Which form of energy transformation would result in the emission of the greenhouse gas methane?

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Burning wood
Burning municipal solid waste (MSW) or wood waste
Disposing ash from waste-to-energy plants
Collecting landfill gas or biogas

Humans need energy for just about any type of function they perform. Houses must be heated, energy is required for industry and agriculture and even within our own bodies a constant flow of energy takes place. All processes that provide us with the luxuries of every day live we can no longer live without require energy generation. This is an industrial process that can be performed using various different sources. These sources can be either renewable or non-renewable. Renewable energy sources are replaced in time and will therefore not run out easily. However, non-renewable energy sources are threatening to run out if our standard of usage becomes too high.

Nowadays many renewable energy sources are available for use, for example solar and wind energy and water power. Ironically, we still gain most of our energy from non-renewable energy sources, commonly known as fossil fuels (figure 1). The non-renewability of these sources will probably cause prices to rise up to a point where they are no longer economically feasible.

Figure 1: fossil fuel combustion is part of the carbon cycle (right)

Fossil fuels consist of deposits of once living organisms. The organic matter takes centuries to form. Fossil fuels principally consist of carbon and hydrogen bonds. There are three types of fossil fuels which can all be used for energy provision; coal, oil and natural gas. Coal is a solid fossil fuel formed over millions of years by decay of land vegetation. When layers are compacted and heated over time, deposits are turned into coal. Coal is quite abundant compared to the other two fossil fuels. Analysts sometimes predict that worldwide coal use will increase as oil supplies become scarcer. Current coal supplies could last for 200 years or more. Coal is usually extracted in mines. Since the middle of the 20th century, coal use has doubled. Since 1996 its application is declining again. Many developing countries depend on coal for energy provision because they cannot afford oil or natural gas. China and India are major users of coal for energy provision.
Oil is a liquid fossil fuel that is formed from the remains of marine microorganisms deposited on the sea floor. After millions of years the deposits end up in rock and sediment where oil is trapped in small spaces. It can be extracted by large drilling platforms. Oil is the most widely used fossil fuel. Crude oil consists of many different organic compounds which are transformed to products in a refining process. It is applied in cars, jets, roads and roofs and many other. Oil cannot be found everywhere on earth and consequentially, there have been wars on oil supplies. A well-known example is the Gulf War of 1991.
Natural gas is a gaseous fossil fuel that is versatile, abundant and relatively clean compared to coal and oil. Like oil, it is formed from the remains of marine microorganisms. It is a relatively new type of energy source. Until 1999, more coal was used than natural gas. Natural gas has now overtaken coal in developed countries. However, people are afraid that like oil, natural gas supplies will run out. Some scientists have even predicted this might happen by the middle or end of the 21st century. Natural gas mainly consists of methane (CH4). It is highly compressed in small volumes at large depths in the earth. Like oil, it is brought to the surface by drilling. Natural gas reserves are more evenly distributed around the globe than oil supplies.

Figure 2. Source: Energy Information Administration

Energy gained from burning fossil fuels is converted to electricity and heat in commercial power plants. When fossil fuels are burned carbon and hydrogen react with oxygen in air to carbon dioxide (CO2) and water (H2O). During this reaction heat is released which further amplifies the reaction. Electricity is generated by transforming mechanical energy (heat) to electrical energy in a turbine or generator. Power plants are very expensive to build, but once they are present efficiency in converting fuel to energy is very high. Most of the time more electricity is created than is actually needed, because electricity cannot be stored. Electricity demands vary throughout the year and provision must meet the peak load, which means the highest possible demand within a year. If demands significantly exceed a power plant's capacity to generate energy this may cause temporary blackouts.

Historically, fossil fuels were available in plentiful supply that was easy to obtain and transport. But now signals are given that the supply is running out and that it will take centuries to be replenished. Both sources and sinks of fossil fuels are limiting in their use. Sources are deep earth layers and sinks are for example air and water, which absorb fossil fuel waste products.
Burning fossil fuels is responsible for environmental issues that are high on the political agenda these days. Examples are greenhouse gas accumulation, acidification, air pollution, water pollution, damage to land surface and ground-level ozone. These environmental problems are caused by release of pollutants that are naturally present in fossil fuel structures, such as sulphur and nitrogen. Currently, oil burning is responsible for about 30% of all carbon dioxide emissions to air. Natural gas does not release as much carbon dioxide because of its methane structure. The largest emissions are cause by coal combustion. Coal may result in underground fires that are virtually impossible to extinguish. Coal dust can even explode. This makes coal mining a very dangerous profession. Oil may end up in soil or water in raw form, for example during oil spills or wars. This has caused many natural disasters in the past.

Why than are we still using fossil fuels to such a large extend? The answer is simple; because this is cheaper than any type of reasonable alternative we now know. Some environmental scientists predict that fossil fuel prices will increase in the coming century because of scarcity. This may cause an eventual transfer to renewable energy sources. According to Bjorn Lomborg, this will eventually happen. The IPCC is not sure whether a fossil fuel future can be entirely ruled out.

Sources

McKinney, M.L. and Schoch, R.M., Environmental Science, Systems and Solutions. Third edition, University of Tennessee, Knoxville USA 2003

Miller, G.T., Living in the Environment: Principles, connections and solutions. Fourth edition, Brooks/ Cole Publishing Company, Pacific Grove, USA 1999

Related pages

Climate change glossary

The greenhouse effect mechanism

Emissions and infrared absorption by greenhouse gases

Explanation of the IPCC SRES scenarios

The IPCC SRES scenarios: causes of climate change

The IPCC SRES scenarios: consequences of climate change

Overview of emission reductions for each country according to Kyoto

Possible policy measures to achieve Kyoto targets

Trading emission permits to achieve Kyoto targets

Discussions of the greenhouse effect

History of global warming

Perspectives on the greenhouse effect

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Biomass and biofuels made from biomass are alternative energy sources to fossil fuels—coal, petroleum, and natural gas. Burning either fossil fuels or biomass releases carbon dioxide (CO2), a greenhouse gas. However, the plants that are the source of biomass for energy capture almost the same amount of CO2 through photosynthesis while growing as is released when biomass is burned, which can make biomass a carbon-neutral energy source.1

Burning wood

Using wood, wood pellets, and charcoal for heating and cooking can replace fossil fuels and may result in lower CO2 emissions overall. Wood can be harvested from forests, from woodlots that have to be thinned, or from urban trees that fall down or have to be cut down.

Wood smoke contains harmful pollutants such as carbon monoxide and particulate matter. Modern wood-burning stoves, pellet stoves, and fireplace inserts can reduce the amount of particulates from burning wood. Wood and charcoal are major cooking and heating fuels in poor countries, but if people harvest the wood faster than trees can grow, it causes deforestation. Planting fast-growing trees for fuel and using fuel-efficient cooking stoves can help slow deforestation and improve the environment.

Switchgrass growing on a test plot for biomass production

Source: Wikimedia Commons (public domain)

Burning municipal solid waste (MSW) or wood waste

Burning municipal solid waste (MSW), or garbage, in waste-to-energy plants could result in less waste buried in landfills. On the other hand, burning garbage produces air pollution and releases the chemicals and substances in the waste into the air. Some of these chemicals, which are mostly related to the combustion of non-biomass materials in garbage, 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, which require waste-to-energy plants to use air pollution control devices such as scrubbers, fabric filters, and electrostatic precipitators to capture air pollutants.

Scrubbers clean emissions from waste-to-energy facilities by spraying a liquid into the combustion gases 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 furnace.

A waste-to-energy furnace burns at high temperatures (1,800°F to 2,000°F), which break down the chemicals in MSW into simpler, less harmful compounds.

Disposing ash from waste-to-energy plants

Ash from waste-to-energy plants 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 burning 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 put 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 could contaminate ground water. Some MSW landfills use ash that is considered safe as a cover layer for their landfills, and some MSW ash is used to make concrete blocks and bricks.

Collecting landfill gas or biogas

Biogas forms as a result of biological processes in sewage treatment plants, waste landfills, and livestock manure management systems. Biogas is composed mainly of methane (a greenhouse gas) and CO2. Many facilities that produce biogas capture it and burn the methane for heat or to generate electricity. This electricity is considered renewable and, in many states, contributes to meeting state renewable portfolio standards (RPS). This electricity may replace electricity generation from fossil fuels and can result in a net reduction in CO2 emissions. Burning methane produces CO2, but because methane is a stronger greenhouse gas than CO2, the overall greenhouse effect is lower.

Biofuels

Biofuels are generally cleaner burning than petroleum fuels made from crude oil, but production and use of biofuels do have effects on the environment. 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 oil palm trees for biodiesel) absorb CO2 as they grow and may offset the CO2 emissions when biofuels are produced and burned.

1 See Note 2: Accounting for carbon dioxide emissions from biomass energy combustion in the Environment section note of the Monthly Energy Review.

Last updated: December 7, 2021