Coagulation and splitting of droplets of coal-water slurry containing petrochemicals and their effect on ignition characteristics

Abstract

The coal-water slurries based on different types of coals and waste petrochemicals or other combustible liquids (CWSP) are very attractive for creation of cheap energetic fuel (for replacement of the coal and gas). Together with evident economic effect, the CWSP is perspective from ecological point of view. It contains a lot of water and therefore the air pollution by nitrogen and sulphur oxides is low. However, the energetic parameters of combustion of such slurry are enough far from excellence. Numerous publications were done during last years about development of new CWSP compositions as well as about improvements of its ignition and combustion techniques. The actual task is stable ignition of the slurry composition at low (critical) temperatures. In this work, we have investigated for a first time the influence of coagulation and splitting of fuel droplets onto the integral parameters of the ignition (using high-speed video-recording technique). These results clearly show the differences of ignition processes of single CWPS droplets and the aerosol flow ignition in combustion chamber. Our experiments gave the understanding of scales of co-influence of fuel droplets onto the ignition parameters. We have used the fuel compositions based on milled (particle size ∼100μm) lignite and bituminous coal, coal-processing wastes, waste industrial oils and mazut. The investigations were done for two-body system where droplet sizes were the same (0.5–1.5mm) at every experiment. The temperature in the combustion chamber was in range from 600K to 1200K. Some sequences of coagulation/splitting were observed: coagulation/splitting inside the combustion chamber; droplet coagulation before injection into the combustion volume. The influence of these processes on the ignition delay time and critical ignition temperature was defined. It was shown that splitting of the CWSP droplets leads to essential (for some times) decrease of the heating stage duration and faster ignition. Otherwise, the coagulation leads to deceleration of fuel heating and therefore to deceleration of gas-phase and heterogeneous ignition. It was shown for a first time that ignition delay time of coagulating droplets can be determined as average value between ignition delays of separate initial droplet and the droplet with the twice bigger volume. The critical ignition temperatures of separate fuel droplets are in range between temperature conditions of the splitting and coagulation. The ignition delay times of fuel fragments which appear after splitting are much lower than for initial droplets. At low temperature ignition the conditions of droplets coagulation and splitting are very important because the ignition delays durations can differ for some times. The influence of the coagulation and splitting effects becomes vanishingly small at oxidant flow temperature more than 1000K. The obtained in this paper results should be accounted for combustion chambers working in low-temperature regime of the CWSP combustion that looks attractive from ecological, technical, economical and fire-safe points of view.