Environ. Sci. Technol. 2007, 41, 4435-4440
SO2 Retention by Reactivated CaO-Based Sorbent from Multiple CO2 Capture Cycles VASILIJE MANOVIC AND EDWARD J. ANTHONY* CANMET Energy Technology Centre-Ottawa, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1
This paper examines the reactivation of spent sorbent, produced from multiple CO2 capture cycles, for use in SO2 capture. CaO-based sorbent samples were obtained from Kelly Rock limestone using three particle size ranges, each containing different impurities levels. Using a thermogravimetric analyzer (TGA), the sulfation behavior of partially sulfated and unsulfated samples obtained after multiple calcination-carbonation cycles in a tube furnace (TF), following steam reactivation in a pressurized reactor, is examined. In addition, samples calcined/sintered under different conditions after hydration are also examined. The results show that suitably treated spent sorbent has better sulfation characteristics than that of the original sorbent. Thus for example, after 2 h sulfation, >80% of the CaO was sulfated. In addition, the sorbent showed significant activity even after 4 h when >95% CaO was sulfated. The results were confirmed by X-ray diffraction (XRD) analysis, which showed that, by the end of the sulfation process, samples contained CaSO4 with only traces of unreacted CaO. The superior behavior of spent reactivated sorbent appears to be due to swelling of the sorbent particles during steam hydration. This enables the development of a more suitable pore surface area and pore volume distribution for sulfation, and this has been confirmed by N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) method. The surface area morphology of sorbent after reactivation was examined by scanning electron microscopy (SEM). Ca(OH)2 crystals were seen, which displayed their regular shape, and their elemental composition was confirmed by energydispersive X-ray (EDX) analysis. The improved characteristics of spent reactivated sorbent in comparison to the original and to the sorbent calcined under different conditions and hydrated indicate the beneficial effect of CO2 cycles on sorbent reactivation and subsequent sulfation. These results allow us to propose a new process for the use of CaObased sorbent in fluidized bed combustion (FBC) systems, which incorporates CO2 capture, sorbent reactivation, and SO2 retention.
Introduction Calcium-based sorbents (limestone and dolomite) are typically used for SO2 capture in fluidized bed combustion (FBC) * Corresponding author phone: (613)996-2868; fax: (613)992-9335; e-mail:
[email protected]. 10.1021/es0629458 CCC: $37.00 Published on Web 05/16/2007
2007 American Chemical Society
systems (1). The global reaction scheme involves calcination (1) and sulfation (2):
CaCO3 f CaO + CO2
(1)
CaO + SO2 + 1/2O2 f CaSO4
(2)
Sulfation is a heterogeneous solid-gas reaction with the formation product (CaSO4) produced at the reacting surface. The product layer obstructs and restricts contact of CaO and SO2, and more importantly, fills and blocks sorbent pores as the molar volume of CaCO3, CaO, and CaSO4 are, respectively, 37, 17, and 46 cm3/mol. As a result, utilization of CaO is relatively low, usually
