(9b) CO2 Enhanced Gasification Of Biomass Fuel

The confluence of three significant events have shown how acutely sensitive the world, and in particular the United States, is to energy supply and security. First, the recent hurricanes in the Gulf of Mexico underscore a transition in the idea of energy security. Second, energy consumption is predicted to increase at least two-fold by 2050. In 1998, global annual energy consumption was 402 exajoules, with the United States portion of that amount corresponding to approximately 25%. Different scenarios have been proposed for future global annual energy needs with values of 837-1041 exajoules estimated for middle to high growth by 2050[1,2].Third, it is now recognized that global temperatures are rising faster than previously recorded [1,3]. If we consider where the world stands today in terms of energy use and where it will be in 2050 assuming continued economic development, we are faced with a daunting challenge of where that energy will come from if our energy profile is to be more CO₂-neutral. One of the best carbon based sources is biomass fuel yielding the greatest energy per unit carbon. A series of experiments were focused on biomass gasification to convert carbon to useful forms of energy while addressing environmental concerns. Using Thermogravimetric Analysis (TGA) we studied the biomass decomposition rates and formation of char residue. The species tested were various pulverized woods and grasses that were comprised of sugar maple, poplar, white pine, spruce, oak, Douglas fir, pine needles, maple bark, alfalfa, beachgrass and cordgrass. Each specimen feedstock was slowly heated at a rate of 10 K min^-1 in a nitrogen-steam atmosphere. The woods and grasses had similar TGA decay curves, resulting in three main decomposition regions, with the third region exhibiting a nearly constant residual mass at about 900^oC. Not until temperatures reached between 900^oC -1000^oC was the mass decomposition complete. In general it was observed that hydrogen evolution began later for the woods by about 100^oC compared to the grasses. Methane concentrations for the woods were typically 40 % of that for the grasses, yet had similar evolution profiles. In addition, methane profiles exhibited a maximum peak concentration 200% higher than the final burnout value of the ligno-cellulosic residual. CO₂ injection demonstrated an increased production of CO evolution beginning at temperatures above 700^oC. The CO₂ was increased in 5% increments from 0% to 50% with the largest enhancement observed to occur at the low concentrations. Comparing the CO evolution relative to no recycle (0% CO₂) at 940^oC for 5%, 20% and 50% CO₂: the enhancements for Douglas fir were 4.6, 5.4 and 7.1 times greater, whereas for beachgrass they were 12.9, 32.0, and 43.7 times greater. To determine the feasibility of enhanced biomass gasification, differing amounts of carbon dioxide to (N₂+steam+CO₂) total line flow gas ratios were introduced (0%, 10%, 20%, 30%, 40%, and 50% CO₂) and they were coupled with online gas chromatography measurements. Four major products were quantified: H₂, CO, CH₄ and CO₂, to determine a carbon dioxide recycle concentration that could optimize the rate of gasification of the solids and simultaneously increase the yield of H₂ and CO. CO levels are increase in the vicinity of 400^oC corresponding to lignin cleavage and elimination reactions and again increase above 750^oC due to the Boudouard and steam gasification reactions and desorption from the char. As the lignin component undergoes thermal cleavage, hydrogen abstraction reactions by intermediate methoxy radicals result in an increase in methane concentration in the vicinity of 400-500^oC that we observed for all types of biomass. As the cellulose and lignin components complete their degradation, the level of H2 abstraction reactions decreases and more H₂ becomes available in the vicinity of 500-650^oC. In this interval the water-gas shift reaction is also significant. We observed this as an abrupt rise in the detected H₂ levels in this interval. Hydrogen abstraction, condensation and radical addition reactions are responsible for the continued production of hydrogen between 700-1000^oC that were confirmed by the gas evolution data. Finally, a mechanistic interpretation will be presented to explain the results that were observed.

Key words: biomass, waste-to-energy, enhanced gasification