OCR Text |
Show resulting from ash deposition or slagging on furnace walls. This larger heat transfer surface in coal-fired designs also is necessary to lower the gas temperature entering the convective tube banks so that deposit buildups can be controlled by sootblowers. In a coal-fired boiler, one of the most pronounced problems is the tendency of coal ash to leave the gas stream, strike a heat transfer tube, and adhere to it. Coal ash can either deposit on tube surfaces dry, or if the temperature of ash particles is high enough, it can become molten, strike the tube surface, and then solidify. Certain volatile ash constituents (alkali sulfates, for example) can also vaporize and recondense on tube surfaces or other ash particles. The larger the ash particle, the more momentum it attains (assuming no difference in ash density with particle size) as it proceeds along in the gas stream. As the gas approaches the convective tubes, the gas stream actually turns to go around the tubes. However, since the larger ash particles have more momentum in the original gas direction, they have more difficulty in changing direction. As a result, some particles impact on the tubes (Figure 1). This phenomenon also results in relatively rapid tube erosion if velocities are high enough. To reduce erosion, the boiler can be enlarged or the gas velocity reduced. Enlarging an existing boiler requires almost complete rebuilding, but gas velocities can be reduced by substantial derating. In a coal-fired boiler, heat transfer tubes in the convection passes are much farther apart than in oil- and gas-fired boilers in order to reduce erosion and the tendency for bridging between adjacent tubes by ash deposits. Thus, if an oil- or gas-fired boiler were to be converted to coal, the convective heat transfer surface would have to be either replaced with fewer tubes spaced farther apart (resulting in substantial derating) or the unit would have to be enlarged. If micronized coal produces ash with a lower tendency to impact the convective tube surfaces its use may alleviate the need for wider convective tube spacing. 9-3 |