Calcination of calcium acetate and calcium magnesium acetate: effect of the reacting atmosphere J. Ada ´nez * , L.F. de Diego, F. Garcı ´a-Labiano Instituto de Carboquı ´mica (C.S.I.C.), Department of Energy and Environment, P.O. Box 589, 50080 Zaragoza, Spain Accepted 25 September 1998 Abstract The calcination process of the calcium acetate (CA) and calcium magnesium acetate (CMA) was investigated as a previous step for coal gas desulfurisation during sorbent injection at high temperatures because the excellent results demonstrated by these sorbents as sulfur removal agents both in combustion and gasification processes. As pore structure developed during calcination is one of the most important characteristic of the sorbent related with the later reaction with the gaseous pollutants, several calcination tests were conducted in a drop tube reactor at temperatures from 700°C to 1100°C, and residence times from 0.8 to 2.4 s. Four different gas atmospheres were used for comparative purposes: inert, oxidising, reducing, and non-calcining (pure CO 2 ). Despite the advantage of the high porous cenospheric structure developed by these sorbents during their injection at high temperature, calcination of the CaCO 3 was not complete even at the longest residence time, 2.4 s, and the highest temperature, 1100°C, tested. An important effect of the reacting atmosphere on the calcination conversion and on the sorbent pore structure was detected. The CO 2 concentration around the particle, both that fed in the reacting gases or that generated by organic material combustion, seems to be responsible for the final calcination conversions obtained in each case, also affecting the sintering suffered by the sorbents.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Calcination; Calcium acetate; Calcium magnesium acetate; Cenosphere 1. Introduction Combustion and gasification are the most usual ways of coal utilisation in power generating systems. The conven- tional pulverised coal fired boiler and steam turbine combi- nations have demonstrated their validity in coal combustion for many years. However, among the advanced power generation options for coal utilisation, the integrated gasifi- cation combined cycle (IGCC) systems offer increased effi- ciency and the greatest ability to meet the stringent environmental emission limits. Nowadays, regulations governing emissions from utility boilers are being more stringent and, in many cases, new standards are obliging to modify existing plants [1]. The main pollutants regarding coal use are the sulphur and nitro- gen compounds, i.e. SO 2 and NO X in the case of combustion and H 2 S in the case of gasification systems. Several gas cleaning options with immediate improvements have been reported. Among them, sorbent injection represents a low cost solution involving relatively simple changes in utility boilers and in operating practice. Different natural calcium-based sorbents as limestones, dolomites and calcium hydroxides have been used in the past as desulfurisation sorbents at high temperatures both in combustion and gasification systems. Specially interest- ing were the results obtained with the calcium acetate (CA) and the calcium magnesium acetate (CMA). These sorbents although more expensive than the natural ones, have exhib- ited excellent results in sulphur removal under oxidising [2– 4] and reducing [5] conditions. Levendis et al. [2] demon- strated the superior SO 2 sorption efficiency of these sorbents compared at the same molar Ca/S ratio to other sorbents including limestones, dolomites and calcium hydroxides. Moreover, the hydrocarbon radicals generated during organic acetate decomposition permits the reduction of the NO X simultaneously to the SO 2 during coal combustion. This can have a significant impact on reducing the capital costs and the complexity of the effluent gas treatment. CA and CMA have the unique property to calcine, form- ing highly cenospheric particles, with thin and porous walls, resembling ‘‘pop-corn’’-like structures. The high porosities exhibited by these particles greatly reduce the importance of the limiting resistance in gas–solid reactions, and increase the sorbent utilisation within typical furnace conditions, as are high temperatures and low residence times. Fuel 78 (1999) 583–592 0016-2361/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0016-2361(98)00186-0 * Corresponding author.Tel.: +34-976733977; fax: +34-976733318. E-mail address: jadanez@carbon.icb.csic.es (J. Ada ´nez)