Performance and Carbonation Kinetics of Modified CaO-Based

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Performance and Carbonation Kinetics of Modified CaO-Based Sorbents Derived from Different Precursors in Multiple CO2 Capture Cycles Long Jiang,† Song Hu,*,†,‡ Syed Shatir A. Syed-Hassan,†,§ Yi Wang,*,† Chao Shuai,† Sheng Su,†,‡ Changyi Liu,† Huangying Chi,† and Jun Xiang†,‡ †

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P.R. China ‡ China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P.R. China § Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia ABSTRACT: Calcium-based sorbent is a suitable sorbent for the solid looping CO2 capture. The loss-in-capacity due to sintering, however, poses a big problem to the direct utilization of natural/pure CaO in the looping process. The issue of attrition, meanwhile, adds a further obstacle to the long-term cyclic utilization of the sorbent. Additionally, the performance of CaO-based sorbent in the long term solid looping CO2 capture depends to a great degree on the type of precursor used to derive the sorbent. This study aims to systematically study the influence of the various calcium precursors on the sorbent performance in the long term carbonation-calcination cycles. Results of this study indicate that the absorption capacity of CaO sorbents was influenced significantly by the type of CaO precursor. It was found that CaO sorbent derived from calcium acetate gave the best absorption capacity of 0.61 mol-CO2/mol-sorbent after 48 carbonation/calcination cycles. The addition of inert supporting matrix of Al2O3 has enhanced sintering resistance of the sorbents. The X-ray diffraction (XRD) results, supported with scanning electron microscopy (SEM) observations and N2 adsorption/desorption analysis, suggest that the formation of mayenite (Ca12Al14O33) helped to strengthen the framework of the sorbents, consequently improved sintering resistance and long-term cyclic stability of the modified sorbents. The incorporation of calcium aluminate cement decreased the attrition of sorbents, over the 48 cycles of carbonation and calcination, from 24.6% to 7.4%. Finally, in this study, carbonation kinetics of the sorbent was studied using a new, more precise three-stage carbonation kinetic model. The calculated results fitted to the experimental results very well.

1. INTRODUCTION Global warming and climate changes are mainly caused by large emissions of the CO2 from burning of the fossil fuels. Carbon capture and storage (CCS), especially from thermal power plants and other energy intensive industries, has been receiving much attention over the past decade as a strategy to reduce carbon emissions to the atmosphere. Recent research show that CaO-based sorbent is very suitable for the solid looping application for the CO2 capture. This is due to its high CO2 uptake capacity, low cost, and widely available in natural minerals (e.g., limestone and dolomite).1−3 Solid looping is a high temperature multi cyclic process (between 650 and 900 °C). The principle chemical reaction taking place in the process is the following reversible gas−solid reaction of carbonation/calcinations: CaO(s) + CO2 (g) ← → CaCO3(s)

widely used supporting matrix due to its higher Tamman temperature.9 Different types of CaO precursors (e.g., calcium hydroxide, calcium nitrate, calcium acetate, and nanosized calcium carbonate) have been used to synthesize CaO-Al 2 O 3 sorbents.2,10−15 Results of the past studies suggest that the performance of CaO-Al2O3 in the solid looping CO2 capture depends to a great degree on the type of precursor used to derive the sorbent. Li et al.11 reported the sorbent of 75% CaO/25% Ca12Al14O33 derived from Al(NO3)3·9H2O precursor showed the best CO2 absorption capacity of 45% (10.2 molCO2/kg-sorbent) in the 13th cycle. A study by Wu et al.14 shows that nano-CaO sorbent derived from nano-CaCO3 precursor without any inert matrix had a reactive sorption capacity of 16.3% (3.7 mol-CO2/kg-sorbent) after 40 cycles. Martavaltzi et al.,16 on the other hand, reported the sorbent of CaO−Ca12Al14O33 derived from calcium hydroxide precursor showed excellent stability but low sorption capacity, while that derived from calcium acetate showed CO2 absorption capacity of higher than 26.4% after 45 cycles. However, the systematical

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A quick review of the published studies4−7 shows that natural/pure CaO sorbents are susceptible to sintering, making it easy to lose its CO2 uptake capacity after several cycles of carbonation/calcination. One way to enhance long-term reactivity of the sorbents is by supporting CaO-based materials with a thermally stable inert matrix.8 So far, Al2O3 is the most © XXXX American Chemical Society

Received: June 6, 2016 Revised: September 2, 2016

A

DOI: 10.1021/acs.energyfuels.6b01368 Energy Fuels XXXX, XXX, XXX−XXX

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long-term performance was identified, and its carbonation kinetics was studied using a new and more precise three-stage kinetic model.

study on the influence of the various calcium precursors on the sorbent performance in the long term carbonation-calcination cycles was hardly found. It thus becomes apparent that a more systematic comparison of the performances of modified CaO sorbents from different CaO precursors is needed in determining a suitable sorbent for industrial application of solid looping process. Apart from the above problems, sorbent loss through attrition is another great challenge in the long term solid looping cycles for the CO2 capture.17−19 The needs for the makeup of fresh sorbent and the disposal of elutriated sorbent may bring negative economic consequences to the solid looping technology.20 The CaO-Al2O3 sorbents discussed above may have good reactive stability over several cycles, but the supporting matrix of Al2O3 does not provide enough mechanical strength to the modified sorbents. The sorbents are therefore prone to attrition or cracking in the long term solid looping carbonation/calcination cycles. Calcium aluminate cement is a promising strengthening material for CaO-based sorbents due to its acceptable attrition resistance, good mechanical strength, and low cost.19−21 Qin et al.19,22 found that the added cement acted with sorbent to produce stable Ca12Al14O33 which has capability of preventing dramatic structure change and providing improved attrition resistance to the sorbent. Kotyczka-Morańska15 pointed out that using mixed alumina and calcium (Ca12Al14O33) as a carrier of calcium oxide led to a significant improvement in regenerative properties of calcium-based sorbent. Manovic et al.20 added calcium aluminate cements to strengthen the structural framework of the sorbents. The modified sorbents showed good performance by having a conversion of higher than 50% after 30 cycles. Further investigation on the strengthening effect of calcium aluminate cement on verities of modified CaO sorbent is useful to develop a highly stable and durable sorbent for the solid looping CO2 capture technology. To further optimize the design and the operation of solid looping reactor, an in-depth understanding of the kinetics of absorption/carbonation is crucial. It is well understood that carbonation can be divided into two well-known stages (corresponding to a different kinetic model): the rapid surface reaction-controlled stage and the slow product layer diffusioncontrolled stage.23 The common two-stage model is usually used to investigate the carbonation kinetics.23,24 However, since the end of chemical reaction-controlled stage and the start of diffusion-controlled stage have no distinct boundary, the two controls could coexist in the middle stage of the reaction (namely, the transition stage). A satisfying result can hardly be obtained by applying the common two-stage model. A more precise kinetic model should therefore take into consideration the existence of the transition stage in the carbonation reaction. In the present study, the influence of the various calcium precursors on the sorbent performance in the long term carbonation-calcination cycles has been systematical studied. Four typical calcium precursors were incorporated into Al2O3 to produce four different CaO-based sorbents. The enhancement in their long-term cyclic stability was investigated. Calcium aluminate cement was added to the sorbents and its effects on the cyclic mechanical properties of sorbents were investigated. The sorption capacity and physical/chemical structure changes of the sorbents before and after multiple carbonation-calcination cycles were studied using various analytical techniques. Finally, the best sorbent in term of its

2. MATERIALS AND METHODS 2.1. Sample Preparation. Four typical calcium precursors, i.e., (i) Ca(OH)2, (ii) Ca(NO3)2·4H2O, (iii) Ca(CH3COO)2·H2O, and (iv) nanosized CaCO3 (70 nm) were incorporated into Al(NO3)3·9H2O to prepare CaO-based sorbents. All reagents were analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai. The derived sorbents were denoted as CH, CN, CA, and CC, correspondingly. CaO sorbent derived from only calcium hydroxide without supporting matrix was denoted as RW. The preparation procedures of the sorbents were as follows, respectively, 10.57 g, 33.71 g, 25.14 g, and 14.29 g (with 0.001 g accuracy) of Ca(OH)2, Ca(NO3)2·4H2O, Ca(CH3COO)2·H2O, and nanosized CaCO3 (each capable of producing 8 g of CaO by calcination) were mixed with 14.71 ± 0.001 g of Al(NO3)3·9H2O (contained 2 g of Al2O3). The mixtures were then mixed with 200 mL of deionized water and stirred for 2 h at ambient temperature. The mixture solution was evaporated at 105 °C for 24 h and subsequently calcined at 750 °C for 1 h in a muffle furnace. The mass ratio of CaO−Al2O3 was 8:2. In order to enhance mechanical strength of the sorbents, each of the calcined sorbent (except for RW) was mixed with calcium aluminate cement (purchased from Zhengzhou Xinxing Special Cement Factory, P. R. China) at a mass ratio of 6:4. The compositions of the cement were 51.98% Al2O3, 33.12% CaO, 7.80% SiO2, 1.8% Fe2O3, 1.47% MgO, and 2.5% TiO2. A small amount of deionized water was added to the mixture and stirred to become slurry. The slurry was dried at ambient temperature for 48 h and then calcined at 900 °C for 2 h in a muffle furnace. Finally, the prepared solid was ground and sieved. The particles within 300−600 μm size were collected for experiments. 2.2. Experimental Procedure. The experiments were conducted in a fixed-bed reactor which had been described elsewhere.12 The differences of the setup for this study were the absence of 1, injection pump (phenol); 2, steam generator; 3, injection pump (water); 11, gas-washing bottle; and 12, ice-salt bath. The inner diameter of the reaction zone was 18 mm, and the length of the reactor was 600 mm. In total, 2.0 ± 0.01 g sample was placed on the quartz sieve of the reactor in each experiment. One cycle of CaO looping comprised of 30 min of carbonation (650 °C, 30 mL/min CO2 and 170 mL/min N2) and 30 min of calcination (heated from 650 to 850 °C in 10 min and held for 5 min, then cooled down to 650 °C in 15 min, 170 mL/min N2). A time controller was used to control CO2 feeding. The carbonation/calcination cycle was repeated 48 times for each run. 2.3. Physical and Chemical Structure Characterization of the Sorbents. To understand chemical structure changes of the sorbents, the sample phase composition was determined by X-ray diffraction (XRD, PANalytical B.V. X’Pert PRO, The Netherlands) using Cu Kα radiation. Diffraction patterns of the sorbent powders were evaluated qualitatively by comparison with reference patterns in the PDF-2 database. To understand physical structure changes of the sorbents in longterm cycles, the morphology of the sorbents (fresh and after 48 carbonation/calcination cycles) was studied by using scanning electron microscopy (SEM) (Phillips Quanta 200, The Netherlands). Specific surface area and pore volume distribution of the sorbents before the first cycle and after 48 looping cycles were determined using the Brunauer, Emmett and Teller (BET) and the Barrett−Joyner− Halenda (BJH) methods of N2 adsorption/desorption (Micromeritics ASAP 2020), respectively. 2.4. Carbonation Kinetics. Carbonation kinetics of the sorbent was investigated by using thermogravimetric analysis (NETZSCH STA449F3, German). Accurately weighed 25 ± 0.5 mg of sorbent was heated at 30 K/min to the desired carbonation temperature (550−650 °C) under purified nitrogen atmosphere (flow rate of 120 mL/min). The nitrogen gas was then switched to pure CO2 at a flow rate of 120 mL/min to conduct carbonation reaction. Reaction time of the carbonation was 35 min. The changes in the weight of the sorbent B

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Energy & Fuels during carbonation reaction were recorded automatically by a computer. The buoyancy effect of TGA was deducted by a blank experiment.

3. RESULTS AND DISCUSSION 3.1. Cyclic Stability of the Sorbents. Figure 1 shows CO2 capture capacity of the five CaO-based sorbents as a function of

Figure 1. Cyclic absorption capacity of different sorbents at 650 °C.

cycle times. As can be seen, the capture capacity of RW was relatively high at the beginning. Upon repeated cycles of carbonation/calcination, however, the capture capacity of RW declined continuously from 0.73 mol-CO2/mol-sorbent in the first cycle to 0.20 mol-CO2/mol-sorbent in the 48th cycle. The deactivation of RW was mainly due to the pore volume loss and sintering of the porous calcium based sorbents.25,26 The volume loss resulted in the capacity decline in the first few cycles, while sintering was responsible for the capacity decline in the following long-term cycles. In contrast, the capture capacity of the sorbents added with supporting matrix showed a slighter decline in the first few cycles (up to 9−10 cycles). Upon further cycles (up to 48 cycles), however, their capture capacity did not show remarkable variation (unlike RW with a continuous decline). The absorption capacity of CH, CN, CC, and CA decreased from 0.66, 0.75, 0.48, and 0.84 mol-CO2/mol-sorbent in the first cycle to 0.38, 0.40, 0.34, and 0.61 mol-CO2/mol-sorbent in the 48th cycle, respectively. It is clearly shown that the type of precursor plays an important role in determining the absorption capacity of a CaO sorbent. Results in Figure 1 also indicate that cyclic stability of the calcium-based sorbents can be enhanced by adding inert supporting matrix. It is believed that the inert supporting matrix (Al2O3) has improved, to a significant extent, the skeleton strength and sintering resistance of the calciumbased sorbents. The sequence of absorption capacity of these four modified sorbents in the long term cycles was CA > CN > CH > CC. With the highest absorption capacity over the 48 cycles (0.61 mol-CO2/mol-sorbent), CA derived from calcium acetate was best under the current experimental conditions. 3.2. Physical/Chemical Structure Evolution of the Sorbents. 3.2.1. Phase Composition of the Sorbents. Figure 2 shows XRD patterns of the different sorbents produced in this study. As is shown in the figure, the XRD results of CaObased sorbents with added calcium aluminate cement always exhibit a more complicated pattern as compared to the one

Figure 2. Phase composition of the sorbents analyzed by XRD.

without the addition (RW), suggesting that more components were present when the CaO sorbents were incorporated with the additive. The main compositions of calcium aluminate cement were gehlenite (2CaO·Al2O3·SiO2), CaAl2O4 and CaAl4O7. For RW sample without adding any additives, only the diffraction peaks corresponding to the phase composition of CaO were detected. In contrast, for CH, CN, CC, and CA samples, the diffraction peaks corresponding to the phase compositions presented in RW and cement samples were also detected, yet the intensities were much lower. More strikingly, the diffraction peaks corresponding to the new phase composition of mayenite (Ca12Al14O33), which were absent in the XRD result of RW, were evidently detected in CH, CN, CC, and CA samples. Apparently, the difference in the cyclic stability between RW and the four modified sorbents discussed earlier was associated mainly to the absence/presence of mayenite. The Tammann temperature of mayenite (∼900 °C)7 is higher than the calcination temperature of this study (850 °C). Therefore, the formation of mayenite has assisted in forming a stable inert matrix that dispersed uniformly in the structure of the sorbents. The formation of this stable inert matrix prevented agglomeration and growth of the sorbent particles and eventually enhanced sintering resistance of the sorbents.27,28 It was also observed that the diffraction peaks of the four modified sorbents (CH, CN, CC, and CA) had little, if at all, difference in their shape and intensity, signifying that their phase composition and quantity were similar. Nevertheless, the absorption capacity and cyclic stability of the four modified sorbents were different (as shown in Figure 1), indicating that C

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Energy & Fuels the physical structure, especially the pore construction, of the four sorbents derived from different precursors was different from one to another. The evaluation of the physical structure of the four sorbents was made to understand the effects of physical structure in the performance of the sorbents. This will be discussed further below. 3.2.2. Porosity of the Sorbents. The specific surface area and pore size distribution of the fresh sorbents are compared with those obtained after the 48th cycle (Table 1 and Figure 3).

Generally, the pores can be classified into three categories: macropores (pore size > 50 nm), mesopores (2 nm < pore size < 50 nm), and micropores (pore size < 2 nm). The specific surface area is dominantly reflected by micropores and mesopores.29 Figure 3 shows that the pore volume of micropores and mesopores of the fresh RW was significantly higher than that of the four modified sorbents before the first cycle. Therefore, it is not surprising that initially the specific surface area of RW was significantly higher than other sorbents (Table 1). After 48 cycles, however, the micropores and mesopores of RW decreased remarkably (Figure 3a), resulting in the significant decrease of specific surface area of RW from 22.69 to 3.89 m2/g (Table 1), leading to obvious changes of absorption capacity of RW (Figure 1). This pore volume loss should be attributed to severe sintering of RW in carbonationcalcination cycles.

Table 1. Specific Surface Area of the Fresh and after 48 Cycling Sorbents (m2/g) samples

RW

CH

CN

CC

CA

fresh after 48 cycling

22.69 3.89

5.09 5.01

6.56 6.3

6.12 6.08

8.36 6.54

Figure 3. Pore volume distributions of the fresh and after 48 cycling sorbents. D

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Figure 4. Surface morphology of the fresh (left) and after 48 cycling sorbents (right).

assuring the stability of the whole framework even though some micropores collapsed after repeated cycling. This has led to the desirable cyclic stability of the modified sorbents within the 48 cycles of carbonation/calcination. Compared to CH, CN, and CC, the specific surface area of CA was larger both before the first cycle and after 48 cycles. Additionally, the growth of macropores was also slighter in CA

On the contrary, after 48 cycles, pore volume loss of the four modified sorbents was much smaller than that of RW. However, as can be seen in Figures 3b−e, the macropores of the four modified sorbents grew evidently after cycling. It provides us an indirect evidence that the inert supporting matrix of Ca12Al14O33, formed through incorporation of an additive, has strengthened the framework of the sorbents, E

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with cement was 7.4%, while that of without cement addition was 24.6%. The results indicate that adding calcium aluminate cement improved mechanical strength of the calcium based sorbent very markedly. Calcium aluminate cement is a promising strengthening material for CaO-based sorbents in practical operation. 3.4. Carbonation Kinetics. 3.4.1. Mathematical Model. The best performance sorbent of CA was selected to study the kinetics of carbonation. The conversion rate of CA was defined as

than in the other three modified sorbents. This indicates that the strength of CA framework was higher than other modified sorbents to result in a better performance of CA in the 48 looping cycles in this study. 3.2.3. Morphology of the Sorbents. The morphology of the fresh and with the 48th cycling sorbents was shown in Figure 4. It can be seen that the particles of the fresh RW had a slight agglomeration, which indicates slight sintering occurred in calcination process of preparing RW. However, these particles were still rich in pore structure. After 48 cycles, the severe agglomeration and sintering of the particles were observed which was in accordance with the severe decrease of the cyclic stability of RW in 48 carbonation/calcination cycles shown in Figure 1. The pore structure of the particles was fused and collapsed, agreeing with the remarkable decline of the specific surface area shown in Table 1. In contrast, the particles of four modified sorbents before subjected to the looping cycle had a good dispersion, and the evidence of agglomeration was not found. These particles were also rich in pore structure. After 48 cycles, the dispersion of the particles of four modified sorbents had negligible change, and agglomeration of only very small particles was observed. However, severe sintering of these particles was not found. Agreeing with the former conclusions, this directly proves that adding inert matrix of Al2O3 notably enhances the sintering resistance of the sorbents. 3.3. Attrition Test. To understand the effects of cement addition on sorbent attrition, attrition test was conducted on CA, the best performance sorbent in this study. Comparison was made between the sample with the addition of cement and that without the addition of cement. The initial particles size of tested samples were between 300 and 600 μm. The sorbent particles were placed in vibrating screening machine to simulate the attrition process. The particles size less than 300 μm was considered as an attrited sorbent, and the attrition rate was then calculated accordingly. The results acquired from different test duration were shown in Figure 5.

X=

(56/44)Δm Δm /44 = mCaO/56 mCaO

(2)

where X represents conversion rate of CA; 44 and 56 are molecular mass of CO2 and CaO, respectively; Δm represents weight change of the sorbent; and mCaO represents nominal weight of CaO in CA sorbent. The rate of carbonation reaction was very high at the initial stage before attenuating rapidly and became stable and low at the final stage. To allow characterization of carbonation behavior to be made as far as possible to the end of the carbonation reaction, a long enough reaction time, i.e., 35 min, which was close to the end of carbonation reaction was chosen in this study. To unify the fitting datum and improve precision of model calculation, the relative conversion rate of Xr was used and defined as

Xr =

X X final

(3)

where Xfinal represents sorbent conversion rate at 35 min carbonation reaction. Carbonation process is generally divided into two stages: chemical reaction-controlled stage at the beginning of reaction and diffusion-controlled stage later in the reaction. It is difficult to describe these two stages using a single mathematical model. Thus, a two-stage mathematical model is usually used to describe these two stages.23 Chemical reaction-controlled stage and diffusion-controlled stage can be described by two separate expressions from the random pore model (RPM) as follows:30,31 k S (C − Ce)t 1 [ 1 − ψ ln(1 − X ) − 1] = RPM 0 b ψ 2(1 − ε0)

S0 1 [ 1 − ψ ln(1 − X ) − 1] = ψ 1 − ε0

(4)

DRPM MCaOC bt 2ρCaO Z (5)

The above expressions require measurements and calculations of complicated structure parameters. A more simplified version of these two expressions are described below.23,31 Simplified Model of Chemical Reaction Controlled-Stage (RPM).

Figure 5. Attrition rate of the sorbent at a different test duration.

As is shown in the figure, the attrition rate increased with the increase of time. However, the rate of increment of the attrition rate gradually became slower when the testing time further increased. The results in Figure 5 also suggest that the attrition rate of the sample with the addition of cement was always much lower than that of the sample without the addition of cement. After 6 h testing time, the attrition rate of the sample added

kψt/4

X = 1 − e−(1+

ψ=

)kt

4πL0(1 − ε0) S0 2

k = k 0 exp( −E /RT ) F

(6)

(7) (8) DOI: 10.1021/acs.energyfuels.6b01368 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels where Ψ represents the structural parameter; L0 and S0 are the initial total length and surface area per unit volume, respectively; and ε0 is the initial porosity. S0, L0, and ε0 can be directly calculated from pore size distribution of the sorbent obtained from the mercury porosimetry or N2 adsorption/ desorption method.31,32 k, k0, E, R, and T represent the reaction rate constant, pre-exponential factor, activation energy, gas constant, and reaction temperature, respectively. Equation 8 can be converted as ln k = ln k 0 −

E1 RT

(9)

k0 and E can be obtained from the slope and intercept by using linear fitting of eq 9. Simplified Model of Diffusion Controlled-Stage (DFM). X r = (1 − eat + b)

Figure 7. Arrhenius plot of RPM in chemical reaction controlled-stage.

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Equation 10 can be converted as ln(1 − X r) = at + b

reaction. Clearly, neither RPM nor DFM was applicable between 25 s and 750 s reaction time. 3.4.2. Determination of Transition Stage Region. As being mentioned before, the end of the chemical reaction controlledstage and the start of diffusion controlled-stage have no distinct boundary, thus these two controls could coexist in the middle stage of the carbonation reaction, namely, transition stage. The inapplicability of RPM and DFM model in the middle stage of the carbonation reaction was due to not considering transition stage in these two models. In this study, the transition stage in carbonation reaction was considered, and a transition model was proposed to describe the transition stage as follows:

(11)

where a and b represent the slope and intercept calculated by using linear fitting of eq 11. Figure 6 shows the experimental data and the fitting curves obtained using the simplified model of chemical reaction

fTran = 1 − τ n

(13)

where n represents the index number, f Tran = (r − r1)/(r2 − r1); τ = (t − t1)/(t2 − t1); r represents reaction rate fitting curve of transition stage. (t1, r1) and (t2, r2) represent starting and terminal points, which separately situate in reaction rate fitting curve of RPM and DF model (as is shown in Figure 8a). The starting and terminal points of transition stage could be, respectively, expressed as Figure 6. Experimental and RPM/DFM calculated relative conversion rate as a function of carbonation reaction time.

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X r(t 2) = β(1 − X r(t0)) + X r(t0)

(15)

where (t0, Xr(t0)) is the intersection point of RPM and DF (as is shown in Figure 8b); α and β are empirical coefficients. The shaded area presented in Figure 8a could be expressed as follows: S = ∫ tt21r dt = Xr(t2) − Xr(t1), and the parameter of n can be calculated by the above shaded area equation. Also as is shown in Figure 6, the relative conversion rate corresponding to the end of chemical reaction controlled-stage (the start of the transition stage) increased with the increase in temperature, thus α was considered as a function of temperature. However, the reaction rate in the start of diffusion controlled-stage (the end of the transition stage) decreases rapidly and will be very low in the terminal points of the transition stage. Thus, the terminal relative conversion rate of the transition stage was not sensitive to temperature and β was considered as constant. It was verified that the simulated results were very satisfied when the α value was selected as 0.33, 0.45, and 0.65 as well as the β value was all selected as 0.4, corresponded to different temperatures of 550, 600, and 650 °C. The α function can be expressed as

controlled-stage (RPM) and diffusion controlled-stage (DFM). As can be seen in the figure, simulated and experimental data calculated from RPM were close only at the initial reaction time of 25 s. Beyond this time, the error between simulated and experimental data became larger, indicating that this model was no longer applicable at later carbonation reaction (beyond 25 s). Figure 7 shows the Arrhenius plot of lnk and 1/T for eq 9. The expression of k was calculated from the above Arrhenius plot as follows: ⎛ 96.01 kJ/mol ⎞ ⎟ k = 1.58 × 104 exp⎜ − ⎝ ⎠ RT

X r(t1) = αX r(t0)

(12)

Figure 6 also shows that simulated and experimental data calculated from DFM were close at later reaction time after ∼750 s. At the reaction time before 750 s, the error between simulated and experimental data was not satisfied, indicating that this model was not applicable at the initial carbonation G

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Figure 9. Experimental and three-stage model calculated conversion rate as a function of carbonation reaction time.

formation of Ca12Al14O33 promoted sintering resistance of the modified sorbents, thus enhancing their long-term cyclic stability. SEM and N2 adsorption/desorption analysis also indicate that the inert supporting matrix of Al2O3 could prevent sintering of the sorbents particles and strengthen the structure framework of the sorbents, thereby enhancing sintering resistance of the sorbents. The addition of calcium aluminate cement decreased attrition rate of the sorbent from 24.6% to 7.4% after 48 cycles. Finally, the calculated results using threestage kinetic model fitted to the experimental results very well, and the fitting error reduced significantly.

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Figure 8. Calculated schematic of fitting curves of transition stage.

α = 1.23 × 10−3 exp(6.78 × 10−3T )

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*E-mail: [email protected]. Phone: 0086-2787559715. *E-mail: [email protected]. Phone: 0086-27-875424178301.

In summary, the new three-stage model including transition stage model can be expressed as ⎧ 1 − e−(1 + kψt /4)kt t ≤ t1 ⎪ t ⎪ r dt t1 < t < t 2 X r(t ) = ⎨ X r(t1) + t1 ⎪ ⎪ at + b t ≥ t2 ⎩ (1 − e )/0.8

AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

∫

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ACKNOWLEDGMENTS The authors gratefully appreciate the financial support for this research from the National Science Foundation of China (NSFC) (Grants 51576072 and 51506070), Program for New Century Excellent Talents in University (Grant NCET-130226), the Foundation of State Key Laboratory of Coal Combustion (Grant FSKLCCB 1513), the China Postdoctoral Science Foundation funded project (Grant 2014M552038), and the Shanxi Science and Technology Major Project (Grant MD2015-03/05). They also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

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Figure 9 shows the experimental and three-stage model calculated conversion rate as a function of carbonation reaction time. As can be seen, the calculated results fitted to the experimental results very well. Thus, this new three-stage model is more precise than the normal two-stage model to express the carbonation reaction of the calcium sorbent.

4. CONCLUSIONS In this study, the influence of the various calcium precursors on the sorbent performance in the long term carbonationcalcination cycles has been systematical studied, and a new and more precise three-stage kinetic model which considered the model description of the transition stage was developed. The results revealed that the inert supporting matrix of Al2O3 could significantly enhance sintering resistance of the sorbents. The modified sorbent derived from calcium acetate acquired the best absorption capacity of 0.61 mol-CO2/mol-sorbent after 48 carbonation/calcination cycles. XRD results indicate that the

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DOI: 10.1021/acs.energyfuels.6b01368 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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DOI: 10.1021/acs.energyfuels.6b01368 Energy Fuels XXXX, XXX, XXX−XXX