Large amounts of whey produced is posing a serious problem all over the world for its proper utilization. Only a few countries have succeeded in utilizing their total whey production [147]. Whey is rich in lactose, a dimer of glucose and galactose unit, and can be fermented only by a selected number of yeasts. Because glucose and galactose are readily fermentable sugars, it is suggested that b-galactosidase treated whey could make a better substrate for industrial fermentation than untreated whey.

9.2.3 Spent Sulfite Liquor

The sulfite process involving the delignification of wood with acid bisulfite is widely used by mills in Europe and North America. While the lignin part is solubilized by combining with HSO3, the wood cellulose largely remains unde­graded. The hemicelluloses are hydrolyzed into monosaccharides. Spent sulfite liquor (SSL), which consists of lignin sulfonates, hexoses and pentoses, polysaccharides, galacturonic and acetic acid, some resins and unconsumed bisulfite, and ash, creates a major pollution problem when discharged into receiving water. Being the source of different types of carbohydrates, it has the potential for conversion into ethanol.

9.2.4 Bioethanol from Algae

The production of motor fuel from algae has been subjected to research for dec­ades. Now, there is an opportunity to produce bioethanol simultaneous to the third — generation biofuel—algae diesel (Oligae). Carbohydrates in algae oil can still be converted into starch that can be used for ethanol production after hydrolysis into simple sugars.

Recently, ethanol production potential of waste bananas has been assessed [63]. Ethanol yield from normal banana was found to be as: ripe whole fruits 0.091, pulp 0.082, and peel 0.0061/kg of whole fruits. The green fruit gave 0.090, normal ripe 0.082, and overripe 0.0691/kg of ethanol. Enzymatic hydrolysis was necessary for higher ethanol yield while dilution with water was not essential for effective fermentation.

9.7.2 Potato Waste

The use of potato peel waste for the production of alcohol has also been made [17]. The acidified peel waste (pH 6) is used for ethanol production.

Biomass can be used to generate different forms of energy (Fig. 1.3). It can be either burnt directly to generate heat, or the flue gases generated during the burning of biomass can be used to provide process heat. The heat generated from biomass can be used to generate steam which can again be used either directly to provide process heat or it can be converted into electricity via steam turbines. As such, biomass is very low in terms of energy density. It can be upgraded into high energy density fuels such as charcoal, liquid fuels (mainly transportation fuels), and gaseous fuels such as hydrogen, producer gas, or biogas. These biofuels form the major, most important product of the bioconversion processes.

Biofuels are classified into four categories depending on the nature of biomass used to produce it. Table 1.1 gives a concise classification of biofuels with rep­resentative examples for each category.

First-generation biofuels are already commercially produced, and an estab­lished technology is available for their production. However, the major problem with first-generation biofuels is that their production largely depends on raw material feedstock that could otherwise be used for food and feed purposes. This food versus fuel controversy gave rise to the development of second-generation biofuels which are produced from non-feed crops, forest residues, agricultural, industrial, and domestic waste. Second-generation biofuels are produced mainly by thermochemical and biochemical methods. The thermochemical methods are more amenable to commercialization as these are based on technologies estab­lished over a number of years. The biochemical methods have not yet been commercialized but these methods have a greater potential for cost reduction. Research efforts toward their optimization are currently ongoing and may soon result in commercialized, low cost alternatives to first-generation biofuels.

Although second-generation biofuels are able to circumvent the food versus fuel controversy, they still need arable land for the generation of feedstock required for their production. Thus land which would otherwise have been used for growing of food crops would still be required. This gave rise to third-

generation biofuels such as biofuels produced from seaweeds and algae. This algal biomass is capable of flourishing in marshy land, sea water, and land which is totally unproductive with respect to cultivation of agricultural crops. Concerted efforts are underway to bring out successful technologies which produce biofuels from algae.

Fourth-generation biofuels are still at a conceptual stage and many more years may be required for these types of biofuels to become a reality. These biofuels are produced by technologies which are able to successfully convert biomass into fuel in such a manner that the CO2 consumed in their generation is much more than that produced as a result of their burning or use. Hence, these biofuels would be instrumental in reducing atmospheric GHGs, thus mitigating the problem of global warming to a significant extent. The technologies for the production of fuels other than first-generation biofuels are yet to prove them­selves as commercially viable alternatives to fossil fuels and are under various stages of development. The following section gives an overview of the different biomass conversion technologies developed till date. These are broadly classified as shown in Fig. 1.4.

An important aspect about the use of biomass as an alternative to fossil fuel for generation of energy is that biomass has a high volatility compared to fossil fuels due to the high levels of volatile constituents present in biomass. This reduces the ignition temperature of biomass compared to that of fossil fuel such as coal. However, biomass contains much less carbon and more oxygen. The presence of oxygen reduces the heat content of the molecules and gives them high polarity.

PROCESSES FOR
CONVERSION
OF BIOMASS INTO
ENERGY

Fig. 1.4 Processes for biomass conversion into energy

Reproduced with permission from [1] a wt% of dry fuel b wt% of dry ash

Hence, the energy efficiency of biomass is lower than that of coal and the higher polarity of the biofuel which is obtained from biomass causes blending with fossil fuel difficult. Table 1.2 gives a comparison between the physicochemical and fuel properties of biomass and coal.

It can be seen from Table 1.2 that the properties of biomass and fossil fuel vary significantly. Although biomass has a lower heating value, the emission problems especially, emission of CO2, NOx, SOx for biomass are much less than those for coal due to the lower carbon, sulfur, and nitrogen contents of biomass.

The technology for conversion of biomass to first generation biofuels is well established and also commercialized. The technologies for second-, third — and fourth — generation biofuels are still at research stage. Hence, the production of second-, third-, and fourth-generation biofuels is presently costlier than the first — generation biofuels. In general, the overall cost of production decreases as the scale of the production unit increases. As the newer biomass conversion tech­nologies reach the stage of maturity required for large-scale production, the costs of production of these second-, third-, and fourth-generation biofuels is likely to become comparable to the first-generation biofuels. The current focus of research is therefore aimed at economizing the production technologies by way of reducing various costs, integrating various technologies on the basis of pinch analysis, increasing the scale of production and diversifying the product range to include value-added products wherever possible. Techno-economic analysis of the dif­ferent individual biomass conversion processes has been carried out. Comparative studies of the different biomass conversion technologies have also been done. Points of cost reduction can be identified and the scope of process integration can studied for the production of biofuels. As there are no commercial-scale produc­tion units for second-generation onwards biofuels, in most cases, the production costs are estimated on the basis of models developed using different production technologies. The entire life cycle right from generation of the biomass to its collection and transportation to the biorefinery/power plant to waste disposal subsequent to the generation of energy is considered for the economic assessment of the biomass conversion process.

Dwivedi et al. [59] have reviewed the economics of ethanol production from cellulose using different conversion technologies. The conversion technology used has a greater impact on the cost of production compared to the type of feedstock used hence, such a study is expected to bring the cost of ethanol production from cellulose feedstock comparable to that from starch-based feedstock. In other words, proper selection and integration of conversion technology is likely to bring the production of second-generation bioethanol comparable in cost to the first- generation bioethanol. The economics of several hydrolysis-based conversion

technologies show that the cost is highest for concentrated acid hydrolysis, neu­tralization, and fermentation technology and lowest for simultaneous saccharifi­cation and fermentation technology (Table 1.14).

Thermoeconomic modeling is carried out to evaluate the various available technologies for a process and select the most suitable one from among them, and to establish optimum operating conditions for the process after identifying critical parameters which will affect the economy of the selected process. This will enable one to assess the competitiveness of different processes and select that or those processes which are likely to offer the greatest economic advantage, energy pro­duction and are at the same time environment friendly and sustainable. Tock et al. [60] have carried out thermo-economic modeling for thermochemical production of liquid fuels (FT fuels, methanol, and dimethyl ether) from biomass with respect to process description and process integration. A thermodynamic model has been developed and used to calculate liquid-vapor and chemical equilibrium; an energy model has been developed to minimize the energy consumption taking place in a process, by carrying out thermodynamic calculations to get feasible energy targets which can be achieved by optimizing the process operating conditions, heat recovery, and energy conversion. This is based on identification and definition of hot and cold streams, temperature-enthalpy profiles, and their minimum approach temperature. Economic model is developed considering the size of all such equipments required and type of construction material required for fabricating them that are responsible for the productivity of the overall process. The cost of equipment is estimated from capacity-based correlations. For evaluating the pro­duction costs, the total annual costs for the system, which include the annual investment cost, cost of operation and maintenance, cost of raw material, and electricity supply and demand are divided by the amount of fuel produced. The electricity and fuel sale price is calculated using the biomass break-even cost (expressed in terms of the expenditure per MWh of biomass) that defines the maximum resource price for which the process is profitable.

Caputo Antonio et al. [61] studied the economics of biomass to energy con­version in combustion and gasification plants with specific reference to the effect of logistics variables with the aim of assessing the feasibility/profitability of direct production of electric energy from biomass. The study was carried out on com­bustion and gasification plants in the capacity range of 5-50 MW. The scale effects were found to be very significant in that profitability of both combustion and gasification systems increased with scale-up of plant size. Also, the influence of logistics on economic performance reduced with increasing plant size. The logistics included purchase and transport cost of biomass, operating labor, main­tenance, and ash transport/disposal costs. The effects of these on the total capital investment and total operating cost were evaluated. In terms of capital and operating costs, combustion-based process showed a lower total capital investment but a higher total operating cost compared to the gasification system. The gasifi­cation system has a lower biomass consumption compared to combustion system and thus, has a lower operating cost. However, in spite of the lower operating cost, the high capital investment, especially in absence of fiscal incentives and adequate financial support, makes the gasification system less profitable than the combustion system. The biomass purchase cost and biomass transportation cost for a gasifi­cation process is much more significant compared to the operational labor, maintenance, and ash transport/disposal costs. It is therefore possible to improve the performance and profitability of a gasification-based approach to the extent that it is comparable to the combustion-based approach by taking advantage of the technological advances and by improving the logistics of biomass procurement and transportation.

With advances in technology and ever increasing fossil fuel and electricity costs, the profits incurred by biorefineries and other biomass conversion technol­ogies is likely to increase enormously due to an added advantage of value-added products generated during the conversion plus the carbon credits earned due the environment friendly processes used, which would give additional monetary and non-monetary benefits to the company. However, the advanced efficient conversion technologies would require a concurrent improvement in the biomass generation collection and transportation efficiencies and improved fuel/energy transport efficiencies. We are gradually moving from carbon neutrality toward carbon negativity, where the amount of carbon generated as a result of con­sumption of the fuel/energy would be significantly less than that used up by the biomass during its generation.

Haibo Xie, Wujun Liu and Zongbao K. Zhao

3.1 Introduction

The impacts of climate change are forcing governments to limit greenhouse gas (GHG) emission through the utilization of sustainable energy, such as solar energy, wind energy, hydrogen energy, etc. Being well recognized as one of the sustainable energy alternatives to petroleum fuels, biofuels are developed from biomass, which are storage of solar energy via photosynthesis by nature. All countries have put the development of biofuels at the top of their agenda on the road to a clean energy system. Traditionally, biofuels were usually produced from corn, sugarcane, and so on. They are recognized as food sources for human and animals. Recently, the overdevelopment of biofuels has simulated concerns about food-based biofuels, and it was regarded as potential threat of food security and strains on natural resources [1]. As the most abundant biomass on the planet, lignocellulose is mainly consisted of cellulose, hemicellulose, and lignin [2]. The utilization of lignocellulosic resources was regarded as one pathway for production of biofuels without occupying plowland and contributing to the greenhouse effect. Additionally, nowadays almost all alternative energy sources have low-energy return on investment (EROI) values, because they require high-energy input [3]. Therefore, the development of energy-efficient conversion technologies is a challenge during the biofuel industrialization process.

Lignocellulosic biomass, primarily being a complex mixture of cellulose, hemicellulose, and lignin, is naturally resistant to breakdown by pests, disease, and weather. This inherent recalcitrance makes the production of monosugars or other valuable chemicals from lignocellulose expensive and inefficient. It is well

H. Xie (H) • W. Liu • Z. K. Zhao

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, CAS, Dalian 116023, People’s Republic of China e-mail: hbxie@dicp. ac. cn

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_3, © Springer-Verlag Berlin Heidelberg 2012 recognized that cellulose crystallinity, covalent interactions between lignin and polysaccharides, and robust hydrogen bond in cellulose microfibrils must be broken before cellulose and hemicellulose are converted to sugars efficiently through pretreatment processes [4]. Lignocellulose pretreatment, which involves many physical, chemical, structural, and compositional changes, is considered to be a central unit in an efficient and economic conversion of lignocellulosic biomass into fuels and chemicals. Presently, there are quite a lot of various physical-, chemical — and biological-based pretreatment technologies for lignocellulosic bio­mass available [5]. However, they still suffer from different problems, such as hash conditions, high cost, and low efficiency. Sometimes, an integration of different pretreatment strategies is needed aiming to a more efficient pretreatment.

The full dissolution of cellulose and lignocellulose in ionic liquids (ILs) was accompanied by the destruction of cellulose crystallinity and inter (or inner) hydrogen-bonding network, partially deconstruction of covalent bonds between carbohydrate and lignin, and decrease in the lignin content in cellulose rich products, all of which are beneficial factors for further chemical or biological conversion of carbohydrates into monosugars and chemicals. This chapter aims to provide an up-to-date progress in the understanding of the fundamental sciences and its relation to enzymatic hydrolysis with the ILs-based strategies at this start point of lignocellulose biorefinery.

Cellulases operate optimally at a specific temperature, and the introduction of IL can shift the optimal temperature. One of the cellulase identified by metagenomics exhibited an optimum activity at 55°C in McIllvaine buffer (0.2 M Na2HPO4 with 0.1 M citric acid, pH 6.5). In 1-ethyl-3-methylimidazolium trifluoroacetate ([EMIM][TfAc]) and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([BMPy][CF3SO3]), the optimum temperature shifted to 37 and 20°C, respectively [126].

Increasing the temperature from 50 to 60-70°C can also accelerate the deac­tivation of cellulases from T. reesei in [EMIM][OAc] [91]. The stability of mix­tures of Celluclast 1.5 l and Novozyme 188 was tested in the presence of [EMIM][OAc] at concentrations ranging from 5 to 30% (volume/volume) in citrate buffer (pH 4.8) with poplar and Avicel [120]. When the hydrolysis was conducted at 4°C for a [EMIM][OAc] concentration of 30%, the activity of the cellulase mixture after 24 h remained above 70% of the activity without [EMIM][OAc]. At 50°C, the drop in cellulase activity dropped further to 31% of the control activity after 24 h in a 30% [EMIM][OAc] solution [120].

Enzyme activity is also pH-dependent [126, 127]. Celluclast 1.5 l hydrolyzes cellulose at an optimum pH between 4.5 and 5. No cellulase activity was observed for pH values below 2 or above 8 [127]. Three cellulases identified with metagenomic libraries have optimal pH values of 5, 7, and 7.5 [126]. The oxi­dation of o-phenylenediamine by lignin peroxidase was most effective at pH 3.2 [128]. A deviation from the optimal pH induced by the introduction of ILs can cause the deactivation of cellulases. The pH of the wood/IL mixture is affected not only by the IL concentration but also IL composition and structure [129]. Increasing the concentration of 1,3-dimethylimidazolium dimethylphosphate [MMIM][DMP] from 0 to 0.5 vol.% in the enzyme solution led to an increase of the pH from 4.8 (optimum for hydrolysis) to 6.5 [67]. Mixtures of water with ILs based on an imidazolium cation and a BF — anion have a pH that increases with the length of the alkyl chain on the cation. The addition of hydroxyl groups increases the acidity of the IL [129]. The pH can also vary during the biomass reaction with the IL. Measurements in wheat straw and pine wood solution in [EMIM][Cl], [BMIM][Cl], and [EMIM][OAc] showed a drop in pH over time. A HPLC analysis showed the formation and the accumulation of acetic acid, which comes from the hydrolysis of acetate groups in the biomass [47].

Various raw materials as a most affecting parameter for biobutanol production are being investigated to find cheaper and highly available alternatives [48-51]. On the basis of different varieties of raw materials, biofuels are classified in two categories

(i) first-generation biofuels (ii) second-generation biofuels. The biofuels of these categories are produced by the consumption of food-related (sugarcane and cereal grains) and non-food (lignocellulosic and wastes) materials, respectively [9,10,27].

Due to the food insecurity worldwide, second-generation biofuels indicates toward the sustainable production of fermentation-based liquid fuels [1].

Previously, cereal grains and sugarcane were the common raw materials for ABE fermentation (Fig. 7.1) [16], but in present world the consumption of these substrates has been criticized because of shortage and prices hiking of edible materials. In present era, only few countries such as Brazil and U. S. have enough

production of food-based materials for the production of first-generation biofuels [13, 21, 52]. Therefore, the main focus of the research turned towards the non-food materials such as lignocellulosic materials. For utilizing lignocellulosic materials, firstly these materials are converted in monomers such as glucose, fructose, mannose, sucrose, lactose, starch, dextrin, galactose, xylose, arabinose, raffinose, melezitose, inulin, minnitol, trehalose, ramnose, malibiose, and glycerol. During the fermentation studies, it was found that glucose, fructose, mannose, sucrose, lactose, starch, and dextrin were completely consumed by clostridial bacteria in butanol production. While, galactose, xylose, arabinose, raffinose, melezitose, inulin, and minnitol were partially consumed, but later on it was observed that xylose and arabinose were also utilized completely by some strains. It could be considered as a milestone point because there is a significant amount of pentose sugar (xylose and arabinose) along with hexose sugar are produced from ligno- cellulosic materials on hydrolysis process [15].

Various starch — (sago, defiberated-sweet-potato-slurry, degermed corn, extruded corn, liquefied corn starch, and cassava) [35, 36, 44, 53] and lactose — (whey per­meate) [38, 54] containing substrate were examined for ABE fermentation. For instance, utilization of liquefied corn starch (a product of corn processing industry) showed significant results when there was a proper removal of product (by gas stripping) and Na2S2O5 (inhibitor for fermentation) was being used through the fermentation process [53]. Lactose-containing substrates (wastes of dairy industry like cheese whey) has also been investigated as feedstocks for butanol production using C. acetobutylicum DSM 792 and C. acetobutylicum AS 1.224. Results pos­tulated that cheese whey produced higher yield than direct lactose solution [54].

Recently, researchers uncovered the suitability of Clostridial bacteria to ferment the lignocellulosic materials as some of them have saccharolytic ability (Fig. 7.2) [55]. Rest of the strains have shown efficient performance toward hexose and pentose sugars producing from lignocellulosic materials through hydrolysis process. Lig — nocellulosic biomass is most abundant renewable source on the Earth [2, 56].

For instance, a developing country like India produces huge amount of biomass (over 370 million tons every year) in the form of direct plants, rice husk from rice mill, saw dust from saw mill, bagasse from sugar mills, etc. [28]. The potentiality of biomass such as wood forestry residues, corn stover, wheat straw, corn fibers, barley straw, and switch grass has been examined at laboratory scale for biobut­anol production (Table 7.1) [3, 22-24, 26, 58, 59]. Still, the endeavors are required for optimizing the processes including hydrolysis and removal of the inhibitors from fermentation broth for utilizing lignocellulosic biomass.

Use of ethanol as a biofuel has several limitations, such as high vapor pressure, low energy density, and corrosiveness, which prevents its widespread utilization [73, 74, 76, 141]. Bioethanol production, higher alcohols, fatty acid derivatives including biodiesels, alkanes, and alkenes are more compatible with gasoline-based fuels and allow direct utilization. Some of these compounds are also important chemical feedstocks. Since native organisms do not produce these compounds naturally in high quantities, metabolic engineering becomes essential for producing these compounds in non-native producing organisms such as E. coli. The four major metabolic systems that allow the production of higher alcohols are the coenzyme-A mediated pathways, the keto acid pathways, the fatty acid pathway, and the isoprenoid pathways which have been discussed in the subsequent sections.

In ultra-rapid pyrolysis, very high heating rates and temperatures of around 1,000°C with short vapor residence times gives predominantly a gaseous product. A rapid quenching of the primary product is done following pyrolysis. The heating is done using a heat carrier solid such as sand. A gas-solid separator separates the gas from the heat carrier solid.

Hydrouspyrolysis and Hydropyrolysis

Hydrouspyrolysis and hydropyrolysis involve thermal decomposition of biomass in the presence of water or hydrogen respectively, under high pressure conditions. The process usually takes place in two stages. The first stage involves treating biomass with water or hydrogen at 200-300°C under pressure; the second stage involves cracking of the hydrocarbon produced in the first stage into lighter hydrocarbons at a temperature of around 500°C. The bio-oil produced by this type of pyrolysis method has reduced oxygen content which is a desirable characteristic.

Vacuum pyrolysis

In vacuum pyrolysis, biomass is heated in vacuum in order to decrease the boiling point and avoid undesirable chemical reactions. Vacuum pyrolysis is carried out at temperatures of 400-500°C and at total pressure of 2-20 kPa. Under these con­ditions, the product of pyrolysis can be rapidly withdrawn from the hot reaction chamber enabling preservation of the primary fragments originating from the thermal decomposition of biomass. Heat transfer, which is a rate limiting factor in pyrolysis, is the major limitation in vacuum pyrolysis. In an actual pilot plant reactor developed by a company called Pyrovac, this has been effected by passing molten salts through hollow heating plates on which the biomass is placed inside the vacuum pyrolysis reactor. The biomass gets heated by conduction as well as radiation thus increasing the heat transfer efficiency. The details of vacuum pyrolysis, with particular reference to the theoretical aspects of heat transfer in vacuum pyrolysis, has been described at length by Roy et al. [15].

The pyrolysis technology is less developed than the combustion or gasification technologies. This is probably due to the fact that the bio-oil obtained from the pyrolysis process costs 10-100% more than fossil fuel and its availability is limited; it is unstable due to the presence of entrained fines of char particles; dedicated liquid handling such as modified pressure filtration is required for removal of these fines; the kinematic viscosity of the pyrolysis oils varies over a wide range depending on the nature of feedstock and temperature of pyrolysis among other factors; bio-oil is acidic in nature with a pH of around 2.5-3.0, making it corrosive to the commonly used construction materials such as carbon, steel, and aluminum. Some of the sealants used may also be affected; bio-oil has a water content of around 15-30% by weight of oil mass, which contributes to the low energy density of the oil—this cannot be removed by conventional methods like distillation. All the above properties of the bio-oil obtained from pyrolysis make it unsuitable for direct application as a transport fuel, as a precursor for generation of chemicals, etc. Upgrading of the bio-oil by methods such as catalytic upgrading, needs to be done before it can be used for the various applications. These are described in detail by Bridgwater [13]. However, fast pyrolysis and flash pyrolysis is advancing very rapidly with a number of commercial level plants being set up across the globe.

Gasification

Gasification of biomass is the thermochemical transformation of biomass at high temperature in the presence of restricted supply of oxygen, which may be supplied as such, or in form of air or steam. It is the latest biomass conversion technology among the thermochemical methods for biomass conversion. The product of gasification is a gaseous product which has applications in electric power gener­ation, manufacturing of liquid fuels, and production of chemicals from biomass. Gasification can be said to be an extension of pyrolysis, and has been optimized to give a maximum of the gas phase at the cost of char or liquid. The gas produced

from the gasification process is a mixture of carbon monoxide, hydrogen, and methane along with carbon dioxide and nitrogen also produced to some extent, and is called producer gas as it can be used as synthesis gas to produce ammonia or methanol, which, in turn, are used to produce synthetic fuel (synthetic petrol) or as a source of hydrogen. It can also be used as such as a heat source, or to generate electricity through gas turbines. Up to 50% efficiency with respect to electricity generation can be obtained if the gas turbine is integrated with a steam turbine in combined cycle gas turbine system (also called biomass integrated gasification combined cycle—BIGCC). In this system the waste gas from the gas turbine is recovered and used to generate steam in a steam turbine. Figure 1.11 shows a schematic flow sheet for such a system. With such a type of integration, biomass gasification plants can be as economical as coal-fired plants for electricity generation.

However, the power output is limited by economic supply of biomass and is generally limited to around 80 MW of electricity [16]. The medium used for gasification, i. e., oxygen, air, steam, etc. greatly affects the heating value of the product obtained. Gasification in the presence of steam has the highest heating value, followed by oxygen and air in that order. Although the product of gasifi­cation in the presence of steam has the highest heating value, higher operating temperatures are required for vaporization of water, increasing the cost of the process. Usually, a mixture of air and steam, with variable inlet ratio is employed. Commercial gasifiers are available in a wide range of sizes and a variety of types
which are capable of using a variety of biomass feedstock such as charcoal, wood, rice husk, and coconut shells. The newer gasification processes such as plasma gasification and hydrothermal gasification are also capable of processing muni­cipal solid waste (MSW). The following sections describe the mechanisms and chemistry of the biomass gasification process and the various types of gasifier designs developed for getting optimum product yields for a given biomass feedstock.

The gasification process increases the H/C ratio of the biomass by adding hydrogen to and removing carbon from the biomass.

Gasification consists of four main stages: preheating and drying, pyrolysis, char gasification, and combustion (also called flaming pyrolysis). These stages may take place either in specific regions or zones of the gasifier equipment (especially in moving bed gasifier designs), or these may take place at a microscopic level, within a particle (especially in the fluidized-bed gasifier designs). As in all thermochemical conversion methods, drying of the biomass is a very important step in the gasification process. Moisture contents in biomass vary over a very wide range (from 30 to 60%, and in some cases, even up to 90%). Every kilogram of moisture in the biomass can consume a minimum of 2,260 kJ of extra energy from the gasifier to vaporize the water, which is not recoverable [11]. Hence, pre-heating of biomass is done where the moisture content of the biomass is brought down to about 25%. The pre-heated biomass further dries in the gasifier when temperatures in the range of 100-200°C are encountered. The surface moisture as well as the inherent moisture present in the biomass is removed. The stage of drying is followed by pyrolysis as the tem­perature increases to 200-700°C. Pyrolysis is the first step in the gasification of biomass. In this stage, large molecules are broken down to smaller gas molecules (condensable as well as non-condensable), carbon char, and tars/oils. This stage is endothermic and does not involve reactions with oxygen or air or any medium. Following the initial pyrolysis of biomass, a number of secondary reactions occur where the products of pyrolysis react with each other and with the medium used for pyrolysis (oxygen, air, or steam), to give CO, CO2, H2, H2O and CH4. The carbon char is further gasified in the presence of restricted air, oxygen or steam to produce additional combustible gases, giving producer gas. The updraft gas­ifier and the downdraft gasifier designs (Fig. 1.12, 1.13) illustrate the different stages in a gasification process.

The overall reactions occurring after the initial pyrolysis of biomass in the gasification process are shown below (source Ref. [11]):

Carbon reactions:

C + CO2 $ 2CO C + H2O $ CO + H2 C + 2H2 $ CH4 C + 5O2 ! CO

A H° = +172kJ/mol ДЯ° = +131 kJ/mol A Я° = —74.8 kJ/mol A Я° = —111 kJ/mol

Oxidation reactions:

C + O2 ! CO2 AH° = -394 kJ/mol

CO + |O2 ! CO2 АH = -284 kJ/mol

CH4 + 2O2 $ CO2 + 2H2O AH°r = -803 kJ/mol H2 + 1O2 ! H2O АH = -242 kJ/mol

Water-gas shift reaction:

CO + H2O $ CO2 + H2 АH = -41.2kJ/mol

Methanation reactions:

2CO + 2H2 ! CH4 + CO2 AH° = -247 kJ/mol

CO + ЗН2 $ CH4 + H2O AH = -206 kJ/mol

CO2 + 4H2 ! CH4 + 2H2O AH° = -165 kJ/mol

Steam reforming reaction:

CH4 + H2O $ CO + ЗН2 AH° = +206 kJ/mol CH4 + 2O2 ! CO + 2H2 AH = — 36 kJ/mol

particle depending on the surrounding temperature and the presence or absence of air/oxygen.

To summarize, gasification produces volatile gases and carbon char. The vol­atile gases are converted to CO, H2 and CH4, whereas the carbon char is com­busted to produce CO. In case of low temperatures and short residence times in the hot zone, medium-sized molecules may escape and condense as undesirable tars and oils. This tar, being viscous, creates problems of fouling in the gasifier, and needs to be removed. This can be done by catalytic cracking of the tar, which gives CO, H2, and H2O. The role of catalysis in cracking is discussed in detail by Bridgwater [13].

The performance efficiency of a gasifier process or a gasifier unit is described in terms of ‘‘% cold gas efficiency” which is defined as follows:

The extent of carbon conversion or fuel utilization can also be determined and related to the production efficiency of the gasifier process.

where

HHVgas = higher heating value of the producer gas, kJ/m3

HHVfuel = higher heating value of the biomass feedstock, kJ/m3

vgas = volumetric rate of producer gas, m3/h

mfuel = input mass rate of biomass fuel, kg/h

mash = mass rate of gas residue exiting the gasifier, kg/h

% Cash = weight percent of carbon in the ash residue, %

% Cfuel = weight percent of carbon in the biomass fuel, %

In general, operating the gasifier with 100% carbon utilization, with simulta­neous maximization of cold gas efficiency, is not possible. The carbon efficiency has to be always sacrificed in order to achieve producer gas of the desired spec­ifications [17].

A solution to the problem of tar entrainment in the gas stream has been found by designing co-current or downdraught gasifiers, in which primary gasification air is introduced at or above the oxidation zone in the gasifier. The producer gas is removed at the bottom of the apparatus, so that fuel and gas move in the same direction.

On their way down the acid and tarry distillation products from the fuel must pass through a glowing bed of charcoal and therefore are converted into permanent gases hydrogen, carbon dioxide, carbon monoxide and methane. Depending on the temperature of the hot zone and the residence time of the tarry vapors, a more or less complete breakdown of the tars is achieved. The main advantage of down­draught gasifiers lies in the possibility of producing a tar-free gas suitable for engine applications. In practice, however, a tar-free gas is seldom if ever achieved over the whole operating range of the equipment: tar-free operating turn-down ratios of a factor 3 are considered standard; a factor 5-6 is considered excellent. Because of the lower level of organic components in the condensate, downdraught gasifiers suffer less from environmental objections than updraught gasifiers.

A major drawback of downdraught equipment lies in its inability to operate on a number of unprocessed fuels. In particular, fluffy, low density materials give rise to flow problems and excessive pressure drop, and the solid fuel must be pelletized or briquetted before use. Downdraught gasifiers also suffer from the problems asso­ciated with high ash content fuels to a larger extent than updraught gasifiers. Minor drawbacks of the downdraught system, as compared to updraught, are somewhat of lower efficiency resulting from the lack of internal heat exchange as well as the lower heating value of the gas. Besides this, the necessity to maintain uniform high temperatures over a given cross-sectional area makes impractical the use of downdraught gasifiers in a power range above about 350 kW (shaft power).