An Innovative Concept for the Complete and Low-NOx Combustion of Non-Carbon Eco-Fuels Using a Thermo-Acoustically-Driven, Hydrogen-Powered Pilot Stage

Abstract

Abstract The problem of climate change, triggered by a high concentration of pollutants in the atmosphere, and the scarcity of fossil resources, increases the need of low emission thermal utilisation of novel, non-carbon eco-fuels such as hydrogen, ammonia (both for energy and propulsion) or hydrogen sulphide (sulphuric acid production). While all of these listed eco-fuels have the potential to decarbonise industry and the energy sector, they also pose demanding challenges regarding combustion. To address these challenges the idea is to combine the advantages of current low-NOx technologies in terms of ultra-lean combustion with a precisely controlled, forced flame turbulence generated by a pulsation actuator. The aim is to develop a safe and highly digitalised combustion technology, including a two-staged burner design for application in the megawatt range with multi eco-fuel capacities and a fully automated control loop for the combustion process. To achieve effective thermo-acoustic modulation an actuator type siren is used. Further improvements towards the demanding application in industry were done leading to the latest siren model, Siren E. A siren is a robust pulsation apparatus that generates powerful noise levels and effective flow pulsations under elevated pressure and temperature conditions [1–3]. By scanning through a certain frequency range during combustion, one is able to detect eigenfrequencies of the flame which increases flame turbulence when excited. The effective flow control of the siren can guarantee a complete burnout of the partially toxic eco-fuels. In addition, gaseous pollutants such as NOx can be reduced to a minimum, since the flame pulsation process provides the opportunity to run reliably a mixture with an otherwise unstable combustion process. The Siren E with its new features is introduced in this paper in detail. To verify the method for eigenfrequency detection, initial combustion tests are first carried out with propane on the so-called MethaNull test rig. There, the main aspect is to meet the same eigenfrequencies as in a previous work. Then, initial combustion tests with premixed hydrogen are performed up to a thermal power of 10 kW. First, different methods for the injection of hydrogen are tested resulting in a prioritised premixed variant. Using this premixing method in the test setup the response to thermo-acoustic excitation via loudspeaker and siren is investigated. Prior to the experiments, a numerical study was performed on a model representing a simplified pilot stage. All numerical and experimental results are presented in detail in this paper.