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The workhorse of America's electric power sector is the coal-fired power plant. Today, coal combustion plants account for more than half of the nation's electric power generation. Largely because of these plants, U.S. consumers benefit from some of the most affordable power rates in the world.

The technology of burning coal has made remarkable advances in the last quarter century, and much of this progress is due to federal research and development partnerships with private sector developers.

In the 1990s, fluidized bed combustion - a process that removes pollutants inside the coal boiler - was termed "the commercial success story of the last decade" by a major power industry publication. The first new coal-fired power plant to be built in Illinois in more than 15 years will employ a new type of "low-emission boiler" technology developed in the federal government's energy program. Innovations in burner designs, refractory materials, and high-temperature heat exchanges are all products of the Department of Energy's research program into cleaner, more efficient ways to burn coal.

A New Focus on "Hybrids" - Topping Off the Technology

	A schematic diagram of a generic "hybrid" combination of a coal gasifier and a fluidized bed combustor. > Click here for larger image

With many of the combustion advances of the late 20th century now moving into commercial use, the Department's Fossil Energy program has refocused its combustion research program to new types of "hybrid" technologies - typically coal-based systems that combine coal combustion and coal gasification into a highly efficient, environmentally clean power-generating technology.

Linking a coal gasifier and a combustor arranged in a "topping cycle" could be an ideal combination of lower-cost capital equipment, high-performance fuel use, and improved environmental performance for future power plants. The combination may be particularly suited for smaller power stations - those in the 200-300 megawatt range - which are likely to become more attractive as power companies develop strategies to deal with the growing uncertainties involved in forecasting future power demands.

In a "hybrid" system, coal is partially gasified in a pressurized gasifier. This produces a fuel gas that can be combusted in a gas turbine - the "top" of the cycle, hence the name. Left behind in the gasifier is a combustible char that can be burned in a fluidized bed combustor or advanced high-temperature furnace to produce steam to drive a steam-turbine power cycle and to heat combustion air for the gas turbine. Heat from the gas turbine exhaust also can be recovered to produce steam for the steam turbine.

This highly integrated system of gasifiers, combustors, gas and steam turbines results in a high overall fuel-to-electricity efficiency, exceeding 55 percent in many advanced concepts (the average efficiency of today's coal-burning power plant typically is around 33-35%). Higher efficiencies mean more affordable electric power for consumers, and because less fuel is required to generate electricity, overall greenhouse gas emissions can be significantly reduced.

"hybrid" systems may also lead to less expensive power plants. Because it is not required to break down coal completely into synthetic gas, a partial coal gasifier can be a relatively simple, compact, and low-cost component. The char combustion system likewise can be a relatively low cost module, and unlike many older coal combustors, which are designed to fire a specific type of coal, fluidized bed combustors can accept a wide range of fuels and would have no trouble burning chars produced from a variety of different coals.

Because of the lower capital costs and the capability to use relatively low cost fuels, a future "hybrid" power plant might be able to produce electricity at costs 25 percent or more lower than today's conventional pulverized coal-fired power plants.

The gasifier-combustor combination also has numerous environmental advantages. Not only does it produce fewer greenhouse gas emissions (because of increased efficiencies), more that 90 percent of sulfur pollutants emitted from the coal can be captured in the gasifier and combustor by adding sulfur-absorbing limestone. Nitrogen oxide emissions are inherently reduced in a fluidized bed combustor because the 'fluidizing' action of the combustion process keeps firing temperatures below the threshold where much of the nitrogen oxides form. With new technology, the fuel gas can be filtered at high temperatures to remove dust and alkalis, avoiding the energy loss of cooling the gas that is necessary in other systems.

The Department's Office of Fossil Energy has set a goal of developing initial concepts for "hybrid" gasifier-combustor power systems by 2010, with more advanced versions ready for large-scale testing by 2015. This timetable would make "hybrid" technology a candidate for the core of the Department's Vision 21 energy plant of the future.