BURNING VELOCITY AND LOWER FLAMMABILITY LIMITS OF HYBRID MIXTURES CONTAINING COMBUSTIBLE DUST AND FLAMMABLE GAS

Abstract

Hybrid mixtures of combustible dust and flammable gas may demonstrate increased burning velocities and reduced lower flammability limits (LFL) over the fuels individually. This can increase explosion likelihood and severity in industrial operations and makes it difficult to develop and implement explosion prevention and protection strategies. The objective of this work is to extend the current knowledge of laminar burning velocity and LFLs of hybrid mixtures. This is achieved using computational fluid dynamics (CFD) modeling to analyze flame structure, burning velocity, and propagation limits. The computational model includes global approximations to molecular transport, and the accuracy of four reaction mechanisms with increasing complexity are explored.

Simulations investigating the structure of coal dust flames, the effect of equivalence ratio on hybrid mixtures, and coupling interaction between gas flame propagation and particle combustion, are explored in this work. These simulations allow combustion regime diagrams to be created for hybrid mixtures. In these diagrams, six regimes are identified: fuel-lean, fuel-rich, volatile-rich, transition flames, kinetic-limited flames, and impeded-gas flames.

Mixing rules for lower flammability limits of methane gas and coal dust mixtures are evaluated based on results from the CFD model. Linear mixing based on Le Chatelier’s law is found to agree with the simulation results for 10 μm coal dust particles. Larger particles with 33μm diameters demonstrated strong flame propagation at concentrations slightly wider than Le Chatelier’s Law, but not as wide as Bartknetch’s curve.

The results from this work provide novel classifications of burning velocity enhancement for hybrid mixtures, illustrate that linear mixing rules approximately delineate LFLs for coal dust and methane gas, and verify the accuracy of the CFD model. These results can be used to guide experimental testing for research programs or hazard assessments, and the CFD model provides an open platform to explore and extend the fundamental knowledge of flame propagation in dust and hybrid mixtures.