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Fluidized bed combustion (FBC) is a combustion technology used in power plants. 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. FBC plants are more flexible than conventional plants in that they can be fired on coal and biomass, among other fuels.

File:Combustion systems for solid fuels.gif

Combustion systems for solid fuels

FBC reduces the amount of sulfur emitted in the form of SO_x 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 NO_x 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.

Benefits

There are two reasons for the rapid increase of fluidized 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 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 (such as scrubbers-flue gas desulfurization). 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); 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 (monoxide?) and polycyclic aromatic hydrocarbons emissions. ^{[citation needed]}

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. Although, it has 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

1-Production of NOx is temperature depended as in FBC temperature is less then other combustion processes Hence it result in low production of NOx.

2-No of production of SOx because SO2,SO3 etc are captured by lime stone.

3-FBC has 10times more heat transfer then other combustion processes because of burning partical Hence it has high combustion efficiency.

4-FBC has low combustion temperature of 750C were as an ordinary boiler operates at 850C.

5-FBC has low centering process (melting of Ash).

6-Lesser area is required for FBC dew to high co-efficient of convective heat transfer.

7-ISO thermal bed combustion as temperature in free belt and active belt remain constant.

8-Low air pollution.

Types

FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating fluidized bed (CFB).

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 (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.

References

@@ Line 1: / Line 1: @@
+'''Fluidized bed combustion''' (FBC) is a combustion technology used in power plants. [[Fluidized bed reactor|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. FBC plants are more flexible than conventional plants in that they can be fired on [[coal]] and [[biomass]], among other fuels.
-{{For2|other topics on fluidization|[[Fluidized bed{{!}}Fluidized bed technology]], [[Fluidized bed reactor]] and [[Fluidization]]}}
-'''Fluidized bed combustion''' (FBC) is a combustion technology used in power plants. [[Fluidized bed reactor|Fluidized beds]] suspend solid fuels in 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.  FBC technology was adapted to burn petroleum coke and coal mining waste for power generation in the early 1980s in the US.  At that time, US regulations first provided special incentives to the use of renewable fuels and waste fuels.  FBC technology spread to other parts of the globe to address specific fuel quality problems.  The technology has proved well suited to burning fuels that are difficult to ignite, like petroleum coke and anthracite, low quality fuels like high ash coals and coal mine wastes, and fuels with highly variable heat content, including biomass and mixtures of fuels.
+[[Image:Combustion systems for solid fuels.gif|right|thumb|350px|Combustion systems for solid fuels]]
-The technology burns fuel at temperatures of {{convert|1400|to|1700|°F|°C|sigfig=2}}, a range where nitrogen oxide formation is lower than in traditional [[Pulverized coal-fired boiler|pulverized coal units]].  But increasingly strict US regulations have led to the use of ammonia DeNOx systems even on FBCs.
+FBC reduces the amount of [[sulfur]] emitted in the form of [[sulfur oxide|SO<sub>''x''</sub>]] 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|NO<sub>''x''</sub>]] 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.
+== Benefits ==
-Fluidized-bed combustion evolved from efforts in Germany to control emissions from roasting sulfate ores without the need for external emission controls (such as scrubbers-flue gas desulfurization). 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 the fuel can be captured inside the boiler by the sorbent.  The sorbent also captures some heavy metals, though not as effectively as do the much cooler wet scrubbers on conventional units.
+There are two reasons for the rapid increase of fluidized 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 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 (such as scrubbers-flue gas desulfurization). 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]] [[atom]]s in the combustion air combine to form [[nitrogen oxide]] pollutants); 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 (monoxide?) and polycyclic aromatic hydrocarbons emissions. {{Citation needed|date=September 2009}}
-Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NO<sub>2</sub> and SO<sub>2</sub> emissions below levels mandated by Federal standards.  FBCs demonstrate different performance characteristics including different locations for erosion on the tubes inside the boiler, uneven temperature distribution if clogs occur in the air inlet of the bed, and long starting times reaching up to 48 hours for problem fuels.
+Commercial FBC units operate at competitive efficiencies, cost less than today's conventional boiler units, and have NO<sub>2</sub> and SO2 emissions below levels mandated by Federal standards. Although, it has 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.
-'''Fluidising Velocity''' At Grimethorpe PFBC the initial tests were with high fluidising velocities - up to 4 metres per second.
-Cold model testing pointed to 1 meter per second as the safe maximum tube bed velocity.
-Tube bank failures tended to occur on the underside of the pipes from rows 3 or 4 above the bubble caps - upwards.
+                                            OR
-The Grimethorpe cold model showed that sand velocity was at its highest as the air bubbles joined together and grew in size ,"agglomerated".
-When a large bubble passes through a tube bundle, the sand sideways velocity can be much larger than upward speed.
-Hence shallow beds [less than 1 meter] combined with low fluidising velocity [less than 1 metre per second]
-are the best way to provide the heating for the tube bundles, with commercially accepted tube wastage.
+-Production of NOx is temperature depended as in FBC temperature is less then other combustion           processes Hence it result in low production of NOx.
+-No of production of SOx because SO2,SO3 etc are captured by lime stone.
-== Types ==
-FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling fluidized bed (BFB) and circulating fluidized bed (CFB).
+-FBC has 10times more heat transfer then other combustion processes because of burning partical Hence it has high combustion efficiency.
-=== 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.
+-FBC has low combustion temperature of 750C were as an ordinary boiler operates at 850C.
-=== Circulating Fluidised Beds ===
+-FBC has low centering process (melting of Ash).
-For 7 years we commissioned and operated two Circulating Fluidised Bed Combustors.
-From this experience I wish to pass on the following:~
-. Errosion corrosion caused tube bank failure after 10 start-ups, that was around 3 months of commissioning.
+-Lesser area is required for FBC dew to high co-efficient of convective heat transfer.
+-ISO thermal bed combustion as temperature in free belt and active belt remain constant.
-A fluidised bed boiler is mainly a box of refractory material; which has to be heated gently through.
-Until the refractory is above 500 - 600 degrees Centigrade the permitted temperature rise is 1 degree per minute. So preheat is a 10 hours process. Time in the Errosion Corrosion zone of 230 - 360 degrees Centigrade, is 2 hours per start up. 10 start ups gives 20 hours before tube failure.
+-Low air pollution.
-It is strongly recommend to all fluidised bed boiler designers and operators to avoid sand circulation in the initial pre - heat stage.
-So first heat the refractory to above 350 degrees Centigrade,
-then start feeding in the sand as the refractory temperature is increased to 650 degrees Centigrade.
-With this procedure it follows that the fluidising air for the low fluidising velocity heat exchanger should have its own start-up burner.
+== Types ==
-Coal is only introduced when the sand and refractory reach 650 degrees Centigrade.
+FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating fluidized bed (CFB).
+=== FBC ===
-So just start introducing sand when the combustion chamber and low velocity heat exchanger tubes reach 350 degrees Centigrade.
+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.
-Grimethorpe had a 150 degree Centigrade rated "Bed material" pressure vessel. Material was sent to and from the combustor by a differential pressure system. With the CFB cold sand is dropped into the "External Heat Exchanger" - a low velocity shallow fluidised bed refractory lined box that houses the evaporator and superheater tube bundles.
-'''COAL FIRING'''
-When coal firing, limestone or dolomite is added to react and absorb the sulphur from with in the coal.
-Coal leaves a residue of around 10 - 15% ash.
-The glass transition temperature of pure SiO 2 is about 1600 K (1330 °C) (1090-1150C).
-In the making of soda glass Limestone/dolomite and ash are added to the sand to reduce the melting temperature to around 1090 - 1175 with 1250 Centigrade as a maximum. Before sand starts to form glass, it takes 10 - 25 minutes to get an even temperature that will begin to fuse the sand/glass.
-'''BLACK GLASS'''
-With a large - multi tube failure in a fluid bed boiler, water flow is suddenly interrupted, resulting in increased bed temperature.     On a few catastrophic failures, the cooling ability of the in-bed tubes disappeared so quickly, that the residual fuel in the bed rapidly increased the bed temperatures to white heat.
-The sand and ash turned to a "black glass"; which took weeks to chip off; before the lengthy tube repairs could start.
-To repeat.... removing the cooling from the bed allows the bed temperature to run away, and can easily reach 1200 Centigrade.
-The mass of the bed can easily maintain 1200 Centigrade for sufficient time for "black glass" to form.
-It is obviously desirable to shut down a boiler at the early signs of a leak to avoid the risk of the bed melting as a whole, or even in local hot spots.
-After a short time the operators of the CFBs noted that the stack water vapour reading would rise as soon a tube leak commenced. After a few more tube leaks it became possible to set a value of stack moisture level to take the boiler out of service and so avoid total failure, and hence preventing the "BLACK GLASS" events.
-The CFBs originally had a small bed of gravel at the base of the high velocity refractory lined combustion tube. However topping up the gravel bed was found to be unnecessary, and this resulted in less refractory damage, without any detrimental effect on the circulating bed.
 === 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.
-=== Grimethorpe Pressurised Fluidized Bed Combustion Project [ 1980 - 1990 ] ===
-[[File:3DsketchWithText.gif|thumb|A 3D sketch of Grimethorpe Pressurised Fluidised Bed Combustion Project as of 1988]]
-Grimethorpe accurately measured the values of tube errosion over a wide range of temperatures.
-'''ERROSION CORROSION'''
-Grimethorpe produced data. Some data has not translated into current fluid bed design.
-Namely ERROSION CORROSION occurs mainly at start up. Errosion Corrosion effects a wide range of materials,
-and is 10 to 20 times greater whilst the tube outer surface temperature is between 230 and 320 degrees Centigrade.
-Once the sand and tube surface temperature is above 360 degrees Centigrade,
-Errosion Corrsion falls to normal - commercially accepted values - becoming almost a self cleaning action.
-For further details on Errosion Corrosion see Grimethorpe PFBC tube bank C2 reports,
-which itemises the scale of Errosion Corrosion for many tube materials, and the temperature of
-maximum Errosions Corrosion values for those tube materials.
-Errosion Corrosion is similar to Fretting Corrosion, in that the actions eats into the metal,
-far more than sand impact zone of the surface. This eating away is partially temperature dependent.
-Whilst studding and finning do reduce Errosion Corrosion, it is cheaper, and makes more sense, to
-keep the bed sand and the tubes apart in the Errosion Corrosion temperate zone of 230 and 360 degrees Centigrade.
-Errosion Corrosion can therefore be most effectively avoided by
-. pre- heating the in bed tube banks in air, to above 360 degrees Centigrade, and
-. pre - heating the initial 10 tonne start up slug of bed sand to 360 degrees Centigrade.
-In other words
-'''KEEP THE TUBES AND SAND APART UNTIL EVERYTHING IS ABOVE 320 DEGREES CENTIGRADE'''
-'''ACID CORROSION PROTECTION'''
-Dolomite limestone is added to coal, in order to capture the sulpher released in combustion.
-Sulpher dioxide dew point is 180 degrees Centigrade.
-Dilute sulphuric acid attacks stainless steel.
-At Grimethorpe all stainless steel expansion bellows and gas turbine blades test pieces where kept above 180 degrees Centigrade in operation and post operation periods.
 '''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 (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.
@@ Line 128: / Line 57: @@
 * [[Grate firing]]
 * [[Pulverised fuel firing]]
+* [[Chemical looping combustion]]
-== External links ==
+== References ==
 * [http://www.netl.doe.gov/technologies/coalpower/Combustion/FBC/fbc-overview.html National Energy Technology Laboratory]
 * [http://www.europa.eu/scadplus/leg/en/lvb/l28028.htm EU regulation: Pollution from large combustion plants]
+* [http://www.cpfd-software.com/applications/cfb_combustor Simulation of a commercial CFB coal combustor]
 {{DEFAULTSORT:Fluidized Bed Combustion}}
@@ Line 137: / Line 68: @@
 [[Category:Energy conversion]]
 [[Category:Chemical engineering]]
+[[de:Wirbelschichtfeuerung]]
+[[es:Combustión en lecho fluido]]
+[[pl:Piec fluidyzacyjny]]
+[[fi:Leijukerroskattila]]
+[[sv:Cirkulerande fluidiserad bäddpanna]]

Revision as of 12:38, 9 January 2014

Benefits

Types

FBC

PFBC

See also

References