OCR Text |
Show (ii) Operating Conditions - described by combustion stoichiometry, coal feed rate, coal to carrier gas ratio, primary and secondary air mass flows and temperature and combustion air oxygen content (iii) Burner Design - described by the recirculation flow time scale mixing time of the primary air and recirculating as flows, the delay time in adding the secondary air and recirculating gas flows (iv) Fire Box Design - described by its size, wall temperature, and wall emissivity. The characteristics of the combustion of alternative fuels can thus be projected as a function of one or more of the above variables. Figure 3 shows an example of a typical form of swirl stabilized recirculating burner. Pulverized fuel combustion is treated by following the detailed time history of a size distribution of coal particles as they and their carrier air mix with hot combustion products from the recirculation zone, and with secondary air. These mixing processes provide respectively the heat source and majority of the oxidant for ignition and combustion of the pulverized fuel. Stable combustion is defined by relating combustion product status predicted, to that assumed as an ignition source of hot fluid from the recirculation zone. Specifically, it is required that the predicted mixed and reacted fluid reach the thermodynamic condition assumed* as an initial condition for the recirculated products, in the time scale corresponding to the recirculation field. Since that is both scale and velocity dependent, a burner scaling capability is inherent. Figure 4 shows a predicted time history of gas * It is generally not equivalent to a complete combustion equilibrium condition, which usually requires a longer time scale than available in the recirculation flow field. -7- _///AVCO EVERETT |