# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 3 Results and Discussion
### Temperature Analysis
#### 3.1.2 Experimental Results  
The use of conventional natural gas burners limits the CO\({}_{2}\) range. Thus, it is essential to find the threshold value of CO\({}_{2}\) content when the conventional burner does not ignite. Figure 8 shows, on the left axis, the temperature decrease registered during the tests. It can be noticed that as the CO\({}_{2}\) percentage increases, the exhaust gas temperature drops considerably. When the ignition succeeds at 40% of CO\({}_{2}\), the temperature registered has strong fluctuations, complicating the data acquisition. The burner does not properly ignite with CO\({}_{2}\) percentages higher than 40%. The oxidant and fuel consumption remain constant during all the tests; consequently, the lambda also remains constant. Both parameters directly affect the lambda, which is the optimal fuel/oxidant proportion to ensure complete combustion. According to Regulation (EU) 2016/426 [46], conventional combustion systems work with a combustion lambda of around 1.3, assuring a 20% of oversupplying air. Figure 8, on the right axis, confirms the effect of CO\({}_{2}\) addition, cutting down the combustion quality. This is explained, as CO\({}_{2}\) reduces the fuel flow entering the burner.  
Figure 7: Operation variables results.  
The burner operation range is then set between 0 and 40% of the CO\({}_{2}\) content. At 40% of CO\({}_{2}\), the combustion is remarkably weak and unstable. Normally, the carbon dioxide content is found to be between 30% and 50% before the treatment is upgraded. Hence, it is interesting to test BG50, BG60, and BG70 with different hydrogen percentages. BG50 cannot ignite in the burner, so the test intends to demonstrate the ability of hydrogen to stabilize the combustion flame and to carry out the combustion of low heating value fuels.  
Despite the simulation results stating the possibility to overcome the temperatures of natural gas, the experimental results plot a different tendency (Figure 9). In every case, the temperature increases at the same time that the hydrogen percentage does. By adding 5% of hydrogen, the temperature rises by approximately 80% for BG70 and 65% for BG60. When the mixture contains more than 10% of hydrogen, temperatures rise more slowly until they reach 20 and 25% of hydrogen, where the temperature increment is stabilized in 5%. If the biogas and natural gas combustion temperatures are compared, it can be noticed that biogas shows temperatures 70% lower than that of natural gas, which is an important negative impact on the combustion behavior. This temperature decrease is translated into a significant decrease in efficiency and, consequently, a lower energy efficiency. Hydrogen addition partially reverses this trend and allows biogas to be burned more efficiently, although none of the tested blends reaches the values obtained from natural gas.  
Figure 9 plots the maximum temperature value for each point. However, large temperature fluctuations are observed during the tests. For mixtures that do not contain hydrogen, variations of up to 85 \({}^{\circ}\)C are observed in BG70, with fluctuations of up to 100 \({}^{\circ}\)C being recorded for BG60. This effect is observed mainly in the first minutes of combustion. As hydrogen is added, the fluctuations decrease considerably. The improvement in register stability is particularly noticeable when 5% hydrogen is added, as the recorded variations decrease up to 20-35 \({}^{\circ}\)C. From 10% onwards, a smaller decrease in fluctuations is experienced, although the combustion temperature increases as discussed above. This effect is directly related to the stability of the flame, which is impaired by the increase in CO\({}_{2}\) in the mixture.  
The results can vary from the bibliographic data, as carbon dioxide and hydrogen additions to methane or natural gas have been normally tested in laboratory devices or low-power appliances, focusing mainly on flame stability [37].  
Figure 8: Effect of CO\({}_{2}\) addition to the exhaust gas temperature and the combustion lambda.