There are a number of technological options available to make use of a wide variety of biomass types as a renewable energy source. Conversion technologies may release the energy directly, in the form of heat or electricity, or may convert it into another form, such as liquid biofuel or combustible biogas. Various methods of conversion of biomass into useful energy gain can be explained as follows:

2.4.1 Direct Combustion Processes

Feedstocks used are often residues such as woodchips, sawdust, bark, bagasse, straw, municipal solid waste (MSW) and wastes from the food industry. Direct combustion furnaces can be divided into two broad categories and are used for producing either direct heat or steam. Dutch ovens, spreader-stoker and fuel cell furnaces employ two stages. The first stage is for drying and possible partial gasification, and the second is for complete combustion. More advanced versions of these systems use rotating or vibrating grates to facilitate ash removal, with some requiring water cooling.

Lignocellulose provides a key sustainable source of biomass for transformation into biofuels and bioenergy products. However, lignocellulosic biomass is recal­citrant to biotransformation to sugars and other value-added products by microbial or enzymatic methods, which limits its use and the economically viable biocon­version [4]. The main goal of pretreatments is to increase the enzyme accessibility and improve the digestibility of cellulose and hemicellulose. Thus, different pretreatment methods and conditions (milling, irradiation, microwave, steam explosion, ammonia fiber explosion, supercritical CO2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv processes, wet oxidation, ozonolysis, dilute — and concentrated-acid hydrolyses, biological and green ILs pretreatments) should be investigated according to the process configuration selected for the subsequent hydrolysis and fermentation steps at low-energy consumption [42]. The lignocellulose crystalline should be accessible to enzymatic surface reactive area, and lignin is the main obstructive factors on enzymatic hydrolysis [43-45].

After the dissolution of cellulose and lignocellulosic materials into ILs, it is possible to recover the samples by simply adding a nonsolvent, such as water or ethanol into the solution. The X-ray spectra of the regenerated material showed that the X-ray diffraction signals from the crystalline regions of cellulose disap­peared after the dissolution-regeneration process with a fully amorphous material obtained, and the crystalline form of cellulose was transformed completely from cellulose I to cellulose II after regeneration from ILs. Such a transformation is anticipated to allow a greater accessibility for the hydrolytic enzymes to rapidly penetrate and hydrolyze the cellulose and hemicellulose. Therefore, the ILs-based pretreatment technologies for monosugars production have received much atten­tion recently. Pioneering investigations in this field suggest that cellulose regen­erated from IL solutions is subject to faster saccharification than untreated substrates, and it was found that the initial enzymatic hydrolysis rates were approximately 50-fold higher for regenerated cellulose as compared to untreated cellulose (Avicel PH-101) [46-49].

This pretreatment technology has also been applied to lignocellulosic materials. It was found that under unoptimized conditions, about 60% of the theoretical amount of glucose was enzymatically released from spruce when predissolved in [Amim] Cl and regenerated by the addition of water as an anti-solvent [21]. Since then, with the aim to develop an economic and efficient pretreatment technology, a lot of efforts have been devoted into optimizing this new technology by modifying the structures of ILs, using different origins of lignocellulosic materials and sim­plifying the sample regeneration and ILs recycle processes.

Due to the good solubility of 1-ethyl-3-methyl-imidazolium diethyl phosphate to cellulose and lignocellulose, Li et al. reported that the enzymatic hydrolysis of wheat straw pretreated by 1-ethyl-3-methyl-imidazolium diethyl phosphate at 130°C for 30 min is enhanced significantly, and the yield of reducing sugars reached 54.8% [50]. In 2010, Nguyen demonstrated that the integration of ammonia and [Emim][OAc] for rice straw pretreatment could reduce the cost of pure ILs pretreatment technology, which may make this emerging technology more adaptable to industrial application in enzymatic hydrolysis of different bio­mass. It was found that the combination treatment exhibited a synergy effect for rice straw with 82% of the cellulose recovery and 97% of the enzymatic glucose conversion. This cooperative effect showed over 90% of the glucose conversion even with a reduced enzyme usage and incubation time. Furthermore, the ILs could be recycled more than 20 times. Compared with the conventional pretreatments of ILs, this combined method for lignocellulosic biomass pretreat­ment was more economical and ecofriendly [51].

Besides using different ILs for the pretreatment, the integration of energy — efficient heating ways into the pretreatment will increase the efficiency, such as ultrasonic and microwave technology, which are widely used in chemistry reaction and biology researches. In 2010, Yang et al. reported that a new approach for in situ enzymatic saccharification of cellulose in ILs (ILs)-aqueous media was presented in which ultrasonic pretreatment was used to enhance the conversion of cellulose. Under optimized reactive conditions, higher conversion (95.48%) of cellulose was obtained in the media of aqueous-[Mmim] [DMP] by conducting the pretreatment of cellulose with ultrasonic heating, whereas the conversion of untreated cellulose was 42.77%. Further analysis of the pretreated sample showed that the application of ultrasonic resulted in the depolymerization of cellulose, which led to more efficient saccharification [52]. Microwave irradiation on cellulose dissolution pretreatment with ILs can not only enhance the solubility of cellulose in ILs, but significantly decrease the degree of polymerization of regenerated cellulose after IL dissolution pretreatment as well. The rate of enzy­matic hydrolysis of cotton cellulose was increased by at least 12-fold after IL dissolution pretreatment at 110°C and by 50-fold after IL dissolution pretreatment with microwave irradiation [53]. Accordingly, the amount of reducing sugars released from regenerated cellulose and [Bmim] [Cl] and [Emim] [OAc] with microwave irradiation were 17.1 and 15.6 mg/ml after 24 h, respectively. This implied that an approximately threefold enhancement in the cellulose hydrolysis yield could be achieved using IL dissolution pretreatment associated with microwave irradiation compared to that of untreated cotton cellulose (5.0 mg/ml after 24 h).

It is essential to minimize sugar losses, to increase solids concentration, and to decrease the cost of the pretreatment step in the biomass conversion. In order to increase sugar yields, efficient conversion and utilization of hemicellulosic sugars have become an important task and an opportunity to reduce the cost of bioenergy production [54]. It is also one of the biggest challenges for biomass pretreatment with ionic liquids, because the IL-pretreated xylan did not show distinct advan­tages on its enzymatic saccharification. On the contrary, some ILs may cause xylan degradation and loss during the dissolution and regeneration steps [55].

Lignin is not only one of the important components in lignocellulosic materials, but also a main barrier for enzymatic hydrolysis of lignocellulose biomass. One main purpose of biomass pretreatment is to partially remove lignin from the lignocellulosic materials. In 2009, Tan et al. demonstrated that 1-ethyl-3-meth — ylimidazolium alkylbenzenesulfonates IL ([Emim] [ABS]) could extract lignin from sugarcane plant waste, and a 93% extraction yield was achieved [56]. Further study by Lee et al. showed that IL [Emim] [OAc] could effectively extract lignin from triticale straw, flax shives, and wheat straw, and in the meantime cellulose digestibility of the recovered residues was significantly enhanced. The ionic liquid [Bmim] Cl was less efficient than [Emim] [OAc] for delignification of straw. It was found that higher temperatures and longer extraction time are beneficial for improved lignin extraction and cellulose hydrolysis of the residues, for example, 52.7% of acid insoluble lignin in triticale straw was extracted by [Emim][OAc] at 150°C after 90 min, yielding >95% cellulose digestibility of the residue in only 2 h. The results implied that the outstanding performance of ionic liquids-based pretreatment technology for enzymatic hydrolysis of lignocellulose is attributed to both the deconstruction of crystal structure of cellulose and the delignification during the dissolution and regeneration process.

Despite the efficiency of ILs-based pretreatment technology, the high cost of ILs and their recycle ability are the main challenges regarding to the scale-up industry application of this emerging technology. Most recently, Shill et al. suc­cessfully developed a three-phase system consisting of [Emim] [OAc], water, and cellulose forms following dissolution of biomass in the IL and subsequent addition of an aqueous concentrated phosphate solution. This process partially separated lignin from the cellulose in Miscanthus, and enhanced the rate of hydrolysis of the precipitated cellulose. ILs and concentrated phosphate solution were recycled and reused [57]. The design presented an ideal concept process for the biomass pre­treatment with ionic liquids.

4.5.1 How Green are ILs?

Complete life cycle assessments of ILs have been attempted to estimate the cumulative energy demand (for synthesis and disposal), the environmental impact and the economic viability (cost of chemicals, energy, disposal, personnel, equipment, and processing). Due to the complexity of the IL life cycle and the limited data available, life cycle assessments have remained a challenge. Previous attempts have instead focused on the optimization of single steps in the IL life cycle, such as the supply of materials and the IL synthesis [138, 139]. It was found that the IL synthesis is expensive [21], and requires large quantities of materials, solvents, energy, and also generates toxic emissions [138]. Therefore, their recy­cling and biodegradability are crucial not only for their economic viability but also to reduce their environmental impact [138].

Fermentation processes are exothermic as its products contain less energy than substrates. Theoretically, mass and energy yield of ABE fermentation is 37 and 94%, respectively calculated on the basis of energy of combustion and products ratio in the fermentation. During the study, it was suggested that yield of ABE fermentation might not be possible to meet its 100%, whereas product yield less than 25% can cause the economical unfeasibility even with any process development [50]. In the account of above fact, strain improvement may be a necessary step to enhance the theoretical yield. In this direction, many endeavors have been made to engineer the strain or transfer the gene in a heterogeneous host organism. But, to time none of genetically engineered strain produced higher yield than native organism [4]. However, the most valuable strain, C. beijerinckii BA101, has been developed through chemical mutagenesis from native organism, C. beijerinckii N-CIMB 8052 [80, 81]. C. beijerinckii BA101 can generate 19-20 g l-1 solvent, which is much higher than the native and other organisms [25, 40]. Through recent endeavors in process development for butanol production using C. beijerinckii BA101, improved solvent concentration (20-30 g l-1), solvent yield (0.30-0.50 g g-1), and reactor productivity (0.30-1.74 g l-1) have been achieved [64]. A high productivity of

15.8 g l-1 h-1 has also been achieved in immobilized reactor. Due to the high concentration of solvents, this organism leads ABE fermentation to be economical. An economic evaluation of ABE fermentation from corn using C. beijerinckii BA101 reported butanol cost of US$0.25 lb-1. The improvement in yield from 0.42 to 0.45 g g-1 resulted in lesser butanol cost of US$0.20 lb-1 [49].

Apart from the yield, other vital factor is feedstock in economics of ABE fermentation, it almost contributes to 60% of the total production cost of butanol [82, 83]. Utilization of none of starch and sugar-containing crops can make this fermentation economically feasible in the present scenario. Moreover the
continuous use of these food materials can cause the food shortage. On the basis of recent studies, cheaper agriculture biomass (lignocellulosic materials) and indus­trial wastes were found suitable for sustainable production of butanol. Still, efforts are being made for scaling-up the process for economical industrial production using lignocellulosic biomass.

In E. coli, the obvious and successful strategy to increase ethanol production has been the expression of the ethanologenic pathway from Z. mobilis, with the genes encoding PDC and ADH II organized in a single plasmid, the PET operon [73, 76], the latter integrated in the chromosome [134]. Subsequent selection of mutants with high ADH activity and disrupted fumarate reductase (for succinate produc­tion) originated KO11 strain that produces ethanol at a yield of 95% [135]. However, this strain is unable to grow in ethanol concentrations of 3.5% [199]. Evolutionary genetic engineering strategies were then, applied during a 3-month period, by alternating selection for ethanol tolerance in liquid media and selection for increased ethanol production in solid medium [199]. The resulting strain, LY01, was able to grow in ethanol concentrations of 5%. Coincidentally, this strain became more resistant to aldehydes (including HMF and furfural), organic acids, and alcohol compounds i. e. found in hemicelluloses hydrolysates [201-203]. However, LY01 strain performed poorly in mineral medium compared to rich medium [199]. To avoid dependence on nutritional supplementation, a new strain was produced from SZ110 [200], while the parental strain KO11 was engineered for lactate production in mineral medium [211].