SMALL WOOD PELLET BOILER 3-D CFD STUDY FOR IMPROVED FLUE GAS EMISSIONS EMPLOYING FLUE GAS RECIRCULATION AND AIR STAGING

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 com-mercial 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 combi-nations of recirculated flue gases and secondary air amount, can serve as useful guidelines for the new innovative design and optimisation of the air and recircu-lated 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 emis-sions.

Automated summary

Wood biomass fuels have become increasingly important for achieving sustainability targets, but the emissions from these fuels need to be addressed. Air staging and flue gas recirculation are commonly used in large scale biomass fuel fired boilers, but their combined application in small scale commercial wood pellet boilers is not common. This study investigates the combustion parameters of a small-scale wood pellet boiler and provides useful guidelines for the design and optimization of the air and flue gas injection process parameters.