June 23, 2013
June 23, 2013
June 26, 2013
Energy Conversion and Conservation
23.173.1 - 23.173.16
An Inexpensive Inverted Downdraft Biomass Gasifier for Experimental Energy-Thermal-Fluids DemonstrationsAbstractTo facilitate experimental introduction of biomass-to-energy technologies in an upper-divisionundergraduate thermodynamics course, a small, inexpensive wood chip gasifier was designed,constructed, and tested. This device, which was built from a metal vacuum-flask-style thermosbottle, was constructed for less than $50. The design is both simple and economical; inexpensiveenough that it could also be implemented as a student project. In the reported experiments, thegasifier processed cedar wood chips (rabbit cage litter – available from any pet store), but itcould also accommodate a variety of other biomass feedstock.Oriented in an ‘inverted downdraft’ configuration, the gasifier motivates teaching opportunitiesthrough experiments in heat transfer, fluid mechanics, thermodynamics, and combustion. Theapparatus could provide rich outdoor demonstrations in a lecture class or it could serve as astudent laboratory exercise (as reported here) in any energy-thermal-fluids course. Within asingle charge of wood chips, there are two reaction zones. The top wood layer smolders,converting chemical energy to heat. This heat conducts downward through the chips into topyrolysis layer. Here, the wood is converted to syn-gas composed of flammable hydrogen andcarbon monoxide as well as inert carbon dioxide. Natural convection drives the syn-gas throughthe gasifier’s burner (the thermos bottle’s neck). In parallel ambient air is drawn up the spacebetween the thermos inner flask and outer wall. At the burner, combustible fuel combines withoxygen in the air to support a flame.To juxtapose biomass gasification and subsequent syngas combustion against directly burningthe wood chips, students also burn an identical mass of cedar wood chips placed in an open-aircontainer. Students measure the initial and final wood chip masses for both combustiontechniques to quantify energy conversion efficiency through ash generation. They findgasification converts more than 99% of the biomass to syn-gas while direct burning leavessubstantial ash and even unburned wood behind. Students also take temperature measurementsinside the flames for both combustion techniques. Experimental results compare qualitatively toadiabatic flame temperature predicted for hydrogen and carbon monoxide versus cellulose andlignin in that the syn-gas produces a much hotter flame than does direct wood chip combustion.
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