated into environmental terms, but forests could also bring economic benefits. For example, the United Nations reports that decreasing the current deforestation rate by 50% by 2030 could avoid 3.7 trillion USD in climate change damages caused by GHG emissions. The report also notes that deforestation and the roles of forests are closely linked to other megatrends, such as food security and biodiversity loss [221].
Pugh et al., revealed that the regeneration of forests through sustainable practices in the PPI leads to more carbon sequestration. Their study shows that younger forests (those of 140 years or less) store 1.17-1.66 billion mt of carbon yearly, while old-growth forests sequester between 950 million-1.11 billion mt [222]. Chazdon et al., studied 43 forests in Latinamerica. Their study shows that even when old-growth rainforests are logged and trees start to grow back -forming second-growth forests-they still have an important role not only in regulating climate but in storing carbon. They conclude that if all these forests continue to grow for the next 40 years, they will store 8.5 Pg of carbon [223]. Another study indicates that recovering one mt of paper results in total forest carbon sequestration of 0.81 mt of CO2-eq [224]. The International Union for Conservation of Nature indicates that bringing 350 million hectares of degraded and deforested land under restoration by 2030 could sequester around 1.7 Gton of CO 2eq /annum [225]. A recent study [226] shows that adding 0.9 billion hectares of forest could remove two-thirds of the 300 Gton of carbon humans have added since the 1800s. The same study notes that those added trees could help sequester 205 Gton of carbon in the coming decade. Such emissions represent about five times the amount of CO 2 emitted worldwide in 2018. Tong et al., show that China has removed a carbon equivalent to 33% of regional fossil CO 2 emissions in the past five years due to the forest growth in harvested forest areas [227]. Perhaps that is why in China, the PPIs own the responsibility to carry out afforestation duties within 30 years, accounting for 0.85%, 6.34%, and 32.16% of the existing planted forest areas [80]. It is under these circumstances that Bellassen and Luyssaert call for more carbon-efficient uses of wood. For instance, harvesting more timber could be a well-designed climate mitigation strategy as well as using wood, which can be recycled, recovered or burnt, as a substitute for bricks, cement or steel [228].

Heating
In the PPI, the drying processes are the main emitters of waste heat. In some cases, steam consumption can account for up to 30% of heat provision [229]. Therefore, boiler technologies for processing heat provision can represent a resourceful approach to mitigate emissions. Laurijssen et al. [8], report that reducing 15% of primary energy is achievable for paper mills implementing heat-recovery practices in multi-cylinder dryers. A case study In Turkey delivered similar results, indicating that energy savings can be up to 16% through improved heat recovery practices. According to the authors, this number translates into mitigating emissions by 12 and 13% [230]. Kong et al., note that integrating a waste heat recovery scheme achieves energy efficiency improvements of 7.3% and decreases energy consumption by 4.6% with profitable investments [231]. The Best Available Techniques (BAT) Reference Document for the Production of Pulp, Paper and Board indicates that waste heat recovery from bleaching can reach steam savings of 30 MJ/t [59]. While Moya and team report that 1.07 GJ/tonne of paper can be saved by implementing the following types of heat recovery: recovery waste heat using heat pumps, heat exchangers, replacing the dryers in the paper machine with stationary siphons and mechanical vapour decompensations [71].
Others indicate that about 20-40% of consumed electricity can be recovered by integrating heat recovery technologies into paper machines with a payback period of less than four years [14]. Svill and colleagues presented similar results suggesting that cost-effective steam savings could reach between 7 and 13% by substituting heat exchangers in three paper machines [232]. Replacing the dryers with stationary siphons in a paper machine reportedly can achieve energy savings of 0.89 GJ/t and operating costs savings of $25,000 ($0.045/t) due to the improved drying efficiency [139]. The IEA argues that the most energy-efficient approach for mechanical pulping is heat recovery produced as a by-product of the thermomechanical process. This technique has a low investment cost of EUR 780/t of pulp per year and leads to energy savings of about 3.5 GJ/t of pulp [233].

Combined heat power
Combined heat power (CHP) has allowed the PPI to reduce natural resource use and improve its energetic profile. The PPI is one of the largest users of CHP since this technology achieves energy savings of around 30%. In 2016 about 95% of the total on-site electricity generation emerging from the European PPI was generated by CHP [173]. That is equivalent to about 50,919 GWh [59]. The PPI in Europe is the third-largest CHP user only after oil refining and chemicals and accounts for 10% of the total CHP capacity in Europe [145]. In the UK, about 65% of the PPI production comes from sites with CHP [74]. In the Dutch PPI, CHP generates a total heat production of about 7.8 PJ, from which 4.1 PJ accounts for electricity production and 1.3 PJ is sold to the grid [155] The CEPI states that due to the large implementation of CHP across Europe, CO 2 emissions have dropped by over 40% since 1990 [13].
CHP is perceived as a crucial technology for bio-energy and as an important intermediate technology for natural gas use [43]. For instance, in 2016, in CEPI members, biomass accounted for 57.7% of PPI's total fuel consumption, with 52.3% of the net electricity demand covered by onsite CHP installations [142]. Regarding efficiency, CHP plants utilizing fossil fuel or bioenergy (which is the case for most pulp mills in the EU) can achieve between 85 and 90% efficiency by employing a back-pressure turbine. However, this percentage can increase to 85-92% when generating CHP through a combined cycle gas turbine unit [59]. Despite these benefits, Möllersten et al., warn that kraft pulp mills with modern CHP systems based on biomass boilers and recovery boilers possess low electrical efficiencies of up to 15% [234].

Electrification
Use of low-carbon electricity is an essential action to decarbonize the PPI [58]. In Sweden, for example, since 1973, electricity has accounted for one-third of the PPI's energy needs [38]. What makes electricity a good alternative is that it can be used to produce low to generate hot water and steam via industrial heat pumps and electric boilers and therefore substitute for combustion of fossil fuels to generate steam for heating purposes [147]. For instance, electric boilers can provide low and medium temperatures of up to 400 • C. Therefore, theoretically, electric boilers could substitute for all fuel-fired boilers in the PPI [16].
Another study suggests that full electrification of drying processes can mitigate emissions by over 70% [235], while others argue that developing a CO 2 -free electricity grid would be the most meaningful way to decarbonize the PPI [132,155].

Carbon capture and storage
The PPI is an energy-intensive sector, with most CO 2 emissions arising from biomass combustion onsite [135]. Therefore, the PPI holds the potential to become carbon neutral or even a negative emitter by storing and capturing CO 2 or by using it as a raw material for other industries [236]. The potential for carbon capture and storage (CCS) in pulp mills is significant, and it is estimated to cover about 3% of the global carbon capture potential from bioenergy [130]. In the short term, the best approach to implement deploying biomass energy with carbon capture and storage (BECCS) in the PPI would be to capture CO 2 from the flue gases from the recovery boiler or through a combination of stacks (e.g. multi-fuel boiler, recovery boiler and lime kiln) [148]. What brings an advantage for BECCS in the PPI is that it can be realized with low additional costs compared to other industries (i.e. iron, glass and ceramics) [237,238]. For instance, there are significant prospects for BECCS within the PPI in Scandinavia due to high energy consumption and the available supply of biomass [170]. This potential is higher in Sweden and Finland, which reported 64 and 51% biogenic emissions, respectively, in 2017 [239]. In Sweden, for instance, deploying BECCS is perceived as a cost-efficient way to mitigate emissions from the PPI due to biomass' abundance, the large forestry industry and huge biogenic emissions [135,240]. While in Finland, the implementation of BECCS could deliver carbon neutrality for the same reasons [151]. Regardless of BECCS' potential, research indicates that successful PPI deployment will depend on energy markets [117] and the impacts of the European emissions trading system [241].
Among carbon capture technologies, post-combustion capture of CO 2 from bark boiler flue gases and the recovery boiler offers the highest CO 2 reduction potential [135,242]. In Europe, the annual BECCS potential for the PPI was estimated at 60 MtCO 2 and in Sweden 20 MtCO 2 [243]. Another study reported that up to 200 million CO 2 tons, or about 5% of 2018 EU emissions, could be abated with biogenic Carbon Dioxide Removal (CDR) from BECCS from existing sources. The same study reports that pulp and paper mills account for over 60% of biogenic CDR potential in Portugal, Sweden and Finland [244]. Another study estimates that the potential for capturing bio-based CO 2 emissions from the recovery boilers of the pulp mills accounted for 13.6 MtCO 2 /annum for a total investment of 2600 M€ [135]. Onarheim et al., argue that it is feasible to retrofit post-combustion CO 2 capture to a pulp and board mill or a pulp mill. Nevertheless, this will mostly depend on power and steam production onsite since retrofitting a CO 2 capture plant could massively increase electricity demand from 2.8 MW e to 14.6 MW e [118].

Alternative approaches
Other approaches to mitigating emissions are not related to technologies but to system optimization, maintenance, and alternative materials. We discuss some of these below.
Kong et al. [164], note after revising 23 technologies, that upgrading and maintaining steam traps, installing stationary siphons and enclosing the paper machine hood results in high costs and energy savings. Laurijssen et al., similarly reports that retrofitting and improving the current conditions of existing pulp and mills represents a cost-effective approach to mitigating emissions and cutting energy costs [55]. Likewise, Kesicki and Yanagisawa indicate that monitoring, management and improvement process of control could reduce energy consumption beyond what is accomplished through a focus on single equipment [249]. Boharb and colleagues [165] show that improving the energy efficiency of specific applications, such as compressed air and furnaces, could deliver savings of around 101.78 MWh of thermal energy and 347.85 MWh of electrical energy, corresponding to energy reductions of 2.22% and 11.48%, respectively. This same technique could mitigate 283.39 t of CO 2 emissions each year. Kong et al. [161] conducted an energy audit for a paper mill in Guangdong, China. Their results show that implementing nine energy efficient measures could mitigate 93, 454 t CO 2 annually with an opportunity for energy conservation of 268, 611.11 MWh. Chiu Lin et al. [22] reported potential energy savings of 211,694.4 MWh for 72 pulp and paper producers. The study identifies that the best approach for energy savings is to improve the facility steam systems. Yugan et al. [182], report that improved process parameters can save around 8% steam. This is equivalent to US$210,853 annually.
Other studies noted that non-wood pulps such as bagasse pulp, straw pulp, bamboo pulp and reed pulp can be used to manufacture tissue paper with lowered environmental impacts. Bamboo is perhaps the feedstock that is attracting the most attention due to its richness in cellulose, easy reproduction, and fast growth and regrowth properties [9,250]. Amode et al. reported that Mauritian hemp is an appropriate non-wood lignocellulosic substitute for biomass to wood for manufacturing printing paper [5]. Another study suggests employing recycled fibre as a raw material for newsprint decreases the amount of waste diverted to landfills, reduces the demand for virgin fibre and cuts the costs of the overall papermaking process. Regarding CO 2 emissions, these decreased from 6.5 t to 5.5 Mt for a mill operating at 30% recycled fiber content [251]. Employing orange tree pruning to acquire lignocellulose nanofibers also contributes to a more sustainable PPI by improving the recycling properties of paperboard products [252]. Manda et al. revealed that utilizing new coatings (nano TiO 2 or micro), combined with the diverse pulp types, brings savings in terms of energy, GHG emissions and compared to traditional paper [253].

Decarbonization options for the PPI transportation and biomass use
PPI products employ plant cellulose from biomass to generate black liquor as a by-product, which contains 65-85% solids [254]. However, black liquor gasification (BLG) is not a novel technique and has been developing since the 1960s. This technique is used as an alternative chemical recovery process within the PPI to substitute for traditional boilers [255]. BLG has become a more recurrent theme within the PPI because many recovery boilers currently in operation are economically obsolete, and implementing a BLG plant ought to have a higher future profit than installing a new recovery boiler [50,256].
The benefits of the use of black liquor and gasification have been amply explored. Al-Kaabi et al. [254], studied the characterization of neutral sulfite semi-chemical black liquor and reported the following benefits: a higher volatile content, neutral pH, and higher heating values. They also noted that using this technique represents a good candidate for biofuel applications and biomaterials. Sussaeta and Rossato report that black liquor can decrease energy costs and mitigate GHG emissions [257]. Naqvi et al., revealed that integrating synthetic natural gas (SNG) with BLG in small pulp mills without chemical recovery leads to significant mitigation of CO 2 emissions and production SNG without external biomass imports [258]. Man and colleagues [159] explored chemical cycle combustion and black liquor gasification in mini-sulfide sulfite anthraquinone pulp-making process. Their results indicate an increase in the energy efficiency of 12.2% and emissions decreasing by 6.2%. Petterson et al. [259], conclude that biomass gasification with dimethyl ether production is profitable and an innovative technique to mitigate emissions if CCS is applied. A similar approach was implemented by Larson et al. [260]. Their results indicate that the production of dimethyl ether, Fischer-Tropsch or mixed alcohols from BLG and biomass reduces capital investments compared to stand-alone plants and represents a more effective use of biomass. Naqvi and team [50] showed that integrating a BLG combined cycle delivered a 60-70% marginal improvement in electrical efficiency compared to heat-only powerhouse configurations.
Others have explored the utilization of sustainable biofuel alternatives produced from lignocellulosic biomass and other non-edible feedstocks to mitigate emissions from the transport sector [261,262]. The urgency to attend this call takes more relevance when Renewable Energy Directive indicates that by the year 2030, at least 14% of the transportation fuel must come from renewable sources, but only 7% can come from first-generation biofuels. The share of biogas and second-generation biofuels must account to least 1% by 2025 and at least 3.5% by 2030 [263]. Some even argue that second-generation biofuels may be a more promising avenue than first-generation biofuels [264,265] since these have less ecological and environmental impacts and improve economic efficiency [266]. However, these benefits do not come without challenges. For example, Panoutsou et al. [267], suggest that the successful transition toward second-generation biofuels depends on adequate incentives and taxation schemes to reduce the price gap with their fossil counterparts. While Gao and colleagues note that this transition could negatively impact small-scale farmers and increase deforestation and competition for water [268]. Other concerns are reported by Mustapha et al. In their study; they show that biofuel production for road transport could augment feedstock costs by 12-35%, depending on the feedstock mix and conversion pathway [269].

Recycling pulp and paper products
There are contrasting views regarding the impact of waste paper recycling in terms of CO 2 emissions. Some argue that mitigating emissions from recycling paper is not profitable in terms of environmental and economic costs [15]. Others argue that CO 2 emissions from use of virgin fiber (harvesting, forest cultivation, and chipping) should not be D.D. Furszyfer Del Rio et al. ignored [270]. For the reasons discussed in this section, we are more prone to support the latter position.
Resource efficiency is a key aspect of the PPI because it reduces urban waste, reduces energy consumption to manufacture products, and supports forest management [6,27,74,105,254,[271][272][273]. For instance, reprocessing waste paper to turn it into new products demands less energy and causes fewer emissions than manufacturing from virgin resources [274,275]. This becomes more evident in developing countries that still rely on coal for manufacturing pulp and paper products [276]. Others note that the benefits of recycling paper are not only in terms of emissions but also in the fact that it requires 70% less water than the production of new paper from trees. In addition, according to the USA Environmental Protection Agency, recycling one ton of paper saves over 3.3 cubic yards of landfill space and 17 trees [277]. Recycling paper is also important for countries with scarce forests and hence need to promote conservation [278]. The benefits of recycling paper are not limited to the PPI alone. The CEPI suggests that over 8% of collected paper is used for other applications, including animal bedding, construction, composting, and energy purposes [13]. Under this premise, recycling paper often leads to a cascade effect. That is, if paper waste is mitigated, this could mitigate manufacturing impacts and the use of fossil fuel sources [279].
Frequent practices for paper disposal include incineration, burning, and landfilling. However, this leads to losing valuable bio-energy resources and creates challenges in terms of environmental safety, energy security, soil degradation, groundwater contamination, economic and social problems, and increased CO 2 and methane emissions [190,194,204,205]. Recycling PPI products impose lower environmental burdens than burning, incineration, or landfilling [280,281]. More specifically, Laurijssen et al. [41] show that in the Netherlands, recycling paper provides about 1 t CO 2 /t paper compared to no recycling, which is equivalent to 3 Mt avoided of CO 2 emissions per year or mitigating from 1100 to 4400 kg CO 2 eq. per ton of paper. While in Switzerland, recycling paper and cardboard led to 25% lower electricity use and reduced water consumption [282]. Hämäläinen and team [113] report that GHG emissions from paper production using virgin pulp are 30% higher than recycling waste paper. Turrini also notes the benefits of recycling paper and shows that the EU recycling efforts in recycling paper prevented releasing three million tons of CO 2 in 2015 [12].
Paper recycling has increased significantly over the past decades, with 50% of waste paper being recycled globally [283]. Keränen and Ervasti break down paper recycling rates globally and note that the most recycled material is container boards, at 50%, and the least is printing/writing papers, at 6% [6]. In Europe, from 1970 to 2010, the recycled paper collection increased from 31 Mt to more than 210 Mt [281]. The EU recycles 72% of all paper and board and 83.5% of paper packaging [284]. The highest recycling rates are in the Nordic countries, ranging from 70 to 80% [285]. In the CEPI member countries, paper fibers are used 3.6 times, while in the rest of the world, the average is about 2.4 times [286]. In the USA, 264.2 Mt of MSW were generated in 2015, of which 68 Mt were paper. From this, 45.4 Mt of paper and paperboard were recycled [287]. During recent decades, China has become the largest producer of recycled PPI products, representing around 40% of the global production [288]. For instance, in 2017, China produced 111.3 Mt of PPI products, from which 88.7 Mt were recovered. The same study indicates that consumption of recycled paper augmented from 2008 to 2017 to 44.39-63.02 Mt, accounting for about 65% of the total pulp consumption [289]. Policies for recycling in China have toughened. For instance, in 2018, they announced the 'Operation National Sword' a regulation that bans imports of recovered mixed paper and other waste streams [290]. Buck suggests that this policy puts in evidence the flaws of the recycling system in which goods assumed to be waste in the Global North are recovered with cheap labor in the South [291].
Recycling PPI products also touch on social aspects. For instance, attitudes toward waste management indicate that recycling paper/ cardboard/beverage cartons is a priority for Austrian, German, and Belgium citizens [292]. This attitude is exemplified by the fact that rich countries often recover more waste paper than low-income countries [293]. Others have reported that training women to run businesses for recycling paper not only empowers them by generating income from waste management practices but also contributes to more sustainable livelihoods and the development of the green economy [294]. Further, waste picking is a form of job creation [295]. In India, for instance, garbage collectors and itinerant waste merchants collect between 30 and 65% of the paper waste [296].
Although paper is far more biodegradable and recyclable than plastics and other materials, it often ends in landfills where the degradation rate is slow. Moreover, in comparison with plastic, paper occupies more space than the same weight of plastic [297]. On top, it is a common practice that paper-based flexible packaging is laminated with aluminium and/or plastic, making the material non or very hard to recycle. Despite these negative attributes, almost all packaging initiatives are solely addressing plastics instead of paper. Rather, packaging manufacturers, as well as governments, ought to explore new alternatives to mitigate the negative effects on the environment during the paper's lifecycle and need to take action in better packaging design to reduce waste. This does not mean that efforts to improve packaging materials have been in vain; regions and countries are taking action by introducing more recycling-friendly flexible packaging solutions and better recycling systems (see Table 5). For instance, the UK, Germany and France have implemented stringent policies, including fees for introducing non-recyclable packaging and only using recyclable packaging materials. In North America, 16 states of the US have enhanced regulation around packaging waste targeting higher recycling targets and banning single-use materials.

Utilization of PPS
The PPI produces large quantities of organic sludges that could be employed elsewhere in new value chains, deliver environmental benefits and generate new jobs and business opportunities [299]. PPS can be used as an alternative for energy recovery through combustion benefits within the PPI (e.g. electricity and heat production). For instance, PPS for energy recovery practices can either be employed directly through Table 5 Global regulations adopting various approaches to minimize packaging waste and improve recycling rates. Source authors, compiled from Ref. [298].  Aq-vane technology The solution includes a thin passive liquid layer (or "Aq-vane") injected into the headbox between neighbouring pulp streams through a narrow hollow channel. In turn, the mix between the layers is avoided, and the process becomes controllable by tuning a set of process parameters. This approach decreases the use of fibrous raw materials and leads to reductions in energy consumption. Moreover, it enhances the paper's properties by improving the paper web surface roughness, layer purity, and the bending stiffness. High consistency papermaking Making a number of modifications to the low-speed high consistency headbox could improve the sheet formation. Applying this technique decreases the energy consumption during the stock preparation and improves the efficiency of the dewatering and vacuum system. Electricity and fibre use savings can reach 8% and 5-8%, respectively, with capital costs decreasing by 10-15%. Water Savings and quality Impulse drying during the wet pressing process This approach decreases water evaporation in the dryer section between 175 and 350 kg of water/t paper. Moreover, impulse drying achieves a reduction in steam consumption between 10 and 25%. Finally, it improves the paper properties such as stiffness, web smoothness, and strength.

Electrochemical sulfide
The application of sulfide improves the quality of wastewater for its later recovery.

CapWa technology
By installing gas separation membranes, water can be separated in a purified form to reuse paper. This technology can also mitigate GHG emissions emerging from the PPI.

Steam box
This technology preheats water to decrease viscosity, improve dewatering efficiency, and allows higher dry contents to be accomplished in the press section. As a result, less water needs to be evaporated. Steam savings can reach up to 4%.

Shoe press
This technology improves the dewatering process and diminishes the demand for thermal drying. With this technology, when applied, a 1% increase in the paper web's dry content delivers 5% in steam savings for the drying section. However, electricity use increases slightly. Other benefits include augmenting production capacity and enhancing product quality.

Water pinch
This technology identifies the minimum freshwater flow rate to synthesize the water consumption and match it with an appropriate flow rate.

Recycling Water
Increasing the use of recycling water in the PPI is necessary to achieve a more sustainable process. Since about 54% of the fresh water introduced into a mill is recycled. Dry debarking Dry debarkers generate 130-660 gallons of wastewater per ton of pulp which is considerably less than the 800 and 2600 gallons when wet debarkers are employed.

Use of byproducts
Directed green liquor utilization pulping This technology decreases both energy consumption by 25% and the effective alkali consumption in digesters up to 30%. Consequently, this decreases the use of kiln fuel consumption. In addition, Directed green liquor utilization in pulping increases pulp yield between 1 and 3% and augments pulp strength by 10% gain. Overall, this technology can mitigate emissions by 95%. Membrane concentration of black liquor.
With this technology, the energy cost for black liquor evaporation is reduced. In addition, it decreases the evaporation volume, the organic content of evaporators and lowers the boiling point with ultra-filtration concentration.

Gasification of black liquor
This technology can generate carbon-neutral energy products such as steam and electricity to use in pulping plants and biofuels to use in transport. Black liquor can either be upgraded to create syngas or used as fuel in onsite facilities to produce steam and electricity.

Biomass gasification
This technology helps reduce the use of fossil fuels and mitigates emissions emerging from pulp and paper mills. This approach mitigates NOx emissions by 30-40% and reduces disposal costs and solids waste. Hemicellulose extraction before chemical pulping This technology improves cooking liquor impregnation, the energy efficiency of kraft pulp mills, and its profitability. In addition, it augments the production capacity for pulp mills and reduces the cooking time.

LignoBoost
Lignin is a renewable organic material that can be used similarly to fossil-based chemicals derived from petroleum products. While LignoBoost enables lignin to be extracted from the Kraft pulping process. Therefore, LignoBoost could substitute fossil fuel in lime kilns and recovery boilers. The technology augments the potential of pulp mills and cuts costs. In addition, it could supply renewable raw products for other industries. Drying Dual-pressure reheat recovery boiler This technology improves electricity generation and steam cycle efficiency. It mitigates GHG emissions due to lowering the use of fossil fuels and increases efficiency.

Steam cycle washing
This approach reduces the consumption of gas and steam by 40% overall and the evaporative load by up to 50%. In terms of water, freshwater usage and plant effluent decrease by 45% and fiber yield increases by 1-2%. It also cuts operational costs by 40-$60/Air-dry-tonne pulp and decreases chemical bleaching consumption. Dry sheet forming This technology improves the drying energy consumption by up to 50%, and around 5 GJ/t paper of fuel is saved. In addition, it eliminates wastewater treatment and effluent.

Gas-fired dryer
This technology reduces the drying energy consumption by 10-20% and augments production by about 20%. In addition, it reaches a higher surface temperature that standard dryers and does not require much time to be installed.

Boost dryer
This technology increases the drying capacity by up to 12%. It also makes certain processes more efficient by reducing specific energy consumption, drying time, and the space required for the dryer section.

Condebelt drying
The main advantage of this technology is that is improves the drying rates by 5-15 times, also reduces steam use by 10-20%, and generates savings of 1.6 GJ/t paper in steam and 20 kWh/t-paper in electricity. It also enhances the paper strength by 20-60%.

Deep Eutectic Solvent technology
This technology could substitute typical mechanical and chemical pulping techniques through the extraction of lignin, wood hemicellulose, and cellulose at atmospheric pressure and at low temperatures. It works for both recovered paper and wood and requires little energy consumption. This technology can mitigate CO 2 emissions by (continued on next page) D.D. Furszyfer Del Rio et al. combustion or indirectly through physicochemical and microbiological processes leading to biofuel (by pyrolysis and bioethanol production) and biogas (by anaerobic digestion) [27,177,189,[300][301][302]. The process for energy recovery often entails the following steps: (i) dewater, (ii) dry the product to increase thermal heat capacity and (iii) recovery of biofuel, thermal conversion, or direct use of the aqueous phase in which PPS is processed for biofuel production. The main advantages of using PPS as fuel are volume reduction, hygienic disposal, and thermal energy recovery by steam production or superheated water for power generation [303]. Nevertheless, this technique can be costly due to the PPS dewatering process and the implementation of combustion facilities [304]. Another approach consists of using PPS for land application purposes [305][306][307]. PPS, in these circumstances, is applied at the beginning or end of the growing season by incorporating it as an organic soil amendment for land reclamation, agriculture and silviculture purposes [177]. The benefits of this approach include improving soil quality after plantation [308,309], a remedy for degraded soils and agricultural sites [310,311]. Note, however, that this technique could entail high costs since pulp and paper mills are often located away from agricultural lands, leading to GHG emissions related to transport and related management costs [177].

Paper waste in the construction industry
Waste emerging from the PPI can also be utilized in the construction sector. For instance, studies indicate that paper sludge ash can contribute to the manufacture of more sustainable mortars [197,312,313]. Another option to employ sludge from paper mills is to manufacture cement and bricks [59]. Paper sludge has also been identified as an option for replacing the mineral filler in many concrete mixes [314] and producing more sustainable pervious concrete [315]. Similar results were reported by Zule and colleagues when they noted that paper sludge brings significant economic and environmental benefits as an alternative hydraulic barrier layer for landfill construction [316]. Leire et al., suggest that waste generated from the PPI, namely, fly ash, biological sludge, and lime mud, provides environmental advantages to the cement industry. Their study shows that Portland cement clinker can be fired at 1390 • C instead of 1450 • C with such additives [317]. Other studies [318][319][320] also note that PPS can operate as a sustainable material in aggregates and concrete. The studies conclude that employing PPS helps to reduce environmental impacts caused by landfill sites, reduces the consumption of raw materials, and facilitates waste recovery practices. Viera et al. show that PPI waste can be employed to prepare ceramic tiles and bricks. Their results indicate that the use of sludge decreases by 3% the firing energy in the manufacturing of these products and complies with relevant environmental and technical standards [321]. Other researchers obtained similar results when they reported that paper sludge could deliver more sustainable clay and brick products [86,188,[322][323][324], adobe bricks [86,325].

Emerging technologies and processes for mitigating the environmental impacts of the PPI
Reductions in energy consumption in the PPI have been achieved through upgrading and maintaining steam traps, more efficient drying techniques, process optimization and other approaches. Table 6 presents 41 innovative methods that decrease emissions from the PPI manufacturing processes.

Barriers facing the decarbonization of the PPI
Although this review has focused on the decarbonization of the PPI and noted many options for it, its decarbonization is not simple, and issues confronting the industry are not all related to decarbonization. By utilizing the latent heat of evaporated moisture and producing steam through the evaporated water, this technology can provide 60-80% of the required heat for drying. Heat recovery from radial blowers used in vacuum systems By reusing the waste heat from the exhaust air, this technology can save 26 kWh/t with a payback period of 1.5 years. Use of thermo-compressors By reducing the condenser losses and with the use of thermo-compressors, this technology enhances the energy efficiency of the drying process, reaching steam savings of 25 kWh/t with a payback period estimated at 0.8 years.

Impingement drying
The main advantages of this technology are that its vertical design allows augmenting the capacity for a fixed floor area, facilitates a quick response to change grades, allows for balancing the evaporation from both sides of the paper, and increases the drying evaporation rate. Through-air drying (TAD) The main advantage of TAD is related to the drying process since it can minimize the drying time and mitigates undesirable effects of drying nonuniformities. Other advantages include improving the sheet's softness, bulk, and absorbance. Flash condensing with steam This approach allows the paper to be formed with a 70% solid content and requires less than 50% less energy for the process. Superheated steam drying By using th steam at a higher temperature than water's boiling point, this technology can achieve up to 25% in energy savings. Dry pulp for cureformed paper Through a viscous solution and fibres that are coated, this process can generate multi-layered products within a single step and generate energy savings of up to 25%. Gas heated cylinder By firing natural gas inside the cylinder either by using an infrared or impingement design, the heat flux of the paper web can increase and deliver drying rates of 2-4 times higher than steam-heated cylinders.

Pulsed combustion
This process can enhance the mass and heat transfer rates in impingement drying. The heat flux utilizing this approach can increase by 2.4 times compared to a traditional impingement system. Supercritical CO 2 With small changes in pressure and temperature and a variation in solvent properties, this technology holds the potential to mitigate CO 2 emissions by up to 45% compared to the 2011 baseline and provide primary energy savings of 20% within the current mill boundaries.

Energy-efficient vacuum systems
This technology generates 20-45% electrical power savings in the vacuum system and water savings of about 95% by recovering a large share of pumping energy through the exhaust air heat energy.

Recycling
Recycled paper fractionation This approach reduces the energy consumption, enhances the efficiency for ink detachment, improves the quality of pulp and reduces virgin fibre consumption.

Artificial intelligence
This technology can use algorithms for waste treatment optimization and improve resource efficiency. Sensors for recovered paper sorting These technologies automate recycled feedstock sorting. But perhaps more importantly, it delivers a more efficient deinking process that leads to electricity and steam savings of 16% and 30% respectively and a reduction of 20% in material loss compared to a typical plant.

D.D. Furszyfer Del Rio et al.
Instead, there are sociotechnical challenges and barriers that the PPI must address that broadly impact sustainability. We present these in the following sections.

The complex nature of the PPI as a barrier to its decarbonization
In this space, we note that the PPI is, by nature, a complex industry to decarbonize, and several elements influence the optimization processes to mitigate emissions. Such aspects should be considered throughout the points raised in the discussion section. In this sense, we first note that each product emanating from the PPI has specific values for energy consumption, as Table 7 depicts.
At the manufacturing stage, other processes influence energy efficiency. For instance, mills manufacturing chemical pulp in Finland and Sweden represent the major share of biofuels use and electricity production in both countries. On the other hand, kraft pulp mills convert about 50% of input wood to pulp; the remaining is often employed as biofuels for energy generation. Finally, the mechanical pulp production market is the most electricity-intensive process [339,340]. Similarly, raw materials preparation also influences the efficiency of the pulping process. For instance, debarking approaches -employed to augment the value of woody biomass by separating wood and bark into two products-impact the final product's energy efficiency and quality. Ultimately, the influence on quality and energy efficiency will depend on the debarking technique, whether this is a cradle, dry or drum debarking process [341]. Moreover, drying during the manufacturing process also requires large amounts of energy and influences the quality of the produced paper grades. Like in other stages, certain elements will affect the energy efficiency of the process; this could range from the fabric and impingement of the fabric -which are often neglected from the drying perspective-to the ventilation, the permeability of the fabric and techniques employed in the papermaking process [146,334] (see section 5).
Other aspects that influence the energy consumption and, more specifically, the heating operation processes is the type of wood used. When wood is employed as an energy source, the most important parameters include moisture content, calorific value, elemental composition and ash content [342]. For instance, wood wastes from mechanical pulp processes, including fines, bark and pines, are often considered a medium heating value fuel since their average calorific value on a dry basis is around 20 MJ/kg [343]. Others note that the calorific heating value of dry matter does not change much from one tree species to another since it often rounds 18.7-21.9 MJ/kg. Values vary to a small extent between species due to the different contents of extractives (e.g. terpenes, resin and fatty acids) and non-flammable substances (e.g. ash) [344]. However, it is slightly higher in coniferous species when compared to deciduous tree species since the resin and lignin contents are higher in the first tree species [345,346].
Similarly, other studies have reported that wood's chemical composition, specifically concerning the polysaccharide fraction and lignin, has a higher value of the heat of lignin combustion than polysaccharides [344]. Meanwhile, Kumar et al. note higher calorific values for the wood of older trees, higher ash contents in the wood of younger trees and a lack of influence of age regarding the carbon content [347]. Other studies have reported that location, ash content, and forest habitat type influence the calorific value of wood [344,347,348].
Another aspect that adds to the complex dynamics of mitigating emissions from the PPI is where resources are extracted from. For instance, there is a big difference between trees resourced from the natural environment and those sourced from plantations. Regarding the latter, these are generally not perceived as destructive to the ecosystem, unlike trees harvested from natural environments. Indeed, single-species plantations are frequently grown on land that would not sustain any other crop. Plantations, in this sense, are managed to supply raw materials for the PPI and usually feature fast-growing species such as acacia, eucalyptus, or conifer species. Therefore, the socioeconomic impacts from where resources are extracted are varied and could range from forest destruction, the loss of homes for indigenous people (as land is removed for conversion to energy crops) to rising food prices and helping control climate [349]. As such, and to push the agenda to expand to a biobased economy, there are policy instruments (e.g. European Union Timer Regulation) and certification processes (e.g. Forest certification and Programme for Endorsement of Forest Certification) that seek proper forest management to avoid conflict and unsustainable actions that harm forests [350].

Financial and economic barriers to decarbonization
Our research identified that among the main barriers to decarbonizing the PPI are those related to financial and economic disincentives. A number of studies identified that traditional investment cycles last between 25 and 40 years for process and utility equipment, and therefore facilities built at the start of this century (i.e. the year 2000) will have a maximum of two further investments cycles by 2050 [55,58,351]. Therefore, mills and machines built at the beginning of the century will still be operating by 2050 or coming to the end of their lives [352]. This may have repercussions on decarbonizing the PPI. Others have noted that critical investments for operations and production are not a priority due to the decline in demand for paper and print products [59,74]. Although energy efficiency is perceived as important, the decarbonization of the PPI is not a priority in the current economic climate [58]. Studies also indicate that initial high costs and the difficulty of recovering investments through sales represent another barrier to decarbonizing the PPI [14,353]. In the Swedish and Dutch PPI, some of the main barriers to transition to a low-carbon future are access to capital, lack of funding, and cost of production disruption from new technology adoption [354]. In Russia, the main barriers are related to deficiencies in legislation, constant changes in forests legislation, bureaucracy, lack of transparency, and insufficient protection of property rights [355].
Lack of investments in research and development operate as a barrier. For example, in the Swedish PPI, R&D shares of total turnover are traditionally below 1%. The same study notes that investments are often oriented toward improving processes instead of product diversification [356]. Financial barriers also represent the main obstacle to developing biorefineries. For instance, Pätäri et al. [357], and Toppinen et al. [23], report that transforming pulp mills into biorefineries calls for strategic planning, given their capital-intensive and risky nature and the years it takes to make such changes. Gregg et al. [358], add that transitioning to biorefineries delivers poor returns to stakeholders, and therefore, they have a low willingness to invest in them.
As mentioned in Section 5, BECCS is a key approach to decarbonizing the PPI. However, there are currently not enough economic incentives for its deployment nationally or internationally, and most likely, governments must intervene for its implementation [239]. Others note that negative CO 2 emissions are not considered in the European Union Emissions Trading System, and therefore, there is no real incentive for the PPI to deploy CCS technologies [118]. Others argue that the retrofit of CCS in the PPI will increase the cost of PPI products in the absence of carbon incentives to uncompetitive levels [240]. Similarly, Zhang and colleagues add that carbon prices between 2005 and 2008 were too low to generate incentives [359]. Others have noted that amine scrubbing, a form of CCS retrofit, has presented some challenges in its implementation, including adverse reactions of solvents with flue gas impurities such as O 2, NO 2, and SO 2 and thermal degradation [236]. In developing countries with low production rates, such as Tanzania, retrofitting with CCS seems like a costly avenue since the PPIs are often limited to a single facility away from other industrial complexes [238]. Others have noted infrastructure issues around the PPI. For instance, Lesson and colleagues [114] note that due to the nature of the PPI feedstock, facilities are typically located near forest areas and thus are not located near heavy industry clusters and potential transport networks. Therefore, building CCS projects of pulp and paper around heavy industry clusters seems unfeasible.

Training, capacity building and lack of knowledge
Trained labour to use complex new technologies is another common barrier to achieving the most energy and carbon efficient options. The existing workforce in the PPI is ageing, and there is no succession planning for technical roles [58]. There are other issues emerging from information and management policies to guarantee continuous efficiency improvements. For instance, equipment configuration remains a struggle for some facilities and obtaining information about more efficient equipment is often difficult [74,360]. The IEA attributes this behaviour to pulp and paper firms since they are often reluctant to share their best practices and develop more efficient technologies [123]. To this, Kong and colleagues report that information regarding new energy efficient technologies is often limited, scattered, and the technologies are not yet fully commercialized [202]. Others have reported that the PPI requires staff to manage a high complexity of production processes that currently lack skills for this [150]. In terms of cooperation, Ericsson et al. [16], indicate that the PPI lacks the knowledge to succeed in the deployment of biorefineries and that PPI firms should start seeking collaborations with chemical producers that, at the moment, are lacking. Expanding their collaborations with chemical producers may enable the PPI to acquire further knowledge regarding distribution and infrastructure channels related to fuel transportation. On the same track, Mäki et al. [361], reports that bioenergy retrofitting in the PPI demands networking with stakeholders beyond typical business partners. Essentially, they argue that retrofitting products emerging from biorefineries may need establishing a novel value chain from production to customer delivery.
Many of these trends make the PPI inefficient, with inefficiency more prominent in certain countries such as China where there is higher water and energy consumption and severe pollution. Even worse, the Chinese PPI relies heavily on fossil fuels, especially coal to conduct its operations, with coal-based thermal power representing over 80% of the energy supply. Although energy consumption in China declined by about 2.7% per year from 2000 to 2010 [82], there is still huge potential for reductions.
Other barriers are related to the diversity and how fragmented the PPI is. Since this is a heterogenous industry, there is no one-size-fits-all approach for its decarbonization. Andersson and Thollander [362] note that the scale of operations and the number of mills using different raw materials are a key barrier. In India, for instance, there are 85 large integrated mills. Each may have different capacities in terms of the scale of operation, levels of capital and skills, and mitigation opportunities [158,363].

Natural resource availability
Another barrier is the availability of natural resources for production. Studies have noted that the transition to biorefineries is obstructed due to the lack of resources and costs [36], mainly since other industries may compete for the same natural resources. For instance Ref. [364], Karlsson et al., suggest that in Sweden, biomass supply will be limited in densely populated regions where biobased heat and power plants have a main role in the delivery of district heating [364]. Toivanen argues that wood harvesting reduces the size of natural carbon sinks, and when the wood is employed to manufacture short-term products (pulp, paper, and bioenergy), carbon is quickly released into the atmosphere. The author argues that it takes up to 20 years for forests to begin storing substantial amounts of carbon [365]. Therefore, pressuring forests with increased harvesting only limits their potential to sequestrate CO 2. This aspect of climate science has challenged the Finnish bioeconomic strategy and questions how sustainable is the transition towards a bioeconomy.

Human and other environmental concerns
Finally, while decarbonization is key to PPI environmental sustainability, other issues arise concerning the social dimension of PPI sustainability. Research has identified androgenic and carcinogenic components along with chloropicrin compounds in pulp effluents [366]. For instance, one study revealed that from a sample of 4247 staff working in the PPI, 380 new cancer cases were observed within a year. From these, 322 patients showed ovarian cancer [367]. Another study reported that staff working in this industry possess a high risk for developing lung cancer along with tumours of the ovaries, prostate, breast, stomach, and nervous system [366]. Research has demonstrated a link between toxicity and risks associated with the use of recycled paper, particularly in food-contact applications [368,369]. Other studies have identified plentiful organic contaminants, such as bisphenols phthalate esters, in printing paper products, which may negatively affect humans' health [370]. Caselli et al. reported that all newspaper stands had high ambient levels of toluene. In some cases, concentrations were up to 100 times higher compared to outdoor levels. Their findings suggest health implications for those operating in newspaper stands [371]. Moreover, gasses that result from PPI products (e.g. sodium sulfides, methyl mercaptan, chlorine dioxides) are responsible for chronic disorders and other health complications such as nausea and headaches [34,372]. Singh and Chandra reported that methyl mercaptan wastewater generated from pulp and paper mills inhibits the cytochrome oxidase in human beings [373].
Bishnoi and colleagues [374] indicated that wastewater from the PPI contains several materials such as pentachlorophenol, trichloroguaiacol, dichoroguaiacol, tetrachloroguaiacol, dichlorophenol, trichlorophenol, copper, lead, nickel and chromium. All of which negatively influence the survival of animals and disturb functions at the molecular, cellular, individual, and population levels. Similarly, Caselli et al. [371] noted that wastewater in surface water bodies generates an unsuitable environment for developing and growing planktons, small fishes, and microbes, consequently affecting the growth of the fish market. At the same time, the use of chlorine-based chemicals coupled with organic content in the paper bleaching process has been reported to cause chronic disorders, respiratory diseases, skin irritation diseases and reproductive and mutagenic damages in aquatic life and terrestrial organisms, including humans [184]. Similarly, Haq and Raj indicate that methyl mercaptan effluent in the wastewater has been reported to cause watering of the eyes and nose, nausea and headaches. This substance is also responsible for causing adverse effects on aquatic life [375].
Other issues emerging from the PPI are related to land use and rights. For instance, indigenous communities continue to fight to have their rights respected across several regions in the northern hemisphere where the PPI industry operates, as reported by the Environmental Paper Network [376]. In China, reports indicate that the PPI has negative social implications for local communities and their employees' working conditions [377]. Brazil, another major player producing pulp and paper products, has conflicts over land acquisition for eucalyptus pulpwood plantations, affecting dozens of indigenous communities. Overall, over 7.5 million hectares of eucalyptus plantations have grown in Brazil, all of them using vast amounts of water, causing agricultural lands to dry up while damaging aquatic systems and affecting water quality. In turn, these lands have been coined 'green deserts' [64]. In Chile, the PPI is involved in conflicts with indigenous communities over lands that were transferred to pulp and paper firms during the Pinochet era, while in the last three decades, millions of hectares of land and forests have been assigned to pulpwood plantations without taking into consideration the communities who have owned and managed those lands [64]. In Uruguay, people also protested against eucalyptus plantations due to violations of communities' rights, negatively impacting rural communities, forests' displacement and lowering water tables [378].
Other studies have warned that women holding leadership positions in the forest industry are underrepresented, despite the increasing share of females enrolling in higher education programs. The study concludes that for women to escalate in this industry, they need to adopt a social position of "being one of the boys" [379]. Another study corroborates this view and notes that only 16% of females hold top management teams and sit on the board of directors in the top 100 pulp paper and packaging companies. The same study indicates that a female proportion on the top of PPI has positive effects on company performance [380].

Gaps and future research agendas
Following our systematic review of the literature, we present gaps in the literature and future areas for research in the next section. In this context, we focus on five areas, namely: digitalization: the sunset of printing media, bioeconomy an emerging business models, coupling to other sociotechnical systems, further studies on waste, land use, and environmental health and cross-cutting solutions.

Digitalization: the sunset of printing media
The dynamics of the PPI production are shifting geographically; most notably, the decline in output from North America and Europe has positioned Asia as the leader providing more than 40% of the world's PPI products [64,152,381]. The decrease in demand is not only attributed to the expansion of other markets but also to digitalization. In fact, Hetemäki and Nilsson, predicted that the internet would replace printed documents and other means of advertising [382]. In turn, companies across Europe and North America have shut down production lines while others are looking for novel market niches to develop new high-added-value products [383,384]. For instance, the number of mills and companies in the EU fell by more than 30% from 2000 to 2017, while the number of employees has also decreased by 37% since 2000 [23,385]. In Finland alone, around 40 paper manufacturing lines have closed since 2005, and the exports from the forest industry have fallen from 29% to 18% [386]. Meanwhile, in the UK, The PPI profit margins have shrunk by 3-10% [58]. More speicifically, in the UK writing, printing and newsprint paper production has experienced annual drops of 2.4% during 2010-17 and 3.9% in 2018 [123].
This transition does not mean the PPI is disappearing in the wake of digitization. Although the graphic paper sector is shrinking, the PPI as a whole is growing since other products (e.g. packaging and hygiene products) are filling the gap left by the reducing demand for graphic paper (see Section 3.2). However, digitalization's negative impacts on the PPI led many companies to navigate away from the industry and position themselves in higher-growth areas, whether through machine conversion or the redirection of investment funds. Consequently, this has led to overcapacity, higher levels of uncertainty and instability within the PPI. Most notably, this has been translated into generating an oversupply of materials in Europe, pushing producers to redouble efforts to export to other markets or to sell regionally [68].
From a brighter perspective, digital products can come with environmental benefits. For instance, Toffel and Horvath [387] report that reading the news on a PDA emits 32-140 times less CO 2 and requires 26-27 less water than reading a newspaper. Reichart and Hischier [388] showed emissions associated with reading a newspaper are about 33% greater than those associated with reading from an internet outlet. The CEPI indicates that energy efficiency improvements through digitalization and automation can mitigate 7 Mt of CO 2 by 2050 [142]. Another study reports that substituting paper with its electronic equivalent eliminates 47 GJ per tonne of office paper replaced [178]. Meanwhile, Calvo and colleagues claim that e-learning products generate fewer emissions and pollution than paper board products [108].
Digitalization could also help mitigate producers' costs through technological developments emerging from learning and automation processes [389,390]. As a result, this could help boost productivity in the PPI via leveraging data production to deliver better insights and outcomes [391]. Digital innovators that implemented such an approach have had material gains of 5-10% and considerable savings on chemicals, energy and materials. According to McKinsey [392], these savings represent between $4 to $6 billion in business opportunities for the PPI, with Fig. 9 depicting areas of opportunity within this industry's value chain. Another benefit emerging from the impacts of digitalization in the PPI is related to the innovation in next-generation bio-products. This could range from composite materials, nanofibers applications and lignin-based carbon fibre to novel processes to extract hemicellulose as feedstock for chemical and sugars and production [393]. Not only that, research in this area has noted that digitalization could increase forestry farms' income by 28% and farmer's life satisfaction by about 10% [394], while Jian et al., indicate that digital tools help fundraisers to understand how and where to launch forestry crowdfunding campaigns [395].

Bioeconomy: an emerging business model in the PPI
We noted there is a potential avenue for future research regarding business model evolution within the PPI. Particularly, how this industry will may end-up competing with the evolving oil refining industry. We see in this research an avenue where the coupling and chaining between industries could deliver fascinating studies in terms of technology and market dynamics.
The global forest sector generates more than US$1298 billion annually [396], from which the PPI made the largest contribution with 43% of the total gross value-added [397]. Nevertheless, and as mentioned previously, the PPI and, more broadly, the forest industry from industrialized countries have experienced a decline in demand for its biggest sector; printing paper. Shifting towards a bioeconomy is a promising solution for pulp and paper firms currently struggling with value creation [23,398], particularly in countries with large forestry sectors [399]. In addition, this change could have an important role in creating new jobs, products and generating new markets [215].
A bioeconomy is defined by the EU as "encompassing the production of renewable biological resources and their conversion into food, feed, bio-based products and bioenergy including agriculture, forestry, fisheries, food and pulp and paper production, as well as parts of the chemical, biotechnological and energy industries" [400]. Unlike the green economy, the bioeconomy centres on emerging bio-based sectors while considering resource and environmental constraints and global challenges to reduce dependence on fossil fuels [106].
Another emerging concept within the PPI is that of the biorefinery. A biorefinery entails a facility that integrates biomass conversion equipment and processes to produce power, fuels, and chemicals [58,401]. Biorefineries produce low-carbon commodities such as biofuels for transport and can also contribute to low-carbon energy supply, for instance, by feeding electricity and excess heat into the grid [16,39,154,402,403]. What makes this complex special is that biorefineries could operate under energy self-sufficiency [42]. In terms of their purpose, biorefineries are divided into two categories. First, to substitute fossil fuel consumption with bioenergy onsite, and second, to produce cleaner fuels or enhance production from process residues [53], with Fig. 10 depicting the forest biorefinery concept. The development of biorefineries and the increasing use of biomass is changing the public perception of the PPI. The industry has passed from being a cause of environmental problems to an industry that is part of the solution [404].
Biorefineries lead to energy efficiency and more profitable and sustainable production processes [42,405]. For instance, Isaksson et al. [406], list the main reasons for integrating biomass gasification and the production of transportation fuels in active PPI mills. These include: expanding the PPI's product portfolio, federal benefits emerging from implementing clean fuel sources, energy security, an increase in local jobs, and added value in the utilization of local resources. To this list of benefits, Brunhoffer et al. [385] and Gupta et al. [27] add that biorefineries offer technological potential that can optimize value chains, reduce waste and close material loops, create new customers and expand markets, revitalize rural areas where PPI mills are often located and reach consumer segments with higher environmental awareness.
Other reasons for integrating biorefineries in the PPI are the capacity to process large volumes of biomass, increase the efficiency for the utilization of biomass feedstocks, provide chemical recovery, increase annual profits, and leverage extensive experience managing biomass along with already established connections with forests owners [42,256,403,[407][408][409][410]. The benefits of biorefineries are so significant that a number of facilities have already been deployed. Perhaps the most prominent examples are in the EU. Among the most notable is the Navigator Company, which invested €55 million in a biomass boiler capable of mitigating 150,000 to 200,000 tonnes of CO 2 yearly. The boiler will consume about 400,000 tonnes of biomass coupled with 200, 000 tonnes of residual forest biomass acquired from abroad. Another notable project is AustroCel where the world's largest wood-based bioethanol plant has been developed with an investment of €40 million.
Within the forest-based bioeconomy, the PPI has a central role in contributing to the EU's GDP with 139 active biorefineries [411]. The CEPI estimates that the value of bio-based products is €2650 billion, Fig. 9. Digital tools to unlock value across the PPI value chain. Source, authors, compiled from [392]. Fig. 10. Visualising a biorefinery. Source [403]. Rio et al. which is equal to 3% of the total value of the forest industry in Europe [412]. In fact, Kunttu et al., indicate that PPI and bioenergy products are the main business structure for the Finnish forest industry [413]. This economic potential is not limited to the EU alone. The forest industry creates more than 513 thousand direct jobs in Brazil, generating socioeconomic benefits for more than 3.8 million people. The forest bioeconomy accounts for 1.3% of Brazil's GDP and represents 6.9% of its industrial GDP [357]. Moreover, with the current diversified product portfolio of the PPI, forest biorefineries are considered fundamental to the decarbonization of the chemical, plastic, and transportation sectors, which may as well generate greater revenues for the industry and new markets [414]. Fig. 11 presents emerging opportunities for the PPI in the bioeconomy. Although, it is worth noting that such processes and products are also located within the scopes of the chemical and oil refining industries.

D.D. Furszyfer Del
Given the plurality of approaches and benefits emerging from biorefineries, Table 8 presents some of the most relevant technologies that can be integrated into PPI mills for transformation into biorefineries. While some of these technologies can be combined, others are mutually excluded.

Coupling to other sociotechnical systems
The global PPI system does not exist in isolation, and like other industries, is linked to other sociotechnical systems [417]. As Fig. 12 displays, the dependence and interconnections from the PPI to many other sociotechnical systems are clear, indicative of a high degree of coupling. These coupled systems range from vital materials in society (e. g., sharing knowledge or expressing discontent) to critical products in the construction sector. Regarding the connections to sociotechnical energy systems, the PPI has an important role in energy and the forests industry, and, therefore, the transition towards a bioeconomy. The PPI even touches on sociotechnical systems such as chemicals, biofuels for transport, biobased materials, and agriculture. Finally, the PPI touches upon national and local recycling schemes, resource extraction, and circularity regulations. Our review notes that these interconnections can create fascinating dependencies and deliver outcomes that are rarely examined. For instance, if the PPI wants to transition to the biorefinery pathway, further research should address collaborations between the forests and chemical industries and how emerging markets will operate. The PPI also touches on the digital world through the use of reading material and even pulping materials used in liquid displays for computer or television screens.
However, this review identified a lack of research in this area of sociotechnical coupling since, for instance, most of the studies focused on how digitalization of media affects paper production, rather than the other way around. We suggest, however, that further research should center on how digitalization impacts the industrial society in terms of tonners, IT equipment, and other materials often located in the office space. Moreover, the strong coupling of paper and pulp to these other sociotechnical systems could reveal previously unseen or obscured pathways to decarbonization (as decarbonizing a coupled system could also result in lower carbon footprints for PPI processes or products). Finally, we recommend that research should address the environmental impacts of reading on screen versus reading from print media given the rapid and continually evolving nature of digital media. Current information on the topic is outdated very quickly. Fig. 11. Emerging opportunities for the existing PPI. In this figure, the black frames represent existing products while the orange and red structures represent new opportunities for processing to address these markets. Source, adapted from [415]. Gasification is flexible in terms of final product and raw materials. The benefits of employing BLG consists in reducing electricity and steam production and increases the capacity of pulp production. Production of ethanol through enzymatic hydrolysis of cellulose or acid followed by fermentation Byproducts and Ethanol (e.g. lignin and acetic acid) Reduces electricity and steam production.

Lignin extraction and refining
Solid fuels, lignin oil, dispersants additives for food production, phenols, carbon fibers, sorbents, binders etc.
Reduces electricity and steam production and increases the capacity in pulp production.
Separation and refining of hemicelluloses from solid biomass or black liquor Butanol, ethanol, acid xylitol, acetic, polymers, dyes adhesives, etc.
Reduces electricity and steam production and increases the capacity of pulp production. Refining and separation of extractives from bark and wood Triglycerides, tall crude oil, turpentine, rosin acids and medical substances, (e.g. antioxidants) Given that in mechanical pulp process, fibres are more intact. This approach enables to recover extractives found in low concentrations. Converting cellulose to alternative products Dissolving ethanol, pulp, biocomposites, and textile fibers Displaces pulp for paper production D.D. Furszyfer Del Rio et al.

Topical studies on waste, land use, and environmental health
Further research should include a focus on management practices to mitigate GHG emissions emerging from landfills where PPS is disposed. We identified a lack of research addressing recycling practices in developing countries, and most of the case studies regarding emissions and energy use are intensively distributed in European countries, Brazil and China. Even fewer studies addressed the role of households regarding resource conservation and paper recycling in developing countries. While most research will naturally focus on the main paper producers, studying other countries would deliver original and enthralling research. Also, we call for a deeper investigation into the role of waste pickers in developing countries and how they contribute to the informal economy. In terms of recycling, the lack of precision in terminology (i.e., recycling rate, collection rate, and utilization) is not defined evenly across regions. Consequently, it is difficult or impossible to compare recycling activities between regions or calculate regional summaries [418]. Further research should focus on trying to homogenize these terms so that comparisons across countries can be made uniformly and benchmarking of best practices is possible.
We echo the call from Mohammadi et al. [307], when they report the current lack of research addressing the effects of hydrochar (char made by hydrothermal carbonization) on wood productivity and soil emissions. We noted little research addressing emissions emerging from transport and waste and the effects of international trade in recycled paper and waste minimization. We believe these two areas show promising future research avenues due to the current economic climate (i.e., lack of demand for prepint paper) and the new legislations implemented in China.

Cross-cutting solutions
Based on this literature review, we argue that there is no single approach to decarbonizing the PPI; instead, a portfolio of technologies or mixed approaches should be implemented to reduce the increasing GHG emissions and energy consumption emerging from the PPI.
For instance, The Forest-based Sector Technology Platform [419], notes that the most efficient measures for reducing CO 2 emissions are implementing the best available techniques (BAT), adopting breakthrough technologies, and shifting from fossil fuels to biomass with integrated CHP. The same study notes that implementing these three measures can mitigate 20 Mt CO 2 emissions by 2050 [145]. The CEPI notes that combining improved energy efficiency technologies with demand-side flexibility and fuel switching can increase the product value by 50% and reduce energy demand in the wood fiber industry by 20% [333]. The CEPI also notes that applying only the BAT in all European mills could mitigate 10 Mt of GHG emissions [43]. Fleiter and colleagues note that combining emerging paper drying technologies and waste heat recovery in paper mills could mitigate emissions significantly [14].
Others have reported that in addition to recycling and waste recovery options, another promising avenue is shifting demand from the most detrimental materials and changing process technologies and feedstocks [36]. Martin et al. showed that emissions could be mitigated by 25% by implementing 45 energy-saving technologies in the PPI [332]. Griffin et al. indicate that the adoption of key techniques, including heat recovery, bioenergy, and energy efficiency improvements, can deliver the greatest results in mitigating emissions [154]. Laurijssen et al. [420] report that reducing heat consumption, increasing recycled fibre and decreasing the filler percentage can deliver savings of up to 15 GJ/ton in the PPI manufacturing processes. Fig. 13, shows how the noted solutions are not isolated to particular aspects of PPI decarbonization, but rather cut across many dimensions of the value chain. For instance, transitioning towards a bioeconomy, resource efficiency and recycling all can impact multiple aspects of the industry's system. We note that addressing cross-cutting paths can influence multiple stakeholders, policy-makers and industries simultaneously. Therefore, more research on cross-cutting options should be pursued in parallel to great focus on sociotechnical system coupling as the two notions are related.

Conclusions
To investigate the decarbonization of the PPI, we employed a systematic searching protocol and the guiding conceptual lens of sociotechnical systems. The systematic review aspect of our study shows that PPI products are intrinsically associated with human development as their products are used in buildings, sanitary and health appliances, packaging materials, and as means to spread knowledge. The PPI is also a key component of fostering a low carbon future through its relationship with forest conservation and afforestation practices, as well as business model evolution toward biorefining. The PPI is closely related to other industries (i.e., chemicals, forests, and transport). Therefore, PPI products are associated with other sociotechnical systems that create compelling interdependencies among industries.
The sociotechnical systems aspect of our study reveals not only the seamless web of economic, educational, legal, administrative, and technical elements involved in decarbonization of the PPI. It also, interestingly, reveals the contingency of a future net-zero industry pathway, one that will depend on the choices made by firms and policymakers (as well as users themselves) over the next few critical decades. Moreover, in this future transition, contests over technical feasibility can also be about social interests and values. That is, some decarbonization options may work fine technically, but fail to achieve social acceptance or political feasibility. Lastly, sociotechnical systems offer a useful framework for how one analyses the industry and industrial net-zero transitions generally. It offers a useful analytical tool that enables one to examine different parts of the system such as finance, training, resources, environmental concerns, and policies. This "checklist" can be fruitfully applied to other industrial transitions or even other transitions as a whole.
PPI products can be highly damaging to social and natural systems during their lifecycle, from the extraction of the raw materials for their production to their end of life. The PPI is among the top five most energy-intensive industries globally and is positioned as the fourthlargest industrial energy user. As stated earlier, the PPI accounts for 6% of global industrial energy and 2% of direct industrial CO 2 emissions, equivalent to 0.45 Gt CO 2 . This industry is also the largest virgin wood user, and it impacts human health and local flora and fauna (including aquatic ecosystems). In this context, Fig. 14 displays (in white) the PPI environmental and social impacts, ranging from the extraction of raw materials (e.g., land and soil degradation and biodiversity loss) to their final disposition.
Regardless of how complex and socially and environmentally detrimental this sociotechnical system can be, Fig. 14 displays possibilities (shown in green) that can help to decarbonize the PPI and ameliorate negative impacts. Options for raw and natural materials vary from resource efficiency to substitution via digitalization. Regarding the second step displayed in Fig. 14 (PPI products manufacturing), our study, in Table 6, provides 41 technologies, processes and emerging options that can help decrease emissions from the PPI. However, as noted in Section 7, there is no consensus on a single approach to decarbonizing the PPI. Instead, our analysis indicates that to mitigate emissions, the PPI must implement cross-cutting strategies that go beyond energy-efficiency initiatives and also consider measures related to recycling, resource efficiency, and even transitioning of business models towards a bioeconomy.
In Fig. 14, we show the barriers to decarbonizing the PPI. Although our review indicates that the main obstacles are financial and economic, we also elaborate on other hindrances. For instance, lack of knowledge and lack of cooperation among firms to develop and share low-carbon processes. Lack of trained staff constitutes another barrier. Finally, natural resource availability (mainly biomass) is yet another obstacle to transition towards a low carbon future via business model evolution. Fig. 14 further summarises the benefits of incorporating our suggested interventions into the PPI sociotechnical system. Most notably, the benefits are translated into more energy-efficient practices and transition towards an emerging business model that would allow the PPI to have a more diverse impact across industries while optimizing the opportunities for circularity given the inherent industry structure.
Our review also suggests promising avenues for future research. For instance, future research could usefully address collaborations between industries, such as chemicals, forestry and even oil refining, and the role of the PPI in emerging economies. Additionally, we suggest future research could explore the PPI from a holistic perspective by considering global energy use, emissions, recycling practices and the evolution of new business models. The promising future avenues discussed in this paper could help in making PPI research more diverse, more robust, more appreciative of sociotechnical coupling, and more seriously committed to cross-cutting solutions that can make it both more sustainable as well as profitable.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Aoife. M Foley third author on this paper is Editor in Chief of RSER, she was blinded during the review process and the paper was handled by another Editor.

Data availability
No data was used for the research described in the article.