* Corresponding author. Tel.: #34-98-5280800; fax: #34-98- 5280800. E-mail address: greca@muniellos.incar.csic.es (G. Marba H n) Chemical Engineering Science 55 (2000) 1661}1674 Decomposition of CaS particles at ambient conditions Marta Garcm H a-Calzada, Gregorio Marba H n*, Antonio B. Fuertes Instituto Nacional del Carbo & n, CSIC, La Corredoria s/n Apartado 73, 33080, Oviedo, Spain Received 1 October 1998; accepted 22 June 1999 Abstract Decomposition of CaS particles (18.6 m size) at ambient conditions was investigated by means of potentiometric analysis, TG, SEM-EDX and XRD techniques. The thermodynamics of the process was also studied. The results show that well dispersed CaS particles are decomposed provoking a high H  S release to the atmosphere during the "rst two days of decomposition. This release was evaluated to be in the range 61}72 mol% on initial sulphur basis. The remaining solid consists of a 66 mol% calcium carbonate, 22 mol% calcium sulphate and 12 mol% elemental sulphur. Trace amounts of thiosulphate were also detected by XRD analysis. The formation mechanisms of the di!erent species are discussed. SEM photographs of decomposed CaS pellets revealed that the decomposition process is di!usion controlled for high loads of particles.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Calcium sulphide; Decomposition; Thermodynamics; SEM; XRD; TGA 1. Introduction An environmental and technological problem asso- ciated with coal gasi"cation in IGCC systems is the formation of H  S and COS in the gasi"er. These corros- ive agents must be removed before the gas stream enters the gas turbine. To do this, desulphurisation systems operating either at low or high temperature can be used. Since the former produce an important loss of e$ci- ency in the IGCC plant, high temperature systems (¹'5003C) are now being researched for future imple- mentation. Among them, the most commonly investi- gated (Westmoreland, Gibson & Harrison, 1977; Yumura & Furimsky, 1985; Wakker, 1992; Naijar & Jung, 1995) are those dealing with regenerable solid sorbents (i.e. CaO, ZnO, Fe  O  ), which are placed between the gasi"er and the gas turbine (external hot desulphurisation). However, the use of non-regenerable sorbents (dolomite and limestone) directly injected in the gasi"ers (in situ desulphurisation) has been the object of attention in the last years as a consequence of their availability and low price (Borgwardt & Roache, 1984; Abbasian, Rehmat, Leppin & Barnejee, 1990; Robin, Wu & Kassman, 1990; Heesink & Van Swaaij, 1995). The e$ciency of these sorbents has been demonstrated in di!erent works (Goyal, Bryan, Rehmat, Patel & Ghate, 1990; Yrjas, Iisa & Hupa, 1995). However, the spent sorbent particles, composed partly of calcium sulphide, are chemically unstable when land"lled at the open air, and provoke the release of part of the H  S captured in the gasi"er (Abbasian, Hill, Wangerow, Rehmat & Barnejee, 1991a; Gee, Chang & Park, 1991; Wheelock, 1995; Kutsovskaya, Heptworth & McGaa, 1966; Yrjas, Hupa & Iisa, 1996). Stabilisation, i.e. via oxidation to CaSO  (Abbasian, Rehmat & Barnejee, 1991b; Yrjas et al., 1996), or regeneration (Wheelock, 1995; Van der Ham, Heesink, Prins & Van Swaaij, 1996; Brooks & Lynn, 1997) of the spent sorbent must be therefore carried out. All the works dealing with stabilisation of the spent sorbent particles (Abbasian et al., 1991b; Yrjas et al., 1996) make use of an oxidation process to produce inert CaSO  . However, as it is not possible to achieve 100% conversion to sulphate (without SO  release) with the commonly used calcareous sorbents, since cessation of reaction due to pore closure occurs before complete conversion, it is not clear whether the partly oxidised particles are stable or not in the long term. Some works in this direction are presently being carried out in our laboratory. Thus, in our opinion the stabilisation process of spent sorbent particles is a subject that needs further 0009-2509/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 4 0 9 - 1