Suggestion on the simultaneous reduction method of CO and NO_x in premixed flames for a compact heat exchanger

This study focused on the design of a domestic gas-condensing boiler, in which the burner and the heat exchanger were contained within a small combustion chamber. A method was suggested for the simultaneous reduction of CO and NO_x by optimum placement of the heat exchangers in the postflame region of the CH₄/air premixed flame. In this study, the effect of the location of heat exchangers onNOand CO emission characteristics was numerically simulated using a quasi-one-dimensional model. To validate this effect, the distance (L₁) between the burner exit and the first heat exchanger and the distance (L₂) between the first and second heat exchangers were varied. The radiative heat loss and the effective heat-loss coefficients (heff, exch) for the conductive and convective heat losses were also considered. In general, the NO mole fraction decreased as the distance L₁ decreased. That is to say, when the burner exit was closer to the flame, it reduced the peak temperature, which decreased the formation of thermal NO. However, the CO mole fraction increased, as opposed to the NO mole fraction, as the distance L₁ decreased, because the temperature was already below 1200 K. The amount of CO production continuously decreased because of the increased oxidation of CO as the distance L₂ increased, whereas the amount of NO production showed little change. Consequently, the location of heat exchangers directly affected the amount of NO and CO production. The NO production was mainly controlled by the distance L ₁, whereas the CO production was controlled by the distance L ₂. The distance L₁ should be set up close to the flame to reduce NO production, and the distance L₂ should be long enough to guarantee the sufficient conversion time from CO to CO₂ for the purpose of reducing CO emissions. Copyrigh