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Energy & Fuels 2007, 21, 2566-2572
Attrition of Limestone by Impact Loading in Fluidized Beds Fabrizio Scala,† Fabio Montagnaro,‡ and Piero Salatino*,§ Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy, Dipartimento di Chimica, UniVersita` degli Studi di Napoli Federico II, Complesso UniVersitario del Monte di Sant’Angelo, 80126 Napoli, Italy, and Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ReceiVed January 30, 2007. ReVised Manuscript ReceiVed May 28, 2007
The present study addresses limestone attrition and fragmentation associated with impact loading, a process which may occur extensively in various regions of fluidized bed (FB) combustors/gasifiers, primarily the jetting region of the bottom bed, the exit region of the riser, and the cyclone. An experimental protocol for the characterization of the propensity of limestone to undergo attrition/fragmentation by impact loading is reported. The application of the protocol is demonstrated with reference to an Italian limestone whose primary fragmentation and attrition by surface wear have already been characterized in previous studies. The experimental procedure is based on the characterization of the amount and particle size distribution of the debris generated upon the impact of samples of sorbent particles against a target. Experiments were carried out at a range of particle impact velocities between 10 and 45 m/s, consistent with jet velocities corresponding to typical pressure drops across FB gas distributors. The protocol has been applied to either raw or preprocessed limestone samples. In particular, the effect of calcination, sulfation, and calcination/recarbonation cycles on the impact damage suffered by sorbent particles has been assessed. The measurement of particle voidage and pore size distribution by mercury intrusion was also accomplished to correlate fragmentation with the structural properties of the sorbent samples. Fragmentation by impact loading of the limestone is significant. Lime displays the largest propensity to undergo impact damage, followed by the sorbent sulfated to exhaustion, the recarbonated sorbent, and the raw limestone. Fragmentation of the raw limestone and of the sulfated lime follows a pattern typical of the failure of brittle materials. The fragmentation behavior of lime and recarbonated lime better conforms to a disintegration failure mode, with an extensive generation of very fine fragments.
Introduction The influence of the progress of calcination and sulfation on the attrition/fragmentation of limestone in atmospheric fluidized bed (FB) combustors has long been recognized.1-5 More limited is the information available on the influence of other physicochemical processes or process conditions on the propensity of Ca-based sorbents to undergo attrition/fragmentation in fluidized beds: water hydration either targeted to reinjection in the fluidized bed furnace6,7 or to ex situ flue gas desulfurization,8,9 * To whom correspondence should be addressed. Tel.: +39-0817682258. Fax: +39-081-5936936. E-mail:
[email protected]. † Consiglio Nazionale delle Ricerche. ‡ Dipartimento di Chimica, Universita ` degli Studi di Napoli Federico II. § Dipartimento di Ingegneria Chimica, Universita ` degli Studi di Napoli Federico II. (1) Scala, F.; Cammarota, A.; Chirone, R.; Salatino, P. AIChE J. 1997, 43, 363-373. (2) Di Benedetto, A.; Salatino, P. Powder Technol. 1998, 95, 119-128. (3) Scala, F.; Salatino, P.; Boerefijn, R.; Ghadiri, M. Powder Technol. 2000, 107, 153-167. (4) Chandran, R. R.; Duqum, J. N. Attrition Characteristics Relevant for Fluidized Bed Combustion. In Fluidization VI; Grace, J. R., Shemilt, L. W., Bergougnou, M. A., Eds.; Engineering Foundation: New York, 1989; pp 571-580. (5) Couturier, M. F.; Karidio, I., Steward, F. R. Study on the Rate of Breakage of Various Canadian Limestones in a Circulating Transport Reactor. In Circulating Fluidized Bed Technology IV; Avidan, A. A., Ed.; American Institute of Chemical Engineers: New York, 1993; pp 672678. (6) Montagnaro, F.; Scala, F.; Salatino, P. Ind. Eng. Chem. Res. 2004, 43, 5692-5701. (7) Li, D.; Zhong, F.; Guo, Q.; Lu, J.; Zhang, J.; Yue, G. Fuel Process. Technol. 2007, 88, 215-220.
steam hydration,10 calcination/recarbonation of lime,11 and pressurized fluidized bed combustion.12 It has been shown that primary fragmentation (decrepitation) may extensively occur after the injection of sorbent particles in fluidized beds operated at atmospheric pressure, as a consequence of thermal stresses and of internal overpressure due to carbon dioxide emission. Primary fragmentation essentially occurs in the dense bed or in the splashing zone of either bubbling or circulating FB combustors, and it results in the generation of either coarse or fine fragments. Further breakage of particles occurs as a consequence of mechanical stresses experienced by the particles during their lifetime in the reactor, whose nature and extent determines the attrition/fragmentation pattern and location. The extensive literature on the attrition of granular solids in fluidized beds13 suggests that in-bed attrition can be related to either of two basic breakage mechanisms: • Surface wear, due to the rubbing of bed solids in the Vigorous bubble-induced sheared flow of the emulsion phase. (8) Lee, S. K.; Jiang, X.; Keener, T. C.; Khang, S. J. Ind. Eng. Chem. Res. 1993, 32, 2758-2766. (9) Chu, C.-Y; Hsueh, K.-W.; Hwang, S.-J. J. Hazard. Mater. 2000, B80, 119-133. (10) Montagnaro, F.; Pallonetto, F.; Salatino, P., Scala F. AIChE J. 2006, 52, 4090-4098. (11) Stanmore, B. R.; Gilot, P. Fuel Process. Technol. 2005, 86, 17071743. (12) Shimizu, T.; Peglowa, M.; Sakuno, S.; Misawa, N.; Suzuki, N.; Ueda, H.; Sasatsu, H.; Gotou, H. Chem. Eng. Sci. 2001, 56, 6719-6728. (13) Werther, J.; Reppenhagen, J. Attrition. In Handbook of Fluidization and Fluid-Particle Systems; Yang, W. C., Ed.; Dekker: New York, 2003; pp 201-237.
10.1021/ef0700580 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/14/2007
Attrition of Limestone by Impact Loading
This contribution is typical of the bulk of bubbling beds or of the bottom bed of circulating fluidized beds and is generally responsible for the formation of fine particulates of elutriable size. • Impact damage, related to high Velocity collisions between fluidized particles and targets. Bed internals or other bed solids can act as the target. High-velocity impact conditions are experienced by the particles in the grid (jetting) region of FB combustors and are closely related to the design of the gas distributor. The exit region of the riser and the cyclone are other potential locations of impact damage of sorbent particles. Depending on the extent and pattern of attrition, coarse/nonelutriable and fine/elutriable fragments can be generated as a consequence of impact damage. The mechanistic pathways along which the course of calcination and sulfation may affect sorbent attrition have been highlighted.1-3,14,15 The broad variability of the extent of primary fragmentation, as related to the porous structure of the native sorbent, was demonstrated. Moreover, it was shown that the rate of fines generation due to attrition by surface wear presents a pronounced peak in the very early stage of particle processing which decays thereafter as a consequence of particle rounding off. The progress of sulfation eventually leads to a further dramatic decrease of the particle attrition rate due to the formation of a tougher sulfate shell at the periphery of the particle. Neglecting the strengthening effect of the particle outer shell throughout conversion would lead to a significant overprediction of fines generation by attrition. Most of the published work refers to test conditions under which attrition by surface wear, rather than impact damage, was emphasized. The characterization of impact damage was limited.3,16 Scala et al.3 addressed the influence of modifications induced by calcination and/or sulfation on impact damage, whereas the impact damage of raw unreacted limestone at moderate temperatures (
