Effect of hydrogen-blending ratio and wall temperature on establishment, NO formation, and heat transfer of hydrogen-enriched methane MILD combustion

Abstract

Hydrogen-enriched methane moderate or intense low-oxygen dilution (MILD) combustion is a promising technology that offers an attractive pathway to achieve low NOx and CO2 emissions in furnaces and boilers. This paper numerically investigates the influence of hydrogen-blending ratios (XH2 = 0–100 %) and wall temperature (Twall = 1500–300 K) on establishment, NO formation/reduction mechanisms, and heat transfer behaviors of hydrogen-enriched methane MILD combustion with non-preheated air in a laboratory-scale furnace. Results show that hydrogen addition increases the peak Damköhler number from 7.5 to 20.3, which could cause departure from MILD combustion at high wall temperatures. However, lowering Twall can slow down the MILD reaction rate and thus reduce Damköhler number closer to unity, which is beneficial for achieving hydrogen-enriched methane MILD combustion. Interestingly, as XH2 is increased from 0 to 100 %, the threshold wall temperature for achieving MILD combustion is extended from 1170 to 690 K. In other words, MILD combustion could be sustained/stabilized under low Twall conditions (encountered in boilers) when burning pure hydrogen or fuels with a high percentage of hydrogen. Moreover, as Twall is lowered, the major NO formation pathway is changed from thermal-NO to NNH and N2O-intermediate in pure hydrogen MILD combustion; also, H reburning via the pathway of NO→+HHNO→+HNH→+NON2 becomes more important in reducing NO at a lower Twall level. As XH2 is increased from 0 to 100 %, the radiative heat transfer is enhanced by 29.4 % while the convective heat transfer is reduced by 16.8 %, ultimately resulting in an increase of 15.0 % in the total heat transfer (Twall = 1300 K). Along the furnace sidewall, the distribution of heat flux has a low value near the burner exit, peaks in the downstream furnace, and declines further downstream close to the furnace exit in MILD combustion with non-preheated air.