High Furnace Exit Gas Temperature
Coarse coal (poor fineness) results in less surface area on the carbonaceous solids from coal. Higher fineness level and consequently smaller particle sizing of carbonaceous solids increases the surface area of particles to be oxidized and allows the chemical reaction of combustion to be completed. Higher fineness allows for more rapid combustion of carbon char in the lower furnace. When combustion is completed lower in the furnace cavity, the waterwalls have increased time to absorb heat released from the fuel.
Increased heat release in the lower furnace results in a higher proportion of heat absorbed by the waterwalls. Higher heat absorption by the waterwalls relates to a reduction in furnace exit gas temperature. Reduction in furnace exit gas temperature resulting from completion of combustion in the lower furnace with higher fineness results in lower slagging propensities. Poor fineness leads to higher furnace exit gas temperature and a higher slagging propensity is also frequently responsible for higher than desired Reheat or Superheat De-Superheating Spray Flows.