- Email: [email protected]
CO2 capture efficiency and energy requirement analysis of power plant using modified calcium-based sorbent looping cycle
Energy 36 (2011) 1590e1598
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
CO2 capture efficiency and energy requirement analysis of power plant using modified calcium-based sorbent looping cycle Yingjie Li a, b, Changsui Zhao a, *, Huichao Chen a, Qiangqiang Ren a, Lunbo Duan a a b
School of Energy and Environment, Southeast University, 2 Sipailou Street, Nanjing 210096, China School of Energy and Power Engineering, Shandong University, Jinan 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2010 Received in revised form 30 December 2010 Accepted 30 December 2010 Available online 1 February 2011
This paper examines the average carbonation conversion, CO2 capture efficiency and energy requirement for post-combustion CO2 capture system during the modified calcium-based sorbent looping cycle. The limestone modified with acetic acid solution, i.e. calcium acetate is taken as an example of the modified calcium-based sorbents. The modified limestone exhibits much higher average carbonation conversion than the natural sorbent under the same condition. The CO2 capture efficiency increases with the sorbent flow ratios. Compared with the natural limestone, much less makeup mass flow of the recycled and the fresh sorbent is needed for the system when using the modified limestone at the same CO2 capture efficiency. Achieving 0.95 of CO2 capture efficiency without sulfation, 272 kJ/mol CO2 is required in the calciner for the natural limestone, whereas only 223 kJ/mol CO2 for the modified sorbent. The modified limestone possesses greater advantages in CO2 capture efficiency and energy consumption than the natural sorbent. When the sulfation and carbonation of the sorbents take place simultaneously, more energy is required. It is significantly necessary to remove SO2 from the flue gas before it enters the carbonator in order to reduce energy consumption in the calciner. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Modified calcium-based sorbent Calcination Carbonation Post-combustion CO2 capture
1. Introduction Carbon dioxide, the most prevalent of man-made emissions of greenhouse gases, is mainly from fossil fuel burning of power plants. A possible solution to this tension between climate change and fossil fuel consumption fact could be the introduction of the carbon capture and storage (CCS) technology [1, 2]. There are many different CO2 capture processes, however, their techno-economic feasibility for industrial applications must be seriously considered. Calcium looping cycle, i.e. calcium-based sorbent calcination/ carbonation cycle using the reversible reaction between CaO and CO2 in which a pure stream of CO2 ready for geological sequestration and resource utilization can be generated, is a promising method to remove CO2. The technology has attracted a great deal of attention recently, owing to a number of its advantages: the relatively small efficiency penalty which imposes upon a power station (estimated at 6e8 percentage points, including compression of the CO2); the potential use in large-scale circulating fluidized beds; an excellent opportunity for integration with cement manufacture and the extremely cheap sorbent such as limestone [3, 4].
* Corresponding author. Tel./fax: þ86 25 83793453. E-mail address: [email protected] (C. Zhao). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.12.072
The applications of calcium looping cycle for fossil fuel power plants include the pre-combustion CO2 capture and post-combustion CO2 capture. The pre-combustion CO2 capture involves the zero-emission coal technology (ZEC) and HyPr-RING process [5e7]. The aim of these gasification processes is to produce hydrogen. The post-combustion CO2 capture proposed by Shimizu et al. [8] is shown in Fig. 1. CO2 from the flue gas of an existing power plant is removed by CaO derived from limestone with a circulating fluidized bed carbonator, operating at a temperature of 600e700 C. Reacted sorbent is then passed to a second fluidized bed operating at temperatures above 900 C, the calciner where the sorbent is regenerated, before being returned to the carbonator. Fuel is burnt during the oxy-combustion process to produce the heat necessary for regeneration of the sorbent. Oxy-combustion is required to maintain a high concentration of CO2 suitable for sequestration and the requisite oxygen for the process is provided by an external air separation unit, estimated to be approximately 1/3 times of the oxygen volume required for an oxy-combustion power plant [9]. Lower sulfur and ash in fuel in a calciner have a less effect on CaO regeneration. Therefore, biomass with low sulfur and ash is chosen preferentially as the fuel for the calciner. The calcium looping cycle for post-combustion CO2 capture is considered to be one of the most feasible ways for capturing CO2 from large-scale coal-fired power plants [10e13].
Y. Li et al. / Energy 36 (2011) 1590e1598
Nomenclature fitting constant in Eq. (3) content of CaO in initial sample heat capacity, calculated from reference [37], J/(K mol) CO2 capture efficiency fitting constants in Eqs. (1) and (3) makeup flow rate of fresh sorbent, kmol/s flow rate of CO2 produced by coal combustion entering the carbonator, kmol/s ratio of fresh sorbent flow rate to recycled sorbent flow F0/FR rate FR/FCO2 ratio of recycled sorbent flow rate to CO2 flow rate by coal combustion in boiler flow rate of fuel into calciner for regeneration of Ffuel sorbent, kmol/s FCO2, fuel flow rate of CO2 produced by fuel into calciner combustion, kmol/s flow rate of all the other gases, except CO2 into Fgas carbonator, kmol/s flow rate of recycled sorbent excluding fresh makeup, FR kmol/s FSO2, fuel flow rate of SO2 produced by coal combustion entering the carbonator, kmol/s flow rate of coal ash generated in calciner, kmol/s Fash b c cp ECO2 fm, fw F0 FCO2
Low-cost natural calcium-based sorbents such as limestone and dolomite can be used as CO2 sorbents for power plants, however, the CO2 capture capacity of these sorbents is known to decay markedly with increasing calcination/carbonation cycle number [14e17]. The decay trend of the CO2 capture capacity of limestone with the number of cycles was summarized by Abanades and Alvarez [18], as follows: N XN ¼ fm ð1 fw Þ þ fw
(1)
where fm ¼ 0.77 and fw ¼ 0.17 for a wide range of limestone and reaction conditions. Consequently, in order to capture CO2 efficiently in calcium looping cycle, more fresh sorbent is needed due to the deactivation and the sorbent makeup flow also increases. Not only the cost of the process will rise, but the abrasion and erosion in reactors will also be aggravated. Besides, more spent sorbents will be discarded, which will greatly surpass the needed amount for cement manufacture. Therefore, it is significantly necessary to
1591
Calcination reaction heat of CaCO3 at 920 C, kJ/mol heat requirement in the calciner, kJ initial mass of sample, g mass of sample after calcination, g mass of sample after N cycles, g number of cycles mass fraction of CaO derived from fresh calcium-based sorbent entering the carbonator in F0þFR (kmol/s) after N cycle carbonation temperature, C Tcarb calcination temperature, C Tcal WCaO,WCO2 mole mass of CaO and CO2, respectively, g/mol average carbonation conversion of sorbent in the Xave carbonator corrected average conversion, considering Xco-ave desulfurization in the carbonator carbonation conversion achieved after N cycles XN average conversion to CaSO4 XCaSO4 DTcalc-carb temperature difference between calciner and carbonator, K DTcalc-fresh temperature difference between calciner and fresh sorbent, K h energy requirement in the calciner per molar of CO2 entering the carbonaor, kJ/molCO2 hCaCO3 Hreq m0 m1 mN N rN
mitigate deactivation of natural sorbent and minimize the sorbent makeup flow during long-term cycles in process design. Much current and promising work involves the investigation of different methods to reduce the decay of the sorbent in reactivity, in order to boost the long-term reactivity of the sorbent or to reactivate the sorbent [19e27]. Silaban et al. [28], Lu et al. [29,30], Liu et al. [31], Sultan et al. [32] and our group [33,34] all agreed that calcium acetate produced by the acetification reaction between calcium-based sorbent and acetic acid is an excellent CO2 sorbent due to its high reactivity during the long-term cycles. Although the acetic acid is expensive, it can be replaced by the industrial wastes with acetic acid. Also the acetone can be reclaimed from calcium acetate by dry distillation. Thus, the cost of CO2 sorbent will be reduced further [33,35]. The systematic CO2 capture efficiency and energy required are the important parameters for the post-combustion CO2 capture system of power plant, which have a direct effect on power generation efficiency. Therefore, it is significantly necessary to
Fig. 1. Flow sheet of calcium looping cycle for post-combustion CO2 capture.
1592
Y. Li et al. / Energy 36 (2011) 1590e1598
Table 1 The chemical compositions of the natural limestone (wt, %). CaO
MgO
Al2O3
Fe2O3
SiO2
Others
LOI
54.70
0.36
0.05
0.04
1.18
0.52
43.15
investigate CO2 capture efficiency and energy required for the postcombustion CO2 capture of coal-fired power plant using calcium looping cycle. Abanades [14] and Rodriguez et al. [36] investigated the CO2 capture efficiency and heat requirement in a calciner for the power plant using the natural limestone. In recent years, more and more attentions have been paid to the cyclic CO2 capture capacity of different modified calcium-based sorbents, but the research on CO2 capture efficiency and energy requirement for the power plant using the modified calcium-based sorbent has been ignored. Consequently, in this paper, we focus on the analysis of CO2 capture efficiency and energy requirement for the postcombustion CO2 capture system of coal-fired power plant using modified calcium-based sorbent, incorporating a fresh feed of sorbent to compensate for the decay in activity during long-term cycles. The limestone modified with acetic acid solution is taken as an example of modified calcium-based sorbents. At the same time, the comparison between the modified and the natural limestones is made.
about 2 mm and the original sample mass was about 2 g. The reacting gas was set by a flow meter and fed to the reactor. The temperatures were collected by a computer. The calcined sample was readily picked out and stored in an inert atmosphere, and CO2 was again admitted into the furnace for further cycling. The variation of sample mass was measured by Mettler Toledo-XS105DU analytical balance. The repeatability at maximum capacity of the balance is 0.1 mg. The sample may absorb the vapor from air during measurement, hence an increase in weight may occur, but the error is less than 1%. The sample was calcined at 920e1100 C in 80% CO2/20% O2 gas mixture, and carbonated at 550e750 C in 15% CO2/85% N2 gas mixture at atmospheric pressure. In order to investigate the effects of calcination temperature and carbonation temperature on carbonation conversions of the modified limestone after 1 cycle, 920 C, 940 C, 960 C, 980 C,1000 C,1020 C,1040 C, 1060 C, 1080 C, and 1100 C were chosen as the calcination temperature and 550 C, 570 C, 600 C, 620 C, 650 C, 670 C, 700 C, 720 C, and 750 C were selected as carbonation temperature in the experiment. Thus, 81 data points were obtained as shown in Fig. 2. The long-term looping cycle experiments were repetitively performed three times, as shown in Fig. 3.
2. Experimental The cyclic carbonation conversions of the limestone modified with acetic acid solutions were investigated in order to analyze its CO2 capture efficiency. 2.1. Sorbents preparation The natural limestone was modified with 50 vol.% acetic acid solution in a beaker at the ambient temperature and pressure. The reaction time was 2 h. Then the beaker was put into a drying cabinet of 120 C. The molar ratio of acetic acid in the solution to Ca in limestone was 1.5:1. The particle size of the sorbent is below 0.125 mm. The chemical components of the natural limestone are shown in Table 1. The pore structure parameters of the natural and modified limestones after the first calcination at 920 C in 80% CO2/20% O2 gas mixture were analyzed by Micromeritics ASAP 2020-M nitrogen adsorption analyzer. The surface area and pore volume of the natural and modified limestones were calculated by BET method and BJH model, respectively, and the results were displayed in Table 2. It is found that both the surface area and pore volume of the natural limestone increase after the calcination due to the modification. 2.2. Cyclic carbonation/calcination The carbonation conversions of the sorbents were studied in a twin fixed bed reactor system including a calcination reactor and a carbonation reactor designed at the atmospheric pressure [34]. The sample boat containing the sorbent can be shifted between two reactors. The thickness of sample laid in the boat was limited to
Table 2 The pore structure parameters of the natural and modified limestones after the first calcination. Sample
Surface area (m2/g)
Pore volume (cm3/g)
Modified limestone Natural limestone
15.57 8.72
0.15 0.09
Fig. 2. Effect of reaction temperature on carbonation conversions of the natural and modified limestone after 1 cycle. (a) Surface plot, and (b) contour plot.
Y. Li et al. / Energy 36 (2011) 1590e1598
1593
cyclic carbonation behavior of the natural limestone can be described by Eq. (1) in which fm and fw are 0.77 and 0.17, respectively [16]. However, Eq. (1) is not appropriate to fit the experimental data for the modified limestone, because the maximum fitting variance is just 0.926 (Eq. (1), fm ¼ 0.83 and fw ¼ 0.41). Therefore, it is necessary to use a new formula to describe the modified limestone. Eq. (3) is employed to fit the carbonation conversion data for the modified limestone as follows: N XN ¼ fm ðb fw Þ þ fw
(3)
where, b ¼ 0.9, fm ¼ 0.92, and fw ¼ 0.41. The variance of the new fitting equation is 0.993, closer to 1.0 than that of Eq. (1). Obviously, Eq. (3) is more precise to describe the CO2 capture behavior of the modified limestone. Eq. (1) can be viewed as a special case of Eq. (3), and the carbonation conversion of the natural limestone can also be described in form of Eq. (3) (b ¼ 1, fm ¼ 0.77, and fw ¼ 0.17).
Fig. 3. Carbonation conversions and fitting equations of the natural and modified limestones. Tcarb 650 C, and Tcal 920 C.
3.2. CO2 capture efficiency for post-combustion CO2 capture system The CO2 capture efficiency is defined as follows:
In order to prevent calcined samples from recarbonation in the calcination reactor, the gas flow was switched to N2 immediately until all CO2 was purged off, and then the sample boat was moved to the carbonation reactor. The actual temperature of sample was measured with a thermocouple placed in the center of the sample boat. Based on experiments, the carbonation reaction time and the calcination reaction time were 20 min and 15 min, respectively. The carbonation conversion of sample was calculated according to the mass change during the cycles as follows:
XN ¼
mN m1 WCaO $ m0 $c WCO2
(2)
3. Results and discussion 3.1. Carbonation characteristics during the calcination/carbonation cycles The reaction temperature has a significant effect on carbonation conversion of calcium-based sorbent. Fig. 2 presents the carbonation conversions of the natural and the modified limestones with the calcination and carbonation temperatures after 1 cycle. The carbonation conversion of the modified limestone is higher than that of the natural sorbent at the same reaction temperature as shown in Fig. 2(a). The natural limestone achieves a carbonation conversion above 0.78 for carbonation at 640e690 C and calcination below 970 C, as plotted in Fig. 2(b). The modified limestone exhibits the optimum conversion, higher than 0.82 for carbonation at 640e670 C and calcination below 1050 C. The feasible carbonation temperature window for the two sorbents is similar, but the suitable calcination temperature window for the modified limestone is wider than the natural one. Considering the effect of reaction temperature on CO2 capture behavior, the cyclic carbonation conversions of the modified and the natural limestones during the 100 cycles were examined for calcination at 920 C and carbonation at 650 C as demonstrated in Fig. 3. The repetition tests demonstrate that both the natural and the modified limestones retain stable carbonation conversions after about 60 cycles and the conversion of the latter is more than twice higher than that of the former. In order to calculate the average carbonation conversion, the cyclic carbonation conversions of sorbents need to be summarized by mathematical equations. The
ECO2 ¼
F0 þ FR Xave FCO2
(4)
The Eq. (4) is easily understood by Fig. 4 which exhibits the main mass flow in CO2 capture with calcium-based sorbent calcination/ carbonation cycles for the post-combustion CO2 capture. As FCO2 is a constant, the CO2 capture efficiency, determined by F0, FR and Xave, increases with the values of F0, FR and Xave. However, increase in F0 and FR results in growth of the energy consumption in a calciner and the economic cost of CO2 capture process. Obviously, increasing the ECO2 by enhancing F0 and FR is not reasonable. Therefore, it is necessary to improve Xave in order to increase ECO2. Xave is determined by the carbonation conversion and the solid stream of calcium-based sorbent during the CO2 capture process. The mass fraction of particles entering the carbonator in the solid stream F0 þ FR which has circulated N cycles can be calculated from a succession of mass balance. Therefore, the average carbonation conversion of sorbent in the carbonator is given as follows [14].
Xave ¼
NX ¼N
rN XN
(5)
N¼1
rN ¼
F0 FRN1 ðF0 þ FR ÞN
(6)
Incorporating Eqs. (3), (5) and (6), the average carbonation conversion becomes:
Fig. 4. The main mass flow (kmol/s) in CO2 capture with calcium-based sorbent calcination/carbonation cycles.
1594
Y. Li et al. / Energy 36 (2011) 1590e1598
Xave ¼
NX ¼N N¼1
¼
F0 F0 þ FR
FR F0 þ FR
N1
N fm ðb fw Þ þ fw
F0 fm ðb fw Þ þ fw F0 þ FR ð1 fm Þ
(7)
SO2 generated from coal combustion will enter the carbonator and react with CaO. The sulfation of CaO induces the loss of CaO due to the irreversibility. Thus the average carbonation conversion must be corrected. The corrected average carbonation conversion can be calculated as follows.
Xcoave ¼ Xave XCaSO4 ¼ Xave
FSO2 ;fuel F0
(8)
Then the CO2 capture efficiency is obtained:
F0 þ FR Xcoave FCO2 0 1 FCO2 F0 F0 þ 1 f ðb f Þ F m w SO ;fuel F F @FR A ¼ RF þ fw FR $ 2 F0 CO2 0 FCO2 FR þ 1 fm FR
ECO2 ¼
(9)
FR
The CO2 capture efficiency is determined by F0/FR, FR/FCO2, fm, fw, b and FSO2, fuel/FCO2. The value of FSO2, fuel/FCO2 is equal to zero, if SO2 is absolutely removed before entering the carbonator. The ultimate analysis of coal for boiler in power plant and biomass for the calciner are shown in Table 3. A kind of bituminous coal is used by lots of power plants in Shandong province, China. The pine wood is one of the local and common biomass. It is assumed that carbon and sulfur in the coal are converted totally into CO2 and SO2, respectively. Thus, the value of FSO2, fuel/FCO2 is 0.01. The sorbent flow ratios, such as F0/FR and FR/FCO2, have a comparable effect on the average carbonation conversion, as seen in Fig. 5. When no SO2 enters the carbonator, FR/FCO2 has no effect on Xave for the sorbents. Also, Xave of the modified limestone almost doubles that of the natural sorbent as presented in Fig. 5(a). When there is SO2 in the flue gas which enters the carbonator, the result is shown in Fig. 5(b). The average carbonation conversions for the two sorbents increase with improving the values of F0/FR and FR/FCO2. F0/FR has a greater effect on Xco-ave of the two sorbents than FR/FCO2. The modified limestone displays much higher Xco-ave than the natural sorbent at the same F0/FR and FR/FCO2. For example, when F0/FR ¼ 0.05 and FR/FCO2 ¼ 4, Xco-ave of the natural and the modified limestones is 0.22 and 0.53, respectively. The sulfation of CaO induces decay in the average carbonation conversion of the two sorbents. Fig. 6 demonstrates the CO2 capture efficiency of the postcombustion CO2 capture system using the natural and the modified limestones with/without SO2 in the carbonator. The value of ECO2 for the two sorbents without SO2 in the carbonator shows an increase with F0/FR and FR/FCO2 as plotted in Fig. 6(a). When Table 3 Ultimate analysis of coal and biomass (air dry basis, wt.%).
C H S O N Fixed C Moisture Volatile matter Ash LHV (MJ/kg)
Bituminous coal
Pine wood
65.22 2.43 1.74 6.61 1.10 47.96 8.74 29.14 14.16 25.59
51.00 6.00 0.00 35.91 0.08 14.04 6.25 78.95 0.76 19.04
Fig. 5. Effects of sorbent flow ratios on average carbonation conversions. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, ECO2 for the natural and the modified limestones are 0.4 and 0.93, respectively. ECO2 for the modified limestone is 2.3 times higher than that for the natural sorbent. In order to achieve ECO2 up to 0.95 at the value of F0/FR ¼ 0.01, the value of FR/FCO2 for the natural limestone should be 4.7, while that for the modified limestone requires 2.1 which is less than a half of FR/FCO2 for the natural limestone. The effect of sulfation on CO2 capture efficiency is exhibited in Fig. 6(b). The sulfation decreases the CO2 capture efficiency for the natural and the modified limestones. For example, when F0/FR ¼ 0.03 and FR/FCO2 ¼ 1.5 without SO2 in the carbonator, ECO2 for the natural and the modified limestones is 0.41 and 0.87, respectively; while with SO2 presence, at the same F0/FR and FR/FCO2, ECO2 for the sorbents is only 0.16 and 0.61, respectively. It is readily calculated that although the content of sulfur in the coal is only 1.74 wt. %, i.e. the volume ratio of SO2 to CO2 is about 1 vol.%, compared with that without sulfation, the sulfation results in a decrease in ECO2 for the natural and modified limestones by 61% and 30%, respectively. The result demonstrates that the sulfation has less effect on ECO2 for the modified limestone than that for the natural sorbent. For the sake of achieving ECO2 of up to 0.95 at the condition of FR/FCO2 ¼ 3 with SO2 in the carbonator, the value of F0/FR for the natural limestone should be 0.08, whereas that for the modified sorbent is only 0.019. It reveals that the system using the modified sorbent requires much less makeup mass flow of the recycled and the fresh sorbent than that using the natural limestone at the same ECO2.
Y. Li et al. / Energy 36 (2011) 1590e1598
1595
The heat requirement for heating unreacted CaO, formed CaCO3, CaSO4 and inert mass from the carbonation temperature to the calcination temperature are defined as follows:
h H1 ¼ cp CaO FR ð1 Xcoave Þ þ cp CaCO3 FR Xcoave þ cp CaSO4 FSO2 ;fuel i þ cp ash Fash DTcalccarb ð10Þ where, it is assumed that the temperature for carbonation and calcination is 650 C and 920 C, respectively. The ash from coal combustion is removed by the dust collector and does not enter the calciner. Consequently, the SO2 and ash derived from combustion of the pine wood in the calciner is negligible. When no SO2 enters the carbonator, the value of FSO2, fuel is equal to zero. The heat requirement for the calcination of CaCO3 includes the formed CaCO3 during carbonation and the fresh CaCO3 from limestone. It is assumed that CaCO3 does not decompose until it is heated to the calcination temperature. The heat requirement is calculated as follows.
H2 ¼ ðFR Xcoave þ F0 ÞhCaCO3
(11)
The fresh CaCO3 filled into the calciner is heated to the calcination temperature from the ambient temperature and the heat requirement is described as follows:
H3 ¼ cp CaCO3 F0 DTcalcfresh
(12)
where, the temperature of the fresh sorbent is 10 and DTcalc-fresh is 910 C. Incorporating Eqs. (10)e(12), the energy requirement in the calciner is obtained: C,
Hreq ¼ H1 þ H2 þ H3
(13)
h Hreq ¼ cp CaO FR ð1 Xcoave Þ þ cp CaCO3 FR Xcoave i þ cp CaSO4 FSO2 ;fuel þ cp ash Fash DTcalccarb Fig. 6. Effects of sorbent flow ratios on CO2 capture efficiencies. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
3.3. Energy requirement in calciner for post-combustion CO2 capture system The calciner is responsible for regeneration of CaO, and the energy requirement in calciner is an important parameter for postcombustion CO2 capture. Although ECO2 becomes higher with increasing F0/FR and FR/FCO2, the choice of sorbent flow ratios is not arbitrary. That is because the energy balance in the system will
þ ðFR Xcoave þ F0 ÞhCaCO3 þ cp CaCO3 F0 DTcalcfresh
ð14Þ
h is used to describe energy requirement of the calciner per molar of CO2 entering the carbonator for the post-combustion CO2 capture system as follows.
h¼
Hreq FCO2
(15)
Considering Eqs. (8) and (14), the h is obtained further as follows:
! ! F0 F FSO2 ;fuel FCO2 FR FSO2 ;fuel FCO2 FR FSO2 ;fuel FR FR FR0 fm ðb fw Þ FR fm b fw h ¼ cp CaO 1 F þ cp CaCO3 þ cp CaSO4 fw þ $ $ þ fw $ $ F0 0 FCO2 F F F F F F F FCO2 R R 0 CO2 CO2 0 CO2 FR þ 1 fm FR þ 1 fm # ! # " F FCO2 FR FSO2 ;fuel F FR FR0 fm ðb fw Þ F F F F þ 0 $ R hCaCO3 þ cp CaCO3 0 $ R DTcalcfresh þ cp ash ash DTcalccarb þ þ fw $ $ FCO2 FCO2 F0 þ 1 fm FR F0 FCO2 FR FCO2 FR FCO2 "
FR
(16)
impose further limitations on these sorbent flow ratios. Not only fresh CaCO3 from limestone and formed CaCO3 from carbonator but also the unreacted CaO, CaSO4, ash and inert mass are fed into the calciner. The energy requirement in the calciner is comparatively complex. The heat input in the calciner is calculated progressively.
In Eq. (16), fm, fw and b are confirmed for the natural and modified limestones, and cp CaO, cp CaCO3, cp ash, cp CaSO4 and hCaCO3 can also be obtained from the reference [37]. Thus h is calculated at the different values of F0/FR and FR/FCO2. Fig. 7(a) presents the energy requirement in the calciner with F0/FR and FR/FCO2, with no
1596
Y. Li et al. / Energy 36 (2011) 1590e1598
and the modified limestones decreases by 33% and 19% due to the sulfation, respectively, compared with the h without sulfation. It seems that the effect of sulfation on h for the modified limestone is less than that for the natural sorbent. Fig. 8 demonstrates the relation between the CO2 capture efficiency and the energy requirement in the calciner for the natural and the modified limestones. When no SO2 enters the carbonator, both sorbents exhibit an increase in h with increasing CO2 capture efficiency as shown in Fig. 8(a). Also, h for the modified limestone is lower than that for the natural sorbent at the same ECO2. In order to achieve ECO2 of up to 0.95, h for the natural limestone is 272 kJ/molCO2, whereas that for the modified sorbent is 223 kJ/molCO2. It is observed that the difference in h between the modified limestone and the natural sorbent becomes larger with increasing ECO2. The post-combustion CO2 capture system using the modified limestone possesses higher power generation efficiency at the same ECO2 due to lower energy consumption. The sulfation of the sorbents aggravates the raise in the energy consumption for the calciner as presented in Fig. 8(b). When ECO2 ¼ 0.95 and FR/FCO2 ¼ 3, h for the natural and modified limestones are 297 kJ/molCO2 and 249 kJ/molCO2, respectively. The result indicates that when the volume ratio of SO2 to CO2 is about 1 vol.%, compared with cases without SO2 entering the carbonator, h for the natural and the modified limestones increases by 9% and
Fig. 7. Effects of sorbent flow ratios on energy requirement in the calciner. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
SO2 entering the carbonator. For the natural and the modified limestones, h increases with improving the values of F0/FR and FR/ FCO2. It is also found that for the modified limestone h is higher than that for the natural limestone at the same F0/FR and FR/FCO2. That is because the modified limestone possesses greater carbonation conversion, hence more CaCO3 formed in the carbonator enters the calciner. Consequently, more heat is required for the CaO regeneration. For example, when F0/FR ¼ 0.04 and FR/FCO2 ¼ 2, for the natural limestone h is 157 kJ/molCO2, while for the modified sorbent it is 270 kJ/molCO2. However, we cannot conclude that the energy requirement in the calciner for the modified limestone is higher. That is because the ECO2 for the two sorbents is different at the same sorbent flow ratio. h for different sorbents should be discussed at the same ECO2. Moreover, although the energy requirement in the calciner using the modified limestone is higher, the energy released in the carbonator is also greater as the carbonation proceeds. The energy requirement in the calciner with SO2 in the carbonator is plotted in Fig. 7(b). The sulfation decreases the energy requirement in the calciner at the same F0/FR and FR/FCO2. That is because sulfation of the sorbent in the carbonator results in a decrease in the formed CaCO3 which consumes heat to regenerate. At the value of F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, h for the natural and the modified limestones are 105 kJ/molCO2 and 219 kJ/molCO2, respectively. When F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, h for the natural
Fig. 8. Relation between CO2 capture efficiency and energy requirement in the calciner. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
Y. Li et al. / Energy 36 (2011) 1590e1598
11%, respectively. Although the sulfur content in coal for power plants is low, the sulfation results in an increase in the energy requirement in the calciner. Therefore, it is significantly necessary to remove SO2 from the flue gas before entering the carbonator in order to avoid more energy consumption in the calciner. The spent CaO from the calciner can be used to remove the SO2 before they enter the carbonator. Sun et al. [38] and Manovic et al. [39] regarded the spent CaO as a good sorbent to capture SO2 due to the generation of macropores during multiple calcination/carbonation cycles. Since SO2 is emitted from the fossil fuel burning together with CO2, it should be captured. Using the spent CaO for SO2 capture is obviously cheaper than other kinds of sorbents. Also, the heat requirement using the spent CaO for SO2 removal is much less than that using the fresh limestone. 4. Conclusions The CO2 capture efficiency and energy requirement in a calciner for the post-combustion system using the modified calcium-based sorbents were investigated. The limestone modified with acetic acid solution, i.e. calcium acetate is taken as an example of the modified calcium-based sorbents. The repeated tests during 100 cycles demonstrate that the natural and the modified limestones retain stable carbonation conversions after about 60 cycles and the conversion of the latter is more than twice higher than that of the former. The modified limestone exhibits much higher average carbonation conversion and CO2 capture efficiency than the natural sorbent at the same sorbent flow ratios. F0/FR has a greater effect on the average carbonation conversions than the FR/FCO2. The sulfation of CaO induces decay in the average carbonation conversion of two sorbents. The system using the modified sorbent requires much less makeup mass flow of the recycled and the fresh sorbent than that using the natural limestone at the same CO2 capture efficiency. The calciner is responsible for regeneration of CaO, and the energy requirement in calciner is an important parameter. Although CO2 capture efficiency becomes higher with increasing the sorbent flow ratios, the choice of sorbent flow ratios is not at random because of an increase in energy requirement in the calciner with the sorbent flow ratios. The sulfation decreases the energy requirement in the calciner at the same sorbent flow ratios. The effect of sulfation on energy requirement in the calciner for the modified limestone is less than that for the natural sorbent. The energy requirement in the calciner for the modified limestone is lower than that for the natural sorbent at the same CO2 capture efficiency. In order to achieve 0.95 of CO2 capture efficiency without sulfation in the carbonator, the energy requirement in the calciner for the natural limestone is 272 kJ/molCO2, whereas that for the modified sorbent is 223 kJ/molCO2. The limestone modified with acetic acid solution shows greater advantages in the CO2 capture efficiency and energy consumption than the natural sorbent. The result indicates that when the volume ratio of SO2 to CO2 is about 1 vol.%, the energy requirement in the calciner for the natural and the modified limestones increases by 9% and 11%, respectively, compared with the energy consumption without sulfation in the carbonator. Although the sulfur content in coal for power plants is low, sulfation results in an increase in the energy requirement in the calciner. Therefore, it is significantly necessary to remove SO2 from the flue gas before entering the carbonator in order to avoid more energy consumption in the calciner. Acknowledgments Financial supports from the National Basic Research Program of China (2006CB705806), China Postdoctoral Science Foundation Funded Project (20090461205), Special Funds for Postdoctoral
1597
Innovative Projects of Shandong Province (200902019), China and Independent Innovation Foundation of Shandong University (IIFSDU) are sincerely acknowledged. References [1] Naqvi R, Wolf J, Bolland O. Part-load analysis of a chemical looping combustion (CLC) combined cycle with CO2 capture. Energy 2007;32:360e70. [2] Garg A, Shukla PR. Coal and energy security for India: role of carbon dioxide (CO2) capture and storage (CCS). Energy 2009;34:1032e41. [3] Blamey J, Anthony EJ, Wang J, Fennell PS. The calcium looping cycle for largescale CO2 capture. Prog Energy Combust 2010;36:260e79. [4] Nikulshina V, Hirsch D, Mazzotti M, Steinfeld A. CO2 capture from air and coproduction of H2 via the Ca(OH)2eCaCO3 cycle using concentrated solar power-thermodynamic analysis. Energy 2006;31:1715e25. [5] Gao L, Paterson N, Dugwell D, Kandiyoti R. Zero-emission carbon concept (ZECA): equipment commissioning and extents of the reaction with hydrogen and steam. Energy Fuels 2008;22:463e70. [6] Lin SY, Suzuki Y, Hatano H. Developing an innovative method HyPr-RING, to produce hydrogen from hydrocarbons. Energ Convers Manage 2002;43:1283e90. [7] Barelli L, Bidini G, Corradetti A, Desideri U. Production of hydrogen through the carbonation-calcination reaction applied to CH4/CO2 mixtures. Energy 2007;32:834e43. [8] Shimizu T, Hirama T, Hosoda H, Kitano K, Inagaki M, Tejima K. A twin fluidbed reactor for removal of CO2 from combustion processes. Trans I Chem E 1999;77:62e8. [9] Hughes RW, Lu DY, Anthony EJ, Macchi A. Design, process simulation and construction on an atmospheric dual fluidized bed combustion system for in situ CO2 capture using high-temperature sorbents. Fuel Process Technol 2005;86:1523e31. [10] Abanades JC, Grasa G, Alonso M, Rodriguez N, Anthony EJ, Romeo LM. Cost structure of a postcombustion CO2 capture system using CaO. Environ Sci Tehnol 2007;41:5523e7. [11] Ives M, Mundy RC, Fennell PS, Davidson JF, Dennis JS, Hayhurst AN. Comparison of different natural sorbents for removing CO2 from combustion gases, as studied in a bench-scale fluidized bed. Energy Fuels 2008;22:3852e7. [12] Alonso M, Rodriguez N, Gonzalez B, Grasa G, Murillo R, Abanades JG. Carbon dioxide capture from combustion flue gases with a calcium oxide chemical loop. Experimental results and process development. Int J Greenhouse Gas Con 2010;4:167e73. [13] Romeo LM, Lara Y, Lisbona P, Martinez A. Economical assessment of competitive enhanced limestone for CO2 capture cycles in power plants. Fuel Process Technol 2009;90:803e11. [14] Abanades JC. The maximum capture efficiency of CO2 using a carbonation/ calcination cycle of CaO/CaCO3. Chem Eng J 2002;90:303e6. [15] Sun P, Grace JR, Lim CJ, Anthony EJ. The effect of CaO sintering on cyclic CO2 capture in energy systems. AIChE J 2007;53:2432e42. [16] Lu DY, Hughes RW, Anthony EJ. Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Process Technol 2008;89: 1386e95. [17] Lu DY, Hughes RW, Anthony EJ, Manovic V. Sintering and reactivity of CaCO3based sorbents for in situ CO2 capture in fluidized beds under realistic calcination conditions. J Environ Eng-ASCE 2009;6:404e10. [18] Abanades JC, Alvarez D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003;17:308e15. [19] Feng B, Liu WQ, Li X, An H. Overcoming the problem of loss-in-capacity of calcium oxide in CO2 capture. Energy Fuels 2006;20:2417e20. [20] Feng B, Du M, Dennis TJ, Anthony K, Perumal MJ. Reduction of energy requirement of CO2 desorption by adding acid into CO2-loaded solvent. Energy Fuels 2010;24:213e9. [21] Feng B, An H, Tan E. Screening of CO2 adsorbing materials for zero emission power generation systems. Energy Fuels 2007;21:426e34. [22] Li ZS, Cai NS, Huang YY. Effect of preparation temperature on cyclic CO2 capture and multiple carbonationecalcination cycles for a new Ca-based CO2 sorbent. Ind Eng Chem Res 2006;45:1911e7. [23] Florin NH, Harris AT. Reactivity of CaO derived from nano-sized CaCO3 particle through multiple CO2 capture-and-release cycles. Chem Eng Sci 2009;64:187e91. [24] Albrecht KO, Wagenbach KS, Satrio JA, Shanks BH, Wheelock TD. Development of CaO-based CO2 sorbent with improved cyclic stability. Ind Eng Chem Res 2008;47:7841e8. [25] Pacciani R, Muller CR, Davidson JF, Dennis JS, Hayhurst AN. How does the concentration of CO2 affect its uptake by a synthetic Ca-based solid sorbent. AIChE J 2008;54:3308e11. [26] Lu H, Smirniotis PG. Calcium oxide doped sorbents for CO2 uptake in the presence of SO2 at high temperatures. Ind Eng Chem Res 2009;48:5454e9. [27] Dennis JS, Pacciani R. The rate and extent of uptake of CO2 by a synthetic, CaOcontaining sorbent. Chem Eng Sci 2009;64:2147e57. [28] Silaban A, Narcida M, Harrison DP. Calcium acetate as a sorbent precursor for the removal of carbon dioxide from gas streams at high temperature. Resour Conserv Recycl 1992;7:139e53. [29] Lu H, Khan A, Smirniotis PG. Relationship between structural properties and CO2 capture performance of CaO-based sorbents obtained from different organometallic precursors. Ind Eng Chem Res 2008;47:6216e20.
1598
Y. Li et al. / Energy 36 (2011) 1590e1598
[30] Lu H, Reddy EP, Smirniotis PG. Calcium oxide based sorbents for capture of carbon dioxide at high temperatures. Ind Eng Chem Res 2006;45:3944e9. [31] Liu WQ, Low NWL, Feng B, Wang GX, da Costa JCD. Calcium precursors for the production of CaO sorbents for multicycle CO2 capture. Environ Sci Tehnol 2010;44:841e7. [32] Sultan DS, Muller CR, Dennis JS. Capture of CO2 using sorbents of calcium magnesium acetate (CMA). Energy Fuels 2010;24:3687e97. [33] Li YJ, Zhao CS, Chen HC, Liang C, Duan LB, Zhou W. Modified CaO-based sorbent looping cycle for CO2 mitigation. Fuel 2009;88:697e704. [34] Li YJ, Zhao CS, Duan LB, Liang C, Li QZ, Zhou W, et al. Cyclic calcination/ carbonation looping of dolomite modified with acetic acid for CO2 sequestration. Fuel Process Technol 2008;89:1461e9.
[35] Palasantzas IA, Wise DL. Preliminary economic analysis for production of calcium magnesium acetate from organic residues. Resour Conserv Recycl 1994;11:225e43. [36] Rodriguez N, Alonso M, Grasa G, Abanades JC. Heat requirements in a calciner of CaCO3 integrated in a CO2 capture system using CaO. Chem Eng J 2008;138:148e54. [37] Green DW, Perry RH. Perry’s chemical engineers’ handbook. 8th ed. McGrawHill; 2007. [38] Sun P, Grace JR, Lim CJ, Anthony EJ. Sequential capure of CO2 and SO2 in a pressurized TGA simulating FBC conditions. Environ Sci Technol 2007; 41:2943e9. [39] Manovic VM, Anthony EJ. SO2 retention by reactivated CaO-based sorbent from multiple CO2 capture cycles. Environ Sci Technol 2007;41:4435e40.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
CO2 capture efficiency and energy requirement analysis of power plant using modified calcium-based sorbent looping cycle Yingjie Li a, b, Changsui Zhao a, *, Huichao Chen a, Qiangqiang Ren a, Lunbo Duan a a b
School of Energy and Environment, Southeast University, 2 Sipailou Street, Nanjing 210096, China School of Energy and Power Engineering, Shandong University, Jinan 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2010 Received in revised form 30 December 2010 Accepted 30 December 2010 Available online 1 February 2011
This paper examines the average carbonation conversion, CO2 capture efficiency and energy requirement for post-combustion CO2 capture system during the modified calcium-based sorbent looping cycle. The limestone modified with acetic acid solution, i.e. calcium acetate is taken as an example of the modified calcium-based sorbents. The modified limestone exhibits much higher average carbonation conversion than the natural sorbent under the same condition. The CO2 capture efficiency increases with the sorbent flow ratios. Compared with the natural limestone, much less makeup mass flow of the recycled and the fresh sorbent is needed for the system when using the modified limestone at the same CO2 capture efficiency. Achieving 0.95 of CO2 capture efficiency without sulfation, 272 kJ/mol CO2 is required in the calciner for the natural limestone, whereas only 223 kJ/mol CO2 for the modified sorbent. The modified limestone possesses greater advantages in CO2 capture efficiency and energy consumption than the natural sorbent. When the sulfation and carbonation of the sorbents take place simultaneously, more energy is required. It is significantly necessary to remove SO2 from the flue gas before it enters the carbonator in order to reduce energy consumption in the calciner. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Modified calcium-based sorbent Calcination Carbonation Post-combustion CO2 capture
1. Introduction Carbon dioxide, the most prevalent of man-made emissions of greenhouse gases, is mainly from fossil fuel burning of power plants. A possible solution to this tension between climate change and fossil fuel consumption fact could be the introduction of the carbon capture and storage (CCS) technology [1, 2]. There are many different CO2 capture processes, however, their techno-economic feasibility for industrial applications must be seriously considered. Calcium looping cycle, i.e. calcium-based sorbent calcination/ carbonation cycle using the reversible reaction between CaO and CO2 in which a pure stream of CO2 ready for geological sequestration and resource utilization can be generated, is a promising method to remove CO2. The technology has attracted a great deal of attention recently, owing to a number of its advantages: the relatively small efficiency penalty which imposes upon a power station (estimated at 6e8 percentage points, including compression of the CO2); the potential use in large-scale circulating fluidized beds; an excellent opportunity for integration with cement manufacture and the extremely cheap sorbent such as limestone [3, 4].
* Corresponding author. Tel./fax: þ86 25 83793453. E-mail address: [email protected] (C. Zhao). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.12.072
The applications of calcium looping cycle for fossil fuel power plants include the pre-combustion CO2 capture and post-combustion CO2 capture. The pre-combustion CO2 capture involves the zero-emission coal technology (ZEC) and HyPr-RING process [5e7]. The aim of these gasification processes is to produce hydrogen. The post-combustion CO2 capture proposed by Shimizu et al. [8] is shown in Fig. 1. CO2 from the flue gas of an existing power plant is removed by CaO derived from limestone with a circulating fluidized bed carbonator, operating at a temperature of 600e700 C. Reacted sorbent is then passed to a second fluidized bed operating at temperatures above 900 C, the calciner where the sorbent is regenerated, before being returned to the carbonator. Fuel is burnt during the oxy-combustion process to produce the heat necessary for regeneration of the sorbent. Oxy-combustion is required to maintain a high concentration of CO2 suitable for sequestration and the requisite oxygen for the process is provided by an external air separation unit, estimated to be approximately 1/3 times of the oxygen volume required for an oxy-combustion power plant [9]. Lower sulfur and ash in fuel in a calciner have a less effect on CaO regeneration. Therefore, biomass with low sulfur and ash is chosen preferentially as the fuel for the calciner. The calcium looping cycle for post-combustion CO2 capture is considered to be one of the most feasible ways for capturing CO2 from large-scale coal-fired power plants [10e13].
Y. Li et al. / Energy 36 (2011) 1590e1598
Nomenclature fitting constant in Eq. (3) content of CaO in initial sample heat capacity, calculated from reference [37], J/(K mol) CO2 capture efficiency fitting constants in Eqs. (1) and (3) makeup flow rate of fresh sorbent, kmol/s flow rate of CO2 produced by coal combustion entering the carbonator, kmol/s ratio of fresh sorbent flow rate to recycled sorbent flow F0/FR rate FR/FCO2 ratio of recycled sorbent flow rate to CO2 flow rate by coal combustion in boiler flow rate of fuel into calciner for regeneration of Ffuel sorbent, kmol/s FCO2, fuel flow rate of CO2 produced by fuel into calciner combustion, kmol/s flow rate of all the other gases, except CO2 into Fgas carbonator, kmol/s flow rate of recycled sorbent excluding fresh makeup, FR kmol/s FSO2, fuel flow rate of SO2 produced by coal combustion entering the carbonator, kmol/s flow rate of coal ash generated in calciner, kmol/s Fash b c cp ECO2 fm, fw F0 FCO2
Low-cost natural calcium-based sorbents such as limestone and dolomite can be used as CO2 sorbents for power plants, however, the CO2 capture capacity of these sorbents is known to decay markedly with increasing calcination/carbonation cycle number [14e17]. The decay trend of the CO2 capture capacity of limestone with the number of cycles was summarized by Abanades and Alvarez [18], as follows: N XN ¼ fm ð1 fw Þ þ fw
(1)
where fm ¼ 0.77 and fw ¼ 0.17 for a wide range of limestone and reaction conditions. Consequently, in order to capture CO2 efficiently in calcium looping cycle, more fresh sorbent is needed due to the deactivation and the sorbent makeup flow also increases. Not only the cost of the process will rise, but the abrasion and erosion in reactors will also be aggravated. Besides, more spent sorbents will be discarded, which will greatly surpass the needed amount for cement manufacture. Therefore, it is significantly necessary to
1591
Calcination reaction heat of CaCO3 at 920 C, kJ/mol heat requirement in the calciner, kJ initial mass of sample, g mass of sample after calcination, g mass of sample after N cycles, g number of cycles mass fraction of CaO derived from fresh calcium-based sorbent entering the carbonator in F0þFR (kmol/s) after N cycle carbonation temperature, C Tcarb calcination temperature, C Tcal WCaO,WCO2 mole mass of CaO and CO2, respectively, g/mol average carbonation conversion of sorbent in the Xave carbonator corrected average conversion, considering Xco-ave desulfurization in the carbonator carbonation conversion achieved after N cycles XN average conversion to CaSO4 XCaSO4 DTcalc-carb temperature difference between calciner and carbonator, K DTcalc-fresh temperature difference between calciner and fresh sorbent, K h energy requirement in the calciner per molar of CO2 entering the carbonaor, kJ/molCO2 hCaCO3 Hreq m0 m1 mN N rN
mitigate deactivation of natural sorbent and minimize the sorbent makeup flow during long-term cycles in process design. Much current and promising work involves the investigation of different methods to reduce the decay of the sorbent in reactivity, in order to boost the long-term reactivity of the sorbent or to reactivate the sorbent [19e27]. Silaban et al. [28], Lu et al. [29,30], Liu et al. [31], Sultan et al. [32] and our group [33,34] all agreed that calcium acetate produced by the acetification reaction between calcium-based sorbent and acetic acid is an excellent CO2 sorbent due to its high reactivity during the long-term cycles. Although the acetic acid is expensive, it can be replaced by the industrial wastes with acetic acid. Also the acetone can be reclaimed from calcium acetate by dry distillation. Thus, the cost of CO2 sorbent will be reduced further [33,35]. The systematic CO2 capture efficiency and energy required are the important parameters for the post-combustion CO2 capture system of power plant, which have a direct effect on power generation efficiency. Therefore, it is significantly necessary to
Fig. 1. Flow sheet of calcium looping cycle for post-combustion CO2 capture.
1592
Y. Li et al. / Energy 36 (2011) 1590e1598
Table 1 The chemical compositions of the natural limestone (wt, %). CaO
MgO
Al2O3
Fe2O3
SiO2
Others
LOI
54.70
0.36
0.05
0.04
1.18
0.52
43.15
investigate CO2 capture efficiency and energy required for the postcombustion CO2 capture of coal-fired power plant using calcium looping cycle. Abanades [14] and Rodriguez et al. [36] investigated the CO2 capture efficiency and heat requirement in a calciner for the power plant using the natural limestone. In recent years, more and more attentions have been paid to the cyclic CO2 capture capacity of different modified calcium-based sorbents, but the research on CO2 capture efficiency and energy requirement for the power plant using the modified calcium-based sorbent has been ignored. Consequently, in this paper, we focus on the analysis of CO2 capture efficiency and energy requirement for the postcombustion CO2 capture system of coal-fired power plant using modified calcium-based sorbent, incorporating a fresh feed of sorbent to compensate for the decay in activity during long-term cycles. The limestone modified with acetic acid solution is taken as an example of modified calcium-based sorbents. At the same time, the comparison between the modified and the natural limestones is made.
about 2 mm and the original sample mass was about 2 g. The reacting gas was set by a flow meter and fed to the reactor. The temperatures were collected by a computer. The calcined sample was readily picked out and stored in an inert atmosphere, and CO2 was again admitted into the furnace for further cycling. The variation of sample mass was measured by Mettler Toledo-XS105DU analytical balance. The repeatability at maximum capacity of the balance is 0.1 mg. The sample may absorb the vapor from air during measurement, hence an increase in weight may occur, but the error is less than 1%. The sample was calcined at 920e1100 C in 80% CO2/20% O2 gas mixture, and carbonated at 550e750 C in 15% CO2/85% N2 gas mixture at atmospheric pressure. In order to investigate the effects of calcination temperature and carbonation temperature on carbonation conversions of the modified limestone after 1 cycle, 920 C, 940 C, 960 C, 980 C,1000 C,1020 C,1040 C, 1060 C, 1080 C, and 1100 C were chosen as the calcination temperature and 550 C, 570 C, 600 C, 620 C, 650 C, 670 C, 700 C, 720 C, and 750 C were selected as carbonation temperature in the experiment. Thus, 81 data points were obtained as shown in Fig. 2. The long-term looping cycle experiments were repetitively performed three times, as shown in Fig. 3.
2. Experimental The cyclic carbonation conversions of the limestone modified with acetic acid solutions were investigated in order to analyze its CO2 capture efficiency. 2.1. Sorbents preparation The natural limestone was modified with 50 vol.% acetic acid solution in a beaker at the ambient temperature and pressure. The reaction time was 2 h. Then the beaker was put into a drying cabinet of 120 C. The molar ratio of acetic acid in the solution to Ca in limestone was 1.5:1. The particle size of the sorbent is below 0.125 mm. The chemical components of the natural limestone are shown in Table 1. The pore structure parameters of the natural and modified limestones after the first calcination at 920 C in 80% CO2/20% O2 gas mixture were analyzed by Micromeritics ASAP 2020-M nitrogen adsorption analyzer. The surface area and pore volume of the natural and modified limestones were calculated by BET method and BJH model, respectively, and the results were displayed in Table 2. It is found that both the surface area and pore volume of the natural limestone increase after the calcination due to the modification. 2.2. Cyclic carbonation/calcination The carbonation conversions of the sorbents were studied in a twin fixed bed reactor system including a calcination reactor and a carbonation reactor designed at the atmospheric pressure [34]. The sample boat containing the sorbent can be shifted between two reactors. The thickness of sample laid in the boat was limited to
Table 2 The pore structure parameters of the natural and modified limestones after the first calcination. Sample
Surface area (m2/g)
Pore volume (cm3/g)
Modified limestone Natural limestone
15.57 8.72
0.15 0.09
Fig. 2. Effect of reaction temperature on carbonation conversions of the natural and modified limestone after 1 cycle. (a) Surface plot, and (b) contour plot.
Y. Li et al. / Energy 36 (2011) 1590e1598
1593
cyclic carbonation behavior of the natural limestone can be described by Eq. (1) in which fm and fw are 0.77 and 0.17, respectively [16]. However, Eq. (1) is not appropriate to fit the experimental data for the modified limestone, because the maximum fitting variance is just 0.926 (Eq. (1), fm ¼ 0.83 and fw ¼ 0.41). Therefore, it is necessary to use a new formula to describe the modified limestone. Eq. (3) is employed to fit the carbonation conversion data for the modified limestone as follows: N XN ¼ fm ðb fw Þ þ fw
(3)
where, b ¼ 0.9, fm ¼ 0.92, and fw ¼ 0.41. The variance of the new fitting equation is 0.993, closer to 1.0 than that of Eq. (1). Obviously, Eq. (3) is more precise to describe the CO2 capture behavior of the modified limestone. Eq. (1) can be viewed as a special case of Eq. (3), and the carbonation conversion of the natural limestone can also be described in form of Eq. (3) (b ¼ 1, fm ¼ 0.77, and fw ¼ 0.17).
Fig. 3. Carbonation conversions and fitting equations of the natural and modified limestones. Tcarb 650 C, and Tcal 920 C.
3.2. CO2 capture efficiency for post-combustion CO2 capture system The CO2 capture efficiency is defined as follows:
In order to prevent calcined samples from recarbonation in the calcination reactor, the gas flow was switched to N2 immediately until all CO2 was purged off, and then the sample boat was moved to the carbonation reactor. The actual temperature of sample was measured with a thermocouple placed in the center of the sample boat. Based on experiments, the carbonation reaction time and the calcination reaction time were 20 min and 15 min, respectively. The carbonation conversion of sample was calculated according to the mass change during the cycles as follows:
XN ¼
mN m1 WCaO $ m0 $c WCO2
(2)
3. Results and discussion 3.1. Carbonation characteristics during the calcination/carbonation cycles The reaction temperature has a significant effect on carbonation conversion of calcium-based sorbent. Fig. 2 presents the carbonation conversions of the natural and the modified limestones with the calcination and carbonation temperatures after 1 cycle. The carbonation conversion of the modified limestone is higher than that of the natural sorbent at the same reaction temperature as shown in Fig. 2(a). The natural limestone achieves a carbonation conversion above 0.78 for carbonation at 640e690 C and calcination below 970 C, as plotted in Fig. 2(b). The modified limestone exhibits the optimum conversion, higher than 0.82 for carbonation at 640e670 C and calcination below 1050 C. The feasible carbonation temperature window for the two sorbents is similar, but the suitable calcination temperature window for the modified limestone is wider than the natural one. Considering the effect of reaction temperature on CO2 capture behavior, the cyclic carbonation conversions of the modified and the natural limestones during the 100 cycles were examined for calcination at 920 C and carbonation at 650 C as demonstrated in Fig. 3. The repetition tests demonstrate that both the natural and the modified limestones retain stable carbonation conversions after about 60 cycles and the conversion of the latter is more than twice higher than that of the former. In order to calculate the average carbonation conversion, the cyclic carbonation conversions of sorbents need to be summarized by mathematical equations. The
ECO2 ¼
F0 þ FR Xave FCO2
(4)
The Eq. (4) is easily understood by Fig. 4 which exhibits the main mass flow in CO2 capture with calcium-based sorbent calcination/ carbonation cycles for the post-combustion CO2 capture. As FCO2 is a constant, the CO2 capture efficiency, determined by F0, FR and Xave, increases with the values of F0, FR and Xave. However, increase in F0 and FR results in growth of the energy consumption in a calciner and the economic cost of CO2 capture process. Obviously, increasing the ECO2 by enhancing F0 and FR is not reasonable. Therefore, it is necessary to improve Xave in order to increase ECO2. Xave is determined by the carbonation conversion and the solid stream of calcium-based sorbent during the CO2 capture process. The mass fraction of particles entering the carbonator in the solid stream F0 þ FR which has circulated N cycles can be calculated from a succession of mass balance. Therefore, the average carbonation conversion of sorbent in the carbonator is given as follows [14].
Xave ¼
NX ¼N
rN XN
(5)
N¼1
rN ¼
F0 FRN1 ðF0 þ FR ÞN
(6)
Incorporating Eqs. (3), (5) and (6), the average carbonation conversion becomes:
Fig. 4. The main mass flow (kmol/s) in CO2 capture with calcium-based sorbent calcination/carbonation cycles.
1594
Y. Li et al. / Energy 36 (2011) 1590e1598
Xave ¼
NX ¼N N¼1
¼
F0 F0 þ FR
FR F0 þ FR
N1
N fm ðb fw Þ þ fw
F0 fm ðb fw Þ þ fw F0 þ FR ð1 fm Þ
(7)
SO2 generated from coal combustion will enter the carbonator and react with CaO. The sulfation of CaO induces the loss of CaO due to the irreversibility. Thus the average carbonation conversion must be corrected. The corrected average carbonation conversion can be calculated as follows.
Xcoave ¼ Xave XCaSO4 ¼ Xave
FSO2 ;fuel F0
(8)
Then the CO2 capture efficiency is obtained:
F0 þ FR Xcoave FCO2 0 1 FCO2 F0 F0 þ 1 f ðb f Þ F m w SO ;fuel F F @FR A ¼ RF þ fw FR $ 2 F0 CO2 0 FCO2 FR þ 1 fm FR
ECO2 ¼
(9)
FR
The CO2 capture efficiency is determined by F0/FR, FR/FCO2, fm, fw, b and FSO2, fuel/FCO2. The value of FSO2, fuel/FCO2 is equal to zero, if SO2 is absolutely removed before entering the carbonator. The ultimate analysis of coal for boiler in power plant and biomass for the calciner are shown in Table 3. A kind of bituminous coal is used by lots of power plants in Shandong province, China. The pine wood is one of the local and common biomass. It is assumed that carbon and sulfur in the coal are converted totally into CO2 and SO2, respectively. Thus, the value of FSO2, fuel/FCO2 is 0.01. The sorbent flow ratios, such as F0/FR and FR/FCO2, have a comparable effect on the average carbonation conversion, as seen in Fig. 5. When no SO2 enters the carbonator, FR/FCO2 has no effect on Xave for the sorbents. Also, Xave of the modified limestone almost doubles that of the natural sorbent as presented in Fig. 5(a). When there is SO2 in the flue gas which enters the carbonator, the result is shown in Fig. 5(b). The average carbonation conversions for the two sorbents increase with improving the values of F0/FR and FR/FCO2. F0/FR has a greater effect on Xco-ave of the two sorbents than FR/FCO2. The modified limestone displays much higher Xco-ave than the natural sorbent at the same F0/FR and FR/FCO2. For example, when F0/FR ¼ 0.05 and FR/FCO2 ¼ 4, Xco-ave of the natural and the modified limestones is 0.22 and 0.53, respectively. The sulfation of CaO induces decay in the average carbonation conversion of the two sorbents. Fig. 6 demonstrates the CO2 capture efficiency of the postcombustion CO2 capture system using the natural and the modified limestones with/without SO2 in the carbonator. The value of ECO2 for the two sorbents without SO2 in the carbonator shows an increase with F0/FR and FR/FCO2 as plotted in Fig. 6(a). When Table 3 Ultimate analysis of coal and biomass (air dry basis, wt.%).
C H S O N Fixed C Moisture Volatile matter Ash LHV (MJ/kg)
Bituminous coal
Pine wood
65.22 2.43 1.74 6.61 1.10 47.96 8.74 29.14 14.16 25.59
51.00 6.00 0.00 35.91 0.08 14.04 6.25 78.95 0.76 19.04
Fig. 5. Effects of sorbent flow ratios on average carbonation conversions. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, ECO2 for the natural and the modified limestones are 0.4 and 0.93, respectively. ECO2 for the modified limestone is 2.3 times higher than that for the natural sorbent. In order to achieve ECO2 up to 0.95 at the value of F0/FR ¼ 0.01, the value of FR/FCO2 for the natural limestone should be 4.7, while that for the modified limestone requires 2.1 which is less than a half of FR/FCO2 for the natural limestone. The effect of sulfation on CO2 capture efficiency is exhibited in Fig. 6(b). The sulfation decreases the CO2 capture efficiency for the natural and the modified limestones. For example, when F0/FR ¼ 0.03 and FR/FCO2 ¼ 1.5 without SO2 in the carbonator, ECO2 for the natural and the modified limestones is 0.41 and 0.87, respectively; while with SO2 presence, at the same F0/FR and FR/FCO2, ECO2 for the sorbents is only 0.16 and 0.61, respectively. It is readily calculated that although the content of sulfur in the coal is only 1.74 wt. %, i.e. the volume ratio of SO2 to CO2 is about 1 vol.%, compared with that without sulfation, the sulfation results in a decrease in ECO2 for the natural and modified limestones by 61% and 30%, respectively. The result demonstrates that the sulfation has less effect on ECO2 for the modified limestone than that for the natural sorbent. For the sake of achieving ECO2 of up to 0.95 at the condition of FR/FCO2 ¼ 3 with SO2 in the carbonator, the value of F0/FR for the natural limestone should be 0.08, whereas that for the modified sorbent is only 0.019. It reveals that the system using the modified sorbent requires much less makeup mass flow of the recycled and the fresh sorbent than that using the natural limestone at the same ECO2.
Y. Li et al. / Energy 36 (2011) 1590e1598
1595
The heat requirement for heating unreacted CaO, formed CaCO3, CaSO4 and inert mass from the carbonation temperature to the calcination temperature are defined as follows:
h H1 ¼ cp CaO FR ð1 Xcoave Þ þ cp CaCO3 FR Xcoave þ cp CaSO4 FSO2 ;fuel i þ cp ash Fash DTcalccarb ð10Þ where, it is assumed that the temperature for carbonation and calcination is 650 C and 920 C, respectively. The ash from coal combustion is removed by the dust collector and does not enter the calciner. Consequently, the SO2 and ash derived from combustion of the pine wood in the calciner is negligible. When no SO2 enters the carbonator, the value of FSO2, fuel is equal to zero. The heat requirement for the calcination of CaCO3 includes the formed CaCO3 during carbonation and the fresh CaCO3 from limestone. It is assumed that CaCO3 does not decompose until it is heated to the calcination temperature. The heat requirement is calculated as follows.
H2 ¼ ðFR Xcoave þ F0 ÞhCaCO3
(11)
The fresh CaCO3 filled into the calciner is heated to the calcination temperature from the ambient temperature and the heat requirement is described as follows:
H3 ¼ cp CaCO3 F0 DTcalcfresh
(12)
where, the temperature of the fresh sorbent is 10 and DTcalc-fresh is 910 C. Incorporating Eqs. (10)e(12), the energy requirement in the calciner is obtained: C,
Hreq ¼ H1 þ H2 þ H3
(13)
h Hreq ¼ cp CaO FR ð1 Xcoave Þ þ cp CaCO3 FR Xcoave i þ cp CaSO4 FSO2 ;fuel þ cp ash Fash DTcalccarb Fig. 6. Effects of sorbent flow ratios on CO2 capture efficiencies. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
3.3. Energy requirement in calciner for post-combustion CO2 capture system The calciner is responsible for regeneration of CaO, and the energy requirement in calciner is an important parameter for postcombustion CO2 capture. Although ECO2 becomes higher with increasing F0/FR and FR/FCO2, the choice of sorbent flow ratios is not arbitrary. That is because the energy balance in the system will
þ ðFR Xcoave þ F0 ÞhCaCO3 þ cp CaCO3 F0 DTcalcfresh
ð14Þ
h is used to describe energy requirement of the calciner per molar of CO2 entering the carbonator for the post-combustion CO2 capture system as follows.
h¼
Hreq FCO2
(15)
Considering Eqs. (8) and (14), the h is obtained further as follows:
! ! F0 F FSO2 ;fuel FCO2 FR FSO2 ;fuel FCO2 FR FSO2 ;fuel FR FR FR0 fm ðb fw Þ FR fm b fw h ¼ cp CaO 1 F þ cp CaCO3 þ cp CaSO4 fw þ $ $ þ fw $ $ F0 0 FCO2 F F F F F F F FCO2 R R 0 CO2 CO2 0 CO2 FR þ 1 fm FR þ 1 fm # ! # " F FCO2 FR FSO2 ;fuel F FR FR0 fm ðb fw Þ F F F F þ 0 $ R hCaCO3 þ cp CaCO3 0 $ R DTcalcfresh þ cp ash ash DTcalccarb þ þ fw $ $ FCO2 FCO2 F0 þ 1 fm FR F0 FCO2 FR FCO2 FR FCO2 "
FR
(16)
impose further limitations on these sorbent flow ratios. Not only fresh CaCO3 from limestone and formed CaCO3 from carbonator but also the unreacted CaO, CaSO4, ash and inert mass are fed into the calciner. The energy requirement in the calciner is comparatively complex. The heat input in the calciner is calculated progressively.
In Eq. (16), fm, fw and b are confirmed for the natural and modified limestones, and cp CaO, cp CaCO3, cp ash, cp CaSO4 and hCaCO3 can also be obtained from the reference [37]. Thus h is calculated at the different values of F0/FR and FR/FCO2. Fig. 7(a) presents the energy requirement in the calciner with F0/FR and FR/FCO2, with no
1596
Y. Li et al. / Energy 36 (2011) 1590e1598
and the modified limestones decreases by 33% and 19% due to the sulfation, respectively, compared with the h without sulfation. It seems that the effect of sulfation on h for the modified limestone is less than that for the natural sorbent. Fig. 8 demonstrates the relation between the CO2 capture efficiency and the energy requirement in the calciner for the natural and the modified limestones. When no SO2 enters the carbonator, both sorbents exhibit an increase in h with increasing CO2 capture efficiency as shown in Fig. 8(a). Also, h for the modified limestone is lower than that for the natural sorbent at the same ECO2. In order to achieve ECO2 of up to 0.95, h for the natural limestone is 272 kJ/molCO2, whereas that for the modified sorbent is 223 kJ/molCO2. It is observed that the difference in h between the modified limestone and the natural sorbent becomes larger with increasing ECO2. The post-combustion CO2 capture system using the modified limestone possesses higher power generation efficiency at the same ECO2 due to lower energy consumption. The sulfation of the sorbents aggravates the raise in the energy consumption for the calciner as presented in Fig. 8(b). When ECO2 ¼ 0.95 and FR/FCO2 ¼ 3, h for the natural and modified limestones are 297 kJ/molCO2 and 249 kJ/molCO2, respectively. The result indicates that when the volume ratio of SO2 to CO2 is about 1 vol.%, compared with cases without SO2 entering the carbonator, h for the natural and the modified limestones increases by 9% and
Fig. 7. Effects of sorbent flow ratios on energy requirement in the calciner. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
SO2 entering the carbonator. For the natural and the modified limestones, h increases with improving the values of F0/FR and FR/ FCO2. It is also found that for the modified limestone h is higher than that for the natural limestone at the same F0/FR and FR/FCO2. That is because the modified limestone possesses greater carbonation conversion, hence more CaCO3 formed in the carbonator enters the calciner. Consequently, more heat is required for the CaO regeneration. For example, when F0/FR ¼ 0.04 and FR/FCO2 ¼ 2, for the natural limestone h is 157 kJ/molCO2, while for the modified sorbent it is 270 kJ/molCO2. However, we cannot conclude that the energy requirement in the calciner for the modified limestone is higher. That is because the ECO2 for the two sorbents is different at the same sorbent flow ratio. h for different sorbents should be discussed at the same ECO2. Moreover, although the energy requirement in the calciner using the modified limestone is higher, the energy released in the carbonator is also greater as the carbonation proceeds. The energy requirement in the calciner with SO2 in the carbonator is plotted in Fig. 7(b). The sulfation decreases the energy requirement in the calciner at the same F0/FR and FR/FCO2. That is because sulfation of the sorbent in the carbonator results in a decrease in the formed CaCO3 which consumes heat to regenerate. At the value of F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, h for the natural and the modified limestones are 105 kJ/molCO2 and 219 kJ/molCO2, respectively. When F0/FR ¼ 0.01 and FR/FCO2 ¼ 2, h for the natural
Fig. 8. Relation between CO2 capture efficiency and energy requirement in the calciner. (a) Without SO2 in the carbonator, and (b) with SO2 in the carbonator (FSO2, fuel/FCO2 is 0.01).
Y. Li et al. / Energy 36 (2011) 1590e1598
11%, respectively. Although the sulfur content in coal for power plants is low, the sulfation results in an increase in the energy requirement in the calciner. Therefore, it is significantly necessary to remove SO2 from the flue gas before entering the carbonator in order to avoid more energy consumption in the calciner. The spent CaO from the calciner can be used to remove the SO2 before they enter the carbonator. Sun et al. [38] and Manovic et al. [39] regarded the spent CaO as a good sorbent to capture SO2 due to the generation of macropores during multiple calcination/carbonation cycles. Since SO2 is emitted from the fossil fuel burning together with CO2, it should be captured. Using the spent CaO for SO2 capture is obviously cheaper than other kinds of sorbents. Also, the heat requirement using the spent CaO for SO2 removal is much less than that using the fresh limestone. 4. Conclusions The CO2 capture efficiency and energy requirement in a calciner for the post-combustion system using the modified calcium-based sorbents were investigated. The limestone modified with acetic acid solution, i.e. calcium acetate is taken as an example of the modified calcium-based sorbents. The repeated tests during 100 cycles demonstrate that the natural and the modified limestones retain stable carbonation conversions after about 60 cycles and the conversion of the latter is more than twice higher than that of the former. The modified limestone exhibits much higher average carbonation conversion and CO2 capture efficiency than the natural sorbent at the same sorbent flow ratios. F0/FR has a greater effect on the average carbonation conversions than the FR/FCO2. The sulfation of CaO induces decay in the average carbonation conversion of two sorbents. The system using the modified sorbent requires much less makeup mass flow of the recycled and the fresh sorbent than that using the natural limestone at the same CO2 capture efficiency. The calciner is responsible for regeneration of CaO, and the energy requirement in calciner is an important parameter. Although CO2 capture efficiency becomes higher with increasing the sorbent flow ratios, the choice of sorbent flow ratios is not at random because of an increase in energy requirement in the calciner with the sorbent flow ratios. The sulfation decreases the energy requirement in the calciner at the same sorbent flow ratios. The effect of sulfation on energy requirement in the calciner for the modified limestone is less than that for the natural sorbent. The energy requirement in the calciner for the modified limestone is lower than that for the natural sorbent at the same CO2 capture efficiency. In order to achieve 0.95 of CO2 capture efficiency without sulfation in the carbonator, the energy requirement in the calciner for the natural limestone is 272 kJ/molCO2, whereas that for the modified sorbent is 223 kJ/molCO2. The limestone modified with acetic acid solution shows greater advantages in the CO2 capture efficiency and energy consumption than the natural sorbent. The result indicates that when the volume ratio of SO2 to CO2 is about 1 vol.%, the energy requirement in the calciner for the natural and the modified limestones increases by 9% and 11%, respectively, compared with the energy consumption without sulfation in the carbonator. Although the sulfur content in coal for power plants is low, sulfation results in an increase in the energy requirement in the calciner. Therefore, it is significantly necessary to remove SO2 from the flue gas before entering the carbonator in order to avoid more energy consumption in the calciner. Acknowledgments Financial supports from the National Basic Research Program of China (2006CB705806), China Postdoctoral Science Foundation Funded Project (20090461205), Special Funds for Postdoctoral
1597
Innovative Projects of Shandong Province (200902019), China and Independent Innovation Foundation of Shandong University (IIFSDU) are sincerely acknowledged. References [1] Naqvi R, Wolf J, Bolland O. Part-load analysis of a chemical looping combustion (CLC) combined cycle with CO2 capture. Energy 2007;32:360e70. [2] Garg A, Shukla PR. Coal and energy security for India: role of carbon dioxide (CO2) capture and storage (CCS). Energy 2009;34:1032e41. [3] Blamey J, Anthony EJ, Wang J, Fennell PS. The calcium looping cycle for largescale CO2 capture. Prog Energy Combust 2010;36:260e79. [4] Nikulshina V, Hirsch D, Mazzotti M, Steinfeld A. CO2 capture from air and coproduction of H2 via the Ca(OH)2eCaCO3 cycle using concentrated solar power-thermodynamic analysis. Energy 2006;31:1715e25. [5] Gao L, Paterson N, Dugwell D, Kandiyoti R. Zero-emission carbon concept (ZECA): equipment commissioning and extents of the reaction with hydrogen and steam. Energy Fuels 2008;22:463e70. [6] Lin SY, Suzuki Y, Hatano H. Developing an innovative method HyPr-RING, to produce hydrogen from hydrocarbons. Energ Convers Manage 2002;43:1283e90. [7] Barelli L, Bidini G, Corradetti A, Desideri U. Production of hydrogen through the carbonation-calcination reaction applied to CH4/CO2 mixtures. Energy 2007;32:834e43. [8] Shimizu T, Hirama T, Hosoda H, Kitano K, Inagaki M, Tejima K. A twin fluidbed reactor for removal of CO2 from combustion processes. Trans I Chem E 1999;77:62e8. [9] Hughes RW, Lu DY, Anthony EJ, Macchi A. Design, process simulation and construction on an atmospheric dual fluidized bed combustion system for in situ CO2 capture using high-temperature sorbents. Fuel Process Technol 2005;86:1523e31. [10] Abanades JC, Grasa G, Alonso M, Rodriguez N, Anthony EJ, Romeo LM. Cost structure of a postcombustion CO2 capture system using CaO. Environ Sci Tehnol 2007;41:5523e7. [11] Ives M, Mundy RC, Fennell PS, Davidson JF, Dennis JS, Hayhurst AN. Comparison of different natural sorbents for removing CO2 from combustion gases, as studied in a bench-scale fluidized bed. Energy Fuels 2008;22:3852e7. [12] Alonso M, Rodriguez N, Gonzalez B, Grasa G, Murillo R, Abanades JG. Carbon dioxide capture from combustion flue gases with a calcium oxide chemical loop. Experimental results and process development. Int J Greenhouse Gas Con 2010;4:167e73. [13] Romeo LM, Lara Y, Lisbona P, Martinez A. Economical assessment of competitive enhanced limestone for CO2 capture cycles in power plants. Fuel Process Technol 2009;90:803e11. [14] Abanades JC. The maximum capture efficiency of CO2 using a carbonation/ calcination cycle of CaO/CaCO3. Chem Eng J 2002;90:303e6. [15] Sun P, Grace JR, Lim CJ, Anthony EJ. The effect of CaO sintering on cyclic CO2 capture in energy systems. AIChE J 2007;53:2432e42. [16] Lu DY, Hughes RW, Anthony EJ. Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Process Technol 2008;89: 1386e95. [17] Lu DY, Hughes RW, Anthony EJ, Manovic V. Sintering and reactivity of CaCO3based sorbents for in situ CO2 capture in fluidized beds under realistic calcination conditions. J Environ Eng-ASCE 2009;6:404e10. [18] Abanades JC, Alvarez D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003;17:308e15. [19] Feng B, Liu WQ, Li X, An H. Overcoming the problem of loss-in-capacity of calcium oxide in CO2 capture. Energy Fuels 2006;20:2417e20. [20] Feng B, Du M, Dennis TJ, Anthony K, Perumal MJ. Reduction of energy requirement of CO2 desorption by adding acid into CO2-loaded solvent. Energy Fuels 2010;24:213e9. [21] Feng B, An H, Tan E. Screening of CO2 adsorbing materials for zero emission power generation systems. Energy Fuels 2007;21:426e34. [22] Li ZS, Cai NS, Huang YY. Effect of preparation temperature on cyclic CO2 capture and multiple carbonationecalcination cycles for a new Ca-based CO2 sorbent. Ind Eng Chem Res 2006;45:1911e7. [23] Florin NH, Harris AT. Reactivity of CaO derived from nano-sized CaCO3 particle through multiple CO2 capture-and-release cycles. Chem Eng Sci 2009;64:187e91. [24] Albrecht KO, Wagenbach KS, Satrio JA, Shanks BH, Wheelock TD. Development of CaO-based CO2 sorbent with improved cyclic stability. Ind Eng Chem Res 2008;47:7841e8. [25] Pacciani R, Muller CR, Davidson JF, Dennis JS, Hayhurst AN. How does the concentration of CO2 affect its uptake by a synthetic Ca-based solid sorbent. AIChE J 2008;54:3308e11. [26] Lu H, Smirniotis PG. Calcium oxide doped sorbents for CO2 uptake in the presence of SO2 at high temperatures. Ind Eng Chem Res 2009;48:5454e9. [27] Dennis JS, Pacciani R. The rate and extent of uptake of CO2 by a synthetic, CaOcontaining sorbent. Chem Eng Sci 2009;64:2147e57. [28] Silaban A, Narcida M, Harrison DP. Calcium acetate as a sorbent precursor for the removal of carbon dioxide from gas streams at high temperature. Resour Conserv Recycl 1992;7:139e53. [29] Lu H, Khan A, Smirniotis PG. Relationship between structural properties and CO2 capture performance of CaO-based sorbents obtained from different organometallic precursors. Ind Eng Chem Res 2008;47:6216e20.
1598
Y. Li et al. / Energy 36 (2011) 1590e1598
[30] Lu H, Reddy EP, Smirniotis PG. Calcium oxide based sorbents for capture of carbon dioxide at high temperatures. Ind Eng Chem Res 2006;45:3944e9. [31] Liu WQ, Low NWL, Feng B, Wang GX, da Costa JCD. Calcium precursors for the production of CaO sorbents for multicycle CO2 capture. Environ Sci Tehnol 2010;44:841e7. [32] Sultan DS, Muller CR, Dennis JS. Capture of CO2 using sorbents of calcium magnesium acetate (CMA). Energy Fuels 2010;24:3687e97. [33] Li YJ, Zhao CS, Chen HC, Liang C, Duan LB, Zhou W. Modified CaO-based sorbent looping cycle for CO2 mitigation. Fuel 2009;88:697e704. [34] Li YJ, Zhao CS, Duan LB, Liang C, Li QZ, Zhou W, et al. Cyclic calcination/ carbonation looping of dolomite modified with acetic acid for CO2 sequestration. Fuel Process Technol 2008;89:1461e9.
[35] Palasantzas IA, Wise DL. Preliminary economic analysis for production of calcium magnesium acetate from organic residues. Resour Conserv Recycl 1994;11:225e43. [36] Rodriguez N, Alonso M, Grasa G, Abanades JC. Heat requirements in a calciner of CaCO3 integrated in a CO2 capture system using CaO. Chem Eng J 2008;138:148e54. [37] Green DW, Perry RH. Perry’s chemical engineers’ handbook. 8th ed. McGrawHill; 2007. [38] Sun P, Grace JR, Lim CJ, Anthony EJ. Sequential capure of CO2 and SO2 in a pressurized TGA simulating FBC conditions. Environ Sci Technol 2007; 41:2943e9. [39] Manovic VM, Anthony EJ. SO2 retention by reactivated CaO-based sorbent from multiple CO2 capture cycles. Environ Sci Technol 2007;41:4435e40.