Fossil-Fuel Technology Basics

Fossil-fuel technologies run the gamut from 19th-century boiler and turbine technologies to recent innovations like microturbines and fuel cells. They also range in size from small units fit for a home to power plants capable of energizing entire cities. To learn more, choose among the following items:

Boiler and Steam Turbine Systems

Most of the electricity produced in the United States today comes from coal. Coal-fired power plants burn coal in a boiler to produce steam, which is then routed to a turbine causing it to spin. The turbine shaft is connected to a generator that converts the mechanical spinning energy into electricity. In some cases, oil, natural gas, or wood is burned in the boiler instead of coal.

Coal-fired power plants can cost twice as much to build as a comparably sized gas plant, but the coal itself is cheaper than natural gas. While gas prices can fluctuate considerably, the cost of producing a megawatt-hour of electricity from natural gas is typically several times the cost of producing the electricity with coal. The volatility of gas prices and supply can make coal more attractive—supplies are plentiful and prices are stable. But coal plants produce more atmospheric pollutants than gas-fired plants: twice as much carbon dioxide and virtually all of the sulfur emissions that lead to acid rain.

To learn more about coal and other conventional boiler and turbine technologies, visit one of the following links:

Combustion (Gas) Turbines

Combustion turbines can run on natural gas or low-sulfur fuel oil and are designed to start quickly to meet the demand for electricity during peak operating periods. The turbines operate on the same general principle as a jet engine. Air enters at the front of the unit and is compressed, mixed with natural gas or oil, and ignited. Combustion produces a hot gas that expands through turbine blades to turn a generator and produce electricity.

Because gas turbines are compact, light weight, quick starting, and simple to operate, they are used widely by industry and others in distributed applications (up to about 25 MW in capacity) as well as for centralized power generation (up to about 250 MW). The exhaust heat, which is normally released into the atmosphere, is sometimes used to boil water, producing steam that powers a second turbine. This is referred to as a "combined cycle" system since two separate processes or cycles are derived from one fuel input to the primary turbine. In other cases, the exhaust heat can be captured and used in cogeneration (combined heat and power) applications for space or water heating, or for running thermally activated equipment, such as absorption chillers. The use of exhaust heat can more than double the efficiency of the system.

Gas-fired power plants can be built more quickly and cheaply than coal-fired plants, and typically get regulatory approval much more easily. To learn more, visit the California Energy Commission's (CEC's) Combustion Turbines Web site.

Reciprocating (Diesel) Engines

The reciprocating, or piston-driven, engine is a widespread and well-established technology. Also called the internal combustion (IC) engine, reciprocating engines require fuel, air, compression, and a combustion source to function. Depending on the ignition source, they generally fall into one of two categories:

  1. Spark-ignited engines, typically fueled by gasoline or natural gas.

  2. Compression-ignited engines, typically fueled by diesel.

Commercially available reciprocating engines for power generation range from 0.5 kW to 6.5 MW in capacity. Reciprocating engines can be used in a variety of applications due to their small size, low unit costs, and useful thermal output. They offer low capital cost, easy start-up, proven reliability, good load-following characteristics, and heat recovery (cogeneration) potential. Possible applications for reciprocating engines in power generation include continuous or prime power generation, peak shaving, back-up power, and remote power. These engines can run on fuel generated by waste treatment (methane) and other biofuels, as long as the fuels are properly cleaned and processed. Reciprocating engines also make up a large portion of the combined heat and power (cogeneration) market.

To learn more, visit the California Energy Commission's Reciprocating Engines Web site.

Microturbines

Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25 kW to 500 kW. They evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines.

Microturbines offer a number of potential advantages over other technologies for small-scale power generation. These advantages include a small number of moving parts, compact size, light weight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. They have the potential to be located on sites with space limitations for the production of power. Waste heat recovery (cogeneration) can be used with these systems to achieve efficiencies greater than 80%.

To learn more, visit the California Energy Commission's Microturbines Web site.

Fuel Cells

A fuel cell is an electrochemical energy conversion device that converts hydrogen and oxygen into electricity and heat. It is very much like a battery that can be recharged while you are drawing power from it. Instead of recharging using electricity, however, a fuel cell uses hydrogen and oxygen. A fuel cell consists of two electrodes, an anode and a cathode, separated by an electrolyte. Power is produced electrochemically by the passing of ions (charged particles) formed at one end of the electrodes with the aid of a catalyst, through the electrolyte.

Fuel cells are quiet, efficient, and have low emissions but are still relatively expensive. The major difference between most fuel cells is in the type of electrolyte used. Some of the electrolyte types are phosphoric acid, molten carbonate, solid oxide, and proton exchange membrane.

To learn more about fuel cells, visit one of the following links:

Cogeneration

Conventional electricity generation is inherently inefficient, converting only about a third of the fuel's potential energy into usable energy. The rest is lost as heat. In applications where thermal energy is needed as well, there is a tremendous efficiency opportunity to combine electricity generation with thermal loads in buildings and industrial applications, converting as much as 85% of the fuel into usable energy. This is called cogeneration or combined heat and power (CHP).

The captured heat can be used for space heating, water heating, steam, process heating/cooling, or for thermally activated equipment, such as absorption chillers. In addition to the efficiency gains, CHP systems produce much less air pollution than separate heating and power systems. CHP systems can incorporate a wide variety of technologies, including most of the power generation options listed on this page.

To learn more about cogeneration, visit the U.S. Combined Heat and Power Association Web site.