Ignition of Coal-Water Fuel Droplets with Addition of Small Amount of Various Alcohols

Abstract

ABSTRACT Coal-water slurries have been actively studied during the last decade due to the fact that much less oxides of sulfur, nitrogen and carbon are formed during their combustion in furnaces of power boilers compared to coal. The main problem that hinders the widespread implementation of such fuels in the energy sector is their long (up to 30–40 s) ignition delay times. Significant (up to 56%) reduction of the ignition delay time is possible when a small (up to 8% by weight) amount of available and relatively inexpensive liquid combustible organic compounds is added to the suspension. The purpose of this work is to substantiate the possibility of significant reduction of the ignition delay times of coal-water fuels by introducing a third component into the coal – water suspension – isoamyl, isopropyl or ethyl alcohol in relatively small (from 3 to 8% by weight) concentrations. Mechanism, characteristics and conditions of ignition of the studied rather promising fuel compositions have been established by the results of experimental studies of ignition process of multicomponent coal-water fuel droplets based on lean coal with alcohol additives in a medium of oxidizer (still air) heated to high temperatures (from 873 K to 1273 K) with recording of heating, ignition and combustion processes by a high-speed video camera (shooting speed up to 105 frames per second). Experiments have shown that stable ignition delay time of the studied suspensions based on lean coal decreases by 11–56% with an increase (from 3 to 8%) in the mass concentration of one of the alcohols in the composition of coal-water fuel at the initial characteristic size of droplets from 1.0 to 2.5 mm. Each studied composition was compared with conventional coal-water fuel burned under similar conditions. Increase in the mass concentration of alcohol in the composition of coal-water fuel reduces the value of the flash point by 27–30%. The viscosity of alcohol-coal-water fuels increases by 7.0 to 40% when the researched alcohols are added to the coal-water slurry. Influence of mass concentration of isoamyl, isopropyl and ethyl alcohols in the composition of coal-water fuel on the characteristics of initiation of ignition and combustion processes established in experiments justifies the possibility of sufficiently effective application of such three-component suspensions in the power industry. Necessary and sufficient conditions for the ignition of alcohol-coal-water fuel droplets have been established based on a mathematical model and experimental data. Increase in the cost of such fuels has been analyzed in comparison with conventional coal-water slurry. It has been shown that the most attractive from the energy and economic points of view is coal-water fuel with isoamyl and isopropyl alcohols in the composition. The obtained results are of significant practical value, as they illustrate the possibility of achieving optimal ignition and combustion conditions for promising multicomponent coal-water suspensions with alcohol additives in the furnaces of boiler plants.