Characteristics of radially propagating smoldering combustion in a sawdust bed

In this thesis, experimental work on smoldering of sawdust beds is presented and discussed. Extensive study has been done on wood burning cook stoves with an emphasis on performance characterization and optimization. Few studies, however, have focused on the smoldering process with the goal of understanding the propagation of the front and the production of emissions. In this study, photographs, temperature and emission measurements on smoldering sawdust clearly showed the evolution of the combustion process: an initial conversion of the raw sawdust to char followed by the conversion of char to ash. In general, the char front propagated symmetrically in the radial direction while the ash front was not symmetric, and typically followed paths where oxygen was most readily available. Further analysis was accomplished by observing the characteristics of the sawdust bed before transition to flaming occurred. Contrary to expected results, flaming did not occur as the air flow was increased, but rather once it was decreased, suggesting that flaming is determined by a balance between generation of volatiles and dilution by incoming air. Experiments with vitiated air, in which the oxygen content of air is diluted by adding nitrogen, were conducted to determine a limit at which combustion was no longer self-sustaining. Experiments showed that vitiated air with 7% oxygen in the supply air did not support self-sustaining combustion. Finally, a comparison between poplar and walnut was conducted to show the effect of wood species. Comparison of temperature, hydrocarbon, and carbon monoxide outputs identified characteristic differences between the poplar and walnut species.