Fluidized bed combustion (FBC) is a combustion technology used in power plants.
FBC plants are
more flexible than conventional plants in that they can be fired on coal, biomass, among other fuels.
Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The
result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides
more effective chemical reactions and heat transfer.
Combustion systems for solid fuels
FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate
out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the
apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct
contact with the tubes(heating by conduction) increases the efficiency. Since this allows coal plants to
burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes
increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the
lower temperatures of combustion (800 °C / 1500 °F ) have other added benefits as well.
There are two reasons for the rapid increase of fluidised bed combustion (FBC) in combustors. First, the
liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to
burn using other technologies, is an important advantage of fluidised bed combustion. The second
reason, which has become increasingly important, is the possibility of achieving, during combustion, a low
emission of nitric oxides and the possibility of removing sulphur in a simple manner by using limestone as
bed material.
Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant
emissions without external emission controls (such as scrubbers). The technology burns fuel at
temperatures of 1,400 to 1,700 °F (750-900 °C), well below the threshold where nitrogen oxides form (at
approximately 2,500 °F / 1400 °C, the nitrogen and oxygen atoms in the combustion air combine to form
nitrogen oxide pollutants). The mixing action of the fluidized bed results brings the flue gases into contact
with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in
coal can be captured inside the boiler by the sorbent. The reductions may be less substantial than they
seem, however, as they coincide with increases in carbon dioxide and polycyclic aromatic hydrocarbons
emissions.
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 (BFB or CFB).
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.
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.
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 stands for combustion-based high performance power system.
