2.1 BIOMASS AS AN ENERGY SOURCE:

TRADITIONAL AND MODERN VIEWS

Biomass energy in its traditional sense is vegetation (mostly woody plants but also sun-dried grasses) and, extrapolating further up the food chain, animal manure, combusted as a direct source of heat for cooking and heating. For commercial purposes, wood was also the major energy substrate before the rapid development of coal extraction in the late eighteenth and nineteenth centuries ushered in the Industrial Revolution. Even in the early twenty-first century, traditional biomass still accounts for 7% of the total global energy demand, amounting to 765 million tonnes of oil equivalents (Mtoe) in 2002, and this is projected to increase to 907 Mtoe by

2030.1 Especially in Sub-Saharan Africa, much of this primary energy demand is unsustainable, as population growth outstrips the biological capacity of increasingly drought — and crisis-damaged ecosystems to replace continuous harvestings of firewood.

Biomass has, however, a much more modern face in the form of substrates for power generation, especially in combined heat and power production in OECD regions. For example, biomass-based electricity was 14% in Finland and 3% in Austria in 2002, and as discussed in chapter 1, the burning of sugarcane bagasse in Brazil is a significant energy source, 3% of national electricity in 2002.1 The use of biomass for power generation is increasingly attractive as a decentralized mechanism of supplying electricity locally or for isolated communities as well as cofiring with coal to reduce CO2 emissions. In contrast, even taken together, hydroelectricity, solar, geothermal, wind, tide, and wave energy may account for 4% or less of total global energy demand by 2030 (figure 2.1).

Thermal conversion (combustion, burning) of lignocellulosic biomass is an ancient but inefficient means of liberating the energy content of the biological material. Compared with solid and liquid fossil fuels, traditionally used biomass has only 0.33-0.50 of their energy densities (e. g., as expressed by the higher heat value).2 The higher the water content, the lower the energy density becomes and the more difficult is the task of extracting the total calorific equivalent, as the gas — phase flames are relatively cool. In a well-oxygenated process, the final products of biomass combustion are CO2, water, and ash (i. e., the inorganic components and salts); intermediate reactions, however, proceed by a complex group of compounds including carbon monoxide (CO), molecular hydrogen, and a wide range of

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Biomass utilization has generally been regarded as a low-technology solution for renewable energy and slow to generate public enthusiasm or investor funding for projects in most OECD countries. Nevertheless, energy analysis calculations have shown that combustion, pyrolytic, and gasification technologies are competitive in their energy conversion efficiencies with biotechnological production of fuel ethanol or biogas, that is, methane (figure 2.2).4 Such biomass-utilizing facilities are highly suitable for inputs that can be broadly described as “waste”; these materials include sawmill residues and forestry, herbaceous agriculture, and construction/ demolition waste that might otherwise be simply burned off or dumped in landfill sites. The appropriateness of thermochemical energy production is well illustrated by the commercialization of the SilvaGas process, originally demonstrated in a 10-ton/day plant at the Battelle Memorial Institute (Columbus, Ohio) and subsequently at a 200-ton/day plant in Vermont. The company Biomass Gas & Electric, LLC (Atlanta, Georgia, www. biggreenenergy. com) announced an agreement in October 2006 to construct a 35-MW plant capable of supplying electricity to 8% of residential properties in Tallahassee, Florida.5 By 2010, this plant will convert 750 tons/day of timber, yard trimmings, and clean construction debris to a medium-energy producer gas by a patented advanced gasification process. The product can substitute for natural gas in most industrial applications, and the process can, therefore, either generate electricity in situ in gas turbines or be injected into the natural gas distribution network. A second (20-MW) facility is under construction northeast

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FIGURE 2.2 Bioconversion efficiencies of thermochemical and microbiological processes for biomass. (Data from Lewis.4)

of Atlanta, adjacent to a landfill site that will divert construction and woody plant material — compared with untreated material, the SilvaGas process reduces solid waste by up to 99%.

To underline the contemporary nature of biomass fuels, U. S. Patent 7,241,321 awarded to Ecoem, LLC (Greenwich, Connecticut) in July 2007 describes an innovative procedure for the production of biomass fuel briquettes: finely ground wood chips, bark, sawdust, or wood charcoal powder can, after drying, be mixed with a vegetable oil to produce a material capable of being pressed into briquettes or ingots. This “organically clean biomass fuel” provides a clean burning, nontoxic fuel, presumably a superior fuel for domestic heating (as well as for igniting the charcoal on barbecues) and offers an alternative route for using vegetable oils as biofuels (chapter 6, section 6.1).

“Biomass” extends well beyond the collection of firewood. Between the late 1960s and the mid-1980s, a program in the United States explored seaweeds as rivals to terrestrial biomass plants because seaweeds have high growth rates, probably more than 100 dry tons/hectare/year, that is, comparable with the highest rates determined for sugarcane and such invasive/problem plant species as water hyacinth.6 The ocean margins are filled with such highly productive species, including the following:

• The giant kelp (Macrocystis pyrifera), whose massive fronds can be iepeatedly harvested and, when managed, a “plantation” might require reseeding only every five to ten years, if at all

• Other kelps, including the oriental kelp (Laminaria japonica), already harvested in tens of thousand of tons annually as food in China and Japan (both of which countries actively have considered this species as a possible biomass source)

• Other brown algae, including Sargassum spp., some of which harbor nitrogen-fixing bacteria and require little artificial fertilizer application

• Red algae such as Gracilaria tikvahiae, once considered as a possible summer crop in Long Island Sound and capable of growth without any attachment in closed or semiclosed cultures

• Green algae, many of which have very high biomass production rates

A feature common to many of these species is that anaerobic digestion with methane­forming bacteria is particularly efficient at converting their biomass to a combustible, widely used fuel gas.6

Research in China and Japan has continued until the present day, exploring the potential for the development of “marine bioreactors.”7 Genetic transformation techniques have also been adapted from the established body of knowledge acquired with terrestrial plants for indoor cultivation systems with such transgenic kelp.

Although the Iogen process relies on acid pretreatment and cellulase digestion, Dan­ish investigators rank other pretreatment methods (with short residence times, 5-6 minutes) as superior for subsequent wheat straw cellulose digestion by cellulase (24 hours at 50°C):8

Steam explosion (215°C) > H2O2 (190°C) > water (190°C) > ammonia (195°C) > acid (190°C)

The degradation of cellulose to soluble sugars was enhanced by adding nonionic surfactants and polyethylene glycol during enzymatic hydrolysis; the best results were obtained with a long-chain alcohol ethoxylate in conjunction with steam explo — sion-pretreated wheat straw, and the additives may either have occupied cellulase binding sites on residual lignin or helped to stabilize the enzyme during the lengthy digestion.8

In addition, the attention of Novozymes (one of the world’s major enzyme pro­ducers) has evidently been attracted post-2004 by wheat straw and the problems of its complete conversion to fermentable sugars:

• Arabinoxylans form an undigested fraction in the “vinasse” (the insoluble fermentation residue) after the end of a wheat-based bioethanol process; a mixture of depolymerizing enzymes from Hypocrea jecorina and Humi — cola insolens could solubilize the insoluble material and release arabinose and xylose — although at different rates with different optimal pH values and temperature ranges for the digestion.9

• A subsequent study mixed three novel a-L-arabinofuranosidases with an endoxylanase and a P-xylosidase to liberate pentoses from water-soluble and water-insoluble arabinoxylans and vinasse; much lower enzyme activi­ties were required than previously, and this may be a technology for pentose release before wheat straw-substrate fermentations.10

• Mixtures of a-L-arabinofuranosidases from H. insolens, the white-rot basidomycete Meripilus giganteus, and a Bifidobacterium species were highly effective in digesting wheat arabinoxylan, the different enzymes acting synergistically on different carbohydrate bonds in the hemicellulose structures.11

Arabinans constitute only 3.8% (by weight) of the total carbohydrate (cellulose, starch, xylose, and arabinose) in wheat straw, and a lack of utilization of all the pentose sugars represents a minor inefficiency. Releasing all the xylose (as a sub­strate for the engineered xylose-utilizing yeast) — xylose constitutes 24% of the total sugars — and completing the depolymerization of cellulose to (insofar as is possible) free glucose are more significant targets for process improvement.

As in Canada, wheat straw has been assessed to be a major lignocellulosic feed­stock in Denmark.[33] Substrate pretreatment studies from Denmark have, unlike at Iogen, concentrated on wet oxidation, that is, heat, water, and high-pressure O2 to hydrolyze hemicellulose while leaving much of the lignin and cellulose insoluble; vari­ous conditions have been explored, including the combining of thermal hydrolysis, wet oxidation, and steam explosion.12-15 The major concern with this type of pretreatment method is — with so many biomass substrates — the formation of inhibitors that are toxic to ethanologens and/or reduce ethanol yield.16-18 These degradation products of lignocellulosic and hemicellulosic polymers include aromatic acids and aldehydes as well as aliphatic carboxylic acids and sugar-derived components; laboratory strains of Saccharomyces cerevisiae exhibit differential responses to the growth inhibitors, and cell-free enzyme preparations with cellulase and xylanase activities were severely inhibited by chemically defined mixtures of the known wheat straw inhibitors, with formic acid being by far the most potent inhibitor.19 The positives to be drawn were, however, that even a laboratory strain could grow in 60% (w/v) of the wheat straw sub­strate and that a focused removal of one (or a few) inhibitors (in particular, formic acid) may suffice to render the material entirely digestible by engineered yeast strains.