(4bc) The Impact of Biomass Fuels On Flame Structure and Pollutant Formation During Biomass Cofiring Combustion

Pulverized coal flames have been optimized for large-scale power generation to minimize criteria pollutant emissions and attain maximum fuel conversion and boiler efficiency.  This traditional way of producing power is facing new challenges due to rising CO₂ emissions, criteria pollutant emissions regulations, and renewable portfolio standards (RPS).  Biomass cofiring is a technology that can maintain this traditional method of power generation, while reducing emissions and potentially meeting RPS.  However, biomass fuels are widely variable and different from coal, which can cause changes in the overall flame structure.   

The volatile flame zone, which is the part of the flame that is dominated by the combustion of the gaseous volatiles, is strongly affected by changes in fuel characteristics.    The effects on volatile flame length in pulverized fuel flames, especially with biomass cofiring and oxyfuel combustion, have been studied recently.  An important contribution of these works is understanding the impact of particle size and volatility on volatile flame length, and the potential that particles can break through the volatile flame zone prior to releasing all of their volatiles. 

In thi work, a model is employed to predict the volatile flame size of pulverized fuel flames, and scaling laws are used to predict particle breakthrough, particularly for conditions associated with biomass cofiring.  Time and length scales of the flame and particle devolatilization processes are calculated and compared to determine to the overall participation of volatiles towards the volatile flame length.   A dimensionless number is developed to characterize conditions of particle breakthrough.