Encyclopedia of Renewable Energy. James G. Speight
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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 as well as the relevant standard test methods to use for each. The common international standard for biodiesel is EN 14214, while ASTM 6751 is most referenced in the United States. In Germany, the requirements for biodiesel are fixed in the DIN EN 14214 standard.
With regards to biodiesel, most of the world uses a system known as the “B” factor to state the amount of biodiesel in any fuel mix, in contrast to the “BA” or “E” system used for bioalcohol. Pure biodiesel is referred to as B100, while fuel containing 20% biodiesel is labeled B20.
The standards ensure that the following important factors in the fuel production process are satisfied: (i) complete reaction, (ii) removal of glycerin, (iii) removal of catalyst, (iv) removal of alcohol, and (v) ensuring the absence of free fatty acids. Basic industrial tests to determine whether the products conform to the standards typically include gas chromatography, a test that verifies only the more important of the variables above. Fuel meeting the quality standards is very non-toxic, with a toxicity rating (LD50) greater than 50 mL/kg.
One of the most important fuel properties of biodiesel and conventional diesel fuel derived from crude oil is viscosity, which is also an important property of lubricants. Ranges of acceptable kinematic viscosity are specified in various biodiesel and crude oil standards. Reducing viscosity is one of the main reasons why vegetable oils or fats are transesterified to biodiesel because the high viscosity of neat vegetable oils or fats ultimately leads to operational problems such as engine deposits. The viscosity of biodiesel is slightly greater than that of petrodiesel but approximately an order of magnitude less than that of the parent vegetable oil or fat. Biodiesel and its blends with petrodiesel display temperature-dependent viscosity similar to that of neat petrodiesel. Influencing factors are chain length, position, number and nature of double bonds, as well as the nature of the oxygenated moieties.
Classic biodiesel has a higher cloud point (temperature at which a fuel becomes hazy or cloudy and starts to gel) than petrodiesel. This makes its use impractical in cooler climates and limits its potential market. Other important chemical and physical properties described in ASTM standards for biodiesel are (i) total acid number, TAN, which indicates the presence of free fatty acids and carboxylic acids present, (ii) corrosion, which is the potential for copper corrosion, (iii) low temperature performance, which is described by the pour point and the cloud point, and (iv) oxidation stability.
In terms of the effects of biodiesel on fuel filters, use of biodiesel blends which do not meet up to the designated specifications, have shown to drastically reduce filter life. Blends greater then B20 may have enough of a solvent to break down the varnish deposits on the walls of existing fuel storage tanks or fuel systems. The breakdown of these varnish deposits will contaminate the fuel with particulate, which can cause fuel filters to plug rapidly.
Another disadvantage of biodiesel is that it tends