To facilitate experimental introduction of biomass-to-energy technologies in an upper-division undergraduate thermodynamics course, a small, inexpensive wood chip gasifier was designed, constructed, and tested. This device was built from a metal vacuum-flask-style thermos bottle, and it was constructed for less than $50. The design is both simple and economical. In the reported experiments, the gasifier processed pine wood chips (rabbit cage litter-available from any pet store), but it could also accommodate a variety of other dry, solid biomass feedstock including other wood types, grass, shredded paper, or leaves. Oriented in an 'inverted downdraft' configuration, the gasifier motivates teaching opportunities through experiments in heat transfer, fluid mechanics, thermodynamics, and combustion. The apparatus provides both enriching outdoor demonstrations for lecture classes and serves as a laboratory exercise (as reported here) viable for any energy-thermal-fluids course. Within a single charge of wood chips, there are two reaction zones. The bottom wood layer smolders, converting chemical energy to heat. This heat conducts upward through the chips into the pyrolysis layer. Here the wood is converted to syngas composed of flammable hydrogen, carbon monoxide, and methane as well as inert nitrogen and carbon dioxide. Natural convection drives the syngas through the gasifier's burner (the thermos bottle's neck). In parallel, ambient air is drawn up the space between the thermos inner flask and outer wall. At the burner, combustible fuel combines with oxygen in the air to support a flame. To juxtapose biomass gasification and subsequent syngas combustion against directly burning wood chips, students also burned identical masses of pine wood chips placed in an open-air container. Students logged mass depletion as a function of time and measured the initial and final wood chip masses for both combustion techniques to quantify energy conversion efficacy through ash generation. They found gasification converted more than 97.8% of the biomass to syngas while direct burning left more than 5% of the initial fuel mass as ash. In some cases direct combustion even left behind substantial unburned wood. Students also took temperature measurements inside the flames for both combustion techniques. Experimental results compared qualitatively to adiabatic flame temperature predicted for syngas versus cellulose and lignin fuels. Despite similar predicted adiabatic flame temperatures, syngas produced by the gasifier yielded a hotter flame than did direct wood chip combustion because the gasifier's fuel consumption rate was higher and neither system adiabatically contained the flames.
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