The previous sections have itemized the various technologies that have been developed to process lignocellulosic materials to mixtures of glucose, pentose sugars, and oligosaccharides suitable for fermentation by microbes with the production of ethanol. But what commercial and economic forces are presently acting to determine (or limit) the choice of lignocellulosic materials for the first large-scale bioethanol facilities?

The most detailed description so far available for the demonstration plant con­structed by the Iogen Corporation in Canada candidly lists the possibilities for start­ing materials:146

• Straws (wheat, barley, etc.) and corn stover as the leading candidates

• Cane bagasse as a localized leading candidate for some tropical locations

• Grass “energy crops” as possible second-generation candidates

• Native forest wood but difficult to process

• Tree farms too expensive because of demands of other markets

• Bark tree waste — cellulose and hemicellulose contents too low

• Sawdust and other mill waste — too expensive because of pulp and paper market demands

• Municipal solid waste and waste paper — too expensive because of paper demand

For a start-up lignocellulosic ethanol facility, there are crucial issues of cost and availability. An industrial plant may require, for example, close to a million tons of feedstock a year, that feedstock should be (for operational stability) as uniform as possible and as free from high levels of toxic impurities and contaminations as possible. Some materials (e. g., wood bark) have compositions that are incompatible with the high yields achieved in starch — and sugar-based materials, and some soft­woods demand unattractively high inputs of cellulase.147 Any lignocellulosic mate­rial is subject to some competitive use, and this may dictate cost considerations (table 2.10). Some of these direct competitors are long-established, mature industries, whereas others have unarguably “green” credentials for recycling waste materials or in renewable energy generation in OECD and non-OECD economies.148 Agricultural waste materials have, in addition, great potential as substrates for the “solid-state” fermentative production of a wide spectrum of fine chemicals, including enzymes, biopesticides, bioinsecticides, and plant growth regulators.149150

The enormous size of the potential supply of lignocellulose is frequently asserted; for example, “Lignocellulose is the most abundant renewable natural resource and substrate available for conversion to fuels. On a worldwide basis, terrestrial plants produce 1.3 x 1010 metric tons (dry weight basis) of wood/year, which is equivalent to 7 x 109 metric tons of coal or about two-thirds of the world’s energy requirement.”69

Some of this “wood,” however, represents trees grown (or harvested) as food crops and used as direct domestic or even industrial energy resources, while most is intimately involved in the global carbon cycle and in stabilizing the CO2 balance in the global ecosystem. Much of what is calculable as available biomass may not, therefore, be commercially harvestable on a short-term basis without the large — scale planting of dedicated “energy crops” such as fast-growing willow trees that are presently planted and harvested in Sweden for burning in district, local, and domestic heating systems.

TABLE 2.10

Competing Uses for Lignocellulosic Biomass Materials Considered for Bioethanol Production

Material Source Uses

Indeed, these were important issues addressed in a multiauthor projection of biomass options for 2030 to replace 30% of U. S. fuel demands.151 Ignoring questions of cost and efficiency of bioconversion, this study identified both forestry and agricultural resources for use in biofuels production. Three types of forest resources were quantified:

1. Primary (logging residues, removal of excess biomass in timberland fuel treatments, and fuel wood extracted from forestlands)

2. Secondary (mill residues)

3. Tertiary (urban wood residues from construction/demolition and recycling)

Similarly, lignocellulosic agricultural resources were divided among

1. Primary (crop residues, perennial grasses, and perennial woody crops)

2. Secondary (food/feed processing residues)

3. Tertiary (municipal solid waste recycling)

Approximately 280 million tons (dry weight) of such resources were estimated to be available by the time of the report on an annual basis (figure 2.12).[17] Augmentation

□ Agriculture ■ Forestry

of this supply could arise from programs to thin native forests strategically so as to reduce fire hazards — in California, for example, more than 750,000 dry tonnes/year could be generated by such activities.152 Data from Sweden (with its relatively low population density and high degree of forestation) suggest that 25% of the country’s gasoline requirements could be substituted by lignocellulose-derived ethanol from existing biomass resources.153

Of the two major immediately available sources of lignocellulosic material, however, field crop residues have the distinct advantage of being generated in close proximity to cereal crops intended (partly or entirely) for ethanol produc­tion. In August 2005, Abengoa Bioenergy (www. abengoa. com) began constructing the world’s first industrial-scale cellulosic bioethanol plant (to use wheat straw as the feedstock) immediately adjacent to its existing 195 million liters/year, Cereal Ethanol Plant (Biocarburantes de Castilla y Leon, BcyL), at Babilfuente, Salamanca, Spain, to dovetail supply trains and technologies. The biomass plant will process more than 25,000 tonnes of wheat straw and other materials to produce 5 million liters of ethanol annually in addition to preparing lignin, pentose sugars, and animal feed products as manufacturing outputs.

As an “energy crop” feedstock for bioethanol production in North America, a grass such as switchgrass (Panicum virgatum) has persuasive advantages—economic, social, and agricultural.154 But the technology for harvesting and processing grasses must be scaled up considerably from that used in, for example, silage fermentation. In the absence of such purpose-dedicated crops, intelligent choices will be mandatory to access sufficiently large supplies of suitable cellulosic feedstocks for start-up facilities.

How much biomass can any nation or region abstract without harming the environment? In a European context, this is an urgent question because any expansion of biomass use for industrial purposes brings in the threat of placing additional pressures on soil and water resources, farmland and forestry biodiversity, and may run counter to extant legislation aiming to encourage environmentally sound farming practices. The European Environment Agency naturally became interested in this issue as projections for large-scale biomass harvesting for bioenergy began to be more ambitious.155 Taking a cautious view of “harming the environment,” that is, with protected forest areas being maintained, residue removal excluded, with no grasslands or olive groves transformed into arable land, and with at least 20% of arable land maintained under environmentally friendly cultivation, the EEA estimated that by 2030 large amounts of biomass could be made available for ambitious bioenergy programs, reaching approximately 300 million tonnes of oil equivalent annually, or 15% of the total primary energy requirements. The calculated increase during the 27 years from 2003 were assumed to be the result of improved agricultural and forestry productivity and liberalization steps leading to more cultivable land being used with higher oil prices and imposed carbon taxes encouraging this expansion. The most revealing aspect of the data presented by the EEA was, however, a ranking of different plant species planted as annual crops with bioenergy as a significant end use: only a mixture of species could ensure that no increased environmental risks were likely, and it is interesting that maize (corn) ranks highest (most potentially damaging) as a monoculture (figure 2.13). The implication was that some of the

Maize Sugar Beet Potato Rape Seed Sunflower Grasses Wheat Mustard Seed Hemp Alfalfa Linseed

favored biofuel crops (maize, sugarbeet, and rapeseed) were in the highest risk category, and careful monitoring of the areas planted with those crops would be advised to minimize environmental damage; grasses were middle ranking purely on the grounds of increased fire risk.