TED-AJ03-616 Extinction Limits of Counterflow Diffusion Flames of CO, H_2,CH_4 and Their Blends

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In order to investigate combustion characteristics of mixed gaseous fuels produced by waste materials in incinerators, extinction limits of counterflow diffusion flames for CO, H_2,CH_4 and their blends, whose gaseous flows are opposed to room-temperature and high-temperature oxidizers up to 733 K mixed with gaseous H_2O, were studied experimentally and numerically. A cylindrical porous burner, Tsuji burner, was used for the experiments. The computations were conducted on a one-dimensional counterflow diffusion flame with detailed chemical reactions and radiative heat losses. The effects of the stagnation velocity gradient, the fuel concentration, the oxygen concentration, and the oxidizer composition and temperature on the flame extinction limit were investigated, and also, the colors of visible flame were observed. The calculated results were generally consistent with the experimental results. For CH_4 / air diffusion flames, the stagnation velocity gradient at extinction increased by approximately on order, and the minimum fuel concentration decreased about in half, when oxidizer temperature increased from a room temperature of 300 K to a high temperature of 773 K. As oxygen concentration decreased, the stagnation velocity gradient at extinction decreased. The minimum oxygen concentration decreased with increasing the oxidizer temperature. Since CO by itself as a fuel cannot burn in dry air, the stagnation velocity gradient at extinction for (CO + N_2) / air diffusion flame increases with H_2 or CH_4 contents in the fuel. However, when the N_2 fraction in the fuel decreases, the H_2 content has a lesser effect on flame extinction. If H_2O is contained in air instead of H_2 being contained in fuel, CO burning is effectively improved due to H atoms resulting from H_2O decomposition in the flame. For CO diffusion flames, the stagnation velocity gradient at extinction in high-temperature air of 773 K is one order higher than that in room-temperature air. Variation of the stagnation velocity gradient at extinction for CO diffusion flames with fuel concentration is more sensitive compared to that for CH_4 flames. Visible CO flames mixed with a small amount of H_2 in the fuel is thicker than CH_4 flames in room temperature air, and emit a luminous blue-white light. In high-temperature air, CO flames emit a bright blue-white light on the fuel side and a red light on the oxidizer side, which is luminescence of H_2O vapor. In high-temperature O_2 + H_2O oxidizer, CH_4 flames are thicker than those in room-temperature air, and emit a red light because of the large amount of H_2O vapor emitted by the flame.[figure]
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TED-AJ03-616 Extinction Limits of Counterflow Diffusion Flames of CO, H_2,CH_4 and Their Blends

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