A Critical Assessment of MILD and Plasma-enhanced Combustion for Net-zero Energy Systems using Green Hydrogen and Ammonia

The transition to sustainable and smart urban energy systems requires combustion technologies that combine high efficiency with near-zero emissions. Moderate or intense low-oxygen dilution (MILD) combustion has emerged as a promising solution, offering volumetric heat release, reduced peak temperatures, and strong NOX suppression, ideal for integrating green hydrogen carriers such as ammonia and ammonia–hydrogen blends into stationary energy systems. While MILD combustion is well-studied for hydrocarbons, its application to carbon-free fuels presents challenges including high ignition temperatures, low reactivity, and potential NOX formation. This review examines the behavior of ammonia-based fuels under MILD conditions, mapping combustion regimes across reactor types and operating parameters. To address ignition and stability issues, the review also explores plasma-assisted MILD combustion (PAMC). Non-equilibrium plasma (NEP) discharges promote radical generation, reduce ignition delay times, and enhance flame stability under lean, highly diluted conditions. Recent experimental and numerical studies demonstrate that plasma activation can reduce ignition delay times by up to an order of magnitude, lower flame lift-off heights by over 30 % in certain configurations, and enhance OH radical concentrations and heat release intensity. The extent of these improvements depends on factors such as plasma energy input, fuel type, and dilution level. This review synthesizes key findings, identifies technical gaps, and highlights the potential of MILD and PAMC as clean, flexible, and scalable solutions for low-emission stationary energy generation in smart city environments.