Ind. Eng. Chem. Res. 2009, 48, 8431–8440
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CO2 Capture from Simulated Syngas via Cyclic Carbonation/Calcination for a Naturally Occurring Limestone: Pilot-Plant Testing Robert T. Symonds,† Dennis Y. Lu,*,† Robin W. Hughes,† Edward J. Anthony,† and Arturo Macchi‡ CANMET Energy Technology CentresOttawa, Natural Resources Canada, 1 Haanel DriVe, Ottawa, Canada K1A 1M1, and Department of Chemical and Biological Engineering, UniVersity of Ottawa, 161 Louis Pasteur Street, Ottawa, Canada K1N 6N5
Experiments were performed using a dual fluidized bed reactor system, operated in a batch mode, in order to investigate the effects of steam and simulated syngas on CO2 capture and sorbent conversion efficiency for a naturally occurring Polish calcitic limestone. In addition, the effect of high partial pressures of CO2 on the calcination process was examined using either oxygen-enriched air or oxy-fuel combustion in the calciner. As expected, calcination under oxy-fuel conditions resulted in decreased carbonation conversion due primarily to particle sintering and pore pluggage. On average there was a decrease in carbonation conversion of approximately 36.5 and 33.4% for carbonation with steam and steam/simulated syngas, respectively, compared to similar experiments using oxygen-enriched air. However, during the carbonation of the limestone with steam present in the feed gas, it was observed that the high CO2 capture efficiency period was significantly extended compared to carbonation with only CO2 present. This resulted in increased CaO conversion from approximately 16.1 to 29.7% for the initial carbonation cycle. A further increase in carbonation conversion, from 29.7 to 46.9%, was also observed when simulated syngas conditions (CO, H2) were used in the carbonator. Analysis of the outlet gases also confirmed that the calcined limestone catalyzes the water gas shift reaction, which we believe results in enhanced CO2 concentration levels at the grain surfaces of the sorbent. 1. Introduction The use of solids for CO2 capture has recently been the subject of significant research,1 as it possesses many potential advantages over other capture technologies, including reduced energy penalties, avoidance of liquid wastes, and the relatively inert nature of the solid wastes.2 The reversible reaction between CaO and CO2 can be used for CO2 removal from combustion or gasification systems at elevated temperatures ranging between 500 and 800 °C.3 Specifically, in the case of CO2 removal from syngas, naturally occurring calcitic-based sorbents can simultaneously remove CO2 and enhance the production of H2 via the water gas shift reaction (reaction 1).4 CO(g) + H2O(g) T H2(g) + CO2(g)
(1)
limestone, which was able to enhance the shift reaction over a temperature range of 580-700 °C at atmospheric pressure.6 Oxy-fuel combustion in a circulating fluidized bed (CFB) is one of the technologies that can be used to operate the calciner, and is already being explored to control CO2 emissions from fossil-fuel-fired power plants.7,8 In oxy-fuel combustion, the fuel is burned in oxygen with recirculated flue gas instead of air. Oxy-fuel combustion can achieve high CO2 levels (>90 vol %), allowing economic compression and piping of CO2 for sequestration in a suitable geological site.9 To date, work on simultaneous CO2 capture and H2 production via the use of naturally occurring calcitic-based sorbents has been limited to laboratory-scale thermogravimetric analyzers (TGAs) and bench-scale reactors such as tube reactors. This paper reports findings on CO2 capture performance under
Since the water gas shift reaction is limited by thermodynamic equilibrium, sorption of CO2, which lowers its partial pressure, moves the equilibrium toward the product side, as described by the following overall reaction (reaction 2). CO(g) + H2O(g) + CaO(s) T CaCO3(s) + H2(g)
(2)
Figure 1 gives a simplified schematic for such a process. In a study by Harrison et al.,5 a coal-derived syngas was “shifted” and the CO2 was captured by calcined dolomite. The reactor was operated at 5.0 MPa and 550 °C, and no shift catalyst was used, which indicates that MgO and possibly CaO can catalyze the water gas shift reaction. Experimental results also showed that the reaction equilibrium could be closely approached. We have recently confirmed this using a calcitic * To whom correspondence should be addressed. Fax: (613) 9929335. E-mail:
[email protected]. † Natural Resources Canada. ‡ University of Ottawa.
Figure 1. Simplified schematic for hydrogen production from syngas with CO2 capture.
10.1021/ie900645x CCC: $40.75 2009 American Chemical Society Published on Web 08/04/2009
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Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009
Figure 2. Schematic of the CANMET pilot-scale dual fluidized bed sorbent looping facility. Table 1. Particle Size Distribution and XRD Chemical Analysis of Polish Limestone (wt %) mean aperture size (µm) 1205
1410
S/V mean diametera (µm)
0.0186
0.0003
0.0002
512
P2O5
CaO
MgO
SO3
Na2O
K2O
LOF
