Most power plants built in the last decade have been fueled by natural gas due to its availability, low cost, and low emissions. Going forward, natural gas-based plants are expected to continue to help meet our nation's new capacity requirements, due to lower capital costs, higher fuel efficiency, and shorter construction lead times.
About Natural Gas
Natural gas is a fossil fuel formed when layers of buried plants and animals are exposed to intense heat and pressure over thousands of years. The energy that the plants and animals originally obtained from the sun is stored in the form of carbon in natural gas.
In its pure form, natural gas is colorless, odorless, and highly combustible. When burned, it produces heat. Natural gas is a mixture of hydrocarbon gases, primarily methane. The term 'dry' is used to describe natural gas that is almost pure methane. The term 'wet' is applied to natural gas that contains other hydrocarbon gases, such as ethane, butane, and propane. When burned, natural gas produces fewer emissions than coal or oil.
Natural gas is used to generate electricity by three main processes:
- Combustion turbines use natural gas directly to fire the turbine, which drives the generator shaft;
- Steam turbines burn natural gas to create steam in a boiler, which is then run through a steam turbine; and
- Combined cycle units utilize a combustion turbine by burning natural gas (gas cycle); then the hot exhaust gases from the combustion turbine are used to boil water that operates a steam turbine (steam cycle) to generate electricity.
Natural Gas Technologies
Two technologies that are transforming the natural gas industry are the increased use of liquefied natural gas and natural gas fuel cells.
Liquefied Natural Gas
Cooling natural gas to about -260°F at normal pressure results in the condensation of the gas into liquid form, known as Liquefied Natural Gas (LNG). LNG can be particularly useful for the transportation of natural gas, since LNG takes up about one-six hundredth the volume of gaseous natural gas.
While LNG can be costly to produce, advances in technology are reducing the costs of liquification and regasification. When vaporized to gaseous form, LNG only burns in concentrations of between 5 and 15 percent mixed with air. In addition, LNG, or any vapor associated with LNG, does not explode in an unconfined environment. The increased use of LNG is allowing for the production of natural gas deposits that were previously economically unrecoverable. Although it currently accounts for only about one percent of natural gas used in the United States, it is expected that LNG imports will provide a steady, dependable source of natural gas for U.S. consumption.
Natural Gas Fuel Cells
Fuel cells powered by natural gas are a promising new technology for the clean and efficient generation of electricity. Fuel cells have the ability to generate electricity using electrochemical reactions as opposed to the combustion of fossil fuels.
Fuel cells work by passing streams of fuel (usually hydrogen) and oxidants over electrodes that are separated by an electrolyte. This produces a chemical reaction that generates electricity without requiring the combustion of fuel, or the addition of heat. When pure hydrogen is used as fuel, and pure oxygen is used as the oxidant, the reaction that takes place within a fuel cell produces only water, heat, and electricity. The use of fuel cells results in very low emission of harmful pollutants and the generation of high-quality, reliable electricity. The use of natural gas-powered fuel cells has a number of benefits, including:
- Clean Electricity: Fuel cells provide the cleanest method of producing electricity from fossil fuels. While a pure hydrogen, pure oxygen fuel cell produces only water, electricity, and heat, fuel cells in practice emit only trace amounts of sulfur compounds, and very low levels of carbon dioxide. However, the carbon dioxide produced by fuel cell use is concentrated and can be recaptured, without being emitted into the atmosphere.
- Distributed Generation: Fuel cells can come in compact sizes, allowing for their placement wherever electricity is needed. This includes residential, commercial, industrial, and even transportation settings.
- Dependability: Fuel cells are completely enclosed units, with no moving parts or complicated machinery. This means a dependable source of electricity, capable of operating for thousands of hours.
- Efficiency: Fuel cells convert the energy stored within fossil fuels into electricity more efficiently than traditional generation of electricity using combustion. This means that less fuel is required to produce the same amount of electricity. The National Energy Technology Laboratory estimates that when used in combination with natural gas turbines, fuel cell generation facilities can be produced that will operate in the 1- to 20-megawatt (MW) range at 70 percent efficiency, higher than the efficiencies that can be reached by traditional generation methods within that output range.
While the best current gas-based industrial steam boilers are 83-85 percent thermally efficient, achieving a targeted 95 percent or better thermal efficiency will require new technologies for heat recovery and transfer, as well as new sensors and controls that ensure real-time optimization of boiler performance.