Status and challenges for realizing low emission with hydrogen ultra-lean combustion

As fossil fuels currently account for approximately 85 % of the world's energy demands, the rapid growth of the global population and the economy is projected to increase greenhouse gas emissions. Consequently, there is a growing interest in the concept of ultra-lean combustion. This advanced combustion technology has the potential to effectively reduce both fuel consumption and emissions, offering a more environmentally sustainable approach. This comprehensive review delves into hydrogen ultra-lean combustion, specifically exploring the flame ball phenomenon by specifically investigating computational solutions in uniform and non-uniform mixtures. Noteworthy is the attention given to the emergence of stable flame balls within quiescent premixtures, especially in microgravity conditions. The review elucidates how these flame balls achieve stability through thermal radiation loss. A thorough examination of the role of the Lewis number (Le) in flame instability is conducted, considering its integral connection with diffusive-thermal instability across various experimental setups. Beyond the theoretical aspects, this review encompasses the practical domain of stable spherical flames originating from ultra-lean premixed reactants based on hydrogen. The exploration extends to comprehending the intricate mechanisms governing lean flammability limits, particularly the interplay between low-speed counterflow premixed flames and flame balls. It sheds light on how the concept of flame balls contributes to a more profound understanding of emission characteristics in ultra-lean combustion. Lastly, the review provides valuable insights into potential avenues for further research. It outlines the challenges that lie ahead, which encapsulates experimental observations and in-depth investigations into flame balls and the propagation of hydrogen ultra-lean flames.