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Response to Comment on “Calcium Precursors for the Production of CaO Sorbents for Multicycle CO2 Capture” Liu and Kong (1) made some interesting comments on our recent paper (2) regarding calcium precursors for the production of CaO sorbents for multicycle CO2 capture. The main comments could be summarized as follows: (i) There appears to be a linear relationship between specific surface area and Ca% and a linear relationship between pore volume and Ca%. Therefore the authors suggested selection of larger molecular-weight organometallic compound with low Ca% as the adsorbent precursor; (ii) Liu and Kong (1) also found that the carbonation conversion capabilities of the OGM-CaO adsorbents to be closely related with the Ca percentage in the organomettalic precursors. Therefore they concluded that it was not suitable to explain the carbonation reactivity of these OGM-CaO adsorbents in the same way as for the INORG-CaO adsorbents. We appreciate the comments and our comments are as follows: (i) The linear relationships between specific area, pore volume and Ca% were interesting observations that we have overlooked. However we would not use Ca% directly as an indicator or rule for the selection of OGM-based calcium precursors yet because of insufficient data and lack of fundamental support. Only five OGM-derived CaOs were tested in our work. These limited data do not justify the use of Ca% as a general rule yet because there are so many OGMs. Also in order to use Ca% as a performance indicator we need to show that there is a direct relationship between Ca% and the performance parameters. The performance parameters that we should use are reaction rate (or CaO conversion after a certain reaction time period) and its stability after cycles, not surface area or pore volume. This is because although surface area is normally closely related with the performance parameters, fundamentally surface area does not represent the true contact surface between CaO and CO2. Also pore volume affects the reaction rate in a very complex way (through diffusional effects) and there are many cases in which pore volume does not have a clear relationship with reaction rate. Actually if we look at Figure 2 in Liu and Kong (1) we can see the relationship between conversion and Ca% is not a very linear one. (ii) We disagree with the conclusion that “it was not suitable to explain the carbonation reactivity of these OGM-CaO adsorbents in the same way as for the INORG-CaO adsorbents”. Figure 2 in Liu and Kong (1) does not support this conclusion. “The value of R2 for the 4th cycle was nearly equal to that for the 9th cycle” does not support this conclusion either. In fact, if we agree that there is a linear relationship between conversion and Ca% in Figure 2, we could derive a conclusion that there is a linear relationship between conversion and surface area (because there is a linear relationship between surface area and Ca% in Figure 1). This would suggest that a reduction in surface area could result in a reduction in reaction rate or a reduction in reaction rate could be explained by the reduction in surface area, which is contradictory with the above conclusion. It is
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clear from Figure 2 that the conversion at the 9th cycle is lower than that at the 4th cycle. No matter what the sorbent is, OGM-CaO or INORG-CaO, the reaction between the sorbent and CO2 is a typical gas-solid reaction. The reduction in the reaction rate after cycles is, therefore, possibly due to two main reasons: chemical or physical, or combined. The material could have undergone a change (for example becoming more organized) resulting in a change in the chemical reaction rate constants. The reduction could also be due to the reduction of the surface available for CO2 contact as a result of crystal growth (physical change). Or it could be due to a combined effect. We did not have sufficient data in our original paper to justify any of these. Therefore we did not discuss it in great depth. However in the literature there are a lot of papers showing the evidence of physical change, as pointed out by Liu and Kong (1). But it is not clear whether such a physical change would cause a change in chemical property of the sorbent. An OGM-CaO is less compact in structure than an INORG-CaO because of a lower apparent density; therefore the speed of crystal growth (reduction of surface area) would be slower. However the reduction of reactivity of both could be due to exactly the same reasons. In summary, the comments by Liu and Kong (1) suggested a need for further work in the area, that is, whether Ca% can be used as a general selection rule for calcium precursors, and the mechanism of reactivity reduction of CaO after cycles. It is perhaps worth mentioning that in our recent work (3) we found CO2 sorbents with high reversibility could be manufactured using any of the OGMs discussed in Liu and Kong (1) as a precursor and the cyclic performance was found to be insensitive to the precursors used. The sorbents were manufactured using a simple method, that is, wet mixing of a calcium precursor and a support precursor followed by drying and calcination, and it was found that in the produced sorbents nanosized CaO particles were physically separated by the inert support material. Even though we still do not have sufficient evidence to conclude on the mechanism of reactivity reduction (could be chemical or physical or both), the results demonstrated that it is possible to solve this problem by physically limiting the growth of CaO crystals in the sorbent product.
Literature Cited (1) Liu, Y. S.; Kong, S. F. Comment on “Calcium precursors for the production of CaO sorbents for multicycle CO2 capture”. Environ. Sci. Technol. 2010, DOI: 10.1021/es1005384. (2) Liu, W. Q.; Low, N. W.; Feng, B.; Wang, G. X.; Diniz Da Costa, J. C. Calcium precursors for the production of CaO sorbents for multicycle CO2 capture. Environ. Sci. Technol. 2010, 44, 841–847. (3) Liu, W. Q.; Feng, B.; Wu, Y.; Wang, G.; Barry, J.; Diniz da Costa, S. F. Synthesis of sintering-resistant sorbents for CO2 capture. Environ. Sci. Technol. 2010. (DOI: 10.1021/es903436v).
Feng Bo* and Liu Wenqiang The University of Queensland, School of Mechanical and Mining Engineering ES100943G
10.1021/es100943g
2010 American Chemical Society
Published on Web 04/12/2010
