## 2 Modeling Methods
### Reaction Kinetics  
The biomass gasification in the presence of steam includes a series of complex and competing reactions, including homogeneous and heterogeneous reactions. These reactions occur simultaneously in four overlapping steps, i.e., drying, thermal decomposition, oxidation, and reduction. The overall reaction of biomass steam gasification is described as follows:  
\[\text{Biomass}+\text{H}_{2}\text{O}\rightarrow\text{H}_{2}+\text{CO}+\text{CO}_{ 2}+\text{CH}_{4}+1\,\text{hydrocarbon}+\text{T}\text{ar}+\text{Char} \tag{1}\]  
Initially, the moisture content of biomass reduces to less than 5% [35]. Then, devolatilization and cracking of weaker chemical bonds takes place at a temperature ranging from 250 to 700 \({}^{\circ}\)C [36], producing various fractions: gas, a liquid/condensed, and a solid [37; 38; 39; 40; 41]. In this step, the biomass converts into solid char which can range from 5% to 10% for fluidized bed reactors, or 20% to 25% for fixed-bed reactors [37; 38; 39; 40; 41]. The volatiles and solid char react with limited oxygen in the oxidation stage to produce mainly CO, CO\({}_{2}\) and H\({}_{2}\)O, and the heat from this stage can supply to the endothermic reactions ((2) and (3)). The unreacted char is converted by steam and CO\({}_{2}\) to form the final gaseous products [36; 42]. The yield of hydrogen from steam gasification is significantly higher than that of fast pyrolysis followed by a steam reforming of char. \[\text{C}+\alpha\text{H}_{2}\text{O}\rightarrow(2-\alpha)\,\text{CO}+(\alpha-1 )\,\text{CO}_{2}+\alpha\text{H}_{2}\,\text{(Water-gas reaction)} \tag{2}\]  
\[\text{C}+\text{CO}_{2}\rightarrow\text{2 CO}\,\text{(Boudour reaction)} \tag{3}\]  
where \(\alpha\) has been experimentally determined in the range of 1.5-1.1 at 750-900 \({}^{\circ}\)C [43]. For the proposed model, the selected value of \(\alpha\) was 1.3, showing good agreement with experimental data. Water-gas shift reaction and steam reforming reaction occur simultaneously according to gasification conditions, playing a key factor for hydrogen production:  
\[\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2} \tag{4}\] \begin{table}
\begin{tabular}{c c c} \hline \hline  & Fluidized Bed Reactor & \\ \hline Temperature (\({}^{\circ}\)C) & 800–900 \\ Pressure (bar) & 0.92 \\ Diameter (m) & 0.0545 \\ Height (m) & 0.55 \\ \hline \hline  & Bed Material (Silica Sand) & \\ \hline Mean particle size (m) & 177 \(\times\) 10\({}^{-6}\) \\ Density (kg/m\({}^{3}\)) & 2650 \\ Mass weight (kg) & 0.8 \\ \hline  & Steam & \\ \hline Temperature (\({}^{\circ}\)C) & 300 \\ Flow rate (kg/h) & 0–0.084 \\ Concentration (\%) & 0–33.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental setup parameters used in the simulation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **Method** & **Range** & **Unit** & **Rel. Error in \%** \\ \hline CO\({}_{2}\) & Infrared & 0–100 & Vol.\% & \textless{}0.5 \\ CO & Infrared & 0–20 & Vol.\% & \textless{}0.5 \\ CH\({}_{4}\) & Infrared & 0–5 & Vol.\% & \textless{}0.5 \\ H\({}_{2}\) & Paramagnetic & 0–20 & Vol.\% & \textless{}0.5 \\ O\({}_{2}\) & Paramagnetic & 0–25 & Vol.\% & \textless{}0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The method for continuous measurement of the gas composition. \[\text{CH}_{4}+\text{H}_{2}\text{O}\rightarrow\text{CO}+\text{H}_{2} \tag{5}\]  
The carbon in the char is gasified with steam and CO\({}_{2}\) ((2) and (3)).