Heat-up, devolatilization, and ignition of coal particles using point-particle DNS

Pulverized coal combustion remains a key energy source due to its availability and low cost. Reducing its carbon dioxide emissions requires retrofitting plants with oxy-fuel combustion technology which fundamentally alters combustion conditions. This paper examines coal particles that cluster in reactive turbulence using point-particle-based direct numerical simulation. In accordance with typical burning conditions, the mass loadings range from 0.006 to 0.025, the Stokes numbers range from 1 to 20, and the Taylor-scale Reynolds number is approximately 50. In a precursor simulation, particles are randomly seeded in a turbulent base flow to obtain statistically homogeneous and isotropic initial conditions. Combustion simulations are conducted in air and in mixtures of oxygen and carbon dioxide, with oxygen volume ratios ranging from 0.25 to 0.35. Replacing nitrogen with carbon dioxide lengthens the ignition delay, yet an oxygen volume fraction of 0.30 restores air-fired timing and peak temperature while raising the cumulative heat release by roughly 30%. Increasing the mass loading intensifies heat-release rates and drives a transition from particle-scale diffusion flames to cluster-scale partially premixed burning without materially changing the global ignition delay. Lower Stokes numbers foster dense particle clustering and earlier, hotter ignition, whereas higher Stokes numbers produce later, weaker, more dispersed diffusion flames.