OCR Text |
Show 1. INTRODUCTION Burner design for stable combustion of pulverized fuels is based at present on an extensive background of engineering experience. Such factors as fuel physical and chemical properties, air preheat and stoichiometry, and burner flow and radiation fields are of recognized importance, for both flame stability and fuel utilization. However, few attempts have been made to provide an organized mathematical model structure capable of relating these factors, to give predictive or parametric exploration capability. The goal of this work is to provide a basis for such parametric exploration, aimed at defining optimum (1 2) conditions for effective combustion. The approach taken is development of a relatively straight-forward engineering model, using a one-dimensional or (3) time history approach to combustion behavior. This allows description of major hydrodynamic effects as time evolutions of fluid stream mixing, based on cold flow measurements or (4) . . . , clean-fueled combustor flow data. This is both much simpler computationally and potentially more accurate than use of totally theoretical flow models, involving a two or three dimensional viscous recirculating flow solution. Pulverized coal combustion is treated as occurring in three major stages : (1) the release of volatile matter upon heating the coal particles, (2) the combustion of the volatiles in the gas phase, and (3) the burning of the char residue from the prior devolatilization step. Each of these stages is dependent upon the quantity and rate of volatile release during the initial heating of the coal particles as this value determines the amount of combustibles than can burn in subsequent homogeneous gas phase reactions and the amount of residual char to be consumed in heterogeneous gas/solid reactions. This information along with data on the heating value of the volatiles released determines the necessary residence time for the coal particles, the size of the combustion chamber and the design of the flow field and/or the need of a flame holder. -2- . AVCO EVERETT |