# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 4 Conclusions  
The results obtained provide relevant conclusions concerning the combustion of the enriched biogas, which are presented below. The experimental data confirmed the difficulty of burning biogas in conventional systems. Its low calorific value and the unfavorable effect of CO\({}_{2}\) increase the flame instability and, therefore, the combustion quality decrease. When biogas contains more than 40% of CO\({}_{2}\), an ignition failure occurs in the burner. Therefore, BG50 cannot be tested in this equipment in its pure state. The combustion of BG60 and BG70 was poor and unstable, although it was possible to burn it directly in the burner.  
As expected, the addition of hydrogen considerably improved the combustion properties of biogas, even in combustion systems of this power capacity. The main increase occurred by adding 5% hydrogen, as the fuel exhaust temperature rose by 80% for BG70 and 65% for BG60. The addition of 10% also had consequential effects; however, the improvement trend flattened out for addition percentages between 20% and 25%.  
The poor combustion behavior was confirmed by the study of O\({}_{2}\) and CO\({}_{2}\) emissions, as well as the combustion lambda. Although the values obtained after enriching the mixtures with hydrogen did not reach the ones recorded from natural gas, the improvement in combustion characteristics was remarkable. A temperature increase may have a negative effect on NOx generation, so it would be interesting to look further into this aspect in future research.  
Based on the results of this study, it can be concluded that biogas is not a suitable fuel for natural gas high-power heating combustion systems. However, optimal operating conditions are established when biogas is enriched with hydrogen. These conditions are obtained for hydrogen percentages between 5 and 10%, provided that the carbon dioxide content is higher than 60%.  
The results importance of this study lies in the possibility of taking advantage of conventional combustion systems to burn alternative fuels, such as biogas. Thanks to the addition of small amounts of hydrogen, it is possible to work with equipment designed to consume natural gas. This can be an advantage for today's industry, which must invest heavily to be able to meet the environmental restrictions that will continue to increase over the years.  
Conceptualization--I.A. and M.F.O.; data curation--I.A.; formal analysis--M.J.G.-M. and D.B.; funding acquisition--M.F.O., I.A. and B.L.; investigation--B.C. and I.A.; methodology--I.A.; experimentation--I.A. and J.L.L.; project administration--M.F.O. and I.A.; resources--D.B., M.J.G.-M., J.G.-T., M.F.O., B.L. and J.L.L.; software--M.F.O.; supervision--J.L.L.; validation--I.A. and B.C.; visualization--I.A.; roles/writing--original draft, B.C. and I.A.; writing--review and editing, M.F.O. and D.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Fundacion Iberdrola, under project grant \"Smart BioH2\".  
The authors would like to thank the Fundacion Iberdrola for supporting this research under project grant \"Smart BioH2\". We would also like to thank [PERSON] for her contribution as a translator and editor of the text.  
The authors declare no conflict of interest.