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BiomassFrequently Asked Questions :: Biomass

(sources: State Environmental Resource Center, USDOE/EERE)

Q: What is biomass?

A: Biomass is the organic matter produced by plants. The solar energy trapped by these plants can be converted to electricity or fuel.

Q: How is biomass used?

A: People have used biomass for heating and cooking for thousands of years. With today's technology, plant materials can be used to generate electricity, heat, or liquid fuels for motor vehicles that have substantially lower environmental impacts than traditional fossil fuels.

Q: What are the benefits of using biomass for energy generation?

A:As with many other renewable energy sources, biomass is capable of simultaneously addressing the nation's energy, environmental, and economic needs. Increased use of biomass for energy would lead to reduced greenhouse gas emissions, reduced dependence on foreign oil, an improved U.S. balance of trade, an improved rural economy, and the creation of a major new American industry.

Q: What economic benefits might a biomass program produce for my state?

A: Because biomass feedstocks - the actual crop to be used in energy generation - are typically bulky and costly to transport, conversion facilities will likely be located where the crop is grown. That means local jobs - rural economies will grow because of the development of a local industry to convert biomass to either electricity or transportation fuel. In addition, farmers will see their income rise thanks to the creation of new markets for their products - such as agricultural wastes and crops that can be grown on marginal land. Furthermore, increased investment in biomass conversion technologies can create high-skill, high-wage jobs for the producers of these technologies and the industry or utility that uses them.

Q: What are the environmental benefits of biomass?

A: The use of biomass energy provides a multitude of environmental benefits. It can help mitigate climate change; reduce acid rain; prevent soil erosion and water pollution; minimize pressure on landfills; provide wildlife habitat; and, help maintain forest health through better management.

The use of biomass will greatly reduce the nation's greenhouse gas emissions. Fossil fuels emit vast quantities of carbon dioxide into the atmosphere upon combustion, carbon that would ordinarily remain trapped underground. Biomass also releases carbon dioxide as it burns, but the plants need CO2 to grow - thus creating a closed-carbon cycle. All the CO2 released during the combustion of biomass materials is recaptured by the growth of these same materials. Unlike fossil fuels, with biomass combustion there is no net increase in carbon dioxide released into the atmosphere. In addition, substantial quantities of carbon can be captured in the soil through biomass root structures, creating a net carbon sink.

Biomass has other environmental benefits as well. The nation has many vast tracts of unused agricultural land - the byproduct of increasingly efficient agricultural techniques - that might otherwise be converted to residential or industrial use. These lands could instead be used to grow biomass crops that will restore soil carbon, reduce erosion and chemical runoff, and enhance wildlife habitat.

Of course not all forms of biomass produce all of these benefits and the use of some forms can actually produce significant environmental damage. Defining environmentally preferable biomass is a crucial step in capturing these benefits.

Q: What impact could biomass really have on our domestic energy supply?

A:Biomass currently provides about 2% of the electricity produced in the U.S., and, according to the American Biomass Association, it could easily supply 20%. As a result of the available land and agricultural infrastructure this country already has, biomass could conceivably replace all of the power that nuclear plants generate and do so in a sustainable fashion.

Q: How is electricity created with biomass?

A:Direct combustion is the simplest and most common method of capturing the energy contained within biomass. Usually these facilities (boilers) produce steam to use either within an industrial process, or to produce electricity directly. They can also produce heat, which is then captured for one purpose or another.

Direct combustion technology is very similar to that used for coal. Biomass and coal can be handled and burned in essentially the same fashion because coal is simply fossilized biomass heated and compressed over millions of years. The process which coal undergoes as it is heated and compressed deep within the earth adds elements like sulfur and mercury to the coal - elements which produce noxious emissions when burned. Since biomass does not contain these dangerous elements, combusting it produces no dangerous emissions.

Gasification is another method to generate electricity from biomass. Instead of simply burning the fuel, gasification captures about 65-70% of the energy in solid fuel (as compared to 20-30% for traditional combustion) by converting it first into combustible gases. This gas is then burned, as if it were natural gas, to create electricity, fuel a vehicle, power industrial applications, or be converted to synthetic fuels.

Q: Is biomass really a renewable source of energy?

A:Yes. If biomass is cultivated and harvested in a way that allows regrowth without depleting nutrient and water resources, it is a renewable resource that can be used to generate energy on demand, with little or no net contributions to global greenhouse gas emissions.

By burning biomass fuels we release no more carbon dioxide than would have been produced in any case by natural processes such as crop and plant decay and no more than will be absorbed by the plants as they regrow. Secondly, provided our consumption of biomass does not exceed our ability to continually supply the biomass feedstock we use, we have a renewable energy source whose use does not substantially disturb the natural biochemical cycle on a human time scale.

Q: Is municipal solid waste (MSW) considered biomass?

A: No. Although MSW is burned in the U.S., Europe, and elsewhere to generate electric power and heat, it contains inorganic materials such as plastics and metals and therefore cannot properly be considered biomass. It also contains a variety of potentially toxic materials such as creosote-treated wood, batteries that contain mercury, and other hazardous products, and therefore cannot be called biomass and is certainly not environmentally sound. MSW incineration also conflicts with recycling goals, diverting resources from more environmentally sound uses.

MSW incineration should be distinguished from landfill methane. Using methane captured from landfills to fuel power plants is far superior to allowing the methane and other air toxics generated by landfills to escape into the atmosphere (where the methane has a global warming potential 21 times that of carbon dioxide) or simply flaring the gas.

For a more extensive list of Frequently Asked Questions relating to biomass and bio-energy, visit the U.S. Department of Energy's Bioenergy Feedstock Information.

Q: Does ethanol require more energy to produce than it delivers as a fuel?

A: Ethanol has a positive energy balance. The energy content of ethanol is greater than the fossil energy used to produce it. This balance is constantly improving with new technologies. Over the last 20 years, the amount of energy needed to produce ethanol from corn has significantly decreased because of improved farming techniques, more efficient use of fertilizers and pesticides, higher-yielding crops, and more energy-efficient conversion technology.

Q: How does biofuels production affect food and feed demand and costs?

A: DOE's efforts on biofuels focus exclusively on developing non-food/feed based cellulosic feedstocks and ethanol production technologies.

Corn and soybeans, the major commodity crops, are only one possible source of biofuels. As researchers develop new, cost-effective methods for converting biomass material to liquid transportation fuels, a significant amount will be made from more abundant cellulosic biomass sources, including crop and forestry residues, energy crops such as switchgrass and sorghum, and sorted municipal wastes.

Crops grown to produce biofuels in the United States can also utilize a variety of agricultural lands. Future cellulosic crops will have the added benefit of being able to grow on marginal soils not suited for traditional agriculture. Less than one percent of farm land globally is currently used to grow biofuels crops.

DOE's approach to biorefining seeks simultaneously to maximize biopower and side-stream bioproduct production within the cellulosic fuel production system. Most corn demand results from cattle and poultry production. One major bioproduct of traditional corn ethanol production has always been Distiller's Dried Grains (DDGs) and/or Distiller's Dried Grain Solubles (DDGS), which can be used as a high-protein animal feed.

In addition, as found in a recent Texas A&M study, a variety of factors not listed above significantly affect food and feed demand and prices:

  • high oil prices (used both in transportation and production of food);
  • increasing demand from developing economies; and
  • speculative fund activities in futures markets.

Q: What is bioethanol?

A: Bioethanol is an alcohol made by fermenting the sugar components of biomass. Today, it is made mostly from sugar and starch crops. With advanced technology being developed by the Biomass Program, cellulosic biomass, like trees and grasses, are also used as feedstocks for ethanol production. Ethanol can be used as a fuel for cars in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions.

What are common feedstocks for bioethanol production?

A: Biomass is material that comes from plants. Plants use the light energy from the sun to convert water and carbon dioxide to sugars that can be stored, through a process called photosynthesis. Organic waste is also considered to be biomass, because it began as plant matter. Researchers are studying how the sugars in the biomass can be converted to more usable forms of energy like electricity and fuels.

Some plants, like sugar cane and sugar beets, store the energy as simple sugars. These are mostly used for food. Other plants store the energy as more complex sugars, called starches. These plants include grains like corn and are also used for food.

Another type of plant matter, called cellulosic biomass, is made up of very complex sugar polymers, and is not generally used as a food source. This type of biomass is under consideration as a feedstock for bioethanol production. Specific feedstocks under consideration include:

  • Agricultural residues (leftover material from crops, such as the stalks, leaves, and husks of corn plants)
  • Forestry wastes (chips and sawdust from lumber mills, dead trees, and tree branches)
  • Municipal solid waste (household garbage and paper products)
  • Food processing and other industrial wastes (black liquor, a paper manufacturing by-product)
  • Energy crops (fast-growing trees and grasses) developed just for this purpose

The main components of these types of biomass are:

  • Cellulose is the most common form of carbon in biomass, accounting for 40%-60% by weight of the biomass, depending on the biomass source. It is a complex sugar polymer, or polysaccharide, made from the six-carbon sugar, glucose. Its crystalline structure makes it resistant to hydrolysis, the chemical reaction that releases simple, fermentable sugars from a polysaccharide.

  • Hemicellulose is also a major source of carbon in biomass, at levels of between 20% and 40% by weight. It is a complex polysaccharide made from a variety of five- and six-carbon sugars. It is relatively easy to hydrolyze into simple sugars but the sugars are difficult to ferment to ethanol.

  • Lignin is a complex polymer, which provides structural integrity in plants. It makes up 10% to 24% by weight of biomass. It remains as residual material after the sugars in the biomass have been converted to ethanol. It contains a lot of energy and can be burned to produce steam and electricity for the biomass-to-ethanol process.

How do methane digesters work?

A: Biodigesters recover methane from animal manure through a process called anaerobic digestion. Here's how it works.

Methane and Anaerobic Bacteria

Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the "natural" gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produce natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or "digest" organic material in the absence of oxygen and produce "biogas" as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.)

Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a "digester" (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.

Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).

Anaerobic Digestion

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

A variety of factors affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances, such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the biodigester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester. In a properly designed system, heating generally results in an increase in biogas production during colder periods. The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex. Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH; for example, when too much "green" or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.

Sludge or Effluent

The material drawn from the anaerobic digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.

Source: DOE Energy Savers

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