(41a) The Impacts of Biomass Particle Size During Air- and Oxy-Cofired Combustion

The cofiring of biomass in pulverized coal boilers for large-scale power generation can only be a viable option if current combustion standards of stability, reliability, emission reduction, and fuel conversion efficiency are maintained and/or improved.  While biomass fuels can be highly variable and differ significantly from coal, generally, they have increased volatile matter content and increased particle sizes, which are due to both fuel preparation methods as well as the structure of the biomass material.  These two characteristics can significantly impact the structure of the volatile flame.  The volatile flame zone is dominated by the combustion of volatiles in the near burner region.  The length and location of the volatile flame envelope is also important not only to flame stability, but also to the formation of pollutants including NOx and SOx. 

 In this work, pulverized coal and biomass cofired flames are investigated under both air and oxyfuel combustion conditions using both experiment and Computational Fluid Dynamics (CFD) to determine the effects of volatile fraction, fuel composition, and particle size on the length of the volatile flame.  Volatile flame length is inferred from experimentally obtained gaseous species concentrations for CO and CO₂ in a 35 kW combustion facility.  The numerical study evaluates the impacts of these variables, both individually and combined, on volatile flame length.

 This study shows the length of the volatile flame envelope is sensitive to the location of volatile matter release and the amount of volatiles available.  For larger particles with high momentum and long heating times, they may pass through the oxygen-lean flame zone and release volatiles outside the volatile flame envelope into areas of higher oxygen.  Particle breakthrough of the volatile flame can result in reduced fuel concentration in the near burner fuel-rich zone, which can reduce the volatile flame length.    Increased volatile matter content, characteristic of biomass fuels, can lead to increased volatile flame length if all volatiles are released in the near burner region.  However, if particles breakthrough the volatile flame envelope, then shorter than expected flame lengths may result. The volatile matter content of the particles that have broken through the flame envelope is determined using thermogravimetric analysis (TGA).  Particles are sampled using a nitrogen-quenched particulate matter sampling probe downstream of the volatile flame.