Application of high-frequency water injection for the performance improvement of waste-heat-recovery boilers

It has been discussed that injecting water into the intake air can increase thermal efficiency and reduce NOx emissions simultaneously in waste-heat-recovery boilers (WHRB). To increase the evaporation rate of the injected water and recover more thermal energy through the heat exchanger, the water injection amount needs to be controlled properly and droplets in the spray need to be distributed homogeneously with the smallest droplets possible. To achieve this, this study introduces a high-frequency injection strategy that can generate strong in-nozzle turbulence, thus promoting spray atomization and supplying continuous water to the heat exchanger. Three different injection frequencies are applied to investigate water spray characteristics. The water spray structure is visualized using the Mie-scattering technique, and the droplet size is measured using the laser diffraction method. In addition, practical boiler tests are performed to confirm the effect of the high-frequency water injection on the boiler performance. The experimental results showed that the mean droplet size decreased, and the spatial and temporal homogeneity of the spray distribution was improved as the injection frequency increased. As a result of the higher water injection frequency, the evaporation rate and thermal efficiency of the boiler increased, and nitrogen oxide (NOx) emissions decreased.