(434d) Effect of Particle Size On Tar Formation in a Bench-Scale Gasifier Using Woody Biomass

Gaston, K. R., National Renewable Energy Laboratory
Nimlos, M. R., National Renewable Energy Laboratory

We employed a four-inch diameter fluidized bed reactor in combination with NDIR/TCD and molecular beam mass spectrometer to study the effect of biomass particle size on the yield of light gas and tar compounds during gasification. White oak was milled into four sphere sizes with diameters of 0.25, 0.5, 0.7, and 1.0 inch. Reactor bed temperatures of 700, 800, and 900°C were used. As expected, temperature had the greatest effect on tar production with yields increasing sharply after 800°C. Furthermore, the trend for the total tar yield as a function of particle size is complex. At 900°C, we observed a peak for 0.5 in. spheres; at 800°C total tar yield was relatively flat; at 700°C the 1 in. spheres produced relatively more total tar. Interestingly, the primary peaks for the PAH compounds, m/z 78 (benzene) and m/z 128 (naphthalene), increase with decreasing particle size, and the peak yields for the 0.5 in. spheres are still observed at 900°C. These initial observations suggest that PAH production is exacerbated by biomass sample geometries with a high surface area to volume ratio, while the total quantity of light gases produced at high temperature remains constant. The lower internal heating rates experienced by large diameter spheres favor an increase in total tar production at the expense of PAH production. The gas pulse observed in both the NDIR/TCD and the MBMS for the smaller, 0.25 in. spheres was on the order of 20 seconds and, for the 1 in. spheres, was between 2 and 3 minutes. This also shows that the smaller spheres are more rapidly heated than the larger ones and hence are subjected to more severe pyrolysis throughout the entire gasification process.