(586f) An Experimental Investigation of Solid Carbon Conversion to Fuels in the Presence of CO2

To meet the challenges of increased energy demand and waste management we must move to an atom economy where every atom is utilized in the best possible manner.  To achieve that goal, a fundamental understanding of the underlying mechanisms and processes of energy and waste generation is necessary.  Furthermore, if we consider where the world stands today in terms of energy use and where it will be in 100 years, there is a significant challenge to meet the expected demand in a more CO₂-neutral way.  Therefore, a primary objective for new energy technology development must be to produce carbon-free energy through the advancement of technologies that use both conventional and alternative sources.

We have recently reported on the impact of CO₂ on biomass gasification and its ability to more effectively gasify biomass than steam.  Continuing this investigation has led to understanding the impact of heating rates and different reaction environments.  This paper will present the results from the gasification of various biomass feedstocks.  The heating rates were varied from 1^oC min^-1 to 100 ^oC min^-1 to ballistic rates (~ 9000^oC min^-1).  The gasification mediums investigated include H₂0/N₂, CO₂/N₂/H₂0, and O₂/N₂.  Global activation energies for pyrolysis were found to be significantly higher than for gasification while those for the grasses were significantly lower than the woods, possibly indicating a catalytic effect during pyrolysis of the high mineral content herbaceous feedstocks.  CO₂ pyrolysis (110-450^oC) activation energy values for lignin, cellulose and biomass averaged  36, 216, and 50 kJ mol^-1 and CO₂ gasification (500-700^oC) values for lignin and biomass were 25 and 33 kJ mol^-1, though cellulose did not exhibit significant gasification behavior.  Using a least squares fit on the rate of mass loss fraction, the global decomposition reaction during pyrolysis for lignin in either medium was found to be third order while that for cellulose was first order and for the various biomass samples either first or second order.  In addition, the impact of pretreatment on biomass will be presented. 

In addition, rapid heating rates (greater than or equal to 700ºC min^-1) were utilized to gasify Clean Wood and two Refuse Derived Fuel (RDF) samples using thermogravimetric analysis coupled to gas chromatography (TGA/GC).  Reaction atmospheres included Air, 5% O₂/95% Ar, 10% O₂/90% Ar, 100% Ar and steam and were used to produce gas evolution profiles for hydrocarbons ranging from H₂ to C₄H₁₀.  While expected results were obtained using Air reaction atmospheres, some interesting results were observed using steam and Ar.  Different concentration profiles and production rates of C₂H₆ compared to C₂H₄ and C₂H₂ enabled some understanding of the reaction sequence occurring during gasification under rapid heating conditions.  Kinetic analysis showed pre-exponential factors of 8.00x10²⁷, 2.02x10²⁹ and 3.71x10²³ (sec^-1 K^1/2) for samples Clean Wood, RDF C (Industrial Solid Waste basis) and A (Municipal Solid Waste basis), respectively.  Furthermore the apparent activation energy was determined to be 22, 71, and 185 (kJ mol^-1) for Clean Wood, RDF C and A respectively indicating that the Clean Wood is slightly more reactive than RDF C and more reactive than RDF A. 

Finally, it will be shown how CO₂ can adjust the products of gasification and fast pyrolysis to a desired output.  This process allows the decoupling of the biomass feedstock from the final product.  Data will be presented focusing on the relation between gasification medium, feedstock selection and major chemical species evolution.