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Energy & Fuels 2004, 18, 830-834
A Study on the Reactivation of Five Fly Ashes from Commercial Circulating Fluidized Bed (CFB) Boilers Y. Wu, J.-P. Charland, E. J. Anthony,* and L. Jia CANMET Energy Technology CentresOttawa, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada Received December 22, 2003
Five different fly ashes and their carbon-free derivatives from commercial-scale circulating fluidized-bed combustion (CFBC) boilers were hydrated with liquid water or steam, to determine whether hydration would improve sorbent utilization in the samples under fluidized-bed combustion conditions. After hydration by water, for one fly ash (FA1) and two carbon-free fly ashes (FA2-A and FA3-A), the capacity of the ashes for taking up SO2 showed limited improvement; however, hydration was not effective in reactivating the remaining samples. Generally, for these fly ashes, reactivation by hydration with either liquid water or steam seemed to be less promising than that for bed ashes, which have been shown to exhibit significant improvement in sulfur capture during resulfation. Hydration, whether by steam or liquid water, is not recommended for fly ash, which has a very limited residence time in the boiler, because of its small particle size. This paper recommends alternative strategies.
Introduction In fluidized-bed combustion (FBC) technology with limestone addition for sulfur capture, fly ash (FA) may often comprise the majority of the solid combustion products (60%-70%). It usually contains ∼20%-30% unreacted CaO, because of the low utilization ( FA2 (finest). Ashing. It has long been known, and has been confirmed in previous work,4 that CaSO4 can react with carbon, e.g., char in the fly ash.5 Therefore, a portion of each original fly ash was re-ashed, to eliminate this complication in subsequent reactivation tests. The re-ashing was performed in a furnace (Lindberg model LBF794C), in air, at 800 °C for 2 h. This treatment also removed any Ca(OH)2 that may have accidentally formed during ash handling and any CaCO3 that may have formed either during the passage of the fly ash through the back end of the boiler or during ash handling. In addition, some FA5 ash was also heat-treated in a thermo-
* Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Anthony, E. J.; Granatstein, D. L. Sulfation Phenomena in Fluidized Bed Combustion Systems. Prog. Energy Combust. Sci. 2001, 27, 215-236. (2) Wu, Y.; Anthony, E. J.; Jia, L. An Experimental Study on Hydration of Partially Sulphated FBC Ash. Combust. Sci. Technol. 2002, 174 (10), 171-181.
(3) Wu, Y.; Anthony, E. J.; Jia, L. Experimental Studies on Hydration of Partially Sulphated CFBC Ash. Can. J. Chem. Eng. 2003, 81, 1200-1214. (4) Wu, Y.; Anthony, E. J.; Jia, L. Steam Hydration of CFBC Ash and the Effect of Hydration Conditions on Reactivation. Fuel, 2004, 83 (10), 1357-1370. (5) Partington, J. R. A Textbook of Inorganic Chemistry; MacMillan: New York, 1951.
CaO + H2O f Ca(OH)2
(1)
CaSO4 + 2H2O f CaSO4‚2H2O
(2)
6Ca2+ + 12OH- + 3SO42- + 2Al3+ + 26H2O f Ca6[Al(OH)6]2(SO4)3‚26H2O (3)
10.1021/ef030192u CCC: $27.50 © 2004 American Chemical Society Published on Web 04/14/2004
Reactivation of Fly Ash from Commercial Boilers
Energy & Fuels, Vol. 18, No. 3, 2004 831
Table 1. Fly Ash Sources fly ash
boiler
fuel
weight fraction of particles
