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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. • Focusing on the phenomenon and characteristics of hydrogen in ultra-lean combustions. • Exploration of the stability mechanisms of ultra-lean combustion flame ball. • Elucidating the role of the Lewis number in flame instability and emission characteristics. • Emphasizing the further investigations into hydrogen based ultra-lean flame propagation. [ABSTRACT FROM AUTHOR]

• Liquid EtOH spray with H 2 enrichment combustion were conducted. • Combustion with global equivalence ratio 0.15(67% EtOH/33% H 2) was achieved. • The effects of ignition timing, injection pressure on lean burn were evaluated. • High jet pressure improves ignition stability but causes incomplete combustion. An experimental study was conducted to determine the effects of ignition timings, injection pressures, and hydrogen enrichment on ethanol stratified ultra-lean combustion. The combustion chamber was a constant volume vessel with an injection and ignition system. The ignition and combustion characteristics of ethanol spray with different ignition timings (0–50 ms after start of injection), injection pressures (5, 15, and 25 MPa), and hydrogen premixed equivalence ratios (0.05, 0.1, and 0.2) under ambient temperature 353 K and 1 atm. The direct injection of ethanol and hydrogen enrichment results in an increase in the stable ignition limit to an extremely lean condition: a global equivalence ratio of 0.15 (67% ethanol/33% H 2). Furthermore, the probability of ignition first increased then decreased with the delay in ignition timings; rose with an increase in the injection pressure and hydrogen ratio. However, the delay in ignition timings and high injection pressures resulting in extremely lean zone in the constant volume vessel, which led to incomplete combustion. The addition of hydrogen not only stabilized ignition process but also improved the combustion characteristics. Stable ignition and combustion were achieved at early ignition timing (2–3 ms), moderate injection pressure (15 MPa), and a minimum global equivalence ratio of 0.2 (50% ethanol/ 50% H 2). [ABSTRACT FROM AUTHOR]