Observations on the combustion behavior of coal water fuels and coal water fuels impregnated with calcium magnesium acetate

Abstract

Combustion studies of single free-falling coal-water fuel (CWF) droplets were conducted in a drop-tube laminar-flow furnace at high heating rates. Most experiments were conducted with predried CWF agglomerates consisting of micronized (3.5 μm mean size) and beneficiated bituminous coal. Agglomerates of known initial size, in the range of 100–600 μm, were burned in air or pure oxygen at furnace temperatures between 1300 and 1500 K. Combustion of CWF agglomerates impregnated with calcium magnesium acetate (CMA), which is being considered as a sulfur capture agent, was also examined under the same conditions. Observations on the devolatilization and char combustion behavior of CWF agglomerates, formed after the evaporation of water, were conducted using pyrometric and cinematographic techniques. Complete time-temperature histories of burning agglomerates were obtained with a three-color near-infrared optical pyrometer, where distinct phases of volatile and char combustion were observed and analyzed. The volatile flame temperatures and the char combustion temperatures in air exceeded the furnace gas temperature by as much as 1000 and 600 K, respectively. The char combustion phase is the most prominent since it accounts for 75%–85% of the burnout time. The volatile combustion phase is also important because of the resulting high-temperature radiant flames. Overall burnout times were between 150 and 600 ms, for the range of agglomerate sizes tested. Char combustion for most agglomerates was controlled by boundary layer diffusion of oxygen. While most experiments involved predried agglomerates, a limited number of experiments were performed with CWF droplets to monitor the time required for water evaporation and subsequent heatup of the resulting agglomerates. Under the oxygen partial pressure and temperature conditions of this study water evaporation took approximately 25% of the total CWF furnace residence time. The swelling behavior of the agglomerates was studied and swelling factors in the neighborhood of 1.12 were determined. Although the bituminous CWF, studied herein, was found to swell and form cenospheric structures during heatup and devolatilization, the addition of CMA catalyst inhibited swelling of the agglomerates. However, the chars containing CMA exhibited occasional splitting or fragmentation during the volatile combustion stage.