Abstract
This work is devoted to the development and validation of a sub-model for the partial oxidation of a spherical char particle moving in an air/steam atmosphere. The particle diameter is 2 mm. The coal particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using semi-global reaction rate expressions taken from the literature. The submodel includes six gaseous chemical species (O 2 , C O 2 , CO, H 2 O , H 2 , N 2 ). Three heterogeneous reactions are employed, along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water-gas-shift reaction. The distinguishing feature of the subgrid model is that it takes into account the influence of homogeneous reactions on integral characteristics such as carbon combustion rates and particle temperature. The sub-model was validated by comparing its results with a comprehensive CFD-based model resolving the issues of bulk flow and boundary layer around the particle. In this model, the Navier-Stokes equations coupled with the energy and species conservation equations were used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration was applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. Good agreement was achieved between the sub-model and the CFD-based model. Additionally, the CFD-based model was verified against experimental data published in the literature (Makino et al. (2003) Combust. Flame 132, 743-753). Good agreement was achieved between numerically predicted and experimentally obtained data for input conditions corresponding to the kinetically controlled regime. The maximal discrepancy (10%) between the experiments and the numerical results was observed in the diffusion-controlled regime. Finally, we discuss the influence of the Reynolds number, the ambient O 2 mass fraction and the ambient temperature on the char particle behaviour.
Highlights
Nowadays, due to the continuous increase in CO2 emissions around the world, traditional power generation using coal as a primary fuel can be considered a waste of carbon
Before we proceed with the formulation of a sub-model, we describe the computational setup used for Computational Fluid Dynamics (CFD)-based simulations
Follows a description of the results validating the subgrid model developed for the partial oxidation of a char particle moving in a hot oxidizing environment
Summary
Due to the continuous increase in CO2 emissions around the world, traditional power generation using coal as a primary fuel can be considered a waste of carbon. The significant contribution to the understanding of char particle behavior in a mixed convective environment came from Higuera [23] He carried out a comprehensive CFD-based numerical study (in the framework of a steady-state approach coupled with a simple chemistry) on the influences of the size and velocity of the coal char particle, the temperature and the gas composition on the burning rate, the particle temperature and the extinction of the flame. It should be noted that recent publications devoted to the CFD-based modelling of chemically reactive coal char particles moving in a hot oxidizing atmosphere (see the works [24, 25, 27]), basically describe the influence of input parameters such as the particle velocity, ambient temperature or particle diameter for single cases. From this point of view, the char combustion/gasification sub-model is of great importance in the successful modeling of large-scale combustors or gasifiers
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