## 2 Materials and Methods
### Modeling Methodology
#### 2.2.2 Rectisol Unit  
The main purpose of Rectisol is to clean up the outlet syngas of gasification to obtain a high-quality syngas that can be used for the methanol synthesis [12]. The Rectisol process uses chilled methanol as the solvent to remove the major impurities, such as CO\({}_{2}\), H\({}_{2}\)S, COS, CS\({}_{2}\), and HCN, producing a cleaner syngas. Thus, the Rectisol system is able to obtain a pure syngas stream and simultaneously adjust its stoichiometric ratio to satisfy the requirements of methanol synthesis. In addition, the methanol used in the purification step can be reused over several cycles, making the process economically feasible.  
It is worth noting that the stoichiometric number (SN) (Equation (1)) for the syngas composition plays a dominating role in methanol synthesis reaction. The ideal value of SN is slightly higher than 2, indicating an excess of H\({}_{2}\) or less CO\({}_{2}\).  
\[\text{SN}=\frac{\text{n}(\text{H}_{2})-\text{n}(\text{CO}_{2})}{\text{n}( \text{CO})+\text{n}(\text{CO}_{2})} \tag{1}\]  
Table 3 lists a brief explanation of the Aspen Plus blocks used in Rectisol system. Firstly, after the gasification process, ash content in syngas (stream 6) was eliminated by the SEP-2 (Cyclone separator). Secondly, the syngas leaving the separator is cooled and pressurized to a temperature of 80 \({}^{\circ}\)C and 40 bar by the C-1 and P-1. Lastly, the RECTISOL block in Aspen Plus is used to simulate Rectisol process where the syngas (stream 10) was fed into Rectisol columns at the bottom. Meanwhile, a 20 kmol/h flow rate of cooled solvent methanol (RE-MEOH), entering from the top of columns, interacts with the syngas. Post-treatment syngas (stream 11) is withdrawn from the top of Rectisol column. Water, impurities and part of the CO\({}_{2}\) are absorbed by the cooled methanol (stream 12), flowing out from the bottom of the Rectisol column. Meanwhile, SEP-4 is used to separate gas (impurities and CO\({}_{2}\)) and liquid phase (methanol and H\({}_{2}\)O) from the liquid phase. Another separator (SEP-5) is used to separate H\({}_{2}\)O and methanol. After purification, the methanol (RE-MEOH) is reintroduced into Rectisol unit.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Flow Sheet Block ID** & **Aspen Plus Block ID** & **Description** \\ \hline R1 & RStoic & Reactor with known conversion rate—used to extract moisture form pyrolysis oil. \\ Operation at 150 \({}^{\circ}\)C. \\ \hline SEP-1 & Sep & Used to separate the moisture from the pyrolysis oil. \\ \hline R2 & RYield & Yield reactor—used to decompose nonconventional pyrolysis oil into its \\ elemental components by FORTRAN statement. Operation at 500 \({}^{\circ}\)C. \\ \hline R3 & RGibbs & Gibbs free energy reactor—used to complete chemical equilibrium by \\ minimizing Gibbs-free energy. Operation at 800 \({}^{\circ}\)C. \\ \hline R4 & RGibbs & Gibbs free energy reactor—used to calculate syngas composition by minimizing \\ Gibbs-free energy. Operation temperature ranges from 400 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Description of Aspen plus blocks used in the gasification model.