Suggestion on the Simultaneous Reduction Method of CO and NOx in Premixed Flames for a Compact Heat Exchanger

Abstract

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 NOx 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 on NO and 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 L1, whereas the CO production was controlled by the distance L2. 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.