Partly because of Government mandates and the public demand for cleaner, more efficient energy production, the industry is looking at different technologies to generate energy cleanly. One promising technology is fluidized bed combustion (FBC). FBC systems use a rising column of air to suspend the coal as it burns. This causes turbulent mixing of the gases and solids, which burns the coal more efficiently. The system evolved from efforts to control emissions internally rather than externally.

FBC systems generate significantly lower quantities of sulfur and nitrogen oxide pollutants than do standard pulverized coal boilers. Nitrogen oxides form in standard boilers because the high operating temperatures (2500ºF) promote the combination of nitrogen and oxygen in the air. FBCs operate at 1400º to 1700ºF, well below the 2500ºF temperature required for NOx formation. Also, limestone or dolomite is added to the FBC chamber to react with the sulfur from the coal and prevent it from leaving in the exhaust gases. The mixing within the chamber works to scrub over 95 percent of the sulfur pollutants out of the coal.

FBC systems are touted for their fuel flexibility. For instance, FBC can efficiently use fuels such as low rank lignites and sub bituminous coals that typically produce a lot of ash and are difficult to use in other types of pulverized coal boilers. FBC boilers are even being used to remove gob piles in some areas.

There are several different kinds of FBC in use or in development:

• Bubbling FBC (BFBC)
• Circulating FBC (CFBC)
• Pressurized FBC (PFBC)
• Advanced Circulating Pressurized FBC Combined Cycle (APFBC)
• Gasification FBC Combined Cycle (GFBCC)
• Combustion High Performance Power System (CHIPPS)

Bubbling FBC System
In a BFBC system, the fluidized bed consists of a sand-like material with limestone added to it. Air is bubbled into the combustion chamber through a perforated plate in the bottom, similar to an aquarium bubbler. At the right air velocity, the particles of sand, limestone, and fuel are suspended in a fluid-like state. Water tubes are immersed within the bed to control the temperature and generate steam. Coal particles are burned away slowly until only an ash is left that leaves the bed with the hot gases. The gases pass through separators where the ash and any particulates are removed and then on to heat exchangers to produce more steam. The thermal efficiency of a BFBC is around 30%, but BFBC systems are limited in size and require a high limestone-to-sulfur ratio for sufficient sulfur removal.

Circulating FBC Systems
CFBC systems use jets of air to suspend the coal and limestone mixture within the hot gases as the coal burns. The air used is at a higher velocity than in BFBC systems, so particles of coal are often lifted up into flue gases. As the flue gases are passed through a separator, the larger particles of coal drop back into the fluidized bed where they continue to burn. Individual particles may be recycled from 10-50 times depending on their size. The relatively clean flue gases then go on to a heat exchanger where they create steam. This design helps reduce the wear on the heat exchanger tubes, improves SO₂ capture, and improves combustion efficiency. These combustors can be up to 40% efficient.

	Click on diagram below to see larger view.
	How the first generation PFBC works.

Pressurized FBC Systems
Both BFBC and CFBC systems operate at atmospheric conditions, which means that the pressure within the system is the same or almost the same as the air outside the system. Pressurized FBC refers to either BFBC systems or CFBC systems performed at a high pressure. While PFBC produces steam from the initial combustion, it also creates a high-pressure hot gas stream that can be fed to a gas turbine, similar to an integrated gasification combined cycle (IGCC) system. The exhaust gas from the gas turbine is used to create more steam, which is used to power a steam turbine. In this system, about 80% of the power is generated from the steam turbine and 20% is generated from the gas turbine. Click the picture to the right for a larger diagram of how the first generation PFBC works.

The stream entering a gas turbine must be at a high temperature for increased efficiency. To help increase the temperature of the gas stream entering the gas turbine, some variations of PFBC systems burn natural gas in a topping combustor to add heat to the stream. However, natural gas is an expensive fuel source, so innovators advanced the technology. The APFBC is a second-generation form of the PFBC system. The APFBC does not use natural gas in a topping combustor. Instead, in this advanced configuration, a pressurized carbonizer or gasifier is added at the beginning of the process train to create syngas and char. The char goes to the pressurized circulating fluidized bed combustor that produces steam and heats the flue gas for the gas turbine. The syngas from the carbonizer is fed into the topping combustor to further heat the gas stream before it enters that gas turbine. The APFBC, then, is entirely coal fueled.

Gasification FBC Combined Cycle System
GFBCC systems use two fluidized beds and stream routing to increase efficiency. In a GFBCC system, the carbonizer is replaced with a partial gasifier PFBC that supplies syngas to the gas turbine topping combustor. The exhaust from the gas turbine is routed to a CFBC to be used as combustion air. The char from the PFBC partial gasifier and additional coal are fed into the CFBC to create steam for a steam turbine.

Combustion High Performance Power System
CHIPPS is another advanced configuration that seeks to improve performance. Similar to a GFBCC system, CHIPPS uses a furnace instead of a CFBC to burn the char from the PFBC partial gasifier. Gas turbine preheater tubes help increase the gas turbine efficiency. These tubes take some heat from the furnace and deliver heat to the topping combustor.

As you can see from the descriptions, the technologies continually advance singularly and can be combined to get the best from multiple technologies.