|Institution:||Washington University in St. Louis|
|Keywords:||Coal; Biomass; Cofiring; Flame Length; Moisture Content; Volatile Breakthrough; Heat Transfer, Combustion; Other Chemical Engineering|
|Full text PDF:||http://openscholarship.wustl.edu/eng_etds/81
Cofiring biomass with coal can contribute to meeting Renewable Portfolio Standards (RPS) and reduce pollutant emissions. The physical characteristics and composition of biomass can vary significantly, which can affect the combustion characteristics. When cofiring biomass with coal, these differences can impact the structure of the volatile flame, the region where combustion of volatiles dominates. The length and location of the volatile flame is important to flame stability and determines the location and extent of volatile release. This has an effect on pollutant emissions, such as NOx (nitrogen oxides). Previously, the effects of parameters such as cofiring ratio, particle size, and air-fired versus oxyfuel conditions on volatile flame length and breakthrough were studied. This work is continued by investigating the effect of moisture variation in the biomass fuel on flame length and volatile breakthrough, a phenomenon whereby fuel particles pass through the volatile flame without completing volatile release. The effects of biomass moisture content on the volatile flame when cofiring PRB coal with oak sawdust was determined. Experiments were done in a 35 kWth combustion facility, with 20 wt.% sawdust, with sawdust moisture contents of 10, 20, and 30%. The volatile flame length was determined by CO and CO2 profile measurements along the centerline of the flame. CFD modeling work was done with ANSYS Fluent 13.0 to simulate experimental conditions and to investigate the mechanisms causing changes in the system with moisture. The results indicate that volatile flame length is inversely correlated to biomass moisture content for 20 wt.% cofiring ratio. Volatile breakthrough is correlated to biomass moisture content, as moisture affects fuel particle size. As the moisture content increases so does the particle size, possibly due to aggregation. Larger particles require a longer heating time associated with their size and have a lower residence time in the flame due to their momentum. This secondary effect of moisture results in less volatile content released in the flame. This results in a smaller flame envelope with increased breakthrough. Results are presented and discussed.