Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: self-heating; smoldering ignition; plywood; bowes. Includes bibliographical references.

This report is a study on the smoldering combustion of natural sawdust from untreated woods. The objective was to develop and test an experimental technique to study the fundamental behavior of sawdust smolder. The experimental setup was an annulus cookstove packed with sawdust between the inner and outer radii creating a central hollow core. The sawdust was ignited by a heating coil wrapped around the inner radius. Thermocouples were embedded in the sawdust bed fanning out in the radial direction, and temperature was recorded throughout the smolder process. Consistent with the literature, the experimental results indicate that wood smolder consists of three pathways, 1) sawdust to char, 2) sawdust to volatiles, and 3) char to ash. Pathways 1 and 3 can be clearly characterized by the temperature profile of the smolder; however, pathway 2 often involves flaming of the sawdust and is beyond the scope of this study. Pathway 1, sawdust to char, is an endothermic reaction that results in a clearly defined char front that propagates across the sawdust bed in the radial direction. As smolder proceeds, the char continues to oxidize and breaks down into non-volatile products such as water vapor and carbon dioxide (CO₂) in further exothermic reactions. Pathway 2, char to ash, is an exothermic reaction that can lead to glowing combustion when exposed to sufficient amounts of oxygen and results in spikes in temperature. In contrast to the clearly defined char front, the ashing zone traces cracks in the sawdust where air can penetrate through, and has no discernible front. Section 1 discusses the motivation behind the study of sawdust cookstoves in third world countries, in particular Ghana, to replace wood with sawdust as a cheap alternative for household fuel. Section 2 details the experimental setup of the cookstove rig and the methodology of the experiments conducted. Section 3 reports the results of the experiments and analyzes the temperature profiles in relation to the three types of chemical reactions as noted above. Section 4 concludes with a summary of the results and discusses efforts in measuring emissions from the smolder and future work to be done. / text

"This paper investigates the thermal conditions at the surface and at depth of 1.8 cm (3/4-inch) maple plywood exposed to heat fluxes between 6 and 15 kW/m2 in the cone calorimeter for up to 8 hours. The minimum heat flux for unpiloted smoldering ignition was 7.5 kW/m2 and compared favorably to classical self-heating theory. The role of self-heating was explored via temperature measurements distributed within the specimens. Elevated subsurface temperature profiles indicated self-heating was an important ignition factor resulting in ignition at depth with smolder propagation to the surface and into the material. The ignition depth was shown to be a function of the heat flux with the depth moving towards the surface as the heat flux increased. Supporting work included sensor calibration testing, mass loss rate analysis, char depth testing and computer modeling. The calibration testing showed optical pyrometer temperature measurements compare favorably to those of surface mounted thermocouples. Mass loss rate analysis was found to be a lagging indicator of smoldering ignition. The char depth tests showed that the rate of change of the temperatures recorded at depth increased around the time the derived char front passed. Computer modeling (HEATING) of a heat flux applied to the plywood for conditions similar to the performed ignition tests compared favorably to experimental data for sub-critical incident heat flux temperature profiles, excepting surface temperatures. For heat fluxes near critical, the model correctly predicted thermal runaway below the sample surface. At higher heat fluxes simulation results indicated surface ignition at times significantly earlier than experimental results."

A MATLAB model was developed for the smoldering combustion of wastewater in the context of a decentralized residential wastewater treatment appliance. Data from a batch process sewage smoldering experiment was simulated using implicit finite-difference approximations, assuming one-dimensional transient conductive heat transfer. The time-dependent temperature profiles within the column represented the main parameters of interest and were used to verify recoverable heat energy estimations. Given that the modeling method used for this thesis represents a unique approach, the assumptions and limitations of this model are thoroughly described in the context of reproducing results for other smoldering setups. A lack of convergence is seen in the model validation section of this report. Consequently, the practicality of this particular model contains significant limitations. Theoretical applications are also discussed and analyzed in terms of comparisons to modern alternatives and prototype feasibility.