## 5 Results and Discussion
### Condensation and Re-Vaporization Sub-Models
#### Condensation:  
Volatiles (gaseous) \(\rightarrow\) Volatiles (liquid) + Heat  
Re-Vaporization:  
Volatiles (liquid) + Heat \(\rightarrow\) CO + H\({}_{2}\) + C\({}_{2}\)H \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **E-Gas** & **E-STR Base** \\ \hline \multirow{4}{*}{Mole fraction (\%)} & CO & 40.1 & 44.1 \\  & H\({}_{2}\) & 18.4 & 16.5 \\  & CO\({}_{2}\) & 9.7 & 5.9 \\  & H\({}_{2}\)O & 21.4 & 24.4 \\ \hline \multicolumn{2}{c}{Exit temperature (\(K\))} & 1891 & 1894 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Exit syngas compositions and temperature of benchmark E-STR case and E-Gas. The volatiles are first released according to the two-competing-rates devolatilization model proposed by [PERSON] [37] as shown in Equations (22)-(24). Typically, the devolatilization and the subsequent thermal cracking of volatiles occur and complete under high temperature near the combustion region in the gasifier. In a low-temperature operation condition, such as in the E-STR, the heavy volatiles or tar vapors have not been thermally cracked to lighter gases. Therefore, they would condense if the local temperature becomes lower than the corresponding condensation temperature of some species, due to the endothermic gasification process. The goals of this study are focused on the development and implementation of these sub-models to envisage the phenomena, which are influenced by the temperatures as well as rates of condensation and re-vaporization. Since the latter is not known at present and finding their rates is beyond the scope of this study, the former is thus selected as the main controlling parameter for facilitating the development of models. A range of \(\pm\)15 K between condensation and re-vaporization is chosen to consider the resistance involved in vapor diffusion and convective process during the condensation and re-vaporization (including the subsequent thermal cracking) processes. The accuracy of the assigned condensation and re-vaporization temperatures is not the focus of this study; rather, functional demonstration of these sub-models is more eminent at present. Although the condensation temperature varies among different compositions of volatiles and tars, for simplicity, one single condensation temperature (780 K) is assigned. Condensation is assumed to occur instantaneously as the local temperature is lower than 780 K and thermal energy due to phase change will be released. On the other hand, a single re-vaporization temperature (810 K) is also assigned, and the re-vaporization is also assumed instantaneously. The 30 K difference between the condensation and re-vaporization temperatures is to serve the following two purposes: (a) to avoid spatially sharp changes from condensation to re-vaporization in a very short distance, and (b) to simulate the resistance and finite time needed to condense and re-vaporize. The second reason is further explained below. Assuming the actual saturation temperature of the volatiles is 795 K, it means that the condensation and evaporation temperatures are the same at 795 K. Thus, the assigned condensation (780 K) and re-vaporization (810 K) temperatures are 15 K lower and higher than 795 K, respectively. This implies that the volatiles need to be sub-cooled 15 K to effectively condense, as well as superheated 15 K to re-vaporize. Typically, condensation will not occur in free space, rather a solid surface is required for condensation to occur and grab onto. A solid surface can be the gasifier wall, duct/pipe surface, or a tiny solid (i.e., a nucleate) surface. Since there are abundant particles in a gasifier, including char dust, ashes, and soot, to serve as numerous nucleate sites for condensation to occur, it is reasonable to assume that condensation can occur at any computational cell once the temperature of that cell drops below the assigned condensation temperature. The volatiles condensation sub-model is first developed and implemented in E-STR without including the re-vaporization model.