Biofuels: The Role of Biomass in Meeting Energy Needs, Exams of Electrical and Electronics Engineering

Download Biofuels: The Role of Biomass in Meeting Energy Needs and more Exams Electrical and Electronics Engineering in PDF only on Docsity! Magazine R115Biofuels Chris Somerville The global energy market provides humans with about 370 exajoules of energy per year, which is equivalent to about 170 million barrels of oil per day (Box 1) or about 11.73 terrawatts (TW) per hour [1]. Approximately 95% of this energy comes from fossil fuels. Additionally, the International Energy Agency suggests that direct combustion of plant biomass provides about one-third of the energy needs in Africa, Asia and Latin America, and as much as 80 to 90% in the poorest countries of these regions [1]. Combining estimates of the magnitude of this form of biofuels consumption with the relatively small amount for which market numbers are available suggests that biofuels currently provide about 10% of all human energy use. There has recently been an upsurge of interest in the use of liquid biofuels for transportation in the developed world. This has been stimulated by a very rapid increase in the price of petroleum, strategic concerns about dependence on politically unstable regions of the world, and concerns about global climate change. The probable trajectory of this interest is beyond the scope of this article. But it is worth noting that there is no compelling evidence that even half of the recoverable petroleum has been used. Also, the regions that consume most of the energy are endowed with abundant coal reserves that are projected to be adequate to meet human energy needs for several hundred years. Coal can be converted into a wide variety of liquid fuels that can substitute for petroleum. Thus, it would be prudent to view the recent discontinuity in historical fossil fuel price trends as a transient imbalance rather than an indicator that fossil fuels are nearing depletion [1]. If concerns about climate change are ignored, there is not a pressing motivation to develop biofuels. Primer The linkage between climate change and biofuels arises from the fact that biofuels can be carbon neutral sources of energy. Energy from sunlight is collected by the photosynthetic system of plants and used to reduce and condense atmospheric CO2 into the chemicals that comprise the body of plants. When plants are burned, the energy resulting from oxidation is released as heat and the CO2 is recycled. If the biomass is simply burned, about 85% of inherent energy is available as heat and if that heat is used to produce steam for generators, approximately 35% of the energy can be recovered as electricity. With highly productive plants, such as Miscanthus giganteus, growing on good soils with adequate rainfall and favorable mean temperature, such as are found in central Illinois (Figure 1), more than 2% of annual incident solar insolation can be harvested as biomass [2]. If we use a value for average solar insolation of 120,000 TW, 2% solar conversion efficiency, and an energy recovery value of 50%, we could meet all human energy needs — 11.73 TW at the present level of consumption — by growing a plant such as Miscanthus on about 3.2% of the terrestrial surface area. Similar numbers can be obtained from actual yield measurements (Box 2). The calculation provides a tangible way of envisioning global bioenergy capacity. Current goals are much more modest, however; the US Secretary of Energy has established a goal for the US of obtaining 30% of transportation fuels from biomass by 2030. The reason for the current focus on using biomass for liquid fuels rather than for direct combustion is that coal is abundant and inexpensive, it is less expensive Box 1 Conversions 1 exajoule = 1018 joules1 1 exajoule = 9.48 x 1014 BTU 1 terrajoule = 1012 joules 1 terrajoule ~ 0.17 barrels of oil 1 kilojoule ~ 0.2777 watt hours 1 hectare = 2.471 acres 1 bu corn ~ 56 lb2 (25.4 kg) 1 bu soybean ~ 60 lb 1 bu canola ~ 49 lb 7.7 lbs vegetable oil ~ 1 gallon 1Other energy interconversions at http://www.mycomponents.co.uk/ energy.htm 2Exact value depends on moisture content of seedFigure 1. Miscanthus giganteus growing at the University of Illinois. Above-ground biomass is harvested in the fall or winter and the crop regrows from rhi- zomes during succeeding growing seasons. Image courtesy of Stephen P. Long and Emily Heaton, University of Illinois. Additional information at http://miscanthus.uiuc.edu/ Current Biology Vol 17 No 4 R116Box 2 Biomass energy yield per acre 1 ton of dry Miscanthus has 17,252 GJ of heat value [2] 1 acre of Miscanthus at 21 dry tons/acre1 ~ 362,292 GJ 1,021,275 acres of biomass ~ 370 EJ Terrestrial surface of earth ~32.123 x 109 acres 370 exajoules could be grown on 3.2% of the surface 1Stephen P. Long, University of Illinois, personal communication.to transport (per joule) and it burns with higher energy efficiency and less ash than biomass. Because trading of carbon credits decreases the effective price of biofuel in Europe, the use of biomass for direct combustion is being encouraged. In the US and other regions, where carbon trading is not yet implemented, economic and political forces favor production of liquid transportation fuels. In the following short overview, I have attempted to outline the prospects and problems associated with moving toward greater reliance on liquid biofuels. Corn and cane ethanol Sugarcane (Saccharum sp.) is a highly productive tropical grass that accumulates sucrose in the stem tissues. The stalks are crushed to produce a sucrose solution that can be fermented to produce a dilute ethanol solution (Box 3). The crushed stalks or ‘bagasse’ are burned to produce heat that is used to distill the ethanol from the fermentation broth and to produce excess electricity. In Brazil, where land suitable for growing sugarcane is abundant, about 4.2 billion gallons of cane ethanol was produced in 2005. The ethanol is mixed with gasoline and now comprises about 40% of all liquid transportation fuel. The automobile fleet in Brazil is largely composed of ‘flex-fuel’ vehicles that can use widely varying ratios of ethanol and gasoline. By contrast, only about 2% of the fleet in the US are flex-fuel vehicles; the remainder of the US fleet can not burn alcohol: gasoline mixtures containing more than 10% ethanol without mechanical modifications. Corn (Zea mays) is the largest US crop with ~81 million acres planted in 2005 yielding about 11.1 billion bushels of corn seed. Approximately 60% of the mass of corn seed is starch. The starch is released by grinding the seed in either a dry or wet process, cooked to gelatinize the starch, then enzymatically hydrolyzed to glucose at low cost and high efficiency and fermented. Following fermentation and separation of ethanol by distillation, the residual slurry of insoluble fiber, protein and lipid, called ‘distiller dry grains with solubles’ (DDGS), is used as animal food. Wet milling allows more complex separation techniques than dry milling and therefore the non-starch components may be used for higher value applications than animal food. However, wet mills are much more expensive to build and operate than dry mills, and are usually much larger in size. Thus, most of the corn processing plants in the US, which are owned and operated by farmer cooperatives, are dry mills. At present there are ~100 corn- to-ethanol plants operating in the US which, in 2005, produced 3.9 billion gallons of ethanol from about 14% of the US corn crop. The US Department of Agriculture and the corn growers association project that production of ethanol from corn grain will increase to ~12 billion gallons per year. Indeed, ~33 new ethanol plants are currently under construction in the US and many existing plants are undergoing expansion. At present the business is very profitable because corn ethanol can be produced for approximately $1 per gallon but, in the summer of 2006, was selling for approximately $3.5 per gallon. In addition, there are some legacy subsidies that add further profit. The technology required for cane or corn ethanol production is mature and most of the technical issues concern improvements in engineering related to the efficient use of heat and water. Unlike cane ethanol, however, which has an energy output:input ratio of about 8, for corn ethanol, calculations of the lifecycle costs of production have stirred substantial scientific debate. These calculations typically include things such as the energy costs of producing and distributing fertilizer, the cost of planting and harvesting, the costs of making the farm machinery and the factories that process the grain, in addition to the costs of converting grain to ethanol per se. The results have been controversial because certain assumptions about things such as heat reuse are inevitable in trying to compile all of the costs. A recent meta-analysis of all such calculations concluded that corn ethanol provides about 25% more energy than is consumed in its production [3]. Because of the low net energy ratio, corn ethanol does not offer an attractive long-term solution to meeting our energy needs in an environmentally sustainable way. But by stimulating the creation of a (profitable) industry, corn ethanol production involves a useful transition technology which will facilitate the development of more environmentally benign technologies for cellulosic ethanol production, as noted below. For instance, the availability of large quantities of corn ethanol provides a rationale to increase the production of flex fuel vehicles. Biodiesel In diesel engines, fuel is injected into a cylinder and then the air–fuel mixture is rapidly compressed so that it heats up to the point where combustion takes place. A wide variety of chemicals or mixtures of chemicals, including biologically produced fatty acids or lipids, will undergo combustion in conventional diesel engines. Glycerolipids, such as triacylglycerol, tend to lead to fouling of engine parts