One reason that, until now, the world has depended so heavily on natural gas and petroleum for energy and the manufacture of most organic materi­als is that gases and liquids are relatively easy to handle. Solid materials such as wood, on the other hand, are difficult to collect, transport, and pro­cess into components that can make desired products for energy. As such, solid materials seriously lack in continuous processability and render logisti­cal problems in their utilization.

Simply speaking, agricultural lignocellulose is inexpensive and renew­able because it is made via photosynthesis with the aid of solar energy. In addition, the quantity of biological materials available for conversion to fuel, chemicals, and other materials is virtually unlimited. Greater bio­mass utilization can also help ameliorate solid waste disposal problems. In 2009, 243 million tons of municipal solid waste (MSWs) were generated in the United States, which is equivalent to about 4.3 pounds of waste per person per day. Of this waste, 28.2% was paper and paperboard, 13.7% yard
clippings, 6.5% wood, and 14.1% food scraps [38]. Considering that some food scraps contain cellulosic materials, about 50% of the total municipal solid wastes is cellulosic and could be converted to useful chemicals and fuels [39].

Although lignocellulose is inexpensive, it involves transformational efforts to convert to fermentable sugars. Furthermore, as shown in Figure 4.4, lig­nocellulose has a complex chemical structure with three major components, each of which must be processed separately to make the best use of high efficiencies inherent in the biological process. The three major components of lignocellulose are crystalline cellulose, hemicellulose, and lignin.

A general scheme for the conversion of lignocellulose to ethanol is shown in Figure 4.5. The lignocellulose is pretreated to separate the xylose and, sometimes, the lignin from the crystalline cellulose. This step is very impor­tant, because the efficiency of pretreatment affects the efficiency of the ensu­ing steps. The xylose can then be fermented to ethanol, whereas the lignin can be further processed to produce other liquid fuels and valuable chemi­cals. Crystalline cellulose, the largest (around 50%) and most useful fraction, remains behind as a solid after the pretreatment and is sent to an enzymatic

hydrolysis process that breaks the cellulose down into glucose. Enzymes, the biological catalysts, are highly specific, hence, the hydrolysis of cellu­lose to sugar does not further break down the sugars. Enzymatic processes are capable of achieving a yield approaching 100%. The glucose is then fer­mented to ethanol and combined with the ethanol from xylose fermentation. This dilute beer (i. e., dilute ethanol-water solution) is then concentrated to fuel-grade ethanol via distillation and further purification such as pressure swing adsorption (PSA).

The hemicellulose fraction, the second major component at around 25%, is primarily composed of xylan, which can be easily converted to the sim­ple sugar xylose (or pentose). Xylose constitutes about 17% of woody angio — sperms and accounts for a substantially higher percentage of herbaceous angiosperms. Therefore, xylose fermentation or conversion is essential for commercial bioconversion of lignocellulose into ethanol or other biochemi­cals. Xylose is more difficult than glucose to convert or ferment to ethanol, based on the current level of science and technology. From the process stand­point, it would be more beneficial to find or develop a more robust and opti­mal micro-organism that can ferment both glucose and xylose to ethanol in a single fermenter with high yield and selectivity. Methods have been identi­fied using new strains of or metabolically engineered yeasts [23], bacteria, and processes combining enzymes and yeasts. Although none of these fer­mentation processes is yet ready for commercial use, considerable progress has been made.

Lignin, the third major component of lignocellulose (around 25%), is a large random phenolic polymer. In lignin processing, the polymer is broken down into fragments containing one or two phenolic rings. Extra oxygen and side chains are stripped from the molecules by catalytic methods and the resulting phenol groups are reacted with methanol to produce methyl aryl ethers. Methyl aryl ethers, or arylmethylethers, are high-value octane enhancers that can be blended with gasoline.