Entrained Pulverized Coal Firing

The concept of burning coal that has been pulverized into a fine powder arose from the belief that if the coal is made fine enough, it will bum almost as easily and efficiently as a gas. The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pnlverized coal fuel is controlled by computers. 

Drying of the fuel can be carried out prior to or simultaneous with crushing and pulverization. The fuel is ground immediately before it is needed to avoid storage and the risks of a serious fire and explosion. The coal is crashed to reduce its size to -1 in., followed by pulverization to 70%-80% less than 200 mesh, 99-h% less than 50 mesh. 

The particulate removal device, such as an electrostatic precipitator, can be placed either before the air preheater (a hot-side electrostatic precipitator) or after the air preheater (cold-side electrostatic precipitator). A fabric filter can be used in place of the electrostatic precipitator. Alternatively, or in addition, a flue-gas-desulfurization (FGD) unit treats the hot gas before it is released to the atmosphere. 

A 1% point improvement in the efficiency of a conventional PCC plant can result in a 2%-3% v/v reduction in carbon dioxide emissions. 

---

WEW Process (24,25). Vereingte Elektrizitatswerke Westfalen is a German electric utility that has developed a unique dry feed, entrained-flow gasification process. The VEW concept relies on partial gasification with preheated air to produce low-Btu-coal-derived gas and char. After waste heat recovery, steam generation, and sulfur removal, the coal gas is fired in a combustion turbine. The char is fired in a pulverized coal boiler with the hot flue gas from the combustion turbine serving as the preheated combustion oxidant. This design requires additional cleanup of the pulverized boiler flue gas to meet emission requirements. 

In a pulverized boiler, the coal is ground to the consistency of talcum powder in a mill, and then entrained in an air stream that is fed through the burners to the boiler combustion chamber.6 Firing, therefore, occurs in suspension. Pulverized boilers can be wet-bottom, which means that coals with low ash fusion temperatures are used, and molten ash is drained from the bottom of the furnace, or can be dry bottom, which means that coals with high ash fusion temperatures are used, and dry ash removal techniques can occur.6... 

SUCTION FIRING. In the suction method, the air and entrained coal are drawn through the pulverizer under negative pressure by an exhauster located on the outlet side of the pulverizer. With this arrangement the fan handles a mixture of coal and air, and distribution of the mixture to more than one burner is attained by a distributor beyond the fan discharge. 

---