ARTICLE pubs.acs.org/IECR
High Temperature Carbonation of Ca(OH)2 Vlatko Materic* and Stuart I. Smedley 69 Gracefield Road, 5040 Lower Hutt, New Zealand ABSTRACT: Steam hydration is reported to be an effective method for reactivating spent sorbents in calcium looping applications; however, uncertainties remain regarding the optimal method of returning the hydrated sorbent to the CO2 capture loop. Carbonation conversions were found to be higher when Ca(OH)2 was directly carbonated at high temperatures compared to conversions reached when Ca(OH)2 was dehydrated prior to carbonation. This observation can lead to improved hydration based reactivation techniques for calcium looping applications. Upon heating in CO2, calcium hydroxide remained stable at temperatures >450 °C and the extent of carbonation was controlled by temperature only. The carbonation mechanism of Ca(OH)2 at high temperatures appears to be more complex than the expected simple mechanism comprising the dehydration reaction of Ca(OH)2 and the subsequent carbonation of the resulting CaO. An alternate mechanism was proposed, involving the formation of liquid like layers of water on the surface of Ca(OH)2.
’ INTRODUCTION An objective of this work is to explore pathways for improving the efficacy of hydration based reactivation techniques for lime in calcium looping applications. This work focuses on the second operation of the reactivation technique, namely the return of the hydrated lime to the CO2 capture loop. The approach was to quantify the CO2 capture performance of the hydrated sorbent when heated in CO2 or N2. The reversible calcinationcarbonation reaction of CaCO3 forms the basis of the limelimestone thermochemical cycle, also referred to as calcium looping. The use of this thermochemical cycle to improve coal gasification was suggested as early as the XIX century.1 More recently, the cycle has been investigated for potential applications in energy storage2 and CO2 capture applications.35 Because of its inherent thermodynamic properties (high carbonation temperature, high CO2 carrying capacity), the limelimestone cycle is recognized to be potentially the lowest cost CO2 capture technology applicable at the required scales.6,7 One of the major limitations of the limelimestone cycle is the observed decay in the carbonation conversion of CaO at every CO2 capture, or carbonationcalcination, cycle24,8,9 due to sintering during every calcination step. A number of solutions for this problem have been proposed,1014 but this work focuses on hydration-based reactivation techniques. Lime was shown to be reactivated when the hydration was performed in humid air at ambient temperatures,15 in saturated steam at elevated temperatures,1618 or in water/ethanol mixtures.19 The reactivation process is performed after a specified number of carbonation calcination cycles and consists of two distinct operations. The first operation consists of hydrating the deactivated (spent) lime to Ca(OH)2, while the second operation consists of returning the freshly formed Ca(OH)2 to the CO2 capture loop to perform further capture cycles. The second operation is hereafter referred to as the Ca(OH)2 return operation. While a number of hydration techniques have been reported to be effective, there is very little discussion about the Ca(OH)2 return operation in the open literature. The hydration reaction is r 2011 American Chemical Society
usually performed at temperatures below 350 °C and the freshly formed calcium hydroxide must be heated to the carbonation temperature prior (or during) its reinjection into the CO2 capture loop. Previous work by the authors17 suggested that the gas in which the heating was performed (N2 or CO2) might make a considerable difference to the performance of the reactivated sorbent. When Ca(OH)2 is heated in N2 or in air, it dehydrates to CaO at about 400 °C, eq 1. The dehydrated CaO can then be returned to the CO2 capture loop and carbonated, eq 2, until the maximum carbonation is reached. When Ca(OH)2 is heated in CO2, it might be expected that slow, direct carbonation of Ca(OH)2, eq 3, would occur until the temperature reaches about 350400 °C. Whereupon Ca(OH)2 would rapidly dehydrate, eq 1, and CaO thus formed would continue reacting with CO2, eq 2, until the maximum carbonation is reached. CaðOHÞ2 f CaO þ H2 O
ð1Þ
CaO þ CO2 f CaCO3
ð2Þ
CaðOHÞ2 þ CO2 f CaCO3 þ H2 O
ð3Þ
It was previously reported20 that the presence of CO2 can greatly increase the temperature of the onset of dehydration. This observation suggests that Ca(OH)2 heated in CO2 might not react as expected and as described above, possibly explaining the difference in the performance of the reactivated sorbent.20 In this work, the importance of the gas composition during the Ca(OH)2 return operation is assessed, and the kinetics of the high temperature carbonation reaction of Ca(OH)2 are investigated. TGA profiles revealed that the rate and extent of carbonation were Received: February 22, 2011 Accepted: March 30, 2011 Revised: March 20, 2011 Published: March 30, 2011 5927
dx.doi.org/10.1021/ie200367w | Ind. Eng. Chem. Res. 2011, 50, 5927–5932
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Conditions for Each Step during Experiments heating rate
temperature
hold time
step
(°C/min)
(°C)
(min)
1
20
200
10
2
6
560
5
3
n/a
560
4
20
5
20
gas
function
N2
drying
N2 or CO2
heating
15
N2
drying
620
15
CO2
carbonation
850
15
N2
calcination
largely independent of time but dependent on temperature. A mechanism is proposed that can explain this observation.
’ METHODS Materials. Two calcium hydroxide materials were used in this study. High purity commercial hydroxide (>95% Ca(OH)2) was purchased from McDonald’s Lime, New Zealand. This material was prepared by water-based industrial slaking techniques and had experienced some carbonation (
