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Title: Attrition-enhanced sulfur capture by limestone particles in fluidized beds

Abstract

Sulfur capture by limestone particles in fluidized beds is a well-established technology. The underlying chemical and physical phenomena of the process have been extensively studied and modeled. However, most of the studies have been focused on the relatively brief initial stage of the process, which extends from a few minutes to hours, yet the residence time of the particles in the boiler is much longer. Following the initial stage, a dense product layer will be formed on the particle surface, which decreases the rate of sulfur capture and the degree of utilization of the sorbent. Attrition can enhance sulfur capture by removing this layer. A particle model for sulfur capture has been incorporated with an attrition model. After the initial stage, the rate of sulfur capture stabilizes, so that attrition removes the surface at the same rate as diffusion and chemical reaction produces new product in a thin surface layer of a particle. An analytical solution for the conversion of particles for this regime is presented. The solution includes the effects of the attrition rate, diffusion, chemical kinetics, pressure, and SO{sub 2} concentration, relative to conversion-dependent diffusivity and the rate of chemical reaction. The particle model results in models thatmore » describe the conversion of limestone in both fly ash and bottom ash. These are incorporated with the residence time (or reactor) models to calculate the average conversion of the limestone in fly ash and bottom ash, as well as the efficiency of sulfur capture. Data from a large-scale pressurized fluidized bed are compared with the model results.« less

Authors:
;  [1]
  1. VTT Technical Research Center of Finland, Jyvaskyla (Finland)
Publication Date:
OSTI Identifier:
20885706
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research; Journal Volume: 46; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; LIMESTONE; DESULFURIZATION; FLUIDIZED-BED COMBUSTION; WEAR; MATHEMATICAL MODELS; SURFACES; PARTICLES; COAL; FLY ASH; ASHES; PRESSURIZATION; SULFATION

Citation Formats

Saastamoinen, J.J., and Shimizu, T. Attrition-enhanced sulfur capture by limestone particles in fluidized beds. United States: N. p., 2007. Web. doi:10.1021/ie060570t.
Saastamoinen, J.J., & Shimizu, T. Attrition-enhanced sulfur capture by limestone particles in fluidized beds. United States. doi:10.1021/ie060570t.
Saastamoinen, J.J., and Shimizu, T. Wed . "Attrition-enhanced sulfur capture by limestone particles in fluidized beds". United States. doi:10.1021/ie060570t.
@article{osti_20885706,
title = {Attrition-enhanced sulfur capture by limestone particles in fluidized beds},
author = {Saastamoinen, J.J. and Shimizu, T.},
abstractNote = {Sulfur capture by limestone particles in fluidized beds is a well-established technology. The underlying chemical and physical phenomena of the process have been extensively studied and modeled. However, most of the studies have been focused on the relatively brief initial stage of the process, which extends from a few minutes to hours, yet the residence time of the particles in the boiler is much longer. Following the initial stage, a dense product layer will be formed on the particle surface, which decreases the rate of sulfur capture and the degree of utilization of the sorbent. Attrition can enhance sulfur capture by removing this layer. A particle model for sulfur capture has been incorporated with an attrition model. After the initial stage, the rate of sulfur capture stabilizes, so that attrition removes the surface at the same rate as diffusion and chemical reaction produces new product in a thin surface layer of a particle. An analytical solution for the conversion of particles for this regime is presented. The solution includes the effects of the attrition rate, diffusion, chemical kinetics, pressure, and SO{sub 2} concentration, relative to conversion-dependent diffusivity and the rate of chemical reaction. The particle model results in models that describe the conversion of limestone in both fly ash and bottom ash. These are incorporated with the residence time (or reactor) models to calculate the average conversion of the limestone in fly ash and bottom ash, as well as the efficiency of sulfur capture. Data from a large-scale pressurized fluidized bed are compared with the model results.},
doi = {10.1021/ie060570t},
journal = {Industrial and Engineering Chemistry Research},
number = 4,
volume = 46,
place = {United States},
year = {Wed Feb 14 00:00:00 EST 2007},
month = {Wed Feb 14 00:00:00 EST 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 themore » 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. 27 refs., 9 figs. 1 tab.« less
  • Mines tested various coals as fuel in fluidized beds of limestone, compared sulfur retention, and measured heat transfer with tubes immersed in the bed. The program involved testing five types of bituminous coal from high volatile A to low volatile, which ranged in ash content from 8 to 24 percent and in sulfur content from 2 to 4 percent.
  • The influence of temperature on attrition of two limestones during desulfurization in a fluidized bed reactor was investigated. Differences in the microstructure of the two limestones were reflected by a different thickness of the sulfate shell formed upon sulfation and by a different value of the ultimate calcium conversion degree. Particle attrition and fragmentation were fairly small under moderately bubbling fluidization conditions for both limestones. An increase of temperature from 850 C to 900 C led to an increase of the attrition rate, most likely because of a particle weakening effect caused by a faster CO{sub 2} evolution during calcination.more » This weakening effect, however, was not sufficiently strong to enhance particle fragmentation in the bed. The progress of sulfation, associated to the build-up of a hard sulfate shell around the particles, led in any case to a decrease of the extent of attrition. Sulfation at 900 C was less effective than at 850 C, and this was shown to be related to the porosimetric features of the different samples. (author)« less
  • Attrited carbon is a large fraction of the overall carbon elutriation from a fluidized coal combustor when gas velocity is significantly above the minimum for fluidization and feed size is coarser than 0.4 mm. Continuous fluidized combustion of a South African bituminous coal of particle size range varying between 0.4-1 and 6-9 mm in beds of sand fluidized at velocities between 0.8 and 1.6 m/s indicated that with the coal tested, the elutriation rate of attrited carbon fines (mostly <100 ..mu..m) was proportional to the surface of char exposed to the rubbing of bed solids. Experiments of fluidized combustion ofmore » the South African coal (1-3 mm size) have been repeated, with various excess air factors, using a miniature 40 mm ID and a pre-pilot 370 mm ID combustor. The layout of these units is similar to that of the 140 mm ID combustor with some exceptions.« less
  • This paper reports on the elutriation rates of unburned carbon from a 20-cm-i.d. bubbling fluidized bed that were evaluated for a variety of coals. Elutriation rates were determined from cyclone catches while carbon loadings in the fluidized bed were evaluated from transient gas and particle emissions. The elutriation rates were well correlated with carbon loading in the bed, but there was no evidence that they depend on superficial velocity through the bed. These results suggest that fragmentation, rather than attrition, is primarily responsible for carbon loss during fluidized bed combustion. The experiments found evidence that the fragmentation rates were dependentmore » on coal rank.« less