## 4 Conclusions  
To increase the overall system efficiency of a combined fuel processor and HT-PEMFC system, the thermal efficiency of the fuel processor must be improved. This study suggests two ways for increasing efficiency:  
1. Optimize the operating condition (temperature of the reactor, S/C ratio, and reactant flow rate) for high thermal efficiency;
2. Recycle the unreacted hydrogen in the anode of HT-PEMFC. The fuel processor consisting of equilibrium reactors and heat exchangers was analyzed by the Aspen Hysys(r) simulator and the operating conditions were optimized to produce the highest efficiency. In the simulation results, the effect of the temperature change of the WGS was less sensitive than that of the steam reformer, and at maximum efficiency, the operating temperature of the HTS was 400 \({}^{\circ}\)C and the operating temperature of the LTS was 200 \({}^{\circ}\)C. As the temperature and S/C ratio of the steam reformer increase, the hydrogen concentration of the reformed gas increases, but the thermal efficiency of the fuel processor decreases. The steam reformer showed maximum thermal efficiency at an S/C ratio of 5 at 500 \({}^{\circ}\)C, S/C ratio of 4 at 650 \({}^{\circ}\)C, and S/C ratio of 3 at 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C. The temperature of the steam reformer was selected to be 700 \({}^{\circ}\)C or higher, which is a condition with a methane conversion of 95% or higher. However, the efficiency of the steam reformer at 900 \({}^{\circ}\)C was excluded in consideration of catalyst life. The reason for the decrease in efficiency is that the energy required to heat the water is greater than the energy from the produced hydrogen. Recycling unreacted hydrogen as combustion fuel increased the overall efficiency of the combined system by a factor of 1.28. However, excessive heat was supplied to the fuel processor when the fuel utilization efficiency was less than 70% and so this condition was also excluded. As a result, an overall efficiency of 43.47-47.31% was confirmed for the following conditions with a 5 kW power output: T\({}_{\text{SR}}\) = 700 \({}^{\circ}\)C, 750 \({}^{\circ}\)C, and 800 \({}^{\circ}\)C, S/C ratio = 3, \(n_{\text{PNG}}\) = 0.8 and 0.9, and \(U_{f}\) = 75% and 80%. By adding a recycling stream, overall efficiency gains and CO reduction are achieved, reducing the operating costs. Waste heat recovery for HT-PEMFC's tri-generation system could also be another good application to increase overall efficiency. HT-PEMFC's have good CO tolerance and good waste heat quality, allowing for a more economical design of energy production for residential buildings. Formal analysis, S.P. ; Investigation, S.P. ; Writing--original draft, S.P. ; Writing--review & editing, S.L. ; Supervision, S.L. All authors have read and agreed to the published version of the manuscript. This research was funded by Korea Institute of Energy Technology Evaluation and Planning grant number 20183010032400. Not applicable. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(U_{f}\) (\%) & \(\mathbf{T_{SR}}\) (\({}^{\circ}\)C) & \multicolumn{2}{c}{**75\%**} & \multicolumn{2}{c}{**80\%**} \\ \hline \(\mathbf{n_{PNG}}\) (mol/min) & & **0.8** & **0.9** & **0.8** & **0.9** \\ \hline \multirow{3}{*}{\(n_{\text{sys}}\) (\%)} & 700 & 43.47 & 43.79 & 46.62 & 46.96 \\  & 750 & 43.74 & 44.05 & 46.90 & 47.24 \\ \cline{1-1}  & 800 & 43.81 & 44.12 & 46.98 & 47.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The efficiency of the system for the selected range of temperature, fuel utilization efficiency, and mole flow rate of reactant natural gas. **Acknowledgments:** The authors are thankful to the Korea Institute of Energy Technology Evaluation and Planning (KETEP) for the support grant funded by the Korean government's Ministry of Trade, Industry, and Energy under the project titled: \"Development on localization technologies of export-purposed stationary fuel cell systems\", numbered: 20183010032400. **Conflicts of Interest:** The authors declare no conflict of interest.