Small wood pellet boiler 3-D CFD study for improved flue gas emissions employing flue gas recirculation and air staging

Abstract

Wood biomass fuels have become increasingly important in terms of achieving future sustainability targets regarding RES, especially to reduce GHG and the use of domestic energy sources, on other hand with these fuels the pulutant air emissions have to be addressed with great attention. Nowadays, air staging and flue gas recirculation are often used as a primary emission reduction measure in large scale biomass fuel fired boilers, but their combined application in small scale commercial wood pellet boilers is not so common. The implementation of this approach to the small scale burning devices can enable further development of small boilers, in order to achieve a more complete combustion and reduction of pollutant emissions, especially CO, NOx, and PM. In the presented work a comprehensive numerical and experimental study on the combustion parameters is performed, in order to determine the overall combustion properties. A commercial 32 kW small-scale hot water wood pellet boiler was modified numerically from the operating point of view. The findings of this study, like different combinations of recirculated flue gases and secondary air amount, can serve as useful guidelines for the new innovative design and optimisation of the air and recirculated flue gas injection process parameters. The scenarios of this study, where recirculated flue gas was added to the secondary air, were beneficial. Optimally, a CO reduction of 63% was achieved by adding 30% of recirculated flue gas to the secondary air stream, and a 22% reduction was achieved for the NO emissions.