5. Insights of MILD Combustion from High-Fidelity Simulations

Simulation of coal combustion remains challenging due to the many physical processes involved which span a large range of length and time scales. Although detailed models exist for devolatilization, char oxidation and gas-phase kinetics, most simulation efforts simplify these models considerably to reduce the high cost of simulation. Three mixture fraction-based chemistry models are evaluated, namely, the steady laminar flamelet, equilibrium, and Burke-Schumann models in the scope of coal volatiles combustion. Coal volatiles are assumed to be composed of “light gasses” (CH\(_4\), CO, etc.) as well as tar, which refers to the various large aromatic compounds released during the devolatilization process. Here, tar is treated as a single empirical species. Each mixture fraction-based model is evaluated by comparing predicted gas phase properties to computations using finite-rate chemistry with a detailed reaction model. The results indicate that the reconstructions for gas phase temperature and composition from steady flamelet model is the most accurate. The Burke-Schumann chemistry model performed very poorly for predicting the gas phase temperature and composition under stoichiometric conditions. We apply the steady laminar flamelet model to Moderate or Intense Low Oxygen Dilution (MILD) combustion. A key requirement for MILD combustion is that mixing rates are sufficiently fast that gas-phase chemistry occurs nearly volumetrically, eliminating visible flame structures. A Well-stirred reactor assumption is applied to MILD combustion due to its characteristic of volumetric reactions. The necessary conditions to achieve MILD combustion, including recirculation rate of flue gas and heat loss, are determined under various mixture fractions and mass fractions of light gas in the fuel stream. We conclude that the increasing the recirculation rate and heat loss are helpful for achieving MILD regime. Additionally, we observe that the recirculation rate and heat loss values required to achieve MILD combustion increase as the fuel stream is enriched in light gases. Steady flamelet computations reveal that MILD combustion can be achieved when reactants are not well-mixed as long as the scalar dissipation rate is sufficiently large. Our considerations indicate that the steady laminar flamelet model provides a reliable method to model MILD combustion in the absence of well-mixed reactants.