Particle temperature and flue gas emission of a burning single pellet in air and oxy-fuel combustion

Pelletization of biomass increases the bulk energy density and the uniformity in size and shape with minimized mechanical degradation from transport, storage, and handling. A pellet can be burned in industrial circulating fluidized-bed or fixed-bed furnaces. This experimental study examines the combustion behavior of single pine wood and empty fruit bunch pellets in a laboratory-scale entrained-flow reactor. Individual biomass pellets were oxidized under both N₂/O₂ (79%/21%) and CO₂/O₂ (79%/21%) to investigate the burning characteristics during oxy-fuel combustion. The gas temperature was set at 860 K, 970 K, and 1090 K to examine its effect on combustion behavior. The burning pellets were directly observed in a visualization window in the reactor throughout the sequential combustion process. Direct observation, optical pyrometry, and flue gas analysis are synchronized for each pellet to interpret the relation between phenomenological events, particle temperature, and flue gas emission. The experimental results shows an increased homogenous ignition delay and a conical, less sooty flame in the presence of CO₂. The particle temperature under CO₂/O₂ atmosphere is slightly lower than that under N₂/O₂, especially for the volatile combustion. The CO emission under CO₂/O₂ is significantly higher and produced from incomplete combustion. These combustion characteristics are attributed to low oxygen diffusion and the high volumetric heat capacity of CO₂. The pine wood displayed a fast, homogeneous ignition and a sooty flame with high NO and SO₂ emissions compared to the empty fruit bunches.