(94b) Hydrogen Production Via Gasification of Solid Carbon Fuels

This presentation will compare experimental and simulation results of an integrated catalytic combustion, solid carbon gasifier to produce hydrogen and electricity. The technology, which is a form of steam reforming, focuses on the combining of catalytic combustion with gasification to generate H2 and CO from a carbon source such as biomass while yielding a CO₂ sequestration ready stream. The H₂ can either be separated from the CO or remain as a mixed stream for use as an IGCC turbine fuel. An Aspen® Plus simulation shows that a steam to carbon ratio of 1.5 provides a hydrogen output of 1.15 kg/hr while generating about 14 kW of electricity (for every one kmol/hr of carbon fed to the reformer) from a SOFC using the portion of the CO generated that was not needed to drive the reforming reactions. Additionally, recycling up to 25% of CO₂ into the reformer produces about 15% more hydrogen, while using 20% less CO for combustion to drive the gasification reactions. Because of the Boudouard reaction an extra 32% electrical capacity (4.5 kW per kmol/hr of carbon) can be generated from an SOFC operating on the CO not used for combustion. The advantage of coupling catalytic combustion with the gasification process is that it enables the heat producing combustion reactions to operate at lower temperatures thus decreasing NO_x and resulting in more energy efficient conversion. A portion of the carbon monoxide is catalytically combusted to carbon dioxide with either stoichiometric amounts of oxygen or air. If combusted with oxygen a pure carbon dioxide stream is produced that can be sent for sequestration with minimal oxygen usage. A series of experiments were done to compare against the ASPEN® simulation results. The experiments were mainly 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. Several biomass sources were analyzed individually to enable comparisons between each species and help adjust the ASPEN® parameters to better reflect biomass conversion in general. The species tested were various pulverized woods and grasses that were comprised of maple, poplar, pine, spruce, oak, green Douglas fir, pine needles, maple bark, alfalfa, switchgrass and cordgrass. Each specimen feedstock was slowly heated at a rate of 10^oC min^-1 in a nitrogen-steam atmosphere. The woods and grasses had similar TGA decay curves, resulting in three main decomposition regions. Not until the third decay region, in which the temperatures reached between 900^oC -1000^oC, was the biomass decomposition complete. In addition, Bituminous coal particles (38, 45, 53, 63 and 106 µm) were tested under the same conditions to ascertain major differences between varieties of carbon feedstocks. Compared to the biomass decomposition thermograms, most of the coal samples were still experiencing mass loss at 1000^oC and thus did not exhibit the plateau observed for biomass. To determine the feasibility of enhanced hydrogen production from gas recycle, differing amounts of carbon dioxide to nitrogen gas ratios were introduced (0%, 25%, 50%, and 75% CO₂) and coupled with online gas chromatography. Four major products were quantified, H₂, CO, CH₄ and N₂, 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. Verification of the numerical simulations was experimentally observed under certain concentrations for solid carbon (i.e. C) gasification at atmospheric pressure. Finally, the residuals of the biomass test samples were analyzed using Atomic Adsorption Spectrometry. A significant quantity of potassium was identified in the grass chars which is likely responsible for observed corrosive behavior of the grasses during gasification in the TGA. The high alkaline content of many grasses is an important consideration in the choice of an appropriate carbon-neutral biofuel and likely will require pre-processing necessary to minimize corrosion of the process equipment. The biomass feedstocks vary in their concentrations of active constituents such as potassium, calcium, magnesium, phosphorus and sulfur that can pose a major problem to downstream equipment.

Keywords: biomass, waste-to-energy, gasification, catalytic combustion, hydrogen generation