Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing

Abstract

This chapter provides an overview of current efforts and advances as well as environmental and economic aspects of fast pyrolysis and hydrothermal processing, which are potential technologies for bioenergy production, mainly bio-oil and syngas. Biomass is presently the primary bioenergy resource in the world. The chapter presents a brief discussion of sources and compositions of biomass. Biomass is converted to various products using thermochemical conversions. Pyrolysis is a thermochemical process that converts biomass into carbon-rich solid residue, condensable vapors, and non-condensable gases in the absence of oxygen. It is a promising technology for converting biomass into renewable biofuels with environmental and economic advantages. Pyrolysis processes are classified based on their operating conditions and desired products. Two thermochemical processes, fast pyrolysis and hydrothermal processing are reviewed. Fast pyrolysis produces a higher quantity and quality of bio-oil and syngas than slow and intermediate pyrolysis processes. Hydrothermal processing converts wet biomass into carbonaceous biofuel. The ability to produce higher-value bioenergy by these pyrolysis technologies depends on the feedstock and operating condition of the pyrolysis processes. This chapter will present the most promising features of fast pyrolysis and hydrothermal processing along with their optimal pyrolysis conditions in maximizing the production of biofuels.

1. Introduction

The current global energy supply is, to a large extent, based on fossil fuels (oil, natural gas, and coal) of which the reserves are finite. As a result of industrialization, population growth, and urbanization, there has been a rapid increase in global energy demand and consumption. The necessity for long-term alternative energy sources is obvious due to the increasing energy consumption, high prices and limited reserves of fossil fuels and evidence of global warming, environmental pollution, and climate change. As a result, there is renewed interest in producing and using renewable energy resources, such as biomass, wind, solar, geothermal, and tidal. Bioenergy is a sustainable form of energy derived from biomass sources [1, 2, 3, 4]. Recently, bioenergy is getting more attention because of its potential advantages, including renewable fuel for boilers, engines, turbines, power generation and industrial processes; inexpensive and CO₂ neutral; utilization of nonfood and waste second-generation biomass feedstocks; easy to store and transport as liquid fuels; high-energy density compared to atmospheric biomass gasification fuel gases [2, 5, 6]. Biomass is a promising eco-friendly alternative source of renewable bioenergy because of its abundant availability, relatively lower price, and zero greenhouse gas emissions in the context of current energy scenarios. However, the only renewable energy resource that can be used to produce transport fuels is biomass [2, 4].

Biomass is plant or animal-based organic matter that is living or was living in the recent past. Various biomass components, such as sugars, starches, and lignocellulosic (non-starch fibrous part of the plant) materials, can be converted to liquid transport fuels, reducing the use of fossil fuels. A promising alternative to reduce environmental issues related to waste disposal and management is converting biomass residues and wastes (such as crop residues, food wastes, animal manure, and municipal solid wastes) into useful bioenergy. Some of the advantages of converting biomass residues and wastes into bioenergy include (a) reducing the burden on waste management, (b) converting waste into valuable energy reduces the dependence on fossil fuels, (c) reducing decomposing waste and associated issues such as water contamination, greenhouse gas emissions, pests and insects breeding, and foul odor. [4, 7, 8, 9, 10].

The biomass feedstocks can be transformed into biofuels through biochemical and thermal conversion processes. The thermal conversion approach, such as pyrolysis, gasification, and torrefaction, are applicable for a wide range of biomass types using different temperatures to breakdown the bonds of organic matter in a relatively short period of time, unlike the biochemical processes [2, 5, 6]. Lignocellulosic biomass, such as agricultural crop residues, wood and forestry residues, are readily available, inexpensive, and promising resources for biofuels. Biomass can be considered one of the best options for sustaining future energy demand. The more efficient biomass production and conversion processes are essential for the efficient utilization of biomass resources [11]. Biomass is a valuable fuel source that is considered renewable as it can be produced year after year. Compared to fossil fuels, biomass has the potential to reduce combustion emissions, such as CO₂, SO_X, and NO_X [12, 13].

2. Biomass

A commonly used biomass classification is based on the origin of biomass, such as agricultural crop residues, forestry and wood processing residues, purposely grown dedicated energy crops, aquatic biomass, animal, food, industrial and municipal waste, sewage sludge, digestate, and industrial crops. Various types of wastes, such as wastepaper, sewage sludge, cow manure, poultry litter, municipal, and many industrial wastes, are treated as biomass because these are a mixture of organic (and nonorganic) compounds. Biomass is also classified based on its chemical composition as carbohydrates, lignin, essential oils, vegetable oils, animal fats, natural resins (gums), etc. Lignocellulosic biomass is the most abundant biomass on the earth and it represents a major carbon source for bioenergy, biofuels, and chemical compounds [2, 4].

2.1 Sources of biomass

Agricultural crop biomasses are natural products of agriculture, including food-based and nonfood-based portions of crops. The food-based portion comprises simple carbohydrates and oils from crops, such as corn, sugarcane, sugar beet, rapeseed, soybean, and sunflower. The nonfood-based portion is commonly discarded, which comprises complex carbohydrates of crops that are not harvested for commercial use or byproducts from harvesting or processing, such as corn stover, sugarcane bagasse, straw residues, waste from food processing, horticulture, and food crops [4, 7, 14]. Forestry and wood processing residues include trees that are not valuable as timber and not harvested during logging, crowns and branches from fully-grown trees that are removed during logging in commercial forests, waste from forest and wood processing (such as wood pellets, woodchips, leaves, lumps, barks, and sawdust) as well as materials removed during forest management operations. Most of the biomass used today are derived from agricultural crop, forestry, and wood biomass [7, 15].

Another expanding and potentially larger source of biomass is dedicated energy crops that are grown specifically for their fuel value on marginal land unsuitable for agriculture. These are high-yield and low-maintenance crops that produce maximum energy yield. There are two types of energy crops, herbaceous and short-rotation woody crops. Herbaceous energy crops include perennial grasses, such as switchgrass, miscanthus, bluestem, elephant grass, bamboo, and wheatgrass, that are harvested annually after maturity. In 2–3 years, herbaceous energy crops reach complete production and do not require replanting for 15 years or more. The drawback of most nonwoody energy crops is that their chemical properties (high ash and salt content) make them less suitable for combustion. Woody crops are grown on short rotations, generally with more intensive management than timber plantations. These fast-growing hardwood trees include poplar, willow, maple, cottonwood, black walnut, and sweetgum. The woody crops are harvested within 5–8 years of planting [4, 14, 15, 16, 17].

Aquatic biomass includes different types of algae, plants, and microbes found in water, such as aquatic plants, water hyacinth, seaweed, kelp, macroalgae, and microalgae [18]. Another primary biomass source is municipal, industrial, food, and animal waste. Municipal solid waste includes waste from commercial, industrial, and residential sectors containing a significant amount of biomass with energy content. The industrial waste includes waste from textile and food processing industries and waste from various industrial and manufacturing processes, such as sugar cane residues and paper sludge. Food waste includes postconsumer waste, animal fat, used cooking oil, residues from food and drink manufacturing, preparation and processing, etc. Animal and human waste includes cooked or uncooked food, fruits, paper, manure of different animals, and waste from farm and processing operations. The problem of disposing of waste is reduced to a certain extent when waste materials are treated and converted to useful energy products. Primarily, animal and human waste are free of harmful materials. In contrast, industrial waste may contain different harmful additives and toxic chemicals [4, 14, 15, 19].

Plant biomass has a carbon-to-oxygen (C/O) ratio of almost one. Because of the high level of oxygen, the energy density of biomass is relatively lower than fossil fuels, which means that issues associated with land use must be considered. The potential benefit of biomass can be reduced by environmental damage due to the expansion of land use for biomass production, leading to a high potential for deforestation, emissions, erosion, nutrient runoff, etc. When sufficient land areas are available, large-scale cultivation of energy crops for bioenergy is feasible. The agricultural lands must be used to grow food crops. Land for energy crops needs to be selected carefully to avoid food versus energy conflict. Identifying lands with minimal disturbance to food production is critical for technically and economically feasible biomass production. To achieve sustainable large-scale biomass production, infertile/marginal or abandoned agricultural land with little fertilizer or pesticides and potentially needing minimal water has been widely considered important. Energy crops are adaptive to infertile/marginal or abandoned agricultural land. Energy crops, such as switchgrass and miscanthus, generally require much less water to grow and are suitable to replace dryland crops partially. Energy crops should not be grown at the expense of biodiversity. Beyond the vast land areas needed to grow energy crops, the long-term impact of soil quality due to repeated removal of biomass and water usage are major concerns [4, 20, 21].

Plants absorb atmospheric CO₂ and produce carbohydrates in photosynthesis that form the building blocks of biomass. Water and sunlight are the other two key ingredients of photosynthesis. The burning of biomass does not add to the total CO₂ inventory of the earth as it releases CO₂ back into the atmosphere that the plants have absorbed recently in photosynthesis producing biomass. Therefore, biomass is considered the most important carbon-neutral or green carbon fuel source. But the overall biomass chain needs to be considered for true carbon neutrality of biomass. Significant cost, energy needs, and CO₂ emissions account for biomass harvesting, drying, handling, transportation, processing, and storage, which need to be considered in life-cycle analysis for sustainability. Biomass plays an integral part in the overall sustainable energy solution. Biochar facilitates the conversion of marginal lands to lands suitable for agriculture by improving soil quality. The impacts of adding biochar to soils may include reduced land area required for food production as a result of increased productivity and making marginal lands economically productive [4, 12, 20, 22].

2.2 Composition of biomass

The chemical composition of biomass is different from fossil fuels. Lignocellulosic biomass is a complex mixture of biopolymers consisting of three key elements, carbon (C), oxygen (O), and hydrogen (H). The percentages in dry matter of C, O, and H are 42–47%, 40–44%, and 6%, respectively, whose total content reaches typically above 95%. In addition, depending on the plant species and environment, plant biomass also contains various macronutrients, micronutrients, trace elements, and other heavy metals [4, 18]. The non-starch fibrous part of the plant (lignocellulosic) material is the major component of plant biomass. Three major constituents of lignocellulosic biomass comprising the cell wall of plants are cellulose, hemicellulose, and lignin. Cellulose, the main component of the plant cell wall, provides structural support. The second most abundant polymer in lignocellulosic biomass is hemicellulose. The third most abundant polymer in lignocellulosic biomass is lignin. Usually, cellulose is the major component in wooden biomasses, whereas hemicellulose is the key component in leaves and grasses and lignin in shells. Hemicellulose is thermally less stable than cellulose. Lignin is the most stable of all three. Knowledge of biomass composition in terms of cellulose, hemicellulose, and lignin can be helpful in controlling the product chemistry [2, 4, 23, 24].

The other compounds present in biomass include inorganic compounds and organic extractives. These nonstructural components include fats, waxes, proteins, terpenes, simple sugars, gums, resins, starches, and essential oils that do not constitute the cell walls or cell layers. Often these compounds are responsible for the smell, color, flavor, and natural resistance to decaying of some species. The inorganic compounds constitute less than 10% by weight of biomass, forming ash in the pyrolysis process. Depending on the type of biomass, the cellulose, hemicellulose, and lignin content fall in the range of 40–60%, 15–30%, and 10–25%, respectively. Fermentable sugars produced by hydrolyzing carbohydrates (cellulose and hemicellulose) can be converted into fuels and chemicals. The content of cellulose, hemicellulose, and lignin in wood biomass is high (~90%), while more extractives and ash are present in agricultural and herbaceous biomass [2, 4, 24].

Analysis of biomass feedstock is an essential part of understanding the behavior of biomass in energy use. The proximate analysis, ultimate analysis, and higher heating value (HHV) of biomass feedstock can provide a clear understanding of its thermochemical conversion characteristics. The proximate analysis gives information on biomass composition in terms of volatile matter (VM), fixed carbon (FC), ash content, and moisture (M) content. VM is the condensable and non-condensable vapors/gases released from biomass during heating. The amount of VM depends on the heating rate and the final biomass temperature. FC is the solid carbon (nonvolatile) that remains in the char after devolatilization. FC and VM indicate the percentage of biomass burned in solid and gaseous states, respectively. Ash is the noncombustible solid residue remaining after biomass is completely burned. These are of fundamental importance for bioenergy use. These data provide the essential information for the furnace design, including sizing and location of primary and secondary air supplies, refractory, ash removal, and exhaust handling equipment. [4, 25, 26].

The ash contains mostly inorganic residues and its composition depends on the biomass type. The inorganics in ash include silica, calcium, iron, aluminum, and small amounts of potassium, sodium, magnesium, and titanium. The content of ash in biomass is generally small. But if biomass contains alkali metals or halides, ash may play a significant role in biomass combustion or gasification. Agricultural residues, grasses, and straw generally contain potassium compounds and chlorides are particularly susceptible to this problem and can cause severe corrosion, fouling, and agglomeration in boilers or gasifiers. Burning biomass at lower temperatures mitigates the problems of corrosion and slagging. The ash produced during biomass conversion does not necessarily come from biomass itself but also from other sources like contamination as well. Biomass can pick up dirt, soil, rock, and other impurities during collection and handling, partly contributing to ash content [4, 14, 25].

The relationship between FC and char yield in biomass is positive, while VM and ash relate negatively to char yield. The greater biomass VM is expected to lead to greater gas production instead of the solid phase. Moisture content has a significant impact on the biomass conversion process. High moisture content is a major concern in biomass conversion. Thermochemical conversion processes generally require biomass with low moisture content. However, biochemical conversion processes can use biomass with high moisture content. Some moisture is required in the gasification process to produce hydrogen and with increasing moisture content, the amount of hydrogen increases. The moisture content can be very high (>90%) in some wet biomass (such as water hyacinth). As the energy used in the evaporation of moisture is not recovered, moisture drains much of the deliverable energy during conversion [4, 25, 26, 27].

The ultimate analysis provides the composition of biomass on a gravimetric basis, including major elements (C, H, O, S, and N), moisture, and ash. The ultimate analysis is usually reported on a dry and ash-free basis. These are useful for performing mass balances on biomass conversion processes. Elemental chemical composition, volatiles, moisture, and ash are essential for thermochemical conversions of biomass. Additionally, information on the polymeric composition of biomass is required for conversions, such as torrefaction, pyrolysis, and gasification. The ultimate analysis helps calculate the quantity of combustion air needed to sustain the combustion reactions. Usually, the sulfur and nitrogen content of biomass is very low and produces minimal pollutants SO_X and NO_X [25, 27].

3. Biomass conversion

Biomass can be converted to end products (such as heat, biofuels, or chemicals) through chemical, biochemical, and thermochemical conversion processes. Selection of the conversion process depends on number of factors, such as the desired form of end products, biomass feedstock available, environmental standards, policy, economic conditions, and specific factors related to the project. In most situations, the selection of the conversion process is based on two factors, the desired form of end products and biomass feedstock available. The moisture content of biomass primarily determines the biomass conversion process. Dry biomasses (such as wood or straw) are required for thermochemical conversions, such as pyrolysis, gasification, or combustion. Low-energy density due to higher moisture content makes wet biomass unsuitable for these processes. Transportation and energy costs significantly increased due to the high moisture content. Hydrothermal and biochemical processing are wet conversion processes that have gained growing attention and are more suitable for processing high moisture content biomass, including aquatic biomass, sewage sludge, food waste, and manure. Compared with thermochemical conversion, biochemical conversion consumes less energy but requires more time. Consequently, cost-effective hydrothermal processing has been given more attention than thermochemical conversion (with drying). If moisture content lies between wet and dry regions, additional parameters (such as cost and feasibility of drying) need to be considered in selecting a suitable conversion process [1, 2, 4, 10, 28, 29, 30].

Thermochemical conversion processes usually offer many advantages over biochemical conversion processes, including better conversion efficiency, handling a wide variety of feedstocks, shorter reaction times, and high-energy efficiency. As a result, thermochemical conversion processes have recently received greater attention for biofuel production. Many thermochemical conversion processes are available to convert biomass into products (solid, liquid, and gaseous). Thermochemical conversion processes use high temperatures to breakdown the bonds of biomass organic matter. These are classified according to the oxygen content used in the process, including combustion (complete oxidation), gasification (partial oxidation), and pyrolysis (thermal degradation in the absence of oxygen). Torrefaction, a mild form of pyrolysis, is also performed in the absence of oxygen. Hydrothermal processing, a thermal degradation in the absence of oxygen, is an alternative route to process wet biomass. The typical products of the thermochemical conversion of biomass are biochar (carbon-rich solid residue), bio-oil (liquid fraction, condensable vapors), and non-condensable gases. The distribution of products (biochar, bio-oil, and gases) depends primarily on the conversion process [2, 4, 9].

4. Pyrolysis

Pyrolysis is one of the thermal decomposition processes conducted in the absence of oxygen to convert biomass into three distinct product fractions—solid residue (biochar), condensable vapors resulting in liquid product fraction (bio-oil), and non-condensable gaseous products. In the absence of oxygen, combustion cannot occur; instead, pyrolysis happens. Pyrolysis processes can be classified as torrefaction, slow pyrolysis, intermediate pyrolysis, fast pyrolysis, flash pyrolysis, microwave pyrolysis, and hydrothermal processing. These pyrolysis processes differ from one another based on the operating conditions such as residence time, heating rate, and pyrolysis temperature, which in turn affect the yield of products (gas, bio-oil, and biochar) [3, 4]. Moderate temperatures and short residence times tend to produce more liquids. The operating conditions of these different thermal conversion processes, along with their product distribution and biomass feed particle size needed, are shown in Table 1.

Mode	Condition	Liquid (bio-oil)	Solid (biochar)	Gas (syngas)	Heating rate	Particle Size (mm)
Slow pyrolysis	Low to moderate temperatures (300–550°C), Long residence time (hours to days)	30%	35%	35%	1–0.8°C/s	5–50
Intermediate pyrolysis	Low to moderate temperatures (450–550°C), Moderate residence time (10–20 s)	50%	25%	25%	1–10°C/s	1–5
Fast pyrolysis	Moderate temperatures (425–600°C), Short vapor residence time (<2 s)	75%	12%	13%	10–1000°C/s	< 1
Flash Pyrolysis	(750–1000°C), (0.5 seconds)				>1000°C/s	<0.2
Microwave- Assisted Pyrolysis	(400–800°C)
Torrefaction	(450–550°C), (< 2 hours)	20%	75%	5%	< 1°C/s
Hydrothermal Carbonization	(<200°C), (minutes to hours)	35–80%			< 1°C/s
Hydrothermal Liquefaction	(200–350°C), 5–20 MPa
Hydrothermal Gasification	(400–600°C), 23–45 MPa, short residence time
Gasification	High temperature (>800°C), Long vapor residence time	5% tar (55% water)	10%	85%

Table 1.

Operating conditions of various pyrolysis processes and their product fractions (bio-oil, biochar, and gas) [2].

The three pathways char formation, depolymerization, and fragmentation describe the primary conversion of biomass during the pyrolysis process. Intra- and intermolecular rearrangement reactions generally favor char formation resulting in higher thermal stability of the residue. The formation of benzene rings and the combination of these rings into an aromatic polycyclic structure characterize char formation. The release of water or non-condensable gas (devolatilization) generally accompanies these rearrangement reactions. The breaking of polymer bonds characterizes depolymerization, a dominant reaction during the initial stages of pyrolysis. When the temperature is sufficiently greater than the activation energies for the bond dissociation, depolymerization occurs, increasing the concentration of free radicals. It is followed by stabilization reactions producing monomer, dimer, and trimer units. These volatile condensable molecules at ambient conditions are found in the liquid fraction. Fragmentation involves breaking polymer bonds and even monomer bonds, resulting in the formation of non-condensable gases and a range of organic vapors that are condensable under ambient conditions [4, 31, 32, 33].

The decomposition of three lignocellulose components (hemicellulose, cellulose, and lignin) releases condensable vapors and non-condensable gases. The condensable vapor includes methanol, acetic acid, acetone (mainly from hemicellulose), anhydrous monosaccharides, hydroxyacetaldehyde (mainly from cellulose), phenols, and heavier tars (from lignin decomposition) apart from water vapor. The water-insoluble heavier tars contain larger molecules obtained from splitting ether and C-C bonds in lignin. The condensable vapors are condensed to form bio-oil (a dark brown and free-flowing organic liquid mixture). It usually contains 15–35 wt.% water resulting from the original moisture and as a pyrolysis product. Pyrolysis temperature determines the degree of devolatilization of biomass. There are significant differences between the pyrolysis behaviors of hemicellulose, cellulose, and lignin, which are responsible for most physical and chemical property modifications during the pyrolysis process. Hemicellulose and cellulose decompose over a narrow temperature range. Lignin decomposes over a wider temperature range than hemicellulose and cellulose [4, 31, 32, 34, 35].

5. Fast pyrolysis

Fast pyrolysis typically involves high temperatures (450 and 550°C), high heating rates (10–1000°C/s), and short residence times (0.5–2 s) [36]. It is the most promising thermal process to produce a higher amount of liquid fuel (bio-oils) than other thermal conversion processes. Fast pyrolysis can produce up to 75 wt% bio-oil [37], which can be used directly or as an energy carrier after upgrading.

Fast pyrolysis suppresses secondary reactions (cracking and repolymerization) by having short vapor residence times (rapid removal and quenching of condensable primary volatile vapors) and maintaining high heating rates, thereby maximizing the yield of condensable vapors (bio-oil). This results from rapid quenching and condensing intermediate degradation products of hemicellulose, cellulose, and lignin to bio-oil without further reactions, such as breaking down larger molecular weight (MW) components into smaller MW gaseous products. The rapid quenching of intermediates results in bio-oil having many reactive species, contributing to its unusual characteristics. Rapid and simultaneous depolymerization and fragmentation of cellulose, hemicellulose, and lignin fractions with a rapid increase in temperature form condensable vapors. Rapid removal and quenching shorten the residence time at high temperatures and trap many of these fractions inhibiting further reactions (depolymerize, decompose, degrade, crack or condense with other molecules) to form more non-condensable gases [4, 38, 39].

The main product of the fast pyrolysis process is bio-oil (65–75%), with smaller amounts of biochar (10–25%) and non-condensable gases (10–20%). The distribution of bio-oil, biochar, and gases depends on the biomass composition, rate, and duration of heating. The fast pyrolysis process has the capability to produce bio-oil with high fuel-to-feed ratios. To strike a balance between thorough devolatilization and minimal secondary cracking of vapors, the optimum pyrolysis temperature range for bio-oil production is 425–600°C, with a maximum yield of around 500°C [10]. Due to the higher cellulose and hemicellulose content in wood than in energy crops and agricultural residues, woody biomass (poplar, sawdust, forest, and wood residue) produces the highest bio-oil yield of around 75%, followed by energy crops (reed) and agricultural residues (wheat straw, flax straw, etc.). Product yield in fast pyrolysis is affected by the feed particle size. Smaller particle size increases the heat transfer rate, thus, increasing bio-oil yield. Feedstock particle size and pyrolysis temperature need to be optimized for maximum bio-oil yield [4, 36, 39]. A finely ground (usually <1 mm) biomass feed is required to achieve very high heat transfer rates, thereby very high heating rates reducing heat and mass transfer limitations. Due to the absence of secondary reactions, the overall fast pyrolysis process is highly endothermic. Fast pyrolysis favors low moisture content biomass (<10 wt.%) to minimize water content in bio-oil. Low moisture content also facilitates grinding the feed to give sufficiently small particles to ensure rapid heating and hence fast pyrolysis [4, 37].

The central part of the pyrolysis process is the reactor used, where the thermal conversion reactions occur. Many reactors are used in the pyrolysis process, such as entrained flow reactor, fluidized bed reactor, fixed bed reactor, autoclave, rotating cone reactor, and plasma reactor [40]. These reactors can be classified into subcategories according to the flow of material and phenomena, such as circulating, co-current, counter-current, and crossflow. The amount of bio-oil depends on the reactors used and the operating conditions. The continuous developments in pyrolysis technologies explore many reactor designs to optimize pyrolysis performance and produce high-quality bio-oil. Because of its moisture contents, a higher heating value (HHV) of the bio-oil produced is half the HHV of crude oil. However, each reactor type has specific characteristics, bio-oil yielding capacity, advantages, and limitations. The crucial characteristic steps of the fast pyrolysis process are: the pyrolysis reaction takes place with high heat and heat transfer rates, thus, the particle sizes of biomass materials need to be small enough to enhance such heat transfer; the pyrolysis reaction temperature ranges from 450 to 550°C in the vapor phase; short residence times for the vapor up to two seconds; rapid quenching and condensing the vapors into bio-oil. Common reactor types used for fast pyrolysis are described below [41, 42, 43, 44, 45].

5.1 Packed bed reactor

The packed bed pyrolysis reactor system contains a reactor with a gas cooling and cleaning system. These reactors are common types of reactors with cylindrical shapes and packed with solid packing materials, such as firebricks, steel, or concrete; they can be packed with catalysts too. The feed enters from one side and the product is obtained from the other. The relatively fine biomass solids move down and contact a counter-current upward-moving product gas stream. The catalyst pellets are packed in a given section and are unmovable where the pyrolysis reactions occur in this section. Some of the advantages of these packed bed reactors are catalyst recovery and recycling, which gives good economic impacts [41, 42].

5.2 Bubbling fluidized-bed reactor

Fluidization is a phenomenon in which fine solids are transformed into a fluid-like state through contact with a gas or liquid. The particles in the fluidized bed are present in a semi-suspended state when the gas velocity maintains a critical value known as the minimum fluidization velocity. The fixed bed transforms into a fluidized bed at this stage when the fluid drag is equal to the particle weight. Bubbles are made at the openings at which the fluidizing gas enters the bed, where the packing solids above the gas entrance are pushed aside until they create a void space through which the gas can enter at the initial fluidization velocity. Uniform mixing, uniform temperature distribution, and operation in a continuous state are the main advantages of bubbling fluidized-bed reactors [43, 44].

5.3 Circulating fluidized-bed reactor

A circulating fluidized-bed reactor works on the same principle as the bubbling fluidized bed except that the bed is highly expanded and solids continuously recycle around an external loop comprising a cyclone and loop seal. In this circulating fluidized bed, the reactor does not contain any bed and does not have any separate upper surface. The most important advantages of circulating fluidized-bed reactors over other reactor configurations include internal recycling of huge bulk particles reaching the top of the vessel back to its bottom, a good void range, and no distinct upper bed surface in the column [42, 45].

5.4 Ablative pyrolysis reactor

Ablative pyrolysis reactor is basically different in concept compared to the other methods of fast pyrolysis. In ablative pyrolysis, biomass is pressed against a heated surface and rapidly moved during which the biomass melts at the heated surface and leaves an oil film behind which evaporates. In the other reactors mentioned above, the rate of reaction is limited by the rate of heat transfer through a biomass particle, that is why fine particles are required. This ablative process uses larger biomass particles and is typically limited by the heat supply rate to the reactor. The rate of reaction is strongly affected by pressure, the relative velocity of biomass on the heat exchange surface, and the reactor surface temperature [41, 42].

6. Hydrothermal processing

Biomass materials are typically wet and have a moisture content range of up to 95 wt.%. Biomass with more than 30 wt.% moisture content is not suitable for pyrolysis. It needs to be dried before being suitable for pyrolysis, which requires a large amount of energy. It becomes a net energy consumption for biomass with high moisture content because the heat available from the biomass is less than the heat of moisture evaporation. Hydrothermal processing involves applying heat and pressure in the presence of water (subcritical or supercritical). Biomass typically with 70 wt.% or more water can be converted into carbonaceous end products without atmospheric oxygen. Water plays an active role as a solvent and reactant in hydrothermal processing. It is a promising technique for converting wet biomass into carbonaceous solids at relatively high yields without energy-intensive drying before or during the process. Depending on the operating conditions (temperature, pressure, and residence time), hydrothermal processes are classified as hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG) [2, 4].

6.1 Hydrothermal carbonization (HTC)

When biomass feedstock in water is heated at temperatures below 200°C in a sealed vessel at autogenous pressure, mostly solids (hydrochar) are formed in a process known as HTC. The residence time of HTC varies from minutes up to several hours. Hemicellulose and cellulose decomposition temperature in subcritical water is usually around 160°C and 180 to 200°C, respectively, while lignin decomposes above 220°C. HTC converts biomass into three distinct product fractions: solid residue (hydrochar), bio-oil mixed with water in liquid fraction (aqueous solution), and a small volume of gases (consisting mainly of CO₂). HTC aims to maximize the hydrochar yield. The three factors (type of biomass, pH, and maximum temperature used) primarily influence the product distribution and characteristics. The other factors, such as solids concentration (in biomass water mixture) and reaction time, have a relatively smaller influence. The overall extent of hydrochar formation from glucose is negligible below 160°C and yield is maximum at 200°C. Hydrochar formation is reduced with the increase in temperature above 200°C as a result of gasification reactions converting part of the hydrochar formed into volatile compounds. Process conditions and the type of biomass feed are the two factors that influence energy requirements and final product composition. Hydrochar has high hydrophobic and homogeneous properties and can be easily separated from the liquid fraction. Dried hydrochar pellets can be produced from the separated solid fraction, which can be used for energy production. The liquid fraction can be used to recover mono sugars. The gas fraction has less CO and CO₂ and is less harmful. The hydrochar has carbon content similar to lignite and the yield of hydrochar varies from 35% up to 80% [2, 4, 46, 47, 48].

The HTC reduces both the oxygen and hydrogen content of the biomass through dehydration and decarboxylation. During HTC, hemicelluloses and cellulose are hydrolyzed to oligomers/monomers, whereas lignin mostly remains unchanged. The reaction mechanism of the HTC process mainly involves dehydration, decarboxylation, and polymerization. Dehydration is favored at temperatures less than 300°C. The hydroxymethylfurfural (HMF) generated from hexose (D-fructose and D-glucose) and furfural generated from pentoses (D-xylose) are well-known dehydration products of sugars. The hydrothermal process under acidic conditions allows the effective conversion of D-glucose to HMF. D-glucose first isomerizes to D-fructose and then undergoes dehydration to form HMF. The HMF, in turn, decomposes into levulinic acid, formic acid, and soluble polymeric carbonaceous material with increasing residence time [49, 50].

Hydrochar has a higher energy content than the feedstock used and lower O/C and H/C ratios than the feedstock. Hydrochar has higher H/C ratios than biochar specifications. HTC is a high-energy-consuming process. Solar energy appears to be an attractive renewable energy source to combine with HTC. HTC can combine with other processes to produce hydrochar with characteristics (morphology, porosity, conductivity, H/C ratio, O/C ratio, energy content, elemental composition, etc.) suitable for applications in many fields such as solid fuel in power generation, soil amendment, adsorbent in water purification and carbon capture. Hydrochar can be further processed to use as carbon electrodes or nanocomposites. HTC process was initially used for the degradation of organic materials, production of liquid and gaseous fuels, and production of basic chemicals. In recent years, the technology gained research interest to produce solid hydrochar and as a technique to synthesize nano- and micro-size carbon particles [46, 47, 51, 52].

The hydrochar produced by HTC directly from carbohydrates or biomass lacks porosity. Only a tiny porosity is developed even after further carbonization at a higher temperature. This is due to hydrochar being pre-carbonized material produced under autogenic pressures and temperatures between 160 and 200°C. For most industrial applications such as adsorption or catalysis, the high surface area and porosity of hydrochar are essential. This would ensure efficient transport and diffusion throughout the material. Different techniques have been developed to improve the porosity of hydrochar [47]. Some of the advantages of the HTC process include low carbonization temperatures, can be synthesized in the aqueous phase (no drying is required), and inexpensive process. Hydrochar obtained from HTC has the following properties: (a) uniform spherical micro-sized particles; (b) oxygenated functional groups at the surface (OH, C=O, COOH groups); (c) controlled porosity can be easily introduced using activation procedures, thermal treatments, etc.; (d) easily controlled surface chemistry and electronic properties via additional thermal treatment; (e) special physicochemical properties can be obtained by adding other components (such as inorganic nanoparticles) to biomass [49, 50].

6.2 Hydrothermal liquefaction (HTL)

At temperatures between 200 and 350°C and pressures of 5–20 MPa, biomass is primarily converted to a liquid fraction (aqueous soluble) in a process known as HTL. Leading reactions in HTL are considered to be free radical and ionic reactions. At ambient conditions, the dielectric constant (a measure of hydrogen bonding) of water is about 80 F/m. It decreases rapidly with increasing temperature, at 250°C and 5 MPa dielectric constant is about 27 F/m and at 350°C and 25 MPa about 14 F/m. Due to decreasing dielectric constant (number of hydrogen bonds), water displays less polar behavior. An increase in temperature increases the dissociation of water. The ionic product of water (pK_w) at 25°C is 14 and decreases to 11 at 250°C. With increasing temperature, mass transfer is enhanced because of accelerated mass-transfer-limited chemical reactions resulting from a decrease in the viscosity of water [4, 53, 54].

The primary conversion of biomass during the HTL comprises three pathways; depolymerization, decomposition, and recombination. Higher MW biomass is depolymerized and decomposed into smaller MW compounds. These compounds are highly reactive and recombined (repolymerized) to form bio-oil, gaseous and solid products. The parameters such as temperature and pressure are important for the depolymerization of long-chain polymer structures to shorter-chain hydrocarbons. The decomposition step involves three steps: dehydration (loss of water molecule), decarboxylation (loss of CO₂ molecule), and deamination (removal of amino acid content). The dehydration and decarboxylation steps remove oxygen from the biomass in the form of H₂O and CO₂, respectively. Macromolecules of biomass are hydrolyzed to form polar monomers and oligomers. Subcritical water at HTL temperatures and pressure breaks down the hydrogen bonds of the cellulose structure to form sugar monomers. It is rapidly degraded by different reactions (such as isomerization, hydrolysis, dehydration, reverse aldol defragmentation, rearrangement, and recombination) into a series of products. Most of the degradation products such as polar organic molecules, furfurals, phenols, glycolaldehyde, and organic acids are highly soluble in water. Recombination and repolymerization of light MW compounds occur due to the unavailability of the hydrogen compound or excess oxygen [53, 55, 56].

During HTL of lignin, hydrolysis and splitting of the ether and C-C bond, demethoxylation, alkylation, and condensation reactions occur. Competition occurs between these main reactions. The gaseous, liquid, and solid yield of HTL of biomass depends on several parameters, including biomass feedstock, temperature, heating rate, residence time, pressure, mass ratio of water/biomass, and catalyst. The main product of HTL is the liquid fraction (bio-oil). The temperature and pressure directly (activation energy, reaction equilibria) and indirectly (solvent properties) impact the reaction. During HTL, the major components of biomass cellulose, hemicellulose, and lignin behave differently. In general, biomass with high cellulose and hemicellulose produces higher bio-oil yields. Higher bio-oil yields have been reported from hardwood samples (cherry) than softwood (cypress). Softwood contains higher lignin than hardwood, hence, lower bio-oil yield [54, 57]. Other studies have also shown that both temperature and lignin contents of wood had a marked effect on bio-oil yield. Bio-oil production was maximum for wood with low lignin contents [58, 59]. Subcritical water in HTL acts as a heat transfer medium to overcome the heat transfer limitations. As a result, biomass particle size has negligible to minimal effects on HTL. Excessive size reduction of biomass feedstock is not required [54, 58].

Usually, the effect of temperature on the bio-oil yield is synergetic due to the increased fragmentation of biomass at higher temperatures. Depolymerization occurs when the temperature is sufficient for bond dissociation. The competition among hydrolysis, fragmentation, and repolymerization reactions describes the role of temperature during the HTL process. Depolymerization is a dominant reaction during the initial stages of HTL. Repolymerization becomes active at later stages of HTL, leading to the formation of hydrochar. Intermediate temperatures usually produce higher bio-oil yields [54, 58]. The increase in HTL temperature not only enhances the reaction rates but also changes the reaction mechanisms. Hence, lower temperatures favor ionic reactions; higher temperatures promote the formation of radicals by homolytic bond breakage. Radical reactions usually lead to a diverse product spectrum and finally to gas formation [54, 60, 61]. Various authors have observed increased bio-oil yields with increasing temperature during the HTL process. Different authors have proposed various optimum temperatures for a variety of biomasses. It can be assumed that the temperature range of 280–350°C would be suitable for the decomposition of biomass under both subcritical and supercritical conditions. Final HTL temperature varies with the type of biomass [54, 58].

The temperature gradients during the heating of biomass are important for the sequence and extent of chemical reactions. Due to the better dissolution and stabilization of fragmented compounds in subcritical water, the effect of heating rates on the product distributions in HTL is minimal compared to pyrolysis. Because of secondary reactions, slow heating rates typically tend to yield solid fraction (hydrochar). Secondary reactions are also dominant at very high heating rates and yield more gases. Furthermore, bio-oil yield is not significantly affected by large variations in high heating rates. Moderate heating rates may be suitable to overcome heat transfer limitations leading to extensive fragmentation and minimal secondary reactions. Many researchers have investigated the effect of residence times on product distribution during the HTL process. Duration of reaction time may characterize the product compositions and the overall biomass conversion. Short residence times are usually preferred during the HTL of biomass. Longer residence times can decompose preasphaltenes and asphaltene into lighter products enhancing the yield of bio-oil and gases. It is essential to inhibit the decomposition of lighter products to obtain a high liquid oil yield. Generally, bio-oil yield attains a maximum before decreasing for extended residence times, whereas gas yield and biomass conversion increase continuously until reaching saturation [54, 55, 56, 58].

Pressure helps maintain single-phase media for HTL to avoid large heat inputs required for phase change. Two-phase systems need a large heat supply to maintain the temperature of the system. Pressure increases solvent density and a high-density medium penetrates effectively into molecules of biomass components resulting in improved decomposition and extraction. Many investigations have been performed to study the influence of different solvents (such as subcritical and supercritical alcohols) on the liquefaction yield of lignocellulosic biomass. Critical temperatures and pressures of alcohols are lower than in water and significantly milder reaction conditions could be used. Alcohols are expected to dissolve relatively high MW products derived from cellulose, hemicelluloses, and lignin due to their lower dielectric constants than water. Ethanol and methanol have been widely employed for biomass liquefaction. The mass ratio of biomass/water is considered a vital parameter for the HTL process. Different authors investigated the effect of water density on HTL yield. All solvolytic conversions are benefitted from the dilution of reactants, intermediates, and products during the reaction. This dilution minimizes cross-reactions and produces a more distinct product spectrum. Higher substrate concentrations inevitably lead to cross-reactions leading to undesirable polymerization of the reaction products. Such processes have been observed for HTL of biomasses, promoting the formation of solid fractions. Catalysts are important in the HTL of biomass. A range of homogeneous catalysts (such as mineral acids, organic acids, and bases) and heterogeneous catalysts (such as zirconium dioxide, anatase, and other materials) has been proposed to tailor the reaction toward a specific product and enhance the reaction rates [54, 55, 56, 62].

6.3 Hydrothermal gasification (HTG)

HTG operates near or above the critical point of water at 400–600°C and 23–45 MPa. The primary product of HTG is a mixture of non-condensable gases (H₂, CO, CH₄, and CO₂), which can produce syngas enriched with H₂. At the critical point (374°C and 22.1 MPa) of water, the conversion efficiency is improved. Biomass polysaccharides split in the presence of supercritical water (SCW). Due to higher reaction temperatures, HTG reactions progress at a faster rate and complete decomposition of biomass is achieved. This is a distinctive feature of HTG compared to other hydrothermal treatments (HTC and HTL). One of the problems with HTC and HTL is the difficulty in byproduct treatment due to undesirable byproducts being occasionally dissolved in the liquid fraction. The conversion rate of HTG is typically higher than 80% that decomposes biomass into gaseous products. Consequently, post-treatment of liquid fraction is not required or easily carried out because only a small amount of organic compounds remain in the liquid. Conventional gasification can be effectively employed when biomass is not wet, but it is ineffective when biomass has a high moisture content (> 80%). The syngas of conventional gasification is partially diluted with nitrogen (due to partial oxidation using air) and contains tar. Syngas from HTG is not diluted with nitrogen and do not contain tars. Tar, if produced, remains in the liquid fraction [2, 4, 63].

The HTG performance is strongly dependent on the operating conditions, including biomass characteristics, temperature, pressure, residence time, feedstock concentration, and catalyst. The rate of hydrolysis and decomposition is relatively fast in the HTG process; hence, short residence times are expected to degrade biomass successfully. Optimization of residence times is required for the efficient destruction of biomass organic compounds. Pressure helps maintain single-phase media for HTG to avoid large heat inputs required for phase change. Two-phase systems need a large heat supply to maintain the temperature of the system. The rate of hydrolysis and biomass dissolution can be controlled by maintaining pressure higher than the supercritical pressure, which may enhance favorable reaction pathways for bio-oil or gas yield. Pressure imparts minor or negligible influence on bio-oil or gas yield in supercritical conditions. This is because, in the supercritical region, the effect of pressure on the properties of water is minimal [54, 58, 64].

SCW exhibits a unique property; the density, viscosity, ionic product, and dielectric constant change significantly when water changes from ambient conditions (25°C and 0.1 MPa) to the supercritical condition. At ambient conditions, the dielectric constant of water is about 80 F/m and water is a polar solvent due to a high dielectric constant (a large number of hydrogen bonds). At supercritical conditions (400°C and 25 MPa), the dielectric constant is about 6 F/m; because of the decrease in the number of hydrogen bonds, water begins to display the behavior of a nonpolar solvent that can completely dissolve many organic compounds, hydrocarbons, and gases (such as CO₂, CH₄, H₂, and N₂). This results in poor solubility of inorganic polar compounds in SCW. Many rapid homogeneous reactions involving organic compounds occur at supercritical conditions due to the absence of phase boundaries. In subcritical water, inorganic polar compounds (such as NaCl, KCl, and CaSO₄) are usually soluble. But these compounds are insoluble in supercritical water and easily separated from the reaction products. SCW exhibits gas-like properties and using SCW as the reaction medium in HTG has several advantages: low viscosity creates a high diffusion coefficient and enhances mass transfer, low density improves the solvation properties, creates a single-phase reaction environment in the reactor by complete miscibility with different organics and gases, enhance mass transfer, prevent poisoning of catalyst (if used) and coke formation and product gas (syngas) does not have tar and has a high heating value. Syngas can be converted to liquid fuels or value-added chemicals via different gas-to-liquid conversions, such as Fischer-Tropsch synthesis or to ethanol and butanol through syngas fermentation using microorganisms [9, 53, 54, 63].

7. Conclusions

Biomass is a sustainable energy source and a promising eco-friendly alternative source of renewable bioenergy. The most abundant biomass, lignocellulose, represents a significant carbon source for bioenergy. Lignocellulosic biomass is a complex mixture of biopolymers. The three major biopolymers are cellulose, hemicellulose, and lignin. Inorganic compounds and organic extractives are among other compounds present in lignocellulosic biomass. The selection of the process to convert biomass to end products depends on several factors, but the desired form of end products and available biomass feedstock are the two key factors usually considered. Thermochemical conversion processes usually offer many advantages over biochemical conversion processes.

Thermochemical conversions convert biomass into liquid, gaseous and solid products. The product distribution depends on the conversion process employed, that is, operating conditions (heating rate, residence time, and temperature). More liquids are produced at moderate temperatures and short residence times. Bio-oil is the desired product in fast pyrolysis and bio-oil yield is maximized at high heating rates, short vapor residence times, and around 500°C. A finely ground dry biomass feed is essential for high heat transfer rates. Due to the higher cellulose and hemicellulose content, woody biomass produces the highest bio-oil yield.

High moisture content is a major barrier in biomass processing for bioenergy. It has a significant impact on the biomass conversion process. High moisture biomass requires a large amount of energy to evaporate moisture to make it suitable for pyrolysis.

Hydrothermal processing is useful for biomass feedstocks with high moisture as it does not require drying, thereby reducing energy costs. Hydrothermal processing has been given more attention in recent years and can be classified into HTC, HTL, and HTG based on the reaction temperature, pressure, and residence time. HTC, HTL, and HTG are aimed to maximize the production of solid (hydrochar), liquid (bio-oil/water), and gas (non-condensable) fractions, respectively. More research is required on hydrothermal processing to investigate reaction kinetics and chemistry, heat transfer, energy and heat recovery, combinations with other technologies, such as solar, technical, and economic aspects and the effect of operational parameters.