Experimental Research on Effects of Combustion Air Humidification on Energy and Environment Performance of a Gas Boiler

To increase the waste heat recovery (WHR) efficiency of gas boiler and decrease NO_x emissions, a flue gas total heat recovery (FGTHR) system integrating direct contact heat exchanger (DCHE) and combustion air humidification (CAH) is put forward. The experimental bench and technical and economic analysis models are set up to simulate and evaluate the WHR performance and NO_x emissions in various operation situations. The results show that when the air humidity ratio elevates from 3 g/kg_{dry air} to 60 g/kg_{dry air}, the dew point temperature increases by 7.9 °C. When the flue gas temperature approaches the dew point temperature, the rate of improvement of the FGTHR system's total heat efficiency notably rises. With spray water (SW) flowrate and temperature of 0.075 kg/s and 45 °C, the WHR efficiency relatively increases by up to 8.4%. The maximum sensible and latent heat can be recovered by 4468 w and 3774 w, respectively. The flue gas temperature can be reduced to 46.55 °C, and the average NO_x concentration is 39.6 mg/m³. Compared with the non-humidified condition, the NO_x and CO₂ emissions relative reduction of the FGTHR system are 61.2% and 8.7%. The payback period of FGTHR system is 2 years. Through simulation, it can be concluded that the decrease in exhaust flue gas temperature and velocity, as well as the increase in exhaust flue gas humidity, has a negative impact on the diffusion of NO_x in the atmosphere.