Experimental study on effects of supply-air humidification on energy and emission performance of domestic gas boilers.

• Experiments on the effect of supply air humidification on the energy and emission performance of domestic condensing boiler is undertaken. • Supply air humidification is beneficial for improving boiler efficiency, which is higher when the humidity of supply air is larger. • More latent heat can be recovered in the condensing heat exchange process of condensing boiler by supply air humidification. The traditional coal-fired boiler used for heating has a low efficiency and emits large amounts of pollutants. Thus, the use of natural-gas-fired boilers has become a popular method for heating all over the world owing to their cleanliness and high efficiency. Condensing domestic gas boilers have higher thermal efficiency than traditional boilers. However, they waste a large amount of heat. Supply-air humidification provides a beneficial effect on the efficiency of heat recovery, but the effects of supply-air humidification on the thermal performance of combustion and the amount of heat recovered from the latent heat remain unclear. Moreover, the effect of air humidification on pollutant emission must be further investigated. In this study, an experiment was conducted to investigate the effects of supply-air humidification on the thermal efficiency and emissions of a domestic boiler. The results indicated that supply-air humidification is beneficial for the improvement of the overall boiler efficiency. The maximum improvements in the boiler efficiency were 3.7% and 3.8% when the excess-air ratios were 1.3 and 1.1, respectively. Supply-air humidification had a significant effect on the condensing heat-exchange process. The maximum increments of the condensed water were 20.2% and 23.47% when the excess-air ratios were 1.3 and 1.1, respectively. When the excess-air ratio was 1.3 and the temperature and relative humidity of the supply air were 41.22 °C and 12.06%, respectively, almost half of the latent heat from the flue gas was recovered. Supply-air humidification reduced the concentration of NO x to a value within the emission limit. However, the amount of humidification needs to be controlled within a reasonable range to avoid massive generation of CO. [ABSTRACT FROM AUTHOR]