(111b) Kinetics of Steam Gasification of Biomass Using Fluidizable Ni/La2O3-gAl2O3 Catalyst

Kinetics of Steam
Gasification of Biomass Using Ni/La₂O₃-γAl₂O₃ Catalyst

Jahirul Mazumder and Hugo I. de
Lasa

Chemical Reactor
Engineering Centre (CREC), Department of Chemical & Biochemical
Engineering, The University of Western Ontario,
London, ON N6A 5B9 (Canada)

Steam gasification of biomass involves a complex
network of heterogeneous reactions [1]. Primary reactions break down the
vaporized biomass molecules, forming permanent gases, higher hydrocarbons and
coke. Secondary reactions crack the higher hydrocarbons into
gases. Furthermore, permanent gases react to alter the gas composition
depending on gasifier conditions.

A Ni-based
catalyst supported on fluidizable γ-Al₂O₃ is one of
the most promising catalysts due to its high surface area, high activity for
tar conversion and affordability [2]. Gasification of glucose (a model compound
for cellulose) and 2-methoxy-4methylphenol (a model compound for lignin) was
conducted using the La₂O₃ modified Ni/γAl₂O₃
catalyst at different steam/biomass ratios, temperatures and reaction times in
a CREC fluidized riser simulator [3]. Significant improvements in dry gas yield
and carbon conversion compare to the non-catalytic and catalytic gasification
using Ni/α-Al₂O₃ catalyst at the same operating
conditions confirmed the high activity of the developed catalyst for reforming
of tars compounds and coke combustion. The trends of gasification products (H₂,
CO, CO₂, CH₄, etc) with the variation of these parameters
are in consistent with the thermodynamics predictions [1]. With the increase in
temperature, H₂ and CO concentrations were increased while CO₂
and CH₄ concentrations were decreased. On the other hand, increasing
CO₂ and decreasing CO profiles were observed with the steam/biomass
ratio and residence. TOC analysis of spent catalyst showed negligible coke
deposition.

Glucose gasification results showed that H₂,
CO, CO₂, CH₄ and H₂O are mainly present in the
product gas with negligible C₂⁺ species and coke
deposited on catalyst surface. Thus, steam reforming of methane, CO methanation and water gas-shift reaction can be considered
as the dominant reactions. Rates of these reactions can be modeled using a
Langmuir-Hinshelwood type rate equation. This approach considers chemical
species adsorption as well as intrinsic kinetics. After simplifications and
linearization it results,

  The
overall rate of formation/disappearance of each chemical species can be written
as:

Knowing that the CREC Riser Simulator is a well
mixed batch reactor, a balance equation for each species “i” can be expressed as follows:

Thus, a set of differential equations representing
the catalytic steam gasification of glucose can be obtained by substituting eq (1)-(4) into eq. (5). Adsorption constants for CO₂ were
calculated using experimental data from the CREC Riser Simulator. Intrinsic
kinetic parameters were estimated from experimental glucose gasification
results using a “nlinfit”
subroutine from Matlab.

Key words: biomass
gasification, La₂O₃ modified γ-Al₂O₃,
Ni catalyst, Langmuir-Hinshelwood equation, CREC riser simulator

References

[1]    
E. Salaices, B. Serrano, H.I. de Lasa, Biomass Steam Gasification Thermodynamics Analysis
and Reaction Experiments in a CREC Riser Simulator, Ind. Eng. Chem. Res. 49
(2010) 6834-6844.

[2]     H.I.
de Lasa, E. Salaices, J. Mazumder, R. Lucky, Catalytic Steam Gasification of
Biomass: Catalysts, Thermodynamics and Kinetics, Chem. Rev. 111 (2011)
5404-5433.

[3]     H.I.
de Lasa, Riser Simulator for Catalytic Cracking
Studies, US patent 5, 102, 628 (1992).