Energy & Fuels 2005, 19, 1927-1934
1927
SO2 Removal and CO2 Capture by Limestone Resulting from Calcination/Sulfation/Carbonation Cycles Yan Li, Steve Buchi, John R. Grace,* and C. Jim Lim Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, Canada V6T 1Z4 Received January 9, 2005. Revised Manuscript Received June 22, 2005
Experiments were conducted in a dual-environment thermogravimetric reactor to investigate the interaction between calcination, sulfation, and carbonation for a limestone that had previously been shown to sulfate primarily in an unreacted core manner. The results indicate that carbon dioxide (CO2) can reactivate partially sulfated sorbent particles, contributing to an increase in overall calcium utilization efficiency. The ability of sorbents to recapture CO2 decreases when cycles of calcination and carbonation are performed, following a pattern similar to sulfation/ hydration cycling.
Introduction and Background Sulfur dioxide (SO2) and carbon dioxide (CO2) emissions are a major concern in combustion processes; the former causes acid rain and the latter has been implicated in global climate change. CO2 from the combustion of fossil fuels is the major contributor to the buildup of greenhouse gases in the atmosphere. At the same time, fuels contain sulfur in varying proportions, and SO2 pollution originates from oxidation of this sulfur during combustion. Calcium sorbents (primarily limestones) are widely used for in situ SO2 capture in fluidized-bed combustion. The key reactions are commonly described by
Calcination: Sulfation:
CaCO3 f CaO + CO2
1 CaO + SO2 + O2 f CaSO4 2
(1) (2)
Direct Sulfation: 1 CaCO3 + SO2 + O2 f CaSO4 + CO2 (3) 2 CO2 is present when sulfur is being captured, both as a result of reaction1 and as a major product of the combustion itself. The calcination reaction augments CO2 emissions associated with combustion when limestone is used as a sorbent for sulfur. Calcium utilization when limestone is used as a sorbent for sulfur capture is limited, because of blockage of the outer pores of calcined sorbent by calcium sulfate. If the utilization efficiency could be improved, not only would solids handling and disposal be reduced, but less fresh limestone would need to be calcined, thereby reducing the overall CO2 emissions and contributing to a reduction * Author to whom correspondence should be addressed. Telephone: 1-604-822-3121. Fax: 1-604-822-6003. E-mail address:
[email protected]. (1) Li, Y.; Nishioka, M.; Sadakata, M. High calcium utilization and gypsum formation for dry desulfurization process. Energy Fuels 1999, 13, 1015-1020.
in overall greenhouse gas emissions, as well as diminishing acid gas emissions. The calcium utilization efficiency is significantly influenced by such limestone particle characteristics as surface area, pore size, and particle size distribution. The total surface area (internal and external) of CaO particles produced from CaCO3 calcination is generally
