OCR Text |
Show NO and NOx emissions in the flue gas were measured by a ThermoElectron analyzer with the proper measuring ranges. Figure 9 shows NOx data versus air/fuel ratio at different firing intensities. All data obtained in the present study have demonstrated that ultra-low NOx emissions from the porous combustion from 1 to 12 vppm (corrected to 0% 02) were achieved. Strong dependence of NOx upon air/fuel ratio is . evidence , while firing intensity has a much less impact on NOx formation. The principle of the porous -combustion mode can explore the mechanism of NOx emission reduction. First, significant heat removal from the combustion reaction by the radiating surface reduces the reaction temperature in the porous media. Second, combustion can be completed during extremely short residence times because very intensive combustion takes place on a great amount of porous surface. Although an accurate reaction time in the porous media is not available, it is believed that this reaction time is much less than that in a conventional gas-phase flame. Therefore, NO formation in the porous combustion is far from equilibrium. Finally, the residence times of c'ombustion gases passing through the porous media are also very short, between 3 to 15 milliseconds, in the present tests. Once the hot gas leaves the radiating surface, its temperature is suddenly dropped. Therefore, further formation of NOx emissions in the post-flame flow is minimized. CONCLUDING REMARKS An experimental study was conducted to investigate the performances of a bench-scale porous combustor ope~ating in the porous-combustion mode. Flame stabilization within the porous bed, radiant heat output from the surface, and combustion emissions were evaluated. It has been proven that the flame can be stabilized within the porous media in a large range of operating parameters. Particularly, the porouscombustion mode is not sensitive to firing rate that provides a remarkable capability of high turndown operation. Although flame stability is relatively sensitive to air/fuel stoichiometric ratio, the range of stable porous-combustion mode is still large enough to meet a variety of applications. The stable porous-combustion performances as follows: mode has demonstrated unique • Very high combustion intensi ty up to 500 MBtujh- ft2 (1.6 MW/m2) and combustion density up to 6 MMBtujh-ft3 (19 MW/m3). • Very high radiant heat output up to 80 MBtujh-ft2 (252 kW/m2) • Higher thermal efficiency of the radiant burner up to 40% |