(575ai) Experimental Kinetic Analysis of CO2 and Steam Gasification of Biomass Fuels

The current investigation is a kinetic study involving a TGA and GC analysis of the mass decomposition and gas evolution from several biomass feedstocks and representative lignin and cellulose structural components. The samples underwent both H₂O/N₂ and CO₂ gasification. Kinetic parameters describing the thermal decomposition rate data under both gasification media were experimentally determined. Activation energy, pre-exponential factor and reaction order were determined for both of the model structural components, Organosolve lignin and microcrystalline cellulose, under heating rates of from 1-100 K/min. Use of CO₂ during the gasification process resulted in enhanced CO and depressed H₂ and CH₄ concentration levels in the gasification products. Thermo-gravimetric analysis and gas chromatography were used to study the mass decay and gas evolution profiles of the various biomass feedstocks that included woods, grasses, agricultural and forestry residues, lignin and cellulose. While similar mass decomposition profiles were produced during pyrolysis for a given heating rate as a result of either CO₂ or steam gasification, the high temperature behavior was significantly different. Though significant residue resulted during steam gasification, nearly complete conversion of the feedstocks occurred during CO₂ thermal processing for all heating rates observed. Using CO₂ as the gasification medium resulted in a significantly more porous structure that enabled the reactive gasifying agent to more easily access the biomass volume and resulted in a more complete processing of the biomass fuel. By varying the heating rate, selective component separation can be achieved, due to the difference in degradation rates of the lignin and cellulose molecular species. Mass decomposition rates of the biomass samples during gasification were found to be intermediate between those of the thermally resistant lignin and the cellulose components though pyrolysis behavior was not framed by these two extreme rates. Many of the biomass fuels, particularly the herbaceous feedstocks, underwent significant pyrolytic decomposition much earlier than the pure structural components indicating a possible coupling of the degradation mechanisms and the presence of a catalytic effect due to the high mineral composition. Though the onset of thermal degradation for the cellulosic component occurs at a slightly higher temperature (250^oC) than the lignitic component (175^oC), corresponding to a higher activation energy for cellulose pyrolysis, the cellulosic fraction undergoes rapid thermal degradation and conversion to gaseous products at a much lower temperature than the thermally resistant lignitic fraction of a biomass sample. Through a suitable thermal treatment involving pyrolysis at a very slow heating rate, the capability exists to gain greater control of the low temperature cellulose decomposition while still retaining the maximum residual lignin available for subsequent processing. By varying the heating rate and the gasification medium, the capability exists to adjust the relative percents of the lignocellulosic components available following pyrolytic treatment at about 450^oC. While the polysaccharide cellulosic component of biomass has gained much attention in the production of biofuels, the aromatic and highly cross-linked lignitic component offers unique opportunities for the production of oxygenated hydrocarbons, aromatic species and other chemical intermediates that can serve as a variety of feedstocks to replace their fossil-fuel derived hydrocarbon counterparts. The available fractions of the lignocellulosic structural components available for chemical treatment or further thermal processing through gasification can be adjusted by varying the heating rate and the gasification medium. It is through this control of the thermal processing parameters, as well as the appropriate design of the reactor and the judicious selection of the biomass feedstock, that optimization of the thermal treatment process can be achieved.