The modelling of the generation of volatiles, H₂ and CO, and their simultaneous diffusion controlled oxidation, in pulverized coal furnaces

Abstract

The purpose of this paper is to contribute to the mathematical modelling of the combustion of coal particles in pulverized coal furnaces. The model deals with the gas and solid phases of the flow. For the coal particles a Lagrangian description is used, taking into account the simultaneous processes of moisture evaporation and devolatilization together with the heterogeneous gasification reactions of the char.

An Eulerian description will be used for the distributions of temperature and concentrations in the gas phase, with the effect of the particles represented by volumetric sources of heat, mass and momentum. The gas phase oxidation reactions of the volatiles, H₂ and CO will be modelled using the assumption of infinitely fast rates; the Burke–Schumann analysis will be generalized to account for the competition for oxygen of CO, H₂ and the volatiles. These reactions may occur, in the form of group combustion, in a gaseous thin diffusion flame separating a region without oxygen, where the coal particles generate volatiles, H₂ and CO, from a region with oxygen, where the reactions may occur inside the particles or, outside, in diffusion flames surrounding the individual particles, even though for small particle sizes the gas phase reactions can be considered as frozen near the particles.

The analysis will provide relations for the volumetric sources appearing in the gas phase description, and for the rates that determine the evolution of the temperature and mass content of moisture, volatiles and char in the particles.

Keywords:

• Char gasification
• CO combustion
• Group combustion
• Pulverized coal
• Pulverized flames

Acknowledgments

Part of this work was supported by MCYT of Spain through research program VEM2003-20069-C03-02. The work of A. Liñán was also supported by the MCYT of Spain under Project BFM2001-3691, and by the MEC under Project ENE2005-09190-C04-01/CON.

Notes

¹For an estimate of the value of the reaction time outside the particle we can use the diffusion time, δ _L ²/, across the flame thickness δ _L of the stoichiometric mixture of the reactant and oxidizer (where typically δ _L is of order of 50 μm).