(743e) A Kinetic Model for Supercritical Water Gasification of Biomass

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
=> C_xH_yO_z

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

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

Char formation through intermediates: C_xH_yO_z
=> w C + C_x-wH_yO_z

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.