Research Article Limestone Attrition under Simulated Oxyfiring Fluidized-Bed Combustion Conditions Limestone attrition by surface wear was studied during the flue gas desulfuriza- tion under simulated fluidized-bed (FB) oxyfiring conditions and hindered calci- nation. Bench-scale experimental tests were carried out using well-established techniques previously developed for the characterization of sulfation and attrition of sorbents in air-blown atmospheric FB combustors. The experimental limestone conversion and attrition results were compared with those previously obtained with the same limestone under simulated air-blown combustion conditions. The differences in the conversion and attrition extents and patterns associated with oxyfiring as compared to air-blown atmospheric combustion were highlighted and related to the different particle morphologies and thicknesses of the sulfate layer. It was noted that attrition could play an important role in practical circulat- ing FB combustor operation, by effectively enhancing particle sulfation under both oxyfiring and air-blown combustion conditions. Keywords: Attrition, Combustion, Desulfurization, Fluidization, Oxyfiring, Particle Received: November 28, 2008; accepted: December 11, 2008 DOI: 10.1002/ceat.200800625 1 Introduction Limestone attrition during the air-blown fluidized-bed (FB) combustion of sulfur-bearing fuels has been thoroughly char- acterized over the last decade [1–7]. Key phenomenological features and mechanistic pathways of sorbent attrition have been determined with the aid of a comprehensive test protocol consisting of different and mutually complementary test pro- cedures [1, 3, 8]. In particular, attrition by abrasion (surface wear) is to be related to mechanical stresses caused by rubbing and collisions with other particles or with the reactor walls or internals. The progress of chemical reactions (calcination, sul- fation) interferes with the attrition process, making the phe- nomenology even more complicated. Recently, Shimizu et al. [5] reported that under pressurized combustion conditions (where large CO 2 partial pressures establish) sulfur capture in a large-scale FB combustor was mostly controlled by the lime- stone attrition rate. In fact, since most of the limestone in the boiler captures sulfur dioxide at a very slow rate, the control- ling process being SO 2 diffusion across the sulfate product layer, attrition of the sorbent particle surface may reduce the thickness of the sulfate layer and, in turn, increase the reaction rate. In the last few years oxygen-blown (oxyfiring) combustion has gained interest as a technology suitable for carbon capture and sequestration (CCS). An almost pure stream of CO 2 can be produced, and the costs of CO 2 separation from the flue gas and its further compression can be substantially reduced [9]. Constraints related to temperature control in the combustor require extensive recirculation of flue gases. Like in pressurized combustion, carbon dioxide partial pressures in the reaction zone under oxyfiring conditions are much larger than those establishing under atmospheric air-blown combustion condi- tions. Although most of the research activity in oxyfiring has concentrated on pulverized coal combustors, recently the application of this technology to circulating FB (CFB) coal combustors has been addressed [10–14]. CFB appear to be particularly suited for oxyfiring conditions because of the fuel flexibility and better temperature control (which allows reduc- ing the amount of recycled flue gas). The feasibility of CFB coal oxyfiring has been successfully demonstrated in pilot- plant tests, and no significant technological barrier exists that could prevent its implementation in the near term. Two alternative strategies can be followed as regards in situ desulfurization with injection of limestone under oxyfiring conditions. The first option is that of operating at tempera- © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Fabrizio Scala 1 Piero Salatino 1,2 1 Istituto di Ricerche sulla Combustione – CNR, Napoli, Italy. 2 Dipartimento di Ingegneria Chimica – Università degli Studi di Napoli Federico II, Napoli, Italy. – Correspondence: Prof. Piero Salatino (salatino@unina.it), Istituto di Ricerche sulla Combustione – CNR, Piazzale Tecchio 80, 80125 Napoli, Italy. 380 Chem. Eng. Technol. 2009, 32, No. 3, 380–385