# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 3 Results and Discussion  
### Temperature Analysis  
#### 3.1.1 Simulation Results  
The combustion temperature can be measured directly from the exhaust or through the refrigeration water, both methods being an indirect measurement of efficiency. However, the temperature analysis may differ depending on the thermocouple position inside the experimental setup. The exhaust gas temperature is a valuable parameter because the obtained results can later be escalated to energy generators through numerical or software simulations. The temperature analysis is first carried out simulating the combustion system conditions, and then by experimental combustion tests.  
Thus, the optimal working conditions are first evaluated by software simulation with the AspenPlus(r) program. The optimization criteria consider two main parameters. The first one is the maximum combustion efficiency expressed as the minimum CO content in the exhaust gas compared to CO\({}_{2}\) generation.  
\[\eta_{\text{-}}\text{comb}=\text{CO}_{2}\text{out}/(\text{COout}+\text{CO}_{ 2}\text{out}) \tag{1}\]  
Figure 4: Wobbe Index analysis of biogas and biogas–hydrogen mixtures: (**a**) influence of CO\({}_{2}\) addition to natural gas Wobbe Index; (**b**) influence of hydrogen addition to Wobbe Index.  
The second one corresponds to hydrogen consumption.  
\[\text{H}_{2}\text{cons}=(\text{H}_{2}\text{in}-\text{H}_{2}\text{burnt-out})/\text{H }_{2}\text{in} \tag{2}\]  
The range of variables used for the simulation are shown in Table 2.  
To evaluate the flame temperature and chamber duty, the parameters range of Table 2 is established. Figure 5 represents the initial results, where it can be seen that higher temperatures produce higher powers. However, higher temperatures do not correspond to the optimal conditions as the highest temperatures reach 2000 \({}^{\circ}\)C, overpassing the material limitations. Regarding hydrogen addition, higher efficiencies are obtained between 0 and 40% of hydrogen consumption. In these conditions, the combustion efficiency would be very low, the hydrogen would not be consumed, and, thus, it would be expelled almost completely out of the chimney.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Range** \\ \hline H\({}_{2}\) mass fraction & 0–0.25 \\ CO\({}_{2}\) mass fraction & 0.3–0.6 \\ Ratio air/fuel & 22–44 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Set variables.  
Figure 5: Initial conditions of the sensitivity analysis.  
Considering that the estimated power for natural gas in the simulation is 30 kW, all the values under this threshold need to be disposed of to guarantee that the system does not lose power. Assuming the abovementioned premise, the simulation is limited to ensure a hydrogen consumption higher than 95% and a power combustion higher than 30 kW. Figure 6 shows the bounded results where the optimal conditions take place when the combustion efficiency is between 40 and 50 kW and hydrogen consumption is about 95-97%, reaching flame temperatures between 2000 and 2200 \({}^{\circ}\)C.  
Figure 7 shows the results after the application of the limitations stated in Figure 6. The greater duties occur when the air/fuel molar ratios are set between 12 and 16. After the simulation results, the optimal enrichment percentage of hydrogen can be defined as between 8 and 12%. On the other hand, the increase in CO\({}_{2}\) content decreases the duty, as expected.  
Figure 6: Optimal operation conditions.