Ethanol fuel is an alternative from petroleum (fossil) based fuels, which is said to be better for the environment. It is manufactured from the distillation/fermentation of carbon-based feedstocks such as starch and sugar crops -- maize, sweet sorghum, rice, potatoes, wheat, sugar beets, sugar-cane, even cornstalks, fruit, and vegetable waste. Production of ethanol from cellulosic biomass such as corn leaves and stalks has the potential to augment the feedstocks in the existing industry and become the technology of the future for fuel ethanol production. Ethanol fuel can be combined with gasoline at different percentages or can be used in its pure form as E100. The enzyme costs of converting cellulosic biomass into sugars for fuel ethanol production have been reduced approximately twenty-fold with newly developed technology.
A large variety of feedstocks is currently available for producing ethanol from cellulosic biomass. The materials being considered can be categorized as agricultural waste, forest residue, MSW, and energy crops. Agricultural waste available for ethanol conversion includes crop residues such as wheat straw, corn stover (leaves, stalks, and cobs), rice straw, and bagasse (sugar cane waste). Forestry waste includes underutilized wood and logging residues; rough, rotten, and salvable dead wood; and excess saplings and small trees. MSW contains some cellulosic materials, such as paper.
Energy crops, developed and grown specifically for fuel, include fast-growing trees, shrubs, and grasses such as hybrid poplars, willows, and switchgrass. Although the choice of feedstock for ethanol conversion is largely a cost issue, feedstock selection has also focused on environmental issues. Materials normally targeted for disposal include forest thinnings collected as part of an effort to improve forest health, MSW, and certain agricultural residues, such as rice straw. Although forest residues are not large in volume, they represent an opportunity to decrease the fire hazard associated with the dead wood present in many National Forests. Small quantities of forest thinning can be collected at relatively low cost, but collection costs rise rapidly as quantities increase.
Agricultural residues, in particular, corn stover, represent a tremendous resource base for biomass ethanol production. Agricultural residues, in the long term, would be the sources of biomass that could support substantial growth of the ethanol industry. At conversion yields of around 60 to 100 gallons per dry ton, the available corn stover inventory would be sufficient to support 7 to 12 billion gallons of ethanol production per year, as compared with approximately 1.4 billion gallons of ethanol production from corn in 1998. However, the U.S. Department of Agriculture (USDA) and other appropriate entities must undertake rigorous research on the environmental effects of large-scale removal of crop residues.
The cost of agricultural residues is not nearly as sensitive to supply as is the cost of forest residues, although the availability of corn stover could be affected by a poor crop year. The relatively low-rise in cost as a function of feedstock use is due to the relatively high density of material available that does not involve competition for farmland. In addition, the feedstock is located in the corn-processing belt, an area that has an established infrastructure for collecting and transporting agricultural materials. It is also located near existing grain ethanol plants, which could be expanded to produce ethanol from stover. Initially, locally available labor and residue collection equipment might have to be supplemented with labor and equipment brought in from other locations for residue harvesting and storage operations, if the plants involved are of sufficient scale.
Eventually, however, when the local collection infrastructure has been built up, costs would come down. Dedicated energy crops such as switchgrass, hybrid willow, and hybrid poplar are another long-term feedstock option.
The use of cellulosic biomass in the production of ethanol also has environmental benefits.
Converting cellulose to ethanol increases the net energy balance of ethanol compared to converting corn to ethanol. The net energy balance is calculated by subtracting the energy required to produce a gallon of ethanol from the energy contained in a gallon of ethanol (approximately 76,000 Btu).
Corn-based ethanol has a net energy balance of 20,000 to 25,000 Btu per gallon, whereas cellulosic ethanol has a net energy balance of more than 60,000 Btu per gallon. In addition, cellulosic ethanol use can reduce greenhouse gas emissions. Argonne National Laboratory estimates that a 2-percent reduction in greenhouse gas emissions per vehicle mile traveled is achieved when corn-based ethanol is used in gasohol (E10) and that a 24- to 26-percent reduction is achieved when it is used in E85. Cellulosic ethanol can produce an 8- to 10-percent reduction in greenhouse gas emissions when used in E10 and a 68- to 91-percent reduction when used in E85.
Depending on the feedstock and process design, ethanol production results in several by-products which may include crop residues, stillage, evaporator condensate, condensed solubles, spent cake and/or distillers grains, all of which have a high potential for methane production. Stillage, a residual of the distillation of ethanol from fermentation liquor, contains a high level of biodegradable COD as well as nutrients and has a high pollution potential. Up to 20 L of stillage may be generated for each liter of ethanol produced. Conversion of stillage to biogas and application of effluent to croplands results in a more sustainable ethanol production system.
Many ethanol plants minimize effluent discharges by evaporation of the stillage to produce evaporator condensate (used partially for make-up water) and condensed solubles. The evaporator condensate contains volatile fermentation products that can inhibit ethanol fermentation. Anaerobic digestion can remove these fermentation products and provide a liquid more suitable for process recycling. The distiller's grains and condensed solubles are normally blended for use in animal feed as dried distillers grains and solubles (DDGS). However, the current rapid expansion of ethanol production could lead to saturation of the feed market with DDGS, affecting the sale value of this by-product. Thus, there is an opportunity for biogas production from these by-products to offset facility energy requirements. In cellulosic ethanol production, non-fermentable hydrolysis products can also be converted to methane. Finally, crop residues may also be harnessed for biogas production, which can greatly improve the energy yield from ethanol production.
Ethanol is a much cleaner fuel than petrol (gasoline):
1. It is a renewable fuel made from plants
2. It is not a fossil-fuel: manufacturing it and burning it does not increase the greenhouse effect
3. It provides high octane at low cost as an alternative to harmful fuel additives
4. Ethanol blends can be used in all petrol engines without modifications
5. Ethanol is biodegradable without harmful effects on the environment
6. It significantly reduces harmful exhaust emissions
7. Ethanol's high oxygen content reduces carbon monoxide levels more than any other oxygenate: by 25-30%, according to the US EPA
8. Ethanol blends dramatically reduce emissions of hydrocarbons a major contributor to the depletion of the ozone layer
9. High-level ethanol blends reduce nitrogen oxide emissions by up to 20%
10. Ethanol can reduce net carbon dioxide emissions by up to 100% on a full life-cycle basis
11. High-level ethanol blends can reduce emissions of Volatile Organic Compounds (VOCs) by 30% or more (VOCs are major sources of ground-level ozone formation)
12. As an octane enhancer, ethanol can cut emissions of cancer-causing benzene and butadiene by more than 50%
Sulfur dioxide and Particulate Matter (PM) emissions are significantly decreased with ethanol.
Ethanol has enjoyed some success as a renewable fuel, primarily as a gasoline volume extender and as an oxygenate for high-oxygen fuels, an oxygenate in RFG in some markets, and potentially as a fuel in flexible-fuel vehicles. A large part of its success has been the Federal ethanol subsidy. With the subsidy due to expire in 2008, however, it is not clear whether ethanol will continue to receive political support. Thus, the future of ethanol may depend on whether it can compete with crude oil on its own merits.
Ethanol costs could be reduced dramatically if efforts to produce ethanol from biomass are successful. Biomass feedstocks, including forest residue, agricultural residue, and energy crops, are abundant and relatively inexpensive, and they are expected to lower the cost of producing ethanol and provide stability to supply and price. In addition, the use of corn stover would lend continued support to the U.S. corn industry. Analysis of NREL technological goals for cellulose ethanol conversion suggests that ethanol could compete favorably with other gasoline additives without the benefit of a Federal subsidy if the goals were achieved. Enzymatic hydrolysis of cellulose appears to have the most potential for achieving the goals, but substantial reductions in the cost of producing cellulase enzymes and improvements in the fermentation of nonglucose sugars to ethanol still are needed. The ban on MTBE in California could provide additional incentives for the development of cellulose-based ethanol. If ethanol were used to replace MTBE in Federal RFG, demand for ethanol in California would increase by more than 550 million gallons per year. California has vast biomass resources that could support the additional demand. In addition, the cost of transporting Midwest ethanol would allow cellulosic ethanol to compete favorably in the market.
Ultimately, ethanol’s future in RFG could depend on whether Congress eliminates the minimum oxygen requirement included in the CAAA90. Without the minimum oxygen requirement, refiners would have more flexibility to meet RFG specifications with blending alternatives, such as alkylates, depending on an individual refinery’s configuration and market conditions. Ethanol would still be valuable as an octane booster, however, and could make up for some of the lost volumes of MTBE.
Significant barriers to the success of cellulose-derived ethanol remain. For example, it may be difficult to create strains of genetically engineered yeast that are hardly enough to be used for ethanol production on a commercial scale. In addition, genetically modified organisms may have to be strictly contained. Other issues include the cost and mechanical difficulties associated with processing large amounts of wet solids. Proponents of biomass ethanol remain confident, however, that the process will succeed and low-cost ethanol will become a reality.