Porous Media Combustion For Low Calorific Gases And Combustion Instabilites

Combustion in an inert porous medium has numerous advantages such as high flame speed, low NOx and CO emissions, extended flammability limit, which can achieve burning of lean combustible mixtures in the superadiabatic combustion regime. This study is devoted to the porous meida combustion theory and mechanism for low calorific value gases. Experimental research and numerical simulation are carried out on the combustion characteristics and combustion instabilities, and the results are appled to the design and development of large-scale low calorific value gases processing and heat utilization system.Experimental study and numerical simulation are carried out in this paper which can be divided into six parts. Stable combustion characteristics includes:①experimental studies on the lean CH4/air combustion characteristics in a two-layer porous medium burner;②numerical studies on the effects of low calorific value gases components on the combustion characteristics in a two-layer porous medium burner. Combustion instabilities includes:③experimental studies on the combustion temperature changes and burner overheating damage when operation conditions changed;④numerical studies on the inclined flame front break of filtration combustion in porous media. Finally, in a large-scale reciprocal flow porous media combustion test system, the devlopment and design of⑤a non-premixed reciprocal flow porous inserted metal-melting burner and⑥ahigh temperature gases generating system based on reciprocal flow porous media combustion are carried out.Combustion characteristics, including flame stability limits, temperature distribution, thermal efficiency, lean burn limit, and pollutant emissions, of low calorific value gas with different components in a two-layer porous burner were experimental and numerical studied. The results showed that the combustion characteristics of low calorific synthesis gases with CO and H2as the main combustible components were different from the lean CH4/air combustion. It was found that different types of porous media burners had different lean burn limits. The selection principle of porous media burners for different low calorific value gases was proposed. Coalbed methane mixtures with methane concentrations greater than3.84%as well as low calorific synthesis gases, of which CO and H2as the main combustible components, can be utilized in a two-layer porous burner. However, Coalbed methane mixtures or coal mine ventilation air with methane concentrations between0.57%～3.84%can only be utilized in a reciprocal flow filtration combustion reactor.In terms of combustion instabilities, the combustion temperature changes and the burner overheating damage when operation conditions changed were experimental studied. The results showed that there was a big temperature fluctuations when the flame front movement direction was changed. A substantial increase in gas inlet velocity can be more prone to the burner overheating damage. The filtration combustion of lean methane-air mixtures in an inert porous medium was investigated by a two-temperature and two-dimensional model in Fluent software. The development of flame inclination was studied by focusing on the flame front break which splits into two or three waves as the combustion wave propagates downstream. Inclined flame front break was determined by the Lewis number, equivalent ratio and gas inlet velocity of the premixed gases. It was confirmed that hydrogen-rich low calorific value gas filtration combustion flame front was prone to break.Based on the laboratory experimental and numerical results above, a non-premixed reciprocal flow porous inserted metal-melting burner and a high temperature gases generating system were developed in a large-scale reciprocal flow porous media combustion test system. The developed non-premixed reciprocal flow porous inserted metal-melting burner can achieve a approximately75%reduction in the NOx emission and a fuel saving of approximately30%compared to the conventional regenerative boiler before. It was most suitable for a lean combustion process at an equivalence ratio lower0.4with NOx and CO emission levels within20-30ppm and10-20ppm, respectively. Finally, the design guidelines of the new non-premixed reciprocal flow porous media metal-melting furnace was given out. A high temperature gases generating system based on reciprocal flow porous media combustion was developed. Combustion of coal mine ventilation air with1.2%methane concentration can produce a stable high temperature gases higher than1000℃, with the system thermal efficiency greater than80%.