Theoretical and experimental study of waste-heat-recovery boiler with water injection

The waste-heat-recovery boiler with water injection (HR-B/W) applies the heat exchange between the intake air and exhaust gas with the water injection into the intake air. Previous theoretical studies have discussed that the HR-B/W would increase the thermal efficiency of the boiler by the active heat exchange between the intake air and exhaust gas. It has also been discussed that the increased fraction of water vapor in the air would reduce the flame temperature which in turn decreases the NOx emission. However, the potential performance of the HR-B/W has not been validated through practical boiler tests by considering the evaporation characteristics of the injected water, which plays a critical role in the performance of the HR-B/W. In this study the effects of water injection into the intake air on the thermal efficiency and pollutant emissions of the waste-heat-recovery boiler are investigated using a commercial 24 kW condensing boiler in full load condition. Thermodynamic analysis is performed to evaluate the adequate amount of water injection and trace the physical properties in the boiler upon the water injection amount and evaporation characteristics. The boiler test results showed water injection can increase thermal efficiency to 4.4% point and reduce NOx and CO emissions by 69% and 33% respectively compared to those without water injection. These advantages can be further enhanced if the atomization and evaporation performance of injected water is improved.