Time-Space Evolutionary Mechanism Of Coupling Multiparameters During The Combustion Of Low-Concentration Coal Mine Methane In Porous Media

Extraction of coal mine methane(CMM) has been regarded as the main method to improve the mining safety. Thus, the CMM extraction volume increased greatly with the increasing of coal production. CMM is not only greenhouse gas but also a great clean energy. However, the utilization rate of extracted CMM was very low and nearly 60% of the CMM volume was discharged to the air. The main reasons include the too low concentration, together with the fluctuant concentration and flow. Thus, the traditional technologies are difficult to utilize the CMM safely and efficiently. In comparison, the premixed combustion in porous media combines several outstanding features, such as high combustion efficiency, extended lean flammability limits, high combustion efficiency, and low pollutant emissions, which are very suitable for the utilization of low concentration CMM. Nevertheless, the present studies are limited to the ideal operating conditions during the experimental design and the conditions based on CMM engineering were rarely considered. Meanwhile, the studying methods were traditional and more practical contents need to be explored deeply.This paper combines the methods of site test, theory analysis, experimental exploration and numerical analysis in order to study the time-space evolutionary mechanism of coupling multiparameters during the combustion of low-concentration coal mine methane in porous media. The main results are showed below:(1) The characteristics of CMM and porous media material are studied. The extracted places of CMM influence the concentration, flow, and the fluctuation of them significantly. Before the utilization of CMM, the adjustment method of operation condition should be established by the design of extracting tubes; The physical parameters of Al2O3 and Si C ceramic foams were obtained by experiments, such as the main components, distribution of pore diameter, mean pore diameter, density, porosity, and so on, which can be used in the combustion model of porous media and the analysis of experimental results.(2) Steady combustion characteristics of low-concentration CMM in porous media are studied. A new burner, which combines the porous structures of reticulated foam and packed bed, is proposed to stabilize the combustion wave; The characteristic of heat transfer in packed bed of 13-mm pellets is similar to that of 10 PPI ceramic foams; The packed bed of pellets can be embedded in the space between the ceramic foam and heat exchange tube, which reduces the effect of space on the heat transfer; The range of velocity limits of the new burner shrinks compared to the single ceramic foam burner; With the increasing inlet velocity, the NOx emissions increase first and then decrease, but the trend of CO emissions is inverse and the HC emissions decrease all the velocities. Meanwhile, the HC emissions of the new burner are between that of the single packed bed and single ceramic foam burners.(3) Unsteady combustion characteristics(upstream) of low-concentration CMM in porous media are studied. The upstream propagation of combustion waves is kept at an even velocity. During the propagation, the combustion products temperatures at the burner outlet gradually decrease, while the peak temperatures are unchanged; When the dispersion effect was considered in the model, the propagation velocity of combustion wave is increased by nearly 19.8%, which is closer to the experimental results; With the increase of methane concentration, the upstream propagation velocity of combustion wave and the peak temperature both increase. As the inlet gas velocity is increased, the upstream propagation velocity decreases but the peak temperature raises; As the heat loss through walls is increased, the upstream propagation velocity and the peak temperature both decrease; With the upstream propagation of combustion wave, the CO emission quantity gradually decreases, while the NO emission quantity first increases slowly and then stabilizes. Moreover, when the peak temperature is lower, the time needed before the stabilization of NO emission quantity is shorter.(4) Unsteady combustion characteristics(downstream) of low-concentration CMM in porous media are studied. In order to obtain two-dimensional(2D) temperature distributions by interpolation method, 2D temperature measuring points were distributed; During the propagation of the combustion wave in the porous medium, unstable phenomena such as flame rupture and inclining will occur. These phenomena are more evident when the flame is closer to the burner’s outlet; The propagation of the combustion wave is accelerated as the flow velocity increases. However, the propagation of the combustion wave is decelerated with the increase of equivalence ratios; The 2D temperature distributions provide a basis for validation of heat dissipation coefficient during numerical modeling; The propagation of the combustion wave is accelerated with the increase in pellet diameter, while the propagation velocity of the combustion wave declines with the increase of burner length.(5) Combustion characteristics of low concentration CMM with water are studied. When the content of water in the CMM increases, the peak temperatures and NO emissions both decrease linearly. However, the CO emissions show apparent quadratic function relation with the content of water; The peak velocity of the gas in the burner is 5.4 times as large as that of the inlet velocity(0.5 m/s) so that the impact force of the gas should be considered during the burner design; As the content of water in the CMM increases, the range of the steady combustion limits shrinks. The change of upper limits are not obvious(0.15~0.2 m/s), while the lower limits decrease apparently; This paper defines the changing rate of peak chemical reaction rates which is used to obtain the reaction steps obviously and slightly influenced by the water.(6) Scale effects of porous media burner on the combustion characteristics of lowconcentration CMM are studied. The NO emissions decrease as the diameter increases. However, the CO emissions increase with the diameter; The NO and CO emissions both decrease with the burner length; When the scale of the burner increases, the velocity limits show the fluctuating characteristics which are more obvious with a larger equivalence ratio. The effect of burner scale on the velocity limits should be considered during the burner design.(7) Combustion characteristics of low concentration CMM in divergent porous media burner are studied. The peak temperatures increase linearly with the equivalence ratios and the stabilized flame positions move to the burner upstream. As the increase of equivalence ratios, the NO emissions gradually increase. However, the CO emissions increase first and then decrease; When the inlet gas is preheated, the peak temperatures in the burner are improved and the NO emissions increase significantly. The level of fresh gas preheating should be determined by the thermal efficiency and pollutant emission comprehensively.The above conclusions contribute to demonstrating the time-space evolutionary mechanism of coupling multiparameters during the combustion of low-concentration coal mine methane in porous media. Meanwhile, the results based on the engineering background provide the theory reference for the practical application and design of porous combustion technology. During the study of this paper, nine academic papers were published. Four papers were collected by SCI database and two papers were collected by EI database. One first prize of the province level was obtained, together with four inventions and five utility model patents were authorized during the study.