Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ after compression, and is approximately between 10 and 40% of the bulk density of most fossil fuels; however, liquid biofuels have comparable values. Since biomass materials are more reactive, with higher ignition stability, they are generally easier to gasify and thermochemically process into methanol and hydrogen (higher-value fuels). Coal ash may contain toxic metals and small amounts of other trace contaminants, while biomass ash may be used as a soil amendment to assist in replacing nutrients removed by harvest. Bioethanol possesses approximately 70% of the heating value of crude oil distillates, but its sulfur and ash contents are appreciably lower.

Ethanol has a lower energy density than gasoline, so for a given volume of gasoline, a larger volume of ethanol is needed to produce an equivalent amount of energy. Unfortunately, when ethanol is used to power vehicles, it leads to lower gas mileage, since energy density is correlated with gas mileage. Ethanol, which has approximately a 30% lower energy density than gasoline, contributes to a reduction in fuel mileage when it is mixed with gasoline, as now commonly practiced in the United States. Another problem with ethanol as an auto fuel is it is extremely hygroscopic. Ethanol is difficult to separate from water and readily absorbs water from its surrounding. This is a problem in the transport and storage of ethanol, as well as the purification of ethanol from water. Ethanol-containing fuels can easily absorb water, and decontamination (i.e., removal of water from fuel) is difficult. The difference between the properties of gasoline and ethanol also causes compatibility problems in engines. Biodiesel is the only alternative fuel to have fully completed the health effects testing requirements of the United States Clean Air Act.

      The use of biodiesel in a conventional diesel engine results in substantial reduction of unburned hydrocarbon derivatives, carbon monoxide, and particulate matter compared to emissions from diesel fuel. In addition, the exhaust emissions of sulfur oxides and sulfates (major components of acid rain) from biodiesel are essentially eliminated compared to diesel. Of the major exhaust pollutants, both unburned hydrocarbon derivatives and nitrogen oxides are ozone or smog-forming precursors. The use of biodiesel results in a substantial reduction of unburned hydrocarbon derivatives. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle of the engine and testing methods used.

      Based on engine testing, using the most stringent emissions testing protocols required by EPA for certification of fuels or fuel additives in the US, the overall ozone-forming potential of the speciated hydrocarbon emissions from biodiesel was nearly 50% less than that measured for diesel fuel. In a review published in 2009,⁴³ the environmental indicators related to biofuel production were examined.

      The postulated superior properties of agrofuels when compared with fossil fuels, as we have seen, must be weighed very carefully among the various factors of cost, environmental impact, energy density, chemical composition and availability, and life cycle of food crops. No doubt, increasing advances in technology will tip the scales in favor of biofuels.

      In terms of fuel properties, one of the largest issues seems to be overall greenhouse gas emissions from the various biofuels when compared with crude oil fuels. To estimate the impacts of increases in renewable and alternative fuels on greenhouse gas emissions, it is necessary to account for the entire fuel lifecycle including fossil fuel extraction or feedstock growth, fuel production, distribution, and combustion.

      The fuels are compared on an energy equivalent or Btu basis. Thus, for instance, for every Btu of gasoline which is replaced by corn ethanol, the total lifecycle greenhouse gas emissions that would have been produced from that Btu of gasoline would be reduced by 21.8%. These emissions account not only for CO₂, but also methane and nitrous oxide.

      It is generally accepted that biofuels have the potential to drastically lower carbon-dioxide emissions than fuels derived from crude oil, but in many instances, this is not the case. For example, ethanol made from corn requires a substantial amount of energy in fertilization, irrigation, harvesting, and fermentation processes and most of this energy comes from fossil fuels. As a result, some ethanol production scenarios emit more lifecycle carbon-dioxide emissions than gasoline. Cellulose-based ethanol, however, allows for more efficient and cost-effective fuel production, and the carbon footprint is decreased.

      However, the use of biodiesel in a conventional diesel engine results in substantial reduction of unburned hydrocarbons, carbon monoxide, and particulate matter compared to emissions from diesel fuel. In addition, the exhaust emissions of sulfur oxides and sulfates (major components of acid rain) from biodiesel are essentially eliminated compared to diesel. Of the major exhaust pollutants, both unburned hydrocarbons and nitrogen oxides are ozone or smog-forming precursors. The use of biodiesel results in a substantial reduction of unburned hydrocarbons. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle of the engine and testing methods used.

      The postulated superior properties of agrofuels when compared with fossil fuels, as we have seen, must be considered and compared very carefully among the various factors of cost, environmental impact, energy density, chemical composition, and availability and life cycle of food crops.

      Biofuels – Second Generation

      Second-generation biofuel production processes can use a variety of non-food crops. These include waste biomass, the stalks of wheat, corn, wood, and special-energy-or-biomass crops (e.g., Miscanthus). Second-generation biofuels use biomass-to-liquid technology, including cellulosic biofuels from non-food crops. Second-generation biofuels include biohydrogen, biomethanol, Fischer-Tropsch diesel, mixed alcohols, and wood diesel.

Second-generation biofuels (also called advanced biofuels) made from nonfood sources hold significant promise as a low-carbon, renewable transportation fuel that can complement traditional crude oil-based fuels in meeting the future energy needs of the world. Technologies that can convert cellulosic biomass, often regarded as a waste material, into transportation fuels are becoming popular. Examples of cellulosic biomass include: (i) agricultural wastes, such as corn stalks and husks, (ii) forestry wastes, such as wood chips and tree trimmings, (iii) fast-growing trees and grasses grown as energy crops, (iv) waste paper, and (v) food processing wastes

      Although using cellulosic biomass as a source of new transportation fuels has obvious advantages, these materials have different chemical structural bonds than food-based crops and are difficult to break down, especially on a large scale. These second-generation fuels may play an important role in diversifying the energy sources of the world and curbing greenhouse gas emissions.

      Cellulosic ethanol production uses non-food crops or inedible waste products and does not divert food away from the animal or human food chain. Lignocellulose is the woody structural material of plants. This feedstock is abundant and diverse, and in some cases represents a significant disposal problem. The discovery of the fungus Gliocladium roseum points toward the production of so-called myco-diesel from cellulose. This organism was recently discovered in the rainforests of northern Patagonia and has the unique capability of converting cellulose into medium length hydrocarbon derivatives typically found in diesel fuel.

      See also: Biofuels – First Generation, Biofuels – Third Generation.

      Biofuels – Specifications and Performance

      ASTM International (ASTM), formally known as the American Society for testing materials, is an international organization which develops and publishes information on the technical standards of various products, materials, systems, and services. It is one of the largest and most highly regarded standards development organizations in the world. The available literature on the performance of biofuels when compared with traditional fossil fuels normally uses ASTM and ISO (International Standards Organization) specifications and parameters. The specifications provide details on requirements for fuel characteristics СКАЧАТЬ