Role of looping-calcination conditions on self-reactivation of thermally pretreated CO2 sorbents based on CaO

Role of looping-calcination conditions on self-reactivation of thermally pretreated CO2 sorbents based on CaO Jose M. Valverde,∗,†Pedro E. Sanchez-Jimenez,‡Antonio Perejon,‡and Luis A. Perez-Maqueda‡ Faculty of Physics, Avenida Reina Mercedes s/n, 41012 Sevilla, and Instituto de Ciencia de Materiales de Sevilla, Americo Vespucio 49, 41092 Sevilla, Spain E-mail: [email protected] Phone: +34 95 4550960. Fax: +34 954239434 Abstract The conversion of thermally pretreated CaO along successive carbonation/calcination cycles has been investigated, as aected by looping-calcination conditions, by means of Thermogravimetric Analysis (TGA). Sorbent samples have been subjected in-situ to a thermal preheating program based on Constant Rate Thermal Analysis (CRTA) by virtue of which decarbonation is carried out at a low controlled rate, which is able to promote self-reactivation in the rst carbonation/calcination cycles. Our observations support a pore-skeleton model according to which solid-state diusion in the rst carbonation stages, which is enhanced by thermal pretreatment, gives rise to a soft skeleton with increased surface area. Yet, the results show that self-reactivation ∗ To whom correspondence should be addressed † University of Seville ‡ CSIC-University of Seville 1 is hindered as looping-calcination conditions are harshened. Increasing the loopingcalcination temperature and/or the looping calcination time period favors sintering of the soft skeleton and eventually self-reactivation is precluded. A model is developed that retrieves the main features of multicyclic conversion of thermally pretreated sorbents in the rst cycles based on the balance between surface area gain due to promoted solid-state diusion carbonation and surface area loss due to sintering of the soft skeleton in the looping-calcination stage, which can be useful to investigate the critical looping-calcination conditions that nullify self-reactivation. The proposed model allows envisaging the behavior of the sorbent performance as a function of the pretreatment conditions. Introduction The Ca-looping (CaL) process, based on the carbonation reaction of CaO to capture CO 2 and the subsequent decarbonation of CaCO 3 to regenerate the sorbent, is at the basis of a promising postcombustion capture technology whose suitability has been demonstrated by sustained CO 2 capture eciencies over 90% in large pilot-scale plants. 13 In practice, carbonation/decarbonation of CaO is carried out in two interconnected uidized bed reactors through which the material is continuously circulated. In the carbonator, CaO particles become carbonated at contact with the postcombustion gas containing CO 2 in a vol% of around 15%. The carbonated particles are driven to the calciner where they are decomposed by calcination at high temperature, which produces a concentrated stream of CO 2 suitable to be compressed and transported for sequestration. By taking into account the tradeo between the reaction equilibrium driving force and the reaction kinetics, carbonation is carried out at optimal temperatures of around 650 ◦ C. On the other hand, decarbonation in the CO 2 rich atmosphere of the calciner requires application of temperatures above 900 ◦ C, which are accomplished by burning coal with a stream of pure O 2 (oxyred combustion). 1,4 Thermogravimetric analysis (TGA) studies show that carbonation of CaO solid particles 2 progresses through two well dierentiated phases. 5,6 A rst kinetically-controlled reaction takes place quickly on the surface of the particles until a thin layer of CaCO 3 (between 30 and 50 nm thick 6 ) is developed. Then, CO 2 sorption becomes controlled by solid-state diusion of CO 2 through this carbonate layer and the reaction rate is slowed down. In practical applications most of the CO 2 sorption is restricted to the fast carbonation phase since the sorbent particles must react with CO 2 at low concentrations and over short contact times. 7 An abundant and inexpensive material to be used as CaO precursor is natural limestone. 710 Typically, CaO is derived from limestone by preheating it up to a temperature high enough to ensure that decarbonation is complete but quickly in order to avoid the sintering of CaO grains after decomposition. Yet, nascent CaO derived from a quick CaCO 3 decarbonation is prone to be signicantly sintered, which decreases the surface area available for fast carbonation causing a drastic drop of CaO conversion in the rst carbonation/calcination cycles. Even though the loss-in-conversion problem may be circumvented by high solid circulation rates, 7 it is recognized that enhancing the regenerability of CaO would favor the eciency of the Ca-looping technology. 9 A proposed method to stabilize multicyclic CaO conversion consists of thermal pretreatment of natural limestones. 1117 By exposing the sorbent to isothermal calcination at high temperature for a prolonged period of time, the sorbent activity in the rst carbonation/calcination cycles may actually increase, a phenomenon referred to as self-reactivation. 12 Conversion reaches a maximum value at a certain number of cycles (usually N < 20 ) after which it decreases with the cycle number at a slow rate. An added benet of thermal pretreatment is the increase of mechanical strength of the sorbent, 18 which enhances its resistance to attrition and thus minimizes the loss of material due to elutriation at high gas velocities in the CO 2 postcombustion capture process. Self-reactivation of thermally pretreated sorbents has been explained by the formation during pretreatment of a hard and stable skeleton in which solid-state diusion carbonation is enhanced in the rst cycle. A quick decarbonation following an enhanced solid-state diusive carbonation stage leads to 3 a renovated porous skeleton with increased surface area for the next carbonation. 9,19 In agreement with this mechanism, it has been reported that self-reactivation is signicantly enhanced if the sorbent is subjected to carbonation periods of prolonged duration and under atmospheres of high CO 2 vol%, which promote solid-state diusion. 13 Self-reactivation has been analyzed as aected by a wide range of experimental variables such as temperature and duration of the pretreatment, presence of additives/impurities in the sorbent skeleton, pre-grinding, pre-hydration and looping-carbonation conditions. 1214 Alonso et al. 20 have recently revisited the suitability of thermal pretreatment as a method to stabilize the conversion of limestones used in pilot-scale plants for CO 2 capture. In contrast with results reported by other authors, these limestones failed to exhibit self-reactivation after thermal pretreatment despite that the concentration of impurities (possibly hampering self-reactivation according to other works 13,14 ) was low and pretreatment conditions were suciently harsh to induce deep sintering for the development of a stable skeleton. 20 As suggested by Alonso et al., 20 a possible reason for this discrepancy was the use of relatively short carbonation periods in their experiments (10 min), which would hamper carbonation in the solid-state diusion phase. A further relevant issue that will be closely examined in the present manuscript is the conditions under which looping-calcination is carried out. Looping-calcination conditions used in TGA tests on pretreated sorbents are similar in the dierent reported studies: around 10 min under 100%N 2 at 800-850 ◦ C in 1214,16 and 30 min under 100%N 2 at 750 ◦ C in. 11,17 Likely, increasing the looping-calcination temperature might compromise self-reactivation since it would enhance sintering of the nascent CaO soft skeleton. Looping-calcination conditions to be expected in practice would be harsh due to the presence of CO 2 in the calciner, which requires to increase the calcination temperature above 900 ◦ C for ecient decarbonation to take place. 1 In this regard, looping-calcination conditions employed by Alonso et al. 20 (10 min at 950 ◦ C in a 10% CO 2 /90% air vol/vol atmosphere) are more realistic than those employed in previous works showing self-reactivation. High loopingcalcination temperatures and the presence of CO 2 would enhance the sintering of the porous 4 soft skeleton developed after solid-state diusion as might be inferred from experimental measurements on the BET surface area of nascent CaO subjected to high temperatures in CO 2 enriched atmospheres. 2123 To further investigate this argument, we analyze in the present manuscript the eect of varying looping-calcination conditions (temperature and time duration) on the multicyclic conversion performance of CaO derived from sorbents pretreated in-situ under a controlled preheating program. Materials and methods In our experiments we have used a Q5000IR TG analyzer (TA Instruments) provided with an infrared furnace heated with halogen lamps, which allows for a very fast change of temperature between cycles (up to 500 ◦ C/min for linear heating range), and with a high sensitivity balance ( < 0.1 µ g) characterized by a minimum baseline dynamic drift ( < 10 µ g). Ca(OH) 2 from Sigma-Aldrich (puriss. p.a.) has been used as CaO precursor free of impurities. After placing the sorbent (around 10 mg) in the balance, it is preheated in-situ up to 900 ◦ C by means of a controlled program in a dry air/CO 2 atmosphere (15% CO 2 vol). Benchmark conditions of subsequent carbonation/calcination cycles consist of carbonation at 650 ◦ C (85% dry air/15% CO 2 vol/vol) and calcination at 850 ◦ C (dry air), both stages for 5 minutes (a shorter period that those used in TGA analysis reported in previous works on thermal pretreatment usually above 10 min 1117,20 ). Further tests were performed varying the looping-calcination temperature between 700 ◦ C and 950 ◦ C and the calcination time period between 1 min and 15 min. The conventional preheating program consisted of a linear preheating program (20 ◦ /min). A novel preheating program applied (as regards the CaL process) was based on a technique known as Constant Rate Thermal Analysis (CRTA), which is an useful technique to derive materials with controlled texture and microstructure. 2433 During the CRTA pretreatment, the evolution of temperature is controlled by means of a feedback mechanism allowing the 5 reactions to occur at a small and constant prexed rate selected by the user. By virtue of this program, similar reaction rates for each grain of the sample are achieved, which minimizes the inuence of mass and heat transfer phenomena typically associated to thermal decomposition reactions. A relevant issue concerning TGA tests regards the heating/cooling rates to pass from carbonation to calcination and viceversa. The use of an infrared halogen furnace in our TGA runs allowed us for heating/cooling the sample very quickly (300 ◦ C min −1 ) minimizing the duration of the transitional periods. An alternative technical approach used by Alonso et al. 20 is to employ a two zones furnace set to dierent temperatures that can be moved up and down by means of a pneumatic piston with respect to the balance. Otherwise, heating rates typically employed in conventional TGA instruments (of about 50 ◦ C min −1 or even smaller 34 ) lead to excessively long transitional periods, which might have a nonnegligible inuence on the multicyclic conversion data. For example, in ref. 15 TGA tests are carried out between 850 ◦ C (calcination in N 2 ) and 650 ◦ C (carbonation in 15%CO2/85%N 2 ) but the heating/cooling rate (20 ◦ C/min) would lead to 10 min transitional periods. This problem has been usually circumvented in previous studies 1114,16,17,35 by testing the multicyclic capture of the pretreated sorbent when subjected to carbonation/calcination cycles isothermally (the temperature during cycling is xed to a value in the range 750-850 ◦ C) and changing the gas between an inert gas for calcination and a CO 2 /air(N 2 ) mixture with a high CO 2 vol% (above 25%) for carbonation. Consequently, the pretreated sorbent is subjected to relatively low looping-calcination temperatures, which would minimize sintering of the soft skeleton, whereas looping-carbonation is carried out at a high temperature in a high CO 2 vol% atmosphere favoring solid-state diusion. 6 Experimental results Figure 1 shows two examples of thermograms illustrating in detail the evolution of temperature, sample mass and sample mass derivative as a function of time during the CRTA preheating program. Firstly, the temperature is linearly increased (20 ◦ /min), which leads to parallel dehydroxilation/carbonation at a quick rate when the temperature reaches a value around 400 ◦ C. Subsequently, decarbonation, which is initiated at a temperature of about 810 ◦ C, is controlled to take place at the small prexed rate (mass variation rate of ∼ 0.4 % /min). In order to maintain the rate of decarbonation constant, the temperature is kept approximately constant at a value slightly above 800 ◦ C for about 2 h. After preheating the sorbent under this program, the resulting skeleton has suered appreciable sintering due to prolonged slow decarbonation in the CO 2 enriched atmosphere as can be inferred from BET surface area measurements and SEM analysis (Fig. 2) of samples treated in a microwave oven by replicating the preheating temperature program used in the TGA tests. In contrast, the linear preheating program, wherein decarbonation takes place at a fast rate near the end of the preheating period, yields a relatively porous skeleton with a higher surface area (see Fig. 2) but that would be prone to be appreciably sintered in the looping-calcination stage of the subsequent cycles. The thermograms plotted in Fig. 1 correspond to samples subjected in-situ to the same CRTA preheating program and cycled after at dierent looping-calcination temperatures. Note the reproducibility of the mass evolution during preheating in dierent experiments, which allows to accurately control the initial state of the sample prior to the carbonation/calcination cycles. As seen in Fig. 1, the evolution of sample mass along the cycles indicates the occurrence in our experiments of notable self-reactivation when looping-calcination is carried out at 700 ◦ C. Yet, self-reactivation is precluded for a looping-calcination temperature of 950 ◦ C. This remarkable feature will be analyzed in detail in our work. Figure 3 shows data on the multicyclic CaO conversion XN of linearly and CRTA preheated samples subjected to the same looping carbonation/calcination conditions (carbona7 tion at 650 ◦ C in 85% air/15% CO 2 vol/vol and calcination in air at 850 ◦ C, both for 5 min). As can be seen, CaO conversion in the rst cycle X0 is notably decreased for the CRTApreheated sorbent as might have been expected from its relatively small BET surface area (Fig. 2). Conversion as a function of time during the 1st carbonation/calcination cycle for both sorbents is shown in Fig. 4a. It is seen that the CRTA thermal pretreatment promotes carbonation in the diusive phase, which yields a contribution to conversion in this phase XD1 roughly equal to conversion in the fast phase XK1 while it is XD1<< XK1 for the nonpretreated sorbent. A rapid decarbonation after the promoted solid-state diusion would thus yield a porous skeleton with a relatively higher surface area leading to an increase of conversion in the 2nd cycle as seen in Fig. 3. Nonetheless, self-reactivation under the loopingcalcination conditions applied in this case (temperature Ts0= 850◦ C, and time period ts0 = 5 min) does not extend beyond the 3rd cycle. Data obtained on the ratio of conversion in the diusive carbonation phase to conversion in the fast phase ( rN=XDN /XKN ) are plotted in Fig. 4b. This plot shows that self-reactivation in the rst cycles occurs in parallel to a decrease of rN with the cycle number. As a main dierence with the CRTA preheated sorbent, the role of diusive carbonation for the linearly preheated sorbent is not relevant in the rst cycle ( rN≃0.2 ). In this case, the initial soft skeleton after preheating would be susceptible to suering progressive sintering in the successive looping-calcinations whereas surface area regeneration due to solid-sate diusion is negligible, which causes a drastic decrease of XN with N along the rst cycles. As the surface area is gradually decreased with N , conversion in the fast phase decreases and thus rN increases with N in contrast with the decreasing trend of rN observed for the CRTA preheated sorbent. Interestingly, rN conforms for both sorbents to the same trend for N&20 . At this cycle number, the eect of the preheating program on renovating the active skeleton by enhanced solid-state diusion would be lost even though the total conversion is kept at a larger value than for the nonpretreated sorbent. Data on the CaO multicyclic conversion for CRTA preheated samples and subjected to dierent looping-calcination temperatures ( 700◦ C < Ts<950◦ C) plotted in Fig. 5a demon8 strate a strong eect of Ts on self-reactivation. While decreasing the looping-calcination temperature below 850 ◦ C enhances self-reactivation, increasing it above Ts= 900◦ C nullies it completely. Figure 5b shows that the decline of self-reactivation is directly correlated to a change of trend of rN . Self-reactivation is only observed as long as rN decreases with N . For the highest looping-calcination temperatures rN increases with N from the rst cycle and self-reactivation does not occur. Data on fast carbonation conversion XKN and diusive carbonation conversion XDN are plotted in Fig. 6. Figure 7a shows a comparison on the evolution of conversion with time for the 10th carbonation/calcination cycle and for dierent values of the looping-calcination temperature Ts . Data on the maximum rate of conversion measured in the fast phase indicating the skeleton reactivity ( dXKN /dt ) are plotted in Fig.7b as a function of the cycle number. Note in Figs. 6a and 7b that the values of conversion XKN and rate of conversion dXKN /dt for the 1st carbonation ( N= 1 ) are approximately the same for all the tests since the samples have been subjected to the same pretreatment. Therefore, the skeleton to be carbonated for the rst time should be similar in all the tests. The values of XKN and dXKN /dt are however greatly aected already in the 2nd carbonation ( N= 2 ) by the looping-calcination temperature Ts used to regenerate the carbonated sorbent. As Ts is increased the values of XKN and dXKN /dt are markedly decreased for N≥2 . In contrast, the results demonstrate that XDN is approximately independent of Ts (see Fig. 6b). Interestingly, Fig. 7b shows that dXKN /dt is mainly enhanced after the loopingcalcination, which indicates that the reactivity of the hard skeleton obtained from thermal pretreatment is rather small as compared to the reactivity of the soft skeleton developed after this 1st calcinations. This enhanced reactivity is kept approximately constant with the cycle number for Ts<850◦ C. Harsher calcination conditions would lead to a reduction of the surface area by sintering and to a decrease of the soft skeleton reactivity as seen in Fig. 7b from the decrease of dXKN /dt with N for N > 2 and Ts>850◦ C. Thus, it may be inferred that the main eect of increasing the looping-calcination temperature Ts for sorbent regeneration is 9 xN≃(1 −a)N N−1 ∏ i=0 (1 + b)1 (i+1)q= (1 −a)N(1 + b)∑N−1 i=0 1 i+1q (18) where it has been used (1+b)1/(1+i)q≈1+b/(i+1)q for b < 1 . The nite sum in the exponent of Eq. 18, which is a special function so-called Harmonic-Number function H(N, q) = ∑N i=1 1/iq , ts well to a power law equation H(N, q)≃Nβ , with β≈e−q . In this way, the model yields a simple analytical equation for the multicyclic CaO conversion of pretreated sorbents, xN= (1 −a)N(1 + b)Nβ (19) In the case of thermally pretreated sorbents showing self-reactivation, the regeneration factor b would be larger than the sintering factor a and it would be q≃ − ln β > 0 as the surface area gain due to diusive carbonation is lessened with the cycle number (Eq. 17). However, if looping-calcination conditions are harshened, the sintering factor would be increased and the surface area gain could be counteracted by its loss in the looping-calcination stage. As the number of cycles builds up, the soft skeleton cannot be fully renovated due to the decline of solid-state diusion. The progressive aging of the soft skeleton would cause a transition to a mechanism determined by the gradual attenuation of its sintering rate as for nonpretreated sorbents. Equation 19 would serve therefore to describe self-reactivation in the rst cycles as usually reported in experimental studies on thermal pretreatment. 1214 Analysis of experimental results As may be seen in Fig. 3 multicyclic conversion data on the nonpretreated (linearly preheated) sorbent can be pretty well tted by Eq. 16 for a=a0= 0.164 and b= 0.024 (looping-calcination conditions: Ts0= 850◦ C and ts0= 5 min). As expected in this case, it is b/a << 1 since most of carbonation occurs via fast carbonation and, thus, regeneration of 16 the soft skeleton due to solid-state diusion is negligible. According to the German-Munir model, the sintering factor a of the initial skeleton would be given by a= (Ksts)1/γs where γs≃2.7 . Using a=a0= 0.164 and ts=ts0= 5 min, it would be Ks0≃10−3 min −1 which ts to the experimental data reported by Borgwardt on the sintering rate of nascent CaO derived from limestone (see Fig. 4 in ref. 21 ). Let us now focus on the analysis of multicyclic conversion data obtained in our work for the CRTA pretreated sorbents in the rst cycles where self-reactivation is observed for mild calcination conditions ( N≤ 10). In this case the soft skeleton will be renovated in each carbonation and will arguably sinter during looping-calcination as it does the soft skeleton of the nonpretreated sorbent in the rst calcination. Accordingly, the sintering factor a for this soft skeleton could be approximated to the sintering factor inferred for the nonpretreated sorbent. However, a necessary requirement to use the sintering factor a0 inferred from conversion data on the nonpretreated sorbents is that looping-calcination conditions are not varied in the multicyclic experiment on the pretreated sorbents, Taking into account that Ks follows a dependence with temperature in accordance with an Arrhenius relationship ( Ks=Aexp(−α/Ts) ), the sintering factor a for given values of Ts (in Kelvin) and ts could be obtained as a(Ts, ts) = a0(ts ts0)1/γ exp (−α γs Ts0−Ts Ts0Ts) (20) where α≃3×103 K for limestone-derived CaO. 21 This leaves the regeneration factor b and the exponent β as the only free parameters in Eq. 19 to t it to multicyclic conversion data on the CRTA pretreated sorbents at varying looping-calcination conditions. The best t curves obtained in this way are plotted in Fig. 5 showing a good t to the data. The best t parameters b and q=−ln β , and the sintering factor a obtained from Eq. 20, are represented in Figs. 11a-11b as a function of the looping-calcination temperature Ts and looping-calcination time period ts . It is seen that the regeneration factor b , as well as the sintering factor a , increase with Ts and ts . The increase of the regeneration factor b with 17 the looping calcination temperature Ts can be explained from the increase of the ratio of conversion due to diusive carbonation to conversion due to fast carbonation with Ts at a given cycle number (Fig. 4b). However, the ratio b/a (Fig. 12) remains approximately constant ( b/a ≃1.5 ) for Ts⩾770◦ C. The case Ts= 700◦ C can be considered as particular since decarbonation takes place very slowly at this looping-calcination temperature as seen in Fig. 7). On the other hand, q becomes close to zero for Ts≃900◦ C ( ts= 5 min) and ts≃15 min ( Ts= 850◦ C) as self-reactivation is impaired as seen in our experimental results, following an exponential decay law with the sintering factor a (Fig. 12). Equation 19 may thus provide an useful tool to foresee the critical conditions at which thermal pretreatment looses eciency in inducing self-reactivation. Conclusions In this paper we have analyzed self-reactivation of thermally pretreated CaO as aected by looping-calcination conditions. Thermal pretreatment has consisted of a Constant Rate Thermal Analysis (CRTA) program, which causes decarbonation to occur at a low prexed value. This type of pretreatment induces self-reactivation of CaO during the rst carbonation/calcination cycles by sintering the sorbent skeleton, which enhances carbonation in the solid-state diusion phase. A series of multicyclic carbonation/calcination experiments have been made on CRTA pretreated samples subjected to varying looping-calcination conditions (temperature Ts and time period ts ). According to the pore-skeleton model, a soft porous skeleton is developed after quick decarbonation at the end of the rst cycle with increased surface area available for enhanced fast carbonation in the next cycle. Conversion at a given cycle would be thus determined by a balance mechanism between the surface area gain due to enhanced diusive carbonation and the loss of surface area of nascent CaO associated to sintering during looping-calcination. By assuming a German-Munir model for the surface area reduction of the nascent CaO in the calcination stage, data on multicyclic conversion 18 in the rst cycles have been well tted by a proposed equation based on this balance. Accordingly, the main eect of harshening looping-calcination conditions is to accelerate the sintering of the renovated soft skeleton. This causes a decrease of conversion in the fast phase whereas conversion in the solid-state diusion phase is practically unaected. As loopingcalcination conditions are further harshened, the renovated soft skeleton is increasingly sintered during the calcination stage of the cycles and self-reactivation is eventually hindered. Self-reactivation is in our experiments fully precluded when the looping-calcination temperature is increased beyond 900 ◦ C ( ts= 5 min xed) or for looping-calcination periods of 15 min ( Ts =850 ◦ C xed) whereas signicant self-reactivation is observed for looping-calcination temperatures below 800 ◦ C or short looping-calcination periods of around 1 min. In order to assess the eciency of thermal pretreatment on stabilizing the multicyclic CaO conversion for CO 2 capture from combustion gases it is thus important to analyze the restrain posed by the looping-calcination conditions, which will foreseeable be harsher than those used in most works showing self-reactivation of thermally pretreated sorbents. Acknowledgement This work was supported by the Consejeria de Innovacion, Ciencia y Empresa (Junta de Andalucia) within the European Regional Development Fund contracts FQM-5735 and by the Spanish Government Agency Ministerio de Economia y Competitividad (contracts FIS2011-25161 and CTQ2011-27626). The authors thank Dr. Francisco Varela from the Microscopy Service of the Innovation, Technology and Research Center of the University of Seville (CITIUS) for his assistance. Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org/ . 19 References (1) Blamey, J.; Anthony, E. 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Applied Catalysis A: General 1993 , 107 , 1  57. 22 0 200 400 600 800 1000 Temperature (°C) 60 80 100 120 (%) 0 50 100 150 200 250 300 350 400 Time (min) -3 -2 -1 0 (%/min) cycles cycles preheating-CRTA preheating-CRTA quick dehydroxylation/carbonation quick dehydroxylation/carbonation slow rate-controlled decarbonation slow rate-controlled decarbonation Temperature Temperature Tcalc Tcalc Tcarb Tcarb a) b) 0 200 400 600 800 1000 Temperature (°C) 60 80 100 120 (%) 0 50 100 150 200 250 300 350 400 Time (min) -3 -2 -1 0 (%/min) Mass Mass Mass Mass Deriv. mass Deriv. mass Deriv. mass (reaction rate) Deriv. mass (reaction rate) Figure 1: Thermograms showing the evolution of temperature, sample mass % and mass % derivative (reaction rate) during CRTA preheating of Ca(OH) 2 samples (9.573 mg in (a), 10.296 mg in (b)) in an air/CO 2 atmosphere and carried out as pretreatment before the carbonation/calcination cycles are initiated. The occurrence of dehydroxylation/carbonation at a quick rate and decarbonation at a slow controlled rate are indicated. Carbonation/calcination temperatures ( Tcarb and Tcalc are indicated) In (a) looping-calcination is carried out at 950 ◦ C and in (b) at 700 ◦ C. 23 0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 2 20 200 Pore Volume (cm³/g·nm) Pore Diameter (nm) linear preheating CRTA preheating linear preheating CRTA preheating Figure 2: BJH Desorption dV/dD Pore Volume and SEM pictures of sorbent samples after being subjected to dierent preheating treatments: linear preheating (BET=9.04 m 2 /g) and CRTA preheating (BET=3.87 m 2 /g). 24 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 10 20 30 40 50 linear preheating CRTA preheating XN N Figure 3: Multicyclic conversion of CaO derived from in-situ linear and CRTA preheating programs. The solid line is the best t curve of Eq. 16 to the data ( a= 0.164 , b= 0.024 ). Looping calcination conditions: Ts0 =850 ◦ C, ts0 =5 min. 25 70 75 80 85 ( (% %) ) 0 2 4 6 8 10 0 2 4 6 8 10 Time (min) 5min 810ºC 1 min 850ºC 5min 900ºC 15min 850ºC 70 75 80 85 Time (min) a) b) 50th cycle 50th cycle Mass Mass Figure 10: Mass gain % as a function of time in the 50th carbonation cycle for samples cycled under dierent looping-calcination temperatures (a) and time periods (b) as indicated. 32 T = 850ºC s q b t (min) s b) q b t = 5 min s a) 0 0.1 0.2 0.3 0.4 0.5 0.6 650 700 750 800 850 900 950 1000 q,b ,a T (ºC) s a 0 0.1 0.2 0.3 0 5 10 15 20 q,b, a a Figure 11: Best tting parameters ( b and q=−ln β ) of Eq. 19 to multicyclic conversion data in the rst 10 cycles as a function of looping-calcination temperature Ts ( ts =5 min) (a) and looping-calcination time period ts ( Ts= 850◦ C). Best t curves for conversion using these parameters are plotted in Fig. 5. 33 0.001 0.01 0.1 1 10 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 q b/a a q (variable T ) s b/a (variable T ) s b/a (variable t ) s q (variable t ) s Figure 12: Ratio of the regeneration to sintering factors b/a and exponent q=−ln β obtained from the best tting parameters of Eq. 19 to multicyclic conversion data in the rst 10 cycles as a function of the sintering factor a calculated from Eq. 20. The solid line is an exponential decay t to data on q ( q= 0.85 exp(−16.9a) ). 34