Reuse of partially sulphated CFBC ash as an SO2 Sorbent

Authored by: Yinghai Wu, Marianna Nobili, Antonio Telesca, Fabio Montagnaro, Lufei Jia and Edward J. Anthony

Corresponding Author:
Yinghai Wu

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The authors of this paper do a very good job of discussing the results of their experiments to reactivate bed and fly ash from a fluid bed combustor for reuse as a sorbent to remove SO3. They discuss the problems and limitations of the techniques they have tried (water, steam and doping with Na2CO3) and show the most practical method and the best expected sulphur removal efficiency. Their findings are important in designing a practical hydration system for reactivation of partially used fluid bed ash.

Keywords: FBC, CFBC ash, reactivation, hydration, sulphation

Summary:
Ashes produced from fluidized bed combustors (FBC) burning high-sulphur fuels often contain 20 – 30 % unreacted CaO because of the limestone added to remove SO2 in situ. This paper presents the results from experiments into reactivating partially sulphated FBC ash (both bed ash and fly ash) with liquid water, steam and sodium carbonate. The water- or steam-hydrated ashes were subsequently re-sulphated in a thermogravimetric analyzer (TGA) with simulated flue gas. The TGA results show that, while liquid water and steam successfully hydrate and reactivate the unreacted CaO in the bed ash, the treated ashes sulphated to widely different extents. Attempts to reactivate fly ash with hydration failed, although fly ash by itself is extremely reactive. A pilot-scale mini-circulating FBC (CFBC) was also used to evaluate the results of reactivation on the bed ash by hydrating with liquid water and admixtures of inorganic salt (Na2CO3) in the form of either powder or solution. When the treated ash was re-injected into the combustor with the fuel, the effect on SO2 removal efficiency was negligible if Na2CO3 was added as powder. Doping with aqueous solution resulted in enhanced SO2 removal; however, the extent was lower than the level achieved if only water hydration was employed. Increasing the amount of water (from 10 % to 30 %) to reactivate the ash did not improve the sulphur capture capacity in the mini-CFBC.  Overall, this study suggests that the most practical way for re-use of the partially sulphated bed ash as a sulphur sorbent is reactivation by water. A proposal for utilization of the fly ash in an economically reasonable way is also discussed.

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