There are several studies being carried out on kinetics modeling for biomass gasifi­cation using air-steam gasification, but limited studies on pure steam gasification. Corella and Sanz (2005) developed a reaction kinetics model based on pyrolysis and gasification in circulating fluidized bed gasifier. Several reactions have been consid­ered in the modeling including fast pyrolysis reaction, oxidations reactions, steam reforming of methane, tar reforming, char reforming, and water gas shift reaction. The char gasification reaction is presented as follows:

char [CH0.20O0.13 ] + 0.38H2O ® 0.54CO + 0.45H2

The kinetics for all reactions has been considered first order based on the easiest or simplest kinetics available in the literature. For example the kinetics for the char gasification reaction was selected from the literature (Gonzalez-Saiz 1988) as follows:

Г10 = k1oCchai2CH20 k10 = 2.0 x105exp (-6,000/T)

Furthermore, the all rate equations for all reactions were solved using Chemical Reaction Engineering rules. For example the overall volumetric rate equation for hydrogen was presented as follows:

RH2 _ 1 + d8 ‘Г8 + d9 ‘Г9 + d10 ‘Г10 — r4

Nikoo and Mahinpey (2008) have presented a comprehensive model for biomass air-steam gasification in fluidized bed using pine saw dust as biomass. Both kinetics and hydrodynamics parameters have been considered with few assumptions.

For the reactions kinetic model, the reaction equations for combustion (CO) and steam gasification (SG) given by Lee et al. (1998) were chosen as follows:

(1 — —со Г

——SG = k exp l-^SG I Pn

d*cO + d^SG Y, Pc£SYC dt dt 0 Mc

Furthermore, the model was validated with the experimental data taken from the literature and the mean error calculated between the experimental value and the predictions. The parametric studies have been done with temperature, steam/bio — mass ratio, equivalence ratio, and particle size on the product gas composition and carbon conversion efficiency.

Lu et al. (2008) considered fluidized bed reactor for kinetics model of biomass air-steam gasification using assumptions of isothermal and steady state conditions. Furthermore, pyrolysis has been considered as instantaneous process. The wood powder has been taken as biomass and the following eight reactions (adopted from Lu et al. (2008)) have been solved in MATLAB.

C + O2 —— CO2

C + CO2 —— 2CO

C + H2O —— CO + H2

C + 2H2 —— CH4

CO + H2O —— CO2 + H

CO + H —— CO + HO
2CO + O2 —— 2CO,

CH4 + H2O —— CO + 3H.

The all kinetics constants (k0-k7) have been chosen from the literature. Furthermore, the model has been validated with experimental data of pine sawdust taken from the literature.

Ji et al. (2009) presented a kinetics model for steam gasification of biomass for enriched hydrogen gas production from biomass. A simplified flow sheet has been also presented to get pure hydrogen based on fluidized bed gasifier, steam reformer, and H2 membrane water gas shift reactor. Several reactions have been considered in all reactors. The rate of reactions for all reactions has been solved using the kinetics data from the literature. Furthermore, the model has been validated with the experi­mental data taken from the literature. The effect of temperature and steam/biomass ratio has been studied on the hydrogen purity and yield. The temperature and steam/ biomass range was taken 960-1,120 K and 0.5-3.0, respectively. The hydrogen purity was predicted more than 60 mol% at 1,023 K and steam/biomass ratio of 3.0. Furthermore, they reported that the lower heating value of the product gas decreased by increasing both temperature and steam/biomass ratio due to the increase of hydrogen in the product gas.