OCR Text |
Show Evaporation of Moisture This phase is endothermic. The localized effect is that the fuel particle will remain at 212 degres F. or less until moisture is completely evaporated. This step is affected by evaporative heat available, surface area, and quantity of moisture. Evaporation of Volatiles Also endothermic, this process begins to some degree, below 212 degrees F., but the major portion of volatiles evaporate above 800 degrees F. Major effectants are surface area, evaporative heat, and volatile characteristics. Combustion of Volatiles Exothermic, this phase occurs at about 1100 degrees F. Effectants are volatile characteristics, temperature, and diffusion of oxygen for combustion. Combustion of Fixed Carbon Oxygen combines with the carbon to form C0«. Exothermic, this reaction is controlled by the diffusion of oxygen ot the carbon at temperatures above 650 degrees F. Requirements controlling the ability and degree of the above mechanisms to occur are as follows. Compromising any of these will, in the most drastic case, prevent combustion and in varying degrees effect complete combustion and heat transfer efficiency. Turbulence Turbulence dictates that sufficient oxygen must be thoroughly mixed with the fuel particle to drive the combustion and diffusion processes. The effectiveness of turbulence will dictate the amount of excess air required. Increasing excess air lowers flame temperature, decreases radiant heat transfer, and increases furnace vessel and stack gas mass flows. i 27-4 |