Using biomass for energy can have positive and negative impacts
Using biomass for energy can have both positive and negative impacts on the environment. Using biomass for energy provides an alternative to using fossil fuels like coal, petroleum, or natural gas. Burning fossil fuels and burning biomass releases carbon dioxide (CO2), a greenhouse gas, but when the plants that are the source of biomass are grown, a nearly equivalent amount of CO2 is captured through photosynthesis.
Burning wood
Using wood, and charcoal made from wood, for heating and cooking can replace fossil fuels and may result in lower CO2 emissions overall. Wood may be harvested from forests or woodlots that have to be thinned, or it may come from urban trees that fall down or that have to be cut down. Wood smoke contains harmful pollutants like carbon monoxide and particulate matter. Burning wood in an open fireplace for heating is an inefficient way to produce heat, and it can also produce air pollution. Modern wood burning stoves and fireplace inserts are designed to reduce the amount of particulates emitted by the appliance. Wood and charcoal are major cooking and heating fuels in poor countries, and the wood may be harvested faster than trees can grow. This results in deforestation. Planting fast-growing trees for fuel and using fuel-efficient cooking stoves can help slow deforestation and improve the environment.
Burning municipal solid waste (MSW) or wood waste
Producing energy by burning municipal solid waste (MSW, or garbage) and by burning wood waste in facilities like waste-to-energy plants means that less waste must be buried in landfills. Waste-to-energy plants produce air pollution when MSW is burned to produce steam or electricity. Burning garbage also releases the chemicals and substances in the waste. Some of these chemicals can be hazardous to people and the environment if they are not properly controlled.
The U.S. Environmental Protection Agency (EPA) applies strict environmental rules to waste-to-energy plants, and it requires that waste-to-energy plants use air pollution control devices, such as scrubbers, fabric filters, and electrostatic precipitators to capture air pollutants.
Scrubbers clean emissions from these facilities by spraying a liquid into the chemical gas to neutralize the acids present in the stream of emissions. Fabric filters and electrostatic precipitators also remove particles from the combustion gases. The particles—called fly ash—are then mixed with the ash that is removed from the bottom of the waste-to-energy plant's furnace.
A waste-to-energy furnace burns at high temperatures (1,800°F to 2,000°F) that make complex chemicals break down into simpler, less harmful compounds.
Disposing of ash from waste-to-energy plants
Ash can contain high concentrations of various metals that were present in the original waste. Textile dyes, printing inks, and ceramics, for example, may contain lead and cadmium.
Separating waste before combustion can solve part of the problem. Because batteries are the largest source of lead and cadmium in municipal waste, they should not be included in regular trash. Florescent light bulbs should also not be included in regular trash because they contain small amounts of mercury.
The EPA tests ash from waste-to-energy plants to make sure that it is not hazardous. The test looks for chemicals and metals that would contaminate ground water. Ash that is considered safe is used in municipal solid waste landfills as a cover layer. Ash is also used to build roads and to make cement blocks.
Collecting landfill gas or biogas
Biogas is composed mainly of methane and CO2 that forms as a result of biological processes in sewage treatment plants, waste landfills, and livestock manure management systems. Many facilities that produce biogas also capture and burn the biogas for heat or electricity generation. The electricity generated from biogas is considered renewable and it is used in many states to meet state renewable portfolio standards (RPS). This electricity may replace electricity produced by burning fossil fuels and could result in a net reduction in CO2 emissions.Liquid biofuels: ethanol and biodiesel
Ethanol and biodiesel were the fuels used in the first automobile and diesel engines, but lower-cost gasoline and diesel fuel made from crude oil became the dominant vehicle fuels. The federal government has promoted ethanol use in vehicles to help reduce oil imports since the mid-1970s. In 2007, the government set a target to use 36 billion gallons of biofuels by 2022. As a result, nearly all gasoline now sold in the United States contains some ethanol.
Biofuels may be considered carbon-neutral because the plants that are used to make biofuels (such as corn and sugarcane for ethanol, and soy beans and palm oil trees for biodiesel) absorb CO2 as they grow and may offset the CO2 produced when biofuels are made and burned.
Growing plants for biofuels is controversial because the land, fertilizers, and energy used to grow biofuel crops could be used to grow food crops instead. Also, in some parts of the world, large areas of natural vegetation and forests have been cut down to grow sugar cane for ethanol and soybeans and palm-oil trees to make biodiesel. The U.S. government supports efforts to develop alternative sources of biomass that do not compete with food crops and that use less fertilizer and pesticides than corn and sugar cane. The U.S. government also supports methods to produce ethanol that require less energy than conventional fermentation. Ethanol can also be made from waste paper, and biodiesel can be made from waste grease, oils, and even algae.
Ethanol and gasoline blended with ethanol burn cleaner and have higher octane ratings than pure gasoline, but they have higher evaporative emissions from fuel tanks and dispensing equipment. These evaporative emissions contribute to the formation of harmful, ground-level ozone and smog. Gasoline requires extra processing to reduce evaporative emissions before it is blended with ethanol. Biodiesel combustion produces fewer sulfur oxides, less particulate matter, less carbon monoxide, and fewer unburned and other hydrocarbons, but it does produce more nitrogen oxide than petroleum diesel.