Fluidized bed combustion
Fluidized bed combustion (FBC) is a combustion technology used in power plants. 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.
The technology burns fuel at temperatures of 1,400 to 1,700 °F (760 to 930 °C), a range where nitrogen oxide formation is lower than in traditional pulverized coal units. But increasingly strict US regulations have led to the use of ammonia DeNOx systems even on FBCs.
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.
Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NO2 and SO2 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.
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).
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.
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 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.
- FutureGen zero-emissions coal-fired power plant
- JEA Northside Generating Station (Jacksonville)
- Fluidized bed
- Fluidized bed reactor
- Chemical looping combustion
- Grate firing
- Pulverised fuel firing