# Experimental Study of Biogas-Hydrogen Mixtures Combustion in Conventional Natural Gas Systems
## 1 Introduction  
Climate instability produced by anthropogenic activities reinforces the necessity to seek new renewable sources to minimize greenhouse gas (GHG) emissions. The biggest contributors to global warming are carbon dioxide and methane emissions [1]. Consequently, all efforts to mitigate the current environmental crisis are focused on reducing carbon emissions. The ambitious environmental targets proposed by the European Commission and the 2050 targets are forcing a technological revolution to reduce human impact. This revolution is currently supported by public and private investments, although these resources are not within the reach of all companies [2]. However, thanks to the \"green recovery\" plan promoted by the European Union and other countries around the world, the COVID-19 crisis can be a great opportunity and a turning point for energy transition [3]. Hence, in the last decades, renewable energy research has been in the scientific spotlight, mainly focused on the fight against climate change, the fast growth of energy demand, and the depletion of fossil fuels. Renewable gases, notably hydrogen, biomethane and biogas,cover a wide range of possibilities. The use of renewable gases may solve some of the most important environmental problems registered in the automotive or energetic sector, and it could help reach the ambitious \"zero emissions objective\" [4; 5; 6; 7]. Energy supply and transport sector are some of the greatest contributors to GHG emissions with 24.9% and 18.1%, respectively [8]. In this scenario, biogas is an alternative and sustainable solution. biogas is produced by the fermentation or anaerobic digestion of organic waste, such as wood, agricultural products, manure, etc., which is, in other words, biodegradable waste [9]. It is precisely the origin of biogas that makes it a potentially carbon-neutral resource, despite containing a high percentage of carbon dioxide, which can vary between 30% and 50% [10; 11]. Biogas consumption and electricity generation from organic wastes are developed technologies already available in the current market. Nevertheless, in Europe, only about 28% of the organic waste is valorized or recycled, which means that there is a great potential for improvement [9]. The main reason for this low valorization lies in the strong limitation of biogas utilization, due to its low heating value, caused by the carbon dioxide content and the strong fuel variabilities [12]. Additionally, carbon dioxide affects the combustion process, hampering flame creation and flame stabilization in conventional systems [13]. It is also important to mention the need for proper reactor design to optimize biogas production, as well as the long residence times required to complete the process [14]. All the above challenge the promotion of the use of biogas, which has to overcome significant technical and economic barriers to become a reliable renewable energy source. Conversely, biomethane is composed mainly of methane (96-99% of CH\({}_{4}\)); therefore, it can be considered a natural gas. Currently, biomethane injection into the natural gas grid has the largest potential use in Europe. Its versatility makes it possible to reuse the current pipeline infrastructures and the conventional combustion systems [15]. However, some local studies showed that composting and biogas production gather the same perspective level [16] and, consequently, the present research focuses on biogas optimization for distributed direct uses. Besides biogas, hydrogen has been gaining great importance in the past decades due to its versatility and its high yield rates, which may boost energy transition. Currently, approximately 95% of hydrogen can be labeled as blue or grey hydrogen [17]. However, to contribute to the decarbonization of conventional energy systems, hydrogen needs to be produced from renewable sources. Both biogas and hydrogen need large investments in transport and distribution infrastructures, so are currently limited to distributed generation direct uses.