Experimental And Numerical Study On Low Calorific Value Gas Premixed Flame

Biomass derived gases from gasification, pyrolysis, and landfills are renewable and CO₂ neutral fuels that have great potential to be usable in internal combustion engines, gas turbines, and industrial furnaces. It is important to know well about the base characteristics of the gases with low calorific value. Laminar burning velocities of five biomass derived gases have been measured at atmospheric pressure over a range of equivalence ratios, using the heat flux burner. Experimental studies about the stabilization of partially premixed turbulent flames with different biomass derived gases are carried out in a conical burner. Flame stabilization behavior with and without the cone is investigated and significantly different stabilization characteristics are observed in flames. Planar laser induced fluorescence imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are conducted to gain further understanding of the flame/flow interaction in the cone.For measurements of laminar burning velocity, the results of the bio-methane flame are generally in good agreement with data in the literature and the prediction using GRI-Mech 3.0. The measured laminar burning velocity of the industrial gasification gas is generally higher than the predictions from GRI-Mech 3.0 mechanism but agree rather well with the predictions from GRI-Mech 2.11 for lean and moderate rich mixtures. For rich mixtures, the GRI mechanisms underpredict the laminar burning velocities. For the model gasification gas, the measured laminar burning velocity is higher than the data reported in the literature. The peak burning velocities of the gasification gases/air and the co-firing gases/air mixtures are in richer mixtures than the biomethane/air mixtures due to the presence of hydrogen and CO in the gasification gases. The GRI mechanisms could well predict the rich shift for the pure gasification gases but failed for the cofiring gases mixtures. The laminar burning velocities for the bio-methane at elevated initial temperatures are measured and compared with the literature data.For flame stabilization and flame structure measurement, the data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.The stabilization characteristics and local extinction structures of partially premixed bio-methane/air flames are studied using simultaneous OH-PLIF/PIV techniques. The stability regime of flames is determined for different degree of partial premixing and Reynolds numbers. It is found that in general partially premixed flames are more stable when the level of partial premixing of air to the fuel stream decreases. For the studied burner configuration at high Reynolds numbers there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes develop leading to eventual flame blowout. Local flame extinction is found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.