Ind. Eng. Chem. Res. 2003, 42, 3245-3249
3245
Effect of CaSO4 on the Strength Development of Sorbents in CFBC Boilers Firing High-Sulfur Fuels Lufei Jia and Edward J. Anthony* CETC, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1
Karen Laursen Kyoto University, Department of Chemical Engineering, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Agglomeration due to the sulfation process itself occurs in CFBC boilers firing high-sulfur fuels such as petroleum coke. This study examines the effect of CaSO4, the effect of limestone reactivity, and the potential effect of reactivation on strength development in such deposits. A very unreactive crystalline limestone showed a lower sulfation level over longer times (days to weeks), but the extent of sulfation did not correlate to strength development. This work also shows that CaSO4 by itself cannot produce significant strength development in the ash samples. Finally, reactivation by hydration did not contribute to strength development under the conditions of this study. Introduction Limestone is used in circulating fluidized bed combustion (CFBC) boilers to reduce SO2 emissions from high-sulfur fuels such as petroleum coke and some coals. At atmospheric conditions, the limestone first calcines and then sulfates in a process that can be described by a two-step global reaction scheme:
CaCO3 f CaO + CO2
(1)
CaO + SO2 + 1/2O2 f CaSO4
(2)
The sulfation (conversion) level of spent sorbent leaving CFBCs is typically limited to 30-50%. Incomplete conversion occurs because CaSO4 has a greater molar volume than either CaCO3 or CaO, resulting in the formation of a dense sulfate shell that blocks the interior of the particle, leaving a significant amount of CaO in the sorbent-derived particles unconverted.1 Recent work has clearly shown that this picture does not fully describe the sulfation behavior of limestones, which are capable of sulfating in at least three different modes: via the core/shell pattern as noted above; via a network of cracks and macropores in the sorbent; and via homogeneous sulfation.2 Moreover, most limestones will show some combination of these patterns, although the core/shell arrangement seems the most common for larger sized particles (probably >0.1 mm). However, universal FBC boiler experience is that sorbent conversions are well below quantitative levels.1 Anthony and co-workers3-5 have identified sulfation as a cause of fouling in boilers firing high-sulfur fuels (normally petroleum coke). Specifically, they have found monolithic deposits, often with characteristic lengths of meters, in the upper part of the furnace, in the cyclone, and on the superheaters. These deposits have been shown to consist of almost pure CaSO4 (i.e., 80-95%) or mixtures of CaCO3 and CaSO4 in cooler regions of * To whom correspondence should be addressed. Phone: 613-996-2868.Fax: 613-992-9335.E-mail:
[email protected].
CFBC boilers (
