*5.3.3 Intermediate pyrolysis*

In intermediate pyrolysis, the reaction is faster than slow pyrolysis but slower than fast pyrolysis. Intermediate pyrolysis differs from fast pyrolysis in terms of heat transfer to biomass feed. It occurs in the temperature range of 450–550°C and even lower temperatures are used (350°C). The heating rates are much lower than fast pyrolysis (100 to 500°C/min) with residence time ranging from 10 to 30 s and produce less biochar than slow pyrolysis [65, 77]. Intermediate pyrolysis occurs at controlled heating rates, thus, inhibiting the formation of high MW tars and yielding products (biochar, bio-oil and gases) with different product qualities. In intermediate pyrolysis, the biomass particles sizes and shapes are less critical than in fast pyrolysis. It can process a wider variety of biomass, larger particles up to pellets, chips and dust and also material with a water content of up to 40%. High cooling rates are needed for the vapors to reduce thermal post-decomposition reactions. A higher amount of bio-oil than slow pyrolysis can be produced through intermediate pyrolysis. More controlled chemical reactions occur and, thus, the reaction conditions offer a broad range of variation for process optimization. The typical product distribution of intermediate pyrolysis is 40–60% of bio-oil, 20–30% non-condensable gases and 15–25% biochar [77].

The biochars produced by intermediate pyrolysis have a high carbon and low volatile contents. The bio-oil produced by intermediate pyrolysis has a high calorific value; the oil fraction easily separates into organic and aqueous phases. Compared to bio-oil generated from fast pyrolysis, the liquid fraction from intermediate pyrolysis has some beneficial characteristics, including low tar yield, improved viscosity and heating value. These characteristics may result from a relatively long residence time and contact with biochar [78]. Intermediate pyrolysis at 400°C yielding 35% solid (biochar), 46% liquid and 19% gaseous products has been reported. The liquid fraction separates into an aqueous phase (38% with 50% water; HHV of 7 MJ/kg) and organic phase (8% pyrolytic lignin, phenols, etc., HHV of 24 MJ /kg) [79].

## *5.3.4 Fast pyrolysis*

The fast pyrolysis typically involves high heating rates (10–1000°C/s), short residence times (0.5–2 s) [56] and temperatures between 450 and 550°C. It decomposes biomass quickly to generate liquid (bio-oil), solid (biochar) and gaseous products. The bio-oil, biochar and gas yields are typically 60–70 wt%, 12–15 wt% and 13–25 wt% on a dry basis, respectively [55].

Fast pyrolysis suppresses secondary reactions from taking place by having short vapor residence times (rapid removal and quenching of the condensable primary volatile vapors) and maintaining high biomass heating rates, thereby maximizing the yield of condensable vapors (bio-oil). In this way, secondary reactions of cracking and repolymerization are prevented. The intermediate products of flash degradation of hemicellulose, cellulose and lignin are rapidly quenched and condensed to bio-oil before further reactions break down higher MW components into gaseous products. This freezing of intermediates results in bio-oil containing many reactive species, contributing to its unusual characteristics. Condensable vapors are formed by rapidly and simultaneously depolymerizing and fragmenting cellulose, hemicellulose and lignin fractions with a rapid increase in temperature. Rapid quenching traps many of these products that would further react (depolymerize, decompose, degrade, crack or condense with other molecules) to form more non-condensable gases if the residence time at high temperature was extended [61, 80].

The distribution of products (bio-oil, biochar and gases) depends on the biomass composition and rate and duration of heating. If bio-oil is the product of interest, the optimum pyrolysis temperature range is 425–600°C, with the peak temperature below 650°C to strike a balance between thorough devolatilization and minimal secondary cracking of vapors. The optimum yield in vapor products translates to the quantity of bio-oil formed. However, the peak temperature can be up to 1000°C if gas production is of primary interest [5]. Woody biomass (poplar, sawdust, forest and wood residue) produces the highest bio-oil yield (around 75%). The second highest bio-oil yield is from energy crops (reed), followed by agricultural residues (wheat straw, flax straw etc.). This is due to the higher cellulose and hemicellulose in wood than energy crops and agricultural residues. Product (bio-oil, biochar and syngas) yields in fast pyrolysis are affected by the feed particle size. The heat transfer rate decreases with increasing particle size, thus, increasing biochar yield and decreasing bio-oil and syngas yield. Smaller particle size is better for internal heat transfer, which increases bio-oil yield. Both pyrolysis temperature and feedstock particle size need to be optimized for maximum bio-oil yield [56, 80].

A finely ground biomass feed (usually <1 mm) is required to achieve very high heat transfer rates and thereby very high heating rates, which reduce the mass and heat transfer limitations. The biochar yield in fast pyrolysis is generally low as only primary char is being produced (secondary reactions are suppressed) and high reaction rates also minimize biochar formation. The overall fast pyrolysis process is highly endothermic due to the absence of secondary reactions. Fast pyrolysis prefers biomass with low moisture content (< 10 wt%) in order to minimize the water in the product bio-oil. Low moisture content also facilitates grinding the feed to give sufficiently small particles to ensure rapid heating and fast pyrolysis [65].

### *5.3.5 Flash pyrolysis*

Flash pyrolysis aims to maximize the liquid yield (bio-oil). It is characterized by high temperatures, higher heating rates (> 1000°C/s) and shorter residence times (< 0.5 s). Very fine particles of biomass feed (< 0.2 mm) are usually required. Flash pyrolysis is extremely fast, thus, leading to a reduced time for processing of the feedstock. It occurs in the temperature range of 800–1000°C [77]. The product containing condensable and non-condensable gas is cooled, thus increasing the liquid yield while reducing biochar production. The main product distributions of flash pyrolysis are similar to that of fast pyrolysis. The small particle sizes of biomass feed result in small particles of biochar. The liquid (bio-oil) yield is typically 75–80 wt% and biochar yield is 12–13 wt%. Biochar particles need to be removed because it can catalyze the polymerization of some of the products and increase bio-oil viscosity. Special reactors, such as appropriately designed fluidized bed or entrained flow reactors, are typically required [21].

### *5.3.6 Microwave-assisted pyrolysis*

In conventional heating, heat is transferred to the material surface (by convection, conduction and radiation) and subsequently from the surface to the interior of the material by conduction as a result of temperature gradients. On the other hand, microwave energy is delivered directly into materials within an electromagnetic field. The electromagnetic field enters the material and generates thermal energy throughout the penetration depth by dielectric heating through interaction with polarizable dipoles present in the material and heat the material from

*Recent Advances in Thermochemical Conversion of Biomass DOI: http://dx.doi.org/10.5772/intechopen.100060*

inside. Microwave heating requires a material with a high dielectric constant. The dielectric constant is a measure of the ability of a material to absorb microwave energy. Biomass has a relatively low dielectric constant. As a result, microwave pyrolysis requires catalysts as well as microwave absorbers to improve the heating. The presence of water in biomass may increase the heating rate of microwave pyrolysis due to the high dielectric constant of water in comparison with biomass. Microwave-assisted pyrolysis usually operates in the temperature range of 400–800°C [72, 81].

Some of the advantages of microwave-assisted pyrolysis over conventional pyrolysis include uniform heating throughout, rapid heating rate, cleaner products due to no agitation, volumetric and selective heating. Microwave heating provides ease of operation by instant on/off control and improves the yield and quality of the products. It reduces the formation of hazardous products and minimizes pollutants emission, making the technique environmentally friendly [72, 81]. The other advantages include high heating efficiency as heating is in situ, the ability to handle wet biomass without drying and the ability to pyrolyse large biomass particles. The disadvantage of microwave-assisted pyrolysis is that it requires electricity which is expensive high-quality energy compared to the heat generated by the combustion of pyrolysis gases and vapors in conventional pyrolysis. Microwave pyrolysis generally needed pre-treatment and catalysts before heating [21].

### **5.4 Hydrothermal processing**

Most biomass materials are wet and have moisture contents range up to 95 wt%. Biomass with more than 30 wt% moisture content requires energy costly drying operation prior to pyrolysis, which is one of the leading technical barriers in using wet biomass. For high moisture content biomass, the heat of moisture evaporation is greater than the heat available from the biomass, thus becoming a net energy consumption. Wet biomass, typically with 70 wt% or more water, can be converted using hydrothermal processing, which involves applying heat and pressure to convert biomass in the presence of water into carbonaceous biofuel. In hydrothermal processing, water plays an active role as a solvent and reactant. It uses subcritical or supercritical water to convert biomass into end products in the absence of atmospheric oxygen. Hydrothermal processing is a promising technique to convert wet biomass into carbonaceous solids at relatively high yields by omitting the energy-intensive drying before or during the process. Hydrothermal processing can be classified into three processes: hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG) based on reaction parameters such as temperature, pressure and residence time [21].

### *5.4.1 Hydrothermal carbonization*

When biomass feedstock in water is heated at low temperatures (< 200°C) in a sealed vessel at autogenous pressure, mostly solids (called hydrochar) are formed in a process known as HTC. The decomposition temperature of hemicellulose is usually around 160°C in subcritical water, while cellulose and lignin decompose in the temperature range 180 to 200°C and above 220°C in subcritical water, respectively. The three products of HTC are hydrochar (solid fraction), aqueous solution (bio-oil mixed with water) and a small volume of gas (mainly CO2). HTC aims to maximize the yield of hydrochar. The product distribution and characteristics will mostly depend on three factors: type of biomass, the pH and the maximum temperature used. The reaction time and the solids concentration in biomass water mixture

also has a relatively smaller influence. A minimum HTC temperature of 160°C is needed for the hydrochar formation from glucose. The overall HTC reaction extent is negligible below these processing conditions. The maximum hydrochar yield is obtained at 200°C and then it decreases gradually. A decreasing trend of hydrochar formation with the increase in temperature is due to the higher temperatures favoring gasification reactions. Consequently, part of hydrochar is lost in the formation of volatile compounds. The process energy requirements and final product composition depend on the input biomass feed and the process conditions. The main product of HTC is hydrochar which can be easily separated from the liquid fraction due to its high hydrophobicity and homogeneous properties. The solid fraction can be used to produce dried hydrochar pellets for energy production and mono sugars can be recovered from the liquid fraction. HTC generates less harmful gases such as CO and CO2 and produces hydrochar mass yields varying from 35% up to 80%, with hydrochar carbon content similar to lignite. Residence time varies from minutes up to several hours [21, 70, 82, 83].

### *5.4.2 Hydrothermal liquefaction (HTL)*

At elevated temperatures (between 200 and 350°C) and pressures (5–20 MPa), HTL takes place and the biomass feedstock is mainly converted into a liquid product (aqueous soluble). Free radical and ionic reactions are considered to be the main reactions in HTL [21, 84]. The dielectric constant of water decreases rapidly with increasing temperature. At ambient conditions, the dielectric constant (a measure of hydrogen bonding) of water is about 80 F/m. It drops to about 27 F/m at 250°C and 5 MPa and to about 14 F/m at 350°C and 25 MPa. Water starts to display less polar behavior due to decreasing number of hydrogen bonds. The dissociation of water also increases with the increase of temperature. The ionic product of water (pKw) decreases from 14 at 25°C to 11 at 250°C. The viscosity of water decreases with increasing temperature. Thus, mass transfer is enhanced and any mass-transfer-limited chemical reactions are accelerated [84, 85].

### *5.4.3 Hydrothermal gasification (HTG)*

HTG operates near and/or above the critical point of water at temperatures of 400–600°C and pressures of 23–45 MPa. The biomass is mainly converted into a mixture of non-condensable gases (H2, CO, CH4 and CO2). HTG is capable of producing syngas enriched with H2. The conversion efficiency is highly improved when water reaches the critical point (374°C and 22.1 MPa) [21]. HTG involves the splitting of biomass polysaccharides with supercritical water (SCW). Due to higher reaction temperature, HTG progresses at a faster rate and complete decomposition of biomass is achieved. This is a distinctive feature of HTG in comparison with other hydrothermal treatments (HTC and HTL). The difficulty of byproducts treatment is one of the problems with HTC and HTL. Undesirable byproducts produced by HTC and HTL are occasionally dissolved in the water phase. HTG typically decomposes biomass into gas with a conversion higher than 0.8. The amount of organic compounds in the liquid fraction is low; hence, post-treatment is unnecessary or easily carried out. HTG usually requires wet biomass; other biomasses can also be used. Conventional thermal gasification technologies are available when biomass is not wet. Conventional thermal gasification cannot be effectively employed when the feedstock is wet or has a high moisture content (> 80%). Conventional thermal gasification is achieved by partial oxidation using air. Syngas is partially diluted with nitrogen in addition to tar production. Syngas from HTG does not contain tars; even if produced, they remain in the liquid phase and are not diluted with nitrogen [21, 86].
