Ind. Eng. Chem. Res. 2005, 44, 8199-8204
8199
The Hydration Behavior of Partially Sulfated Fluidized Bed Combustor Sorbent Jinsheng Wang,* Yinghai Wu, and Edward J. Anthony CANMET Energy Technology Centre-Ottawa (CETC-O), Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada
The efficiency of limestone sorbent utilization in fluidized bed combustors (FBCs) is low, because of incomplete sulfation of CaO. Hydration of the FBC ash can reactivate the partially sulfated sorbent, and the hydrated ash can be reinjected into the combustors as the SO2 sorbent. In this work, the rate of hydration, which is of primary importance in the reactivation process, is studied for FBC ash. The effects of major rate factors, temperature and particle size, are analyzed. At ambient temperature, the degree of hydration is well below 100% after 4 h of testing, suggesting the existence of a barrier to the complete conversion of CaO. The particle size effect on hydration rate appears to be complex, but can be interpreted in terms of the effects of heat and mass transfer. A model is proposed that gives a consistent description of the hydration behavior, and methods for enhancing the hydration are discussed. 1. Introduction About 65% of electrical power in North America is produced from coal combustion, and similar figures are seen worldwide.1 Fluidized bed combustion is a highly efficient technology that can utilize a variety of coals, with low emissions of pollutants. For removal of sulfur released from the coal, limestone is typically used as the sorbent in FBCs, and the related sulfur-capture process is described by two global reactions:
CaCO3 ) CaO + CO2
(1)
CaO + SO2 + 1/2O2 ) CaSO4
(2)
Unfortunately, in FBCs the sulfation reaction (reaction 2) does not go to completion and the degree of conversion of CaO to CaSO4 is generally below 45%.2-5 In consequence, an excess of limestone (the molar ratio, Ca/S > 2) is required to achieve SO2 capture of 90%. This not only adds to the overall cost of boiler operation, as substantial amounts of excess limestone must be transported, but also leads to increased CO2 emissions, as excess CO2 is produced as a result of reaction 1. Another problem related to the low degree of sulfation is the disposal of large amounts of the resultant ash. Disposal of coal ash from conventional combustors is already a growing problem because of concern over contamination of groundwater and surface waters and the decreasing availability of landfill sites.6 Additionally, FBC ash poses additional difficulties, in that the high content of CaO means that the ash reacts exothermically with water and must be conditioned before it goes to a landfill. Furthermore, the pH of the leachate from the landfill is very high and the leachate must be neutralized before discharge. Other problems include significant bulk expansion in the landfill, which can damage liners and increase the amount of leachate to be treated by several orders of magnitude.7-9 Ash treatment and disposal costs, therefore, add signifi* To whom correspondence should be addressed. E-mail:
[email protected].
cantly to the cost of operating FBC boilers burning a high-sulfur fuel. Reactivating FBC ash would help to reduce costs associated with both the use of excessive amounts of limestone and ash disposal, while mitigating the burden to the environment. Ash hydration appears to be a promising method.10-12 It is generally believed that, during the sulfation reaction in FBCs, a CaSO4 product layer forms on the surface of CaO particles, preventing penetration of SO2 to the unreacted CaO core. This results in incomplete sulfation.4,13,14 Hydration of the ash with water leads to the formation of Ca(OH)2 via
CaO + H2O ) Ca(OH)2
(3)
Because Ca(OH)2 has a molar volume larger than that of CaO, the CaSO4 layer would crack and expose fresh surface.13,15,16 When the hydrated ash is reinjected into the combustors, Ca(OH)2 dehydrates back to CaO, which can again capture SO2. There is also limited work to show that the degree of reutilization of the spent sorbent is proportional to the degree of hydration of the ash.17 Other reactions in the ash are also possible: CaO is known to react with Al2O3, SiO2, and Fe2O3 in the presence of water. However, such reactions would be less important in a short hydration period. Hydration of CaO has been studied for a very long time.18-20 However, hydration of partially sulfated FBC ashes is still insufficiently understood. In particular, the rate-controlling factors have not been adequately investigated. There is only limited information in the literature.21 Moreover, the hydration behavior has not been well described. Because hydration of FBC ashes is relatively slow, and can take hours to complete at ambient conditions,4,10 an analysis of the hydration rate is valuable for the development of effective hydration processes. In this work, we study the hydration behavior of partially sulfated ashes from an industrial FBC unit, focusing on the dependence of the hydration rate on temperature and particle size. A study of the effect of temperature and particle size on the hydration rate of similar ashes was carried out by Couturier et al.,21 but
10.1021/ie0507124 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005
8200
Ind. Eng. Chem. Res., Vol. 44, No. 22, 2005
Table 1. Composition of FBC Ashes SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO SO3 CO2 Na2O K2O BaO SrO V2O5 NiO MnO Cr2O3 LOFa sum a
bottom ash
fly ash
8.24 2.76 0.91 0.10 0.033 49.84 0.62 32.95 0.51
