## 3 Results and Discussions
### Influence of the Burners Turned on or off
Figure 8 compares the effect of the burners on/off combinations on the hydrogen average mole fractions at the catalyst tube outlets (hydrogen yields). It is clear hydrogen yields decrease in the corresponding areas of burners turned off. The hydrogen yields reduce to a higher extent when more burners are turned off. Table 2 compares the average hydrogen yields using different burners on/off combinations. It can be observed that the hydrogen yields decrease by about 3.2%, 6.4%, or 10%, respectively, when one group, two groups, or three groups of burners are turned off. The result also shows that the hydrogen yields have greater reductions when upstream burners are turned off. On the other hand, the hydrogen yields have less reduction when central burners are turned off. The trend is similar to that of the outer surface temperatures of the catalyst tubes. Figure 7: _Cont._  
Figure 8: Comparison of average hydrogen yields using different burners on/off combinations. Figure 7: Outer surface temperature distributions of the catalyst tubes using different burners on/off combinations: (**a**) Fully opened; (**b**) Group 1 turned off; (**c**) Group 2 turned off; (**d**) Group 3 turned off; (**e**) Group 4 turned off; (**f**) Group 5 turned off; (**g**) Group 6 turned off; (**h**) Group 1 & 2 turned off; (**i**) Group 3 & 4 turned off; (**j**) Group 5 & 6 turned off; (**k**) Group 1, 2 & 3 turned; (**l**) Group 4, 5 & 6 turned off. The lifetime of a catalyst tube is not only significantly affected by the catalyst tube temperature but also closely connected with the pressure inside the catalyst tube. Figure 9 compares the effect of the burners on/off combinations on the inner surface average pressures of the catalyst tubes. The result shows that the pressures have greater reductions in areas where the burners are turned off. The pressures reduce to a higher extent when more burners are turned off. Compared with Figure 6, it can also be seen that the inner surface pressures and outer surface temperatures of the catalyst tubes have similar trends. \begin{table}
\begin{tabular}{c c} \hline
**Operating Mode** & **Hydrogen Yield** \\ \hline Fully opened & 0.670 \\ Group 1 off & 0.641 \\ Group 2 off & 0.647 \\ Group 3 off & 0.655 \\ Group 4 off & 0.654 \\ Group 5 off & 0.652 \\ Group 6 off & 0.644 \\ Group 1 \& 2 off & 0.616 \\ Group 3 \& 4 off & 0.637 \\ Group 5 \& 6 off & 0.629 \\ Group 1, 2 \& 3 off & 0.602 \\ Group 4, 5 \& 6 off & 0.604 \\ \hline \end{tabular}
\end{table} 
Table 2: Average hydrogen yields using different burners on/off combinations. Figure 9: Average inner surface pressures of the catalyst tubes using different burners on/off combinations. The lifetimes for the catalyst tubes of different burners on/off combinations can be estimated from the calculated maximum outer surface temperatures and maximum inner surface pressures of the catalyst tubes. The result is shown in Figure 10. It can be observed that turning off some burners is beneficial to the catalyst tube lifetime, especially for the catalyst tubes in the corresponding areas of burners turned off. The catalyst tube lifetime will be longer when more burners are turned off. Turning off the upstream burners is more efficient to the catalyst tube lifetime while turning off the central burners is less efficient. Comparing Figure 6 and Figures 8-10, it can be observed that turning off some burners can reduce the catalyst tube temperatures and pressures, and increase the catalyst tube lifetimes. The main drawback is the reduction in hydrogen yields. performance and lifetime can be improved. The results of this study reveal that obvious difference exists between the simulation results of the prototype and the periodic model. Using the periodic boundary conditions, the temperature and hydrogen yield obtained are much lower than the experimental values and the inner surface pressures of the catalyst tubes are much lower than those using the real model. This may result in overestimating the catalyst tube lifetime and underestimating the reformer operation risk.