MILD combustion of low calorific value gases

Abstract

The utilization of low calorific value gases (LCVG) in combustion devices presents particular challenges in terms of ignition and sustained combustion stability due to the presence of non-combustible components. Moderate or intense low-oxygen dilution (MILD) combustion has emerged as a promising technology for LCVG combustion, offering numerous advantages such as high combustion efficiency, reduced pollutant emissions, and increased fuel flexibility. However, the current body of research in this area is fragmented, making it challenging to draw meaningful comparisons between studies and hindering its practical application. This paper provides a comprehensive review of conventional and MILD combustion of LCVG. To understand the impact of composition on combustion, the fuels are classified based on their composition of hydrogen, carbon monoxide, methane, carbon dioxide, nitrogen, and water. We also delve into the chemical and physical effects of composition, including reaction kinetics and turbulence mixing, and provide an overview of the burners and methods used in establishing MILD combustion. Furthermore, computational fluid dynamics (CFD) models and chemical kinetics in MILD combustion are also thoroughly discussed.

The presence of a large amount of dilution gas in LCVG increases the self-ignition temperature and ignition delay time of the mixture, making preheating the reactants a critical consideration. In MILD combustion, it is crucial to have an inlet reactant temperature higher than the self-ignition temperature ($T_{\text{in}}>T_{\text{si}}$) to mitigate the difficulties associated with ignition and unstable combustion. The heat release in MILD combustion should be moderate to ensure that the combustion temperature does not become too high. The non-combustible components of LCVG are beneficial in this regard, as they allow for a temperature increase of less than the self-ignition temperature ($\Delta T<T_{\text{si}}$). Hydrogen is the most reactive component in LCVG, and its content directly impacts the establishment, efficiency, and pollutant emissions of MILD combustion. Carbon dioxide, nitrogen, and water act as diluents, helping to reduce NOx emissions in MILD combustion. Although a burner may have the potential to be used for MILD combustion, it must be optimised for LCVG with variable composition in order to achieve the lowest pollutant emissions. Further research is necessary to verify and improve simulation models and chemical kinetics. This article provides theoretical support for the practical application of MILD combustion of LCVG with variable composition.