**5.2 Gasification**

Biomass gasification is a thermochemical process of converting solid biomass into a gaseous fuel known as synthesis gas or producer gas under a reduced oxygen atmosphere to avoid complete combustion [59, 60]. Gasification aims to maximize the conversion of biomass feed into useable gases. In the gasifier, feed is exposed to a high temperature atmosphere, which heats the biomass leading to thermal decomposition. In contrast to pyrolysis, the feed is brought into contact with a gasifying agent (air). At the gasifier temperature, reactions between oxygen and carbon take place. A mixture of many gases, primarily carbon monoxide and hydrogen, is released as the output product of the gasification process. The gas contains various percentages of CO, H2, CH4, CO2, H2O and N2 depending on the quality of the biomass used and the way gasification is conducted. It also produces liquids (oils, tars, and other condensates) and solids (char, ash) from solid biomass feedstocks [5, 61, 62]. A simple way of representing the gasification reaction is shown below

$$\begin{array}{l}\text{Biomass} + \text{O}\_{2}\text{(g)} \rightarrow \text{CO}\text{(g)} + \text{H}\_{2}\text{(g)} + \text{CO}\_{2}\text{(g)} + \text{CH}\_{4}\text{(g)}\\ + \text{Tar}\,(l) + \text{H}\_{2}\text{O}\,(l) + \text{Char}\,(s) + \text{Trace}\,\text{Specific} \end{array} \tag{1}$$

Gasification processes are designed to generate fuel or synthesis gases as the primary product that can be used in internal and external combustion engines as well as fuel cells, offering a viable solution to overcome energy demands. Currently, such syngas is used as fuel to generate heat and electricity or as a feedstock for many products in the petrochemical and refinery industries, like methanol, ammonia, synthetic gasoline, etc. The overall gasification process is endothermic, requiring energy input for the reactions to proceed, most of which operate between 600°C and 1500°C [59, 63]. The energy needed for this endothermic reaction is obtained by oxidation of part of the biomass through a direct heating (autothermal) or an

indirect heating (allothermal) phase. The main operating parameters of gasification include type and design of gasifier, gasification temperature, flow rates of biomass and oxidizing agents (air or steam), type and amount of catalysts, and biomass type and properties [64]. In addition to the operating conditions of the gasifiers, the properties of biomass such as size, shape, density, chemical composition, energy content and moisture content affect biomass gasification. Gasifier reactors are simple in construction and their designs are generally categorized into the following types: downdraft, updraft, entrained flow, and fluidized bed.

If air is used as the gasifying agent, the producer gas is usually diluted by atmospheric nitrogen. As a result, producer gas has a relatively low calorific value of 4–6 MJ/m3 (normal cubic meter) compared to the calorific value of natural gas of 39 MJ/m3 . Because of its low calorific value, larger volumes of producer gas are required to achieve a given energy output compared to natural gas. In some more applications, oxygen-enriched air, oxygen or even steam may be used as the gasifying agent, resulting in the production of syngas with higher calorific value in the range of 10–15 MJ/m3 due to the absence of diluting nitrogen [16].

### **5.3 Pyrolysis**

Pyrolysis is a thermal decomposition process that takes place in the absence of oxygen to convert biomass into three distinct product fractions: solid residue (biochar), condensable vapors resulting liquid product fraction (bio-oil) and noncondensable gaseous products. Once oxygen is removed, combustion cannot occur; instead, pyrolysis happens. Pyrolysis temperatures are usually between 300 and 700°C, depending on the pyrolysis process employed. Pyrolysis is the most promising technique to convert biomass into biochar and bio-oil. Lower pyrolysis temperatures and longer residence times tend to produce more biochar. High temperatures and longer residence times increase the production of gas. Moderate temperatures and short residence times tend to produce more liquids. Higher pyrolysis temperatures tend to produce a higher proportion of aromatic carbon [65, 66].

Depending on the operating conditions (heating rate, solid residence time and temperature), pyrolysis processes are classified as torrefaction, slow (conventional) pyrolysis, intermediate pyrolysis, fast pyrolysis, flash pyrolysis, and microwave pyrolysis. Various operating conditions are used in these processes; residence time can vary from less than 1 second to hours, heating rate can vary from less than 1°C/s to more than 1000°C/s and temperature ranges from 300 to 700°C or higher. As each type of pyrolysis produces different proportions of the three types of products (biochar, bio-oil and gas), careful selection of the pyrolysis process is essential to obtain the final desired product [21].

The primary conversion of biomass during the pyrolysis process can be described by three pathways; char formation, depolymerization and fragmentation. Char formation is generally favored by intra- and intermolecular rearrangement reactions resulting in higher thermal stability of the residue. This pathway is characterized by the formation of benzene rings and the combination of these rings into an aromatic polycyclic structure. These rearrangement reactions are generally accompanied by the release of water or non-condensable gas (devolatilization). Depolymerization is a dominant reaction during the initial stages of pyrolysis, characterized by the breaking of polymer bonds. This occurs when the temperature is sufficiently greater than the activation energies for the bond dissociation. Depolymerization increases the concentration of free radicals. It is followed by stabilization reactions to produce monomer, dimer and trimer units. These volatile molecules are condensable at ambient conditions and found in the liquid fraction. Fragmentation consists of breaking polymer bonds and even monomer bonds result

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

in the formation of non-condensable gases and a range of organic vapors that are condensable at ambient conditions [55, 58, 67].

The decomposition of cellulose, hemicelluloses and lignin releases a mixture of condensable vapors and non-condensable gases. The condensable vapor contains (apart from water vapor) methanol, acetic acid, acetone (all three mainly from hemicellulose), hydroxyacetaldehyde, anhydrous monosaccharides (both mainly from cellulose), phenols and heavier tars (from lignin decomposition). The heavier, water-insoluble tars contain larger molecular fragments obtained after splitting the ether and C-C bonds in lignin. The resulting complex mixture, once condensed, is referred to as bio-oil. The main parameter that determines the degree of devolatilization of the biomass is the pyrolysis temperature. Yang et al. [68] observed great differences among the pyrolysis behaviors of the three components, cellulose, hemicellulose and lignin. These three forms of polymers are responsible for most of the physical and chemical property modification during the pyrolysis process. The mechanisms of pyrolysis of these polymers are chemically different from biomass species to species. Cellulose and hemicellulose decompose over a narrower temperature range, whereas lignin degrades over a wider temperature range than cellulose and hemicellulose. Pyrolysis of lignin is known to produce more biochar than pyrolysis of cellulose and hemicellulose. Biomass pyrolysis consists of three main stages: (a) initial evaporation of moisture, (b) primary decomposition and (c) secondary reactions (oil cracking and repolymerization). At the initial heating stage, when the biomass temperature is increased to about 100°C, the mass of biomass decreases due to the evaporation of free water. Bound water is then removed in heating the biomass to temperatures up to 160°C. At this stage, the heating value of pyrolysis gases is negligible. Thermal decomposition of biomass begins with devolatilization/ decomposition of extractives at temperatures <220°C. Hemicellulose is the least stable polymer and breaks down first at temperatures of 220 to 315°C with maximum mass loss at 268°C [23, 38, 55, 67, 68].

The reactions are endothermic between 180 and 270°C, sometimes becoming exothermic at temperatures above 280°C. The nature of pyrolytic decomposition reactions explains this phenomenon. Devolatilization and decomposition in pyrolysis is not a single step reaction and a difference can be made between primary and secondary reactions. The gas and vapor products of primary conversion are unstable under pyrolysis temperatures and, with sufficient residence time, can undergo secondary reactions such as cracking and/or repolymerization of primary volatile compounds. Cracking reactions consist of the breaking of volatile compounds into lower MW molecules. Repolymerization involves combining volatile compounds into higher MW molecules, which may not be volatile under pyrolysis temperatures. Repolymerization reactions become effective at later stages of pyrolysis, leading to the formation of char. It also results in the formation of secondary char. Primary char can act as a catalyst to the secondary reactions. Primary reactions are highly endothermic, while secondary reactions are exothermic and result in the production of secondary char and non-condensable gases at the expense of volatiles in the vapor phase. Decomposition of vapors to coke and secondary vapors has been suggested as the reason for exothermicity. The extent of secondary decomposition reactions determines the overall exothermicity of the pyrolysis reaction and the overall char (primary and secondary) yield. The occurrence of primary and secondary reactions in the thermal decomposition of biomass highlights the fundamental difference between fast pyrolysis and slow pyrolysis [38, 55, 67].

Cellulose has a high degree of polymerization and exhibits higher thermal stability. It decomposes in the temperature range 315 to 400°C. The secondary reactions continue to occur within the solid matrix with further increasing of the temperature. At temperatures above 400°C, the less volatile components are gradually driven off from solid char residue resulting in higher fixed carbon content and lower volatile carbon content of the solid char residue. As the temperature increases above 600°C, the condensable vapor components undergo cracking and polymerization reactions, resulting in a lower bio-oil yield. Lignin is the most difficult component to pyrolyse, which decompose in a wide temperature range from 160 to 900°C, the rate of lignin degradation reactions is slower than cellulose and hemicellulose [23, 38, 55, 68].

The combination of low heating rate and longer residence time allowed for repolymerization reactions to maximize biochar yield. A low temperature, high heating rate and short gas residence time would be required to maximize bio-oil yield. A high temperature, low heating rate and long gas residence time would be preferred to maximize the gas yield. As a result of high heating rates and short residence times, fast pyrolysis tends to yield higher proportions of bio-oils. In contrast, slow pyrolysis produces higher proportions of biochars because of slow heating rates and longer residence times. Pyrolysis requires relatively dry feedstock (usually moisture content <30 wt%, but moisture contents of ~10 wt% are preferred) and ground to different particle sizes based on the type of pyrolysis. Feedstock with high moisture content consumes more energy to account for increasing heat of vaporization during the heating of biomass towards the pyrolysis temperature. Additionally, the gases and vapors produced in pyrolysis using a high moisture feedstock are diluted with steam and have a lower calorific value [21, 61].

The molar H/C and O/C ratios of LCB are approximately 1.5 and 0.7, respectively. During pyrolysis, the biomass undergoes devolatilization and the solid portion gets enriched in carbon. The H and O are preferably removed over C and the H/C and O/C ratios tend to decrease as biomass undergo its transformation into biochar. The H/C and O/C ratios are used to assess the degree of aromaticity and maturation. Low-temperature chars have high H/C and O/C ratios, the values close to the original biomass. After pyrolysis, a significant decrease in the H/C and O/C atomic ratios is reported and it decreased straightly with increasing pyrolysis temperature. When the pyrolysis temperature is below 500°C, the reduction in H/C and O/C is mainly attributed to major decomposition reactions of biomass, including dehydration (water removal), decarboxylation (CO2 removal) and decarbonylation (CO removal). Above 500°C, the H/C ratio decreases drastically compared to the O/C ratio, which indicated direct dehydrogenation and demethanation of the chars occurred [38, 69].

### *5.3.1 Torrefaction*

Torrefaction is a pretreatment for upgrading biomass primarily for energy production. Torrefaction, a mild or incomplete form of pyrolysis, involves heating the feedstock to temperatures of 200 to 300°C at slow heating rates (less than 1°C/s) in the absence of air under atmospheric pressure conditions. The residence time depends on the particle size and ranges between several seconds and an hour. Torrefaction removes water as well as superfluous volatiles and partly decomposes the biopolymers (cellulose, hemicelluloses, and lignin) by giving off organic volatiles. It tends to yield higher proportions of solid (torrefied biomass) in addition to liquid and non-condensable gaseous products [21]. Decomposition can be further subdivided into (a) drying, (b) depolymerization and recondensation, (c) limited devolatilization and carbonization, (d) extensive devolatilization and carbonization. Hemicellulose is the least stable of three major polymers, soften between 150 and 200°C and break down via various dehydration, deacetylization and depolymerization reactions at processing temperature range 200–300°C. Predominant hardwood hemicellulose is xylan, while predominant softwood hemicellulose is

glucomannan. Xylan tends to break down more quickly than glucomannan at lower temperatures. Therefore, hardwood has a higher mass loss (higher breakdown of hemicellulose) than softwood when treated at the same temperature. This indicates that different biomass species have different torrefaction kinetics. The cellulose and/or lignin degradation during torrefaction is small. A greater degradation has been reported at temperatures above 270°C. Different biomasses have different physical properties such as porosity, specific heat capacity, thermal conductivity, particle size distribution etc., which results in non-homogenous torrefied biomass. A narrower particle size distribution is required for efficient torrefaction and optimized product quality [23, 70].

As specified by the EBC [71], the molar oxygen to carbon (O/C) ratio of biochar should be less than 0.4. But torrefied biomass tends to have higher oxygen to carbon (O/C) ratio than the ECB specification of biochar. Therefore, torrefied biomass cannot be referred to as biochar. Torrefied biomass has physicochemical properties in between that of raw biomass and biochar. Torrefaction is a pretreatment method that is used primarily for moisture removal and densification of biomass, which will reduce the cost of transportation and increase the heating value of biomass. Torrefaction has higher conversion efficiencies compared to slow pyrolysis. Torrefied biomass yield has a maximum of 35 wt% (on a weight basis of dry, ash-free biomass feedstock). A typical mass yield from torrefaction is 70–80% and energy yield is 80–90%. The lower heating value (LHV) of torrefied biomass is about 20.4 MJ/kg compared to the LHV of charcoal between 28 and 33 MJ/ kg. Torrefaction increases the hydrophobicity, stability, grindability and reduces biodegradability compared to the untreated biomass feedstock. Torrefied biomass can be stored long-term without degradation [21].

The heating value of torrefied biomass on a weight basis increases compared to its original biomass. Its heating value on a volume basis is not necessarily increased as torrefied biomass has relatively low bulk density. Torrefied biomass can be pelletized or briquetted to account for low bulk energy density. Consequently, bulk energy densities between 14.9 and 18.4 GJ/m3 can be achieved.

## *5.3.2 Slow pyrolysis*

Slow pyrolysis is characterized by moderate temperatures (300–550°C), slow heating rates (0.1–0.8°C/s) and longer residence time (5–30 min or even 25–35 h) [56]. Slow pyrolysis aims at maximizing the yield of biochar by promoting secondary reactions, which is achieved by longer vapor residence times. The biochar produced in slow pyrolysis consists of both primary and secondary char. The slow heating rate with moderate pyrolysis temperatures also promotes the production of biochar. Biochar yield and physicochemical properties depend on the feedstock properties and pyrolysis conditions such as processing temperature, heating rate and pyrolysis environment. In addition, moisture content and particle size also significantly affect biochar yield [28, 56, 72].

The biomass composition plays a significant role in the resulting biochar yield and the physicochemical nature of the biochar. Biomass cell wall constituents (cellulose, hemicellulose and lignin) behave differently in terms of decomposition and devolatilization in pyrolysis. As lignin decomposes at lower reaction rates and contains aromatics, it is known to contribute to high biochar yields in slow pyrolysis. Consequently, if high biochar yield is required, then lignin-rich biomass feedstocks are preferable. The extractives in biomass will evaporate and end up in the vapor phase or may be cracked, thereby contributing to char and non-condensable gas formation [38]. High ash contents in the biomass affect the slow pyrolysis process and the physicochemical properties of biochar in multiple ways. Alkaline and earth alkaline metals exhibit catalytic activity in pyrolysis. They catalyze secondary reactions of primary vapor components favoring higher yields of non-condensable gases and biochar or they may catalyze different primary decomposition reactions altogether. Biomass containing more minerals yields less biochar [67, 72–74]. Most of the constituents in ash (mainly the alkaline and heavy metals) are non-volatile within the range of temperatures typically employed in slow pyrolysis processes. Thus, ash remains in the solid biochar product, potentially affecting the use of biochar in downstream processes. If biochar (with high ash content) is used in combustion or gasification processes, slagging and equipment fouling occurs. Chlorine and sulfur lead to higher corrosion. Some downstream applications, such as soil amendment, can benefit from higher ash contents in biochar (nutrient recycling). High ash content biochar can be treated with leaching, which involves soaking of biochar in hot water or hot dilute acid. Washing with pure water at 80°C is sufficient to remove about 90% of the potassium found in biochar, produced at 550°C. The leaching process has been successfully applied to treat potassium and chlorine of biochar. Extra dewatering and drying processes are required after leaching in addition to handling and treatment of leachate, which may contain heavy metals extracted from biochar [38].

The process variables that affect the biochar yield and properties include pyrolysis temperature, heat transfer (to and in the biomass), biomass residence time and operating pressure. The primary biomass constituents (hemicelluloses, cellulose and lignin) undergo decomposition and devolatilization over different temperature ranges. Decomposition of hemicellulose occurs at temperatures below 300°C. The resulting gas and vapor products include non-condensable gases (such as CO, CO2, H2 and CH4), water vapor and low MW oxygenated organic compounds (mainly acetic acid, formic acid, methanol, acetone and furfural). Extensive devolatilization of lignin and cellulose occurs at temperatures higher than 300°C (slow pyrolysis temperatures). Peak devolatilization occurs at temperatures around 500°C, yielding typical vapor products including levoglucosan, hydroxyacetaldehyde, acetic acid and hydroxymethyl furfural (HMF) all originate from cellulose [38, 74, 75].

Phenolic compounds (both in monomeric and oligomeric form) are typically liberated from lignin [76]. Further increase in temperatures causes secondary vapor phase cracking reactions to dominate, yielding additional non-condensable gases and secondary biochar. The higher the pyrolysis temperature, the lower the biochar yield. The additional secondary char formation occurring at higher temperatures is offset by further devolatilization of the primary biochar. The net effect is a decreasing biochar yield with increasing slow pyrolysis temperature. Slow pyrolysis uses low heating rates resulting in biomass conversion being rate limited by heat transfer. Additionally, the slow devolatilization resulting from a slow heating rate ensures maximum secondary char formation. The heat transfer is of less critical importance in slow pyrolysis (compared to fast pyrolysis) as very long biomass residence times are used [38]. Biochar yield increases with increasing residence time, decreasing pyrolysis temperature and lower heating rate. With increasing residence times, vapors are restrained and reacted with solid-phase extensively for more biochar yield [28, 56, 72].

Slow pyrolysis favors biochar production at the expense of bio-oil production. The biochar, bio-oil and gas yields are typically 35 wt%, 30 wt% and 35 wt%, respectively. The overall slow pyrolysis process can generally be exothermic due to the extensive occurrence of secondary reactions. Slow pyrolysis can accept a wide range of particle sizes (5–50 mm). Large biomass particles are frequently used when rapid heating rates are not required or when the desired product is biochar [21, 55].
