Chemical Engineering Journal 96 (2003) 157–169 An experimental and modeling study of the contribution of coal ash to SO 2 capture in fluidized bed combustion Borislav Grubor a,∗ , Vasilije Manovic b , Simeon Oka a a Institute for Nuclear Sciences Vinca, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro b Mining and Geology Faculty, Belgrade University, Djusina 7, 11000 Belgrade, Serbia and Montenegro Abstract The process of sulfur self-retention (SSR) occurs as a result of the reactions between the mineral matter in coal ash and the SO 2 evolved during coal combustion. Consequently, the emission of SO 2 may be significantly reduced. The results of experimental investigations and modeling of SSR is presented in this work. The transformations of sulfur forms during devolatilization are taken into account via a correlation for the amount of sulfur that remains in the char, after devolatilization. A novel approach has been applied for modeling SSR during char combustion, closely related to the grain model used for SO 2 retention by limestone as a sorbent. It is assumed that SSR is a result of the reaction between SO 2 and CaO in the form of uniformly distributed micro-grains in char. An unreacted shrinking core model is adopted for the reactions between the CaO micro-grains and SO 2 . The comparison with the experimentally obtained values in a fluidized bed reactor and in a laboratory oven, using coals of different rank (fixed carbon over volatile matter ratio, C fix /VM = 0.75–7.40), content of sulfur forms (total 0.84–6.04%, organic 0.71–4.71%, pyritic 0–2.57%) and molar Ca/S ratio (0.34–3.17), has shown that the model can adequately predict the kinetics of the process, the levels of the obtained values of SSR efficiencies, as well as the influence of temperature, coal particle size and the surrounding conditions. © 2003 Elsevier B.V. All rights reserved. Keywords: Coal combustion; Sulfur retention by ash; Modeling 1. Introduction In the course of development of fluidized bed combus- tion (FBC) boilers for domestic coals, in the Laboratory for Thermal Engineering and Energy of the Institute for Nuclear Sciences Vinca, special attention was paid to the fundamen- tal research on single coal particle behavior in fluidized bed, in order to reveal the influence of coal characteristics on boiler concept and design. These investigations [1,2], which started from 1976, encompassed the processes of bed to coal particle heat and mass transfer, kinetics of devolatilization, particle fragmentation, ignition temperature and start-up temperature and combustion kinetics of coal and char. The experimental research was supported by mathematical modeling of the particle heating and particle temperature distribution and history, particle fragmentation and char combustion. Initially, investigations in our Laboratory in relation to sulfur retention were limited to experimental characteriza- tion of domestic limestones with the aim of determining the influence of the limestone type, particle size, temperature ∗ Corresponding author. Tel.: +381-11-455695; fax: +381-11-453670. E-mail addresses: grub@vin.bg.ac.yu, ite@vin.bg.ac.yu (B. Grubor). and SO 2 concentration on the degree of sulfation [3]. In due course, when a number of stationary FBC combustion tests showed that in the case of some coals substantial sulfur retention can be achieved without the addition of limestone, our interest for the process of sulfur self-retention (SSR) by coal ash itself was enhanced. This new orientation is also in accordance with our opinion, that processes in and around coal particle are crucial for FBC boiler design and overall behavior. Investigation of sulfur self-retention completes our fundamental interest in coal particle behavior under FBC conditions. Also, the numerous investigations of SSR under FBC conditions by other authors [4–7] have shown that a sub- stantial part of sulfur may be retained in the ash, decreasing the needed amount of limestone to be added. Puff et al. [4] reported that around 60% of sulfur is retained due to SSR, while values near 90% were obtained with fly ash recirculation or combustion of coal rich with tailings. In our earlier investigations [8] no direct correlation could be found between the molar Ca/S ratio in coal and the amount of sulfur retained in the ash and this fact initiated further investigations in order to determine the influence of coal characteristics and combustion conditions on the SSR process. The results of the recent experimental investigations 1385-8947/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2003.08.021