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Biomass is just a fancy word for plant or animal materials, and biomass fuels are combustable products made from biomass (usually from plant matter). Biomass fuels, or biofuels for short, include wood chips or other waste materials that are burned to generate heat and/or electricity, methane gas generated by microbes in landfills and animal waste, and liquid fuels used for transportation.
Liquid biofuels fall into two broad categories: ethanol made from plant sugars or starches, and biodiesel made from lipids.
What is it, and how is it made? Ethanol is a 2-carbon alcohol, which when combined with oxygen (combustion), releases energy and reaction products water and carbon dioxide (CO2):
In the U.S. , almost all ethanol is made from corn (exceptions include a plant in Idaho that uses, naturally, potato waste!) (Renewable Fuels Association, 2006). Brazil produces significant quantities of ethanol from sugar cane, and in Europe there have been limited efforts to produce ethanol from sugar beet and wheat. In theory, and demonstrated at small scale, it also is possible to make ethanol by digesting and fermenting the sugar building blocks of cellulose, the woody fiber that provides support to plant cell walls. Although research continues on so- called “cellulosic ethanol,” there currently are no production-scale plants that produce ethanol from this source.
Ethanol produced from corn kernels requires conversion of the corn starch to sugars, which are then fermented to produce alcohol. The majority of ethanol production (about 79%) in the U.S. is via the dry milling process (RFA, 2006). Corn kernels are ground, the starch is extracted and converted to sugars, and the sugars are fermented to form alcohol. The resulting product is purified to remove water, and a denaturant is added (to ensure ethanol is not diverted to direct human consumption) and shipped to blenders for mixing with gasoline.
Ethanol is added to gasoline as an oxygenate, to enhance combustion and reduce pollutant emissions. This use of ethanol as a gasoline additive (often 10% ethanol) is required for areas of the country that are not meeting air quality standards for ground-level ozone, a primary component of smog.
More recently, ethanol has been mixed with gasoline in an 85:15 percent combination (called E85) and used as a transportation fuel in Flexible Fuel Vehicles. General Motors, for example, produced quite a few FFV, or "ethanol-ready" vehicles, over the years but drivers of these vehicles usually operated them on standard gasoline.
Pros and cons. Supporters of ethanol stress its “renewable energy” credentials, home-grown source, and clean-burning properties relative to gasoline. Research is progressing into methods for converting annual grasses or agricultural and municipal wastes (so-called “cellulosic” sources) into ethanol, though commercial-scale production is not yet feasible. Amid the chorus of excitement over ethanol, however, there are growing rumbles of dissent. Opponents of ethanol fuels point to the amounts of petroleum-based energy required to produce each gallon of ethanol, as well as environmental impacts from intensive agriculture—use of pesticides, herbicides, fertilizer, and water, and conversion of natural terrestrial ecosystems —to produce ethanol feedstocks. Availability of high ethanol content fuels, such as E85, remains limited primarily to the Midwest because of distribution and cost factors. Vehicles capable of burning high ethanol-blend fuels remain a small percent of the total vehicle fleet.
Diesel is a petroleum product containing a mixture of hydrocarbons of varying carbon-chain lengths (i.e., with varying numbers of carbon atoms per molecule). Biodiesel contains a similar mix of long-chain carbon molecules, derived from plant oils. In some cases, used restaurant fryer oils can be burned in vehicles. [More info to come on the types and sources of biodiesel...]
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