A biorefinery using forest residues as its feedstock is called a forest biorefinery. The enormous scope of using biomass generated from forests for energy genera­tion has been excellently highlighted by Klass [45] by way of the statistics pre­sented in his book titled ‘‘Biomass for renewable energy, fuels and chemicals’’. According to this statistics, forests cover only about 9.5% of the earth’s surface or about 32% of the total land area but account for 89.3% of the total standing biomass and 42.9% of total annual biomass production. In terms of energy, forests alone could produce 1,030 quadrillion BTU/year which is equivalent to more than double the world’s total primary consumption of about 460 quadrillion BTU in 2005. Thus, forest biomass can be considered to be a very important source for feedstock of a biorefinery. Forest biomass can be categorized into two categories as shown in Table 1.10.

The use of forest biomass for energy generation had taken a back seat until a few years ago due to the depletion of forests as a result of felling trees for pro­ducing forest products like lumber, paper, and other items. However, this excellent source of fuel has again gained importance since the development of biorefinery concept as it is now being recognized as an attractive alternative for pulp and paper mills which see in it, the incentive of increasing their revenue by producing biofuels and other chemicals in addition to their core products, as also the forest waste can be processed in a typical forest biorefinery to yield a number of valuable products including biofuels, without jeopardizing the forest vegetation.

Forest biomass is mainly lignocellulosic in nature. Lignocellulose consists of cellulose (40-47%), hemicellulose (25-35%), lignin (16-31%), and other extrac­tives (2-8%), where the polysaccharides cellulose and hemicellulose are tightly cross-linked with lignin via ester and ether linkages with the purpose of providing structural rigidity to higher plants and trees and protecting the cell walls of plants from various external physical and chemical hazards. Therefore lignocellulose, in its native form, is highly refractory in nature and resistant to most hydrolytic processes which aim at extracting cellulose for further hydrolyzing it to fermen­table sugars which can be converted to biofuels. Hence, pretreatment of lig — nocellulose is essential before it can be used for other conversion processes in a biorefinery. Table 1.11 gives a list of the types of pretreatment that can be done on lignocellulose before it can be used in a biorefinery [46, 47].

The pretreatment of lignocellulosic feed stock serves the basic purpose of converting the native lignocellulosic biomass into a form where hydrolysis can be effectively achieved. Among these, the biological methods of pretreatment have a number of advantages in that, the equipment requirement is modest, no environ­mentally damaging waste products are generated, and hazardous chemicals and conditions are avoided. All this results in a significant amount of cost saving. However, the total pretreatment time required is very lengthy and there are chances of degradation of polysaccharide which may reduce the total fermentable substrate. Newer methods of pretreatment aim at not only improving hydrolysis but also carrying out fractionation, where the lignocellulosic biomass is converted to its core components—cellulose, hemicelluloses, and lignin. A lot of work still needs to be done, however, to carry out fractionation of lignocelluloses in a manner that is technically feasible and at the same time, economically viable. The conceptual ideas for the purpose have been proposed by a number of researchers. FitzPatrick et al. [47] have reviewed these techniques at length.

A majority of lignocellulosic biomass including forest biomass is used in a kraft mill where the lignocellulosic feedstock is processed to paper pulp, which serves as an important intermediate material for the generation of a variety of paper products. Figure 1.26a and b shows a schematic of a typical Kraft mill and how the biorefinery concept can be integrated into such a kraft mill to get multiple products including energy products and other value-added products [48].

The key requirement of integrating the biorefinery concept into a kraft mill is the recovery of hemicellulose which will enable its conversion into ethanol and/or other products. Mao et al. [49] introduced a ‘‘near neutral’’ process prior to pul­ping, for extraction of hemicellulose which otherwise ends up in the black liquor. However, this ‘‘near neutral’’ process modifies the energy balance of the kraft pulp mill. During the pretreatment process, approximately 10% of hemicellulose and lignin is extracted. This reduces the calorific value of the black liquor which is used for production of steam. Thus, less steam is produced whereas the steam consumed in the extraction process is greater. Marinova et al. [48] studied the effect of introducing a hemicellulose extraction and conversion stage into a Canadian hardwood Kraft pulp mill on the energy supply and demand of the mill. On the basis of their studies, they have proposed process optimization methods

have been shown to reduce the steam consumption in a Kraft mill by 5.04 GJ/Adt, thus making the process more cost-effective and economical.

The lignin, which remains after processing has the potential to serve as an important precursor for a wide variety of products. The US DOE gives a com­prehensive data regarding the possible products that can be obtained out of pro­cessing of this residual lignin [37]. Presently, very few Kraft mills separate and use lignin for producing other products.

Biorefinery Based on Industry (Process Residues and Leftovers), and Municipal Solid Waste

Residues and wastes comprise of the following:

— Municipal solid waste

— Municipal sewage sludge

— Animal waste

— Crop residues

— Industrial waste

— Forest waste.

Use of crop residues and forest waste for biomass conversion has already been discussed in the earlier sections. This section will focus specifically on the major source of biorefinery feedstock viz. municipal solid waste and industrial waste. Municipal soild waste also includes municipal green waste such as tree trimmings and gardening wastes, waste wood, and paper component of domestic rubbish.

Municipal solid waste can be defined as a combination of domestic, light industrial, and demolition solid waste generated within a community [6]. There are established priorities so far as disposal of municipal solid waste is concerned. Recycling, if it is economical to do so, enjoys the first priority. The green com­ponent of MSW can be separated out and used for compost or as mulch. The use of municipal green waste for energy production has the advantage that it reduces the waste load to municipal landfills which consequently reduces GHG methane arising from its decomposition. The anaerobic conversion process used for such conversions has already been discussed in ‘‘Anaerobic Digestion’’ The biorefinery integration into the waste conversion process is described in Fig. 1.27 which shows a current waste biorefinery and how an advanced future biorefinery could be developed to maximally tap the potential of waste-to-energy technology.