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Ind. Eng. Chem. Res. 2003, 42, 1162-1173
Advanced Fluidized Bed Combustion Sorbent Reactivation Technology E. J. Anthony,*,† A. MacKenzie,‡ O. Trass,§ E. Gandolfi,| A. P. Iribarne,⊥ J. V. Iribarne,⊥ and S. M. Burwell# CANMET Energy Technology Centre, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1, A.M. MacKenzie Consultants Ltd., 2800 Robert Murphy Drive, Halifax, Nova Scotia, Canada B3L 3T4, Chemical Engineering Department and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5, General Comminution Inc., P.O. Box 70, Station P, Toronto, Ontario, Canada M5S 2S6, Department of Physics, University of Toronto, 60 George Street, Toronto, Ontario, Canada M5S 3A7, and Suzanne Burwell Enterprises, 17 Treanor Crescent, Georgetown, Ontario, Canada L7G 5H8
A new technique for simultaneous grinding and hydrating of fluidized bed combustion (FBC) bottom ash has been developed. This method has been shown to be effective in hydrating the CaO component of the ash, so that the sorbent is reactivated. Careful control of water levels is required to prevent energy demand increases for grinding. No problems associated with the potentially exothermic reaction of water with FBC bottom ash have been observed during grinding. When excess water (over that required by hydration) is used, the resulting material is a slurry and, while quantitative conversion of CaO in the solids is achieved, using the slurry for the sorbent would require a redesign of the limestone feed system. Therefore, coal or unreacted ash is added to the mixture after grinding. The resulting dry product contains the spent bed material in a completely hydrated form. The reactivated ash produced has been evaluated for sulfur capture using thermogravimetric analysis and a CFBC pilot plant. Conversion rates of almost 100% are achieved for ash after grinding hydration. An industrial demonstration of the technology has supported its viability with no decrease in sulfur capture, while limestone requirements decreased by 18%. The economic implications of the industrial applicability of the technology are outlined in a case study using the Point Aconi CFBC unit. Decreased limestone usage is calculated to net savings in the order of $500000/year. The project is calculated to have an equity payback of less than 1 year. 1. Introduction Fluidized-bed-combustion (FBC) technology is an effective method of burning a variety of fuels for power generation, industrial steam production, or space heating with low pollutant emissions. Limestone or dolomite addition to the bed minimizes SO2 emissions from highsulfur fuels by capturing the SO2 in situ. The sulfur capture process can be described by the following reactions:
CaCO3 ) CaO + CO2
∆H ) 182.1 kJ/gmol (1)
CaO + SO2 + 1/2O2 ) CaSO4 ∆H ) -481.4 kJ/gmol (2) One of the limitations of this technology, however, is relatively inefficient use of the limestone sorbent (30-40%).1 In comparison, wet flue gas desulfurization (FGD) processes typically achieve limestone utilization of over 90%.2 FBC ashes contain the excess sorbent in * Corresponding author. Telephone: +1-613-996-2868. Fax: +1-613-992-9335. E-mail:
[email protected]. † Natural Resources Canada. ‡ A.M. MacKenzie Consultants Ltd. § Chemical Engineering Department and Applied Chemistry, University of Toronto. | General Comminution Inc. ⊥ Department of Physics, University of Toronto. # Suzanne Burwell Enterprises.
the form of CaO or, following conditioning, Ca(OH)2, which is highly reactive with water. The FBC ash landfill site must then be managed with attention to high-pH leachate and considerable potential for expansion, which might damage the integrity of the site.3 As a result, ash disposal is a costly component associated with FBC technology. Disposal costs range from approximately US$10/ton for the 165 MWe unit at Point Aconi, Canada, to US$21/ton for some units in the U.S.4 The economics of FBC technology would benefit from reductions in both the ash volume produced and ash reactivity. There have been several sorbent reactivation methods proposed over the past decade; however, technical and economic considerations have prevented widespread industrial use of the processes. On the basis of previous work and experience, a method of wet-grinding FBC ash has been developed. The technical development and economic considerations of this technology follow. 2. Background 2.1. Limits to Sulfation of FBC Ash. Limestone utilization of 45% is considered excellent for industrial FBC units (i.e., about 90% sulfur capture for a Ca/S molar ratio of about 2).5 Pore plugging during the sulfation process limits utilization of the sorbent. The molar volumes of calcite (the most common form of CaCO3 in limestone6), CaO, and CaSO4 (as anhydrite) are 36.9, 16.9, and 46 cm3/ mol, respectively.7 Under typical operating conditions
10.1021/ie020305h CCC: $25.00 © 2003 American Chemical Society Published on Web 02/22/2003
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Figure 1. Typical sulfation pattern for limestone particles.
in a FBC, limestone calcines from a relatively nonporous material (with a natural porosity between 0.3 and 12%) to an extremely porous solid, with a porosity of over 50%.5 However, the product of sulfation (CaSO4) forms an impenetrable shell, particularly on larger particles. This prevents further reaction between unreacted CaO in the no-longer-accessible particle core and SO2 (Figure 1), and this core-shell structure is typical of limestone sulfation for the majority of calcitic limestones.8 Based on the molar volumes of the chemical species involved, the maximum possible sulfation of a chemically pure nonporous limestone would be 69%.5 This is not typical of FBC ash. This model of sulfation of calcined limestone has been obtained from energy-dispersive X-rays (EDX) of bubbling FBC ash samples, which show a particle consisting of a CaO core surrounded by a CaSO4 shell.8-10 Examination of ashes from the Chatham CFBC11 suggests that smaller particles (approximately
