Other Biomass Options 351 Ethanol

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The chapter to this point has focused on converting coal, coal plus biomass, and biomass by thermochemical approaches to liquid fuels that fit directly into the existing fuel infrastructure. Biochemical approaches, such as converting corn grain or sugar cane (starch and sugar) to ethanol by fermentation, or by conversion of lignocelluloses to ethanol by depolymerization to sugars and then fermentation to ethanol are not evaluated in as much detail in this chapter. However, these latter processes may become important for producing ethanol for transportation fuel from biomass and for addressing CO2 emissions in transportation. Ethanol has a high octane number and contains 80,000 Btu per gallon vs. gasoline, which has 115,000-119,000 Btu per gallon. Thus, about 1.45 gal of ethanol are required to replace 1 gal of gasoline. The cost to produce grain ethanol, which is a mature technology, in 2007 was estimated at about $2.50 per gallon on an energy equivalent basis [26].

Biochemical conversion of starch from corn or wheat involves conversion of starch to six-carbon sugars and its fermentation to ethanol using natural yeast. Sugar from sugar cane is fermented directly. Fermentation technology is mature, commercially robust, and highly optimized. Sugar cane to ethanol is mature and practiced in large volumes in Brazil. Sugar cane-based ethanol has a LC GHG emission that is ~85% lower than petroleum-based fuels. This is in contrast to grain ethanol, as produced in the U.S. from corn, which produces about a 25% reduction in CO2 emissions over petroleum-derived fuels on an energy-equivalent basis because of all the fossil fuels used in grain ethanol production [27, 28]. Furthermore, ethanol from sugar cane could provide a roughly 90% reduction in petroleum imports vs. petroleum-derived fuels on an energy equivalent basis [27]. However, grain ethanol can reduce petroleum imports by a larger amount also because limited petroleum fuels are used in growing and harvesting corn, as with sugar cane ethanol. However, recent analysis of the full life-cycle of ethanol production from corn grain in the U.S. indicates that it produces only a small net energy gain over fossil fuel energy needed to produce it [27, 28]. Another major issue for grain ethanol is the volume challenge. The U.S. (in 2007) used 25-30% of its corn crop to produce about 3% of its transportation fuels, and this affected world food prices, etc. This is not sustainable.

Production of ethanol from the majority of the plant mass, the lignocellulosic portion, provides a larger feedstock source to convert; and if it is not grown as an energy crop on arable land, it need not have a negative impact on the food chain. Crop wastes can be used without affecting food production, if the not he proper agronomy practices are used [17]. Lignocellulose conversion is more difficult than converting starch in corn or sugar from sugar cane to ethanol, because it first requires that the sugar molecular components must be broken out of the hemicellulose and cellulose structures of the biomass. This produces six-carbon and five-carbon sugars which offer additional challenges to convert to ethanol than starch or sugar cane. This makes the process more challenging and more costly than grain ethanol production. Even at high crude oil prices, grain ethanol still required public subsidies to make it economic because increased demand drove up corn price as crude oil price increased. Cellulosic ethanol has the advantage of cheaper feedstocks and thus potentially, lower production costs. However today, much of this is offset by more process steps and longer process residence time, which drive up cost. Enzyme costs are also high but are expected to be reduced with volume; feedstock costs remain a major fraction of ethanol costs and are not expected to make major reductions. Several cellulosic ethanol commercial demonstration plants are under construction, and the next few years will probably experience much development in the technology.

For cellulosic ethanol, cost remains a major issue. Cost reductions are expected, but the extent achievable will be better known with the first round of demonstration plants. Paustain et al. [29] estimated the cost of cellulosic ethanol at $1.95 ± 0.65 per gallon ethanol for biomass costing $35 per dry tonne. This is about $2.90 ± 1.0 on an energy equivalent basis. At a more realistic biomass cost [17], the ethanol cost would be in the range of $3.0-$4.0 per gallon on an energy equivalent basis. If CO2 emissions in the transportation sector have a price associated with them, the relative economics would improve.

Other options, driven by the rapid advances that are occurring in the biological sciences and in synthetic biology, could markedly change biochemical production of liquid transportation fuels. Some of this is summarized briefly below; and if one or more are successful, the biofuels picture could change substantially.

3.5.2 Butanol

Biobutanol (butanol or butyl alcohol) is another potential entrant into the liquid transportation fuel (biofuel) market. Butanol is a four-carbon alcohol vs. the two-carbon alcohol, ethanol. Butanol made from biomass is typically referred to as biobutanol. Its longer hydrocarbon chain makes it fairly non-polar and thus more similar to gasoline. Butanol has a number of attractive features as a fuel. Its energy content is closer to that of gasoline (105,000 Btu/gal for butanol, vs. 115,000-119,000 Btu/gal for gasoline); it has a lower vapor pressure; it is not hygroscopic; and thus, it is not sensitive to water, and does not pose the problems of ethanol and water in the distribution system. It is less hazardous to handle and less flammable than gasoline, and it has an octane similar to gasoline. Thus, it can go directly into the existing fuel distribution system. It has been shown to work in gasoline engines without modification [30].

Several technologies to produce butanol are in the R&D phase. The one receiving the most attention is the acetone-butanol-ethanol (ABE) process, which was initially used to produce acetone for making cordite in 1916. The process produced about twice as much butanol as acetone, and also produced acetic, lactic and propi-onic acids in addition to ethanol and isopropanol. As currently being commercialized, this process involves the bioconversion of sugars or starches using a genetically engineered microorganism, Clostridium beijernickii BA101, which shows greater selectivity to butanol. There are also efforts to develop improved microorganisms that have increased reaction rate and selectivity in the conversion of sugars to butanol. This includes microorganisms or enzymes that can efficiently convert the different sugars that are obtained from cellulose and hemicellulose. Because butanol is toxic to the producing organism, the butanol concentration is limited to about 15-18 g/l even for the native organism that produces it. This problem is similar to the self-inhibition and resulting ethanol concentration limits that occur in grain ethanol and cellulosic ethanol production. Overcoming these limitations and increasing the conversion rate would positively impact these technologies.

Isobutanol is less toxic and is also a good fuel component. It has a higher octane than gasoline but needs to be used in blends because of its high melting point (78°F or 25.5°C). As such, a potentially more-promising approach to improving the process is to engineer organisms that produce mixtures of butanol and isobutanol. Atsumi et al. [31] have recently engineered E. Coli to produce isobutanol in high yield with high specificity from glucose.

An extension of this technology is the conversion of cellulose to butanol. This depends on the development of biotechnologies for the depolymerization of cellulose and hemicelluloses into the basic sugars. These are the same problems faced by cellulosic ethanol. These sugars can then be converted to butanol. The most important development would be microorganisms that could depolymerize the biomass components into sugars and then convert the sugars into butanol, in the same reactor to reduce capital cost. The cellulose approach to butanol is being studied, but the technology is in the research stage and is far from commercial.

Currently, butanol's main drawback is cost. To attack the cost challenge and initiate market entry, Dupont and BP have joined forces to retrofit an existing sugar-based ethanol plant to produce butanol using Dupont-modified biotechnology [32]. Improved next-generation bioengineered organisms could be available within the next few years [32]. According to DuPont, existing ethanol plants can be cost-effectively retrofit to butanol [30].

Because butanol production should markedly reduce some of the highly energy intensive operations associated with grain ethanol production, it should have improved LC GHG performance compared with grain ethanol. If the production route starts with lignocelluloses, the LC GHG reductions should be similar to those of cellulosic ethanol or sugar cane-based ethanol because in these processes the energy-intensive separations involve biomass-generated energy rather than fossil-based energy.

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Guide to Alternative Fuels

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