OCR Text |
Show 1. INTRODUCTION Coal and chars derived from coals are complex heterogenous materials. Their combustion behaviour is made even more complex by the coupled mechanisms of diffusion to and reaction within the particles, respectively. To simplify the task, we shall concern ourselves with the combustion of a single coal char particle. The overall processes are well understood. However, given the heterogeneous nature of a char particle and the impossibility of characterizing its internal morplology in any exact manner, most mathematical models are forced to make many assumptions regarding the details of the physical processes during combustion. Experimental data are needed to test such assumptions. It has long been recognized that particle temperature is one of the most sensitive parameters that reflects the balance between the actual competing physical processes that occur during combustion. The temperature of the particle is governed by the energy balance on the particle. Included in such a balance is the chemical reaction source term which in turn is determined by the intrinsic chemical kinetics and local reactant concentration. The local reactant concentration is governed by the diffusion of reactant into the particle. Thus it depends, largely, on the particle internal morphology. The overall energy balance also includes loss terms involving all three modes of heat transfer to the external ambient. Thus particle temperature is expected to be a sensitive function of all the major variables involved in combustion. Much effort has therefore been directed by experimentalists to accurately measure single particle temperatures. Pulverized coal particles are usually in the range 40-200J,Lm. Char~ derived from them after devolatilization are also similarly sized. In a typical combustion environment at 2000K, the burntime of such chars is in the range 0.02-1.5 seconds. While thermocouples can be used to measure gas and wall temperatures, clearly their usefulness in measuring temperatures of moving particles is limited. Various types of disappearing filament pyrometers can be used provided there is a steady stream of particles. However they only measure average temperatures over many 2 |