Agricultural crops
Sugarcane, sugarbeet, corn, and sweet sorghum are agricultural crops presently grown commercially for both carbohydrate production and animal feeds. Sugarcane, corn and sweet sorghum are efficient at trapping solar energy because they are all "C4" plants. They use special biochemical pathways to recycle and trap carbon dioxide tht is lost through photorespiration. Sugarbeets are efficient because they store their carbohydrate in the ground. Sugarcane was the basis for the World's first renewable biofuel program in Brazil. Corn is the basis for the present renewable ethanol fuel industry in the United States. The sugars produced by these crops are easily fermented by Saccharomyces cerevisiae. The sucrose produced by sugarcane, sugarbeet, and sweet sorghum can be fermented directly after squeesing them from the crop. Corn traps its carbohydrate largely in the form of starch which must first be converted into glucose through saccharification with glucoamylase. The residues left over after removing fermentable sugars can also be utilized. In some cases they end up as animal feeds, but many agricultural residues can be converted into additional fermentable sugars through saccharification with cellulases and hemicellulases. The hemicellulosic sugars are not fermentable by S. cerevisiae, and must be converted to ethanol by pentose fermenting yeasts or genetically engineered organisms.
Bioenergy crops
Bioenergy crops include fast growing trees such as hybrid poplar, black locust, willow, and silver maple in addition to annual crops such as corn, sweet sorghum, and perennial grasses such as switchgrass. Many other crops are possible, and much more information is available from the Oak Ridge National Laboratory the National Renewable Energy Laboratory, and various USDA regional laboratories. The optimal crop will vary with growing season and other environmental factors. Most fast-growing woody and annual crops are high in hemicellulosic sugars such as xylose.
Agricultural residues
Corn stalks and wheat straws are the two agricultural residues produced in the largest quantities, however, many other residues such as potato and beet waste may be prevalent in some regions. In addition to quantity it is necessary to consider density and water content (which may restrict the feasibility of transportation) and seasonality which may restrict the ability of the conversion plant to operate on a year-round basis. Facilities designed to use seasonal crops will need adequate storage space and should also be flexible enough to accommodate alternative feedstocks such as wood residues or other wastes in order to operate year-around. Some agricultural residues need to be left in the field in order to increase tilth and to reduce erosion. But some residues such as corncobs can be removed and converted without much difficultly.
Wood residues
Softwood residues are generally in high demand as feedstocks for paper production, but hardwood timber residues have less demand and fewer competing uses. In the past, as much as 50% of the tree was left on site at the time of harvest. Whole tree harvest systems for pulp chips recover a much larger fraction of the wood. Wood harvests for timber production often generates residues which may be left onthe site or recovered for pulp production. Economics of wood recovery depend greatly on accessibility and local demand. Underutilized wood species include Southern red oak, poplar, and various small diameter hardwood species. Unharvested dead and diseased trees can comprise a major resource in some regions. When such timber has accumulated in abundance, it comprises a fire hazard and must be removed. Such low grade wood generally has little value and is often removed by prescribed burns in order to reduce the risk of wildfires.
Waste streams
Numerous waste streams could be exploited for ethanol production. They are often inexpensive to obtain, and in many instances they have a negative value attributable to current disposal costs. Some principal waste streams currently under consideration include mixed paper from municipal solid waste, cellulosic fiber fines from recycled paper mills, baggasse from sugar manufacture, corn fiber, potato waste, and citrus waste, sulfite waste liquors and hydrolysis streams from fiber board manufacture. Each waste stream has its own unique characteristics, and they generally vary from one source or time to another. Waste streams with lower lignin contents and smaller particle sizes are easier to deal with than those with higher lignin contents and larger particle sizes. Waste paper that has been treated by a chemical pulping process is much more readily converted than is native wood or herbaceous residue.
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