Advances in upgradation of pyrolysis bio-oil and biochar towards improvement in bio-refinery economics: A comprehensive review

https://doi.org/10.1016/j.eti.2020.101276Get rights and content

Highlights

  • Provides critical analysis of pyrolysis process and bio-oil properties.

  • Value addition in bio-refinery concept.

  • Extraction of valuable chemicals from bio-oil.

  • State of the art information on bio-oil upgradation and biochar utilization.

  • Techno-economic studies highlight the reduction in bio-oil derived fuel cost in case of integrated bio-refineries.

Abstract

Depletion of fossil fuel reserves and ever-increasing energy demands necessitates exploring green and renewable biofuels. Though biomass-based fuels have the potential to replace fossil fuels but the low quality of fuels along with high process economics limits direct application. Concept of integrated bio-refinery is the unparalleled solution to this problem which involves the production of hydrocarbon grade fuels along with valuable chemicals from the pyrolysis derived bio-oil. This paper reviews recent advances in upgradation techniques of bio-oil along with moisture removal techniques and recovery of valuable chemicals from bio-oil. It also reviews current advances in the utilization of co-product such as biochar to increase the overall economics of the process. It also elucidates the challenges encountered as well as scope for future work. The current review is peculiarly based on pyrolysis based bio-refinery with detailed literature on the techniques of conversion of pyrolysis derived bio-oil into valuable biofuels and industrial grade chemicals with a view to improve the overall economics of the process through biochar utilization, which is not covered in other reviews. As compared to other published studies, this work provides more comprehensive information on the topic, which will give better understanding to a new researcher as well as help to plan and develop new techniques on bio-refinery for attaining the aim of cleaner and sustainable future.

Introduction

With the increase in technological advancement and industrialization, demand for energy is increasing worldwide leading to the problem of energy crisis. As stated by International Energy Agency (IEA), around 82% of the global energy requirements are fulfilled through fossil fuel reserves of which are depleting day by day (International Energy Agency (IEA), 2018). According to the world energy resources survey 2016, about 63% requirement of transportation fuels is attained only through conventional hydrocarbon fuels (World Energy Resources Survey, 2016). Moreover, fossil fuel produces CO2 emissions and other emissions like NOX, SOX, etc. to the atmosphere leading to the pollution problem. In 2015, the United Nations General Assembly (UNGA) adopted Sustainable Development Goals (SDG) to provide a framework for international collaboration to achieve a sustainable future for the earth. ‘Agenda 2030’ has 17 SDG with 169 targets to fight injustice and inequality, end poverty and protecting the earth’s environment. Among all, sustainable energy or SDG 7 is the central goal for ‘Agenda 2030’ to increase renewable energy share in the global energy mix, improve energy efficiency, and achieve affordable, reliable, and universal access to new energy services (Gielen et al., 2019). In 2015, about 194 countries signed the Paris Climate Agreement to reduce global temperature rise by 2 °C by cutting down the harmful greenhouse gas emissions and exploring new alternative energy resources (Saravanan et al., 2018). Thus, there is an urgent need to search for an alternative renewable and clean energy source that can reduce fossil fuel consumption. Among all other renewable sources of energy, biomass seems to be an effective and emerging alternative, which grows continuously on earth and accounts for 40% of the primary energy needs (British Petroleum Report, 2018). It is one of the most renewable energy sources which have great potential for solving energy-related problems such as the “greenhouse” effect and the depletion of non-renewable energy resources. Combustion of biomass emits roughly the same amount of CO2 as taken up during plant growth; therefore, it does not contribute to a buildup of CO2 in the atmosphere (Cherubini et al., 2011). National Policy on Biofuels was approved in India in 2018 to increase the replacement of fossil fuels by clean and renewable biomass-based fuels, aiming to achieve 20% blending of biofuels with fossil-based fuels by the year 2030 (Saravanan et al., 2018). Among all other thermochemical methods of extracting energy from biomass such as combustion, pyrolysis, and gasification; pyrolysis is the most suitable pathway to drive a wide variety of energy-rich products from biomass at a comparatively lower temperature.

Pyrolysis is the process of thermochemical conversion of biomass in which biomass is heated in inert atmosphere leading to the production of liquid (bio-oil), solid (biochar) and gaseous (non-condensable gases) products (Cherubini et al., 2011, Açıkalın and Karaca, 2017). Pyrolysis has been utilized from ancient times to produce charcoal, but the production of transportation fuels is a recent development. All three products have their own commercial applications, making it more effective for converting biomass. Bio-oil is the main product of pyrolysis that can potentially be utilized as a transportation fuel in place of hydrocarbon fuels like diesel and other heavy fuels (Onay, 2007). Temperature, heating rate, residence time, N2 flow rate, etc. are important process variables affecting yield and properties of products obtained through pyrolysis (Park et al., 2008, Salehi et al., 2011). Gasification is another thermochemical conversion process, which drives energy from biomass in the form of gaseous fuel. Pyrolysis has distinct advantages over gasification. Pyrolysis products are much easier to transport and store as compared to syngas produced via gasification. Pyrolysis produces wide range of products such as bio-oil, biochar and gases, having range of applications in various fields. Pyrolysis is quite flexible in terms of its operating conditions. It can be easily optimized depending on the desired product, while gasification is a rigid process that requires a detailed adjustment of process conditions. Also, lower temperature during pyrolysis as compared to gasification makes it a bit cheaper process (Tripathi et al., 2016). Additionally, lesser emission during pyrolysis compared to gasification, too makes it superior to gasification (Tripathi et al., 2016). Depending on the operating parameters, pyrolysis can be of different types, as shown in Table 1, with different distributions of product. Thus, the pyrolysis process can be selected based on the main product requirement. Pyrolysis of biomass produces fuels containing a lower amount of sulfur and nitrogen than fossil fuels (Miao and Wu, 2004).

Fast pyrolysis is the most appropriate and widely used process among all the processes for the production of bio-oil from biomass. It can give the highest yield of bio-oil, about 65%–70%, through high heat and mass transfer rates and rapid removal and quenching of volatile vapors (Xianwen et al., 2000, Arni, 2018). Although the higher yield of oil is obtained through flash pyrolysis, studies showed that low-quality bio-oil is obtained through flash pyrolysis with low thermal stability, high solid content, high water content and high corrosivity. Its viscosity and corrosive property increases during storage (Kumar and Nanda, 2016).

Key problems associated with direct use of bio-oil in combustion engines are low energy density, high viscosity, corrosiveness, high solid content and high thermal instability compared to hydrocarbon fuels (Zheng et al., 2008, Gupta et al., 2019). Thus, it requires significant upgradation so that its properties reach up to that of hydrocarbon fuels. Further, bio-oil contains large number of undesirable oxygenated compounds such as aldehydes (like acetaldehyde, formaldehyde), acids (such as acetic and propanoic acid), ketones (including acetone, cyclopentanone), alcohols (like methanol and ethanol), phenols (including phenol, methyl phenol, dimethyl phenol), furans (such as furfurol, furfural), sugars (1,6-anhydroglucose), miscellaneous oxygenates (glycolaldehyde, acetol) and many more (Gupta and Mondal, 2020, Lu et al., 2008). It also contains a high amount of water around 30%–40% (m/m or v/v) and many of these chemicals are present in the aqueous phase of the bio-oil.

Thus, there is an urgent need to improve the properties of pyrolysis oil before its commercial utilization. There are many methods available for upgrading bio-oil, but the process of evolving maximum high-value products with minimum waste is desirable. To achieve this target, an integrated bio-refinery concept is being investigated. Firstly, in this process, moisture is separated from the bio-oil as aqueous phase using different available methods and then valuable chemicals are extracted from the aqueous phase to improve process economics. Some chemicals may also be recovered from the organic phase. The residual organic part of bio-oil is then upgraded to transport fuel using several methods such as deoxygenation, catalytic cracking, hydrocracking, emulsification as well as blending with heavy fuel oils followed by refining. This leads to the way towards the improvement of overall process economics by extracting highly valuable chemicals from bio-oil and upgrading the residual oil into transport fuel.

Numerous review articles have been published on the concept of integrated bio-refinery aiming to produce fuels and valuable chemicals with zero-waste production. Most of the reviews on this subject give general overview of the concept describing different type of bio-refineries (such as pyrolysis based bio-refinery, biosyngas based bio-refinery, hydrothermal based bio-refinery and fermentation based bio-refinery) highlighting their importance and barriers associated with them but very few are based on any single type of bio-refinery process providing more exhaustive review (Demirbas, 2009, Banu et al., 2020, Neves et al., 2020). Several reviews on the conversion of different industrial wastes such as food processing wastes, winery waste and eucalyptus wood waste based on the concept on integrated bio-refinery through thermochemical and biochemical conversion into several high-value bioproducts, fuels, organic acids, organic fertilizers, etc. have been published (Teigiserova et al., 2019, Ahmad et al., 2020, Penín et al., 2020). Most of the studies have discussed on the recent developments and challenges in microalgal based hydrolysis, fermentation as well as combined thermochemical processes for the conversion of microalgae into energy rich biofuels (ethanol, butanol) and valuable bioproducts (biogas, biofertilizers, bio-oil, biochar) based on zero waste bio-refinery concept (Cuevas-Castillo et al., 2020, Fan et al., 2020, Rajak et al., 2020). A recent review by Hassan et al. (2019) provides critical update on future of bio-refinery industries along with technical challenges. It provides emphasis on its role in bioeconomy. Among most of the recently published reviews, a review based explicitly on the bio-refinery model of algal biomass to produce biofuels and bio-products discussed in detail different processes such as transesterification, oil extraction, fermentation, anaerobic digestion, hydrothermal liquefaction, pyrolysis and gasification (Kumar et al., 2020b). A recent review on activation and functionalization of biochar to produce biochar supported catalysts for numerous applications such as pollutants degradation, bio-syngas reforming, tar removal, biodiesel and value-added chemicals production has been published (Kumar et al., 2020c). Kumar et al. (2020a) also presented an innovative approach to produce bio-diesel from fatty acids and triacylglycerol (TAG) produced from bacterial metabolism using waste materials as carbon source. Another review of Nie et al. (2020) on microalgal technology discussed the bioremediation of pesticides containing water. Briefly, it introduced waste microalgae recycling by converting biodiesel and biochar through the bio-refinery approach. But it involves very little discussion on the bio-refinery model of microalgae oil conversion to biodiesel by focusing only on transesterification. Thus, a lot of work has been done in the field of integrated bio-refinery conversion but till date, there has been no study that is individually based on pyrolysis based bio-refinery with an exhaustive review on the different available technologies for conversion of pyrolysis derived bio-oil into valuable biofuels and chemicals. The present study is different from most of these recently published reviews. The current review involves detailed, exhaustive discussion on bio-refinery model of specifically pyrolysis process with different biomass feedstocks. It discussed the conversion and utilization of pyrolysis derived bio-oil into biofuels and industrial grade chemicals along with advanced biochar utilization pathways in a view to present a more complete model of bio-refinery which will help to improve the overall process economics.

This current study reviews detailed literature and recent advances on each of the different techniques involved for product utilization and concepts of moisture removal, valuable chemical extraction, emulsification, bio-oil co-processing in FCC refinery and bio-oil deoxygenation techniques. It also discusses in detail the advancements in biochar utilization in different fields, including in catalysis and bioprocessing. Also, this review explored the possibility of utilizing biochar in the process of upgrading bio-oil to make the process more integrated and economically feasible. A critical analysis of different methods has been discussed in detail. Challenges and scope for further development to make the process technically and economically viable have also been identified and discussed. Thus, as compared to published literatures including latest ones (Ahmad et al., 2020, Fan et al., 2020, Neves et al., 2020), this work will aid to the knowledge of the readers working in the area of pyrolysis and its utility, further this review will help to plan their future research more effectively, towards the aim of achieving cleaner and sustainable future.

Section snippets

Pyrolysis products properties and composition

In physical appearance, bio-oil is a dark brown color containing some solid particles, highly viscous liquid with a pungent smell. Water is present as principal constituent occupying about 30%–40% of bio-oil. Presence of water is mainly due to the internal moisture content of biomass or dehydration reaction occurring during pyrolysis. Water has both positive as well as negative effects on the utilization of bio-oil as fuel. Presence of water reduces the viscosity of bio-oil and improves its

Scope of value additions through oil and char upgradation

Application of bio-oil directly in place of diesel and gasoline fuel causes the following operational challenges (Oasmaa and Czernik, 1999):

  • Does not produce sufficient heat due to very low calorific value, large number of oxygenated compounds and high water content.

  • Provides flow barrier during its flow through injectors and engines due to high viscosity.

  • Results in corrosion and damage to engines due to the presence of acidic compounds in bio-oil.

  • Creates problem for starting of the engine.

Advances in moisture removal techniques

As discussed in the earlier sections, the presence of water content in bio-oil decreases its heating value and consumes a large amount of latent heat of vaporization. It also influences the performance of the bio-oil upgradation to liquid transportation fuels. To solve the water issue in bio-oil, different approaches have been tested in recent years, including pretreatment of raw material and modification in the process.

Hussain et al. (2018) adopted three strategies to reduce the content of

Chemical recovery from aqueous phase as well as organic phase

Upon separation of the water-soluble and insoluble fractions, various techniques including chromatography, solvent extraction, and distillation have been developed for the separation and recovery of aqueous and organic phase chemical compounds. Among these methods, solvent extraction is found to be more effective as it can be carried out at ambient temperature and pressure. Earlier studies also revealed that solvent extraction with one solvent is not enough to remove all the value-added

Advances in upgradation techniques of residual organic phase

Bio-oil can be upgraded via emulsification with hydrocarbon fuels such as diesel and surfactants as well as co-processing with heavy gas oils in FCC units. Both raw bio-oil and aqueous phase separated bio-oil can be used for this process. However, bio-oil used after aqueous phase separation gives better results in terms of the properties of the final product and the percentage of bio-oil blended. The organic phase is more miscible with hydrocarbon fuels due to the presence of a relatively

Advances in biochar utilization

During the thermochemical or hydrothermal conversion of lignocellulosic biomass, char is a common byproduct, which is also called as “Biochar”. Biochar is a carbon-rich, fine-grained, highly porous solid material obtained from the thermochemical or hydrothermal conversion of biomass in an oxygen-limited atmosphere (Qian et al., 2015). In recent times, biochar utilization has received considerable attention due to its physicochemical, biological properties, low-cost, and sustainable nature.

Techno-economic evaluation of the integrated bio-refinery concept

Techno-economic analysis (TEA) evaluates both the technical as well as economic aspects of the product with the quantification of all involved costs and technologies involved in the process. Evaluation of technical as well as economic prospects of the bio-refinery concept is an important study for checking its viability for commercialization. Minimum fuel selling price (MFSP) is an indicator used to test the economic feasibility of fuel towards commercialization. As shown in many studies, the

Challenges and future scope for development

Bio-oil seems to be a viable option for boilers and engines in place of hydrocarbon fuels for energy production and often acts as a resource for extracting value-rich chemicals. But the use of bio-oil is not an easy task and commercialization of bio-oil as fuel and chemical source possess the following challenges:

  • Bio-oil with low heating value, high viscosity, instability and corrosivity is produced through pyrolysis, which is quite inferior to gasoline and diesel fuels.

  • Upgrading processes such

Summary

Biomass seems to be a viable option to combat environmental problems and increasing energy demands across the globe. Raw bio-oil obtained through pyrolysis cannot be used directly for energy production in engines and boilers due to the low calorific value, high viscosity, corrosiveness and tendency to form gummy compounds on storage.

Integrated refinery technique is the most suitable technique for upgrading bio-oil which produces a large number of valuable chemicals, gasoline grade fuel along

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are likely to express their gratitude towards Ministry of Human Resource Department, Government of India, Department of Chemical Engineering, Indian Institute of Technology, Roorkee, India and Department of Chemical and Biological Engineering, University of Saskatchewan, Canada for providing assistance during the research study.

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