New Feedstock, Technologies, and Prospects
The various biomass feedstock used for producing biofuels can be grouped into two basic categories. The first is the currently available “first-generation” feedstock, which comprises various grain and vegetable crops. These are harvested for their sugar, starch, or oil content and can be converted into liquid fuels using conventional technology. The yields from the feedstock vary considerably, with sugar cane and palm oil currently producing the most liters of fuel per hectare. (See Figure 5.)
By contrast, the “next-generation” of biofuel feedstock comprises cellulose-rich organic material, which is harvested for its total biomass. These fibers can be converted into liquid biofuels only by advanced technical processes, many of which are still under development. Cellulosic biomass such as wood, tall grasses, and crop residues is much more abundant than food crops and can be harvested with less interference to the food economy and potentially less strain on land, air, and water resources. Promising energy crops include fast-growing woody crops such as willow, hybrid poplar, and eucalyptus, as well as tall perennial grasses such as switchgrass and miscanthus. Another potential “next-generation” feedstock is the organic portion of municipal solid waste.
The use of “next-generation” cellulosic biomass feedstock has the potential to dramatically expand the resource base for producing biofuels in the future. Over the next 10–15 years, lower-cost sources of cellulosic biomass, such as the organic fraction of municipal waste and the residues from biomass processing, crops, and forestry, are expected to provide the initial influx of next-generation feedstock. Dedicated cellulosic energy crops, such as switchgrass, poplar, and other fast- growing plants, are expected to begin supplying feedstock for biofuel production toward the end of this period, then expanding rapidly in the years beyond.