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The paper elaborates the influence of the non-equimolar and equimolar counterdiffusion on combustion of single coal char particles in the oxy-fuel combustion conditions. The phenomenon of the non-equimolar counterdiffusion is usually neglected and superseded by the equimolar counterdiffusion. Such a replacement can lead to results, e.g. to char burnout, particle temperature and species concentrations, which do not agree with the real process. The paper presents the numerical results of single coal char particle combustion in the oxy-fuel combustion conditions at which the effect of the equimolar and non-equimolar counterdiffusion has been taken into consideration. It has been assumed that the char carbon heterogeneously reacts with O2, CO2 and H2O forming CO or CO2 and H2 depending on the combustion conditions. The reactions in the gas phase have been neglected to achieve the state of pure diffusion. The combustion of the particle is described by the mass and energy conservation equations commonly used in Euler–Lagrange computations of pulverized coal combustion. Numerical simulations performed for various values of the particle diameter and reagent concentrations clearly show that the use of the equimolar counterdiffusion model always overpredicts the non-equimolar one. Since the molar fluxes of the equimolar counterdiffusion are not coupled each other, the mass transfer towards the particle burning is higher which gives particle temperature and reaction rates too high compared to the non-equimolar counterdiffusion. Occurring discrepancy further develops during combustion also for other quantities describing the process, i.e. char burnout and gas concentrations at the particle surface. A simple correction which is proposed consists in reducing the value of the equimolar mass transfer coefficient that decreases the mass transfer and consequently letting the equimolar counterdiffusion model to be effortlessly used and get results which well follow the non-equimolar model. Such a simplified treatment of complex non-equimolar counterdiffusion can be easily implemented into numerical codes and needs no numerical solution of coupled non-linear equations describing the non-equimolar counterdiffusion.

Oxy-fuel combustion is considered as a promising solution to reduce greenhouse-gases and pollutant emissions. The main advantage of oxy-fuel combustion over other technologies for pollution reduction from pulverized coal combustion is that it can be applied to the existing coal-fired power plants. However, switching from conventional to oxy-fired coal combustion brings significant challenges. One of the most important is change of pulverized coal ignition characteristics. This paper presents the results of experimental and numerical analysis of ignition phenomena under oxy-fuel conditions. The main focus of the presented paper is to evaluate the effectiveness of the mathematical devolatilisation sub-model, in predicting the ignition point of pulverized coal flames under oxy-firing conditions. Regarding this, the performance of several devolatilisation models, from simple to more complex ones, in predicting ignition point position have been investigated. Numerically determined values of the ignition point position, and ignition temperature for various O-2-N-2 and O-2-CO2 conditions were compared with experimental data from the laboratory ignition test facility. Obtained results pointed out that network devolatilisation models (CPD and FG) give more accurate results in comparison with standard devolatilisation models (single rate and two competing rates). The best performance is achieved using FG devolatilisation model. Thus, newly implemented FG model will be used for future numerical simulations of oxy-fuel pulverized coal combustion on 0.5 MW pilot plant facility. (C) 2011 Elsevier Ltd. All rights reserved. 8th European conference on coal research and its applications, Sep 06-10, 2010, Leeds, England