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.