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Title: CO{sub 2} capture capacity of CaO in long series of carbonation/calcination cycles

Abstract

Calcium oxide can be an effective sorbent to separate CO{sub 2} at high temperatures. When coupled with a calcination step to produce pure CO{sub 2}, the carbonation reaction is the basis for several high-temperature CO{sub 2} capture systems. The evolution with cycling of the capture capacity of CaO derived from natural limestones is experimentally investigated in this work. Long series of carbonation/calcination cycles (up to 500) varying different variables affecting sorbent capacity have been tested in a thermogravimetric apparatus. Calcination temperatures above T > 950{sup o}C and very long calcination times accelerate the decay in sorption capacity, while other variables have a comparatively modest effect on the overall sorbent performance. A residual conversion of about 7-8% that remains constant after many hundreds of cycles and that seems insensitive to process conditions has been found. This residual conversion makes very attractive the carbonation/calcination cycle, by reducing (or even eliminating) sorbent purge rates in the system. A semiempirical equation has been proposed to describe sorbent conversion with the number of cycles based on these new long data series.

Authors:
;  [1]
  1. CSIC, Zaragoza (Spain)
Publication Date:
OSTI Identifier:
20847714
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research; Journal Volume: 45; Journal Issue: 26
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; CARBON DIOXIDE; CAPTURE; CALCIUM OXIDES; CALCINATION; LIMESTONE; BENCH-SCALE EXPERIMENTS; ADSORBENTS; EQUATIONS

Citation Formats

Grasa, G.S., and Abanades, J.C. CO{sub 2} capture capacity of CaO in long series of carbonation/calcination cycles. United States: N. p., 2006. Web. doi:10.1021/ie0606946.
Grasa, G.S., & Abanades, J.C. CO{sub 2} capture capacity of CaO in long series of carbonation/calcination cycles. United States. doi:10.1021/ie0606946.
Grasa, G.S., and Abanades, J.C. Wed . "CO{sub 2} capture capacity of CaO in long series of carbonation/calcination cycles". United States. doi:10.1021/ie0606946.
@article{osti_20847714,
title = {CO{sub 2} capture capacity of CaO in long series of carbonation/calcination cycles},
author = {Grasa, G.S. and Abanades, J.C.},
abstractNote = {Calcium oxide can be an effective sorbent to separate CO{sub 2} at high temperatures. When coupled with a calcination step to produce pure CO{sub 2}, the carbonation reaction is the basis for several high-temperature CO{sub 2} capture systems. The evolution with cycling of the capture capacity of CaO derived from natural limestones is experimentally investigated in this work. Long series of carbonation/calcination cycles (up to 500) varying different variables affecting sorbent capacity have been tested in a thermogravimetric apparatus. Calcination temperatures above T > 950{sup o}C and very long calcination times accelerate the decay in sorption capacity, while other variables have a comparatively modest effect on the overall sorbent performance. A residual conversion of about 7-8% that remains constant after many hundreds of cycles and that seems insensitive to process conditions has been found. This residual conversion makes very attractive the carbonation/calcination cycle, by reducing (or even eliminating) sorbent purge rates in the system. A semiempirical equation has been proposed to describe sorbent conversion with the number of cycles based on these new long data series.},
doi = {10.1021/ie0606946},
journal = {Industrial and Engineering Chemistry Research},
number = 26,
volume = 45,
place = {United States},
year = {Wed Dec 20 00:00:00 EST 2006},
month = {Wed Dec 20 00:00:00 EST 2006}
}
  • A series of carbonation/calcination tests consisting of 1000 cycles was performed with CaO-based pellets prepared using hydrated lime and calcium aluminate cement. The change in CO{sub 2} carrying capacity of the sorbent was investigated in a thermogravimetric analyzer (TGA) apparatus and the morphology of residues after those cycles in the TGA was examined by scanning electron microscopy (SEM). Larger quantities of sorbent pellets underwent 300 carbonation/calcination cycles in a tube furnace (TF), and their properties were examined by nitrogen physisorption tests (BET and BJH). The crushing strength of the pellets before and after the CO{sub 2} cycles was determined bymore » means of a custom-made strength testing apparatus. The results showed high CO{sub 2} carrying capacity in long series of cycles with an extremely high residual activity of the order of 28%. This superior performance is a result of favorable morphology due to the existence of large numbers of nanosized pores suitable for carbonation. This morphology is relatively stable during cycles due to the presence of mayenite (Ca{sub 12}Al{sub 14}O{sub 33}) in the CaO structure. However, the crushing tests showed that pellets lost strength after 300 carbonation/calcination cycles, and this appears to be due to the cracks formed in the pellets. This effect was not observed in smaller particles suitable for use in fluidized bed (FBC) systems.« less
  • CaO is being proposed as a regenerable sorbent of CO{sub 2} via a carbonation/calcination loop. It is well known that natural sorbents lose their capacity to capture CO{sub 2} with the number of cycles due to textural degradation. In coal combustion systems, reaction with the SO{sub 2} present in flue gases also causes sorbent deactivation. This work investigates the effect of partial sorbent sulfation on the amount of CaO used in systems where both carbonation and sulfation reactions are competing. We have found that SO{sub 2} reacts with the deactivated CaO resulting from repetitive calcination/carbonation reactions. Therefore, the deactivation ofmore » CaO as a result of the presence of SO{sub 2} is lower than one would expect if one assumes that SO{sub 2} reacts only with active CaO. This work shows that changes in the texture of the sorbent due to repetitive carbonation/calcination cycles tend to increase the sulfation capacity of the sorbents tested. This suggests that the purge of deactivated CaO obtained from a CO{sub 2} capture loop could be a more effective sorbent of SO{sub 2} than fresh CaO.« less
  • 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 (CO{sub 2}) can reactivate partially sulfated sorbent particles, contributing to an increase in overall calcium utilization efficiency. The ability of sorbents to recapture CO{sub 2} decreases when cycles of calcination and carbonation are performed, following a pattern similar to sulfation/hydration cycling. 16 refs., 10 figs., 3 tabs.
  • Process analysis of CO{sub 2} capture from flue gas using Ca-based carbonation/calcination cycles is presented here. A carbonation/calcination system is composed essentially of two reactors (an absorber and a regenerator) with Ca-based sorbent circulating between the two reactors (assumed here as fluidized beds). CO{sub 2} is, therefore, transferred from the absorber to the regenerator. Because of the endothermicity of the calcination reaction, a certain amount of coal is burned with pure oxygen in the regenerator. Detailed mass balance, heat balance and cost of electricity and CO{sub 2} mitigation for the carbonation/calcination cycles with three Ca-based sorbents in dual fluidized bedsmore » were calculated and analyzed to study the effect of the Ca-based sorbent activity decay on CO{sub 2} capture from flue gas. The three sorbents considered were: limestone, dolomite and CaO/Ca{sub 12}Al{sub 14}O{sub 33} (75/25 wt %) sorbent. All results, including the amount of coal and oxygen required, are presented with respect to the difference in calcium oxide conversion between the absorber and the regenerator, which is an important design parameter. Finally, costs of electricity and CO{sub 2} mitigation costs using carbonation/calcination cycles for the three sorbents were estimated. The results indicate that the economics of the carbonation/calcination process compare favorably with competing technologies for capturing CO{sub 2}.« less
  • Capturing carbon dioxide is vital for the future of climate-friendly combustion, gasification, and steam-re-forming processes. Dry processes utilizing simple sorbents have great potential in this regard. Long-term calcination/carbonation cycling was carried out in an atmospheric-pressure thermogravimetric reactor. Although dolomite gave better capture than limestone for a limited number of cycles, the advantage declined over many cycles. Under some circumstances, decreasing the carbonation temperature increased the rate of reaction because of the interaction between equilibrium and kinetic factors. Limestone and dolomite, after being pretreated thermally at high temperatures (1000 or 1100{sup o}C), showed a substantial increase in calcium utilization over manymore » calcination/carbonation cycles. Lengthening the pretreatment interval resulted in greater improvement. However, attrition was significantly greater for the pretreated sorbents. Greatly extending the duration of carbonation during one cycle was found to be capable of restoring the CO{sub 2} capture ability of sorbents to their original behavior, offering a possible means of countering the long-term degradation of calcium sorbents for dry capture of carbon dioxide. 12 refs., 12 figs., 2 tabs.« less