# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 3 Results and Discussion
### Combustion Behavior
#### 3.2.2 Exhaust Gas Analysis  
Unlike laboratory devices designed to evaluate flame behavior, the combustion chamber geometry also affects the combustion behavior [49]. Spatial distribution is essential to boost the energy transmission to the carried fluid or to propitiate the greater harnessing of the exhaust enthalpy. The thermic phenomenon occurring inside the combustion chamber can diverge from bibliographic results using smaller devices. Combustion behavior is also evaluated from the exhaust gas concentration variations point of view. According to [PERSON] et al., O\({}_{2}\) emissions decrease while the carbon dioxide proportion increases [47]. The carbon dioxide content is reduced while H\({}_{2}\) is added, as hydrogen dilutes the gas mixture. This effect is also observed in Figure 11b, where the percentage of O\({}_{2}\) contained in the exhaust is reduced by around 37% from 0 to 25% of hydrogen for BG70 and BG70. From a practical point of view, the lower the O\({}_{2}\) content in the exhaust, the more complete the reaction. Typically, the O\({}_{2}\) content in the chimney is about 3% [50]. Figure 11b shows a singular point, which coincides with an increase higher than expected in the CO\({}_{2}\) emissions. At this point (15%H\({}_{2}\)), lambda does continue with the expected trend. Hence, since fuel and combument consumption remain constant during all the tests and their repetitions, this can be considered an anomalous point. Figure 10: Stability map. Figure 11: Cont. The connection between O\({}_{2}\) emissions and CO\({}_{2}\) emissions is explained through fuel consumption. The oxidant flow remains constant on every test. A reduction in the O\({}_{2}\) percentage necessarily implies an improvement of the combustion behavior. Hydrogen displaces the carbon dioxide content in the blend and increases the heating value of the biogas, increasing, too, the O\({}_{2}\) consumption. Next to this, the percentage of CO\({}_{2}\) increases as the combustion reaction kinetics enhances. In Figure 11a, the CO\({}_{2}\) content registered for BGA60 and BG70 enriched with 12% of hydrogen is 5.33 and 6.29, respectively, which is still far from the 9.06 registered from natural gas. The lambda tendency confirms the improvement on combustion behavior related to the hydrogen addition. However, as it is observed with CO\({}_{2}\) and O\({}_{2}\) emissions, the difference between 20 and 25% of hydrogen addition is practically insignificant, observing a flattening of the curve. BG50 does not fail to ignite when it contains less than 15% of hydrogen, and, at this point the flame is extremely weak, hindering the data acquisition. Consequently, Figure 11a-c just reflects the points of BG50, where the combustion can be maintained in time. The tendency is similar to BG60 and BG70, but the limited data available prevent the correct evaluation of hydrogen addition to BG50. After the results evaluation, the expected trend, also observed by other authors in the literature [13; 32; 47], is confirmed. The combustion characteristics of biogas can be significantly improved by the addition of small amounts of hydrogen. In this study, this effect was observed on a conventional industrial burner capable of working up to 100 kW. Unlike smaller equipment, these conventional systems are built to operate under stable operating conditions, so variations in the fuel seriously affect the combustion of the burner. In this case, it was confirmed that working with lean biogas containing CO\({}_{2}\) concentrations above 40% is not possible. The combustion is very unstable and leads to safety failures and equipment shutdown. However, for the BG60 and BG70, the addition of hydrogen stabilizes combustion and allows the burner to operate without failure for long periods of time. This effect is especially remarkable for hydrogen enrichment percentages between 5 and 10%, as it is precisely at these proportions that the greatest percentage improvement is obtained.