Ignition and Combustion of Utility Grind Coal-Water Slurry in a Lean, Confined, Turbulent Diffusion Flame

摘要

A coal-water slurry (CWS) designed for boiler firing typically contains 30 wt % water. Although the heat required to vaporize this water is only a small fraction of the overall heat of combustion, it approximately doubles the heat required for ignition of the coal particles. The additional time and distance needed to vaporize the water before ignition of the coal residues is detrimental to the stability of CWS flames. In order to assess the importance of this effect, measurements of gas velocities, temperatures, and oxygen mole fractions were made near the burner during combustion of CWS in a confined turbulent diffusion flame. The trajectories and temperature histories of 10 to 100 mu m slurry droplets injected into this flow were calculated using a one-dimensional model. The distances required for vaporization of water, ignition, release of volatiles, and char burnout were then estimated. Consistency of the procedure was checked by comparison of calculated and measured temperature and oxygen profiles. The heat for ignition of the spray is supplied by entrainment of hot combustion products into the fuel and air jets, and from combustion of the volatiles and char residues of the smallest droplets. The droplet velocity leaving the atomizer, and the gas velocity in the region of the fuel spray have important effects on the spatial distribution of ignition and combustion through their determination of the droplet trajectories. The presence of 32 wt % water in a slurry droplet results in an increase of about 50% over the distance required for ignition of an equivalent coal particle under the conditions in the flame. 34 refs., 11 figs., 6 tabs. (ERA citation 10:043704)