Biofuels: Technology Overview
Ethanol
Ethanol, also called ethyl-alcohol or grain alcohol, is an alcohol that can be easily produced from common agricultural feedstocks such as corn and sugarcane. While ethanol has been widely used in a variety of non-energy related industries for many years, its favorable characteristics as a cool-burning, clean, renewable fuel have recently caused energy applications to dominate ethanol consumption and drive ethanol production.
Ethanol is most commonly produced through a dry milling procedure. The biomass feedstock is milled to a fine powder and slurried with water. This causes the starch component in the biomass feedstock to break down into its simple sugars (glucose). With the addition of yeast, these simple sugars are then fermented into ethanol. After fermentation, the mash is distilled to 200 proof. To make the ethanol undrinkable as well as to avoid any alcoholic beverage excise taxes, a denaturant (usually gasoline) is added to the ethanol.
Due to federal legislation, increased demand, and other market drivers, ethanol production has increased dramatically over the last two decades. Production has increased from 50 million gallons in 1980, to 2.81 billion in 2003. Correspondingly, ethanol production facilities are being constructed all across the
United States, with most new facilities having a production capacity over 50 million gallons per year. As of 2004, there are 78 ethanol production facilities operating in the
United States, and ten new facilities under construction.
Applications
Since ethanol can be used in most spark ignition engines with little to no engine modifications, ethanol use can directly displace gasoline use. Ethanol is already commonly used as a low percentage blend in automobiles; however, recent efforts from the ethanol industry are pushing to market higher percentage ethanol blends such as E85, which contains ethanol as 85 percent of the total fuel volume. In general, ethanol is suitable for any application in which gasoline is used. While this primarily pertains to the transportation sector, there is a variety of power production applications in which ethanol would be a suitable replacement for gasoline or natural gas.
Resource Availability
While most of the ethanol produced in the
United States today is derived from corn, ethanol is also produced from agricultural feedstocks that are high in simple sugars such as sugarcane and sugar beets. Currently, the sugar or starch components of plants are primarily used for ethanol production. It is also possible to utilize the more fibrous parts of biomass, such as the cellulose, hemicellulose polymers, and lignin to produce ethanol. While the sugar polymers in hemicellulose and cellulose are more resistant and difficult to break down using conventional dry milling processes, other production processes are being developed that allow these components to be fully utilized. Researchers have focused their efforts on acid hydrolysis and enzymatic hydrolysis technologies that are capable of breaking down or hydrolyzing the sugar polymers in lignocellulosic biomass such as trees, grasses, and waste biomass. Processes are also under development that gasify organic feedstock (including municipal waste) and synthesize ethanol from the product gas. These alternative processes hope to expand the biomass resource base and lower feedstock cost in ethanol production.
Environmental Impacts
Ethanol is a renewable, environmentally friendly fuel that is inherently cleaner than gasoline. Using ethanol reduces emissions of carbon monoxide, particulate matter, oxides of nitrogen, and other ozone-forming pollutants. Ethanol blended fuel can reduce carbon monoxide emissions by as much as 25 percent and greenhouse gas emissions by as much as 35-45 percent.
While the actual energy balance of ethanol was debated for several years, recently released results from a USDA study indicate that corn ethanol yields 67 percent more energy than what is required to produce it. It is further noted that the fossil fuels used in the process of producing ethanol are usually of domestic origin (coal and natural gas), rather than imported fuels. While the USDA’s study focused specifically on ethanol produced from corn, it is likely that ethanol production from other feedstocks can yield similar results.
Biodiesel
Biodiesel is a non-toxic, biodegradable, and renewable fuel that can be used in diesel engines with little or no modification. Biodiesel can be produced from oils and sources of free fatty acids such as animal fat, vegetable oil, and waste greases. Biodiesel is produced by removing excess hydrocarbons from these oils to create a shorter chain molecule that is chemically more comparable to diesel fuel. Sodium methoxide is added to the oil causing the mixture to settle into two simpler constituents: glycerin and methyl ester. The methyl ester is collected, washed and filtered to yield biodiesel. The glycerin has several commercial uses, the most common one being the manufacture of soap.
The actual facilities where biodiesel is created are relatively simple and easily scaled to meet local needs. Two kinds of biodiesel production facilities are in operation today: batch plants and continuous flow plants. Batch plants tend to be much smaller than continuous flow plants, and produce discrete “runs” of biodiesel. Continuous flow plants are usually much larger, run continuously, and are capable of implementing more efficient processes than those used in batch operations. Compared to ethanol, production of biodiesel is still in its infancy.
Applications
Biodiesel can directly displace diesel fuel in many applications. Biodiesel requires some special handling and storage procedures, and is limited to use during warm or temperate seasons/climates due to its viscous nature at low temperatures. No engine modifications are required for most static internal combustion (IC) engine applications. While there has been little study of biodiesel’s performance in gas turbine engines, there has been extensive research and testing of the fuel’s performance in traditional four-stroke IC engines. As such, biodiesel is already used in a variety of operations throughout the United States.
Biodiesel’s greatest market potential lies within the transportation sector. However, diesel is generally the fuel of choice for most IC engine power production, as such, there is substantial potential for biodiesel to replace diesel fuel in the energy sector. A variety of stationary engine products are available for a range of power generation market applications and duty cycles including standby and emergency power, peaking service, intermediate and base load power, and combined heat and power. Reciprocating engines are available for power generation applications in sizes ranging from a few kilowatts to over 5 MW.
Diesel engines have historically been the most popular type of reciprocating engine for both small and large power generation applications. However, in many industrialized nations, diesel engines are increasingly restricted to emergency standby or limited duty-cycle service because of air emission concerns. While biodiesel does improve the emissions of a diesel engine, the improvements are small when compared to the emissions reduction provided by natural gas powered engines.
Resource Availability
The most basic feedstock for biodiesel is vegetable oil, a long chain hydrocarbon. The oil can be derived from a variety of sources including: soybeans, cotton, palm, rapeseed, sunflower seeds, and restaurant waste greases. These feedstocks are generally categorized as virgin (fats and oils that have not been previously used) and recycled (fats, oils, and greases that have been previously used). While recycled feedstocks tend to have lower costs, they are limited by their availability and a variety of socioeconomic factors that may not be completely controllable. Virgin feedstocks are controlled by the available agricultural resources.
In Europe, rapeseed oil is the leading feedstock for biodiesel production. Pork and beef industries dictate the supply of white grease and tallow that is available for biodiesel production. The supply of recycled fats and oils is largely determined by the demand for fried food products, lubricants, and other oil dependent industries. While biodiesel demand has been known to have moderate impacts on agricultural production, it is unlikely that increases in the demand for biofuels will significantly impact the supply of animal fats or recycled greases.
Environmental Impacts
When compared to petroleum diesel, biodiesel offers a variety of benefits. Testing has shown that biodiesel has lower sulfur emissions and particulate emissions than regular diesel fuel. While biodiesel yields significantly lower sulfur emissions, particulate matter, and unburned hydrocarbons, emissions of nitrogen oxides can be higher for biodiesel than diesel depending upon engine configurations. Not only does biodiesel emit few harmful gases when combusted, but in almost every circumstance, fewer greenhouse gases are emitted in the production and transportation of biodiesel than are released in the production, transportation, and refinement of petroleum diesel. In addition to the aforementioned benefits, biodiesel boasts higher full-fuel cycle efficiency, and, in certain niche applications, a lower cost than petroleum diesel