Progress in lignocellulosic biomass valorization for biofuels and value‐added chemical production in the EU: A focus on thermochemical conversion processes

The demand for sustainable energy sources has increased owing to environmental concerns, such as climate change, rising energy demand and rapid industrialization. Biomass utilization for bioenergy and value‐added chemical production has become essential for creating a circular low‐carbon bioeconomy and sustainable waste management techniques. This review focuses on thermochemical processes and analyzes the trends of biomass utilization for energy production in the EU over the last decade. It discusses the current state‐of‐the‐art of gasification, pyrolysis, advanced combustion and liquefaction technologies, and presents the individual challenges and prospects of each process. A comprehensive overview of previous studies related to each conversion technology is provided to pave the way for future thermochemical, biochemical and integrated biomass valorization studies. Additionally, the review discusses biomass thermochemical conversion processes, which are combined with carbon capture, such as gasification, liquefaction and pyrolysis. These processes offer the potential to achieve negative net atmospheric carbon emissions, which can contribute to global warming mitigation efforts.


Introduction
T he rapidly increasing global energy demand owing to industrialization and population growth is a major challenge of the modern era.With a projected world population of 9 billion by 2050, energy demand is expected to increase by 50%. 1 Despite this, traditional resources like fossil fuels still provide 80% of the world's energy supply and 66% of electricity generation, yet they emit large amounts of greenhouse gases (GHGs) into the atmosphere.
Atmospheric CO 2 levels have risen consistently over the past three decades, from 1.42 ppm during 1985-1995 to 1.86 and 2.06 ppm from 1995 to 2005 and from 2005 to 2015 respectively. 2In 2019, the amount of atmospheric CO 2 was recorded at 410.5 ± 0.2 ppm, which is 130 ppm higher than in the preindustrial period. 3The World Health Organization reports that air pollution causes the deaths of 3 million people annually and only 10% of people live in areas that meet air quality standards.The Intergovernmental Panel on Climate Change predicts that without action to mitigate the problem, GHG emissions will increase dramatically and reach levels of between 750 and 1300 ppm CO 2 equivalents and potentially higher by 2100. 4 This increase in GHG emissions leads to global warming and climate change.In 2015, the UN countries agreed at the Paris Climate Conference to limit the global temperature increase to below 2°C compared with preindustrial levels. 5The average temperature in 2020 was reported to be 1.2 ± 0.1°C higher than the baseline of 1850-1900. 6wing to the pressing issue of excessive dependence on non-renewable fossil fuels, the substitution of these traditional fuels with sustainable and renewable energy sources can help mitigate the problem. 3In 2018, the European Union (EU) Renewable Energy Directive declared that 20% of the EU's energy demand should be met by renewable energy sources by 2020. 7By 2019, 19.7% of the energy consumed in EU countries was generated from renewable sources, leaving only a 0.3% gap to reach the 2020 target. 8Energy Roadmap 2050 projects that 30% of the EU's total energy consumption could be derived from renewable energy sources by 2030. 9iofuels, which have similar properties to petroleum-based fuels, are widely recognized as a crucial sustainable energy source for various applications, such as transportation, energy generation, and industrial boilers. 10A life cycle assessment of biofuels shows that the amount of CO 2 released during the entire biofuels production process, from raw material procurement to the final combustion in engines, is lower compared with traditional fuels. 11The production of biofuels in Europe has significantly increased over the past two decades, rising from 29.2 to 649.8 petajoules in 2019.Germany and France were the leading biofuel producers, generating 143.4 and 113 petajoules, respectively, in 2019. 11he production of biofuels in various European countries is depicted in Fig. 1.
The efficient development of biofuel production is highly dependent on the type of feedstock used.Biofuels can be categorized into different generations based on the biomass source.First-generation biofuels, derived from food sources, have been linked to numerous environmental and economic challenges, including increased land use, food crises, elevated food prices, ecological degradation and elevated 757 GHG emissions. 12As a result, the European Commission Renewable Energy Directive reduced the number of foodbased biofuels allowed in the 10% renewable energy target to 5% in 2012. 13The growing conflict between food supplies and first-generation biofuel production has led to an increased focus on second-generation biofuels, which are made from lignocellulosic biomass and non-edible materials.
Lignocellulosic substrates can be further divided into various groups, including agricultural residues (e.g.rice straw, rice husks, corn stover, sugar cane bagasse and wheat bran), energy crops (e.g.switch grass, cotton stalk, miscanthus and jatropha curcas), industrial residues (e.g.newspaper, pulp, wood residues, black liquor, skins and waste from palm oil mills) and agro-industrial residues (e.g.wood chips, potato and orange peel, paper pulp and spent coffee grounds).The significant advantage of lignocellulosic biomass is that the feedstock source is not in competition with the food supply and does not require arable land for cultivation.Additionally, lignocellulosic biomass is abundant worldwide and relatively inexpensive.In contrast, third-generation biofuels are biofuels made from algae, while fourth-generation biofuels are biofuels made from synthetic biology and metabolic engineering.Third-generation biofuels offer benefits such as higher biofuel yields per unit area, the ability to grow in saltwater, and carbon dioxide removal during growth.However, they also have disadvantages like being an emerging technology with limited commercial viability, high costs of production and difficulties in harvesting and processing.Fourth-generation biofuels are still in the development stage and have ethical, safety and regulatory hurdles to overcome.
Both third-and fourth-generation biofuels aim to improve traditional biofuels but face significant challenges before they can be fully commercialized.
Despite the advancement and plethora of studies on biomass valorization for biofuels and value-added chemicals, there are limited reviews on progress and advancement in thermochemical conversion technologies, especially in the EU context.To address the knowledge gaps, the present study provides an overview of advanced thermochemical conversion technologies in the EU context.An overview of biomass utilization in the EU is presented, following the advantages and limitations associated with thermochemical processes such as gasification, pyrolysis and liquefaction.It is expected that the present study will be a useful resource for researchers or practitioners working on biorefining.

Overview of lignocellulosic biomass distribution in the EU
In many developing countries, the main source of energy is still provided by the burning of biomass, which is not a sustainable and environmentally friendly method.With the emergence of new technologies, biomass can be converted into different forms such as solid, liquid and gaseous, and utilized to produce energy more effectively.Lignocellulose biomass is one of the most abundant and sustainable resources for biofuel production and has gained significant attention in recent years.It is estimated that there is a global production of 181.5 billion tons of lignocellulose waste Review: A focus on thermochemical conversion processes 758 each year. 14Different groups of lignocellulose substrates are illustrated in Fig. 2. Lignocellulose materials used for producing biofuels are primarily sourced from agricultural and forestry residuals and dedicated energy crops. 15he major sources of lignocellulose biomass are wheat, maize, rice and sugarcane, which produce up to 5300 million tons of dry lignocellulosic waste annually, while other agricultural residues constitute a smaller proportion of the total global biomass. 16Corn stover, with a global yield of 1.0 kg/kg corn grain or 4.0 ton/acre, is a leading source for biofuels production, obtained from the residues of harvested corn kernels such as stalks, leaves, cobs and husks. 17Wheat straw has a production rate of 1-3 tons/acre annually, while rice straw, with an estimated production rate of 731 million tons/year, is also a promising source of agricultural residues. 18agasse, produced from the processing of sugarcane, has a global yield estimated to be in the range of 317-380 million tons/year. 19The global yield of plant biomass is around 20 million tons per year, although only about 8-20 million tons of this biomass is available for biofuel production. 18n recent years, several studies have focused on the resource availability of lignocellulosic biomass in Europe, considering geographical constraints.Central Europe has potential sources of forestry-derived lignocellulosic biomass such as aspen, alder, birch, willow and poplar. 20Willow is mainly found in Denmark, the Netherlands, the UK and Ireland, while poplar is predominantly produced in warmer regions like France, Italy and Spain. 21Among dedicated energy crops, miscanthus is considered a suitable source for biofuel production in Europe, given its wide climate adaptability. 22However, only a small portion of cropland in the EU countries is dedicated to lignocellulosic crops such as miscanthus, switchgrass, willow and poplar. 23Wheat straw is recognized as the most abundant lignocellulosic biomass from plant residues. 24Agricultural crop residues, including stalks and leaves of cereals, sugar and vegetable crops, have an estimated realistic potential of 74.89 Mt per year in the EU, while forestry residues (stumps, branches, treetops, needles and leaves) have lower potential. 25The estimated production of brewer's spent grain in the EU is around 3.4 million tonnes per annum. 26Considering environmental constraints, the estimated annual amount of straw and stover in the EU-27 is in the ranges 45-214 Mt in 2017, 115-185 Mt in 2020 and 110 -165 Mt in 2030. 27In the UK and Europe, miscanthus and short rotation coppice are the primary focus for biofuel production from herbaceous and woody crops. 28Italy mainly utilizes sorghum and short-rotation forestry as feedstock. 29In Germany, maize

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is the primary source of lignocellulosic biomass for energy production. 30n Europe, switchgrass, reed canary grass, miscanthus and giant reed are identified as the most promising perennial grasses for biofuel production. 31The yield of different lignocellulosic biomass used for biofuel production is variable, probably owing to environmental conditions and the potential of various species to produce biomass.For instance, the yields of switchgrass, reed canary grass, miscanthus and giant reed across Europe are in the ranges 9-25, 3-14, 10-30 and 7-61 Mg/ha, respectively. 32The yield level of some agricultural crop residues in EU-27 is represented in Tables 1  and 2. 25 The substantial variation in these biomass yields is attributed to their adaptability to regional climate conditions.Reed canary grass is reported to be more adapted to northern European climates, while switchgrass and miscanthus have higher adaptability in northern and central Europe.On the other hand, the giant reed seems to be better suited for the Mediterranean region. 33

Overview of biofuel production technologies
Lignocellulosic biomass can be converted into biofuels through two approaches: biological processes and thermochemical conversion processes.Biological processes involve the use of different microorganisms to produce biofuels and green chemicals through the anaerobic digestion of organic substrates or fermentation of sugars.On the other hand, thermochemical conversion encompasses a range of processes such as gasification, pyrolysis, liquefaction and carbonization, which require more energy input. 34Figure 2(c) presents the various technologies of second-generation biofuels generated from lignocellulose materials.It should be mentioned that the focus of this study is on thermochemical conversion processes.The thermochemical processes have some advantages over biological processes in converting lignocellulosic biomass into biofuels.Some of these advantages are listed below: • High conversion efficiency -Thermochemical processes typically have higher conversion efficiencies compared with biological processes, meaning that a larger portion of the biomass is converted into biofuels.• Flexibility -Thermochemical processes are more flexible in terms of feedstock selection and can process a wider range of lignocellulosic biomass, including waste materials, compared with biological processes, which may have limitations in terms of feedstock type and quality.• Lower processing time -Thermochemical processes tend to have a shorter processing time compared with biological processes, which may take several days to weeks to complete.• Low maintenance costs -Thermal conversion processes have relatively low maintenance costs compared with biological processes, which require the maintenance of microorganisms and an optimal environment for their growth.• Lower operating costs -In general, thermochemical processes have lower operating costs compared with biological processes, owing to their lower energy requirements, lower maintenance costs, and higher conversion efficiencies.
It is important to note that both thermochemical and biological conversion processes have their advantages and limitations, and the best approach will depend on the specific requirements of the application, such as feedstock availability, end-product specifications and cost considerations.As a result, Okolie et al. 35 study detailed a comprehensive analysis of the advantages and limitations of each technology.The authors also provided a discussion on the integration of both processes to attain a circular economy, minimize material wastage and promote effective energy optimization.

Thermochemical conversion technologies
Biomass is a crucial component of the global carbon cycle and can serve as a cost-effective, environmentally friendly and clean source for energy production and the creation of value-added chemicals. 37Its potential to replace fossil fuels in energy processes, such as the production of heat, electricity and transportation fuels, makes biomass a promising and attractive energy resource. 38As a result, the utilization of biomass is playing a crucial role in meeting the EU's Green Deal targets for net-zero energy.In 2019, bioenergy applications accounted for roughly half of the renewable energy sources in the EU, and a significant increase in biomass use and demand for bioenergy is projected owing to energy and climate change strategies. 38he European bioeconomy is now valued at over €621 billion and contributes 4.2% to the EU GDP, employing 18 million people. 39Biomass feedstocks can be transformed into clean energy and value-added chemicals through a variety of thermal conversion technologies, including pyrolysis, hydrothermal conversions, gasification, combustion, advanced combustion and torrefaction.These technologies will be discussed in the next section.
There are still several challenges that hinder the commercialization of bioenergy and bioproducts through these technologies.These challenges include sourcing biomass, differences in physical, chemical and biological structures, lack of cost-competitive bioproducts, insufficient 760 biomass refinery technologies, scalability limitations and limited or unstable supply of biofuels and bioproducts. 40he variations in the chemical and biological composition of different biomass feedstocks can lead to significant differences in their handling, biochemical composition and resulting biochar structures. 40,41he selection of the most appropriate biomass valorization routes involves several factors, including the type and characteristics of biomass, its location, quantity, environmental regulations, economics and the desired end products (solid, liquid or gas).Gas products can be utilized to synthesize value-added chemicals through methods such as Fisher-Tropsch (alkanes), oxosynthesis (alcohols/aldehydes), fermentation (ethanol), isosynthesis (isobutane) and water-gas-shift (hydrogen/methanol). [42][43][44] Similarly, liquid products can be transformed into valuable chemicals and transportation fuels through cracking (thermal or catalytic cracking) 41,45 or anaerobic digestion. 46iochars (or hydrochars) offer versatility and have a broad range of applications, such as energy production, 47 water purification, 48 soil amendment, 49 CO 2 capture 50 and nanoparticles (for making composites) owing to their physicochemical properties. 51The choice of processing route has a direct impact on the potential applications of the

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primary gas, liquid and solid products.An illustration of thermal conversion technology routes is provided in Fig. 3.

Gasification
Gasification is a thermochemical conversion technique used for the production of synthesis gas (syngas) from different organic or carbonaceous materials using gasifying agents under high temperatures and an inert atmosphere. 52here has been significant work on biomass gasification, with ~18 329 publications using the keywords 'Biomass Gasification' published according to the Web of Science between 2010 and 2023.Among these publications, ~6899 papers were published from countries in Europe.Most of the biomass gasification studies (~2182 publications) were specifically detected using the keyword 'Biomass Steam Gasification'; ~430 publications were detected using the keyword 'Biomass Supercritical Water Gasification' .In addition to these two common gasification technologies, there are ~170 publications on 'Biomass Partial Oxidation Gasification' and ~117 publications on 'Biomass Autothermal Gasification' .The process of biomass gasification is complex and involves several endothermic and exothermic reactions that complement each other. 53Gasification can typically be divided into two categories: (i) conventional gasification and (ii) supercritical water gasification (SCWG) also known as hydrothermal gasification.The difference between both types of gasification has been meticulously described elsewhere. 35urthermore, a detailed review of the role of gasification in a carbon-negative economy has been recently published. 53he reactions involved include pyrolysis, oxidation, partial oxidation, reduction, steam reforming and water-gas shift reactions. 54,55Pyrolysis is an endothermic decomposition process at high temperatures in an inert atmosphere (absence of air or steam) that produces gaseous, liquid tar and residual solid char products. 56During oxidation or partial oxidation

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reactions, the products from the pyrolysis reaction react with oxygen to produce more gas and release heat.The final step in gasification involves upgrading the gases produced via steam reforming, in which low molecular weight hydrocarbons, such as methane (CH 4 ), are converted into carbon monoxide (CO) and hydrogen (H 2 ).The carbon monoxide further reacts with water vapour through a water-gas shift reaction to generate H 2 and CO 2 .Other side reactions, such as methanation, also occur in gasification, but to a lesser extent. 57Table 3 lists the main reactions that occur during gasification to produce gaseous products.Syngas, which is the primary product of biomass gasification, contains mainly non-hydrocarbon (CO, H 2 , CO 2 ) and hydrocarbon (CH 4 ). 52The gas may also contain some traces of hydrocarbons such as C 2 H 2 , and C 2 H 6 , the amount of which is dependent upon the gasifier configuration and biomass type. 61The syngas H 2 :CO ratio flexibility enables its wide range of applications, including power generation, heating, alternative transportation fuel and the production of synthetic value-added chemicals such as ammonia, acetic acid, dimethyl ether and methanol. 58In addition, high-purity H 2 derived from syngas is also used in fuel cell applications.Syngas is produced at a low cost and has reduced emissions of harmful gases to the environment.In addition to the syngas, tar-like liquid products and solid carbon residue known as char are also formed during gasification. 59,60Theoretically, nearly all types of biomass materials containing moisture content in the range of 5-30 wt% can be used as feedstock for gasification.Nevertheless, it is known that biomass characteristics, such as particle size, specific surface area, particle shape, carbon content, moisture content, volatile matter, carbon content, gasifier configuration, mode of operations, gasification process conditions and agents are variables that significantly influence the extent of gasification.

Steam gasification of biomass
Steam gasification of biomass is an environmentally friendly process for the production of biofuels and sustainable materials. 62This process yields syngas with a higher hydrogen (H 2 ) content and a higher heating value of 12-14 MJ m −3 . 63,64Additionally, the residence time is shorter, and the formation of tar and char is reduced. 65Biomass steam gasification is mostly driven by carbon gasification reactions such as primary steam reforming (Eqn 2), secondary steam reforming (Eqn 3) and methane steam reforming (Eqn 3), respectively. 66enerally, municipal solid waste and agricultural waste produced a higher amount of CO and CO 2 than other waste feedstocks during gasification. 67Theoretically, nearly all types of biomass containing moisture content in the range of 5-30 wt% can be used as feedstock for gasification. 67Biomass feedstock containing a higher amount of moisture, however, needs to be oven-dried or dewatered to a satisfactory limit before conventional gasification using either steam or nitrogen. 68In addition to the above drawback, the gasification

Biomass Primary products Integrated process Output Conversion tech. (Thermal)
Hydrothermal Conversion

Liquid products
Biooil Liquified chemicals

Metals
Cracking (Thermal or Cataly c)

Energy
Electricity, Heat, Power

Water Treatment
Dye, Metal, and Pharmaceu cal Removal 763 process emits sulfur in the form of H 2 S which could lead to complexity in gas separation and treatment.This necessitates an efficient gas treatment unit for high-sulfur-containing biomass. 67A major challenge with this process is the amount and number of contaminants in the produced gas that need to be removed before utilizing the gas.These gas contaminants are key to the design and operation of downstream gas conditioning equipment, where it is crucial to limit fouling. 69n particular, gas cooling is a challenging step, during which the equipment can become clogged with ash, particles or tar, or a mixture of all three. 70

Supercritical water gasification of biomass
Supercritical water gasification is a promising conversion technology that appears to provide a feasible solution to the challenges of conventional gasification.Supercritical water gasification is mostly carried out within the temperature range of 375-700°C and pressures of >22.1 MPa with or without a homogenous/heterogeneous catalyst. 71upercritical water gasification is capable of transforming waste biomass to H 2 -rich syngas with low CO and CH 4 content at high temperatures and elevated pressures in single-step gasification, thereby eliminating downstream shift or reforming.The production of H 2 at high pressure eliminates the need for downstream compression of the gas.The process permits easy product purification and collection of H 2 .Another advantage of the SCWG is that inorganic impurities become insoluble, and heteroatoms like S, N and halogens become soluble in supercritical water, which eases their removal. 72The process does not require biomass drying and can handle biomass with up to 80 wt% moisture, thus eliminating additional drying costs. 73Compared with conventional gasification processes, SCWG requires a lower temperature to produce H 2 .The gaseous product from SCWG can be burnt directly in engines or turbines to produce electricity. 74Supercritical water gasification has the favourable advantage of avoiding water evaporation and thereby enabling the water to serve as an effective solvent media for organics and some gas synthesis, thus making the process more energy-efficient compared with thermal gasification. 75The process is characterized by less tar and char formation.It produces a significant amount of additional H 2 from the water used as a reaction medium.One of the drawbacks of the process is that supercritical water exhibits high acidity, and thus, the reactor wall is susceptible to corrosion.The process is also disadvantaged because of its high capital investment costs.The presence of supercritical water facilitates the degradation of hydrolysis, pyrolysis, hydrogenation, dehydration, Boudouard reaction, condensation, decarbonylation, decarboxylation and dehydrogenation to produce H 2 , CO, CO 2 , CH 4 and other gases.The main reaction governing this process includes a steam reforming reaction (Eqn 2), water-gas shift (Eqn 3) and methanation of CO (Eqn 12) and CO 2 (Eqn 13), hydrogenation (Eqn 17) and Boudouard reaction (Eqn 1). 76,77quation ( 16) represents the overall SCWG reaction The biomass elemental molar ratios n and m defined by H/C and O/C respectively are prime determinants of syngas ( 16)

Review of non-catalytic gasification
According to a study, 75 high temperatures and long residence time are known to enhance char formation through polymerization.The Inconel material that makes up the reactor has a catalytic impact for suppressing repolymerization and provides higher yields at high temperatures with long residence times.Thus, a higher temperature (750°C) and a long residence time (300 s) promote gasification reactions of black liquor resulting in cold gas efficiency (87.8%) and H 2 yield (24.92 mol/kg dry-ash-free) respectively.Another study 75 found that H 2 yields increased with temperature until an efficiency level of about 100% was attained at a temperature of 560°C and a residence time of 6 s, while the feed concentration rates had the contrary effect; thus, H 2 yields decreased with an increase in biomass concentration. 74Kumar and Reddy 78 demonstrated that the presence of a high amount of lignin in biomass feedstock inhibits H 2 yield when they reported H 2 yields of 9.5 and 13.8 mol H 2 kg −2 biomass from mosambi peels and sugarcane bagasse containing 23 and 20 wt% respectively.In the study of the SCWG of the wheat straw, optimum conditions of 550°C with 20 wt% feed concentration for 60 min resulted in higher yields of H 2 (2.98 mmol g −1 ) and total gases (10.6 mmol g −1 ) respectively. 79An optimum H 2 yield of 112.5 ± 6.2 mol H 2 /kg waste dry condition was obtained from the SCWG of olive mill oil waste.In another study, it was found that the maximum H 2 yields were obtained at higher temperatures, longer residence times and lower feed concentration and flow rates.The author stated that the optimum yield was obtained at 700°C, 230 bar, a flow rate of 3 g min −1 , a 40.8 s residence time and 7.8 ± 0.1 g O 2 l −1 initial COD from olive mill wastewater. 80Batch SCWG of different types of 1 wt% concentration of biomass samples at 440°C and 25 MPa and a residence time of 15 min reaction time revealed that the lignin content was inversely proportional to total gas yields while the cellulose content showed a directly proportional relationship with the total gas yields.The total gas yields were in order of walnut shell < almond shell < barley straw < rice straw < wheat straw < canola stalk. 81In another related study, it was found that the SCWG of different types of biomass at 440°C and 25 MPa produced the highest H 2 yields of 4.1, 4.6 and 7.3 mol kg −1 , obtained after a reaction time of 15, 20 and 10 min from the almond shell, walnut shell and wheat straw, respectively.The study demonstrates that wheat straw containing higher cellulose and the least amount of lignin produced the highest H 2 yield within the shortest reaction time. 82Sivasangar  83 showed that an increase in SCWG reaction time of 0-8 min results in a significant increase in H 2 yields from 0 to 50 mmol mL −1 .However, a further increase to 32 min only increased the H 2 yield to 73 mmol mL −1 with no appreciable increase beyond 32 min during the SCWG of empty palm fruit bunches at 380°C, 24 MPa using 0.3 g of biomass and 8 mL of water.During the SCWG of alkaline black liquor in a continuous flow system, the increase in temperature and residence time and the decreasing feeding concentration favour the gas yield during SCWG.Supercritical water gasification of beech sawdust, municipal solid waste, hydrothermal char and malt spent grains in a different reactor (stainless steel, autoclaves and Inconel 625, ceramic with internal alumina inlay) for 16 h batch tests at 400°C with real biomasses and model compounds showed that the three devices have similar performance patterns in terms of gas yield.The ceramic reactor produced higher yields of C 2+ hydrocarbons owing to the ability of Al 2 O 3 to promote the cracking reaction of intermediate compounds at the expense of observed intergranular corrosion from scanning electron microscope observation. 84

Review of catalytic gasification
The major drawback of steam gasification compared with SCWG is the high probability of condensable heavy hydrocarbon (tar) formation during the process. 86Tar is a complex mixture of aldehydes, alcohols, aromatic and phenolic compounds. 87The formation and accumulation of tar in the biomass gasifier might cause serious operational challenges such as pipeline clogging, corrosion and thereby lowering the gasification efficiency, 57 and increasing operational and maintenance costs. 87Table 5 shows different catalysts and their effect on the product yields from the gasification of biomass.The use of catalysts during steam gasification is known to be a viable approach that promotes tar cracking and reforming of the heavy condensable hydrocarbon fractions. 85In contrast, SCWG is highly energy intensive, which leads to high processing costs.Catalysts are therefore used in the SCWG process to maximize the gas yield and increase carbon gasification while enhancing the selectivity of H  89 reported that the catalytic activity of NiFe 2 O 4 favours water gas shift reaction and steam reforming reactions.The authors found that the H 2 yield was enhanced by 45% when 2 g of NiFe 2 O 4 catalyst was used during SCWG of eucalyptus wood chips compared with the noncatalytic test.After the third reaction cycle, the results of XRD demonstrated the formation of coke which caused the deactivation of the NiFe 2 O 4 and consequently, a 13.6% reduction in H 2 mol% and a 5.6% reduction in biomass conversion.The combination of K 2 CO 3 with CaO showed excellent performance for high-yield H 2 production with 71.9% H 2 selectivity during the optimization of SCWG of sorghum and lignite coal blends.
It has been observed that total gas volume and H 2 volume were directly affected by the water volume in the reactor and the coal ratio of the coal-biomass mixtures.The highest total gas and H 2 volumes can be achieved under the conditions where the higher levels of the water volume of the reactor and lower levels of coal percentage of the coal/biomass mixture were combined. 90In the study of the catalytic SCWG of eucalyptus wood chips using NiFe 2 O 4 optimum conditions of 450°C, 2 g of NiFe 2 O 4 catalyst and 60 min gave the highest H 2 mol%.The increase in the NiFe 2 O 4 concentration promoted degradation of the eucalyptus wood chips (eucalyptus wood milled or ground into smaller particles) compounds water gas shift reaction, leading to higher H 2 and CH 4 yield and lower CO 2 production.Char also diminished in the presence of NiFe 2 O 4 .Nearly 66% of the carbon compounds in wood chips were converted to gaseous products in the presence of 2 g of NiFe 2 O 4 catalyst at 500°C and 60 min, with CH 4 becoming the most abundant gas at this condition. 96omparative evaluation of the different metal-doped (Ru/Al 2 O 3 and Ni/Si-Al 2 O 3 ) catalyst on SCWG of wheat straw shows that H 2 and total gas yields were optimal with maximum values up to 4.18 and 5.1 mmol g −1 for H 2 yield, along with a corresponding total gas yields of 15 and 18.2 mmol g −1 .This value is relatively high compared with the H 2 yield of 2.98 mmol g −1 and the yield of total gases of 10.6 mmol g −1 obtained for the non-catalytic gasification under similar conditions. 79In another study by Peng et al. 94 a commercial 5% Ru/C catalyst exhibited good catalytic performance during the continuous catalytic SCWG of Chlorella vulgaris throughout 55 h.Characterization of the spent 5% Ru/C catalyst revealed that the deactivation of the catalyst was primarily caused by sulfur poisoning and sintering.The further application of zinc oxide adsorbent upstream of the catalyst bed, together with a F Güleç et al.
Review: A focus on thermochemical conversion processes 766

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higher ruthenium loading, significantly improved the performance of the continuous catalytic supercritical water gasification process since the zinc oxide adsorbent showed high mechanical stability and satisfactory sulfur adsorption performance under supercritical conditions except for a drop in specific surface area.The co-feeding of H 2 with biomass during catalytic SCWG has been proven to be an efficient way of stimulating CH 4 formation during the catalytic hydrothermal gasification of wet biomass in a continuous test rig using a feed of 10 wt% glycerol in water and a fixed bed of a carbon-supported ruthenium catalyst.The gas produced is composed of about 86 vol% CH 4 . 95The investigation of the catalytic effect of cerium oxide nanocatalyst towards the hydrothermal conversion of black liquor revealed that the catalyst is efficient for the conversion of black liquor owing to the increase in H 2 production and inhibition of coke formation. 97The investigation of the effect of process parameters on SCWG of fruit pulp using 5 wt% Ru on activated charcoal as a catalyst revealed that the amount of H 2 produced increased nearly five times as temperature increased from 400 to 600°C.The optimum H 2 yield (54.8 mol H 2 kg −1 fruit pulp) was obtained at a biomass ratio of 2.5% with a corresponding higher gasification efficiency (150.8%),carbon gasification efficiency (88.1%) and H 2 gasification efficiency (213.5%)respectively. 93

Hydrothermal conversion technologies (carbonisation and liquefaction)
Hydrothermal processing facilitates the physio-chemical transformation of carbonaceous material under high temperature and -pressure liquid (sub-critical and supercritical) water to produce varieties of green fuel and chemical products. 98The products of the hydrothermal process include different forms of biofuels (bio-char, biooil or biocrude, gaseous hydrogen and methane) and other value-added products (ethanol, acetone and acetic acid).The dominant products obtained are purely dependent upon the choice of operating conditions. 99The hydrothermal operation takes place in the following states: supercritical (hydrothermal gasification) and subcritical for hydrothermal liquefaction (HTL) temperatures and hydrothermal carbonisation (HTC), 99,100 following the phase diagram of water and its different regions above vapour pressure and critical temperature, as depicted in Fig. 4.
Hydrothermal processing is one of the most promising technologies, as it can use the high inherent moisture of biomass to its advantage. 101For other processing techniques, such as pyrolysis and combustion, the high moisture content needs to be removed, which requires a significant amount of energy for drying processes.In contrast, hydrothermal conversion of biomass in hot-compressed water is a viable, scalable and energy-efficient thermochemical route for converting biomass into synthetic solid, liquid or gaseous fuels and chemicals. 102In hydrothermal treatments, water can be a solvent, a reactant and/or a catalyst in the hydrolysis reactions.The process also leads to by-products that can be used for power generation and the recovery of useful nutrients.In this process, biomass conversion is carried out by several complex reactions depending on the physical properties of the water, which are usually manipulated by changing the temperature, pressure and contact time of the water-biomass in order to obtain the desired products.4][105][106] There has been significant work on hydrothermal conversion technologies, with ~5500 publications using the keywords 'Hydrothermal carbonisation' or 'Hydrothermal carbonisation' , ~2880 publications using the keywords 'Hydrothermal liquefaction' , and ~1478 publications using the keywords 'Hydrothermal gasification' published according to the Web of Science between 2010 and 2021.Among these publications, 1572 (for HTC), 785 (for HTL), and 535 (from Hydrothermal gasification) are from countries in the EU and European Economic Area.

Hydrothermal carbonisation
Hydrothermal carbonisation (180-250°C, 15-40 bar) is a thermochemical process for the pre-treatment of high moisture content biomass to make it viable for energy production.It uses relatively low temperatures and is suitable for any kind of biomass feedstock.Hydrothermal   107 and also produce liquid products that contain organic and inorganic value-added chemicals.The HTC hydrochars exhibit lower O/C and H/C ratios compared with dry torrefaction and turn into more lignin or coal-type materials. 108HTC hydrochars can be used in a wide range of processes such as soil amendment, 49 CO 2 capture, 50 nanoparticles (for making composites), energy production 47 and water purification 48 thanks to their physicochemical properties. 51Although the lab-scale research on HTC of various biomass feedstocks has recently progressed and provided significantly promising results, the HTC process needs further investigations in terms of process and reactor types, biomass feedstocks and conditions owing to the complex reaction mechanisms and operational barriers to make this technology commercial technology.For a continuous HTC process would be one of the key components for a potential industrial application of HTC, as most HTC research have carried out in batch.

Hydrothermal liquefaction
Hydrothermal liquefaction (250-370°C, 50-240 bar) is the wet processing route for high-moisture biomasses to produce liquid fuel (bio-crude or bio-oil), 103 which is similar to petroleum crude and can be upgraded to a range of petroleum-derived fuel products.Since HTL involves the direct conversion of biomass into bio-crude in the presence of a solvent, it eliminates the high drying costs.The HTL process could be operated under subcritical or supercritical conditions for producing bio-oil, water-soluble organics, gaseous products and char. 109Hydrothermal liquefaction is usually conducted at a moderately higher temperature (280-370°C) and pressure (4-25 MPa) in the presence of a solvent medium. 98,100This process can be done using either a batch or a continuous reactor.Catalyst addition has been reported to show significant effects on liquid yield and quality.Figure 5 shows a diagrammatic overview of the reaction pathway.
During HTL large biomass undergoes depolymerization to produce smaller monomeric fractions which are subsequently repolymerized into varieties of compounds that form the biocrude.The high operating conditions of the process facilitate the conversion of all lipids, proteins and carbohydrates present in the biomass into the desired bio-oil with a higher yield. 110The products of HTL constitute biocrude or bio-oil, an aqueous phase containing hydrophilic organics, a gas and solid char (Tables 6 and 7).The bio-oil derived from HTL contains a lower amount of oxygen and has miscibility with water in comparison with bio-oil from other thermochemical processes and therefore can easily be subjected to hydrotreatment.Typically, the oxygen content reduces to a value of 10-15% from 40% contained in the biomaterial. 111The bio-oils can be substantially purified and used as fuels in burners, boilers and turbines.Bio-oils can also be further upgraded into transportation fuels like automotive gas oil (diesel), premium motor spirit (gasoline) and other important end products such as polymers, asphalt and lubricants.The bio-oil which is the main product of HTL is known to contain several value-added compounds as summarized in Table 6.
The main advantage of HTL is that the energy-consuming drying stage is eliminated.Another cutting-edge benefit of this HTL process is it consumes only 10-15% of the energy in the feedstock biomass, yielding an energy efficiency of 85-90%.Hydrothermal liquefaction can recover more than 70% of the feedstock carbon content that can be utilized for carbon capture procedures.The other technical edge of this process is that the bio-oil that is obtained from this process does not require sophisticated treatment/upgrading procedures for commercial utilization. 113The details on hydrothermal gasification are presented under Section 3.1.1.Table 7 presents literature studies on the hydrothermal conversion of biomass to value-added products and biofuels.

Pyrolysis
The limitation to biomass valorization in Europe largely depends on factors ranging from utilization to the market (demand and supply), and eventually government policies.The enormous availability of geographically distributed biomass in the EU makes the drive for valorization ready to be ignited.There has been significant work on biomass pyrolysis, with ~31 982 publications using the keywords 'Biomass Pyrolysis' published according to the Web of Science between 2010 and 2023.Among these publications, ~9201 papers were published from countries in Europe, ~1816 publications were specifically detected using the keyword 'Biomass Fast Pyrolysis' , ~795 publications were detected using the keyword 'Biomass Slow Pyrolysis' and only ~310 were detected using the keyword 'Biomass Flash Pyrolysis' .The waste-to-energy legislation in the EU proposed a 35% reduction of landfilled wastes by the year 2020 when compared with the levels in previous years. 125Just like municipal solid waste, other biomass including forest residues, can be valorized into valuable products, hence the term biomass-to-energy.In previous times, incineration was the adopted process required to convert wastes to carbonbased products.However, the process involves the complete oxidation of 'combustible materials' operating at extremely high temperatures above 850°C.The drawback associated with incineration is mainly the high temperature and 769 oxidative environment, as well as the combustible material required for burning.Gasification can surmount these challenges, as partial oxidation is required, with little (or no) access to air, but this operates at high temperatures (around 500-1800°C).Pyrolysis (just like gasification) allows for the use of various feedstocks and moderately lower temperatures, in the absence of air.The pyrolysis of biomass is capable of generating products in gaseous, liquid and/or solid form, generally known as pyrolysis gas (or pygas), pyrolysis oil (biooil) and biochar (or just char), respectively.
Although several reports have shown that numerous biomasses can be valorized via pyrolysis, limited use and interests have been geared toward organic wastes.In the UK (no longer a Member State of the European Union) but a previous member of EU, the drive to improve electricity generation motivated companies to seek to utilize organic wastes for electricity generation.In energy generation, biomass offers an excellent choice for valorization, as it is a natural source of carbon.Several companies and institutions in the UK such as Future Blends Ltd, Anergy Ltd and Torftech Energy Ltd have adopted pyrolysis for the generation of electricity, with plant/forest biomass being the key candidate required for processing.In 2016, the total electricity generated (from bioenergy) via pyrolysis in the UK amounted to 30 042 GWh, where about 63% of the raw materials required for processing were sourced from plant biomass. 125iomass pyrolysis is made possible either by slow pyrolysis or by fast pyrolysis, which can be either catalytic or noncatalytic.Fast pyrolysis is mostly adopted, because of the enormous advantages compared with slow pyrolysis.Slow pyrolysis requires moderate temperatures of around 400-500°C, a longer residence time typically within minutes and a heating rate of 0.1-1°C s −1 , while fast pyrolysis requires higher temperature, a shorter residence time and a heating rate of 10-200°C s −1 , and generates a higher yield of bio-oil, which is suitable for a further upgrade to hydrocarbon-like fuels. 126Reports on the pyrolysis of food waste have revealed the yield of individual products at different variations as shown in Fig. 6.Biochar is a carbon-rich product that holds significant potential for carbon sequestration, acting as a  113 770 stable and long-term sink for atmospheric CO 2 .Its porous structure and high surface area enable the trapping of CO 2 , and when incorporated into soil, it not only sequesters carbon but also enhances soil fertility, increasing water retention and improving microbial activity.This makes it a multifaceted solution that contributes to both climate change mitigation and sustainable agriculture.
Biochar can also serve as industrial fuel.Moreover, it is characterized by high heating values.In contrast, bio-oil is mostly utilized for bioenergy purposes since it can be upgraded via hydrodeoxygenation or decarboxylation, to generate on-road vehicles or even jet fuels.The percentage yield of pyrolysis products can be optimized to selectively improve the yield of the desired product.Important processing parameters such as temperature, biomassmixing ratios, feed rate, residence time and atmospheric conditions, and so forth, are factors that influence the yields of the products from pyrolysis.Generating liquids from fast pyrolysis typically requires higher heating and heat transfer rates using finely ground biomass, properly controlled reaction temperatures around 500°C shorter vapour residence times, usually within seconds, and rapid cooling.Other by-products that can be generated include organics, ash and water.Although fast pyrolysis is fast gaining attention, more research is needed to address important issues such as scale-up, cost reduction, yield improvement and drop-in or infrastructural compatibility of the bio-products.
The type of biomass and the process parameters determine the properties of the biochar, which is produced in large quantities by slow pyrolysis.However, as stated earlier, more bio-oil is produced with fast pyrolysis, with a lower heating value, which is because of the presence of moisture content existing as H 2 O, O 2 and other carboxylic acids.The pyrolysis gas is a mix of H 2 , C 1 -C 4 hydrocarbon gases, CO 2 , CO and H 2 S.An optimum yield is obtained when the biomass has a low moisture content, since the presence of water may result in the formation of tar and an increase in the energy cost of drying the biomass.Additionally, the use of wet biomass can result in a 40% increase in H 2 content in the pyrolysis gas product. 126In recent times, most companies based in Europe have developed and adopted reactor designs suitable for pyrolysis.The different reactors used in pyrolysis are fixedbed reactors, rotary kilns, fluidized bed reactors, microwaveassisted reactors and batch and semi-batch reactors. 126he bubbling fluid beds have attractive features such as simple construction and operation, moderate temperature control, easy to scale and high liquid yields.As a result, several companies in both Spain (Union Fenosa) and the UK (Wellman), who have built and operated 200 kg h −1 and 250 kg h −1 units, respectively, have recently selected bubbling fluid beds for development.Ensyn in both Finland (VTT) and Spain (ENEL) has developed other reactor designs for pyrolysis, such as circulating fluid beds and transported beds for commercial use, at 20 and 650 kg h −1 , respectively. 127he main components of biomass are cellulose, hemicellulose and lignin.The most prevalent biomass class is lignocellulosic biomass, which is a plant-derived biomass that can be classified into hardwood, softwood, farm wastes, and grasses.Among the main components of biomass, lignin has been shown to be the most recalcitrant constituent of the plant cell wall, acting as a protection against chemical and enzymatic degradation of plant cells.Unlike hemicellulose and cellulose, lignin is not a carbohydrate.The thermal processing of biomass (via pyrolysis) helps in degrading these structural blocks that makeup plants.Pre-treatment through acidic or alkaline media can help improve the hydrocarbon and energy contents, thereby decreasing the high moisture generally associated with the process of pyrolysis -which can be a bane to the pyrolysis process.
Shown in Table 8 is the biomass available, and the processing modes required for pyrolysis upgrade in several European countries.According to the European Biomass Industry Association, most biomass ranging from waste wood, fresh wood chips, and industrial sawdust contain Table 6.Major product compounds of bio-oil obtained by the hydrothermal liquefaction (HTL) process. 113,114oducts Compounds

Gasification with CO 2 capture
Among different thermochemical pathways, gasification has the potential to decarbonize fuels and support a circular economy.The potential for BECCS using gasification with CO 2 removal and capture has been evaluated in many studies.Based on previous studies, an integrated gasification and carbon capture with a combined cycle gas turbine was developed with a cost of electric power of 0.082 $ (kW h) −1 , mitigation of 140 gC (kW h) −1 , a CO 2 capture efficiency of 44% and a thermal efficiency of 28%. 130According to Uddin and Baretto, 131 a 200 MW combined heat and power plant could mitigate 200 kgC with an 85% capture efficiency.Another scenario presented by Kraxner et al. 132 could potentially achieve a 90% capture efficiency with 2.5 tons of carbon per year per ha.Converting biomass into hydrogen through gasification, intended for transportation and power production, holds appeal owing to its potential for significant carbon capture. 53wever, this process encounters various obstacles including the challenge of procuring a sufficient amount of biomass on a scale that ensures cost-effective plant operation, as well as the complexities of cost-efficiently and dependably storing CO 2 in either ocean reserves or geological structures. 53here are several carbon capture separation methods, including absorption, adsorption, membrane separation and chemical looping.A simulation study was conducted by Shahbaz et al. using Aspen Plus for steam gasification of palm kernel shells with CO 2 capture through a sorbent (CaO). 133The maximum hydrogen content is predicted to be 79.32 vol% and the minimum CO 2 content to be 5.42 vol% under operating conditions which include a temperature of 700°C, a steam/biomass ratio of 1.5 and a CaO/biomass ratio of 1.42. 133Cormos et al. evaluated membrane technology both independently and in combination with gas-liquid absorption.Overall, the results of this study indicated that the CO 2 capture costs for membrane technology are about 50% lower than for chemical or physical absorption. 134hemical looping gasification, a novel gasification technology, is capable of achieving 100% capture efficiency and a low energy penalty.Similar to oxy-fuel combustion, chemical looping involves the replacement of oxygen introduced with the fuel with a metal oxide (e.g.Fe 2 O 3 , NiO, Mn 2 O 3 ), which acts as an oxygen carrier.Metal oxide and fuel redox reactions, resulting in two different streams of gases, one rich in hydrogen and one rich in CO 2 . 129Acharya et al. found that 71% hydrogen and nearly 0% CO 2 were present in the product gas produced in a batch-type fluidizedbed steam gasifier with CaO as the sorbent. 135According to He et al., 136

Liquefaction with CO 2 capture
In BECCS systems, liquefaction is an important process since it does not necessarily capture CO 2 owing to its nature.Liquefaction can mitigate carbon dioxide by addressing the following issues: (i) by converting biomass into a valuable liquid fuel without removing moisture, biomass can be used as a replacement for fossil fuel-based oils; and (2) Pre-treatment is not required, thereby reducing the CO 2 emissions associated with it.Furthermore, owing to the elimination of the drying process, this process produces no CO 2 emissions. 129sing HTL of forestry residues and Selexol™ as a physical adsorbent, 95% purity CO 2 was obtained at a capture cost of US$75 t −1 . 144Furthermore, The HTL process with CCS is shown to be capable of reducing greenhouse gas emissions by 102-113% compared with fossil fuels. 144

Pyrolysis with CO 2 capture
In bioenergy systems, pyrolysis has been widely adopted, although it has not been as prevalent in BECCS systems. 129otably, the pyrolysis process directly mitigates CO 2 , as biochar, the by-product of pyrolysis, offers a potential BECCS option since very little CO 2 is released per unit of char, and carbon can be sequestered when the biochar is buried on land. 146Biochar derived from pyrolysis presents an appealing option for carbon sequestration owing to its enduring presence in soils and the potential benefits it offers to ecosystems. 53The gradual pyrolysis method generates substantial quantities of biochar, akin to the carbon capture potential of biofuels produced through gasification. 53In contrast, rapid pyrolysis yields a smaller amount of biochar, but it boasts more favourable economics since the resulting bio-oil can be transformed into high-energy, carbon-negative liquid fuels or alternative products. 53A study was conducted to examine the potential of biochar soil application in BECCS, and the results indicated that biochar soil application has a significant potential to reduce greenhouse gas emissions (0.7 Gt CO 2 eq per year worldwide). 147It is estimated that integrating CCS with pyrolysis would result in a reduction of 2 Mt of CO 2 by 2030. 1480][151] Owing to the relatively high cost of organic calcium compounds, calcined carbide slag can be used in negative-carbon pyrolysis as an alternative to commercially available CaO. 152 Chen et al. found that biomass pyrolysis with calcined carbide slag at a mass ratio of 1:1 was the most effective method for minimizing CO 2 production and upgrading products. 146he integration of CCS and biomass pyrolysis has been studied from a techno-economic perspective.The study by Cheng et al. examined four thermochemical conversion technologies (hydrothermal treatment, pyrolysis, gasification and conventional combustion) in combination with CO 2 capture under a variety of processing conditions. 146Their results demonstrated that slow pyrolysis of crop residues and woody wastes has the potential to be a negative emissionsproducing technology that would lead to global warming potential values ranging from −470 to −200 kg CO 2 eq t −1 (without substitution) and from −1050 to −770 kg CO 2 eq t −1 (with two substitution benefits: exporting heat generated and using biochar as a fertilizer. 146In another study, various conversion technologies were compared by Cheng et al., 153 and gasification and combustion with CCS were found to have the highest net negative carbon emissions (1400 kg CO 2 eq t −1 biomass on dry weight).It was found that slow pyrolysis of woody wastes and crop residues has the potential to be an economically viable negative emission technology, even without carbon incentives, whereas other technologies require carbon incentives to be successful. 153Sun et al. compared the technical and economic feasibility of different capture technologies appropriate for pyrolysis (monoethanolaminebased chemical absorption, temperature swing adsorption, calcium looping (CaL), and chemical looping combustion (CLC). 154In comparison with other methods, CLC is found to capture more CO 2 with the lowest energy and exergy penalty, whereas CaL has the highest energy and exergy penalty.It was found that CO 2 capture potential can be directly impacted the reaction time of the pyrolysis reaction.Furthermore, the levelized costs of CO 2 (LCOC) for CLC and CaL are about 56$/tCO 2 , while monoethanolamine based chemical absorption shows the highest, at 83$/tCO 2 . 154

Conclusion
This review is centred around the exploration of thermochemical methods, specifically examining the patterns of biomass usage for energy production within the EU during the past 10 years.It goes into detail about the modern and cutting-edge technologies in the field, including gasification, pyrolysis, advanced combustion techniques and liquefaction, all of which are integral to the process of converting biomass into usable energy.Within the discussion of these technologies, the review highlights the unique challenges and opportunities associated with each method, thus offering a nuanced understanding of their individual characteristics.
Furthermore, the review provides an extensive overview of previous research related to each conversion technology, synthesizing the existing knowledge to create a foundation for future studies.This encompasses not only thermochemical but also integrated approaches to biomass valorization, emphasizing the importance of diverse and multifaceted research in this area.
An additional significant aspect of the review is its exploration of biomass thermochemical conversion processes that incorporate carbon capture strategies, like gasification, liquefaction and pyrolysis combined with CO 2 capture.By integrating carbon capture into these processes, the potential for achieving negative net atmospheric carbon emissions Review: A focus on thermochemical conversion processes 775 is introduced.This offers an exciting possibility for not only producing energy but also actively contributing to the reduction of global carbon levels.In the broader context of climate change and global warming, these processes symbolize promising paths forward, aligning with worldwide efforts to mitigate the adverse impacts of increasing atmospheric carbon concentrations.
Thermochemical processes, such as gasification, pyrolysis, advanced combustion and liquefaction, have been pivotal in advancing technological developments in the biomass sector.These processes enable the efficient conversion of various biomass materials into biofuels and valuable chemicals, laying the groundwork for continued innovation and improvements in this technology.Thermochemical conversion technologies offer flexible and efficient pathways for the utilization of diverse biomass feedstocks, including algae and agricultural waste, thereby supporting feedstock diversity.This advancement is also aligned with economic feasibility, as these technologies are continually being refined to reduce costs and optimize large-scale production.Furthermore, the integration of carbon capture with these processes contributes to sustainability by offering potential solutions for global warming mitigation.
The progress in thermochemical processes also supports the broader framework of market dynamics and regulatory compliance within the EU.By enhancing efficiency and reducing the costs of biofuel and value-added chemical production, these technologies cater to the current and future market demand, shaping the potential for market growth.Concurrently, understanding and aligning with existing policies and regulations within the EU is crucial.Thermochemical processes, through their innovation and adaptability, are poised to navigate the complex regulatory landscape, ensuring that biomass valorization not only meets the technical and economic needs but also adheres to evolving policies, thus contributing to the overall robust and responsible growth of the industry.
In summary, this review serves as a comprehensive guide and resource for those interested in the intricate interplay between thermochemical processes, biomass utilisation and carbon management, setting the stage for continued innovation and progress in these critical fields.

Figure 1 . 8 F
Figure 1.Biofuel production in selected European countries 2019.8

Figure 2 .
Figure 2. Second-generation biofuels production from lignocellulose materials: (a) schematic of lignocellulose biomass; 36 (b) different groups of lignocellulosic substrates; and (c) different ways of Lignocellulosic biomass conversion to biofuels.

Figure 3 .
Figure 3. Thermal conversion technologies that can convert biomass into products.

Figure 4 .
Figure 4. Hydrothermal processing conditions in a water phase diagram.40

Figure 5 .
Figure 5. (a) Process flow diagram of the hydrothermal liquefaction (HTL) process of lignocellulose biomass adopted from and (b) process flow diagram of HTL process of algal biomass/wet biomass.These figures were adopted from Biller and Ross 112 and Gollakota et al.113

Table 1 .
25e yield level of some agricultural crop residues in EU-27.25

Table 2 .
25e yield level of some agricultural crop residues in the EU-27.25

Table 4 .
Summary of studies related to the non-catalytic supercritical water gasification (SCWG) of biomass.
2 and minimizing heat requirements through operating temperature reduction.Catalysts commonly used in both gasification routes include alkaline earth metallic catalysts [KOH, NaOH, K 2 CO 3 , KHCO 3 , Na 2 CO 3 , Na 3 PO 4 , MgO and Ca(OH) Practicable catalysts speed up the cleavage of the C-C, C-O, C-H, and O-H bonds to yield an H 2 -rich gas mixture.Borges et al.
88], metal-based catalysts (e.g.Ni, Ce, La, Fe, Ce) and natural mineral catalysts such as carbonates (MgCO 3 •CaCO 3 ) and olivine (2MgO•SiO 2 ).These catalysts can be added by direct mixing with the raw precursor or placed in another secondary reactor to crack newly formed tar during gasification.88

Table 7 .
Biomass to value-added products via hydrothermal processing.
128Review: A focus on thermochemical conversion processes 772 moisture contents of 10-30, 40-60 and 40-60%.128These higmounts of moisture content propose the initial cost challenge associated with most wood biomass in Europe for the initial drying of the biomass before processing.
biomass-chemical looping gasification coupled with water/CO 2 -splitting and NiFe 2 O 4 oxygen carrier can achieve 93.84% carbon conversion at an optimal CO 2 /H 2 O mole ratio of 0.15.It is currently possible to successfullyFigure 6.Typical product yields generated from the pyrolysis, gasification and carbonisation of wood.

Simulations 8 .
145e selected EU countries and their industrial biomass wastes available for pyrolysis.Review:A focus on thermochemical conversion processes 774 using HTL, based on lignocellulosic biomass waste and urban biomass using Selexol™, indicate that CO 2 purity between 90 and 98 mol% can be obtained for 40-53 EUR t −1 and 98-99 mol% purity is achievable for 57-77 EUR t −1 .145Although the liquefaction process is still being studied at the lab scale, further research and development are required to make it feasible for commercial use, as it can be an invaluable component of the BECCS process that produces liquid fuel and reduces CO 2 emissions.