Impact of hydrogen admixture on combustion processes – Part I: Theory

Climate change is one of today’s most pressing global challenges. Since the emission of greenhouse gases is often closely related to the use and supply of energy, the goal to avoid emissions requires a fundamental restructuring of the energy system including all parts of the technology chains from production to end-use. Natural gas is today one of the most important primary energy sources in Europe, with utilization ranging from power generation and industry to appliances in the residential and commercial sector as well as mobility. As natural gas is a fossil fuel, gas utilization is thus responsible for significant emissions of carbon dioxide (CO2 ), a greenhouse gas. However, the transformation of the gas sector, with its broad variety of technologies and end-use applications is a challenge, as a fuel switch is related to changing physical properties. Today, the residential and commercial sector is the biggest end user sector for natural gas in the EU, both in terms of consumption and in the number of installed appliances. Natural gas is used to provide space heating as well as hot water and is used in cooking and catering appliances, with in total about 200 million gas-fired residential and commercial end user appliances installed. More than 40 % of the EU gas consumption is accounted for by the residential and commercial sector. The most promising substitutes for natural gas are biogases and hydrogen. The carbon-free fuel gas hydrogen may be produced e.g. from water and renewable electricity; therefore, it can be produced with a greatly lowered carbon footprint and on a very large scale. As a gaseous fuel, it can be transported, stored, and utilised in all end-use sectors that are served by natural gas today: Power plants, industry, commercial appliances, households, and mobility. Technologies and materials however need to be suitable for the new fuel. The injection of hydrogen into existing gas distribution for example will impact all gas-using equipment in the grids, since these devices are designed and optimized to operate safely, efficiently and with low pollutant emissions with natural gas as fuel. The THyGA project1 focusses on all technical aspects and the regulatory framework concerning the potential operation of domestic and commercial end user appliances with hydrogen  / natural gas blends. The THyGA deliverables start with theoretical background from material science (D2.4) and combustion theory (this report), and extend to the project’s experimental campaign on hydrogen tolerance tests as well as reports on the status quo and potential future developments on rules and standards as well as mitigation strategies for coping with high levels of hydrogen admixture. By this approach, the project aims at investigating which levels of hydrogen blending impact the various appliance technologies to which extent and to identify the regime in which a safe, efficient, and low-polluting operation is possible. As this is in many ways a question of combustion, this report focuses on theoretical considerations about the impact of hydrogen admixture on combustion processes. The effects of hydrogen admixture on main gas quality properties as well as combustion temperatures, laminar combustion velocities, pollutant formation (CO, NOx), safety-related aspects, and the impact of combustion control are discussed. This overview provides a basis for subsequent steps of the project, e.g. for establishing the testing program. A profound understanding of the impact on hydrogen on natural gas combustion is also essential for the development of mitigation strategies to reduce potential negative consequences of hydrogen admixture on appliances.