# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 1 Introduction
This study investigates the possibility of combining both and, thus, accelerate its technical implementation. Hydrogen enrichment is a technique that may allow the harnessing of renewable fuels into conventional combustion systems [18; 19]. In recent times, enriched natural gas combustion behavior has been widely studied. Hydrogen injection into conventional gas fuels reduces heat release oscillations and improves flame stability [20; 21; 22], while enhancing turbulent explosion parameters [21]. It has, indeed, been noticed that with 5% hydrogen, the flame expands greatly, inhibiting combustion instability [23]. All studies highlight that hydrogen concentration is a crucial parameter in all flammability and combustion characteristics [24]. Small percentages of hydrogen improve global efficiency in some combustion processes, while large amounts may change the flame typology, decreasing the combustion quality [25; 26]. This also implies an increase in pollutant emissions despite the numerous studies that stated that GHG emissions are reduced when hydrogen-methane mixtures are used [27]. Usually, the studied percentages of hydrogen content are between 0 and 50% [28]. The enrichment of biogas, as well as the dual interaction of H\({}_{2}\)/CO\({}_{2}\) mixtures, has been recently studied [29]. Flame stability is one of the biggest disadvantages of biogas combustion. However, [PERSON] et al. [18] observed a favorable effect of hydrogen addition on flame stability and a detrimental effect of carbon dioxide concentration. This might be partly explained by the widening of the biogas flammability limits caused by the added hydrogen. The favorable results of previous studies has encouraged the research of biogas-hydrogen blends in other combustion systems, such as combustion engines or power-to-methane cycles [30; 31]. The results agree with former research works, proving the increase in detonation characteristics of biogas by adding hydrogen percentages between 0 and 20% [32]. Most of the research works focused on investigating the intrinsic properties of combustion by using experimental laboratory devices. However, this study intends to evaluate the combustion behavior in conventional combustion systems currently available in the market. The importance of tests on conventional energy systems resides in the necessity to boost the circular economy by reusing current equipment to burn renewable fuels. Industrial processes are large heat and electric energy consumers; therefore, their decarbonization is a key point to reduce greenhouse emissions [33]. Combustion systems in industry are typically expensive, due to their generation capacity and, consequently, their size. The replacement of these systems implies large investments that sometimes companies cannot afford. Then, the possibility to reuse conventional burners, boilers, or engines for energy conversion could be the key to a sustainable future, and an easier circular economy. Additionally, the replacement of conventional equipment for systems adapted to renewable fuels is also expensive for end consumers. As mentioned above, combustion tests are generally carried out on small-scale laboratory equipment [18; 34; 35; 36]. [PERSON] et al. [37] investigated the influence of renewable gas content on the operating performance of conventional combustion devices, such as room furnaces. They evaluated the effect of adding CO\({}_{2}\) or H\({}_{2}\) to natural gas in a representative furnace with a heating load from 15.4 kW to 22.0 kW. The results reflected the necessity to test the combustion parameters in high-power heating devices [37]. Hence, the present study addresses a conventional combustion system behavior, consisting of a burner and a combustion chamber of 100 kW fed with biogas-hydrogen mixtures instead of natural gas. Combustion efficiency parameters, such as exhaust gas temperature, emissions, and ignition behavior, were investigated.