Role of particle diameter in laminar combustion regimes for hybrid mixtures of coal dust and methane gas

Burning velocity in hybrid mixtures depends on coupling interaction between the gas flame, particle heating, and particle combustion. The objective of this work is to use computational fluid dynamics to investigate the role of particle diameter on hybrid mixture burning velocity and to develop laminar combustion regime diagrams at different particle diameters. Analysis of the particle heating, devolatilization, and surface reaction timescales demonstrates that combustion is limited by particle heating with the model parameters used. Furthermore, comparison to the premixed gas flame residence timescale (flame thickness divided by burning velocity) shows that a maximum diameter exists above which particle combustion cannot contribute to enhancing hybrid flame propagation. Combustion regime diagrams constructed for 10 μm particles demonstrate coupling between the gas flame and dust combustion throughout the entire concentration range investigated. Combustion regime diagrams constructed for 33 μm particles illustrate that burning velocity enhancement terminates above initial gas equivalence ratios of 0.68 using the current model. Combustion regime diagram for 33 μm coal particles mixed with methane gas. The colour contours denote burning velocity and the solid lines denote specific contours in 5 cm/s increments. The open circles denote maximum burning velocity at each gas equivalence ratio and annotations indicate different combustion regimes outlined in this work. [Display omitted] •Coupling between dust combustion and gas flame depends on physical timescales.•Six flame coupling regimes are identified through timescale and simulation analysis.•Small particles (10 um) show coupling between dust and gas for all gas concentrations.•Larger particles (33 um) show coupling up to gas-phase equivalence ratios of 0.68.