Advanced fluidized-bed combustion (FBC) technology offers a viable power generation option for the post-2000 time frame.  Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NOx and SO₂ emissions below levels mandated by Federal standards. 
FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling or circulating fluidized bed.

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1st Generation PFBC

1½ Generation PFBC

2nd Generation PFBC

FBC.  Atmospheric fluidized beds use a sorbent such as limestone or dolomite to capture sulfur released by the combustion of coal.  Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid.  These boilers operate at atmospheric pressure.

PFBC.  The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion.    However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine.  Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system.

A 1-1/2 generation PFBC system increases the gas turbine firing temperature by using natural gas in addition to the vitiated air from the PFB combustor.  This mixture is burned in a topping combustor to provide higher inlet temperatures for greater combined cycle efficiency.  However, this uses natural gas, usually a higher priced fuel than coal.

APFBC.  In more advanced second-generation PFBC systems, a pressurized carbonizer is incorporated to process the feed coal into fuel gas and char.   The PFBC burns the char to produce steam and to heat combustion air for the gas turbine.  The fuel gas from the carbonizer burns in a topping combustor linked to a gas turbine, heating the gases to the combustion turbine's rated firing temperature.   Heat is recovered from the gas turbine exhaust in order to produce steam, which is used to drive a conventional steam turbine, resulting in a higher overall efficiency for the combined cycle power output.  These systems are also called APFBC, or advanced circulating pressurized fluidized-bed combustion combined cycle systems.  An APFBC system is entirely coal-fueled. 

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	GFBCC.  Gasification fluidized-bed combustion combined cycle systems, GFBCC, have a pressurized circulating fluidized-bed (PCFB) partial gasifier feeding fuel syngas to the gas turbine topping combustor.  The gas turbine exhaust supplies combustion air for the atmospheric circulating fluidized-bed combustor that burns the char from the PCFB partial gasifier.
GFBCC
	CHIPPS. A CHIPPS system is similar, but uses a furnace instead of an atmospheric fluidized-bed combustor.  It also has gas turbine air preheater tubes to increase gas turbine cycle efficiency.
CHIPPS

Programs.  Research being conducted in several FBC subprograms is demonstrating advanced features of FBC and developing the technology base to lower capital and production costs.  Thrusts include simplification of FBC systems and components, incorporation of alternative feed and withdrawal systems, and incorporation of advanced subsystems and steam cycles.

Results from system studies will guide future R&D, determine optimum turbine-compressor configuration, and lead to the demonstration of first-generation PFBC systems.   Optimum configurations of second-generation PFBC for Vision 21 power plants with fuel cells and CO₂ sequestration options are also being developed.  Gas turbine studies will be performed on gas compositions and heat capacities specific to PFBC, which can lead to higher allowable turbine blade temperatures.