Experimental study of flue gas condensing heat recovery synergized with low NOx emission system

Abstract

To improve overall thermal efficiency while simultaneously reducing the NOx emissions of gas boilers, a novel flue gas condensation heat recovery and low NOx emission system that integrates a direct contact heat exchange unit with a combustion air humidification unit is proposed. A well-designed experimental system was established and a test bench was set up to study the effects of variable multi-factor operating conditions on the waste heat recovery and NOx emission properties. The synergistic enhancement principle between the flue gas waste heat recovery process and low nitrogen emission is described in detail. The dew point temperature of the flue gas is improved significantly by humidifying the combustion air, which contributes to both low nitrogen emission and full waste heat recovery. The experimental results show that when the combustion air humidity ratio increased from the zero-humidification condition to 49.0 g/kgdry air, the dew point temperature of the flue gas increased by 6.4 °C. At a spray water flow rate of 0.83 m3/h and with a heat supply network return water temperature of 45 °C, the flue gas waste heat recovery efficiency and the waste heat utilization efficiency of the heat supply network increased by up to 12.2% and 6.9%, respectively, and the average NOx emissions decreased to 50.0 mg/m3. In the NE mode, with a spray water flow rate of 1.03 m3/h, the combustion air humidity ratio is increased to 101.2 g/kgdry air, the NOx emission reduction procedure is more effective and can be reduced to 23.3 mg/m3. The experimental system offers advantages in terms of energy savings, environmental protection, and economic benefits, and also outperforms other competing systems with respect to these issues.