(177d) Understanding the Gasification Reactivity of Biomass Derived Chars

Biomass gasification comprises of two processes in series: pyrolysis and char gasification. Since char gasification is the slowest step in the overall biomass gasification process, extensive research efforts are being made to understand the fundamental descriptors of char reactivity and, thus, to derive a design model to predict char gasification conversion. However, the major hurdles to achieve these objectives are the inherent complexity of biomass, widely different char properties generated from different types of biomass and under different pyrolysis conditions. Concurrent variations in many char properties prevent the deconvolution of the effect of different char properties on char gasification reactivity. To partially overcome these limitations, three biomass feedstocks are used in this study (sugarcane bagasse, switchgrass and loblolly pine) in an effort to segregate the effect of different inorganics, ash content and specific surface area while still maintaining the inherent biomass feedstock properties. Biomass chars were generated in a pressurized entrained flow reactor (PEFR) operating at high heating rates (≥ 10^3 oC/sec) and short residence times (1-30 sec). Char gasification reactivity was studied in a thermogravimetric analyzer (TGA).

We present here the results on char physiochemical properties (namely, surface area from N₂ and CO₂ physisorption; ash content and composition determined by ICP-OES, active surface area of chars measured by CO₂ chemisorption, and long-range order of char by XRD) and their effect on gasification reactivity. It was found that the char specific surface area alone does not correlate well with char reactivity from different biomass, indicating that the concentration of specific active sites rather than the total surface area might be a better descriptor, since inorganics varied from one biomass char to another.  However, even the K and Ca content may not correlate well with reactivity for all biomass chars because silica present in some biomass (like bagasse) reduces catalytic activity of potassium. Thus, a combination of surface area, inorganics content and composition, and thus dispersion of inorganics together may be a better descriptor of char reactivity. Active surface area of chars was measured by CO₂ chemisorption, which can quantify the cumulative effect of above factors.