Numerical Study of Pulverized Coal MILD Combustion in a Self-Recuperative Furnace

A numerical study of pulverized coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions is presented in a parallel jet self-recuperative MILD combustion furnace. The Reynolds-Averaged Navier-Stokes equations, in a three-dimensional axisymmetric furnace domain, were solved using the Eddy Dissipation Concept model for the turbulence-chemistry interaction. The main aim of this study is to understand the influence of devolatilization models on the prediction accuracy of pulverized coal combustion under MILD combustion conditions. In particular, three devolatilization models are analyzed: a conventional single-rate model, a two-competing-rates model, and an advanced chemical percolation devolatilization (CPD) model based upon structural networks of the coal with a global kinetics mechanism. In addition, a new simplified numerical model is developed for Australian black and/or hard coal and optimized for the MILD combustion conditions. The modeling cases are validated with the experimental results of MILD combustion of high volatile Australian brown coal from Kingston using N-2 as a carrier gas and low volatile Australian black coal from the Bowen basin using CO2 as a carrier gas in a MILD combustion furnace. From the comparison, the advanced CPD devolatilization model with a three-step global kinetic mechanism gives, as expected, the best agreement with the experimental measurements for the Kingston brown coal case, while all the models tested provide similar results for the Bowen basin black coal case. The heterogeneous reactions on the char burnout rate for pulverized coal combustion under MILD combustion conditions are presented and discussed. The characteristics of three NO formation mechanisms, namely, thermal-NO, prompt-NO, and fuel-NO, together with the NO reduction mechanism, named NO-reburning, are evaluated using the postprocessing modeling approach. The results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.8% to the total NO emissions, while the fuel-NO route dominates the NO emissions under MILD combustion conditions. It is also found that the total NO emission is reduced by up to 47% for the brown coal case and up to 39% for the black coal case via the reburning mechanisms.