Fluidized bed combustion is a combustion technology used to burn solid fuels.
In its most basic form, fuel particles are suspended in a hot, bubbling
fluidity bed of ash and other particulate materials through which jets
of air are blown to provide the oxygen required for combustion or gasification.
The resultant fast and intimate mixing of gas and solids promotes rapid heat
transfer and chemical reactions within the bed. FBC plants are capable of
burning a variety of low-grade solid fuels, including most types of coal and
woody biomass, at high efficiency and without the necessity for expensive fuel
preparation. In addition, for any given thermal duty, FBCs are smaller than the
equivalent conventional furnace, so may offer significant advantages over the
latter in terms of cost and flexibility. 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. The heated precipitate
coming in direct contact with the tubes 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 have other added benefits as well.
Benefits There are two reasons for the rapid
increase of 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 fluidized 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 sulfur 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. The
technology burns fuel at temperatures of 1,400 to 1,700 °F, well below the
threshold where nitrogen oxides form; it also avoids the ash melting problems
related to high combustion temperature. The mixing action of the fluidized bed
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 dramatic increases in
carbon and polycyclic aromatic hydrocarbons emissions.
Commercial FBC units operate at competitive efficiencies, cost less than
today's conventional boiler units, and have NO2 and SO2 emissions below levels
mandated by Federal standards. However, they have some disadvantages such as
erosion on the tubes inside the boiler, uneven temperature distribution caused
by clogs on the air inlet of the bed, long starting times reaching up to 48
hours in some cases. OR
Production of NOx is temperature depended as in FBC temperature is less
than other combustion processes Hence it results in low production of NOx.
Lower production of SOx because SO2, SO3 etc. are captured by limestone.
FBC has 10 times more heat transfer then other combustion processes because of
burning particle, hence it has a high combustion efficiency.
FBC has a lower combustion temperature of 750 °C whereas an ordinary boiler
operates at 850 °C. FBC has low sintering process.
Less area is required for FBC due to high coefficient of convective heat
transfer. Iso-thermal bed combustion as
temperature in free belt and active belt remain constant.
Low air pollution. Types
FBC systems fit into essentially two major groups, atmospheric systems and
pressurized systems, and two minor subgroups, bubbling and circulating
fluidized bed. = FBC=
Atmospheric fluidized beds use 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. Advanced PFBC
A 1½ 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 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.
See also Chemical looping combustion
Fluidized bed Fluidized bed reactor
FutureGen zero-emissions coal-fired power plant
Grate firing JEA Northside Generating Station
Pulverised fuel firing References
National Energy Technology Laboratory EU regulation: Pollution from large
combustion plants Simulation of a commercial CFB coal
combustor