OCR Text |
Show The furnace bottom of the coal-fired steam generator must be inclined to handle oversized particles that become disentrained and wall slag shed during operation. Thus most oil-fired units must be modified to accommodate larger quantities of furnace ash. Convection tube spacing must be larger in coal-fired units to avoid erosion by the high loading of ash in the flue gas as well as to avoid premature plugging by ash that will deposit on the tube surfaces. Provision must be made for soot blowing and on-line removal of ash from the convection pass tube surface. Adjustments must also be made in the heat-recovery area to provide sufficient air preheat to the coal to ensure adequate drying during pulverizing. Figure 13 illustrates the difference in size and design of coal-, oil-, and gas-fired steam generators. Ash chemistry plays a significant role in determining how much larger a coal-fired steam generator must be than an oil-fired steam generator to provide equal capacity, or conversely, how much derating must be expected in an oil-fired steam generator when retrofitted for coal firing. With eastern fuels, iron as pyrites is the dominant influencing mineral acting as a fluxing agent for the otherwise innocuous alumina and silica. Pyrites are particularly sensitive to reducing conditions and residence time for complete oxidation. In western fuels the alkali-bearing minerals are the dominant constituents whose concentration influences the temperature at which they condense and hence the furance exit temperature. 1-44 |