Operation of a Mixing Seal Valve in Calcium Looping for CO2 Capture

Article pubs.acs.org/EF

Operation of a Mixing Seal Valve in Calcium 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 Gómez 15, 50018 Saragossa, Spain ABSTRACT: Calcium looping has attracted a great deal of attention among researchers investigating CO2 capture systems. Energy consumption in the regeneration reactor is one of the most important issues. However, as a high-temperature process, calcium looping enables an efficient heat recovery in the capture process itself. The objective of this study is to reduce the energy consumption in the calciner by increasing the temperature of the solids entering this reactor. A calcium looping system including a mixing seal valve is modeled and analyzed to determine the potential advantages this configuration entails. The influence of different seal valve aeration gases, carbonator inventories and solid circulation between reactors is assessed. The reduction of the fuel consumption when the mixing seal valve is aerated with flue gas tends to dilute the CO2 stream. When using CO2, the achievement of substantial energy savings may imply an important increase of the solid flows in the system. Aeration of the mixing seal valve with both gases, so that each gas aerates one exit, is also proposed to address these issues. Results show a significant potential in terms of coal and oxygen savings and reduction of the CO2 generated in the capture system.

1. INTRODUCTION Calcium looping is a promising technology for CO2 capture that has attracted a great deal of attention in the past 10 years.1−6 It makes use of calcium-based sorbents, mainly CaO, to separate the CO2 from the flue gas by means of a carbonation reaction, eq 1. Carbonated sorbent is regenerated by calcination in a separate reactor. A highly concentrated CO2 stream, ready for compression, transport, and storage, is then released in the calciner. CaO + CO2 ↔ CaCO3 + Q (1)

degradation related to sintering in the regeneration process reduces the capture capacity of the sorbent to a residual value.11 Several options such as thermal preactivation,12,13 hydration, 14−16 doping, and chemical pretreatment17−19 are proposed to enhance the activity of the sorbent. Another key parameter is the important energy requirement of the process, which is the main focus of this study. Fuel consumption in the calciner is mainly related to the regeneration reaction. However, an extra amount of energy is also needed to heat up the particles coming from the carbonator, which operates at up to 300 °C below the calciner temperature. An intense sorbent flow circulating between reactors is required to achieve an adequate CO2 capture capacity. The heating up of the solids therefore entails considerable energy consumption. The heat recovery from the gaseous and solid streams leaving the calciner to increase the temperature of the particles entering the calciner has been proposed to reduce the energy consumption in this reactor.20,21 Two different configurations aligned with this target, which include the internal heat integration of both streams ́ respectively, were presented by Martinez et al.21 and initial analysis showed promising results specially when using the gaseous stream. The configuration containing a cyclonic preheater, to heat up the entering solids with the gas leaving the calciner, was also tested as a precombustion capture solution obtaining good results.22 A further assessment of the cyclonic preheater configuration concluded that it may reduce the coal and oxygen specific consumptions, as well as the extra CO2 generated in the calciner.23 The mixing seal valve configuration was proposed to facilitate the heat transfer between the solids leaving the calciner and those entering this reactor. Initial results showed that the important solid flows needed to compensate for the particles

Carbonation is an exothermic reaction that takes place at around 650 °C, whereas calcination is endothermic and takes place at temperatures up to 950 °C, for the CO2 partial pressures at which the reactors operate. Energy for the regeneration is provided by oxy fuel combustion in the calciner to avoid the dilution of the CO2 with the N2 in the air. Circulating fluidized beds have been extensively proposed as reactors to enhance the gas−solid contacting, to achieve a homogeneous temperature, and to facilitate circulation of the solids along the cycle.6,7 Calcium looping has several advantages compared to other CO2 capture technologies:2,8−10 i. The sorbent is relatively cheap and wide available. As a high-temperature sorbent, it enables an efficient heat recovery in the capture process itself or in a steam cycle of a power generation system. The energy penalties of the CO2 separation stage may be therefore reduced to some extent. ii. The use of circulating fluidized beds, a mature technology at large scale, may facilitate the scale-up of the process. iii. It presents excellent opportunities of integration with cement plants that may potentially allow for decarbonisation of both power and cement industries. However, the competitiveness of this technology is still conditioned to the improvement of some key issues. Sorbent © 2014 American Chemical Society

Received: December 18, 2013 Revised: January 30, 2014 Published: February 17, 2014 2059

dx.doi.org/10.1021/ef402487e | Energy Fuels 2014, 28, 2059−2068

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Table 1. Hypothesis Summary23

mixing may counteract the effect of the temperature gradient reduction.21 The achievement of adequate capture efficiencies, with the formerly proposed carbonation model24 used in that study, was restricted to the increase of the CaO/CO2 ratio, that entails the intensification of the solids circulation. However, advanced models for carbonation24,26 introduce the influence of other operational parameters that enhance the extent of reaction, such as the sorbent inventory in the reactor. The dependency between CO2 capture efficiency and solid flow is therefore reduced. Advanced models are here applied for carbonation and calcination reactions. The effect of the aeration gas in the mixing seal valve is evaluated. Seal valves aeration is usually carried out using the same gas as that used in the corresponding bed to avoid gas mixing. In this case, the single seal valve feeds two beds containing different gases and the adequate aeration gas has to be determined. The mixing seal valve configuration is modeled and simulated, and the influence on relevant parameters, such as fuel and oxygen specific consumption, is analyzed in this study.

Power Plant 500 MWe 40% electrical efficiency 20% oxygen excess Coal composition and low heating value: 72.04%db C; 4.08%db H; 1.67%db N; 0.65%db S; 7.36%db ash; 8.1% H2O 25,372 kJ/kg Tflue gas = 180 °C Carbonator 650 °C Carbonation model, mainly based on active space time:25−27

ηCR = k CR φfa,CR τCR Xave.( vCO2 − veq)

⎛ −t * ⎞ RC ⎟ fa,CR = 1 − exp⎜ ̇ ⎠ ⎝ nCa /nCa

2. MODEL DESCRIPTION A system comprising the calcium looping process for CO2 capture with a mixing seal valve, Figure 1, is modeled, simulated, and analyzed

* = tCR

Xave. − X in k CR φXave.( vCO2 − veq)

τCR =

nCa nCO ̇ 2

Maximum carbonation efficiency limited only by equilibrium Calciner 950 °C Calcination model:28

ηCL =

⎛ −t * ⎞ CL ⎟ fa,CL = 1 − exp⎜ ̇ ⎠ ⎝ nCa /nCa

Figure 1. Diagram of the calcium looping system with a mixing seal valve. to gain knowledge about its operating features and to evaluate the potential advantages over the ordinary configuration. The model includes a 500 MW power plant that generates the flue gas and the CO2 capture system. The latter is mainly composed by the carbonator, where the gas is captured; the calciner, where the sorbent is regenerated; and the mixing seal valve, where the solids from carbonator and calciner exchange heat and are recycled to both reactors, preserving the pressure loop. The main hypotheses of the model are summarized in Table 1 and may be found elsewhere.23 In that paper, 23 we modeled the ordinary configuration and a configuration with a cyclonic preheater. Despite the assumptions regarding the distinctive equipment, i.e., the cyclonic preheater and the mixing seal valve, the models are accordant. 2.1. Average Capture Capacity in the Mixing Seal Valve Configuration. One of the differences in this model is the calculation of the age distribution of the sorbent, rN, that is required to estimate its average capture capacity, Xave., eq 2.

* = tCL

∑ rNXN N=1

3X in k CL(Ceq − CCO2)

Coal composition and low heating value: 72.04%db C; 4.08%db H; 1.67%db N; 0.65%db S; 7.36%db ash; 8.1% H2O 25,372 kJ/kg 60%v entrance oxygen fraction Tpurge = 200 °C TCO2 = 180 °C General Hypotheses Attrition is neglected Complete desulfurization

ordinary configuration, this reaction only takes place in the regeneration reactor. However, solids in the mixing seal valve contain a fraction of carbonated sorbent and temperature may be high enough to promote calcination. In this configuration, sorbent aging may therefore take place in the mixing seal valve besides the calciner. A typical characteristic of the mixing seal valve configuration regarding the age distribution of the sorbent is that this parameter varies from one stream to another due to the distribution of mixed

∞

Xave. =

fa,CL ln⎡⎣1/(1 − fa,CL )⎤⎦

(2)

Each fraction of sorbent is assumed to age, that is, to reduce its capture capacity from XN to XN+1, whenever it is calcined. In the 2060


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The mass balance in the mixing seal valve is divided in two parts corresponding to each exit to both reactors. The reasoning to define the equations is the same as in the calciner. It should be notice that two different calcination efficiencies are assumed for both exits. This hypothesis is required in case different gases are used for aeration and then, the fraction of sorbent calcined in the mixing seal valve is different for streams S4 and S5. Equations 10−12 refer to the exit directed to the carbonator (S4), and eqs 13−15 are related to the stream entering the calciner (S5).

particles from the seal valve to both reactors. Sorbent population in streams S3 and S4, exit and entrance of the carbonator, has the same rN because aging requires calcination reaction and it is unfeasible in the carbonator, eq 3. As well, sorbent in streams S2 and S6, calciner exit and purge, has also the same rN since no segregation is assumed in the calciner and both streams are consider having the same composition, eq 4. The remaining streams (S1 and S5) may have different age distributions. The age distribution in stream 1, the make-up flow of fresh sorbent, is given by eq 5. rN |S3 = rN |S4 = rN |carb

(3)

rN |S2 = rN |S6 = rN |calc

(4)

rN |carb =

XCaCO3|Si − r0|Si Xave.|Si

+

(6)

Fcarb + Fcalc

(12)

Fcarb + Fcalc

Fcarb + Fcalc

rN |calc = (7)

+

(13)

Fcarb[r0|carb ηCL|S5 + r1|carb (1 − fCR |carb ηCL|S5)]

+

Fcalc[r0|calc ηCL|S5 + r1|calc (1 − fCR |calc ηCL|S5 )] Fcarb + Fcalc

(14)

Fcarb[rN − 1|carb fCR |carb ηCL|S5 + rN |carb (1 − fCR |carb ηCL|S5 )] Fcarb + Fcalc

Fcalc[rN − 1|calc fCR |calc ηCL|S5 + rN |calc (1 − fCR |calc ηCL|S5 )]

∀N>1

Fcarb + Fcalc (15)

Instead of using an explicit expression for the sorbent age distribution, rN|i, and consequently for the average capture capacity, Xave.|i, the age distribution is calculated including in the model eqs 7−15. The average capture capacity is estimated by means of a numerical sum, eq 2, for N = 1−500, that is accurate enough with a negligible error, lower than 10−4%. 2.2. Mixing Seal Valve Assumptions. The objective of implementing this device is to transfer heat between particles, to increase the temperature of the solids entering the calciner, while providing the pressure seal required in the circulating fluidized beds. The indirect heat transfer between the two independent seal valves feeding both reactors was previously proposed as an ideal case.21 The heat exchange between 930 and 650 °C in a closed loop with a severe atmosphere of particles makes that system unfeasible since best available technology is not developed enough to properly carry out this process. To overcome this technical limitation, the use of a single seal valve (mixing seal valve) is proposed. The mixing of carbonated and regenerated sorbent in the seal valve is an unavoidable disadvantage of this configuration since a completely regenerated flow is preferred in the carbonator. Different characteristics of carbonated and regenerated particles could lead to segregation. This phenomenon could be used to separate the particles and reduce to some extent the mixing of carbonated and regenerated sorbent by means of a suitable design of

+ fCR |S5 (1 − ηCL,calc)]}/(F0 + Fcalc) (F0 + Fcalcr0|S5 )ηCL,calc + Fcalcr1|S5(1 − fCR |S5 ηCL,calc) (8)

Following the same reasoning, the fraction of sorbent that has been calcined N > 1 times may be calculated with eq 9. Fcalc[rN − 1|S5fCR |S5 ηCL,calc + rN |S5(1 − fCR |S5 ηCL,calc)]

∀N>1

Fcarb[rN − 1|carb fCR |carb ηCL|S4 + rN |carb (1 − fCR |carb ηCL|S4 )]

Fcarbr0|carb (1 − ηCL|S5) + Fcalcr0|calc (1 − ηCL|S5 )

r1|calc =

(F0 + Fcalcr0|S5)(1 − ηCL,calc)

(11)

Fcarb + Fcalc

r0|calc =

F0 + Fcalc

F0 + Fcalc

Fcarb + Fcalc

Fcalc[rN − 1|calc fCR |calc ηCL|S4 + rN |calc (1 − fCR |calc ηCL|S4 )]

(F0r0|FS + Fcalcr0|S5 )(1 − ηCL,calc)

F0 + Fcalc

Fcalc[r0|calc ηCL|S4 + r1|calc (1 − fCR |calc ηCL|S4 )]

∀N>1

r1|calc = {(F0r0|FS + Fcalcr0|S5 )ηCL,calc + Fcalcr1|S5 [(1 − fCR |S5 )

rN |calc =

Fcarb + Fcalc +

Following the same reasoning, the fraction of sorbent that has been calcined once at the calciner exit may be calculated with eq 8. The molar flow of sorbent in its first cycle at the calciner exit is the sum of the make-up flow that has been calcined in this reactor, F0r0|FSηCL,calc; the sorbent coming from the mixing seal valve that had never been calcined before but it has been calcined in the calciner, Fcalcr0|S5ηCL,calc; the sorbent coming from the mixing seal valve in its first cycle that cannot to be calcined since it is already CaO, Fcalcr1|S5(1 − f CR|S5); and the sorbent coming from the mixing seal valve in its first cycle that is susceptible to age one cycle if it is calcined but it is not calcined in the calciner, Fcalcr1|S5 f CR|S5(1 − ηCL,calc).

=

(10)

Fcarb[r0|carb ηCL|S4 + r1|carb (1 − fCR |carb ηCL|S4 )]

r1|carb =

Separated mass balances are required in calciner and mixing seal valve to determine the sorbent age distribution in each stream. Starting with the calciner, the fraction of sorbent leaving this reactor that has never been calcined may be calculated with eq 7. F0 + Fcalc is the total molar flow of calcium compounds, CaO and CaCO3, leaving the calciner. The molar flow of sorbent that has never been calcined is the sum of the make-up flow that has not been calcined in this reactor, F0r0|FS(1 − ηCL,calc), and the fraction of sorbent coming from the mixing seal valve that had never been calcined before and has not been calcined in the calciner, Fcalcr0|S5(1 − ηCL,calc).

=

Fcarb + Fcalc

(5)

As a consequence, streams with different age distribution have different average capture capacities, Xave.|Si. Regarding the proportion of carbonated sorbent, f CR, even streams with equal age distribution may have different f CR|Si, eq 6. This is the case of streams S3 and S4 that have the same age distribution, rN|carb, and average capture capacity, Xave.|carb, but the fraction of carbonated sorbent is higher in stream S3, at the carbonator exit, than in stream S4, at the entrance of the reactor.

r0|calc =

ncarb(Fcarb + Fcalc) Fcarbr0|carb (1 − ηCL|S4 ) + Fcalcr0|calc (1 − ηCL|S4 )

=

⎧1 ∀ N = 0 rN |S1 = rN |FS = ⎨ ⎩0 ∀ N > 0

fCR |Si =

ncarb[Fcarbr0|carb (1 − ηCL|S4 ) + Fcalcr0|calc (1 − ηCL|S4 )]

r0|carb =

F0 + Fcalc (9) 2061


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A parametric study was carried out modifying the fraction of solids directed to each reactor from the mixing seal valve and the sorbent inventory in the carbonator. The solids distribution from the mixing seal valve to the reactors was varied from 10% to 90% of particles directed to the carbonator, ncarb. Sorbent inventories in the carbonator (400−900 kmol) higher than that in the ordinary configuration (333.5 kmol) were required to counterbalance the effect of the mixing of carbonated and regenerated sorbent, and the possible sorbent extra degradation.

the seal valve. As there are no previous experimental analyses in this regard and an accurate assessment of the sorbent segregation in the mixing seal valve, which would require a final design of the sealing device, is beyond the scope of this study, complete mixing of solids in the seal valve is assumed as a conservative hypothesis. The mixing seal valve has two entrances in the upper zone of the standpipe and two exits through both recycle chambers that feed carbonator and calciner, Figure 2. Fluidization of the particles in the

3. RESULTS AND DISCUSSION 3.1. Temperature in the Mixing Seal Valve When Aerating with a Single Gas. As expected, the mixing of solids in the seal valve increases the temperature of the particles entering the calciner, Figure 3. This effect is enhanced as the

Figure 2. Sketch of the mixing seal valve. valve, specifically the aeration in the recycle chambers, is needed for the solids circulation.29 Aeration in both exits is independent to permit a different distribution of solids between carbonator and calciner. The fraction of gas directed to each exit is therefore supposed to be in accordance with the distribution of solids between reactors. The flue gas from the power plant or the highly concentrated CO2 stream may be used for the seal valve aeration, Figure 1. The use of both gases at the same time so that each gas aerates its corresponding exit is also analyzed in this study. In this case, both gases are assumed to keep unmixed. It should be noticed that the fraction of gas which circulates upward the standpipe may be neglected since, in practice, most of the aeration flow goes through the recycle chamber to the reactor.29−31 As mentioned before, carbonation or calcination may take place in the mixing seal valve. The same models as those used in carbonator and calciner, Table1, are implemented in this case. When the same gas aerates both recycle chambers all particles react to the same extent. When different gases are used in both recycle chambers, each fraction of sorbent get into contact with a different atmosphere since, as mentioned above, aeration gases keep unmixed.29−31 This fact entails different conversion for both the streams leaving the mixing seal valve. Carbonated sorbent in the seal valve could suffer recarbonation when it is aerated with highly concentrated CO2. This fact would entail sorbent reactivation increasing the average capture capacity.32 Although this phenomenon could improve the operation of the system, it is not applicable in this study since particles leaving the carbonator are only partially carbonated, and fast reaction stage should come to an end before recarbonation is accomplished. Moreover, the CO2 partial pressure evolves to values lower than that required for the re-carbonation process.32 2.3. Calculation. The ordinary configuration modeled and ́ simulated by Martinez et al.23 is used as a base case for the purpose of comparison. As mentioned before, both models are in accordance; the power plant that generates the flue gas is equal and the same kind of sorbent is used. In addition, the flow of fresh sorbent in the ordinary configuration, 42.1 kg/s, is assumed to be the same in the mixing seal valve configuration to set the absolute consumption of sorbent. Simulations using the flue gas or the highly concentrated CO2 as aeration gas in the mixing seal valve were carried out. The volumetric flow was assumed to be 10% of the flue gas volumetric flow entering the carbonator.33 The gas flow is distributed between the reactors in the same proportion as solids.

Figure 3. Temperature at the entrance of the calciner for different aeration gas in the mixing seal valve and solid inventories in the carbonator.

fraction of solids directed to the carbonator diminishes, since the amount of solids circulating between the mixing seal valve and the calciner becomes more important. In this case, the fraction of solids entering the mixing seal valve from the regenerator, at a high temperature (950 °C), is greater than that coming from the carbonator, at a lower temperature (650 °C). Hence, the temperature of the mixture of solids increases. Regarding the composition of the mixing seal valve aeration gas, Figure 3 shows an increase of the temperature of the solids entering the calciner when using highly concentrated CO2 with respect to that obtained using flue gas. The reason is associated with the extent of carbonation or calcination reactions taking place in the mixing seal valve. As mentioned before, the carbonation/calcination equilibrium depends on the temperature and on the CO2 partial pressure. Lower CO2 partial pressure in the flue gas promotes higher extents of calcination and lower extents of carbonation than the highly concentrated CO2 stream. As carbonation is exothermic and calcination is endothermic, higher temperatures are reached when aerating with the highly concentrated CO2 stream. Slight differences are found when modifying the sorbent inventory in the carbonator. 3.2. Chemical Reactions in the Mixing Seal Valve When Aerating with a Single Gas. At lower temperatures, for high values of ncarb, carbonation will occur in the mixing seal valve. The extent of carbonation reaction diminishes as the fraction of solids directed to the carbonator is reduced and the temperature increases. Calcination reaction appears when operating at high temperatures. The lower values of ncarb, the more intense calcination. If the flue gas is used for the aeration of the mixing seal valve the transition from one reaction to another occurs when solids are distributed in a balanced way, ncarb = 0.5, and the 2062


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temperature of the solids entering the calciner is 776 °C. Otherwise, if highly concentrated CO2 is used as aeration gas, the transition takes place at a higher temperature, 883 °C, when most of the solids are directed to the calciner, ncarb = 0.16−0.19. 3.3. Sorbent Mixing and Extra Degradation When Aerating with a Single Gas. Carbonation in the mixing seal valve implies sorbent degradation with no other benefits such as an increase of the CO2 capture efficiency. The extent of this reaction when the seal valve is aerated with highly concentrated CO2 is more important than that using flue gas. The average capture capacity is therefore lower in this case due to the extra degradation of the sorbent, Figure 4.

average capture capacity. The fraction of CaO and the solid circulation is therefore lower for nCa = 900 kmol than for nCa = 400 kmol. 3.4. Purity of the CO2 Stream to Storage When Aerating with a Single Gas. Dilution of CO2 in the ordinary configuration is mainly related to the composition of the fuel and the purity of the oxygen. Given the coal used in the model and assuming almost pure oxygen feeding, the fraction of CO2 leaving the calciner in the ordinary configuration is 92%. Using CO2 as aeration gas in the mixing seal valve has a negligible effect on the purity of the CO2 stream leaving the calciner, whereas the use of flue gas dilutes the atmosphere in the calciner and, hence, the leaving stream presents a lower purity, from 91%, for ncarb = 0.9, to 74%, for ncarb = 0.1. This fact becomes more important for low values of ncarb, when the flow of solids directed to the calciner is greater and consequently, the flow of flue gas from the mixing seal valve to the regenerator reactor is also significant. The dilution of the stream leaving the calciner will increase the penalty related to CO2 purification. 3.5. CO2 Capture Efficiency When Aerating with a Single Gas. The CO2 capture efficiency in the carbonator is assumed to reach equilibrium. Besides temperature, it depends on the gas composition, which results from the mixture of the fraction of flue gas directed to the carbonator and the gas entering this reactor from the mixing seal valve. The real CO2 capture efficiency is defined as the fraction of CO2 from the power plant that is captured in the system. In the ordinary configuration, the CO2 capture efficiency of the carbonator and the real CO2 capture efficiency are identical since the flue gas coming from the power plant and the gas entering the carbonator are the same stream. In the mixing seal valve configuration, a fraction of the flue gas from the power plant may be directed to the calciner via the seal valve and, as well, the fraction of gas entering the carbonator from the seal valve may have a composition different from that of the flue gas. Ordinary and mixing seal valve configurations are compared on the basis of the real CO2 capture efficiency since this parameter better represents the objective of the system. When aerating the mixing seal valve with flue gas, the capture efficiency is higher than that obtained in the ordinary configuration, ηCR = 93.01%. The increase is almost negligible when 90% of solids are directed to the carbonator, ηCR = 93.05%, and it becomes slightly more significant when ncarb = 0.1, ηCR = 93.64%. This augmentation with respect to the ordinary configuration is due to the fact that a fraction of the flue gas is directed to the calciner via the mixing seal valve aeration gas. The CO2 from this fraction of gas is not separated but is captured together with the rest of the substances in the flue gas, diluting the concentrated CO2 stream. If the highly concentrated CO2 stream is used as the aeration gas in the mixing seal valve, the capture efficiency is reduced with respect to the ordinary configuration. In this case, a fraction of gas leaving the calciner is directed to the carbonator through the mixing seal valve. The flow of gas entering this reactor increases and, although equilibrium is also reached since it is an assumption of the model, the amount of CO2 leaving the carbonator is greater. Hence, the real CO2 capture efficiency diminishes. The reduction is negligible when only a 10% of the solids in the mixing seal valve is directed to the carbonator, ηCR = 93%, and it is slightly higher when ncarb = 0.9, ηCR = 92.83%.

Figure 4. Average capture capacity at the entrance of the carbonator for different aeration gas in the mixing seal valve and solid inventories in this reactor.

Particles mixing reduces the fraction of regenerated sorbent, CaO, in the carbonator inlet stream. Higher sorbent inventories in the carbonator and solid flows circulating in the cycle are then required to achieve an adequate CO2 capture efficiency. Figure 5 shows the flow of solids circulating in the cycle, which

Figure 5. Flow of solids circulating in the cycle for different aeration gas in the mixing seal valve and solid inventories in this reactor.

is the sum of the solid flows entering each reactor. The more unequal particles distribution, the more significant solid flows. For high values of ncarb, the solid circulation increase is required to compensate for the reduction of the fraction of CaO entering the carbonator. On the other hand, the reduction of the fraction of particles directed to the carbonator leads to an increase of the solid flow, since there is a minimum sorbent flow in stream S4 required for the carbonation even when the fraction of CaO, XCaO, is near 100%. It should be also highlighted that for high sorbent inventory, nCa = 900 kmol, the requirement of regenerated sorbent, CaO, in the carbonator is lower than that for nCa = 400 kmol, since the sorbent inventory may compensate for the reduction of the 2063


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3.6. Coal and Oxygen Consumption When Aerating with a Single Gas. Figure 6 shows the specific fuel

lower than 100 mbar, even for the higher molar inventories and for fluidization velocities up to 7 m/s. As well, increasing the sorbent inventories above 900 kmol presents no significant improvements in the operation of the system. 3.9. Energy Efficiency When Aerating with a Single Gas. The energy efficiency of the cycle is defined as the ratio of the heat flow available in the system to the thermal heat provided by the coal combustion in the calciner. Results show slight differences in this parameter since there is no significant energy penalty associated with the mixing seal valve. 3.10. Aeration of Mixing Seal Valve with Both Gases. The aeration of the mixing seal valve with flue gas has the disadvantage of the CO2 dilution (yCO2,db = 74−91%) and, if highly concentrated CO2 is used, the problem is the important solid flows required (mS = 5086−49143 kg/s). A possible answer to this issue is the use of two different gases to recycle each solid stream leaving the mixing seal valve. The idea is to use the flue gas to recirculate the fraction of solids entering the carbonator, and the highly concentrated CO2 for the particles entering the calciner. Figure 7 shows the temperature of the solids leaving the mixing seal valve to the carbonator (S4) and to the calciner

Figure 6. Coal to CO2 captured from flue gas ratio for different aeration gas in the mixing seal valve and solid inventories in the carbonator.

consumption of the system, defined as the coal to CO2 captured from flue gas ratio. As expected, for low values of ncarb, when the temperature of the solids entering the calciner is high, the specific coal consumption diminishes. If mixing seal valve is aerated with flue gas, the coal consumption is always lower, 0.32−0.43 kgcoal/kgCO2, than that in the ordinary configuration, 0.45 kgcoal/kgCO2. Also the oxygen consumption may be reduced from 1.03 kgO2/kgCO2 for the ordinary configuration to 0.76−0.99 kgO2/kgCO2 when aerating the mixing seal valve with flue gas. This case has the advantages of keeping the average capture capacity similar to that in the ordinary configuration and increasing the temperature of the solid stream entering the calciner. However, the stream of captured CO2 is diluted by the seal valve aeration gas especially for low values of ncarb, as mentioned before. If the mixing seal valve is aerated with the highly concentrated CO2 stream, the sorbent degradation related to the carbonation in the seal valve makes it necessary an important increase of the solid inventory in the cycle that is more significant for high values of ncarb, when the temperature of the solids entering the calciner is lower. The energy consumption in the calciner is therefore higher in this case. The specific coal consumption is only lower than that in the ordinary configuration when most particles in the mixing seal valve are directed to the calciner and the temperature in this device is high enough to avoid carbonation and the consequent sorbent degradation, ncarb < 0.2. The coal to CO2 and the O2 to CO2 ratios may reach values as low as 0.39 kgcoal/kgCO2 and 0.91 kgO2/kgCO2 for ncarb = 0.1 and nCa = 900 kmol. 3.7. CO2 Emissions When Aerating with a Single Gas. Besides the CO2 generated in the power plant, the coal combustion in the calciner also contributes to the CO2 production. All this CO2 may be captured avoiding its emission to the atmosphere, but it has to be compressed, transported and stored increasing the related costs and the storage requirements. The coal savings obtained with the mixing seal valve may imply a reduction of the CO2 produced in the whole system of up to a 13.1%, when aerating with flue gas; or a 6.4%, when aerating with CO2, with respect to the ordinary configuration. 3.8. Sorbent Inventory in the Carbonator When Aerating with a Single Gas. It should be highlighted that the pressure drop in the carbonator remains at moderate levels,

Figure 7. Temperature at the entrance of the reactors when aerating with both streams for different solid inventories in the carbonator.

(S5). Streams have different temperatures since they contact a different gas. Thus, the equilibrium displacement, carbonation or calcination, or the extent of the reaction may change. As expected, the solids aerated with highly concentrated CO2 reach a higher temperature since this gas promotes a higher extent of carbonation, which is an exothermic reaction. The temperatures of the solids entering the calciner are similar to those obtained when aerating the whole mixing seal valve with the highly concentrated CO2 stream. Carbonation in the mixing seal valve takes place for high values of ncarb. When the temperature increases, for low values of ncarb, a fraction of the sorbent may be calcined. The temperature at which the transition between carbonation and calcination takes place varies for streams S4 and S5, since they contact gases with different CO2 concentration. The transition temperatures and the corresponding distributions of solids, ncarb, are equivalent to those obtained when the whole mixing seal valve is aerated with the same gas: 776 °C and ncarb = 0.5 for the solids entering the carbonator (S4) and 883 °C and ncarb = 0.16−0.19 for the solids entering the calciner (S5). As each fraction of the solids in the mixing seal valve is aerated with one of the gases previously studied, intermediate results are obtained regarding the sorbent degradation. Figure 8 shows the average capture capacity, which is lower than that in the ordinary configuration due to the degradation related to the 2064


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Figure 10. Coal to CO2 captured from flue gas ratio when aerating with both streams for different solid inventories in the carbonator.

Figure 8. Average capture capacity at the entrance of the carbonator when aerating with both streams for different solid inventories in the carbonator.

when using flue gas, for high values of ncarb. Specific coal consumptions lower than that in the ordinary configuration may be achieved if the sorbent inventory in the carbonator is high enough, nCa = 600−900 kmol. For these molar inventories, the coal to CO2 ratio ranges 0.37−0.44 kgcoal/kgCO2 and the oxygen specific consumption ranges 0.85−1.01 kgO2/kgCO2. This coal savings imply a reduction of the CO2 generated in the capture system of 0.7−8.7%. As in the previous analysis, the energy efficiency slightly varies if compared to that obtained in the ordinary configuration. 3.11. Results Summary. Table 2 shows some key operating points of the calcium looping cycle for the ordinary and the mixing seal valve configurations. Results from three tests are presented for the case of mixing seal valve aeration with flue gas. Two of them constitute the best CO2 purity and specific coal consumption respectively, and the other is an intermediate point in which solids flow in the cycle and CO2 purity reduction are moderate. Regarding mixing seal valve aeration with the highly concentrated CO2 stream and the use of both gases, two operating points confronting solids flow and specific coal consumption are presented in each case. Results from the previously proposed cyclonic preheater configuration23 are also summarized in Table 2 to compare both proposals. A reduction of the energy consumption in the calciner entails a decrease of coal and oxygen demand, as well as a diminution of the CO2 generated in the capture system. Besides the coal savings, reductions of the energy requirement related to the air separation unit (ASU) and the CO2 compression train are therefore obtained. Table 3 shows the energy consumptions per tonne of avoided CO2 from the power plant in the ordinary, cyclonic preheater and mixing seal valve configurations assuming that the ASU consumes 220 kWhe/tO2,34,35 and the CO2 compression energy requirement is typically 105 kWhe/ tCO2.36,37 The energy savings related to fuel, ASU, and compression train involves a reduction of the corresponding investment and operating costs. As well, the capital costs of the steam cycle in which excess heat flows are integrated is expected to diminish. It should be highlighted that the fuel saving reduces the available heat but the energy efficiency proved to keep almost constant. On the other hand, solid flow increase may increment the operation costs. The augmentation of the reactors inventory is not expected to significantly affect the costs since the pressure drop in the reactors remains at moderate levels, below 100 mbar. The integration of the CO2 capture system in a

carbonation in the mixing seal valve. However, this effect is less important under this scenario compared to the case in which high concentrated CO2 is used alone as aeration gas, since only a fraction of the solids contact this gas in the mixing seal valve. When most of the solids are directed to the calciner, average capture capacity tends to reach the same value obtained when only highly concentrated CO2 is used. In contrast, for high values of ncarb, results approach that obtained when only flue gas is used. A similar effect is observed for the flow of solids circulating in the cycle, Figure 9, since this parameter is intimately related to

Figure 9. Flow of solids circulating in the cycle when aerating with both streams for different solid inventories in the carbonator.

the sorbent average capture capacity. It should be highlighted that reasonable solids flows are required for a wide range of ncarb, if both gases are used for the aeration of the mixing seal valve. Particle flow becomes significantly high only when the solids distribution is very unequal. The purity of the CO2 stream for compression and transport is equivalent to that obtained in the ordinary configuration since the solids from the mixing seal valve fed to this reactor are recirculated with a fraction of this gaseous stream. There only exist slight variations related to the reactions taking place in the mixing seal valve. This represents one of the advantages of the use of both gases for the aeration of the seal valve. As the entire flue gas flow is directed to the carbonator either straight or via the mixing seal valve, and no other gas is introduced in this reactor, the real CO2 capture efficiency is equal to that in the ordinary configuration. Figure 10 shows the specific coal consumption when aerating with both gases for different sorbent molar inventories in the carbonator. Similarly to sorbent capture capacity and solids flow, results approach those obtained when aerating with highly concentrated CO2, for low values of ncarb; and those obtained 2065


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Table 2. Summary of the Operating Points ordinary configuration cyclonic preheater mixing seal valve/ flue gas mixing seal valve/ highly concentrated CO2 mixing seal valve/ both gases

ncarb (%)

nCa (kmol)

yCO2,db (%)

mS (kg/s)

mmake‑up (kg/s)

mcoal/mCO2 (kg/kg)

− − 0.9 0.75 0.1 0.15 0.1 0.45 0.15

333.5 346.3 900 900 900 900 900 900 900

92 92 91 87 74 93 92 93 93

2221 2147 20735 4602 5233 5525 7139 3572 4802

42.1 44.9 42.1 42.1 42.1 42.1 42.1 42.1 42.1

0.45 0.40 0.43 0.40 0.32 0.41 0.39 0.43 0.38

Table 3. Summary of the Energy Consumption ordinary configuration cyclonic preheater mixing seal valve/ flue gas mixing seal valve/ highly concentrated CO2 mixing seal valve/ both gases

ncarb (%)

coal (kWhth/tCO2)

ASU (kWhe/tCO2)

CO2 compression (kWhe/tCO2)

− − 0.9 0.75 0.1 0.15 0.1 0.45 0.15

3172 2819 3031 2819 2255 2890 2749 3031 2678

227 202 213 205 167 209 200 216 194

273 225 230 225 205 228 222 232 219

noticed, as well, that there is a broad operating window at which solid flow remains at relatively moderate values while giving satisfying energy savings. The comparison between cyclonic preheater and mixing seal valve configurations is not straightforward since they are at a different levels of development. The cyclonic preheater is widely used in the cement industry, whereas the mixing seal valve still needs to be experimentally analyzed. Although both systems present good results in terms of energy savings, the mixing seal valve presents a higher potential, and further experimental research should be of great interest. The design of the mixing seal valve presents some uncertainties, mainly related to the aeration flow required for the recirculation of solids and the possibility of keeping both gases completely unmixed. Further experimental work would be needed to shed light on the mixing seal valve design and requirements. It may be concluded that the addition of a mixing seal valve may reduce the specific coal consumption of the CO2 capture plant. As well, the use of both flue and CO2 gases for the aeration of the mixing seal valve may contribute to improved performance of the system.

steam cycle is required to carry out a rigorous economic analysis. The mixing seal valve entails a modification of the available heat streams compared with the ordinary configuration. Therefore, existing integration schemes cannot be used and the development of a new integration configuration is beyond the scope of this study.

4. CONCLUSIONS A calcium looping system including a mixing seal valve to preheat the solids entering the calciner was modeled and simulated. Different aeration gases, sorbent inventories in the carbonator, and distributions of solids between reactors were tested to assess the operation of the system and determine the potential energy savings of the configuration. Mixing of carbonated and regenerated sorbent reduces the fraction of CaO entering the carbonator. An increase of the flow of particles in the cycle, especially important when solids are unequally distributed, is therefore necessary to achieve an adequate CO2 capture capacity. The required increment may be moderated to some extent by enlarging the sorbent inventory in the carbonator. Using a fraction of the flue gas as aeration gas in the mixing seal valve has the disadvantage of diluting the CO2 in the calciner. This issue becomes less important as the fraction of solids directed to the calciner diminishes. However, solid circulation requirements increase significantly, and the energy savings are reduced in this case. If the mixing seal valve is aerated with a fraction of the highly concentrated CO2 stream, the operating window in which the specific coal consumption is lower than that in the ordinary configuration is restricted to low fractions of solids directed to the carbonator. Energy savings are accompanied by considerable solid flow increments. The use of both gases for the aeration of the mixing seal valve is also proposed in this study. In this case, CO2 dilution may be avoided, and solid flows may be reduced to some measure, also obtaining good results in terms of coal savings. It should be

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for A.M. 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) dx.doi.org/10.1021/ef402487e | Energy Fuels 2014, 28, 2059−2068

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Ceq=concentration of CO2 in equilibrium conditions (kmol/ m3) 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 carbonation fast reaction regime Fcalc=molar flow of CaO and CaCO3 circulating between mixing seal valve and calciner (kmol/s) Fcarb=molar flow of CaO and CaCO3 circulating between mixing seal valve and carbonator (kmol/s) f CR=maximum proportion of carbonated sorbent in the cycle kCL=kinetic constant of CaCO3 calcination (m3/kmol·s) kCR=surface carbonation rate constant (s−1) mmake‑up=make-up flow of fresh sorbent (kg/s) mS=flow of solids circulating in the cycle; sum of streams of carbonator and calciner (kg/s) N=number of cycles accomplished by a volume of sorbent nCa=molar sorbent inventory in the carbonator (kmol) ncarb=fraction of solids in the mixing seal valve that are directed to the carbonator ṅCa=inlet molar flow of CaO and CaCO3 (kmol/s) ṅCO2=inlet molar flow of CO2 (kmol/s) rN=age distribution of particles; fraction of particles that has accomplished N carbonation/calcination cycles Si=stream i tCL * =time for full calcination under calciner operating conditions (s) t*CR=time for maximum fast kinetic-stage carbonation, Xave. (s) Tpurge=temperature of the purge after gas preheating (°C) TCO2=temperature of the highly concentrated CO2 flow leaving the system after cooling (°C) vCO2=average volume fraction of CO2 veq=volume fraction of CO2 in equilibrium conditions Xave.=average maximum capture capacity of the sorbent XCaCO3=molar fraction of CaCO3 with respect to CaO and CaCO3 XCaO=molar fraction of CaO with respect to CaO and CaCO3 Xin=inlet molar fraction of CaCO3 with respect to CaO and CaCO3 XN=capture capacity of a fraction of sorbent that has accomplished N carbonation−calcination cycles yCO2,db=volumetric fraction of CO2 leaving the calciner in dry basis ηCL=calcination efficiency, fraction of CaCO3 calcined ηCR=carbonation efficiency, fraction of CO2 captured τCR=carbonator space time; molar inventory of calcium compounds (CaO and CaCO3) per molar flow of CO2 (s) φ=gas−solid contacting effectivity factor

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Subscripts

calc=calciner carb=carbonator FS=fresh sorbent MSV=mixing seal valve Si=stream i Acronym

ASU=air separation unit 2067


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Operation of a Mixing Seal Valve in Calcium Looping for CO2 Capture

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