Operation of a Cyclonic Preheater in the Ca-Looping for CO2

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Operation of a Cyclonic Preheater in the Ca-Looping for CO2 Capture Ana Martínez,* Yolanda Lara, Pilar Lisbona, and Luis M. Romeo CIRCE (Research Centre for Energy Resources and Consumption), Universidad de Zaragoza, Mariano Esquillor 15, 50018 Saragossa, Spain ABSTRACT: Calcium looping is an emerging technology for CO2 capture that makes use of the calcium oxide as a sorbent. One of its main issues is the significant energy consumption in the calciner, where the regeneration of the sorbent takes place. Nevertheless, as a high temperature looping technology, the surplus heat flows may be used to reduce the energy needs in this reactor. The addition of a cyclonic preheater similar to those used in the cement industry is proposed in this work. A calcium looping system was modeled and simulated to assess the advantages and disadvantages of the inclusion of a cyclonic preheater. Despite the negative effect on the maximum average capture capacity of the sorbent, a reduction on the coal and oxygen consumptions and on the extra CO2 generated in the calciner is obtained. may reach up to 300 °C. This temperature variation, coupled with the intense particles flow circulation between both reactors, makes the solids heat up a considerable energy penalty. The use of the gaseous and solid streams leaving the calciner to increase the temperature of the particles entering the calciner and, thus, reduce the energy consumption in the calciner, is already patented.7 Martı ́nez et al.8 presented and modeled different configurations for this internal heat integration obtaining promising results in terms of fuel savings. The use of a cyclonic preheater (Figure 1a), as those used in the cement industry,9 to transfer energy from the hot gas leaving the calciner to the particles entering this reactor (Figure 1b) is proposed, modeled and analyzed in this work. The mixing of a highly concentrated CO2 stream (already separated) with a partially carbonated sorbent stream, that is still able to react in the fast reaction phase, may allow its carbonation in the cyclonic preheater. On the other hand, if the temperature is higher than that for which CO2 concentration yields equilibrium, the particles might be precalcined in this device. The effect of the cyclonic preheater in the operation and the capture and energetic efficiencies is analyzed. In addition, the three different preheaters are tested to determine their suitability in the system.

1. INTRODUCTION Calcium looping is a promising technology for CO2 capture that makes use of the carbonation−calcination equilibrium, eq 1, to separate the CO2 from the flue gas of the power plant.1 CaO + CO2 ↔ CaCO3 + Q

(1)

The system mainly consists of two reactors. They are usually circulating fluidized beds to enhance the gas−solid mixing, to homogenize the temperature and to facilitate the circulation of the solids. The CO2 sorption takes place in the carbonator that is fed with the power plant flue gas and the sorbent flow, CaO. It operates at around 650 °C and releases the gaseous stream almost free of CO2 and the partially carbonated sorbent. The regeneration of the sorbent takes place in the calciner where the highly concentrated flow of CO2 is produced. The formed CaO is redirected to the carbonator to close the loop. The calciner has to operate at higher temperatures, up to 950 °C, to allow the calcination reaction.2 The wide availability and low cost of the natural limestone, as well as the fact that it is a high temperature sorption cycle that enables the surplus heat integration in a steam cycle to produce electricity, makes the calcium cycle a promising CO2 capture technology. However, there are still some key issues that have to be improved to make this technology competitive. On the one hand, the sorbent capture capacity diminishes with the cycles and a large number of research studies are focused on the improvement of the sorbent.3−6 On the other hand, there is an important energy penalty in the calciner, which is the focus of this work. The energy consumption in the calciner is mainly related to the regeneration of the sorbent, but there is also a fraction of the energy required to heat up the particles coming from the carbonator. The temperature difference between both reactors © 2013 American Chemical Society

2. MATERIALS AND METHODS A CO2 capture system based on the calcium looping cycle was modeled and simulated to analyze the influence of using a cyclonic preheater in the calciner (Figure 1b). The carbonator, Received: Revised: Accepted: Published: 11335

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Figure 1. (a) Sketch of a four step cyclonic preheater. (b) Diagram of the calcium looping with a cyclonic preheater.

a circulating fluidized bed, operates at 650 °C and is fed with the flue gas from a power plant at 180 °C. The 500 MW coal power plant has a 40% global efficiency and operates with the coal composition shown in Table 1 and a 20% of oxygen excess. Maximum capture is supposed in this reactor, only limited by the carbonation equilibrium at 93.01% efficiency capture.

The fraction of particles with a residence time below a critical reaction time, t*CR, are the particles reacting in the fast reaction regime, fa,CR, eqs 3 and 4.11 ⎛ −t * ⎞ CR ⎟ fa,CR = 1 − exp⎜ n / ̇ ⎠ ⎝ Ca nCa

Table 1. Coal Composition and Low Heating Value

* = tCR

C [%db]

H [%db]

N [%db]

S [%db]

O [%db]

Z [%db]

H2O [%]

LHV [kJ/kg]

72.04

4.08

1.67

0.65

7.36

14.2

8.1

25 372

(4)

The surface lime reaction rate constant, kCR, was found to be around 0.3 and almost perfect gas−solid contacting (φ = 1) was also observed by Charitos et al.10 φ is the gas−solid efectivity factor which has been defined elsewhere.12 The space time parameter, τCR, is defined as shown in eq 5. n τCR = Ca nCO ̇ 2 (5)

The calciner is also a circulating fluidized bed and operates at 950 °C. The energy required for the regeneration is provided by oxy-fuel combustion to avoid the dilution of the CO2. The same coal and oxygen excess as in the power plant are assumed and the combustion is supposed to be complete. A fraction of the calciner flue gas has to be recirculated to allow a proper fluidization of the solids and to reduce the oxygen fraction at the entrance of the calciner, 60%w of oxygen is assumed. The purge and the makeup flow of fresh sorbent are required to remove the ashes and to maintain an adequate sorbent capture capacity. The purge flow is cooled down to 200 °C to preheat the gas entering the calciner. Calcination is almost complete under the operating conditions analyzed in this study. The surplus energy from the system is supposed to be integrated in a new different steam cycle to produce electricity. Energy flows are obtained from the carbonator refrigeration, the clean gas stream, and the solid and gaseous streams from the calciner. The gaseous streams leave the system at 180 °C and the solids enter the carbonator at 650 °C. The implemented carbonation model was developed by Charitos et al.10 It is mainly based on the active space time variation and assumes instant mixing of solids and plug-flow, eq 2. In this model, only a fraction, fa,CR, of the particles of CaO with a sufficiently short residence time are assumed to be active and react in the fast reaction regime.11 Those particles react with a reaction rate dependent on the lime surface reaction rate constant, kCR, the average maximum capture capacity, Xave, and the difference of the actual and equilibrium CO2 volume fraction. ηCR = k CR ·φ ·fa,CR ·τCR ·Xave( vCO2 − veq)

Xave − X in k CR ·φ ·Xave ·( vCO2 − veq)

(3)

The calciner requires high CO2 concentration at the exit and temperatures as low as possible, to reduce the energy penalty associated to the heat up of the entering streams. This fact may affect the sorbent regeneration reaction. In this study, the model developed by Martı ́nez et al.13 is used to calculate the calcination efficiency. It presents a simple but realistic kinetic description of the calcination reaction. The calcination efficiency is calculated with eq 6 ηCL =

fa,CL ⎛ 1 ⎞ ln⎜ 1 − f ⎟ ⎝ a,CL ⎠

(6)

where fa,CL is the fraction of particles with a residence time lower than that needed for full calcination in the calciner operating conditions. This parameter is determined with eq 7. ⎛ −t * ⎞ CL ⎟ fa,CL = 1 − exp⎜ n / ̇ ⎠ ⎝ Ca nCa

(7)

And the time required for the full calcination is calculated from eq 8. * = tCL

3·X in k CL(Ceq,CL − CCO2)

(8)

where Xin, in the calciner, is the average CaCO3 molar fraction of the total flow entering the calciner resulting from mixing FR and F0. The kinetic constant of calcination, kCL, is determined

(2) 11336

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with the kinetic parameters kc0 = 20.5 × 106 m3/kmol·s and Ea = 112 × 103 kJ/kmol obtained by Martı ́nez et al.14 over the temperature range of 820−910 °C. Since the calciner is assumed to operate at 950 °C, a sensitivity analysis was carried out to evaluate the suitability of this approximation. A significant variation on the kinetic constant, kCL, barely modifies the fa,CL and the calcination efficiency because tCL * is various orders of magnitude lower than the residence time in the calciner. The maximum average capture capacity, Xave, takes into account that both carbonation and calcination may be partial, understanding as partial carbonation that in which Xave is not reached. Figure 2 shows a pie chart illustrating the fractions of

calcined, r0, since this is a particular fraction completely composed of CaCO3 as it has never been calcined. fCR =

X max − r0 Xave

(9)

The parameter f CL, eq 10, is the fraction of Xmax that is calcined and thus, it begins a new cycle. This parameter, f CL, is the calcination efficiency of the whole cycle. In the ordinary configuration, where the maximum fraction of CaCO3 is obtained at the calincer entrance (Xmax = Xin,R) it is also the efficiency of the calciner, ηCL, calc. But if a cyclonic preheater is added to the system, f CL might be different from the calcination efficiency of the calciner, ηCL, calc, since the sorbent might be also partially calcined in some steps of the cyclonic preheater before entering the calciner. fCL =

X max − X in,R X max

+

X in,R X max

ηCL,calc

(10)

A mass balance in the calcination region of the system (the calciner and the eventual steps of the cyclonic preheater in which calcination takes place) is carried out to define the age distribution of the particles, rN. The fraction of sorbent that has never been calcined, that is the makeup sorbent that is not calcined, r0, is calculated with eq 11. Taking the calciner output as a reference, F0 + FR represent the total molar flow of calcium compounds, CaCO3 and CaO. The molar flow of sorbent that has never been calcined is the sum of the fresh limestone that has not been calcined in the calciner, F0(1 − ηCL, calc), and the fraction of sorbent circulating in the cycle that had never been calcined at the entrance of the calcination region and has not been calcined in it as well, FRr0(1 − f CL).

Figure 2. Pie chart of an average sorbent volume in the calcium looping cycle.

an average sorbent particle, that are the key parameters in the Xave calculation. In this way, Xave is the average capture capacity of the sorbent, that is, the upper limit fraction of CaCO3. As carbonation is partial, Xmax is the maximum fraction of CaCO3 actually achieved in the cycle and it is lower than Xave. With the ordinary configuration, Xmax is obtained at the carbonator exit. When a cyclonic preheater is added to the system, carbonation may also continue in some steps of the cyclonic preheater and then, Xmax is the fraction of CaCO3 at the exit of the last step in which carbonation has taken place. As calcination may also be partial, the sorbent may never be completely calcined and, thus, there is a minimum fraction of CaCO3 in the cycle, Xmin, that remains at the calciner exit in any configuration. The calculation of the age distribution of the particles, rN, that is, the fraction of sorbent that has suffered N calcinations, is needed to calculate Xave. For this purpose, the carbonation and calcination efficiencies are assumed to affect sorbent of different ages to the same extent. Two parameters, f CR and f CL, have to be defined. As shown in Figure 2, the parameter f CR accounts for the maximum fraction of carbonated sorbent in the whole cycle, Xmax, plus the corresponding fraction of inert sorbent that ages although it is never carbonated, eq 9. The addition of the inert fraction is necessary for the calculation of rN due to the defintion of the capture capacity, XN. It should be noted that, in complete carbination/calcination cycles, the whole particle ages, not only the volume fraction that has been completely carbonated. The maximum fraction of carbonated sorbent and the average maximum capture capacity, have to be corrected with the fraction of sorbent that has never been

r0 = =

F0(1 − ηCL,calc) + FR ·r0(1 − fCL ) F0 + FR

→ r0

F0(1 − ηCL,calc) F0 + FR ·fCL

(11)

Following the same reasoning, the fraction of sorbent in its first cycle, r1, may be calculated with eq 12. The molar flow of sorbent that has been calcined once at the exit of the calcination region is the sum of the fresh limestone that has been calcined in the calciner, F0·ηCL,calc; the fraction of sorbent circulating in the cycle that had never been calcined at the entrance of calcination region but is calcined in it, FR·r0·f CL and the fraction of sorbent circulating in the cycle that belongs to r1 at the entrance of the calcination region, but is not calcined once again either because it is still CaO or due to the incomplete calcination, FR·r1(1 − (Xmax|N=1/X1)f CL). It should be highlighted that the quotient Xmax|N/XN keeps constant and equal to f CR for every fraction rN, with the exception of r0, since carbonation and calcination affect sorbent of different ages equally, as mentioned before. 11337

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Figure 3. Heat transfer and temperature differences in the (a) single step, (b) two steps, and (c) three steps cyclonic preheater.

r1 =

= =

⎡ F0·ηCL,calc + FR ⎢⎣r0·fCL + r1 1 −

(

X max N = 1 fCL X1

)⎤⎦⎥

avoided with an adequate design of the cyclones. If the system is able to operate with the fines, the attrition should not imply a problem.18 Complete desulfurization is supposed in both the carbonator and the calciner since the CaO/SO2 ratio is extremely high.19 The presence of SO2 in the flue gas may decrease the sorbent capture capacity due to the formation of a CaSO4 layer in the particles. The extent of this deactivation depends on the limestone and on the SO2 concentration. As well, carbonation reaction changes the reactivity of the sorbent regarding sulfation.20−22 This fact would affect the degradation curve and also the age distribution of the particles, and then, the average capture capacity. However, the effect of the sulfation on this parameter is not taken into account since it is still under study and beyond the scope of this work. Three different cyclonic heat exchangers comprising one, two and three steps were modeled. Each step is a mixing heat exchanger, although the whole cyclonic heat exchanger system is counter current flow. As mentioned before, carbonation or calcination may happen in the cyclonic heat exchanger. The calculation of the inventory in this device is therefore required as an input for the model. Li et al.23 studied the particle holdup and the average residence time in the cyclone of a circulating fluidized bed and concluded that the particle inventory in the cyclone is 10−40% of the corresponding bed material in the riser. It was initially assumed a 40% of the inventory in the carbonator since it is the most unfavorable case, and a parametric analysis was also carried out to assess the effect of this parameter on the operation of the system. The flow inside a cyclone is characterized by high swirl and turbulent motion that provides excellent heat transfer between gas and solids. Thus, the exit temperature of both solids and gas was assumed to be the same according to the results obtained by Mujumdar et al.9

F0 + FR F0·ηCL,calc + FR ⎡⎣r0·fCL + r1 1 − fCR ·fCL ⎤⎦

(

F0·ηCL,calc

)

F0 + FR + FR ·r0·fCL

F0 + FR ·fCR ·fCL

(12)

Following the same reasoning for the fraction of sorbent that has been calcined twice, eq 13, rN may be calculated with eq 14. r2 = =

rN =

FR ⎡⎣r1·fCR ·fCL + r2(1 − fCR ·fCL )⎤⎦ F0 + FR

(F0·ηCL,calc + FR ·r0·fCL )FR ·fCR ·fCL 2 (F0 + FR ·fCR ·fCL )

(13)

⎛ ηCL,calc ⎞ N−1 N ⎜F · + FR ·r0⎟FRN − 1·fCR ·fCL ⎝ 0 fCL ⎠ N (F0 + FR ·fCR ·fCL )

(14)

According to this, the maximum average capture capacity of the sorbent may be calculated with eq 15 ∞

Xave =

∑ rNXN N =1

⎛ F0(1 − fCL ) ⎞ f ⎟ = ⎜⎜F0ηCL,calc + FR F0 + FR fCL CL ⎟⎠ ⎝ ⎡ a1f12 ⎢ ⎢⎣ F0 + FR fCR fCL (1 − f1 )

b⎤ + + ⎥ F0 + FR fCR fCL (1 − f2 ) F0 ⎥⎦

3. RESULTS AND DISCUSSION Four different configurations were simulated with the model presented in the previous section: an ordinary configuration with no cyclonic preheater, and three configurations with a cyclonic heat exchanger comprising one, two, and three steps. A CaCO2 ratio of 5 and a purge of the 3% of the solids leaving the calciner are assumed since, in the ordinary configuration, minimum coal consumption was obtained in this operating point. Chemical Reastions in the Cyclonic Preheater. Carbonation reaction takes place in the cyclonic preheater.

a 2f22

(15)

where a1 = 0.1045, f1 = 0.9822, a2 = 0.7786, f 2 = 0.7905 and b = 0.07709, are the coefficients fitted by Rodrı ́guez et al.15 in the deactivation curve of the limestone, XN, proposed by Li et al.16 Attrition is neglected in this model although this phenomenon may be significant for some types of limestone.17 The elutriation of the sorbent and the loss of material could be 11338

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nevertheless compensated by increasing either the makeup flow (6.6% higher) or the CaO/CO2 ratio from 5 to 5.34; or having 5.5% more inventory in the carbonator. Coal and Oxygen Consumption. The temperature increase of the particles entering the calciner produces a reduction of the energy requirements in this reactor. Consequently, the coal and oxygen needs diminish. According to the model, the required mass ratio of coal consumed in the calciner to CO2 captured in the carbonator in the ordinary configuration is 0.45 kgcoal/kgCO2, whereas it is a 11.1% lower (0.40 kgcoal/kgCO2) with the system comprising a single step cyclonic heat exchanger, and a 13.3% lower (0.39 kgcoal/kgCO2) when comprising two steps. A reduction on the avoided CO2 cost of around 5.2−6.2 $/tCO2 related to the coal savings may be achieved, assuming a coal price of 104.1 $/tcoal.24 Also the oxygen needs are reduced in a similar proportion, from 1.03 kgO2/kgCO2 for the ordinary configuration to 0.92 kgO2/kgCO2 with the single step cyclonic preheater and 0.89 kgO2/kgCO2 with the two steps one. Inventory in the Cyclonic Preheater. As mentioned before, the cyclone inventory may vary between a 10% and a 40% of the reactor inventory,23 although in the previous results a 40% was assumed. A parametric analysis was carried out to quantify the error this assumption entails. Results show no significant influence of the inventory in the operation of the cyclonic preheater. Energetic Efficiency. The energetic efficiency of the cycle may be calculated as the ratio of the heat flow available for integration in a steam cycle to the thermal heat provided by the coal in the calciner. The results from the simulation show that the system with a cyclonic heat exchanger presents almost the same energetic efficiency as that in the ordinary configuration since there is no significant energy penalty associated with this device. Calciner Temperature. A conservative temperature in the calciner (950 °C) was assumed to facilitate complete calcination, although a lower temperature would reduce the energy required to heat up the inlet flows. However, a slight temperature reduction may be compensated by using higher solid inventories, according to the calcination model. A parametric analysis was carried out to analyze this question. Figure 4 shows the effect of the calciner temperature for the ordinary configuration and for the system with a cyclonic preheater. As expected, the ratio of coal to CO2 captured in the carbonator diminishes as the temperature is reduced. The inventory required in the calciner to achieve complete calcination is represented as the ratio of the calciner inventory in each case to the carbonator inventory in the ordinary configuration. The inventory required in the calciner to achieve complete calcination exceeds the carbonator inventory for temperatures lower than 905 °C (mCL/mCR > 1). Even operating at this low temperature, the coal consumption ratio, 0.41 kgcoal/kgCO2 in the ordinary configuration, may be reduced a 9.8% single step cyclonic heat exchanger and a 11.3% with a two steps one. CO2 Emissions. The coal burned in the calciner to carry out the regeneration of the sorbent implies an increase of the CO2 produced in the system that adds to that generated in the power plant. Even if it is completely captured and the emission to the atmosphere is avoided, it has to be transported and stored. The coal savings related to the addition of a single

Although high temperatures may be reached in this device, CO2 concentration is too high to enable calcination. As a consequence of the carbonation taking place in the cyclones, a fraction of the CO2 produced in the regeneration reactor is captured by the sorbent in the cyclonic heat exchanger and redirected to the calciner. A certain amount of CO2 (about 3% of the CO2 leaving the calciner) remains therefore trapped moving from the calciner to the cyclonic heat exchanger, as a gas, and from the cyclonic heat exchanger to the reactor, as part of the solid sorbent. The carbonation exothermic reaction in the cyclonic preheater increases the temperature of the solids entering the calciner. However, this energy is subsequently required for the regeneration of the extra CaCO3 generated in the cyclonic heat exchanger. The heat produced in the cyclones due to the carbonation is therefore consumed in the reactor due to the calcination of this fraction of the material. It represents approximately 40 MW in one step, as shown in Figure 3. Heat Transfer in the Cyclonic Preheater. Figure 3 shows the heat exchanged and the temperatures in each step of the cyclonic heat exchangers. The continuous arrows represent the gas flow leaving the calciner (Gi) and the dotted arrows represent the solid flow leaving the carbonator (Si). Figure 3a corresponds to a single step cyclonic heat exchanger. The heat absorbed by the solids is the sum of the energy lost by the gas and the heat generated in the carbonation reaction (gray line). The final temperature of the solids is therefore higher than that assuming no carbonation reaction. An equivalent temperature that accounts only for the heat transferred from the gas to the solids may be calculated (dotted gray line). In the single step case, an equivalent temperature of 725 °C may be reached. That means a heat transfer from gas to solids of 105 MW. Figure 3b corresponds to a two steps cyclonic preheater. The solids temperature increase is similar in both steps. In step B (streams S1 and S2 for the solids and G0 and G1 for the gas), the temperature difference between gas and solids is higher than in step A (streams S0 and S1 for the solids and G1 and G2 for the gas), but in the latter, the heat from the carbonation reaction compensates for the lower temperature difference. Carbonation in step B is negligible since the sorbent capture capacity almost reaches its maximum in step A. the equivalent temperature is 11 °C higher than in the single step case, and the heat transfer from gas to solids is 127 MW. The three steps cyclonic preheater operates similarly, Figure 3c. Carbonation is only appreciable in step A (streams S0 and S1 for the solids and G2 and G3 for the gas) where the active sorbent is almost exhausted in step C (streams S2 and S3 for the solids and G0 and G1 for the gas), the temperature difference between gas and solids is higher and the increment of the temperature is more pronounced than in step B (streams S1 and S2 for the solids and G1 and G2 for the gas). The equivalent temperature is slightly higher than in the 2 steps cyclonic preheater, only 2 °C, and the heat transfer from gas to solids is 133 MW. The addition of a third step in the cyclonic preheater only presents minor improvements. Average Activity of the Sorbent. The carbonation reaction taking place in the cyclonic heat exchanger has a negative effect on the average capture capacity of the sorbent since the extent of carbonation during each cycle increases. A higher fraction of sorbent is carbonated and calcined each cycle, and then, the sorbent degrades more rapidly. In the case of one step heat exchanger, Xave is reduced from 21.6%, for the ordinary capture configuration, to 20.5%. This effect may be 11339

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kCL kCR N nCa ṅCa ṅCO2 rN t*CL tCR * vCO2 veq Xave Xmax

Figure 4. Effect of the calciner temperature on the coal to CO2 captured in the carbonator ratio and on the calciner to carbonator inventory ratio for the ordinary configuration and that containing a single step cyclonic preheater.

Xmin Xin

cyclonic preheater imply a 5.3% (6.5% with a two steps one) decrease of the CO2 produced in the whole system and, thus, also a reduction of the transport and storage costs. As a general conclusion, the addition of a cyclonic preheater in the calcium looping system reduces the coal and the oxygen needs, as well as the extra CO2 generated in the calciner. An economic analysis of the complete system comprising not only the capture looping but also the steam cycle where the heat flows are integrated is required to determine whether a single step cyclonic is better than a two step one.

■

Xin,R

XN ηCL ηCR ηCL,calc τCR

AUTHOR INFORMATION

Corresponding Author

*(A.M.) Phone: (+34) 976 762 196; fax: (+34) 976 732 078; email: [email protected].

φ

■

Notes

The authors declare no competing financial interest.

■

Kinetic constant of CaCO3 calcination, (m3/kmol·s) Surface carbonation rate constant, (s−1) Number of cycles accomplished by a volume of sorbent Molar inventory of CaO and CaCO3 (kmol) Inlet molar flow of CaO and CaCO3 (kmol/s) Molar flow of CO2 entering the carbonator, (kmol/s) Age distribution of particles, fraction of particles that has accomplished N carbonation/calcination cycles Time for full calcination under calciner operating conditions, (s) Time for maximum fast kinetic-stage carbonation, Xave, (s) Average volume fraction of CO2 Volume fraction of CO2 in equilibrium conditions Average maximum capture capacity of the sorbent Maximum molar fraction of CaCO3 with respect to CaO and CaCO3 in the cycle Minimum molar fraction of CaCO3 with respect to CaO and CaCO3 in the cycle. Inlet molar fraction of CaCO3 with respect to CaO and CaCO3 Molar fraction of CaCO3 with respect to CaO and CaCO3 entering the calciner from the cyclonic preheater (from the carbonator in the ordinary configuration). Capture capacity of a fraction of sorbent that has accomplished N carbonation-calcination cycles Calcination efficiency, fraction of CaCO3 calcined Carbonation efficiency, fraction of CO2 captured Calcination efficiency in the calciner, fraction of CaCO3 calcined in the calciner Carbonator space time. Molar inventory of calcium compounds (CaO and CaCO3) per Molar flow of CO2, (s) Gas−solid contacting effectivity factor

REFERENCES

(1) Abanades, J. C.; Grasa, G.; Alonso, M.; Rodríguez, N.; Anthony, E. J.; Romeo, L. M. Cost structure of a postcombustion CO2 capture system using CaO. Environ. Sci. Technol. 2007, 41, 5523−5527. (2) Abanades, J. C.; Anthony, E. J.; Wang, J.; Oakey, J. E. Fluidized bed combustion systems integrating CO2 capture with CaO. Environ. Sci. Technol. 2005, 39, 2861−2866. (3) Donat, F.; Florin, N. H.; Anthony, E. J.; Fennell, P. S. Influence of high-temperature steam on the reactivity of CaO sorbent for CO2 capture. Environ. Sci. Technol. 2012, 46, 1262−1269. (4) Qin, C.; Liu, W.; An, H.; Yin, J.; Feng, B. Fabrication of CaObased sorbents for CO2 capture by a mixing method. Environ. Sci. Technol. 2012, 46, 1932−1939. (5) Valverde, J. M.; Perejon, A.; Perez-Maqueda, L. A. Enhancement of fast CO2 capture by a nano-SiO2/CaO composite at Ca-looping conditions. Environ. Sci. Technol. 2012, 46, 6401−6408. (6) Valverde, J. M. Ca-based synthetic materials with enhanced CO2 capture efficiency. J. Mater. Chem. A. 2013, 1, 447−468. (7) Epple, B. 2010. Method and arrangement for separation of CO2 from combustion flue gas. U.S. Patent. 12/584,519, filed September 8, 2009, and issued April 8, 2010. (8) Martínez, A.; Lara, Y.; Lisbona, P.; Romeo, L. M. Energy penalty reduction in the calcium looping cycle. Int. J. Greenhouse Gas Control 2012, 7, 74−81. (9) Mujumdar, K. S.; Ganesh, K. V.; Kulkarni, S. B.; Ranade, V. V. Rotary cement kiln simulator (RoCKS): Integrated modeling of preheater, calciner, kiln and clinker cooler. Chem. Eng. Sci. 2007, 62, 2590−2607.

ACKNOWLEDGMENTS Financial support for A. Martı ́nez during her Ph.D. studies was provided by the FPU programme of the Spanish Ministry of Science and Innovation.

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NOMENCLATURE CCO2 Concentration of CO2 (kmol/m3) Ceq,CL Concentration of CO2 in equilibrium conditions, (kmol/m3) Ea Activation energy of kinetic constant for CaCO3 calcination, (kJ/kmol) F0 Molar flow of fresh CaCO3 entering the calciner, (kmol/s) fa,CL Fraction of particles in the calciner with a residence time lower than t*CL fa,CR Fraction of active sorbent reacting in the fast reaction regime in the carbonator f CL Maximum proportion of calcined sorbent in the cycle f CR Maximum proportion of carbonated sorbent in the cycle FR Molar flow of CaO and CaCO3 between the reactors, (kmol/s) kc0 Pre-exponential factor of kinetic constant of CaCO3 calcination, (m3/kmol·s) 11340

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Environmental Science & Technology

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dx.doi.org/10.1021/es401601k | Environ. Sci. Technol. 2013, 47, 11335−11341