The modeling of the combustion of high-ash coal–char particles suitable for pressurised fluidized bed combustion: shrinking reacted core model Raymond Everson * , Hein Neomagus, Rufaro Kaitano Research Focus Area: Separation Science and Technology, School of Chemical and Minerals Engineering, Private Bag X6001, North-West University, Potchefstroom Campus, Potchefstroom 2520, South Africa Received 2 October 2003; received in revised form 19 August 2004; accepted 18 January 2005 Available online 23 February 2005 Abstract An investigation was undertaken involving the combustion of high-ash coal/char particles under conditions suitable for pressurised fluidised bed combustion, in order to evaluate an overall combustion model. The use of very poor quality feedstocks (greater than 40% ash, low calorific value and high sulphur content) in conventional pulverised fuel combustors (PFC) could be technically difficult and un- economical, and has the associated disadvantage of generating gaseous pollutants. Pressurised fluidised bed combustion (PFBC) which is an attractive alternative process and which uses millimetre-sized coal particles is increasing in use on a commercial scale and is the basis for several clean coal technology processes. A Thermogravimetic Analyser (TGA) was used for the experimentation, which was capable of handling relatively large coal/char particles at high pressures and temperatures. Experimentation with prepared coal/chars particles with a diameter of 3 mm at a pressure of 487 kPa and temperatures between 750 and 950 8C was carried out. For the determination of the overall kinetics of combustion it was found necessary to deviate from the established methods (surface-based reaction) and that it was essential to incorporate diffusion in the overall reaction model. Also, the concept of carbon concentration variation in the particle is introduced to account for the effect of high ash content (a mixture of carbon and minerals), instead of assuming pure carbon. This model, which consists essentially of a shrinking reactive core, was found to agree very well with experimental results and all relevant parameters required for an overall rate equation were evaluated. It is also shown that at high temperatures the shrinking reacted core model results approached the results obtained from the conventional shrinking unreacted core model. q 2005 Elsevier Ltd. All rights reserved. Keywords: High ash coal; Shrinking reacted core model; Pressurised combustion 1. Introduction Advanced technologies, such as pressurised fluidised bed combustion (PFBC), and integrated gasification combined cycle (IGCC), have been identified as the most viable alternatives for the clean utilisation of coal [1–3]. These clean coal technologies have gained increased technological and scientific interest because of higher efficiencies and minimal environment impacts. The higher operating pressures used in these processes will inherently result in an increase in coal throughput, and an increase in the overall reaction rate [4,5]. Fluidised beds can process solids more efficiently which can favour the removal of ash resulting from the combustion of high ash coal and the handling of sorbents used for sulphur compound removal. Also the lower temperatures used is beneficial for in situ sorption of sulphur compounds and reduction of nitrogen oxides. The combustion of medium sized (2–5 mm) coal particles in these processes, which is not uncommon, has obvious coal preparation and handling advantages but requires the examination of diffusional effects and structural changes [4,6,7]. The analysis of the combustion of coal/chars have been based on the shrinking unreacted core model with a surface controlling reaction (shell progressive burn-off) by many investigators [8]. This model is applicable to coal/char with very low porosities, such that the coal/char is practically impervious to the oxygen, and that the reaction will occur at the surface of the coal/char particle or at the interface between the unreacted coal/char and the porous ash layer Fuel 84 (2005) 1136–1143 www.fuelfirst.com 0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.01.011 * Corresponding author. Tel.: C27 18 299 1986; fax: C27 18 299 1535. E-mail address: chirce@puknet.puk.ac.za (R. Everson).