How quickly large biomass particles can ignite and burn out when transported into a pulverized-fuel (pf) furnace and suddenly exposed to a hot gas flow containing oxygen is very important in biomass co-firing design and optimization. In this paper, the ignition and burnout of the largest possible biomass (pine wood) particles in a pf furnace (a few millimeters in diameter) are studied experimentally in a single particle combustion reactor rig, in which the ambient gas temperature and oxygen concentration can vary in the ranges of 1473-1873 K and 5-20%, respectively. A one dimensional (1D) transient model is also developed to predict their conversion, in which the key processes inside the particle and in the boundary layer outside the particle are properly considered. For the pine wood particles in which large temperature gradients exist, the primary heterogeneous ignition is always detected for all the test conditions. As the particle is further heated up and the volume-weighted average temperature reaches the onset of rapid decomposition of hemicellulose and cellulose, a secondary homogeneous ignition occurs. The model-predicted ignition delays and burnout times show a good agreement with the experimental results. Homogeneous ignition delays are found to scale with specific surface areas while heterogeneous ignition delays show less dependency on the areas. The ignition and burnout are also affected by the process conditions, in which the oxygen concentration is found to have a more pronounced impact on the ignition delays and burnout times at lower oxidizer temperatures.
Energy and Fuels, 2013, Vol 27, Issue 2, p. 1061-1072