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A Kinetic Model for Supercritical Water Gasification of Biomass

Fernando L. P. Resende and Phillip E. Savage. Chemical Engineering Department, University of Michigan, H.H. Dow Building, 2300, Hayward St., Ann Arbor, MI 48109-2136

The use of renewable sources of energy such as biomass is an alternative that has the potential to reduce our reliance on imported oil while addressing environmental concerns. Among the technologies that convert biomass into fuels, Supercritical Water Gasification (SCWG) has been suggested to process wet feedstocks because of the ability of water to dissolve organic components of plant materials at supercritical conditions, promoting formation of gases such as H2 and CH4, and limiting the amount of tar and char formed as residues.

We gasified cellulose and lignin as model compounds for biomass in supercritical water, and avoided catalytic effects from the reactor walls by using quartz reactors. The experimental results were used to develop a kinetic model able to describe formation of the gas products in SCWG. The reactions are drawn from the literature. For example, the reactions in the cellulose SCWG model include:

Cellulose Hydrolysis: (C6H10O5)n + n H2O => n C6H12O6

Glucose Decomposition: C6H12O6 => CxHyOz

Steam-reforming I: CxHyOz + (x-z) H2O => x CO + (x z + y/2) H2

Steam Reforming II: CxHyOz + (2x-z) H2O => x CO2 + (2x- z+ y/2) H2

Char formation through intermediates: CxHyOz => w C + Cx-wHyOz

Water-Gas Shift: CO + H2O <=> CO2 + H2

Methanation: CO + 3 H2 <=> CH4 + H2O

Experimental results at different temperatures, times, biomass loading and water densities were used to determine the best-fit kinetics parameters.

The kinetic parameters obtained in this model were used to extrapolate experimental results and predict which conditions optimize H2 yield and energetic content of the product gas. This type of model can be used to simulate the performance of a SCWG plant and determine its thermal efficiency, in order to establish a comparison with current technologies for biomass processing.