Hydrothermal carbonization: Sustainable pathways for waste‐to‐energy conversion and biocoal production

Hydrothermal carbonization (HTC) technology emerges as a sustainable method to convert wet biomass, including food waste and municipal solid waste into high‐energy dense biocoal. This process, conducted at temperatures ranging from 180 to 260°C and pressures of 10–50 bar, effectively transforms the organic material in wet biomass into solid, liquid, and gaseous outputs. The solid product, biocoal, possesses a high carbon concentration and heating values on par with lignite coal, presenting a cleaner alternative to traditional fossil fuels. Despite operational commercial‐scale HTC facilities globally, further adoption across various feedstocks can improve waste management and energy production. The process can achieve energy yields up to 80%, particularly at temperatures favoring the generation of secondary char with higher heating values. HTC not only aids in reducing greenhouse gas emissions through carbon sequestration in solid waste but also promotes environmental sustainability by yielding nutrient‐rich by‐products for agriculture. As a versatile and energy‐efficient solution, HTC technology is a pivotal innovation in waste‐to‐energy conversion, addressing the imperative for sustainable waste management. Other supplementary benefits are presented; they include higher employability, reduction of a nation's reliance on imported energy, and better waste control, therefore considering all pillars of sustainability. Future research should focus on optimizing process efficiency and exploring the broader applicability of HTC to various biomass feedstocks, enhancing its role in the global pursuit of sustainable energy solutions.


| INTRODUCTION
In the contemporary landscape, the exigency to mitigate the environmental burden of waste management intersects with the imperative to curtail reliance on nonrenewable energy sources (Barman et al., 2022;Gil, 2022;Kristia & Rabbi, 2023).The burgeoning volumes of wet biomass, an underutilized resource, present both a challenge and an opportunity (Bin Abu Sofian et al., 2023;Wang, Zhai, Zhu, Li, et al., 2018).Conventional disposal methods such as landfilling is becoming increasingly unsustainable, struggling under the weight of urban expansion and the demands of environmental stewardship (Iravanian & Ravari, 2020;Nanda & Berruti, 2021;Vaverková, 2019).The efficient conversion of this biomass into energy, without the prerequisite of drying, remains an obstacle yet to be surmounted effectively at scale.This scenario is further complicated by the economic considerations of deploying new technologies that have yet to be proven sustainable commercially.As nations vie for a sustainable future, the need for innovative solutions that can navigate the complexities of economic viability, environmental responsibility, and scalability is pronounced (Bin Abu Sofian, Lim, Siti Halimatul Munawaroh, et al., 2024).Within this context, hydrothermal carbonization (HTC) emerges as a beacon of potential, poised to address these multifaceted challenges by transforming wet biomass into a sustainable and versatile energy source known as biocoal (Cavali et al., 2023;Fakudze & Chen, 2023;Supee et al., 2023).Throughout the literature, the solid product of HTC is alternatively referred to as 'hydrochar'; however, in this work, we use 'biocoal' to align with terminologies that emphasize its energy content and applications similar to traditional coal.
HTC is a method by which numerous wet biomass feedstocks can be transformed into biocoal, a substance resembling coal in appearance and calorific value (Hoekman et al., 2018;Kruse et al., 2013).Bergius (1913) initiated the experimental investigation of HTC in 1913.The diminished attractiveness of biomass fuel production due to the identification of plentiful fossil carbon reserves led to a decline in interest in this technology during the succeeding decades (Marinovic et al., 2015).Subsequently, in the 1980s, Bobleter et al. (1981) rediscovered and revitalized HTC; since then, additional research on the subject has expanded substantially (Marinovic et al., 2015).HTC is a waste treatment method that finds particular application in food waste and various other waste streams, including wet agricultural residues, sewage sludge, algae and aquaculture residues, and the organic fraction of municipal solid waste (OFMSW) (Afolabi et al., 2015;Libra et al., 2011;Lu & Berge, 2014;Volpe et al., 2016).Ingelia SL, a Spanish corporation, possesses the intellectual property associated with the HTC technology.Recent reports have confirmed the availability of commercial-scale continuous HTC plants, showcasing successful nutrient recovery and operational feasibility (Lucian et al., 2021(Lucian et al., , 2022)).
HTC simulates the natural processes involved in the formation of coal.The oxygen and hydrogen contents of the feedstock are diminished through the process, which primarily involves dehydration and decarboxylation.HTC utilizes moist food waste (75%-90% moisture content) as opposed to other thermal conversion technologies that require drying of biomass feedstock; this transforms a complex process into one that is environmentally safe (Funke & Ziegler, 2010).Hydrogen-reducing reactions occur within an industrial reactor containing a suspension of liquid water at low to 225°C and high pressures (typically less than 20 bar), with a residence time ranging from 0.5 to 8 h (Figure 1).Direct execution or catalyst-assisted execution utilizing metal ions and citric acid of the HTC process can affect the quantity and quality of the final product (Hu et al., 2008).Biocoal has numerous applications besides fuel (Gamgoum et al., 2016;Messineo et al., 2012) for established coal-using sectors, such as utilities, cement, and brick manufacturers (Kruse et al., 2013); it can also be utilized as a raw material for the production of activated carbon (Titirici, 2013;Zhang et al., 2014), hydrogen storage, and electrochemical energy storage with supercapacitors or lithium-ion batteries (Bin Abu Sofian, Imaduddin, et al., 2024;Libra et al., 2011;Unur et al., 2013).
Furthermore, In the process of HTC, the aqueous liquid fraction contains dissolved organic compounds that can be harnessed for biogas production.By recirculating this liquid fraction back into the anaerobic digestion (AD) system within the waste management plant, these organic compounds can serve as an additional substrate for the broader applicability of HTC to various biomass feedstocks, enhancing its role in the global pursuit of sustainable energy solutions.

K E Y W O R D S
biocoal production, bioenergy, environmental sustainability, greenhouse gas reduction, hydrothermal carbonization, socioeconomic impact, waste-to-energy microbial consortia, thereby enhancing the methanogenesis process and increasing biomethane yield (Ghavami et al., 2024).This integration leverages the synergies between HTC and AD, optimizing the overall energy recovery from waste and contributing to a more sustainable and efficient waste-to-energy conversion process.In addition, HTC is also beneficial for soil amendments, as shown in the various applications of hydrovhar, particularly from agro-industrial and waste sources (Bona, Lucian, et al., 2023;Volpe et al., 2023).For instance, the work by Bona, Bertoldi, et al. (2023) found that mixing hydrochar with compost and then subjecting it to aerobic stabilization significantly improved plant growth, with the hydrochar-compost mixture doubling plant growth compared with compost alone.This mixture was tested at various doses, with the most effective being between 15 and 75 Mg/ha dry matter.Chemical analysis revealed that hydrochar has a high dissolved organic matter content, including small, easily degradable molecules, which correlates negatively with plant growth, especially at high doses.While hydrochar has potential phytotoxic effects due to its chemical composition, its judicious use in combination with compost and proper stabilization can effectively enhance soil organic matter and fertility, promoting sustainable agriculture practices.
Considering the sustainable potential of HTC technology, the purpose of this work is to examine HTC's requirements and the potential benefits of HTC processes over conventional technologies in terms of energy efficiency, environmental impact, and economic viability.To assess the quality of the biocoal generated, the authors also compare the effectiveness of utilizing various raw materials for HTC.In the ambit of this comprehensive review, the principles and particularities of HTC technology are explained, emphasizing the critical operational parameters and the distinct qualities of the resultant biocoal.The global intrigue surrounding HTC stems from its capacity to address waste management issues while producing high-quality biocoal.The influence of process variables, such as temperature, pressure, heating rate, residence time, pH, and catalyst incorporation on HTC are thoroughly examined.The adaptability of HTC to various feedstocks, including food waste, municipal solid waste (MSW), sewage sludge, and algal residues, is rigorously assessed.The energy dynamics of the HTC process are discussed in detail, showcasing its remarkable energy efficiency, and bolstering its viability as an alternative to traditional waste treatment methods.The economic analysis of HTC is meticulously scrutinized, considering both the prospective financial return and the broader economic impacts.Environmental considerations, particularly HTC's sustainability and carbon emission reduction capabilities, are carefully evaluated, underscoring the method's alignment with ecological preservation.The socioeconomic influences and potential benefits of HTC's widespread implementation are also articulated, reinforcing its integral contribution to achieving sustainability objectives.This review provides a nuanced understanding of HTC, highlighting its scientific, environmental, and socioeconomic dimensions.The ultimate goal is to affirm HTC's stature as a preeminent solution for waste management, particularly in contexts such as Malaysia, accentuating its crucial role in championing a sustainable future.The synthesis of extant research and discerning analysis aims to empower stakeholders, from policymakers to industry veterans and scholars, to navigate the nexus of innovation and sustainable progress.

HTC process
The HTC process needs to be analyzed to determine the effect of process parameters, that is, reaction temperature, water, residence time, heating rate, and catalyst, on the system.The following sections discuss the influence of each process parameter on the properties of biocoal as fuel, along with yield and quality (i.e., proximate and ultimate analysis).

| Temperature
Temperature is the most critical parameter of the HTC process.Elevated temperatures promote dehydration and decarboxylation reactions, reducing hydrogen-to-carbon and oxygen-to-carbon ratios in the processed feedstock.Consequently, this causes an increase in the carbon concentration of the biocoal.A 15%-60% reduction in the solid phase (commonly food waste biomass by dry weight) results from increased liquid and gaseous yields at elevated temperatures during the reaction.Erdogan et al. (2015) observed that although response time had no effect on yield, increasing the temperature of orange pomace (the pulpy waste remaining after the fruit is crushed) reduced HTC biocoal yields.Moreover, elevated temperatures may result in the degradation of a portion of the biocoal produced, thereby reducing the overall output (Xiu et al., 2010).The majority of polysaccharides remain undecomposed at comparatively low temperatures, producing biocoal that bears a resemblance to the initial raw material.
The study by Pecchi et al. (2022) explored HTC of biomass at 250°C, producing hydrochar with distinct solid yields based on feedstock type.For lipid-rich feedstocks like food waste, up to 50 wt% secondary char was obtained using ethanol, which is rich in liquid fuel precursors.In contrast, the primary char displayed enhanced coal-like properties.Conversely, substrates rich in carbohydrates, proteins, and lignocellulose yielded less secondary char.Solvents like acetone and dichloromethane were pivotal in maximizing the yield of primary char by effectively removing the oily secondary phase.This research underscores the critical role of processing temperature and solvent selection in determining the quantity and quality of hydrochar phases, with temperature being a key factor in optimizing the hydrochar's composition and energy recovery potential.
Moreover, the study by Lucian et al. (2018) investigates the effects of HTC on MSW, revealing that higher temperatures generally reduce solid yield but increase the high heating value (HHV) of hydrochar.Specifically, at 220-280°C, energy yields of up to 80% were achieved.The presence of secondary char, more reactive due to amorphous characteristics, was noted, particularly at intermediate temperatures around 220-240°C, where organic acids and furfurals in secondary char extracts peaked.Secondary char displayed a higher HHV than primary char, indicating its potential as a biofuel source.This underscores the temperature's pivotal role in optimizing the solid mass yield and energy content of hydrochars made from MSW.
In conclusion, temperature profoundly influences the HTC process, significantly impacting the yield and quality of hydrochars produced from various feedstocks.Elevated temperatures foster dehydration and decarboxylation reactions, reducing hydrogen-to-carbon and oxygen-tocarbon ratios, and enhancing the carbon content of the biocoal.However, higher temperatures can decrease solid yields due to increased liquid and gaseous phase production and potential biocoal degradation.Studies have demonstrated that temperature optimization can enhance the hydrochar's energy content and lead to the formation of secondary chars with higher energy values, indicating their potential as biofuel sources.Therefore, temperature dictates the HTC process's efficiency and the composition and energy recovery potential of the resulting hydrochar. 1.1.2| Pressure   Due to its significant reliance on temperature, pressure is considered an intermediary process variable.In low-temperature HTC, pressure is frequently generated using water vapor; therefore, the water content and the autoclave volume are significant factors.Consequently, elevated temperatures give rise to augmented pressure, which explains why pressure data are generally absent from published works (Inagaki et al., 2014).Maintaining the water in the liquid phase requires high pressure (10-50 bar); under these conditions, hot-pressured water generates a greater number of ions than ambient water, which functions as a solvent, reactant, catalyst, or product, in addition to serving as a precursor to the acid/base catalyst.Furthermore, elevated pressures have been observed to impede the typical mechanisms involved in dehydration and decarboxylation (Yu et al., 2023). 1.1.3

| Heating rate
The rate of heating, which significantly influences the formation of intermediates and the distribution of final products, is, in theory, one of the most critical factors for HTC.As opposed to a rapid rate of heating (20°C per min).Brand et al. (2014) discovered that solid residue with an increased HHV and a reduced O/C and H/C ratio benefited from a moderate heating rate of 2°C/min.It has been observed that continuous industrial HTC systems for food waste reach their maximum temperature in a comparable amount of time, ranging from 1 to 2 h.The reintegration of intermediates and the degradation of biomolecules can both be facilitated by a moderate heating rate that permits sufficient reaction time.It is apparent that the cooling rate, generally considered excessive when water/air cooling is employed, encounters an identical outcome.Berge et al. (2011) identify three prevalent heating methods utilized in HTC: an autoclave reactor equipped with an external electric heater, an oil bath, and superheated steam (Yoshikawa & Prawisudha, 2014). 1.1.4

| Residence time
The HTC process for food waste typically takes a few hours.Tradler et al. (2018) found that a residence time of 4-6 h at 200°C is appropriate for converting wet restaurant food waste into high-quality biocoal with fuel qualities similar to those of lignite fossil coal.Similar residence times have been reported on continuous industrial HTC systems (Personal communication with Andrew Gill).Increasing the residence duration of food waste in the HTC reactor often increases heating value and biocoal carbon content (Erlach et al., 2012;Erlach & Tsatsaronis, 2010;Heilmann et al., 2011;Krause, 2010;Müller & Vogel, 2012;Xiu et al., 2010).The residence time will only add to costs by a small amount, along with other costs in the form of heat loss from the equipment.However, the HTC reactor is well insulated, significantly reducing heat loss. 1.1.5| pH   As many works have suggested (Orem et al., 1996), the pH generally falls during an HTC reaction owing to the creation of several acidic compounds, including acetic, formic, and lactic acids.On a small-scale batch HTC unit for food waste, the pH could be as low as 5 (weak acidic conditions; Personal communication with Andrew Gill).Typically, acidic conditions accelerate biomass carbonization (Titirici et al., 2007), aiding the hydrolysis of cellulose.Funke and Ziegler (2010) stated that weak acidic conditions enhance the overall HTC reaction rate and, nevertheless, the HTC reactions may be inhibited by pH values that are too low (Krause, 2010).Although an acidic pH may aid in breaking down the lighter organics for a longer equipment lifespan, a more neutral pH is ideal. 1.1.6| Catalyst   Empirical evidence supports the notion that incorporating a catalyst into the initial process water improves HTC.This phenomenon may be attributed to the influence of the acid catalyst, which induces a decrease in pH and promotes dehydration and polymerization, ultimately leading to char formation (Bi et al., 2017).Biocoal production has been increased by several HTC techniques employing metal salt catalysts, acids, and alkalis (Lynam et al., 2011;Muppaneni et al., 2017;Neyens et al., 2003;Yang et al., 2014).Escala et al. (2013) reported that the HTC of stabilized sludge utilized citric acid as a catalyst, which accelerated the carbonization processes and generated biocoal with an HHV.
Furthermore, the study by Ma et al. (2023) investigated the catalytic effects of surfactants and citric acid on the HTC of pomelo peel for solid fuel production.Incorporating SP80/citric acid as a catalyst notably increased the mass yield of hydrochar to 63.45% and improved its HHV to 31.302MJ/kg.This catalytic combination enhanced the solid yield by agglomerating organic intermediates and significantly improved the combustion performance when co-fired with coal, showing better combustibility and synergistic effects.The findings highlight the pivotal role of surfactants in modulating the composition and properties of hydrochar, thereby enhancing the energy density and efficiency of biomass conversion processes in HTC.
Moreover, the study by Faradilla et al. (2020) focused on enhancing the HTC of nanoscale cellulose using citric acid as a catalyst.Citric acid significantly impacted the hydrochar's morphology and thermal properties, producing a uniform and spherical shape in the nanodimensioned material.The yield of hydrochar from softwood pulp increased from 32.2% to 50.5% with citric acid, demonstrating its effectiveness in improving the yield and quality of hydrochar.Additionally, acetone rinsing post-HTC increased the surface area of the hydrochars, indicating the method's potential to produce high-quality hydrochars suitable for various applications, including bioenergy and carbon sequestration.
Numerous methods have been proposed for repurposing moist agricultural waste, including corn cobs.To generate bio-oil from maize cobs, a sodium hydroxide catalyst loaded at 1.03%-1.56%was utilized in HTC with a maximum yield of 41.3% (Gan & Yuan, 2013).Lynam et al. (2011) found in a separate study that the energy value of the recovered biocoal is enhanced by the addition of acetic acid and lithium chloride.

| End products of HTC
HTC products can be observed in the following three states: solid, liquid, and gaseous (Kumar et al., 2018).In particular, their proportions and characteristics (primarily temperature and residence time) are influenced by the process parameters and the input component (which may contain varying amounts of carbon and moisture).In general, the solid phase (50-80 wt%) is produced predominantly by the HTC reaction at temperatures ranging from 200 to 225°C and pressures of approximately 30 bars.The char produced via the HTC method is referred to as biocoal, which signifies the presence of water throughout the procedure.HTC char is called wet char, biochar, and HTC char, among other names.This essay contains an application of the term biocoal.This is followed by the liquid phase (5-20 wt%), also known as process water.Finally, a minuscule quantity of gaseous products, predominantly CO 2 (2.5-5 wt%), is introduced.The product distribution of an ordinary HTC process is about 80% solid, 16% liquid, and 4% gas (Libra et al., 2011).

| Solid
Lignite, bituminous, anthracite, and subbituminous coal are the four primary types of fossil coal.(1) Hard coal, also called anthracite, represents the highest quality of coal.It has a low volatile matter (VM) content, a high fixed carbon content (92-98 wt%), and a HHV (35-40 MJ/kg).( 2) Boiler of intermediate rank, bituminous coal contains 60-80 wt% fixed carbon.It is the most prevalent type of coal utilized in electricity generation and has an average heating value of 24-35 MJ/kg.Subbituminous coal, with a fixed carbon content ranging from 60 to 70 wt%, exhibits a higher HHV of 19-26.7 MJ/kg compared with lignite (10-20 MJ/kg).( 4) Lignite coal, also known as brown coal, is the lowest grade coal and contains the least quantity of fixed carbon (35-45 wt%).However, the foremost and most urgent issue associated with fossil fuels is the climate catastrophe, to which coal remains the primary contributor to CO 2 emissions.
HTC biocoal is a solid, stable, hydrophobic, cleanburning substance with a fuel value comparable to lignite fossil coal (Funke & Ziegler, 2010).The largest solid biocoal product by weight percentage-wise is the carbonization by-product.An important attribute of this substance is its considerable fixed carbon content (Lynam et al., 2015).Consequently, it finds utility across a wide range of applications, including but not limited to serving as an energy provider, storing hydrogen (Berge et al., 2011;Libra et al., 2011), eliminating antibiotic resistance genes and pathogens from livestock mortalities, treating plant mortality with biocoal (Ducey et al., 2017), and extracting antimony, arsenic, copper, cadmium, and lead from water (Han et al., 2017).Additionally, biocoals can sorb pesticides, consequently mitigating water resource contamination (Sun et al., 2016;Wang et al., 2016).

| Liquid
Following filtration, the liquid phase (process water) remains the biocoal suspension.The liquid comprises various organic acids, including furfurals, phenols, and their derivatives.These acids are generated through the hydrolysis process of simple sugars (Berge et al., 2011;Funke & Ziegler, 2010;Reza et al., 2014).Especially when a brief residence time is implemented, the HTC process water creates favorable conditions for AD, which can generate biogas.Furthermore, the process water contains inorganic ions such as potassium and phosphate and short-chain carboxylic acids, both of which are beneficial for plant development (Bevan et al., 2021).
Nevertheless, the liquid phase can potentially cause complications both within the HTC process apparatus and when it naturally discharges.Thirteen of the 680 organic contaminants assessed for their presence in the process water were detected in trace amounts (Bevan et al., 2021).To reduce the TOC and other nutrients, process water must be treated anaerobically or aerobically (Funke & Ziegler, 2010).After this, it can be regarded as a liquid fertilizer designed to promote plant development (Berge et al., 2013).
Studies into the fusion of two techniques, such as creating AD-HTC hybrids, can address the issue of one technology using the other's by-product (process water).According to a study conducted by (Reza et al., 2014), processing AD digestate through HTC produces 20% and 60% more energy per kilogram of raw biomass than processing HTC alone or AD alone, respectively.Hence, as businesses that previously invested in AD aspire for higher energy outputs, HTC applications combined with existing AD facilities may increase during the ensuing decades. 1.2.3 | Gas   This component of the carbonization product has the lowest weight percentage and economic value.As a result of the decarboxylation process, the predominant gas generated during HTC is carbon dioxide.Based on the substrate and reaction intensity, it has been determined that CO 2 comprises the predominant portion (90%) of the gaseous effluents produced (Ramke et al., 2009).The residual constituents consist of several insignificant substances, such as CH 4 and H 2 (Sharma, Jasrotia, et al., 2020).There has been a lack of research regarding the capture, extraction, and utilization of the gaseous effluent produced by the HTC process.However, the potential of the HTC process to predominantly generate carbon dioxide while minimizing the release of hydrocarbons like methane and hydrogen has been recognized for its environmental benefits (Child, 2014).Therefore, by utilizing an HTC plant, the production and discharge of substantial quantities of hazardous greenhouse gases that result from the landfilling or combustion of renewable biomass could be avoided (Bevan et al., 2021).

| Effect of different biomass feedstock types on mass yield and specific attributes of HTC end products
The mass yield and specific attributes of HTC end products are significantly influenced by the type of biomass feedstock used.While process parameters like temperature and retention time play a crucial role in determining the physiochemical properties of the resulting biocoal, the nature of the biomass feedstock, including its moisture content and organic constituents, is equally important (Ischia & Fiori, 2021;Nizamuddin et al., 2017;Sharma, Sarmah, et al., 2020).
Process parameters, such as temperature and retention time, exert a greater influence than other factors on the physiochemical properties of the final products during the production of biocoal.The biomass feedstock and its constituents hold equivalent significance (Cao et al., 2013), notwithstanding the substantial influence of all process parameters outlined in Section 1.1 (Requirements for HTC) on biocoal, liquid, and gas production and quality (Yan et al., 2009).
HTC prefers feedstock with substantial initial moisture (Berge, Flora, et al., 2015).The moisture content of the initial feedstock is critical to the energy balances associated with carbonization.The energy required for water evaporation is greater than that required for heating.Berge, Flora, et al. (2015) assessed the input energy requirements for converting wet feedstock with varying moisture contents via incineration, pyrolysis, and HTC to illustrate this theory.To maintain a moisture content of 65% for the HTC process, water is supplied as a wet weight.The investigation results indicate that HTC requires less energy input than incineration and pyrolysis when the moisture content of the feedstock is greater than 8% (by weight) and 30% (by weight), respectively.
In addition, the work by Gómez et al. (2020) on HTC of biomass reveals significant findings in transforming wet biomass into energy-rich hydrochar.HTC operates at temperatures between 180 and 250°C, with mass yields of hydrochar ranging from 35% to 60%.This process produces a carbon-enriched solid and reduces the oxygen and hydrogen content through dehydration and decarboxylation, enhancing its fuel quality.Continuous HTC processes have demonstrated improved efficiency, potentially yielding higher quality hydrochar compared with batch processes.For instance, the continuous process of wood chips followed by entrained flow gasification showed that hydrochar gasification is more efficient than raw wood.Furthermore, life cycle analysis (LCA) highlighted the environmental benefits of hydrochar usage, indicating lower greenhouse gas intensity compared with coal-generated electricity.The simulation model developed provides a comprehensive understanding of the mass and energy balances in HTC, offering insights into optimizing the process for commercial application.
The study on the effect of different biomass feedstocks on the HTC process underlines the pivotal role of feedstock characteristics in determining the efficiency and outcome of carbonization.The moisture content of the feedstock is a critical factor, influencing the energy balance and the efficacy of the HTC process.Research findings demonstrate that HTC can efficiently process various biomass types, including food waste and aquaculture residues, into high-quality hydrochar with enhanced energy properties.
1.3.1 | Food waste and agricultural waste Explorations into HTC have shown its potential in transforming various food and agricultural wastes into biocoal, a sustainable energy source.Tradler et al. (2018) conducted a study examining various restaurant feedstocks, such as vegetable-based food (50 wt%), carbohydraterich food (33 wt%), and animal-based food (17 wt%).The study's findings revealed that feedstocks with high protein and lipid content yielded substantially lower amounts of biocoal than feedstocks rich in carbohydrates (>60%).In addition, carbon beads, which indicate the formation of coal, formed in response to carbohydrate-rich meals (Benavente et al., 2015;Dinjus et al., 2011;Hu et al., 2008;Simsir et al., 2017).
Several household and commercial organic wastes were carbonized efficiently by Ramke et al. (2009).It was observed that the biocoal produced maintained a carbon content ranging from 75 to 80 wt% of the initial input.Biocoal is a captivating fuel alternative due to its comparable composition of constituents and calorific value to brown coal.Based on their break-even point calculations, the authors determined that a 50 L volume capacity HTC plant operating on a small scale for restaurants could achieve profitability within 8 years.Saqib et al. (2018) reported comparable results, indicating that the carbon content of food waste HTC exhibited a 62-73 wt% increase compared with that of the raw feedstock.By the results, a food waste biocoal mixture improved low-rank coal's ignitability and devolatilization characteristics.Wang, Zhai, Zhu, Peng, et al. (2018) validated these characteristics with biocoal pellets produced from restaurant food waste, which consisted primarily of cooked meat, rice, noodles, condiments, and paper cups.How the mixture burned was significantly influenced by the biocoal content.A synopsis of food waste and an inventory of elements detected in solid phase biocoal are presented in Table 1.
Enzymatic hydrolysis was applied to uncooked food waste from a "multi-ethnic food court," including fruit peels and vegetable bits, as well as condiments including salad dressing, ketchup, and cocktail sauce (Kaushik et al., 2014).It was observed that the carbon contents and calorific values of biocoal produced using enzymeassisted pretreatment exhibited a range of 43.7%-65.4% and 17.4-26.9MJ/kg, respectively.In contrast, the pretreated biocoal displayed no variation in these parameters, with values spanning from 38.2% to 53.5% and 15.0-21.7 MJ/kg, respectively.According to the authors, the co-production of biocoal and bio-oil from food waste is a potential application of the enzyme-assisted hydrothermal process that merits consideration.The HTC method has been demonstrated in recent studies (Berge  , 2011;Hwang et al., 2012;Ramke et al., 2009) to be highly suitable for the co-combustion with coal conversion of food waste into a solid carbon material suitable for a wide range of energy generation applications.Li et al. ( 2013) evaluated the carbonization of food wastes (including cooked and uncooked food, condiments, and packaging materials, including plastic, paper, and condiment containers) to produce energy.The authors state that alterations in reaction temperature had a negligible effect on the distribution of carbon.However, changes in the initial solids' concentration did have an effect due to the heightened solubilization of the compounds.As the proportion of packaging materials increases, the energy content of recovered solids diminishes due to the low energetic retention of packaging materials.They determined that HTC operates most efficiently when exposed to food waste consisting of 32% solids by wet weight at the time of receipt and 275°C.Hwang et al. (2012) utilized dog kibble to recover solid fuel via HTC and discovered that the resulting biocoal contained a carbon concentration exceeding 75%.Orange waste and olive pomace were both utilized as feedstock for the HTC and pyrolysis processes utilized in the production of an adsorbent.The adsorption capacities of adsorbents produced from orange waste via pyrolysis and HTC were equivalent, while those fabricated from olive pomace demonstrated a significantly higher adsorption capacity (Pellera et al., 2012).Zhang et al. (2018) investigated the feasibility of power generation using moist agricultural waste, specifically fruit waste.They demonstrated that after HTC, the amount of ash in substances, including rotting apples, apple chips, apple juice, and grape pomace, decreased significantly.Debris accumulation in ash may result in fouling, slagging, and corrosion complications (Libra et al., 2011).As the temperature rose, the carbon content of all samples increased while the oxygen concentration decreased, as determined by the scientists.With increased HTC process temperature, the energy yield, HHV, and fixed carbon content all exhibited an upward trend, while the VM content demonstrated a downward trend.
In their study, Hoekman et al. (2013) established that biocoal produced using woody feedstock had energy levels ranging from 28 to 30 MJ/kg at 255°C, equivalent to subbituminous coal.This study's results apply to grape pomace and winery waste (Pala et al., 2014).Xiao et al. (2012) researched cornstalk HTC and found that the HHV of biocoal was 66.8% higher than that of the fresh feedstock.In contrast, Oliveira et al. (2013) applied the HTC method to various combinations of agricultural wastes using the co-hydrothermal carbonization technique, which produces biocoal with almost no ash and a high HHV (Lang et al., 2019).Biocoal quality improves as the proportion of low molecular weight carbohydrates in waste mixtures increases, as determined by the authors through analysis of mass and energy losses during the processing stage.
Furthermore, the study by Parshetti et al. (2013) showcases the effectiveness of HTC in processing palm oil empty fruit bunch (EFB) into hydrochar, focusing on its utility in co-combustion with low-rank coal.Achieving a high-energy density of over 27 MJ/kg, the hydrochar retained 68.52% of the original biomass energy when treated at 350°C.This process exemplifies a method for converting agricultural waste into valuable resources and highlights its potential in energy recovery, showcasing an energy retention efficiency of 68.52%-78.18%across different temperatures.The study's comprehensive analysis, including FE-SEM, FT-IR, XRD, and BET characterizations, underpins the carbon-rich nature of the hydrochar and enhanced combustion properties.Notably, the research delineates the HTC's role in sustainable waste management and energy production, underlining its capability to treat food and agricultural wastes efficiently, hence offering a promising avenue for advancing green technology.
Moreover, the study by Yan et al. (2019) investigated the hydrothermal treatment (HT) of EFBs to enhance its utility as a renewable energy resource.The work highlights the transformation of EFB, notorious for its high moisture content and low calorific value, into a superior fuel through HT.Conducted at temperatures ranging from 120 to 220°C, the HT process significantly boosted the fixed carbon content and energy density of EFB, with the heating value reaching 19.47 MJ/kg.Notably, the VM to fixed carbon ratio decreased from 0.82 to 0.68, indicating a more stable and efficient fuel form.Moreover, the treatment effectively diminished the ash and alkali metal content, addressing common issues related to combustion and gasification processes.The subsequent pyrolysis of HT-EFB produced gases richer in CO, CH 4 , and H 2 , alongside a diverse array of valuable liquid compounds like alkanes, phenols, and aromatic groups.This study establishes HT as a viable method for enhancing EFB's fuel characteristics, thereby contributing to the sustainable utilization of agricultural waste in energy production.
Studies on HTC of food and agricultural wastes demonstrate its efficacy in enhancing the energy value and carbon content of the resulting biocoal, making it comparable to brown coal.The process offers a sustainable method for waste management and contributes to the generation of renewable energy sources.The consistent findings across various feedstocks underline HTC's role in improving ignitability and reducing VM in co-combustion with coal, reinforcing its potential as a key technology in the transition toward greener energy alternatives.
1.3.2| MSW and sewage sludge MSW is typically used to describe garbage gathered from urban and suburban businesses, neighborhoods, and residences.The socioeconomic status, consumption patterns, and collection sources of the agglomerations, regions, and nations all impact the MSW composition (Khiari et al., 2006).It typically has a high proportion of VM and a medium to low amount of FC.Nevertheless, depending on the mineral concentration of the feedstock, ash content can range from extremely low (0.28%) (Lin et al., 2016), to high (43.11%)(Gai et al., 2016) percentages.According to Lu et al. ( 2012), the MSW HTC causes a 6.39-9.0times increase in initial energy density.
According to the carbon fractionation data from this carbonization research, the biocoal created from MSW carbonization may be a sizeable carbon sink.Once organic matter is digested anaerobically in wastewater treatment plants, sewage sludge is produced (Vasco-Correa et al., 2018).Its output has significantly increased in recent years as a result of the population expansion and urbanization that is occurring at an accelerated rate.According to Song et al. (2019) research, the HTC procedure altered the sewage sludge's characteristics by boosting its hydrophobicity, lowering its oxygen content, and increasing its energy density.In the HTC samples, three different organic acids-oxalic, citric, and tartaric-were utilized at the same concentration of 20 moles, and the removal effectiveness of phosphorus pentoxide was 93.5%, 86.7%, and 55.6%, respectively. 1.3.3| Algae and aquaculture waste Microalgae and cyanobacteria comprise proteins, lipids, non-cellulosic polysaccharides, and nucleic acids rather than lignocellulosic.Heilmann et al. (2010) demonstrated that the biocoal production by HTC of microalgae resulted in products with distinctive compositions and energy contents comparable to coal.The HTC method had very gentle conditions, such as a reaction time of 0.5 h and a temperature of 200°C, which led to acceptable carbonization levels and algal biocoal materials yields (Table 2).The authors also concluded that a continuous process may be created for the HTC processing of algal waste, given the batch processing's comparatively quick reaction time.The HTC of lignocellulosic feedstock has been the subject of several investigations in the past to produce different gas and liquid fuel goods (Elliott et al., 1988;Knezevic et al., 2010;Peterson et al., 2008).Dote et al. (1994) described one of the groundbreaking studies using this conversion approach with microalgae.In that work, Botryococcus braunii, a high hydrocarbon content microalga, was employed, and a maximum oil production of 64 wt% was achieved at 300°C with a 1 h reaction period and the inclusion of sodium carbonate catalyst.The main reason for the high oil output and much higher HHV (40-50 MJ/kg) was the composition of B. braunii.Furthermore, research by Garcia Alba et al. ( 2012) demonstrated that the HTC of Desmodesmus sp. was examined at various reaction circumstances (i.e., 175-450°C and a 1-h reaction period).The product yield distribution throughout the oil, gas, aqueous phase, and solid residue was identified with correct mass balance closure.At 375°C and a 5-min reaction time following HTC, up to 75% of the calorific content of the algal feedstock could be recovered.Nevertheless, high nitrogen concentration in the oil (up to 6%) accompanied significant oil output.

AND ENERGY BALANCE
The economic viability of a process is highly dependent on its energy efficiency.In biocoal research, energy yield is typically used to assess the energy balance or performance.The amount of energy maintained in the product, or the amount of energy produced from the product per unit energy input in the feedstock, is determined by the energy yield of the product (Weber & Quicker, 2018).Using the experimental data, two methods may be used to determine the energy balance of any process (Ábrego et al., 2018).By deducting each product's standard enthalpies of formation, an energy balance is first calculated, taking into account the products (biocoal, bio-oil, and gases) and feedstock at standard temperature and pressure.Each product's HHVs and the enthalpies of the related combustion products are used to compute its standard enthalpies.Second, process heat is calculated by estimating energy fluxes and sensible heat transported by all products (and latent heat of the condensable vapors).Because complete heat recovery is not achievable, the second method of calculating the energy balance is more realistic (Ábrego et al., 2018).Thus, the energy balance of the generation of biocoal may be determined using Equation (1).HTC's energy balance offers a thorough benchmark for financial investments and business opportunities.After initial heating, the proportion of input energy will  be greatly reduced since HTC is an exothermic process, and the released energy can keep the process temperature constant.The energy balance for the HTC process of solid sludge at various temperatures and residence durations was examined by (Zhao et al., 2014).According to their research, the procedure could be sustained at 200°C for 30 min.And 52.4% of the energy produced by burning biocoal may be used to operate the HTC and the mechanical dehydration process.In comparison, 47.6% of this energy might be retrieved as heat or electricity.Danso-Boateng et al. ( 2015) examined human feces heated to 200°C for 30 min to study the energy efficiency of the HTC process.
The results of this study showed that the quantity of solids in the feedstock could affect how efficiently the process used energy, (ii) no external energy was required for the entire HTC process, which included dehydrating the feedstock, and (iii) the feedstock's moisture content was crucial to the HTC process and a low moisture level was necessary for high-energy recovery for the sustainability of the HTC.
Determining the energy balance in the HTC process is crucial to assessing the plant's energetic and economic feasibility.Bevan et al. (2021) have demonstrated this in their calculation (Table 3).

DISADVANTAGES OF HTC
The advantages and disadvantages of the HTC technology are summarized in Table 4.

| HTC versus other thermochemical processes
Table 5 summarizes the various thermochemical reactions' common process parameters and product distribution.
Nonetheless, it should be emphasized that different reactor conditions will be utilized based on the reactor size, the feedstock type, the product's intended use, and the technology manufacturer (Bevan et al., 2021).The primary differentiation among identified thermochemical processes lies in the capacity of reactors to process feedstock with a moisture content ranging from 75% to 90%.
Any substance with a high moisture content would require thermal drying prior to pyrolysis, an energyintensive process (Bevan et al., 2021).Given the unwelcome energy required for intensive biomass pretreatment, eliminating biomass feeds with high moisture content is the more pragmatic course of action.This underscores the importance of HTC, a bioenergy technique capable of processing food waste and other moist biomasses containing considerable moisture.

RETURN ON INVESTMENT
HTC is a promising technology for converting food waste with high water contents (75%-90%; Feng et al., 2019).More so, the technology is useful for treating other problematic biomass streams like animal manure and urban waste, that is, sewage sludge, which represents a serious environmental and human well-being issue in developing nations (Pham et al., 2015).Economic feasibility and environmental impacts are the two main components and decision factors when considering waste conversion technologies.Return on investment (ROI) measures returns associated with money that has been invested in the business in a particular time frame.To calculate the ROI, the total expected financial gains are divided by the total costs of the project multiplied by 100.
In a LCA of Co-Formed Coal Fines and biocoal by Liu et al. (2013), they concluded that the ROI of the HTC system would depend on the selling price of the co-formed T A B L E 3 Energy balance estimation.

| 13 of 25
SINGH et products and that the minimum selling price for the break-even operation depends upon the cost of uncarbonized biomass feedstock, HTC plant size, and blend ratio.Given the situation in Malaysia, costs associated with securing food waste and transportation are likely to be major determinants.In a similar study, Medick et al. (2017) concluded that the location of the HTC plant and the logistics are crucial determinants of the profitability of the HTC system.They further reported that the use of HTC biocoal as a bridging technology might be most suitable as it offers a flexible and demand-oriented supply of energy from coal.

| Economics of HTC (biocoal) production
When biocoal is first brought to the market, economic research will be crucial in determining its viability.Economic analysis takes operational expenses, feedstock collection costs, equipment costs, and production costs into account to determine the total cost of any facility.The labor cost and transportation costs for collecting make up the collection cost (Kumar et al., 2020).According to Kumar et al. (2020), it is crucial to comprehend these expenditures (labor, transportation, insurance, etc.) because they change annually.Without considering environmental advantages like a decrease in GHG emissions like CO 2 and methane, biofuel coal cannot compete with fossil fuel coal (Saqib et al., 2019).Yet, several studies have demonstrated the economic viability of HTC and biocoal (Lucian & Fiori, 2017).Analyzed the cost of producing pelletized biocoal from a facility with a 20,000 t/year capacity for grape marc.Whereas the predicted biocoal break-even price was 226 $/t, the production cost was 177.41 $/t.This value corresponded to 9.38 $/GJ HHV in terms of energy.The overall capital investment was repaid over 10 years.
A proposed plant with a 20-year lifespan and a capacity of 2000 tons of food waste per day underwent a techno-economic study by Mahmood et al. (2016).

Advantages References
HTC can be applied to various biomass feedstocks (animal manure and sewage sludge) with high moisture content (75%-90%) without pre-drying With the use of an economic model, economic analysis links an examination of the mass and energy balance in a fully integrated biomass processing to an assessment of the overall cost of production.In all three situations, the techno-economic comprises expenditures for raw materials, transportation, utilities, operation, and cleaning (Table 6).The authors concluded that the minimum selling prices of the products were much lower than the market price levels using the sensitivity analysis of the non-enzyme pretreated hydrothermal oxidative refining process (for instance, the minimum selling price of biocoal was 30 $/t, which is more than half of the market price of coal, which is 85.68 $/t).For a 1-ton rice husk fuel biorefinery, Unrean et al. (2018) investigated the techno-economic performances of three distinct technologies (HTC with pelletization, pyrolysis, and anaerobic co-digestion).HTC was more economically feasible than the other two processes, pyrolysis and AD, whose respective process economies were 0.013, 0.043, and 0.055 $/MJ, respectively Kumar et al. (2020).All three procedures were more profitable than the current fossil fuel technology (0.070 $/MJ) (Pourhashem et al., 2013).Shao et al. (2019) used the price of input energy and the price of biocoal to determine the economic potential of the biocoal produced from green waste.The input cost to produce biocoal was 50 $/t, less expensive than the market price of biocoal, which is 108 $/t.
T A B L E 6 Process economy for converting rice husk to fuel using hydrothermal carbonization (HTC), pyrolysis, and anaerobic digestion (AD) (Unrean et al., 2018).parameters all significantly contribute to increased economic efficiency.The data on production from laboratory-scale research serves as the foundation for most of the HTC's current economic analyses.Despite these results, numerous obstacles remain to the commercialization and widespread implementation of HTC, making more studies on the technology crucial if it is to be used as a sustainable environmental solution.
4.1.1| Economic benefits In areas where coal and other fossil fuels are already employed, the generation of hydrochar using HTC of waste biomass might offer energy security.Malaysia has to cease constructing fossil coal-fired power stations since it consumes the 24th most fossil coal in the world (Singh et al., 2022).Operating HTC plants (for energy security via renewable technologies) in Malaysia would provide energy security in the event of interruption/termination of supply.They will contribute to greater economic stability in the future as revenue can be generated from the direct sales of biocoal.The specific cost for biocoal production ranges between 9.67 and 14.68 €/GJ for small plant capacity .However, the overall costs of the produced biocoal decrease with increasing plant capacity due to the benefits of economic scaling.For a plant capacity of 28.65-55.75(MWHHV), the cost for biocoal ranges between 7.94 and 11.11 €/GJ (Reza et al., 2014).Production costs can be changed significantly by raw material prices and waste process water disposal costs, which are site-specific and fluctuate over time.Furthermore, gate fees might be obtained from biomass discharge, and process effluent could be marketed for fertilizing crops.
Hence, an opportunistic margin exists for increased profits by utilizing the waste produced from the HTC process.
For 1 ton of processed rice husk, HTC was demonstrated to cost US$ 0.013/MJ as compared with pyrolysis (US$ 0.043/MJ) and AD (US$ 0.055/MJ) (Unrean et al., 2018).This shows that HTC is more cost-effective than the conventional waste management technologies currently in use.However, biocoal will need to be blended with fossil coal or other products as the sole application of biocoal may not be economical.

| Environmental impact
There is much more Malaysia can do in the advanced green technology sector.Food waste recycling is still mostly done via landfills and incinerators that burden the environment.Therefore, introducing HTC technology will accelerate the development of green technology in the country, thereby improving environmental performance in the long run.
Moving food waste from landfills and/or incineration facilities to HTC facilities can also help lower CO 2 emissions.In addition to reducing climate change and global warming's effects, HTC technology could decrease groundlevel air pollution.Shifting from the burning of fossil fuels and inefficient biofuel processes to HTC technology can reduce emissions of combustion-related pollutants, such as hydrocarbons, carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter, which have proven adverse effects on human health and contribute to ground-level air pollution (Bevan et al., 2021).
LCA studies showed that HTC demonstrated the lowest damage to human health, ecosystems, and resource availability as compared with pyrolysis (by 3.5-fold) and AD (by 7-fold) (Berge, Li, et al., 2015).Furthermore, HTC is the least energy-consuming approach compared with pyrolysis (lower by 4-fold) and AD (lower by 7.8-fold).Hence, HTC has the best environmental performance (i.e., mitigating climate change), followed by pyrolysis and AD.Hence, shifting financial resources away from fossil fuelbased businesses and toward developing renewable energy sources, particularly in Malaysia, might open up new societal chances in the form of employment opportunities and advantages for human health (Bevan et al., 2021).
To determine if the finished product may be utilized in agriculture for crop production, HTC feasibility studies on sewage sludge have been carried out.The fundamental idea is that the HTC product must adhere to rules to safeguard human health and the environment.Although sewage sludge contains beneficial agronomic qualities (phosphorus and nitrogen), it is frequently loaded with harmful organic compounds, bacteria, and heavy metals, which may be extremely damaging to the environment.HTC on sewage sludge was the subject of a feasibility study by (Escala et al., 2013).In contrast to normal coal, HTC biocoal exhibits greater H/C and O/C ratios and poorer carbon efficiency (CE), ranging from 60% to 73%.
The authors concluded that additional research and experiments are required to optimize the HTC process parameters (such as temperature and residence time) to better understand how these variables affect the composition of the biocoal and the liquid phase (process water), which will then be treated effectively to produce the best fertilizer.To sum up, the numerous uses and potential advantages of HTC end products make them appealing for usage in developing nations.Huge amounts of food waste-one of the biggest issues with Malaysia's MSW-can be converted into a useful energy source (replacing conventional fossil fuels) for lower emissions and pollution.

| impact of HTC
In the twenty-first century, scale operations of the new HTC technology are being established all over the world.Despite growing interest in its use, it should be remembered that HTC technology is new and only just beginning to be used (Bevan et al., 2021).The use of HTC to carbonize food waste in Malaysia is a potential system that offers many positive, sustainable impacts (Figure 2).The introduction of this green technology will not only reduce the environmental burden but will also benefit different levels of society.Implementing such technology promises job opportunities and will create new business avenues.In addition, developing a new renewable energy sector will help diversify the country's skill base, boost its industrial development, and support society's broad development.Additionally, exportation (of the product biocoal) can improve the gross domestic product (Kumar et al., 2020).

| Social benefits and impact
A profitable HTC facility can improve the regional economy if local companies own, build, and operate the plant (Bevan et al., 2021).Figure 3 shows the positive socioeconomic impacts of HTC.The introduction HTC in rural areas can also increase rural development if the plant is located near sites such as landfills, farms, and waste treatment areas.HTC could potentially stimulate investment in a large number of decentralized high-tech biomass conversion plants at landfill sites or food waste collection centers, as well as transportation equipment, due to the high availability of food waste.The operation of HTC conversion plants is also closely connected to the creation of highly skilled jobs in the fields of chemical engineering, mechanical engineering, business administration, and unskilled jobs in separation, product packing, etc., regardless of gender.
Furthermore, the benefits of HTC may positively contribute to public acceptance of food waste recycling, which will further increase the availability of raw materials for HTC (Sivaprasad et al., 2021).Interestingly, Suwelack et al. (2017) conclude that the socioeconomic impact of HTC, especially with waste feedstocks, is questionable compared with alternatives like microalgae and agricultural side products.Despite environmental benefits highlighted in the 2012 Leopoldina Survey, waste feedstock conversion systems fall short on many socioeconomic indicators.
F I G U R E 2 Sustainability: energy recovery from hydrothermal carbonization of food waste-promoting social and economic wellbeing while protecting the environment.

F I G U R E 3
The potential positive socioeconomic impacts of hydrothermal carbonization (HTC) from sustainable and renewable sources.

| Sustainability of the HTC
Taking the necessary steps to make restaurant, and industrial food waste a sustainable renewable energy source (Figure 4) is crucial.In fact, making proper use of food waste and other waste is both socially and environmentally responsible ways to safeguard our planet for future generations.The use of food waste as a fuel source depends on the availability of the feedstock.If the harvest rate of food waste is more than the growth rate, then the availability of feedstock to be harvested and used in the HTC system in successive harvests will be reduced.
As a result, other feedstock from other regions/states that would have otherwise ended up in landfills would need to be sourced to compensate for the demand, which makes the whole process of using food waste feedstock alone as fuel challenging (Dillen et al., 2013;Djomo et al., 2015).Moreover, the transportation cost in the feedstock conversion process also increases when the feedstock has higher bulk volume and insufficient energy density.However, if the system is designed to overcome the above-mentioned challenges, then the ambiguity of food waste feedstock being more sustainable can be solved.
Another possible route for recovering nutrients is food waste.A conceivable approach to sustainability is to reclaim nutrients from waste streams.The wastewater industry has been the sole sector to recover nutrients from waste streams over the years.Food waste is another waste stream with a lot of potential for nutrient recovery.Food waste is a plentiful stream that constitutes a sizeable portion of the MSW dumped in Malaysia.Food waste is a substantial source of nutrients (Opatokun et al., 2015;Pleissner et al., 2013;Zhang et al., 2007).For instance, according to Opatokun et al. (2015), the nitrogen concentration of food waste from various nations ranges between 1.5% and 16%.

| Managing liquid stream (process water recirculation) and gas emission
If the liquid stream and gas emissions are properly managed, the HTC technology could significantly reduce environmental impact (Benavente et al., 2017;Berge et al., 2015).The water formed post-HTC is rich in organic content (Weiner et al., 2014), which is useful if HTC is being enhanced by a catalyst (Hoekman et al., 2013).The HTC technology allows this process water to be recirculated back into the HTC system, eliminating the need for costly acids and bases as catalysts, creating a closed-loop sustainable system.Recirculating carbonization process water was used in the experiments conducted by Stemann et al. (2013), who report that changes in the quality of the initial process water catalyze dehydrogenation and that organic material in the liquid stream may be further polymerized, expanding the energy and carbon content of the recoverable solids.
Furthermore, a study by Berge, Flora, et al. (2015) reported that activated sludge and landfill leachate are acceptable alternative supplemental liquid sources for HTC, ultimately imparting minimal impact on the evaluated carbonization product characteristics and yields.The authors concluded that using these alternative liquid sources can greatly increase the sustainability of the HTC process by reducing water consumption.to laboratory findings, HTC's liquid process water has high biochemical oxygen demand (BOD), which raises the possibility that HTC might break down the liquid to create biogas and boost energy production.The anaerobic treatment of processing water from wet agricultural waste that has undergone HTC has been covered in several studies (Oliveira et al., 2013).The cumulative methane production they found ranged from 6 to 21 mL CH 4 /g of fresh materials.This exhibits the biodegradability of HTC process water by showing that anaerobic treatment converted more than 80% of the original BOD to biogas.
To add, the study by Picone et al. (2021) examined the recirculation of PW during HTC of waste biomass, finding that PW recirculation increased hydrochar mass yield by an average of 7%, with a peak increase of 20% for certain biomasses.It was observed that recirculating PW could raise the HHV of hydrochar by nearly 20% for specific feedstocks, enhancing the energy density of the biofuel produced.Challenges highlighted in the research include the need for precise control of operating conditions to maximize the benefits of PW recirculation, and the necessity for further studies to understand the chemical dynamics and optimize the process.This investigation underlines the potential of PW recirculation to improve both the environmental and economic aspects of HTC, by increasing carbon and energy recovery rates significantly.
When the energy-intensive pre-drying process used during processing food waste is abolished, significant energy savings can be achieved.Nonetheless, this costsaving measure is applied to drying the biocoal, the ultimate product.Yet, compared with pre-drying food waste, this biocoal uses less energy during drying due to the biocoal's improved hydrophobicity, making dewatering simple and affordable.Moreover, most environmental savings come from burning biocoal to produce electricity, which makes up for the environmental cost of energy use during the HTC process, gas emissions, and liquid discharge (Benavente et al., 2017).We must maximize biocoal and energy production to achieve this environmental saving and to make the HTC process more energy-beneficial.

| FUTURE RESEARCH DIRECTION
The burgeoning field of HTC offers fertile ground for future research, particularly as the global community intensifies its search for sustainable energy solutions.The future research should focus on refining HTC technology to enhance its applicability, economic viability, and environmental benefits.Further exploration is warranted into optimizing operational parameters beyond the conventional realms of temperature, pressure, and residence time (A.Shukla et al., 2024).Investigations into the intricate interplay between these variables and the composition of feedstocks could unlock higher efficiencies and superior quality of biocoal.Moreover, probing into the kinetic pathways of the HTC process will be indispensable in transitioning from batch to continuous processing, a leap necessary for industrial-scale applications.Furthermore, it is also important to explore technologies such as machine learning as they have been demonstrated to optimized various technology related to energy and sustainable materials development (Bin Abu Sofian, Lim, Chew, et al., 2024;Bin Abu Sofian, Sun, et al., 2024).
While touted to be favorable, the environmental impact of HTC processes necessitates a deeper and more nuanced understanding.Comprehensive life cycle assessments comparing HTC to traditional waste management and energy production methods are crucial.These studies should quantify greenhouse gas emissions and evaluate other ecological footprints, such as water usage and biodiversity impacts.In the economic arena, rigorous cost-benefit analyses are required to establish HTC's commercial viability.The feasibility of integrating HTC within existing industrial infrastructures and the potential for retrofitting or repurposing current facilities for HTC should be assessed.The economic models must also consider the potential revenue streams from byproducts, such as nutrient-rich aqueous phases suitable for agricultural use.
A critical yet often understudied aspect of HTC technology is its socioeconomic impact.Future studies should scrutinize the potential for job creation, the skill requirements for a workforce adept in HTC operations, and the socioeconomic ripple effects on communities where HTC plants could be established.Moreover, understanding public perception and acceptance of biocoal as an alternative energy source will be critical for widespread adoption.The scalability of HTC technology presents another research avenue.The challenge lies in maintaining biocoal quality while scaling up the process.Investigations into modular designs and decentralized systems could provide insights into scalable models that are efficient and adaptable to different regions and feedstock availability.
On the policy front, research should delve into the incentives and regulatory frameworks that could foster the adoption of HTC.Analysis of international case studies where HTC or similar technologies have been successfully implemented can offer valuable lessons and best practices.Developing standards and certifications for biocoal could also aid in market acceptance and integration into global energy mix.Lastly, research should into developing new applications for HTC-products.The potential of biocoal in various industrial processes, beyond its use as a solid fuel, warrants investigation.The compatibility of biocoal with emerging technologies in materials science, such as the production of advanced carbon materials for electronics and energy storage, could significantly enhance its market potential.
In summary, the trajectory of HTC technology research should embrace a multidisciplinary approach, integrating scientific, technological, economic, environmental, and social perspectives.Such a holistic examination will pave the way for HTC to transition from a promising technology to a cornerstone of a sustainable future.

| CONCLUSIONS
The utilization of HTC exhibits encouraging outcomes on a significant scale, as it reduces production costs and demonstrates greater cost-effectiveness compared with prevailing waste management technologies that are extensively employed in Malaysia.The present analysis has examined HTC's prerequisites for comprehensive capital, energy, and operational efficiency.The HTC technology is widely regarded as a more environmentally friendly alternative to AD, pyrolysis, and landfills for waste disposal due to its reduced environmental impact.Landfills produce substantial quantities of greenhouse gas emissions and hazardous leachates, exacerbating pollution and global warming.HTC has the potential to serve as a viable solution, as it operates on an energy-efficient methodology to generate a high-energy biocoal product that exhibits clean combustion and can be produced within a relatively short timeframe.Nevertheless, the commercialization of HTC will encounter numerous obstacles due to the reliance on the public's acceptance of this novel method for recycling food waste, which greatly affects the availability of raw materials for the process.Scaling HTC to a continuous industrial process may be hindered by the absence of research on the kinetic pathways of the process, which presents a challenge in overcoming technical barriers.Enhanced investment and the redirection of incentives toward research and development within HTC would not only aid in eliminating subsidies to fossil fuel industries but also aid in the gradual reduction of reliance on fossil fuels.

F
Hydrothermal carbonization (HTC) and factors affecting the production of coal and other by-products.

T A B L E 1
Agricultural waste and the overview of elements identified in solid phase biocoal.

( 1 )
Energy balance = (Output heat) − (Input heat + Sensible heat + Latent heat).T A B L E 2 Overview of the characteristics of biocoal produced from different wet waste.
Cao et al. (2007),Libra et al. (2011),  Liu et al. (2013)    HTC biocoal is similar to natural coal, providing an opportunity for HTC biocoal to be used as a substitute for fossil fuel Cao et al. (2007), Libra et al. (2011) HTC reaction is carried out at a comparatively low temperature compared with incineration (200-225°C and 400products, especially CO 2 , due to limited exposure to oxygen in the reactor and dissolved oxygen in the water Berge, Flora, et al. (2015) HTC can eliminate pathogens and inactivate organic contaminants like pharmaceuticals, making the end products sterile and hygienic Libra et al. (2011) HTC takes only hours (usually 1-12 h) instead of days or months required for biological conversion technologies Berge, Flora, et al. (2015) HTC is carbon efficient, and most of the original carbon present in the substrate stays bound in the final biocoal product Titirici et al. (2007) Disadvantages Lack of reaction kinetic data, including reaction pathway and mass transfer, which are important parameters for process optimization Munir et al. (2018) Detailed process intensification and integration are required to enhance the limited industrial batch-scale HTC process's economic and environmental advantages Venna et al. (2021)HTC's carbonaceous requires thermal drying, an extremely energy-intensive process to remove excess moisture before the pelletization of the solid biocoalBevan et al. (2021)

F
Life cycle of a sustainable hydrothermal carbonization (HTC) process.