Numerical Analysis on Microscopic Characteristics of Pulverized Coal Moderate and Intense Low-Oxygen Dilution Combustion

Moderate and intense low-oxygen dilution (MILD) combustion is regarded as a new clean combustion mode, which can simultaneously improve combustion efficiency and decease NO emission. The computational fluid dynamics (CFD) modeling on pulverized coal MILD combustion was conducted to Simulate the flue gas velocity, temperature and species concentration fields with two turbulence-chemistry interaction models, which were well-validated by the experimental data of International Flame Research Foundation (IFRF) furnace no. 1. Then, macro- and microscopic characteristics of pulverized coal MILD combustion were quantitatively analyzed to emphasize the unique MILD combustion regime compared to the conventional flame combustion. The eddy dissipation concept (EDC) model with a four-step global reaction can well reproduce turbulence-chemistry interaction behavior of pulverized coal MILD combustion. Strong turbulent mixing and entrainment of hot flue gases is found in the furnace because of high momentum secondary air jets. The internal flue gas recirculation ratio in the furnace is over 5, Which makes local oxygen concentrations lower than 5% before the coal devolatilization proceeds and makes the fuel temperature higher than its autoignition temperature. Furthermore, the pulverized coal MILD combustion regime is first depicted with the calculated Damkohler number and Karlovitz number on the basis of different regimes of turbulent non-premixed flames. The numerical results confirm that pulverized coal MILD combustion is in a slow chemistry regime (D-aI < 10 and Ka >> 1) and disperses throughout the whole furnace volume. Pulverized coal MILD combustion is a unique combustion mode with strong turbulent mixing and entrainment, high internal flue gas recirculation ratio, and slow reaction rate under a low local oxygen concentration with respect to the traditional flame,combustion mode.