1.2.1.1 Thermochemical Processes

Biomass conversion technologies can be broadly classified into primary conver­sion technologies and secondary conversion technologies. The primary conversion technologies such as combustion, gasification and pyrolysis involve the conversion of biomass either directly into heat, or into a more convenient form which can serve as an energy carrier such as gases like methane and hydrogen, liquid fuels like methanol and ethanol, and solids like char. The secondary technologies convert these products of primary conversion into the desired form which may be an energy product such as transportation fuel or a form of energy such as elec­tricity. The different thermochemical conversion processes are given in Fig. 1.5.

These processes involve high temperature and sometimes high pressure pro­cessing of biomass. The combustion process for generation of heat and/or power involves heating the biomass in the presence of excess oxygen. It is responsible for over 97% of the world’s bioenergy production [1]. The other processes such as torrefaction, pyrolysis and gasification involve heating in the presence of restricted or controlled oxygen to produce liquid fuels, heat, and power.

The thermochemical processing of biomass produces gas, liquid, and solid. The gas produced primarily comprises carbon monoxide, carbon dioxide, methane, hydrogen, and some impurities such as nitrogen. This gas is called synthesis gas which can be used as fuel, or can be upgraded or converted to more valuable and/or useful products such as methanol or methane. The liquid product contains mainly noxious and a highly complex mixture of oxygenated organic chemicals consisting of volatile components and non-volatile tars. The solid contains ash and carbon or char.

The suitability of biomass for thermal/thermochemical conversion processes, and the products obtained as a result of these biomass conversion processes, depend greatly on the composition and properties of the biomass used. Physicochemical characterization of biomass is therefore an important step in biomass conversion. This involves the determination of particle size and bulk density; proximate anal­yses such as determination of moisture content, volatile matter, fixed carbon, ash content; ultimate analysis such as determination of carbon, hydrogen and oxygen content; determination of ash deformation and fusion temperature; calorific value; biomass composition; equilibrium and saturation moisture content; and biomass pyrolysis characteristics. There have been a number of projects undertaken the world over, wherein a systematic characterization of different varieties of biomass and species has been undertaken. The output of these systematic studies has, in many cases, resulted in a database on biomass fuel characteristics. Biobank is a set of three databases giving the chemical composition of biomass fuels, ashes, and condensates from flue gas condensers from actual installations. The data set was originally compiled by Biosenergiesysteme GmbH, Graz, Austria. It is continu­ously expanding, using data inputs from other member countries of IEA Bioenergy Task 32. It currently contains approximately 1,000 biomass samples, 560 ash samples, and 30 condensate samples [2]. Another database—BIOBIB has been developed by the Institute of Chemical Engineering, Fuel and Environmental Technology, Vienna, Austria, which gives similar data for European plants. This database covers different types of biomass such as energy crops, straw, wood, wood waste from wood processing industries, pulp and paper industry, and other cellu — losic waste such as that from the food industry. It currently has 331 different biomass fuels listed [3]. Phyllis is yet another database which is designed and maintained by the Netherlands Energy Research Foundation containing informa­tion about composition of biomass and waste fuels [4]. Over 250 biomass species from different parts of India have been characterized with respect to the above properties under the MNES sponsored Gasifier Action Research Project at the Biomass Conversion Laboratory of the Chemical Engineering Department at the Indian Institute of Technology Delhi [5]. An overview of the different thermal and thermochemical conversion processes is given in the following sections.

Biomass is the only renewable organic resource available in great abundance. If exploited to its fullest extent, it has the capacity to completely replace fossil fuels for energy generation, simultaneously maintaining a clean environment, free from the greenhouse gases. Technologies for the production of the third — and fourth- generation biofuels are likely to have a very great impact on reducing the problem of global warming caused by the GHGs and in taking us from an era of carbon neutral environment to a carbon-negative environment. These include biofuels produced by upgraded pyrolysis and gasification technologies and solar-to-fuel technologies. The concept of biorefineries has already made the biomass conver­sion technology a great attraction among industry investors because biorefineries have the potential of reaping great profits by generating costly fuels as the main product, and in addition to this, costlier value-added products such as chemicals, as by-products, the original cost of the initial raw material being almost negligible. The future biorefineries would use efficient feedstock upgrading processes, where the raw materials are continuously upgraded and refined. Fractionating the biomass into its core constituents before using it as feedstock will give the much lacking uniformity in the biomass, making the processing in a biorefinery all the more efficient. Only the residue remaining after all the useful components are converted, should be used for generation of heat and electricity. This will ensure complete usage of the biomass. The catalytic cracking/upgrading technologies used in the thermochemical conversion methods are likely to improve with the use of nano­particle-based catalysts. Simultaneously, the development of biocatalysts will enable biomass conversions under milder conditions, and with greater efficiencies, leading to more environment friendly ‘‘green’’ processes. Bioethanol and biodiesel are the two biofuels that have the potential of replacing gasoline. The rapid advances and the unlimited scope of the biochemical conversion technologies and the algal conversion processes are likely to make this a reality in the near future. Genetic manipulation of microorganisms to improve production of efficient cel — lulases and hemicellulases will go a long way in improving yields and reducing conversion times in the biochemical conversion of lignocellulosic biomass. Recombinant DNA technology is being applied to bacteria and fungi in order to achieve this. Strains of microorganisms which have the ability to co-ferment different types of substrates simultaneously, will improve the economy and effi­ciency of the biochemical conversion processes. On the other hand, transgenically modified plants can be grown which will have a reduced lignin content and an upregulated cellulose biosynthesis. ‘‘Plant factories’’ can be set up, where such genetically modified plants can be grown which have the capacity to capture and store more carbon so that the overall energy density of the biomass increases. The bright future of biomass conversion into energy is clearly evident from the large number of integrated biorefineries which have already come up in different parts of the world.

For a long time, the full dissolution biomass is one of the biggest barriers for the homogeneous utilization of biomass. In 2002, Rogers et al. first reported that 1- methyl-3-butyl-imidazlium chloride was able to dissolve cellulose with capability of 10-25 wt% depending on heating methods [6]. Since then, the soluble behaviors of most of carbohydrates and biopolymers have been studied [7], such as chitin, chitosan [8, 9], lignin [10], silk fibroin [11], and wool keratin [12]. In 2007, Kilpelainen et al. first investigated the details of woody lignocellulosic materials in ILs. It was found that the lignocellulosic materials were harder to dissolve com­pared to the soluble behavior of cellulose by ILs, which needed higher tempera­ture, longer time, and more intense stirring. As a result, a 7 wt% spruce wood solution was achieved at 130°C in 8 h. Further study showed that the 1-methyl-3- ethyl-imidazolium acetate was a better solvent for lignocellulosic materials. After the dissolution of cellulose or wood in ILs, it was anticipated that all the chemical bonds and functional groups on these biopolymers are totally open to external chemicals and catalysts, which rationally facilitates the conversion of chemical bonds and functional groups. All of the pioneering work has stimulated a growing research effort in this field to investigate the potential of this new homogenous platform [13]. Another reason for the passion in biomass utilization in ILs is that the process is the combination of the application of biorenewable resources as raw materials and sustainable solvents for the production of valuable materials and chemicals, which will contribute to the foundation of bio-based sustainable chemical industry [14].

The activity of commercial cellulases extracted from Trichoderma viride and Aspergillus niger on carboxymethylcellulose and Avicel generally increased at high pressure up to 500 MPa (above atmospheric pressure) [130]. The activity of cellulases from Aspergillus niger was assessed on carboxymethylcellulose in 10% [BMIM][Cl] at 30°C at hydrostatic pressures up to 675 MPa (above atmospheric pressure). The activity increased by 70% at a pressure of 100 MPa, compared to the activity at atmospheric pressure; then decreased for pressures above 200 MPa. The activity at 600 MPa was comparable to the one at atmospheric pressure. Although the cellulases lost 50% of their activity in 10% [BMIM][Cl] at atmo­spheric pressure, their activity in 10% [BMIM][Cl] at 100 MPa is about 85% of the one in acetate buffer at atmospheric pressure. This result suggests that high pressure can limit the de-activation of cellulase in ILs [123].

The efficacy of different types of fermentation processes, namely batch, fed-batch, and continuous, at laboratory scale has been investigated with various raw mate­rials and clostridia species for butanol production (Table 7.2).

n-Butanol and isopropanol are the two higher alcohols which are overproduced in nature by Clostridium species. The fermentative pathway in this organism starts from acetyl-CoA. The enzyme acetyl-CoA acetyltransferase condenses two mol­ecules of acetyl-CoA to one molecule of acetyl-CoA. This molecule branches the pathway into isopropanol and n-butanol. For the biosynthesis of isopropanol, an acetoacetyl-CoA transferase transfers the CoA group away from acetoacetyl-CoA

Butyraldehyde

-NADH

adhE2

NAD*

n-Butanol

to acetate or butyrate, forming acetoacetate. Acetoacetate is decarboxylated to acetone by an acetoacetate decarboxylase. Then, acetone is reduced to isopropanol by a NADPH-dependent secondary alcohol dehydrogenase [64]. For n-butanol biosynthesis, acetoacetate has to go through four steps of NADH-dependent reduction and one step of dehydration as shown in Fig. 9.10.

Isopropanol and n-butanol are produced by Clostridium species. However, production by this procedure is difficult to handle and optimize, because of

Fig. 9.11 Schematic illustration of higher chain alcohol production via keto acid pathway. keto acid decarboxylase (KDC), alcohol dehydrogenase (ADH) complex physiological features, such as oxygen sensitivity, slow-growth rate, and spore-forming life cycles of Clostridium. Therefore, E. coli has been metabolically engineered to produce acetone, the immediate precursor of isopropanol [15] and n — butanol production by using the traditional CoA-dependent pathway originated from C. acetobutylicum [8].

A variety of gasifier designs have been developed depending upon the nature of the gasification process involved, nature of feedstock used, scale of operation, and the product specifications required. These designs can be classified either on the basis of the manner in which the feedstock is handled in the gasifier or, on the basis of the manner in which heat is supplied to the gasifier. The first category of gasifiers are the fixed-bed gasifiers, fluidized-bed gasifiers, and the entrained flow gasifiers. The salient features of each of these are summarized in Table 1.6. The oxidation reactions taking place in the gasifier are generally exothermic reactions.

In most gasifiers the energy released as a result of these reactions is used to serve a dual purpose: first, to fuel the endothermic reactions taking place in the gasifier, and second, to maintain the high temperatures required in the gasifier. Such a gasification process, in which the heat released in one portion of the gasifier is partly or fully utilized to propel other endothermic gasification reactions taking place in the equipment, is a called direct gasification process. If no oxidizing agent is added, there will be no exothermic reactions taking place in the gasifier, and the heat required for the gasification processes will have to be supplied from some external source of heat. Such systems where the heat requirements for the gasification process are supplied externally are called indirect gasification systems or allothermal gasification. Figure 1.14 shows a schematic diagram of the direct and indirect gasification sys­tems. As the direct gasifiers use a part of their input stream to drive other reactions taking place in the system, the overall efficiency of such systems is reduced.

On the other hand, as the indirect gasifiers use an external source of energy for the purpose, such gasifiers are expected to be more energy efficient, especially if solar energy is used as the source. Use of sunlight to drive an endothermic gasi­fication reaction increases the calorific value of the initial biomass, with an added advantage of being a renewable source. An optimized design of such an indirect gasification system may even increase the energy content of the product stream beyond that of the feedstock. The gasification systems in which the heat producing processes or reactions are separated from the processes which consume heat are

Fluidized bed gasifier

• Uses inert material such as sand to

mix solid fuel with gas phase

• High operating temperatures

(1,000-1,200°C)

• Gasifier zones at microscopic levels

in individual particle

• Uniform temperature distribution

• Better solid-gas contact and heat

transfer rates

• Equipped with cyclone separators at

the top for removal of particulates from product

• Suitable for feedstocks with low ash

fusion temperature

• Ash removed as slag or dry

Updraft gasifier

• Can tolerate more moisture in feedstock

• Producer gas exits from top and at

lower temperature (130-150°C)

• Product contaminated with tars, oils and

particulate matter from incoming fuel

• Suitable for direct heating applications

only

Downdraft gasifier

• Radiant and conductive heat transfer

from lower pyrolysis and combustion zones provide heat for drying of biomass

• Properly designed and positioned

‘‘throat’’ increase velocity of gas and promote heat and mass transfer

• Gives least amount of tar

• Widely used for small-scale

applications
Cross-flow gasifier

• High temperature (>1,500°C) reached

in the combustion zone

• Reaction zone is small with low thermal

capacity

• Short start-up time and response time

• Tar production is low

• Generally used for gasification of

charcoal (with very low ash content)

• Suitable for small-scale biomass

gasification units Bubbling fluidized bed gasifier

• Exit gas temperatures usually

700-800°C

• Residence time is short

• Suitable for medium-sized units

(25MWth)

• Suitable for treated MSW biomass Circulating fluidized bed gasifier

• Provides long residence time

• Suitable for fuels with high volatiles

• Capacity of 60 MWth achieved

Table 1.6 (continued)

Entrained flow gasifier

• Operate at higher temperatures

(1,200-1,600°C) and higher pressures (2-8 MPa)

• High oxygen demand

• Require small and uniform particle

size distribution (<0.4 mm) in feedstock (not suitable for fibrous materials)

• High reactivities and high capacities

• Low higher hydrocarbons and low tar

formation

• Product low in methane content

hence, better suited for synthesis gas production

• High operating temperatures causes

ash to get converted into slag which may be corrosive

• Not suitable for high ash content

feedstocks

• Preferred for IGCC plants

Based on Direct gasifiers

method of • Use portion of product or input stream heat to drive gasification reactions

supply

Indirect gasifiers

• The combustion process is separated

from the gasification process

• Energy efficiency greater than direct

gasifiers

• Produces medium calorific value

product and flue gases

• Complete conversion of biomass is

possible

• High investment and maintenance

cost

also categorized under indirect gasification systems. Such systems therefore con­sist of two reactors connected by an energy flow. Typically, the oxidation or combustion reactions which are exothermic in nature are separated from the pyrolysis and gasification reactions which require heat. The heat from the com­bustion reactor is provided to the gasification reactor by means of hot sand which is circulated between the two reactors. The different designs of gasifiers, along with their salient features, advantages and disadvantages, have been reviewed by a number of authors [10, 11, 17-19]. A summary of these is given in Table 1.6.

In addition to the gasifier designs summarized in Table 1.6, there are other gasifiers which are modifications of the existing designs. Transport gasifier, twin reactor system, and chemical looping gasifier designs are modifications of the circulating fluidized-bed gasifier. The transport gasifier is a hybrid of entrained flow and fluidized-bed gasifier systems. Its construction and process design attri­butes to it, a higher throughput, better mixing and consequently, higher heat and mass transfer rates. However, it is more suitable for gasification of coal; its suit­ability for gasification of biomass is yet to be proven [11]. The twin reactor system is a dual fluidized-bed gasifier where the combustion process in the gasification of biomass is separated from the gasification process using separate fluidized-bed reactors. Such a design prevents the dilution of the product mix by nitrogen which is released subsequent to air combustion in normal fluidized-bed gasification process. These are used for gasification of coal as well as biomass. Different industrial-scale units are described by Basu [11]. The chemical looping gasifier process is a relatively recent development in which a bubbling fluidized-bed gasifier is coupled with a circulating fluid bed regenerator to obtain a continuous stream of hydrogen from agricultural feedstock. In — situ sequestration of carbon dioxide, formed during the gasification process, is done by using calcium oxide which reacts with carbon dioxide to form calcium carbonate, which is then reconverted into calcium oxide in the circulating bed regenerator [20].

Plasma gasification technology is a newly developed self-sustaining technology which is used especially for conversion of municipal solid waste into electric power. It is relatively insensitive to the quality of feedstock which is the most desired feature for MSW processing. The garbage is converted into a finely shredded mass and is then fed into a plasma chamber which consists of a sealed stainless steel vessel which is filled with either ordinary air or nitrogen. A 650 V electric current is passed between two electrodes which tears off electrons from air to create plasma. The energy generated in the process is sufficient to disintegrate the garbage into its constituent elements, forming syngas along with other by-products depending on the nature of MSW used as feedstock. The syngas, which leaves the plasma chamber at a temperature of * 1,200°C is fed to a cooling system, where the heat transferred from the syngas to the cooling water generates steam, which can be used in a steam turbine to generate electricity. The gas, after appropriate clean-up, can be used either for the usual applications of syngas, or in a gas generator to generate electricity. Part of the electricity generated is used to generate plasma in the plasma chamber. Figure 1.15 shows a schematic of the process.

Hydrothermal gasification involves gasification of biomass in an aqueous medium using supercritical water, i. e., water at a temperature and pressure beyond the critical point of water (374.29°C and 22.089 MPa). The conventional thermal methods of biomass conversion are cost-effective only when the moisture content of biomass feedstock is low. However, certain biomass such as aquatic biomass and MSW may contain moisture even up to 90% weight basis. In such cases, either the biomass has to be dried separately or, process heat is used to dry the excess moisture, both of which reduce the efficiency of the process. Another alternative in such cases is the use of biochemical methods for biomass conversion. However, these methods suffer from a major disadvantage of being very slow, having a low efficiency and producing only methane and no hydrogen. In order to get hydrogen, steam reforming process is required to be carried out. In contrast, the hydrothermal gasification process is relatively rapid and can tolerate very high moisture contents without compromising on the efficiency of the process [21]. Supercritical water has some unique properties which make it suitable for biomass gasification:

— it causes rapid hydrolysis of biomass

— the intermediate reaction products, including gases have a high solubility

— single-phase reactions are possible, thus eliminating interphase barriers for mass transfer

— being non-polar, supercritical water is a good solvent for substances like lignin which show low solubility in ordinary water.

Hydrothermal gasification causes splitting of the organic molecules present in the biomass by hydrolysis and oxidation reactions. The biomass gets broken down to methane, hydrogen, carbon monoxide, and carbon dioxide. The major advan­tages of hydrothermal gasification are that it is suitable for biomass with high moisture contents; the product formed is rich in hydrogen; char and tar formation is low; the tar that is formed gets cracked and dissolves in the supercritical water. In addition, automatic separation of the product gas from the liquid containing char and tar takes place which obviates the need for a separate gas cleaning process which is usually required for all the conventional gasification or, for that matter for all thermochemical conversion processes. Elements such as sulfur, nitrogen, hal­ogens, etc. leave the process along with the aqueous effluents. However, due to this, corrosion of the reactor is a major problem as the presence of water causes the elemental by-products to get converted into acids which can corrode the reactor. Products such as bio-oil, methanol, hydrogen and a range of chemicals including phenol can be obtained from hydrothermal gasification of biomass. Hydrothermal gasification is most suitable for processing of MSW, and other biomass having very high moisture content as the efficiency of this process is independent of moisture content.

Cross-draught gasifiers are an adaptation for the use of charcoal. Charcoal gasi­fication results in very high temperatures (1500 °C and higher) in the oxidation zone which can lead to material problems. In cross-draught gasifiers insulation against these high temperatures is provided by the fuel (charcoal) itself. Advan­tages of the system lie in the very small scale at which it can be operated. Installations below 10 kW (shaft power) can under certain conditions be economically feasible. The reason is the very simple gas-cleaning train (only a cyclone and a hot filter) which can be employed when using this type of a gasifier in conjunction with small engines.

A disadvantage of cross-draught gasifiers is their minimal tar-converting capabilities and the consequent need for high quality (low volatile content) charcoal. It is because of the uncertainty of charcoal quality that a number of charcoal gasifiers employ the downdraught principle, in order to maintain at least a minimal tar-cracking capability.

The nature of the anion played a major role in the dissolution of biomass. For example, [EMIM][OAc] was more effective than [EMIM] [Cl] in the dissolution of southern yellow pine [36]. The chloride anion combined with the [BMIM] cation was effective in the dissolution of maple wood flour. Substitution of the chloride anion with the tetrafluoroborate or hexafluorophosphate anions made the maple wood flour insoluble [25]. Maple wood flour pretreated in [EMIM][OAc] and [BMIM][OAc] at 90°C for 6 h resulted in a decrease in cellulose crystallinity, higher glucose, and xylose yields. In contrast, pretreatment with [BMIM][MeSO4] had little effect on the biomass cell structure, sugar yields, and cellulose crystal­linity, compared to untreated wood flour [38].

The ability to dissolve biomass was related to the anion basicity. [EMIM] [OAc] was a better solvent than [BMIM][Cl] for southern yellow pine and red oak (particle size 0.125-0.250 mm), due to the increased basicity of the acetate anion and also its lower viscosity and melting point [36]. Cork powder remained insoluble in [EMIM] [Cl] and [BMIM][Cl] after 4 h at 100°C. Replacing the chloride anion with a lactate or ethanoate anion improved the cork dissolution significantly. ILs based on the cholinium cation and alkanoate anions were more effective in the cork dissolution. Among the alkanoate anions included in the study, the increasing alkyl chain length (ethanoate, butanoate, hexanoate) led to an increase in biomass dis­solution efficiency, attributed to an increase in the basicity of the anion [39].

The structure of the cation plays a role in the melting point of the IL. Alkyl groups on the imidazole tend to lower the melting temperature, and enhance wood liquefaction and processability of the wood solution. For example, wood disso­lution was more effective in 1-butyl-3-allylimidazolium chloride ([BAIM][Cl]), then in 1-methyl-3-allylimidazolium chloride ([MAIM][Cl]) [16].

MgCO3CaCO3 (Dolomite) is a magnesium ore widely used in biomass gasification since the tar content of the produced gases during the biomass conversion process is significantly reduced in the presence of Dolomite [8-10]. In addition, this cat­alyst is relatively inexpensive and disposable, so it is possible to use it in bed reactors as primary catalysts as well as in secondary, downstream reactors. The studies related to the catalytic effect of dolomite during biomass gasification are mainly focused on reformation of higher molecular weight hydrocarbons (tar). Steam gasification of biomass in the presence of dolomite leads to the efficient removal of coke formed on the catalyst surface and thus product selectivity is significantly enhanced. On the other hand, olivine [(Mg, Fe)2SiO4], another effective catalyst for biomass gasification, is also an attractive material regarding stability in fluidized bed reactors [11] due to its attrition resistance. Olivines also possess very low surface areas (about 0.4 m2 g-1), normally being an order of magnitude less than those of dolomites. The advantages of both catalysts are their low price and high attrition resistance. However, olivines and dolomites have higher calcination temperatures and this restricts the effective use of both catalysts.

In fact, calcination of both materials leads to several unwanted phenomena such as losing tar conversion activity and catalyst stability, reducing surface area, etc.