OPTICAL DIAGNOSTIC MEASUREMENTS OF COAL SLAG PARAMETERS IN COMBUSTION MHD SYSTEMS.

摘要

nherent in the direct combustion of pulverized coal is the transformation of included mineral matter to ash droplets and ultimately the deposition of these droplets on flow train surfaces to form a slag layers. The presence of slag layers on MHD generator walls profoundly influences the thermal and electrical performance of the generator; while the size and concentration of the ash droplets directly affects the formation and replenishment of the slag layers as well as the emissive properties of the plasma and the performance of seed recovery and particulate cleanup equipment. Hydrodynamic models have been developed to predict the behavior of slag layers on MHD generator walls and these have indicated a strong dependence of slag layer thickness on a number of system parameters. The first objective of this thesis was the development of a diagnostic technique capable of making continuous, in-situ measurements of slag layer thickness, and the application of this technique in an MHD system for comparison with the predictions of one of the existing hydrodynamic models. The second objective of this thesis was the development of a diagnostic technique capable of making in-situ measurements of ash droplet size and concentration at the exit of an MHD combustor, and the study of the sensitivity of these quantities to various combustion parameters.;A slag layer surface position monitor employing a laser triangulation method has been developed and used to measure variations in slag layer thickness as a function of substrate temperature for a variety of ash loadings and total reactant flowrates. These measurements, on both eastern and western coals, are in excellent agreement with theoretical predictions based on estimated properties of these slags. Slag layer thickness is seen to decrease with increasing substrate temperature and plasma velocity. Increased deposition rate, slag viscosity and thermal conductivity all lead to increased slag layer thickness.;The measurement of mean size and concentration of suspended ash droplets in the plasma exiting the M-8 combustor has been accomplished utilizing a two-wavelength transmissometer developed for this purpose. This instrument incorporates several unique features which were required to overcome the difficulties imposed by the MHD environment. Illumination is provided by a HeCd laser at a wavelength of 3250 (ANGSTROM) and by a HeNe laser at 3.39 (mu)m. This allows for spectral filtering of the thermal emissions from the plasma and ash droplets, and for measurement of a wide range of mean diameters. A special beam chopping technique in conjunction with specially constructed signal processing electronics permits accurate measurement of very small fractional attenuation. Also, a mirrored, conical light pipe provides the spatial uniformity of detection system response required to cope with turbulence-induced beam wander. Measurements obtained with this device show that mean ash droplet diameter is relatively independent of ash loading, combustor residence time and combustion stoichiometry, but is a decreasing function of plasma temperature. The Sauter mean diameter for Illinois