Numerical Simulation Of Combustion Of Low Concentration Coal Bed Gas In Metal Fiber Burner

Low concentration coal bed gas refers to those gas mined under coal mine withconcentration of CH4less than30%.When the concentration of CH4is within5%-16%,which is the explosion concentration of CH4, there would be a fatal potential safetyhazard for normal combustors to use it. Limited by the technique at present, lowconcentration coal bed methane is mostly forbidden to use directly, so it is usually to beemitted to the air. However, too much methane emitted to the air would give rise toenvironment pollution and waste of energy sources. Therefore, it is of great significancefor energy saving and emission reduction to find ways to recycle these coal bed methanesafely and efficiently. Due to its low quantity of pollutant discharged, high combustionefficiency, and strong load adaptability, metal fiber burner has received high attention ofthe researchers. Burning the methane gas within explosion range concentration safelyand efficiently by the metal fiber burner has been regarded as a promising research area.In the thesis, the combustion of methane gas within explosion concentration wassimulated numerically on the surface of five metal fiber burners located symmetricallyin a3D torch barrel. Experimental set was proposed as well for further experimentmeasurement.In the simulation, finite-rate/eddy-dissipation model and the normal k-ε modelwere employed for turbulence flow. Analyses of speed, temperature and concentrationfield of the burning procedure indicate the flow and combustion characteristics in thetorch barrel and metal fiber burner with different premix gas speeds and CH4concentrations. The results show that under cold condition, the premix gas flowed outuniformly from the surface of metal fiber burner, which makes it clear that the metalfiber burner based on the experiment has a better flow characteristic for the premix gasunder cold condition. Under reacting condition, when the CH4concentration reached10%in the premix, the efficiency of the burner could attain more than99.5%for all theflow speeds of0.5m/s,0.7m/s,0.9m/s and1.1m/s. Particularly, when the flow speedis1.1m/s, the strength of the turbulence motion and conflict between gases current werelargest, which leads to the highest combustion efficiency of CH4and generation rate ofreactants among all the four speeds mentioned above. However, the flame front was faraway from the wall of the burner barrel which means longer torch barrel are required,and the combustion temperature was high across a wide area which would strengthenthe generation of NOX. With flow speed of0.5m/s and0.7m/s, combustion reaction proceeds slowly, and the combustion efficiency of CH4is lowest with flow speed of0.5m/s while the case with flow speed of0.7m/s has a lowest generation rate ofreactants, although flame front were closely located to the wall of burner barrel andareas with high temperature were small. Still, there is hardly any difference of the peakvalue of temperature and combustion efficiency between those four premix gas speeds.This indicates that the premix burner has a strong adaptability for difference burningload. Since a best premix gas speed has been obtained, further simulations have beencarried out for cases that CH4concentration was6%,8%and10%respectively, all withpremix gas speed of0.9m/s. The results show that the combustion efficiency reached ahighest value when the CH4concentration was6%, with lowest pollutant emission andsmallest high temperature area. Besides, no backfire and blow-off were observed whichmeans a highly stable combustion. Generally, the case with premix gas speed of0.9m/sand CH4concentration of6%is considered to be the best combustion conditions. Thefollow-up experiment is carried out based on this simulation result.An experiment system for metal fiber burner was designed and established by theauthor. Further research would be carried out by comparing the results betweenexperiments data, simulation results and industrial experiments to give a generallystatement about the safe reliability and pollutant emission characteristics of the metalfiber burner for methane gas within explosion concentration. It is promising that goodfundaments of theory and technique will be settled by this project for metal fiber burnerfor low concentration coal bed gas to be widely used in industry.