9.2.1 Wheat Straw

Wheat (Triticum aestivum L.) is the world’s most widely grown crop, cultivated in over 115 countries under a wide range of environmental conditions. Over the past 100 years, the yields of wheat have been increased and the annual global pro­duction of dry wheat in 2008 was estimated to be over 650 Tg [10]. Assuming residue/crop ratio of 1.3, about 850 Tg of wheat residues are annually produced which include straw as the major waste. The straw produced is left on the field, plowed back into the soil, burnt, or even removed from the land depending on the convenience of the landowner. Disposal of wheat straw by burning is viewed as a serious problem due to the increased concern over the health hazards of smoke generated [93]. Burning of wheat straw also results in production of large amounts of air pollutants including particulate matter, CO, and NO2 [110]. Thus, finding an

alternative way for disposal of surplus wheat straw is of paramount interest and an immediate necessity.

Wheat straw like any other biomass of lignocellulosic nature is a complex mixture of cellulose, hemicellulose, and lignin as three main components (Table 9.4), and a small amount of soluble substrates (also known as extractives) and ash. The overall chemical composition of wheat straws could slightly differ depending on the wheat species, soil, and climate conditions. The cellulose strands are bundled together and tightly packed in such a way that neither water nor enzyme can penetrate through the structure [104, 179]. Hemicellulose serves as a connection between lignin and cellulose fibers, and it is readily hydrolyzed by dilute acid or base, as well as hemicellulase enzyme. Lignin is covalently linked to cellulose and xylan (predominant hemicellulose carbohydrate polymer in wheat straw) such that lignin-cellulose-xylan interactions exert a great influence on the digestibility of lignocellulosic materials [104]. Due to this structural complexity of the lignocellulosic matrix, ethanol production from wheat straw requires at least four major unit operations including pretreatment, hydrolysis, fermentation, and distillation. Unlike sucrose or starch, lignocellulosic biomass such as wheat straw need to be pretreated to make cellulose accessible for efficient enzymatic depolymerization.

9.6.2.1 Conventional Batch Fermentation

Batch cultures are simple, closed systems. In this system, all the substrates are added at the beginning, before inoculation, and neither anything is added or taken out during the fermentation. A typical growth curve is followed by the organism (Fig. 9.12a) in this type of fermentation. In industrial processes, generally, the actively growing inoculum is added to avoid any lag phase as it leads to the wastage of time (Fig. 9.12b) The batch fermentation has certain limitations like exhausting of nutrients, accumulation of antagonists, product inhibition, etc. which eventually affects the product formation.

The product is recovered at the end of the growth phase. This involves emptying the fermenter out and processing the medium to get the product out. The fermenter has to be cleaned, refilled, resterilized, and then, reinoculated. Such operations are called turn-round and the time it takes to do it is called down time.

Figure 9.13 depicts the batch fermentation equipment layout incorporating heat exchangers and chemical sterilization systems. Most of the currently practiced alcohol fermentations are based on the traditional processes described above. But many advanced methods have been developed in order to increase the produc­tivity, reduce the capital investment, and better utilization of energy. Such advances are the use of continuous fermentations, the increase of yeast population by recycling, and the removal of ethanol during fermentations.

Shared heat exchanger

cooling bank

Fig. 9.13 Batch fermentation equipment layout incorporating teamed heat exchanger and chemical sterilization systems. Source [52]

9.6.2.2 Continuous Fermentation

In continuous fermentation, fresh medium is continuously pumped into the fer­menter and an equal volume of the fermented liquid is continuously pumped out for recovery of ethanol and yeast. This is an open system. The rate at which medium is added or at which the fermenter liquid is withdrawn is expressed as the dilution rate D which is the ratio of withdrawn liquid (F) to the volume of total liquid in the fermenter (V) i. e. D = F/V (Units of D are h-1).

Feed is pumped continuously into the fermenter displacing beer which then overflows from the vessel. The composition of the produced beer is the same as the composition inside the fermenter. Therefore, the fermenter is to run at a relatively slow rate to obtain a higher concentration of alcohol because it will allow complete utilization of sugar and growth of new yeast cells in the fermenter to replace

washed out cells [139]. Stirring is an important factor for successful continuous flow fermentation. The modification of the continuous fermentation process is the Biostil process (Fig. 9.14).

Souring prices of petroleum, concern over secured supply beside climate change are major drivers in the search for alternative renewable energy sources. The use of biomass to produce energy is an alternative source of renewable energy that can be utilized to reduce the adverse impact of energy production on the global environment.

Current biomass resources comprise primarily industrial waste materials such as sawdust or pulp process wastes, hog fuel, forest residues, clean wood waste from landfills, and agricultural prunings and residues from plants such as ligno — cellulosic materials. The increased use of biomass fuels would diversify the nation’s fuel supply while reducing net CO2 production (because CO2 is with­drawn from the atmosphere during plant growth) and reduce the amount of waste material that eventually ends up in landfills. It is important that biomass uses have a high process efficiency to increase the overall resource productivity from past commercial applications. Biomass is considered carbon neutral because the amount of carbon it can release is equivalent to the amount it absorbed during its lifetime. There is no net increase of carbon to the environment in the long term when combusting the lignocellulosic materials. Therefore, biomass is expected to have a significant contribution to the world energy and environment demand in the foreseeable future.

This new book entitled ‘‘Biomass Conversion: The Interface of Biotechnology, Chemistry and Materials Science’’ assembles 14 chapters authored by renowned specialists. This book provides an important review of the main issues and tech­nologies that are essential to the future success of the production of biofuels, bioenergy, and fine-chemicals from biomass, and the editors and authors are to be applauded for constructing this high quality collection. The scientific and engi­neering breakthroughs contained in this book are the essential building blocks that construct the foundation and future development of biomass conversion with interface of biotechnology, bioengineering, chemistry, and materials science.

This book therefore reviews the state of the art of biomass conversion, along with their advantages and drawbacks. By disseminating this information more widely, this book can help bring about a surge in investment in the use of these technologies and thus enable developing countries to exploit their biomass resources better and help close the gap between their energy needs and their energy supply.

I am delighted that the editors, Dr. Baskar, Dr. Shikha, and Dr. Dhillon, took their strong involvement in this enterprise, and the authors, whose liberally con­tributed expertise made it possible and will guarantee success.

March 2012 Prof. D. S. Chauhan

Vice Chancellor Uttarakhand Technical University Dehradun, Uttarakhand India

Though the second-generation biofuels overcome the disadvantages of using edible food crops as feedstock, the cultivation of non-edible crops—which serves as the feedstock for the second-generation biofuels—still require land and other resources which could otherwise be used for cultivation of food crops. Hence, the

Processes common to both current and future waste biorefinery

third- generation of biofuels offers an excellent alternative to the first — and second — generation biofuels in that they do not use arable land for their generation but use algae and seaweeds, which can be cultivated on completely nonproductive land and use significantly less water than terrestrial crops. Sea weeds/algae thrive on sea water using merely sunlight and some simple nutrients present in the sea water. About 75% of the earth’s surface is covered by water, and seawater comprises about 97% of total water present on the earth. Hence, there is an immense potential for cultivation of algae/seaweeds. Algae as feedstock for production of biofuels include all unicellular and simple multicellular organisms such as prokaryotic microalgae (e. g. cyanobacteria), eukaryotic microalgae (e. g., green and red algae), and diatoms. Millions of years of evolution have enabled algae to develop an efficient system which is capable of capturing unlimited amounts of solar energy continuously via photosynthesis and converting simple inorganic molecules to complex organic compounds such as carbohydrates, fats, and proteins. The pho­tosynthetic efficiencies of algae are much higher than most terrestrial plants, hence algae can absorb higher amounts of CO2 from atmosphere and as a result, provide higher amounts of these complex molecules, which can be converted to biofuels (bioethanol) and other molecules. Figure 1.28 shows the variety of compounds that can be obtained from algae.

Singh et al. [51] have comprehensively reviewed all the aspects of using algae as a potential feedstock for the generation of third — generation biofuels. Other than the advantages of higher photosynthetic efficiencies and nonrequirement of arable land, use of algae as feedstock for biorefineries offer many other advantages

compared to plant biomass feedstock. Algae have an almost exponential growth potential-doubling of biomass in as short a time as 3.5 h can be possible. In addition, more than five harvests can be obtained in a year [52]. Another major advantage of algae is that they thrive on nutrients such as nitrogen and phos­phorous, which can be obtained from wastewater, and on organic effluent from agro-food industry, thus serving a dual advantage of utilization of waste water and enhanced cultivation of algal feedstock. Algae also do not require fertilizers, herbicides, and pesticides like their plant counterparts for their sustained cultivation.

Microalgae are rich in carbohydrates. These can be fermented to produce bioethanol. Chung Sheng et al. [52] have reviewed the potential of such a biore­finery in playing the role of a sustainable energy provider for efficient production of bioethanol. A number of flow charts have been proposed for efficient production of biofuel from algae.

The common steps in all processes involve collection of algae, extraction, purification, and separation of polysaccharides, hydrolysis, fermentation, and final purification (Fig. 1.29).

In order to successfully compete with a petroleum refinery, the efficiency of biofuel production of an algal biorefinery can be enhanced significantly by developing good strains of algae with increased carbohydrate content which will give a high yield on fermentation. Simultaneous production of biogas using methane fermentation technique is also possible. The residual biomass can be reprocessed to make fertilizers.

Algae also contain a lipid fraction which can be used for production of bio­diesel. Microalgal lipids are neutral lipids having a lower degree of unsaturation (similar to fossil fuels). By integrating processes such as transesterification, cracking, etc., into an algal biorefinery, a range of products other than bioethanol, which is normally obtained by hydrolysis and fermentation of carbohydrates, can be obtained [51]. Such an integrated algal biorefinery is shown in Fig. 1.28.

In addition to the biofuels viz. bioethanol, biodiesel, and biogas, other products such as food supplements, pigments, etc. can also be obtained from an algal biorefinery. There are a number of algae-based biorefineries in different regions of the world, producing the products stated above. Mussgnug et al. [53] have investigated six species of freshwater and saltwater algae and cyanobacteria for their suitability as substrates for production of biogas. They showed that the methane content of biogas from microalgae was 7-13% higher compared to that obtained out of maize fermentation. They also reported that drying as pretreatment step decreased the amount of biogas production to approximately 80%. Chla — mydomonas reinhardtii has the ability to produce hydrogen via hydrolysis of water during illumination. The hydrogen production cycle induces an increase in the amount of starch and lipids within the cells which increase the fermentative potential of the algal biomass. Thus, a two-step biorefinery process where hydrogen is produced in the first step by sulfur deprivation method, and subse­quently, the remaining biomass, after production of hydrogen is used as substrate for anaerobic fermentation was found to increase the biogas production to 123%, compared to the use of fresh algal biomass. This synergistic effect gives a dual advantage of providing an environment friendly gaseous fuel hydrogen, and an increased amount of biogas.

The most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. Digesters typically can accept any biodegradable material; however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application. The more putrescible the material the higher the gas yields possible from the system. Substrate com­position is a major factor in determining the methane yield and methane pro­duction rates from the digestion of biomass. Techniques are available to determine the compositional characteristics of the feedstock, while parameters such as solids, elemental and organic analyses are important for digester design and operation.

Anaerobes can break down material to varying degrees of success from readily in the case of short chain hydrocarbons such as sugars, to over longer periods of time in the case of cellulose and hemicellulose. Anaerobic microorganisms are unable to break down long chain woody molecules such as lignin. Anaerobic digesters were originally designed for operation using sewage sludge and manures. Sewage and manure are not, however, the material with the most potential for anaerobic digestion as the biodegradable material has already had much of the energy content taken out by the animal that produced it. Therefore, many digesters operate with co-digestion of two or more types of feedstock. For example, in a farm-based digester that uses dairy manure as the primary feedstock the gas production may be significantly increased by adding a second feedstock; e. g. grass and corn (typical on-site feedstock), or various organic by-products, such as slaughterhouse waste, fats oils and grease from restaurants, organic household waste, etc. (typical off-site feedstock).

A second consideration related to the feedstock is moisture content. Dryer, stackable substrates, such as food and yard wastes, are suitable for digestion in tunnel-like chambers. Tunnel style systems typically have near-zero wastewater discharge as well so this style system has advantages where the discharge of digester liquids are a liability. The wetter the material the more suitable it will be for handling with standard pumps instead of energy intensive concrete pumps and physical means of movement. Also the wetter the material, the more volume and area it takes up relative to the levels of gas that are produced. The moisture content of the target feedstock will also affect what type of system is applied to its treatment. In order to use a high solids anaerobic digester for dilute feedstocks, bulking agents such as compost should be applied to increase the solid content of the input material. Another key consideration is the carbon:nitrogen ratio of the input material. This ratio is the balance of food a microbe requires in order to grow. The optimal C:N ratio for the ‘food’ of a microbe is 20-30:1. Excess N can lead to ammonia inhibition of digestion.

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters has significant levels of physical contaminants such as plastic, glass or metals, then pre-processing will be required in order for the material to be used. If it is not removed then the digesters can be blocked and will not function efficiently. It is with this that mechanical biological treatment plants are designed. The higher the level of pre-treatment a feedstock requires, the more processing machinery will be required and hence the project will have higher capital costs.

Cellulose can be regenerated from the IL/biomass solution with an anti-solvent, such as acetone, water, dichloromethane, or acetonitrile, in excess [4, 7, 36].

Lignin and the IL can be washed away with NaOH [48]. Lignin can be precipitated with HCl [48] or H2SO4 [27]. Cellulose and lignin can also be extracted separately: cellulose was precipitated with ethanol and lignin with water after dissolution of wheat straw and pine wood with [EMIM][Cl] [47]. After regeneration, cellulose usually becomes amorphous or changes into its cellulose II structure [7, 36].

Changes in crystallinity have been characterized in purified cellulose substrates and native biomass by X-ray diffraction [8, 10, 25, 38, 71, 91]. The intensity of the (002), (110), and (1-10) reflections from cellulose usually decrease in intensity after the IL pretreatment and regeneration, indicating a loss in crystallinity [2, 14, 17, 67, 78, 149]. This was the case for cellulose in maple flour dissolved in [BMIM][OAc] and [EMIM][OAc] [38], and in spruce sawdust dissolved in [AMIM][Cl] [7]. The lower crystallinity led to improved access for hydrolytic enzymes and an improved glucose yield after hydrolysis [7].

However, it is possible for cellulose to retain its native cellulose I structure. Upon application of [EMIM][OAc] on poplar microtome section, the (002), (110), and (1-10) reflections of cellulose I disappeared and the diffraction pattern was dominated by a diffuse ring from the [EMIM][OAc]. When the pretreated sample was exposed to water, the IL was expelled from the wood and the diffraction pattern of cellulose was gradually recovered (Fig. 4.3). This recovery of cellulose I is in contrast to studies on biomass dissolution at high temperatures, where the regenerated cellulose is either amorphous or in the cellulose II phase. This is explained by the partial solubilization of cellulose microfibrils. Those microfibrils that retained their cellulose I structure acted as nucleation sites for cellulose I recrystallization after expulsion of [EMIM][OAc] [71].

The transformation of unsaturated fatty acids was studied at temperature of 300°C in hydrogen or in argon atmosphere [33, 36]. The results indicate that with increasing unsaturation of the feedstock there is a decrease in deoxygenation activity of Pd/C catalyst. As mentioned before (see Sect. 6.2.2.3), deactivation of the catalyst can occur due to formation of aromatic compounds. In deoxygenation experiments with stearic and oleic acid, the initial deoxygenation rates are the same, but for oleic acid reaction there can be visible deactivation of the catalyst with time [36]. Rate of linoleic acid deoxygenation is much lower compared to stearic and oleic acids which indicate that extensive deactivation occurs from the beginning of the reaction (Fig. 6.6).

The deoxygenation of tall oil fatty acids was performed over 1 wt% Pd/C catalyst the temperature range between 300 and 360°C and with different hydrogen content in the reaction atmosphere [29]. Tall oil fatty acids are mixture of free fatty acids derived from wood biomass. The main components are linoleic and oleic acid, whose amounts are varying depending on the origin of crude tall oil used in the distillation. An increase in deoxygenation was achieved with increase of temperature, but at the same time selectivity decreased due to increase of aromatic compounds production, which could be inhibited by increase of hydrogen content in reaction atmosphere, as mentioned before (see Sect. 6.2.2.3).

The deoxygenation of fatty acid esters over Pd/C catalyst was studied in the semibatch reactor [31, 32]. For the transformation of ethyl ester over Pd/C catalyst, the yield of hydrocarbons was lower compared to that achieved with stearic acid [31]. The main product was stearic acid which is an intermediate in the reaction. A higher conversion can be achieved with increase of hydrogen content in the reaction atmosphere than under inert atmosphere [32]. Ethyl stearate transforma­tion was also successfully demonstrated in the fixed bed reactor over Pd/C catalyst [37].

The triglycerides were deoxygenated over 5 wt% Pd/C catalyst at 360°C and

1.2 MPa pressure of 5% hydrogen in argon [7]. The results indicated that a total conversion of tristearine was achieved with hydrocarbon fraction of 64 wt% of the reaction mixture. The n-heptadecane was the main product in the mixture of C17 hydrocarbons isomers.

The renewable diesel, because of its composition, has worse low temperature properties compared to conventional diesel. One of the ways to improve low temperature properties of long-chain hydrocarbons in renewable diesel is skeleton isomerization. Therefore, Pd/SAPO-31 catalyst was studied in the one-pot deox­ygenation and skeleton isomerization of sunflower oil at the temperature range of 310-360°C and 2 MPa of hydrogen [38]. Deoxygenation over Pd/SAPO-31 cat­alyst shows good selectivity to branched hydrocarbons. The ratio between bran­ched and linear hydrocarbons is the highest at temperature between 320 and 350°C. Isomerization of hydrocarbons can be increased as well by the increase of the residence time in the fixed bed reactor. Despite good isomerization properties, Pd/SAPO-31 catalyst deactivates in time on stream showing extensive decrease in selectivity toward branched hydrocarbons after 14 h with the optimal isomeriza­tion conditions (T = 340°C, WHSV = 0.9/h). It is worth to mention that with increase of temperature and residence time, the selectivity toward long (C17 and C18)-chain hydrocarbons decreases from 91 (T = 310°C, WHSV = 0.9/h) to
28 wt% (T = 360°C, WHSV = 0.9/h), which is caused by extensive cracking at higher temperatures.

The different approach for deoxygenation of triglycerides, fatty acids and their esters was shown in their transformation over a PtSnK/SiO2 catalyst [39]. The idea of the process was to obtain olefins and paraffins, which could be used as a diesel fuel or could serve as substitute for petrochemical feedstocks for specialty chemicals. To avoid side reactions of unsaturated products (aromatization, olig­omerization) and to increase selectivity toward olefins, the reactive distillation process was used. It was observed that PtSnK/SiO2 catalyst has high selectivity toward olefins, which also increased with a decrease of product residence time inside the reactor.

Among hexoses, glucose is the immediate metabolizing sugar that can be fer­mented through different pathways such as glycolysis. The orientation of the — H and -OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. When the — OH group on this carbon is on the right side, the sugar is the D — isomer; when it is on the left, it is the L-isomer. Most of the monosaccharides occurring in mammals are D sugars (Fig. 9.1), and the enzymes responsible for their metabolism are specific for this configuration. In solution, glucose is dex­trorotatory—hence the alternative name dextrose, often used in clinical practice. Other important hexoses like galactose and mannose are first either converted into

glucose before fermentation or their products after initial metabolism join the glycolytic sequence. Figure 9.2 shows the pathway of glucose degradation.

9.4.1.1 Sucrose

This disaccharide is most commonly used as the carbon and energy source by fermentative microorganisms. It is a non-reducing sugar consisting of one mole­cule each of D-glucose and D-fructose linked through a-1, b-2 glycosidic bond (Fig. 9.3). In the fermentation process, sucrose is first hydrolyzed by invertase (sucrase) to D-glucose and D-fructose. D-glucose directly enters the glycolysis while fructose joins the main stream after phosphorylation with ATP in a hexo — kinase-catalyzed reaction. Sucrose can also be fermented through its initial breakdown by sucrose phosphorylase (Fig. 9.4).

This is the most common design used in biomass combustion systems. The bio­mass fuel is fed at the upper part of the furnace where pre-drying of the biomass takes place. The dried biomass slowly tumbles down over the sloping grate onto a reciprocating grate in the lower portion of the furnace, where combustion takes place. The grate is either water cooled or air cooled which obviates the require­ment of an insulating ash layer in order to protect it from abrasion. Thus, this type of a combustion design is suitable for biomass with a lower ash content.

Fluidized-bed combustion

In this type of system, finely comminuted biomass particles are fed onto a bed of coarse sand particles present at the bottom of the furnace. Fluidizing air is passed through this bed in an upward direction through uniformly distributed perforations on which this bed rests. The velocity of this air is critically controlled such that it is just sufficient to fluidize the fuel particles in the air above the bed. The bed appears like a bubbling liquid at this air velocity. The coarse sand particles assist the mixing of the fuel with the air and also increase the heat transfer to the fuel for initial drying and subsequent ignition. Figure 1.8 shows a schematic diagram of a typical fluidized — bed combustion system designed for a boiler. The critical air velocity or the minimum fluidization velocity at which fluidization occurs is a function of the biomass particle size, density and pressure drop across the bed. An increase in air velocity beyond the minimum fluidization velocity causes the bed to become turbulent, and subsequently to circulating. This results in increased recycling rates of the material in suspension. Commercial designs are either bubbling fluidized-bed or (BFB) or, circulating fluidized-bed (CFB). The entire system may operate at atmospheric pressure or may be pressurized. Air or oxygen may be used for fluidization.

BFB system uses air velocities of 1-3 m/s. The primary air supply is through nozzles beneath the bed, whereas the secondary air flow enters the furnace above the bed. The ratio of the primary to secondary air supply controls the bed tem­perature. The bed temperature can also be controlled by recirculating some of the flue gases that are formed as a result of combustion of the biomass.

In the CFB systems, a higher air velocity of 4-9 m/s is employed. This causes the bed material to circulate within the furnace. As in BFB, here also, there are

primary and secondary air supplies. Due to the higher air velocities used, the smaller biomass particles tend to get entrained along with the flue gases generated as a result of combustion. Cyclone separators are provided to collect the biomass and sand particles which are then returned to the feed bed. CFB designs are more expensive than the BFB ones. However, CFBs operate at lower operating tem­peratures than the BFBs, which reduces the NOx emissions significantly.

Fluidized-bed combustion systems are much more versatile compared to fixed — bed design systems. A wide range of biomass with varying compositions such as higher moisture contents and varying ash properties can be handled without encountering slagging problems. Varying loads, ranging from full capacity to as low as 35% of full capacity can be handled. At any given time, compared to the fixed-bed design, only a small quantity of fuel is present in the combustion chamber, hence, giving good conversion efficiencies. However, the fluidized-bed designs are costly compared to the fixed-bed designs and are suitable for large — scale operations only.

Pyrolysis

Pyrolysis is the thermochemical decomposition of organic material at high temper­atures in the absence of oxygen, producing gas and liquid products and leaving behind a carbon-rich residue. It is invariably the first step in combustion and gasi­fication of biomass. If sufficient oxygen is provided subsequent to initial pyrolysis, it can proceed to combustion or gasification. The liquid products obtained from pyrolysis include water and oils, whereas the gaseous products include carbon monoxide, carbon dioxide, and methane. A solid residue that is left behind is a carbonaceous solid, i. e., charcoal. The solid residue can be used as such for heating. The gas produced can be processed through a gas burner and under a restricted air supply can be used as a heat source for the pyrolyzer, or it can be used in gas turbines or gas boilers for production of electricity. The liquid product, bio-oil can have multiple uses: it can be used as such for heating, or for power generation, or it can be upgraded to transportation fuel, or can be used for conversion into suitable chemicals. Figure 1.9 shows the different energy products/forms that can be obtained from pyrolysis. Figure 1.10 shows a general schematic diagram of a pyrolysis process.

The fact that one of the products of pyrolysis is a liquid product (viz. bio-oil) makes this process very important because liquid fuels are easy to transport and hence, it is possible to have the conversion plant remote from the point of use, which is not possible in case of the combustion process. Pyrolysis is not an exothermic process like combustion. It is an endothermic process where heat is required to be supplied for the process. Different types of pyrolysis processes, resulting in different types of products, are possible depending on the temperature and the rate of heating employed. The nature of the biomass also largely affects the

Fig. 1.10 Schematic diagram of a pyrolysis process (Adapted from [11]) yield of pyrolysis, as the rate of heating depends on the nature of biomass. Typically, lignocellulosic materials such as wood, stalks, straw, etc. are poor heat conductors. Hence these materials require pretreatment such as size reduction before they can be used for pyrolysis, so that an acceptable yield can be obtained. Lower processing temperatures and longer vapor residence times are favorable for production of charcoal (solid product); higher processing temperatures and longer vapor residence times favor production of gas, whereas under moderate temper­atures and short vapor residence times, a liquid product is obtained. The product dependence on the processing conditions and vapor residence times can be explained on the basis of the composition of the biomass and the chemical nature of pyrolysis [10]. Biomass mainly comprises polymers in which large chains of carbon atoms are linked with each other, or to oxygen atoms, or sometimes to other elements like nitrogen or sulfur, to form macromolecules. The most com­monly occurring macromolecules in biomass are hemicelluloses and cellulose.

Unprocessed biomass consists of a small number of such large polymers or macromolecules. Cellulose is a linear chain polymer, whereas hemicellulose is a branched chain polymer with side chains or branches present at random locations along the chain. As heat is supplied, the chemical bonds linking the monomer units in the large polymer begin to break off. In cellulose, the bonds are broken ran­domly along the chain whereas in hemicelluloses, first the side chain or branches break off followed by breaking of the straight chains. As more heat is supplied, a large number of smaller molecules are generated i. e. the degree of polymerization (Dp) reduces. When Dp reduces to <10, the polymer is no longer a polymer but an oligomer. These oligomers (especially those having Dp less than around 8), are volatile. These are generated at typical pyrolysis temperatures between 400 and 800°C. These oligomers, comprising anhydro sugars, evaporate from the solid mass as volatiles. These are required to be removed from the solid biomass. If they are not removed, under continued influence of high temperature, they undergo thermal fragmentation to produce highly reactive, small intermediates. These fragments, if removed and quenched immediately, can be used as such as chem­icals or as fuels. If, again, these are not removed from the solid biomass, they undergo chemical reaction with the remaining solid material to form new polymers or, accelerate breakdown of original chains. These reactions are exothermic and thus accelerate the overall pyrolysis reaction. Thus, depending on the processing conditions and vapor residence times, different types of pyrolysis processes have been developed, which result in a different product mix. These are summarized in Table 1.4.

A brief description of the different variants of the pyrolysis process is given below.

A modern practice which has allowed biomass feedstocks an early and cheap entry point into the energy market is the practice of co-firing a fossil fuel (usually coal) with a biomass feedstock. It refers to the blending of biomass with coal in the furnace of a conventional coal-fired steam cycle electric power plant. This is currently one of the simplest ways of utilizing biomass to displace fossil fuels, requiring no new investment or specialized technology. Between 5 and 15% biomass (by heat content) may be used in such facilities at an additional cost estimated at <0.5 cents/kWh (compared with coal-firing alone). Co-firing is known to reduce carbon dioxide emissions, sulfur dioxide (SOx) emissions, and potentially some emissions of nitrogen oxides (NOx) as well. Many electric util­ities around the US have experimented successfully with co-firing, using wood chips, urban waste wood and forestry residues.

Co-firing has a number of advantages, especially where electricity production is an output. First, where the conversion facility is situated near an agro-industrial or forestry product processing plant, large quantities of low-cost biomass residues are available. These residues can represent a low-cost fuel feedstock although there may be other opportunity costs. Second, it is now widely accepted that fossil-fuel power plants are usually highly polluting in terms of sulfur, CO2 and other GHGs. Using the existing equipment, perhaps with some modifications, and co-firing with biomass may represent a cost-effective means for meeting more stringent emis­sions targets. Biomass fuel’s low sulfur and nitrogen (relative to coal) content and nearly zero net CO2 emission levels allows biomass to offset the higher sulfur and carbon contents of the fossil fuel. Third, if an agro-industrial or forestry processing plant wishes to make more efficient use of the residues generated by co-producing electricity, but has a highly seasonal component to its operating schedule, co-firing with a fossil fuel may allow the economic generation of electricity all the year round.

Agro-industrial processors such as the sugarcane sugar industry can produce large amounts of electricity during the harvesting and processing season; however, during the off-season the plant will remain idle. This has two drawbacks, first, it is an inefficient use of equipment which has a limited lifetime, and second, electrical distribution utilities will not pay the full premium for electrical supplies which cannot be relied on for year-round production. In other words the distribution utility needs to guarantee year-round supply and may therefore have to invest in its own production capacity to cover the off-season gap in supply with associated costs in equipment and fuel. If however, the agro-processor can guarantee electrical supply year-round through the burning of alternative fuel supplies, then it will make efficient use of its equipment and will receive premium payments for its electricity by the distribution facility.