The results of the theoretical and experimental studies of the ignition processes of a completely new class of fuel – Bio water-coal (Bio-WCF) based on charcoal and coal. Based on the results of a high-speed video recording of the ignition process, the main stages of thermal preparation (evaporation of moisture from the surface layer of the particle, thermal decomposition of the organic part of the coal) and the ignition of Bio-WCF drops have been identified. The experiments have been carried out on the setup providing a low level of error (not more than 11.2%) of the main recorded values (ignition delay time (t_ign), a characteristic size of the fuel particles at the initial time d, temperature of the oxidizing agent T_e). The temperature regime has varied over a wide range (873≤ Te≤1273 K) and corresponded to the actual operating conditions of the steam and hot water boilers. It is established that the addition of charcoal to the structure of a water-coal suspension leads to a significant acceleration of the ignition process. It has been shown that at relatively low temperatures (Te = 873 K) the addition of charcoal to the WCF structure accelerates the ignition process by 40%. Under conditions of high oxidizer temperatures, the ignition delay times of Bio-WCF and WCF are identical. According to the results of the experiments, it has been found that when particles of bio-coal-carbon fuel based on high-volatile A bituminous coal are ignited, dispersion of the surface fuel layer is possible. In the case of the ignition of Bio-WCF based on semi-anthracite, such an effect has not been registered. Based on a detailed analysis of the video frames, a mathematical model of the ignition process of suspension fuel based on water, coal, and charcoal is developed. The numerical simulation of the ignition process has been carried out and the ignition delay times have been calculated. A comparative analysis of the theoretical and experimental values of t_ign has shown their good agreement. According to the results of a mathematical modeling, the values of the concentrations of the components of the gas mixture at the moment of ignition of a particle of bio water-coal fuel have been established. It is shown that a vapor-gas mixture is formed in the particle boundary layer with a large concentration gradient and with a complex topology caused by a convective-diffusive motion in the “volatile-oxidizer” system.

MATEC Web of Conferences, open access proceedings in Materials science, Engineering and Chemistry

The article presents the experimental research results of the flame propagation processes in the small vicinity of Bio-WCF particles during their ignition. According to the experimental researches, there have been obtained characteristic ignition delay times (t_ign) and times of full engulfment of the fuel particle in fire (t_sf). We have analyzed the influence of metamorphism degree of fuel basic component (coal) and sort of biomass on characteristics of these processes. It has been shown that increase in concentration of biomass in fuel leads to decrease in ignition delay times and fire propagation in boundary layer of the Bio-WCF particle. We have recognized that the oxidizer temperature (T_g) strongly influences the fire propagation dynamic in small volume vicinity of fuel particles. Also, we have observed that the effect of influence of oxidizer temperature (T_g) on the time of full engulfment of the fuel particle in fire (t_sf) differs from the predicted once. Increase in T_g in a certain period of changes in its values (T_g = 1073–1273 K) leads to increase in time of full engulfment of the Bio-WCF particle in fire. The hypothesis describes the influence of ambient temperature on t_sf was formulated. We have substantiated the hypothesis using the results of mathematical modeling. This hypothesis describes the physical mechanism of the influence of water vapors, which are formed during intrapore moisture evaporation on characteristics and conditions of volatile combustion in the boundary layer of the fuel particle.