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Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping
FUPROC-04601; No of Pages 9 Fuel Processing Technology xxx (2015) xxx–xxx
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping Xin Xie, Ying-Jie Li ⁎, Chang-Tian Liu, Wen-Jing Wang School of Energy and Power Engineering, Shandong University, Jinan 250061, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Received in revised form 14 June 2015 Accepted 14 June 2015 Available online xxxx Keywords: HCl absorption Carbide slag CaO/Ca3Al2O6 sorbent Calcium looping CO2 capture
a b s t r a c t The synthetic CaO/Ca3Al2O6 (prepared from carbide slag, aluminum nitrate enneahydrate, and glycerol water solution by combustion synthesis) as a CO2 sorbent discharged from the CO2 capture cycles using the calcium looping was used to remove HCl after a series of carbonation/calcination cycles. The effects of chlorination temperature, HCl concentration, presence of CO2, and cycle number on the HCl absorption by the CaO/Ca3Al2O6 sorbent after the repetitive carbonation/calcination cycles for CO2 capture were discussed. In addition, the HCl absorption capacities of the CaO/Ca3Al2O6 sorbent and the carbide slag were compared. The chlorination products of the CaO/Ca3Al2O6 sorbent after 1 h HCl absorption are CaClOH, CaO, and Ca3Al2O6 detected by XRD analysis. Among 600–800 °C, the CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles achieves the highest HCl absorption capacity at 700 °C. The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent rises as HCl concentration increases. CO2 is adverse to the HCl absorption by the cycled CaO/Ca3Al2O6 sorbent in the chlorination process. With the carbonation/calcination cycle number increasing from 0 to 50, the CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the highest HCl absorption capacity. HCl absorption capacities of the cycled CaO/Ca3Al2O6 sorbent after 20 and 50 cycles were 0.18 and 0.13 g/g, which were 2.3 and 2.6 times as high as those of the cycled carbide slag after the same cycles, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The CO2 emission from fossil fuel-fired power plants and hydrogen production systems is widely accepted as the predominant contributor to global warming and various methods for CO2 capture have been investigated [1–5]. The calcium looping technology, i.e., the carbonation/ calcination cycles of CaO, is considered to be one of the most potential and feasible methods to capture CO2 for storage [6–8]. The cyclic process is based on the reversible reaction between CaO and CO2 shown in Eq. (1). However, it has been proved that the CO2 capture capacities of natural calcium-based sorbents such as limestone and dolomite show a rapid decline with the cycle number due to sintering of the sorbents [9–11]. CaOðsÞ þ CO2 ðgÞ↔CaCO3 ðsÞ:
ð1Þ
Lots of methods have been developed to enhance the cyclic CO2 capture capacity of calcium-based sorbents, minimize their loss in CO2 capture capacity, increase the sintering resistance, and promote their mechanical stability in the multiple CO2 capture cycles using calcium looping [12–16]. The criterion for the CO2 sorbent design is to increase active surface area, pore structure stability, and mechanical stability of the sorbent, which would increase CO2 capture capacity of the sorbent.
⁎ Corresponding author. E-mail address: [email protected] (Y.-J. Li).
Material chemistry methods oriented to this criterion are generally focused on the use of rigid porous materials as carriers of the calciumbased sorbents, use of additives to improve the sorbent thermal stability, reduction of the sorbent particle size down to the nanometer scale, and use of synthetic precursors to produce novel sorbents with a rich micropore structure [6]. The development of some novel methods of modifying calcium-based sorbents and synthesizing new ones with enhanced CO2 capture capacity, thermal stability, and mechanical strength is a strong active area of research [6,11,12]. The synthetic sorbents prepared from CaO and Al2O3 precursors under different conditions exhibited a significant enhancement in CO2 capture durability, sintering resistance, and pore structure in the calcium looping cycles [17–26]. The synthetic sorbents were composed of CaO and inert supporters (calcium aluminates) such as Ca12Al14O33 [17–20], Ca9Al6O18 [21–23], Ca3Al2O6 [24], and Ca3Al10O18 [15]. The difference in inert supporters depended on not only the calcium and aluminum precursors but also the synthetic methods. Li et al. [17] prepared a CaO/Ca12Al14O33 sorbent by adding CaO and Al(NO3)3·9H2O into the mixture of 2-propanol and distilled water (wet mixing method) and the solution was calcined in air at 800–1300 °C after being stirred and dried. Martavaltzi and Lemionidou [18] used Ca(CH3COOH)2, Al(NO3)3·9H2O, and 2-propanol water solution by mixing method to synthesize a CaO/Ca12Al14O33 sorbent through calcination at 900 °C. Wu et al. [19] mixed nano CaCO3, aluminum sol, and sodium hexametaphosphate together to prepare nano CaO/Ca12Al14O33 pellet by heating and pressing. Using Ca(NO3)2·4H2O, Al(NO3)3·9H2O, citric acid, and ethylene glycol as raw materials. Mastin et al. [20] obtained single phase Ca3Al2O6
http://dx.doi.org/10.1016/j.fuproc.2015.06.028 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
under the controlled conditions and they proposed a new synthesis route of a CaO/Ca12Al14O33 sorbent through the decomposition of Ca3Al2O6 at different calcination temperatures (900–1100 °C). Zhou et al. [21,22] investigated the effect of calcium and aluminum precursors on CO2 capture performance of the obtained synthetic sorbents by wet mixing method and the result showed that CaO/Ca9Al6O18 prepared from calcium citrate and aluminum nitrate exhibited the highest CO2 capture capacity. Radfarnia and Iliuta [23] used limestone, citric acid, and aluminum nitrate or aluminum acetate to prepare a CaO/ Ca9Al6O18 sorbent. Zhang et al. [24] adopted a four-step heating mode to synthesize a CaO/Ca3Al2O6 sorbent through the citrate preparation route using citric acid, aluminum nitrate, and CaCO3 as raw materials and they pointed out that Ca3Al2O6 in the synthetic sorbent prevented the sintering of CaO particles. CaO/Ca12Al14O33 with high reactivity for CO2 capture at high temperature was synthesized by Luo et al. [11,25] using a standard sol–gel process with citric acid as the chelation agent. Koirala et al. [26] employed the single nozzle flame spray pyrolysis (FSP) method to prepare a CaO/Ca12Al14O33 sorbent from calciumnaphthenate, aluminum acetylacetonate, and xylene. These synthetic sorbents all exhibited high CO2 capture capacity and durability in the multiple cycles. Carbide slag which is mainly composed of Ca(OH)2, is a by-product of the PVC production process and is always discharged from the chlor-alkali plants as an industrial solid waste. Li et al. [27,28] found that carbide slag showed a good CO2 capture performance in the calcium looping cycles. The carbide slag was effectively recycled to capture CO2, thus the cost of CO2 capture was also decreased. Our research group synthesized a new CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis method [29]. The new synthetic sorbent prepared from a much cheaper industrial solid waste, i.e., carbide slag as CaO precursor and glycerol was investigated. As a solvent which is a byproduct from the preparation of the biodiesel fuel, glycerol is also very cheap. Thus, the preparation of the new synthetic sorbent has a much lower cost [29]. We found that when the mass ratio of CaO/Ca3Al2O6 was 73:27, the synthetic sorbent exhibited the highest CO2 capture capacity of 0.38 g/g after 50 cycles, which was 1.8 times as high as that of the carbide slag (carbonation: 700 °C, 15% CO2, 30 min; calcination: 850 °C, N2, 10 min) [29]. Because of the plentiful availability and environmental benefits, more attentions have been paid to the combustion and gasification of biomass fuels and refuse-derived fuels (RDFs). However, the chlorine contents in these fuels are much higher than those in the common coals [30,31]. Consequently, the flue gases of biomass-fired and RDFs-fired boilers often contain hydrogen chloride (HCl) with the
CO2 rich gas
concentration of 200–1500 ppm [31–34], which cannot meet the emission limit without the addition of sorbents. HCl is a poisonous air pollutant and might turn into some corrosive compounds. The carbide slag not only can be used to remove HCl, but also capture CO2 from the flue gases of biomass-fired and RDFs-fired boilers. Wang et al. [35] found that the presence of HCl sharply decreased CO2 capture capacity of calcium-based sorbent after a dozen CO2 capture cycles using the calcium looping in the simultaneous CO2/HCl capture process. Therefore, in order to avoid the adverse effect of HCl on CO2 capture of the sorbent in the multiple cycles, we have proposed that the carbide slag discharged from CO2 capture cycles using the calcium looping could be used to remove HCl from the flue gases of biomass-fired and RDFs-fired boilers in our previous research [36]. And then CO2 and HCl can be sequentially captured by the carbide slag, as shown in Fig. 1. Not only the spent carbide slag discharged from the CO2 capture cycles will be reused, but also the cost for HCl removal will be reduced. The HCl absorption capacity (HCl uptake by per unit mass sorbent, g HCl/g sorbent) of the carbide slag after 1 and 50 carbonation/ calcination cycles were 0.18 and 0.05 g/g in the previous research [36], respectively. It is necessary to improve the HCl absorption behavior of the cycled carbide slag from the CO2 capture cycles using calcium looping due to the lower HCl absorption capacity. The CO2 capture capacity of the CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis could achieve 1.8 times as high as that of the carbide slag after 50 cycles due to the more porous structure, larger surface area, and pore volume in the multiple cycles [29]. The calcium-based sorbent with more porous structure, larger surface area, and pore volume could achieve higher HCl absorption capacity [37–39]. Therefore, the CaO/Ca3Al2O6 sorbent may be used to sequentially capture CO2 and HCl from the flue gases of the biomass-fired and the RDFs-fired boilers. The CaO/Ca3Al2O6 sorbent as a CO2 sorbent is repetitively performed in a carbonator and calciner. A part of the discharged CaO/Ca3Al2O6 from the CO2 capture cycles is used to remove HCl from the flue gases of biomass-fired and RDFs-fired boilers. The HCl absorption by the CaO/Ca3Al2O6 sorbent from the CO2 capture cycles using calcium looping is worth to be studied. In this work, the HCl absorption performance of the cycled CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis was investigated. And the effects of chlorination temperature, HCl concentration, presence of CO2, and cycle number were discussed. Moreover, the HCl absorption performance of the CaO/Ca3Al2O6 sorbent was compared with that of the carbide slag from the CO2 capture cycles using calcium looping.
flue gas (CO2 free) CaO
carbide slag
calciner 850-950 0 oC
flue gas (HCl free)
carbonator 600-700 oC
air
biomass-fired biomass or RDFs-fired boiler or RDFs
CaCO3 fuel O2 discharged sorbent (for HCl removal) discharged sorbent (for cement production)
chlorination product and CaO, ash
Fig. 1. Schematic of HCl removal from flue gas of biomass-fired or RDFs-fired boiler by the calcium-based sorbent from CO2 capture cycles using calcium looping.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx Table 1 Chemical components of carbide slag (wt.%). CaO
MgO
SiO2
Al2O3
Fe2O3
Na2O
Ti2O
Others
Loss on ignition
71.09
0.09
2.51
1.96
0.16
0.02
0.04
1.28
22.85
2. Experimental 2.1. Sorbent preparation The carbide slag (sieved to size b0.125 mm) used in the experiments of this work was sampled from a chlor-alkali plant in Shandong Province, China. The chemical components of the carbide slag were analyzed by X-ray fluorescence (XRF) and listed in Table 1. Aluminum nitrate enneahydrate (Al(NO3)3·9H2O, Shanghai Qingxi Chemical Technology Co., Ltd, China) and glycerin (C3H8O3, Tianjin Kemiou Chemical Reagent Co., Ltd, China) used in the preparation of the synthetic sorbent were analytically pure (N99%). Since the synthetic sorbent containing the mass ratio of CaO derived from the carbide slag to Al2O3 derived from Al(NO3)3·9H2O = 90:10 (namely the mass ratio of CaO/Ca3Al2O6 = 73:27) exhibited the highest CO2 capture capacity in the carbonation/calcination cycles [29], we prepared this synthetic sorbent on the basis of the preparation procedure reported by Li et al. [29], as follows: 10 g carbide slag and 4.56 g Al(NO3)3·9H2O were completely dissolved in the mixture of glycerin (50 mL) and deionized water (50 mL). This solution was stirred at 80 °C and then was combusted at 800 °C for 1 h under air atmosphere in the muffle furnace to synthesize the sorbent. The obtained synthetic sorbent was ground and sieved to size b 0.125 mm.
3
3 min). Then the mass of sorbent sample was measured by a Mettler Toledo-XS105DU electronic balance with a resolution of 0.1 mg. The sorbent samples after 0, 1, 2, 3, 4, 5, 10, 15, 20, 35, and 50 carbonation/calcination cycles in TFBR were sent into the chlorinator for HCl absorption, respectively. 600–800 °C was chosen to be the chlorination temperature range. The HCl purity of the HCl gas cylinder was 3000 ppm (N2 as an equilibrium gas) and the gas flow was mixed with N2 or CO2 to get the specified HCl concentrations. The reaction atmosphere was switched to the gas mixture consisting of HCl (500–2500 ppm) and N2 (balance). Additionally, 15 vol.% CO2 was added into the reaction gas in order to investigate the effect of the presence of CO2 on HCl absorption by the cycled sorbents. The sorbent sample after HCl absorption was cooled in N2 to room temperature (about 3 min) and then was weighed by the electronic balance. For the sorbent sample after each number of cycles at each HCl absorption condition, three identically repetitive experiments were performed and the error bars of data were less than 1%. The average value of the data in the three repetitive experiments was taken to guarantee the accuracy of this research. The HCl absorption capacity of the sorbent sample from carbonation/ calcination cycles is calculated as follows: C HCl; N ¼ mchl; N ðt Þ−mcal =mcal
ð2Þ
where CHCl,N denotes the HCl absorption capacity of the sorbent sample after N carbonation/calcination cycles, g (HCl)/g (sorbent); t is the reaction time, min; mchl,N (t) represents the mass of the chlorinated sorbent sample after N carbonation/calcination cycles at t, g; and mcal is the sorbent sample mass after complete calcination, g.
2.2. HCl absorption by sorbents from calcium looping cycles 2.3. Phase and microstructure analysis As shown in Fig. 2, a triple fixed-bed reactor (TFBR) comprises a carbonator, a calciner, and a chlorinator, which can be operated at atmospheric pressure, and the temperatures of the reactor can maintain at specified values in the range of 600–850 °C. Before being introduced into the reactor, each gas (HCl, N2, and CO2) was controlled by a needle valve and the mass flow rate of the each gas was regulated by a mass flowmeter. All of the gases were mixed in a blending tub and the gas mixture was then sent to TFBR. The sample boat containing 500 mg synthetic sorbent was firstly put into the carbonator at 700 °C in a 15 vol.% CO2/85 vol.% N2 gas mixture for 20 min. After the carbonation, the reaction atmosphere was switched to pure N2 and the sample boat was carried into the calciner at 850 °C in pure N2 for 10 min. Then the first carbonation/calcination cycle was finished. The sorbent sample after various carbonation/calcination cycles was picked out from the calciner and sent into a drier for cooling in N2 to room temperature (about
mass flowmeter
The phase components of the CaO/Ca3Al2O6 sorbent under different conditions were identified by a D/Max-IIIA X-ray diffraction (XRD). A SUPRATM 55 field emission scan electron microscope (SEM) was used to observe the surface morphologies of the CaO/Ca3Al2O6 sorbent after the various carbonation/calcination cycles and the chlorinated CaO/Ca3Al2O6 sorbent. The elements on the surfaces of the CaO/ Ca3Al2O6 sorbent were detected by an Oxford INCA sight X energy dispersive spectrometry (EDS). A Micromeritics ASAP 2020-M nitrogen absorption analyzer was utilized to measure the pore structure parameters of the CaO/Ca3Al2O6 sorbent after the various cycles. And the pore structure parameters of the CaO/Ca3Al2O6 sorbent and the carbide slag in the cycles were compared. The surface area and pore volume were calculated according to Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) model, respectively.
blending tub
electric balance computer
absorption bottle
valve temperature controller
NaOH solution
sample boat HCl/N2 N2
CO2
carbonator
calciner
chlorinator
Fig. 2. Schematic of TFBR.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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3. Results and discussion
(a)
o
600 C, CaO/Ca3Al2O6 o
650 C, CaO/Ca3Al2O6
3.1. XRD analysis of chlorinated synthetic sorbent from calcium looping cycles
o
700 C, CaO/Ca3Al2O6
0.18
o
3.2. Effect of chlorination temperature on HCl absorption by cycled CaO/Ca3Al2O6 sorbent
CHCl,5 (g/g)
750 C, CaO/Ca3Al2O6 o
800 C, CaO/Ca3Al2O6
0.12
o
600 C, carbide slag
0.06
o
650 C, carbide slag o
700 C, carbide slag o
750 C, carbide slag o
800 C, carbide slag
0.00 0
20 40 Chlorination time (min)
0.22
60
(b)
0.20
CHCl,5 (g/g)
Many studies on HCl absorption by calcium-based sorbents have been implemented on the basis of the assumption that the reaction product was CaCl2 [34,39–41]. However, the product of the reaction between CaO and HCl was reported to be CaClOH [36,42–44]. Moreover, some researchers pointed out that a variety of intermediates (CaClOH and CaCl2·2H2O) and end products (CaCl2) were formed in the absorption process of HCl [45] and the reaction product depended on the chlorination reaction time [46]. CaO and the inert support substance Ca3Al2O6 are found in the synthetic sorbent and the mass ratio of CaO to Ca3Al2O6 is 73:27 according to the XRD quantitative analysis, as exhibited in Fig. 3(a). Thus, the synthetic sorbent is called the CaO/Ca3Al2O6 sorbent. During the 1 h HCl absorption by the CaO/Ca3Al2O6 sorbent after 0 cycle, Ca3Al2O6 remains stable and a fraction of CaO turns into CaClOH, as illustrated in Fig. 3(b). The chlorination product of the CaO/Ca3Al2O6 sorbent after 5 cycles is still CaClOH after 1 h chlorination, as presented in Fig. 3(c). The results show that the cycled CaO/Ca3Al2O6 sorbent from the calcium looping cycles absorbs HCl to generate CaClOH after 1 h chlorination and Ca3Al2O6 as the supporter remains stable.
0.18
CaO/Ca3Al2O6 carbide slag
0.16 0.14 0.12
It has been widely accepted that the reaction temperature is of great importance in HCl absorption by calcium-based sorbents [47,48] and there exists an optimum temperature range of 550–700 °C for HCl absorption. Our previous paper reported that the optimum temperature for HCl removal by the carbide slag undergone various carbonation/ calcination cycles was 700 °C [36]. As depicted in Fig. 4(a), with increasing the chlorination temperature in the range of 600–800 °C, the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent experienced 5 carbonation/calcination cycles increases firstly and then decreases. Among 600–800 °C, the cycled CaO/Ca3Al2O6 sorbent shows the highest HCl absorption capacity at 700 °C. It indicates that the optimum chlorination temperatures for CaO/Ca3Al2O6 sorbent and the carbide slag from CO2 capture cycles are the same. The CaO/Ca3Al2O6 sorbent after 5 cycles exhibits higher HCl absorption capacity than the carbide slag at the same chlorination 25000
CaO Ca3Al2O6
20000 15000
(a) Mass ratio of CaO to Ca3Al2O6=73:27
10000
Intensity (CPS)
5000 0 400 300
CaO Ca3Al2O6
200
CaClOH
(b)
0.10 600
650 700 750 o Chlorination temperature ( C)
800
Fig. 4. Effect of chlorination temperature on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles: (a) CHCl,5 vs. chlorination time; (b) CHCl,5 at 60 min vs. chlorination temperature (chlorination: 1500 ppm HCl/N2 balance).
temperatures, as presented in Fig. 4(b). CHCl,5 of the CaO/Ca3Al2O6 sorbent after 1 h chlorination at 700 °C and 800 °C are 0.21 and 0.19 g/g, which are 42% and 84% higher than that of the carbide slag, respectively. It is found that the chlorination temperature exhibits little impact on the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent. By contrast, CHCl,5 of the carbide slag varies with the chlorination temperature. As the chlorination temperature increases from 700 to 800 °C, CHCl,5 of the CaO/Ca3Al2O6 sorbent decreases by 9% merely, while CHCl,5 of the carbide slag decreases by 30%. It indicates that the feasible temperature window for HCl absorption by the cycled CaO/Ca3Al2O6 sorbent is wider than that of the cycled carbide slag. The cycled CaO/Ca3Al2O6 sorbent shows higher HCl absorption capacity than the cycled carbide slag at high temperature above 700 °C. 3.3. Effect of HCl concentration on HCl absorption by cycled CaO/Ca3Al2O6 sorbent
100 0 300
(c)
CaO Ca3Al2O6
200
CaClOH
100 0
10
20
30
40
50
o
60
70
80
90
2 () Fig. 3. XRD spectra of the synthetic sorbent before and after chlorination: (a) synthetic sorbent; (b) chlorinated synthetic sorbent after 0 carbonation/calcination cycle; (c) chlorinated synthetic sorbent after 5 carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl/N2 balance, 60 min).
The HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles is significantly affected by the HCl concentration, as plotted in Fig. 5(a). The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent increases rapidly with increasing the HCl concentration from 500 to 1500 ppm, but it increases slowly with the HCl concentration above 1500 ppm. For example, CHCl,5 of the cycled CaO/Ca3Al2O6 sorbent at 1 h under 1500 ppm HCl is 2.8 times as high as that at 1 h under 500 ppm HCl. But as the HCl concentration rises from 1500 to 2500 ppm, CHCl,5 of the cycled CaO/Ca3Al2O6 sorbent at 1 h only increases by 13%. As exhibited in Fig. 5(b), the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent is greater than that of the cycled carbide slag for
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
0.25
500 ppm, CaO/Ca3Al2O6 1000 ppm, CaO/Ca3Al2O6
Intensity (CPS)
2500 ppm, CaO/Ca3Al2O6 500 ppm, carbide slag 1000 ppm, carbide slag 1500 ppm, carbide slag 2000 ppm, carbide slag 2500 ppm, carbide slag
0.15
0.10
CaO Ca3Al2O6
500
2000 ppm, CaO/Ca3Al2O6
CHCl,5 (g/g)
600
(a)
1500 ppm, CaO/Ca3Al2O6
0.20
5
0.05
CaClOH CaCO3
400 300 200 100 0 10
0.00 0
20 40 Chlorination time (min)
0.25 CaO/Ca3Al2O6
60
(b)
carbide slag
20
30
40
50 60 o 2 ()
70
80
90
Fig. 6. XRD spectrum of the HCl absorption product of the CaO/Ca3Al2O6 sorbent after 5 cycles in the presence of CO2 (chlorination: 700 °C, 1500 ppm HCl/15 vol.% CO2/N2 balance, 60 min).
0.15
0.10
0.05 500
1000 1500 2000 HCl concentration (ppm)
2500
Fig. 5. Effect of HCl concentration on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles: (a) CHCl,5 vs. chlorination time; (b) CHCl,5 at 60 min vs. HCl concentration (chlorination: 700 °C).
the same HCl concentration. CHCl,5 of the CaO/Ca3Al2O6 sorbent at 1 h under 1000 and 2500 ppm HCl are 1.43 and 1.17 times as high as those of the carbide slag, respectively. And the difference between HCl absorption capacities of the CaO/Ca3Al2O6 sorbent and carbide slag after 5 cycles becomes smaller under higher HCl concentration. 3.4. Effect of the presence of CO2 on HCl absorption by cycled CaO/Ca3Al2O6 sorbent Grasa et al. [49] pointed out that the CO2 capture capacity of calciumbased sorbents was relatively high at 650–720 °C for the carbonation. And this temperature range is also appropriate for HCl absorption. Therefore, the carbonation and chlorination of calcium-based sorbents can take place simultaneously during the HCl absorption process in the presence of CO2. The XRD analysis result shown in Fig. 6 indicates that CaClOH and CaCO3 are simultaneously generated in the HCl absorption by the cycled CaO/Ca3Al2O6 sorbent at 700 °C in the presence of CO2. It can be seen from Fig. 7 that the presence of CO2 shows an adverse influence on the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles. CHCl,5 of the CaO/Ca3Al2O6 sorbent after 1 h chlorination in the presence of 15 vol.% CO2 is 19% lower than that in the absence of CO2. The volume fraction of CO2 (15 vol.%) in the reaction atmosphere is 100 times as high as that of HCl (1500 ppm), so CO2 competes against HCl to react with CaO. The molar volume of CaO (16.8 cm3/gmol) is much lower than that of CaCO3 (36.9 cm3/gmol), so the CaCO3 product layer formed during the carbonation of CaO is compact and almost nonporous, which is not beneficial for HCl absorption of the cycled CaO/Ca3Al2O6 sorbent. Duo et al.
[47,50] also found that the reaction between CaCO3 and HCl was very slow and CaCO3 exhibited worse dechlorination performance than CaO. Compared with the cycled carbide slag, the cycled CaO/Ca3Al2O6 sorbent exhibits better HCl absorption performance under the same reaction conditions as shown in Fig. 7. CHCl,5 of the CaO/Ca3Al2O6 sorbent in the presence of CO2 is even higher than that of the carbide slag in the absence of CO2. In addition, CHCl,5 of the carbide slag at 1 h in the presence of 15 vol.% CO2 is 27% lower than that in the absence of CO2, while CHCl,5 of the CaO/Ca3Al2O6 sorbent at 1 h decreases by 19% with increasing the CO2 volume fraction from 0 to 15 vol.%. It reveals that the effect of the presence of CO2 on HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent is less than that of the cycled carbide slag. 3.5. Effect of number of carbonation/calcination cycles on HCl absorption by cycled CaO/Ca3Al2O6 sorbent The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent varies with the number of carbonation/calcination cycles in the range of 0–50, as presented in Fig. 8. As the cycle number rises from 0 to 5, the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent increases and CHCl,5 of the CaO/Ca3Al2O6 sorbent is 16% higher than CHCl,0. With the number of carbonation/calcination cycles increasing from 5 to 50, the HCl absorption capacity declines slowly. Therefore, the CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the highest HCl absorption capacity
0.24
CaO/Ca3Al2O6, in the absence of CO2 CaO/Ca3Al2O6, in the presence of 15 vol.% CO2
0.20
carbide slag, in the absence of CO2 carbide slag, in the presence of 15 vol.% CO2
0.16
CHCl,5 (g/g)
CHCl,5 (g/g)
0.20
0.12 0.08 0.04 0.00 0
20 40 Chlorination time (min)
60
Fig. 7. Effect of the presence of CO2 on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl/0 or 15 vol.% CO2/N2 balance).
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
0.20
CHCl,N (g/g)
during the 50 cycles, which is attributed to the pore structure of the CaO/Ca3Al2O6 sorbent from the calcium looping cycles. The cycled CaO/Ca3Al2O6 sorbent possesses higher HCl absorption capacity than the cycled carbide slag from the calcium looping cycles, especially when the cycle number exceeds 20. CHCl,20 and CHCl,50 of the CaO/Ca3Al2O6 sorbent are 0.18 and 0.13 g/g, which are 2.3 and 2.6 times as high as those of the carbide slag, respectively. Ca3Al2O6 is a stable inert support substance formed in the synthetic sorbent by the combustion synthesis. Ca3Al2O6 provides a stable framework to inhibit the severe sintering of CaO [24], so the CaO/Ca3Al2O6 sorbent shows higher HCl absorption capacity than the carbide slag.
CaO/Ca3Al2O6 carbide slag
0.15
0.10
3.6. Microstructure analysis
0.05 0
10
20 30 Cycle number
40
50
Fig. 8. Effect of cycle number on HCl absorption capacity of CaO/Ca3Al2O6 sorbent after carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl//N2 balance, 60 min).
The deterioration in the HCl absorption performance of the carbide slag compared to the CaO/Ca3Al2O6 sorbent is attributed to the difference of their microstructures. As shown in Fig. 9(a) and (b), the surfaces of the CaO/Ca3Al2O6 sorbent and the carbide slag after 0 cycle observed by SEM seem fluffy and porous. As presented in Fig. 9(c) and (e), it is
Fig. 9. SEM images of sorbents after various carbonation/calcination cycles: (a) carbide slag after 0 cycle; (b) CaO/Ca3Al2O6 sorbent after 0 cycle; (c) carbide slag after 20 cycles; (d) CaO/Ca3Al2O6 sorbent after 20 cycles; (e) carbide slag after 50 cycles; (f) CaO/Ca3Al2O6 sorbent after 50 cycles.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
7
Fig. 10. SEM images of chlorinated CaO/Ca3Al2O6 sorbent after various carbonation/calcination cycles: (a) after 0 cycle; (b) after 50 cycles (chlorination: 700 °C, 1500 ppm HCl/N2 balance, 60 min).
noted that the pores on the surface of the cycled carbide slag are blocked and the CaO grains aggregate with the number of cycles due to the sintering, which inhibits the HCl diffusion in the sorbent. As illustrated in Fig. 9(d) and (f), the CaO/Ca3Al2O6 sorbent after the multiple cycles appears still more porous and interconnected compared with the carbide slag, which is beneficial to HCl absorption. This is a reason why the cycled CaO/Ca3Al2O6 sorbent exhibits higher HCl absorption capacity than the cycled carbide slag. During the combustion synthesis process of the CaO/Ca3Al2O6 sorbent, the glycerin burns rapidly to release CO2 and H2O [29], which leads to the formation of the porous structure. The SEM analysis shows that the CaO/Ca3Al2O6 sorbent has higher sintering resistance during the multiple carbonation/calcination cycles. Compared with the original CaO/Ca3Al2O6 sorbents exhibited in Fig. 9(b) and (f), the pores on the surfaces of the chlorinated sorbents after 0 and 50 cycles are blocked by chlorination product, as observed in Fig. 10(a) and (b). The element distributions in the marked regions in Fig. 10(a) and (b) were detected by EDS, as shown in Fig. 11(a) and (b), respectively. The high chlorine content is detected on the surfaces of the chlorinated sorbents due to the formation of CaClOH on the basis of XRD analysis. The chlorine element content in the region of the chlorinated CaO/Ca3Al2O6 sorbent after 0 cycle is higher than that after 50 cycles. Gupta and Fan [51] and Ghosh-Dastidar et al. [52] believed that the reactivity of calcium-based sorbents was mostly determined by their surface areas and pore volumes. It has been widely reported that the calcium-based sorbents with larger surface area and pore volume exhibit better HCl absorption behavior [37–39]. Gupta and Fan [51] found that sufficient surface area and pore volume benefited the gaseous diffusion and led to the rapid reaction kinetics of CaO–HCl. As presented in Fig. 12, the cycled CaO/Ca3Al2O6 sorbent shows larger surface area and pore volume than the cycled carbide slag after the same number of carbonation/calcination cycles. It suggests that the formation of Ca3Al2O6 due to the reaction of CaO and Al2O3 increases the surface area and pore volume of the synthetic sorbent and improves
the sintering resistance in the multiple carbonation/calcination cycles. Since the larger surface area and pore volume of calcium-based sorbents are more favorable to the HCl absorption, the cycled CaO/Ca3Al2O6 sorbent exhibits higher HCl absorption capacity than the cycled carbide slag. It should be noted that the CaO/Ca3Al2O6 sorbent after 5 cycles shows the largest surface area and pore volume, which possibly results in the highest HCl absorption capacity of the synthetic sorbent. Adanez et al. [53] pointed out that not only the specific surface area but also the pore size distribution affected the sorbent reactivity. The pore size distributions of the CaO/Ca3Al2O6 sorbent and the carbide slag after the various cycles are also investigated as shown in Fig. 13. The volume of pores in the entire measured pore size range for the CaO/Ca3Al2O6 sorbent is higher than that for the carbide slag after the same cycles. The pores in the range of 2–10 nm may be an important area for HCl absorption by CaO derived from the carbide slag [36] (the size of HCl molecule is about 0.4 nm). For the CaO/Ca3Al2O6 sorbent, the volume of pores in the entire measured pore size range after 5 cycles is higher than that after 0 cycle. Therefore, HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 0 cycle. It should be noted that for the CaO/Ca3Al2O6 sorbent, although the volume of pores in the range of 2–10 nm after 20 cycles is higher than that after 5 cycles, the volume of pores in 30–100 nm after 20 cycles is obviously smaller than that after 5 cycles. The pores in the range of 30–100 nm are helpful for HCl diffusion into the inner of the sorbent. In addition, the surface area of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 20 cycles. For these reasons, HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 20 cycles. Therefore, it can be understood why the highest HCl absorption capacity of the CaO/Ca3Al2O6 sorbent from CO2 capture cycles is achieved after 5 cycles. The cost of the CaO/Ca3Al2O6 sorbent is higher than that of the carbide slag or the natural calcium-based sorbents, but the consumption of CaO/Ca3Al2O6 sorbent is obviously smaller for the same CO2 capture efficiency. The more sorbent consumption would possibly pose a
Fig. 11. EDS spectrograms of chlorinated CaO/Ca3Al2O6 sorbent in Fig. 9: (a) marked region in Fig. 9(a); (b) marked region in Fig. 9(b).
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
11 10 9 8 7 6 5 4 3 2 1 0
0.16
3
surface area of carbide slag pore volume of CaO/Ca3Al2O6
pore volume of carbide slag
0.08
0.04
0
5
10 Cycle number
15
-1
0.12
surface area of CaO/Ca3Al2O6
Pore volume ( cm g )
2
-1
Surface area ( m g )
8
0.00 20
Fig. 12. Surface areas and pore volumes of CaO/Ca3Al2O6 sorbent and carbide slag after various carbonation/calcination cycles.
serious problem since the size of the carbonator and calciner required should be larger. Moreover, the energy requirement in the calciner would increase. Therefore, as the sorbent consumption decreases, both equipment cost and energy consumption would be reduced significantly. Besides, as opposed to the fragile natural calcium-based sorbents [54], Pacciani et al. [55] found that the formation of calcium aluminates due to the reaction of CaO and Al2O3 could strengthen the synthetic sorbent structure. Thus the synthetic CO2 sorbent may mitigate the severe attrition problem under the fluidized bed operation conditions. Therefore, the CaO/Ca3Al2O6 as a CO2 sorbent may be promising in the calcium looping technology. Valverde [6] pointed out that while the economic aspect was of course a priority for the use of synthetic sorbents at the industrial level, their studies were still interesting from a fundamental perspective of understanding the physics and chemistry responsible for the enhancement of the CO2 capture performance. In this work, the CaO/Ca3Al2O6 sorbent discharged from the CO2 capture cycles is recycled to remove HCl efficiently, which will further reduce the operation cost. 4. Conclusions
2.4 CaO/Ca3Al2O6 after 0 cycle CaO/Ca3Al2O6 after 5 cycle
2.0
CaO/Ca3Al2O6 after 20 cycle
3
-1
-1
Pore size distribution ( mm g nm )
A CaO/Ca3Al2O6 sorbent (mass ratio of CaO/Ca3Al2O6 = 73:27) as a CO2 sorbent in the calcium looping cycles was prepared from carbide slag, aluminum nitrate enneahydrate, and glycerol water solution by the combustion synthetic method. The cycled CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles for CO2 capture was proposed to remove HCl from biomass-fired and RDFs-fired boilers. The XRD
carbide slag after 0 cycle
1.6
carbide slag after 20 cycle
1.2 0.8 0.4 0.0 1
10 Pore size (nm)
100
Fig. 13. Pore size distributions of CaO/Ca3Al2O6 sorbent and carbide slag after various carbonation/calcination cycles.
analysis reveals that the reaction products of the cycled CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles are CaClOH, CaO, and Ca3Al2O6 after 1 h chlorination. The optimum temperature for HCl absorption by the cycled CaO/Ca3Al2O6 sorbent is 700 °C and the feasible chlorination temperature window (600–800 °C) of the cycled CaO/Ca3Al2O6 sorbent is wider than that of the cycled carbide slag. The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent rises with the HCl concentration. The presence of CO2 decreases the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent. The number of the carbonation/calcination cycles presents a significant effect on the HCl absorption performance of the CaO/Ca3Al2O6 sorbent. The CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the greatest HCl absorption capacity with the cycle number increasing from 0 to 50. The CaO/Ca3Al2O6 sorbent after 5 cycles shows the largest surface area and pore volume and possesses more pores in 30–100 nm in diameter, which maybe lead to the highest HCl absorption capacity. The CaO/Ca3Al2O6 sorbent appears a promising sorbent for sequential CO2 and HCl removal in the calcium looping. Acknowledgments Financial support from the National Natural Science Foundation of China (51376003) is gratefully appreciated. References [1] Y. Huang, S. Rezvani, D. Mcllveen-Wright, A. Minchener, N. Hewitt, Technoeconomic study of CO2 capture and storage in coal fired oxygen fed entrained flow IGCC power plants, Fuel Process. Technol. 89 (2008) 916–925. [2] J.M. Valverde, A. Perejon, L.A. Perez-Maqueda, Enhancement of fast CO2 capture by a nano-SiO2/CaO composite at Ca-looping conditions, Environ. Sci. Technol. 46 (2012) 6401–6408. [3] M.U.M. Junaidi, C.P. Leo, S.N.M. Kamal, A.L. Ahmad, T.L. Chew, Carbon dioxide removal from methane by using polysulfone/SAPO-44 mixed matrix membranes, Fuel Process. Technol. 112 (2013) 1–6. [4] C.W. Zhao, X.P. Chen, E.J. Anthony, X. Jiang, L.B. Duan, Y. Wu, W. Dong, C.S. Zhao, Capturing CO2 in flue gas from fossil fuel-fired power plants using dry regenerable alkali metal-based sorbent, Prog. Energy Combust. Sci. 39 (2013) 515–534. [5] M. Rydén, P. Ramos, H2 production with CO2 capture by sorption enhanced chemical-looping reforming using NiO as oxygen carrier and CaO as CO2 sorbent, Fuel Process. Technol. 96 (2012) 27–36. [6] J.M. Valverde, Ca-based synthetic materials with enhanced CO2 capture efficiency, J. Mater. Chem. A 1 (2013) 447–468. [7] H.C. Chen, C.S. Zhao, M.L. Chen, Y.J. Li, X.P. Chen, CO2 uptake of modified calciumbased sorbents in a pressurized carbonation-calcination looping, Fuel Process. Technol. 92 (2011) 1144–1151. [8] J.M. Valverde, F.J. Duran, F. Pontiga, H. Moreno, CO2 capture enhancement in a fluidized bed of a modified Geldart C powder, Powder Technol. 224 (2012) 247–252. [9] Y.J. Li, R.Y. Sun, H.L. Liu, C.M. Lu, Cyclic CO2 capture behavior of limestone modified with pyroligneous acid (PA) during calcium looping cycles, Ind. Eng. Chem. Res. 50 (2011) 10222–10228. [10] H.C. Chen, C.S. Zhao, L.B. Duan, C. Liang, D.J. Liu, X.P. Chen, Enhancement of reactivity in surfactant-modified sorbent for CO2 capture in pressurized carbonation, Fuel Process. Technol. 92 (2011) 493–499. [11] C. Luo, Y. Zheng, C.G. Zheng, J.J. Yin, C.L. Qin, B. Feng, Manufacture of calcium-based sorbents for high temperature cyclic CO2 capture via a sol–gel process, Int. J. Greenhouse Gas Control 12 (2013) 193–199. [12] J.M. Valverde, F. Pontiga, C. Soria-Hoyo, M.A.S. Quintanilla, H. Moreno, F.J. Duran, M.J. Espin, Improving the gas–solids contact efficiency in a fluidized bed of CO2 adsorbent fine particles, Phys. Chem. Chem. Phys. 13 (2011) 14906–14909. [13] F.N. Ridha, V. Manovic, A. Macchi, M.A. Anthony, E.J. Anthony, Assessment of limestone treatment with organic acids for CO2 capture in Ca-looping cycles, Fuel Process. Technol. 116 (2013) 284–291. [14] J.M. Valverde, F. Raganati, M.A.S. Quintanilla, J.M.P. Ebri, P. Ammendola, R. Chirone, Enhancement of CO2 capture at Ca-looping conditions by high-intensity acoustic fields, Appl. Energy 111 (2013) 538–549. [15] H.C. Chen, C.S. Zhao, W.W. Yu, Calcium-based sorbent doped with attapulgite for CO2 capture, Appl. Energy 112 (2013) 67–74. [16] A. Coppola, P. Salatino, F. Montagnaro, F. Scala, Hydration-induced reactivation of spent sorbents for fluidized bed calcium looping (double looping), Fuel Process. Technol. 120 (2014) 71–78. [17] Z.S. Li, N.S. Cai, Y.Y. Huang, Effect of preparation temperature on cyclic CO2 capture and multiple carbonation calcination cycles for a new Ca-based CO2 sorbent, Ind. Eng. Chem. Res. 45 (2006) 1911–1917. [18] C.S. Martavaltzi, A.A. Lemionidou, Parametric study of the CaO–Ca12Al14O33 synthesis with respect to high CO2 sorption capacity and stability on operation, Ind. Eng. Chem. Res. 47 (2008) 9537–9543.
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Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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Research article
HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping Xin Xie, Ying-Jie Li ⁎, Chang-Tian Liu, Wen-Jing Wang School of Energy and Power Engineering, Shandong University, Jinan 250061, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Received in revised form 14 June 2015 Accepted 14 June 2015 Available online xxxx Keywords: HCl absorption Carbide slag CaO/Ca3Al2O6 sorbent Calcium looping CO2 capture
a b s t r a c t The synthetic CaO/Ca3Al2O6 (prepared from carbide slag, aluminum nitrate enneahydrate, and glycerol water solution by combustion synthesis) as a CO2 sorbent discharged from the CO2 capture cycles using the calcium looping was used to remove HCl after a series of carbonation/calcination cycles. The effects of chlorination temperature, HCl concentration, presence of CO2, and cycle number on the HCl absorption by the CaO/Ca3Al2O6 sorbent after the repetitive carbonation/calcination cycles for CO2 capture were discussed. In addition, the HCl absorption capacities of the CaO/Ca3Al2O6 sorbent and the carbide slag were compared. The chlorination products of the CaO/Ca3Al2O6 sorbent after 1 h HCl absorption are CaClOH, CaO, and Ca3Al2O6 detected by XRD analysis. Among 600–800 °C, the CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles achieves the highest HCl absorption capacity at 700 °C. The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent rises as HCl concentration increases. CO2 is adverse to the HCl absorption by the cycled CaO/Ca3Al2O6 sorbent in the chlorination process. With the carbonation/calcination cycle number increasing from 0 to 50, the CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the highest HCl absorption capacity. HCl absorption capacities of the cycled CaO/Ca3Al2O6 sorbent after 20 and 50 cycles were 0.18 and 0.13 g/g, which were 2.3 and 2.6 times as high as those of the cycled carbide slag after the same cycles, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The CO2 emission from fossil fuel-fired power plants and hydrogen production systems is widely accepted as the predominant contributor to global warming and various methods for CO2 capture have been investigated [1–5]. The calcium looping technology, i.e., the carbonation/ calcination cycles of CaO, is considered to be one of the most potential and feasible methods to capture CO2 for storage [6–8]. The cyclic process is based on the reversible reaction between CaO and CO2 shown in Eq. (1). However, it has been proved that the CO2 capture capacities of natural calcium-based sorbents such as limestone and dolomite show a rapid decline with the cycle number due to sintering of the sorbents [9–11]. CaOðsÞ þ CO2 ðgÞ↔CaCO3 ðsÞ:
ð1Þ
Lots of methods have been developed to enhance the cyclic CO2 capture capacity of calcium-based sorbents, minimize their loss in CO2 capture capacity, increase the sintering resistance, and promote their mechanical stability in the multiple CO2 capture cycles using calcium looping [12–16]. The criterion for the CO2 sorbent design is to increase active surface area, pore structure stability, and mechanical stability of the sorbent, which would increase CO2 capture capacity of the sorbent.
⁎ Corresponding author. E-mail address: [email protected] (Y.-J. Li).
Material chemistry methods oriented to this criterion are generally focused on the use of rigid porous materials as carriers of the calciumbased sorbents, use of additives to improve the sorbent thermal stability, reduction of the sorbent particle size down to the nanometer scale, and use of synthetic precursors to produce novel sorbents with a rich micropore structure [6]. The development of some novel methods of modifying calcium-based sorbents and synthesizing new ones with enhanced CO2 capture capacity, thermal stability, and mechanical strength is a strong active area of research [6,11,12]. The synthetic sorbents prepared from CaO and Al2O3 precursors under different conditions exhibited a significant enhancement in CO2 capture durability, sintering resistance, and pore structure in the calcium looping cycles [17–26]. The synthetic sorbents were composed of CaO and inert supporters (calcium aluminates) such as Ca12Al14O33 [17–20], Ca9Al6O18 [21–23], Ca3Al2O6 [24], and Ca3Al10O18 [15]. The difference in inert supporters depended on not only the calcium and aluminum precursors but also the synthetic methods. Li et al. [17] prepared a CaO/Ca12Al14O33 sorbent by adding CaO and Al(NO3)3·9H2O into the mixture of 2-propanol and distilled water (wet mixing method) and the solution was calcined in air at 800–1300 °C after being stirred and dried. Martavaltzi and Lemionidou [18] used Ca(CH3COOH)2, Al(NO3)3·9H2O, and 2-propanol water solution by mixing method to synthesize a CaO/Ca12Al14O33 sorbent through calcination at 900 °C. Wu et al. [19] mixed nano CaCO3, aluminum sol, and sodium hexametaphosphate together to prepare nano CaO/Ca12Al14O33 pellet by heating and pressing. Using Ca(NO3)2·4H2O, Al(NO3)3·9H2O, citric acid, and ethylene glycol as raw materials. Mastin et al. [20] obtained single phase Ca3Al2O6
http://dx.doi.org/10.1016/j.fuproc.2015.06.028 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
under the controlled conditions and they proposed a new synthesis route of a CaO/Ca12Al14O33 sorbent through the decomposition of Ca3Al2O6 at different calcination temperatures (900–1100 °C). Zhou et al. [21,22] investigated the effect of calcium and aluminum precursors on CO2 capture performance of the obtained synthetic sorbents by wet mixing method and the result showed that CaO/Ca9Al6O18 prepared from calcium citrate and aluminum nitrate exhibited the highest CO2 capture capacity. Radfarnia and Iliuta [23] used limestone, citric acid, and aluminum nitrate or aluminum acetate to prepare a CaO/ Ca9Al6O18 sorbent. Zhang et al. [24] adopted a four-step heating mode to synthesize a CaO/Ca3Al2O6 sorbent through the citrate preparation route using citric acid, aluminum nitrate, and CaCO3 as raw materials and they pointed out that Ca3Al2O6 in the synthetic sorbent prevented the sintering of CaO particles. CaO/Ca12Al14O33 with high reactivity for CO2 capture at high temperature was synthesized by Luo et al. [11,25] using a standard sol–gel process with citric acid as the chelation agent. Koirala et al. [26] employed the single nozzle flame spray pyrolysis (FSP) method to prepare a CaO/Ca12Al14O33 sorbent from calciumnaphthenate, aluminum acetylacetonate, and xylene. These synthetic sorbents all exhibited high CO2 capture capacity and durability in the multiple cycles. Carbide slag which is mainly composed of Ca(OH)2, is a by-product of the PVC production process and is always discharged from the chlor-alkali plants as an industrial solid waste. Li et al. [27,28] found that carbide slag showed a good CO2 capture performance in the calcium looping cycles. The carbide slag was effectively recycled to capture CO2, thus the cost of CO2 capture was also decreased. Our research group synthesized a new CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis method [29]. The new synthetic sorbent prepared from a much cheaper industrial solid waste, i.e., carbide slag as CaO precursor and glycerol was investigated. As a solvent which is a byproduct from the preparation of the biodiesel fuel, glycerol is also very cheap. Thus, the preparation of the new synthetic sorbent has a much lower cost [29]. We found that when the mass ratio of CaO/Ca3Al2O6 was 73:27, the synthetic sorbent exhibited the highest CO2 capture capacity of 0.38 g/g after 50 cycles, which was 1.8 times as high as that of the carbide slag (carbonation: 700 °C, 15% CO2, 30 min; calcination: 850 °C, N2, 10 min) [29]. Because of the plentiful availability and environmental benefits, more attentions have been paid to the combustion and gasification of biomass fuels and refuse-derived fuels (RDFs). However, the chlorine contents in these fuels are much higher than those in the common coals [30,31]. Consequently, the flue gases of biomass-fired and RDFs-fired boilers often contain hydrogen chloride (HCl) with the
CO2 rich gas
concentration of 200–1500 ppm [31–34], which cannot meet the emission limit without the addition of sorbents. HCl is a poisonous air pollutant and might turn into some corrosive compounds. The carbide slag not only can be used to remove HCl, but also capture CO2 from the flue gases of biomass-fired and RDFs-fired boilers. Wang et al. [35] found that the presence of HCl sharply decreased CO2 capture capacity of calcium-based sorbent after a dozen CO2 capture cycles using the calcium looping in the simultaneous CO2/HCl capture process. Therefore, in order to avoid the adverse effect of HCl on CO2 capture of the sorbent in the multiple cycles, we have proposed that the carbide slag discharged from CO2 capture cycles using the calcium looping could be used to remove HCl from the flue gases of biomass-fired and RDFs-fired boilers in our previous research [36]. And then CO2 and HCl can be sequentially captured by the carbide slag, as shown in Fig. 1. Not only the spent carbide slag discharged from the CO2 capture cycles will be reused, but also the cost for HCl removal will be reduced. The HCl absorption capacity (HCl uptake by per unit mass sorbent, g HCl/g sorbent) of the carbide slag after 1 and 50 carbonation/ calcination cycles were 0.18 and 0.05 g/g in the previous research [36], respectively. It is necessary to improve the HCl absorption behavior of the cycled carbide slag from the CO2 capture cycles using calcium looping due to the lower HCl absorption capacity. The CO2 capture capacity of the CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis could achieve 1.8 times as high as that of the carbide slag after 50 cycles due to the more porous structure, larger surface area, and pore volume in the multiple cycles [29]. The calcium-based sorbent with more porous structure, larger surface area, and pore volume could achieve higher HCl absorption capacity [37–39]. Therefore, the CaO/Ca3Al2O6 sorbent may be used to sequentially capture CO2 and HCl from the flue gases of the biomass-fired and the RDFs-fired boilers. The CaO/Ca3Al2O6 sorbent as a CO2 sorbent is repetitively performed in a carbonator and calciner. A part of the discharged CaO/Ca3Al2O6 from the CO2 capture cycles is used to remove HCl from the flue gases of biomass-fired and RDFs-fired boilers. The HCl absorption by the CaO/Ca3Al2O6 sorbent from the CO2 capture cycles using calcium looping is worth to be studied. In this work, the HCl absorption performance of the cycled CaO/Ca3Al2O6 sorbent prepared from carbide slag, Al(NO3)3·9H2O, and glycerin water solution by the combustion synthesis was investigated. And the effects of chlorination temperature, HCl concentration, presence of CO2, and cycle number were discussed. Moreover, the HCl absorption performance of the CaO/Ca3Al2O6 sorbent was compared with that of the carbide slag from the CO2 capture cycles using calcium looping.
flue gas (CO2 free) CaO
carbide slag
calciner 850-950 0 oC
flue gas (HCl free)
carbonator 600-700 oC
air
biomass-fired biomass or RDFs-fired boiler or RDFs
CaCO3 fuel O2 discharged sorbent (for HCl removal) discharged sorbent (for cement production)
chlorination product and CaO, ash
Fig. 1. Schematic of HCl removal from flue gas of biomass-fired or RDFs-fired boiler by the calcium-based sorbent from CO2 capture cycles using calcium looping.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx Table 1 Chemical components of carbide slag (wt.%). CaO
MgO
SiO2
Al2O3
Fe2O3
Na2O
Ti2O
Others
Loss on ignition
71.09
0.09
2.51
1.96
0.16
0.02
0.04
1.28
22.85
2. Experimental 2.1. Sorbent preparation The carbide slag (sieved to size b0.125 mm) used in the experiments of this work was sampled from a chlor-alkali plant in Shandong Province, China. The chemical components of the carbide slag were analyzed by X-ray fluorescence (XRF) and listed in Table 1. Aluminum nitrate enneahydrate (Al(NO3)3·9H2O, Shanghai Qingxi Chemical Technology Co., Ltd, China) and glycerin (C3H8O3, Tianjin Kemiou Chemical Reagent Co., Ltd, China) used in the preparation of the synthetic sorbent were analytically pure (N99%). Since the synthetic sorbent containing the mass ratio of CaO derived from the carbide slag to Al2O3 derived from Al(NO3)3·9H2O = 90:10 (namely the mass ratio of CaO/Ca3Al2O6 = 73:27) exhibited the highest CO2 capture capacity in the carbonation/calcination cycles [29], we prepared this synthetic sorbent on the basis of the preparation procedure reported by Li et al. [29], as follows: 10 g carbide slag and 4.56 g Al(NO3)3·9H2O were completely dissolved in the mixture of glycerin (50 mL) and deionized water (50 mL). This solution was stirred at 80 °C and then was combusted at 800 °C for 1 h under air atmosphere in the muffle furnace to synthesize the sorbent. The obtained synthetic sorbent was ground and sieved to size b 0.125 mm.
3
3 min). Then the mass of sorbent sample was measured by a Mettler Toledo-XS105DU electronic balance with a resolution of 0.1 mg. The sorbent samples after 0, 1, 2, 3, 4, 5, 10, 15, 20, 35, and 50 carbonation/calcination cycles in TFBR were sent into the chlorinator for HCl absorption, respectively. 600–800 °C was chosen to be the chlorination temperature range. The HCl purity of the HCl gas cylinder was 3000 ppm (N2 as an equilibrium gas) and the gas flow was mixed with N2 or CO2 to get the specified HCl concentrations. The reaction atmosphere was switched to the gas mixture consisting of HCl (500–2500 ppm) and N2 (balance). Additionally, 15 vol.% CO2 was added into the reaction gas in order to investigate the effect of the presence of CO2 on HCl absorption by the cycled sorbents. The sorbent sample after HCl absorption was cooled in N2 to room temperature (about 3 min) and then was weighed by the electronic balance. For the sorbent sample after each number of cycles at each HCl absorption condition, three identically repetitive experiments were performed and the error bars of data were less than 1%. The average value of the data in the three repetitive experiments was taken to guarantee the accuracy of this research. The HCl absorption capacity of the sorbent sample from carbonation/ calcination cycles is calculated as follows: C HCl; N ¼ mchl; N ðt Þ−mcal =mcal
ð2Þ
where CHCl,N denotes the HCl absorption capacity of the sorbent sample after N carbonation/calcination cycles, g (HCl)/g (sorbent); t is the reaction time, min; mchl,N (t) represents the mass of the chlorinated sorbent sample after N carbonation/calcination cycles at t, g; and mcal is the sorbent sample mass after complete calcination, g.
2.2. HCl absorption by sorbents from calcium looping cycles 2.3. Phase and microstructure analysis As shown in Fig. 2, a triple fixed-bed reactor (TFBR) comprises a carbonator, a calciner, and a chlorinator, which can be operated at atmospheric pressure, and the temperatures of the reactor can maintain at specified values in the range of 600–850 °C. Before being introduced into the reactor, each gas (HCl, N2, and CO2) was controlled by a needle valve and the mass flow rate of the each gas was regulated by a mass flowmeter. All of the gases were mixed in a blending tub and the gas mixture was then sent to TFBR. The sample boat containing 500 mg synthetic sorbent was firstly put into the carbonator at 700 °C in a 15 vol.% CO2/85 vol.% N2 gas mixture for 20 min. After the carbonation, the reaction atmosphere was switched to pure N2 and the sample boat was carried into the calciner at 850 °C in pure N2 for 10 min. Then the first carbonation/calcination cycle was finished. The sorbent sample after various carbonation/calcination cycles was picked out from the calciner and sent into a drier for cooling in N2 to room temperature (about
mass flowmeter
The phase components of the CaO/Ca3Al2O6 sorbent under different conditions were identified by a D/Max-IIIA X-ray diffraction (XRD). A SUPRATM 55 field emission scan electron microscope (SEM) was used to observe the surface morphologies of the CaO/Ca3Al2O6 sorbent after the various carbonation/calcination cycles and the chlorinated CaO/Ca3Al2O6 sorbent. The elements on the surfaces of the CaO/ Ca3Al2O6 sorbent were detected by an Oxford INCA sight X energy dispersive spectrometry (EDS). A Micromeritics ASAP 2020-M nitrogen absorption analyzer was utilized to measure the pore structure parameters of the CaO/Ca3Al2O6 sorbent after the various cycles. And the pore structure parameters of the CaO/Ca3Al2O6 sorbent and the carbide slag in the cycles were compared. The surface area and pore volume were calculated according to Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) model, respectively.
blending tub
electric balance computer
absorption bottle
valve temperature controller
NaOH solution
sample boat HCl/N2 N2
CO2
carbonator
calciner
chlorinator
Fig. 2. Schematic of TFBR.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
0.24
3. Results and discussion
(a)
o
600 C, CaO/Ca3Al2O6 o
650 C, CaO/Ca3Al2O6
3.1. XRD analysis of chlorinated synthetic sorbent from calcium looping cycles
o
700 C, CaO/Ca3Al2O6
0.18
o
3.2. Effect of chlorination temperature on HCl absorption by cycled CaO/Ca3Al2O6 sorbent
CHCl,5 (g/g)
750 C, CaO/Ca3Al2O6 o
800 C, CaO/Ca3Al2O6
0.12
o
600 C, carbide slag
0.06
o
650 C, carbide slag o
700 C, carbide slag o
750 C, carbide slag o
800 C, carbide slag
0.00 0
20 40 Chlorination time (min)
0.22
60
(b)
0.20
CHCl,5 (g/g)
Many studies on HCl absorption by calcium-based sorbents have been implemented on the basis of the assumption that the reaction product was CaCl2 [34,39–41]. However, the product of the reaction between CaO and HCl was reported to be CaClOH [36,42–44]. Moreover, some researchers pointed out that a variety of intermediates (CaClOH and CaCl2·2H2O) and end products (CaCl2) were formed in the absorption process of HCl [45] and the reaction product depended on the chlorination reaction time [46]. CaO and the inert support substance Ca3Al2O6 are found in the synthetic sorbent and the mass ratio of CaO to Ca3Al2O6 is 73:27 according to the XRD quantitative analysis, as exhibited in Fig. 3(a). Thus, the synthetic sorbent is called the CaO/Ca3Al2O6 sorbent. During the 1 h HCl absorption by the CaO/Ca3Al2O6 sorbent after 0 cycle, Ca3Al2O6 remains stable and a fraction of CaO turns into CaClOH, as illustrated in Fig. 3(b). The chlorination product of the CaO/Ca3Al2O6 sorbent after 5 cycles is still CaClOH after 1 h chlorination, as presented in Fig. 3(c). The results show that the cycled CaO/Ca3Al2O6 sorbent from the calcium looping cycles absorbs HCl to generate CaClOH after 1 h chlorination and Ca3Al2O6 as the supporter remains stable.
0.18
CaO/Ca3Al2O6 carbide slag
0.16 0.14 0.12
It has been widely accepted that the reaction temperature is of great importance in HCl absorption by calcium-based sorbents [47,48] and there exists an optimum temperature range of 550–700 °C for HCl absorption. Our previous paper reported that the optimum temperature for HCl removal by the carbide slag undergone various carbonation/ calcination cycles was 700 °C [36]. As depicted in Fig. 4(a), with increasing the chlorination temperature in the range of 600–800 °C, the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent experienced 5 carbonation/calcination cycles increases firstly and then decreases. Among 600–800 °C, the cycled CaO/Ca3Al2O6 sorbent shows the highest HCl absorption capacity at 700 °C. It indicates that the optimum chlorination temperatures for CaO/Ca3Al2O6 sorbent and the carbide slag from CO2 capture cycles are the same. The CaO/Ca3Al2O6 sorbent after 5 cycles exhibits higher HCl absorption capacity than the carbide slag at the same chlorination 25000
CaO Ca3Al2O6
20000 15000
(a) Mass ratio of CaO to Ca3Al2O6=73:27
10000
Intensity (CPS)
5000 0 400 300
CaO Ca3Al2O6
200
CaClOH
(b)
0.10 600
650 700 750 o Chlorination temperature ( C)
800
Fig. 4. Effect of chlorination temperature on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles: (a) CHCl,5 vs. chlorination time; (b) CHCl,5 at 60 min vs. chlorination temperature (chlorination: 1500 ppm HCl/N2 balance).
temperatures, as presented in Fig. 4(b). CHCl,5 of the CaO/Ca3Al2O6 sorbent after 1 h chlorination at 700 °C and 800 °C are 0.21 and 0.19 g/g, which are 42% and 84% higher than that of the carbide slag, respectively. It is found that the chlorination temperature exhibits little impact on the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent. By contrast, CHCl,5 of the carbide slag varies with the chlorination temperature. As the chlorination temperature increases from 700 to 800 °C, CHCl,5 of the CaO/Ca3Al2O6 sorbent decreases by 9% merely, while CHCl,5 of the carbide slag decreases by 30%. It indicates that the feasible temperature window for HCl absorption by the cycled CaO/Ca3Al2O6 sorbent is wider than that of the cycled carbide slag. The cycled CaO/Ca3Al2O6 sorbent shows higher HCl absorption capacity than the cycled carbide slag at high temperature above 700 °C. 3.3. Effect of HCl concentration on HCl absorption by cycled CaO/Ca3Al2O6 sorbent
100 0 300
(c)
CaO Ca3Al2O6
200
CaClOH
100 0
10
20
30
40
50
o
60
70
80
90
2 () Fig. 3. XRD spectra of the synthetic sorbent before and after chlorination: (a) synthetic sorbent; (b) chlorinated synthetic sorbent after 0 carbonation/calcination cycle; (c) chlorinated synthetic sorbent after 5 carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl/N2 balance, 60 min).
The HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles is significantly affected by the HCl concentration, as plotted in Fig. 5(a). The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent increases rapidly with increasing the HCl concentration from 500 to 1500 ppm, but it increases slowly with the HCl concentration above 1500 ppm. For example, CHCl,5 of the cycled CaO/Ca3Al2O6 sorbent at 1 h under 1500 ppm HCl is 2.8 times as high as that at 1 h under 500 ppm HCl. But as the HCl concentration rises from 1500 to 2500 ppm, CHCl,5 of the cycled CaO/Ca3Al2O6 sorbent at 1 h only increases by 13%. As exhibited in Fig. 5(b), the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent is greater than that of the cycled carbide slag for
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
0.25
500 ppm, CaO/Ca3Al2O6 1000 ppm, CaO/Ca3Al2O6
Intensity (CPS)
2500 ppm, CaO/Ca3Al2O6 500 ppm, carbide slag 1000 ppm, carbide slag 1500 ppm, carbide slag 2000 ppm, carbide slag 2500 ppm, carbide slag
0.15
0.10
CaO Ca3Al2O6
500
2000 ppm, CaO/Ca3Al2O6
CHCl,5 (g/g)
600
(a)
1500 ppm, CaO/Ca3Al2O6
0.20
5
0.05
CaClOH CaCO3
400 300 200 100 0 10
0.00 0
20 40 Chlorination time (min)
0.25 CaO/Ca3Al2O6
60
(b)
carbide slag
20
30
40
50 60 o 2 ()
70
80
90
Fig. 6. XRD spectrum of the HCl absorption product of the CaO/Ca3Al2O6 sorbent after 5 cycles in the presence of CO2 (chlorination: 700 °C, 1500 ppm HCl/15 vol.% CO2/N2 balance, 60 min).
0.15
0.10
0.05 500
1000 1500 2000 HCl concentration (ppm)
2500
Fig. 5. Effect of HCl concentration on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles: (a) CHCl,5 vs. chlorination time; (b) CHCl,5 at 60 min vs. HCl concentration (chlorination: 700 °C).
the same HCl concentration. CHCl,5 of the CaO/Ca3Al2O6 sorbent at 1 h under 1000 and 2500 ppm HCl are 1.43 and 1.17 times as high as those of the carbide slag, respectively. And the difference between HCl absorption capacities of the CaO/Ca3Al2O6 sorbent and carbide slag after 5 cycles becomes smaller under higher HCl concentration. 3.4. Effect of the presence of CO2 on HCl absorption by cycled CaO/Ca3Al2O6 sorbent Grasa et al. [49] pointed out that the CO2 capture capacity of calciumbased sorbents was relatively high at 650–720 °C for the carbonation. And this temperature range is also appropriate for HCl absorption. Therefore, the carbonation and chlorination of calcium-based sorbents can take place simultaneously during the HCl absorption process in the presence of CO2. The XRD analysis result shown in Fig. 6 indicates that CaClOH and CaCO3 are simultaneously generated in the HCl absorption by the cycled CaO/Ca3Al2O6 sorbent at 700 °C in the presence of CO2. It can be seen from Fig. 7 that the presence of CO2 shows an adverse influence on the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles. CHCl,5 of the CaO/Ca3Al2O6 sorbent after 1 h chlorination in the presence of 15 vol.% CO2 is 19% lower than that in the absence of CO2. The volume fraction of CO2 (15 vol.%) in the reaction atmosphere is 100 times as high as that of HCl (1500 ppm), so CO2 competes against HCl to react with CaO. The molar volume of CaO (16.8 cm3/gmol) is much lower than that of CaCO3 (36.9 cm3/gmol), so the CaCO3 product layer formed during the carbonation of CaO is compact and almost nonporous, which is not beneficial for HCl absorption of the cycled CaO/Ca3Al2O6 sorbent. Duo et al.
[47,50] also found that the reaction between CaCO3 and HCl was very slow and CaCO3 exhibited worse dechlorination performance than CaO. Compared with the cycled carbide slag, the cycled CaO/Ca3Al2O6 sorbent exhibits better HCl absorption performance under the same reaction conditions as shown in Fig. 7. CHCl,5 of the CaO/Ca3Al2O6 sorbent in the presence of CO2 is even higher than that of the carbide slag in the absence of CO2. In addition, CHCl,5 of the carbide slag at 1 h in the presence of 15 vol.% CO2 is 27% lower than that in the absence of CO2, while CHCl,5 of the CaO/Ca3Al2O6 sorbent at 1 h decreases by 19% with increasing the CO2 volume fraction from 0 to 15 vol.%. It reveals that the effect of the presence of CO2 on HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent is less than that of the cycled carbide slag. 3.5. Effect of number of carbonation/calcination cycles on HCl absorption by cycled CaO/Ca3Al2O6 sorbent The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent varies with the number of carbonation/calcination cycles in the range of 0–50, as presented in Fig. 8. As the cycle number rises from 0 to 5, the HCl absorption capacity of the CaO/Ca3Al2O6 sorbent increases and CHCl,5 of the CaO/Ca3Al2O6 sorbent is 16% higher than CHCl,0. With the number of carbonation/calcination cycles increasing from 5 to 50, the HCl absorption capacity declines slowly. Therefore, the CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the highest HCl absorption capacity
0.24
CaO/Ca3Al2O6, in the absence of CO2 CaO/Ca3Al2O6, in the presence of 15 vol.% CO2
0.20
carbide slag, in the absence of CO2 carbide slag, in the presence of 15 vol.% CO2
0.16
CHCl,5 (g/g)
CHCl,5 (g/g)
0.20
0.12 0.08 0.04 0.00 0
20 40 Chlorination time (min)
60
Fig. 7. Effect of the presence of CO2 on HCl absorption by CaO/Ca3Al2O6 sorbent after 5 carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl/0 or 15 vol.% CO2/N2 balance).
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
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X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
0.20
CHCl,N (g/g)
during the 50 cycles, which is attributed to the pore structure of the CaO/Ca3Al2O6 sorbent from the calcium looping cycles. The cycled CaO/Ca3Al2O6 sorbent possesses higher HCl absorption capacity than the cycled carbide slag from the calcium looping cycles, especially when the cycle number exceeds 20. CHCl,20 and CHCl,50 of the CaO/Ca3Al2O6 sorbent are 0.18 and 0.13 g/g, which are 2.3 and 2.6 times as high as those of the carbide slag, respectively. Ca3Al2O6 is a stable inert support substance formed in the synthetic sorbent by the combustion synthesis. Ca3Al2O6 provides a stable framework to inhibit the severe sintering of CaO [24], so the CaO/Ca3Al2O6 sorbent shows higher HCl absorption capacity than the carbide slag.
CaO/Ca3Al2O6 carbide slag
0.15
0.10
3.6. Microstructure analysis
0.05 0
10
20 30 Cycle number
40
50
Fig. 8. Effect of cycle number on HCl absorption capacity of CaO/Ca3Al2O6 sorbent after carbonation/calcination cycles (chlorination: 700 °C, 1500 ppm HCl//N2 balance, 60 min).
The deterioration in the HCl absorption performance of the carbide slag compared to the CaO/Ca3Al2O6 sorbent is attributed to the difference of their microstructures. As shown in Fig. 9(a) and (b), the surfaces of the CaO/Ca3Al2O6 sorbent and the carbide slag after 0 cycle observed by SEM seem fluffy and porous. As presented in Fig. 9(c) and (e), it is
Fig. 9. SEM images of sorbents after various carbonation/calcination cycles: (a) carbide slag after 0 cycle; (b) CaO/Ca3Al2O6 sorbent after 0 cycle; (c) carbide slag after 20 cycles; (d) CaO/Ca3Al2O6 sorbent after 20 cycles; (e) carbide slag after 50 cycles; (f) CaO/Ca3Al2O6 sorbent after 50 cycles.
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
7
Fig. 10. SEM images of chlorinated CaO/Ca3Al2O6 sorbent after various carbonation/calcination cycles: (a) after 0 cycle; (b) after 50 cycles (chlorination: 700 °C, 1500 ppm HCl/N2 balance, 60 min).
noted that the pores on the surface of the cycled carbide slag are blocked and the CaO grains aggregate with the number of cycles due to the sintering, which inhibits the HCl diffusion in the sorbent. As illustrated in Fig. 9(d) and (f), the CaO/Ca3Al2O6 sorbent after the multiple cycles appears still more porous and interconnected compared with the carbide slag, which is beneficial to HCl absorption. This is a reason why the cycled CaO/Ca3Al2O6 sorbent exhibits higher HCl absorption capacity than the cycled carbide slag. During the combustion synthesis process of the CaO/Ca3Al2O6 sorbent, the glycerin burns rapidly to release CO2 and H2O [29], which leads to the formation of the porous structure. The SEM analysis shows that the CaO/Ca3Al2O6 sorbent has higher sintering resistance during the multiple carbonation/calcination cycles. Compared with the original CaO/Ca3Al2O6 sorbents exhibited in Fig. 9(b) and (f), the pores on the surfaces of the chlorinated sorbents after 0 and 50 cycles are blocked by chlorination product, as observed in Fig. 10(a) and (b). The element distributions in the marked regions in Fig. 10(a) and (b) were detected by EDS, as shown in Fig. 11(a) and (b), respectively. The high chlorine content is detected on the surfaces of the chlorinated sorbents due to the formation of CaClOH on the basis of XRD analysis. The chlorine element content in the region of the chlorinated CaO/Ca3Al2O6 sorbent after 0 cycle is higher than that after 50 cycles. Gupta and Fan [51] and Ghosh-Dastidar et al. [52] believed that the reactivity of calcium-based sorbents was mostly determined by their surface areas and pore volumes. It has been widely reported that the calcium-based sorbents with larger surface area and pore volume exhibit better HCl absorption behavior [37–39]. Gupta and Fan [51] found that sufficient surface area and pore volume benefited the gaseous diffusion and led to the rapid reaction kinetics of CaO–HCl. As presented in Fig. 12, the cycled CaO/Ca3Al2O6 sorbent shows larger surface area and pore volume than the cycled carbide slag after the same number of carbonation/calcination cycles. It suggests that the formation of Ca3Al2O6 due to the reaction of CaO and Al2O3 increases the surface area and pore volume of the synthetic sorbent and improves
the sintering resistance in the multiple carbonation/calcination cycles. Since the larger surface area and pore volume of calcium-based sorbents are more favorable to the HCl absorption, the cycled CaO/Ca3Al2O6 sorbent exhibits higher HCl absorption capacity than the cycled carbide slag. It should be noted that the CaO/Ca3Al2O6 sorbent after 5 cycles shows the largest surface area and pore volume, which possibly results in the highest HCl absorption capacity of the synthetic sorbent. Adanez et al. [53] pointed out that not only the specific surface area but also the pore size distribution affected the sorbent reactivity. The pore size distributions of the CaO/Ca3Al2O6 sorbent and the carbide slag after the various cycles are also investigated as shown in Fig. 13. The volume of pores in the entire measured pore size range for the CaO/Ca3Al2O6 sorbent is higher than that for the carbide slag after the same cycles. The pores in the range of 2–10 nm may be an important area for HCl absorption by CaO derived from the carbide slag [36] (the size of HCl molecule is about 0.4 nm). For the CaO/Ca3Al2O6 sorbent, the volume of pores in the entire measured pore size range after 5 cycles is higher than that after 0 cycle. Therefore, HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 0 cycle. It should be noted that for the CaO/Ca3Al2O6 sorbent, although the volume of pores in the range of 2–10 nm after 20 cycles is higher than that after 5 cycles, the volume of pores in 30–100 nm after 20 cycles is obviously smaller than that after 5 cycles. The pores in the range of 30–100 nm are helpful for HCl diffusion into the inner of the sorbent. In addition, the surface area of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 20 cycles. For these reasons, HCl absorption capacity of the CaO/Ca3Al2O6 sorbent after 5 cycles is higher than that after 20 cycles. Therefore, it can be understood why the highest HCl absorption capacity of the CaO/Ca3Al2O6 sorbent from CO2 capture cycles is achieved after 5 cycles. The cost of the CaO/Ca3Al2O6 sorbent is higher than that of the carbide slag or the natural calcium-based sorbents, but the consumption of CaO/Ca3Al2O6 sorbent is obviously smaller for the same CO2 capture efficiency. The more sorbent consumption would possibly pose a
Fig. 11. EDS spectrograms of chlorinated CaO/Ca3Al2O6 sorbent in Fig. 9: (a) marked region in Fig. 9(a); (b) marked region in Fig. 9(b).
Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028
X. Xie et al. / Fuel Processing Technology xxx (2015) xxx–xxx
11 10 9 8 7 6 5 4 3 2 1 0
0.16
3
surface area of carbide slag pore volume of CaO/Ca3Al2O6
pore volume of carbide slag
0.08
0.04
0
5
10 Cycle number
15
-1
0.12
surface area of CaO/Ca3Al2O6
Pore volume ( cm g )
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-1
Surface area ( m g )
8
0.00 20
Fig. 12. Surface areas and pore volumes of CaO/Ca3Al2O6 sorbent and carbide slag after various carbonation/calcination cycles.
serious problem since the size of the carbonator and calciner required should be larger. Moreover, the energy requirement in the calciner would increase. Therefore, as the sorbent consumption decreases, both equipment cost and energy consumption would be reduced significantly. Besides, as opposed to the fragile natural calcium-based sorbents [54], Pacciani et al. [55] found that the formation of calcium aluminates due to the reaction of CaO and Al2O3 could strengthen the synthetic sorbent structure. Thus the synthetic CO2 sorbent may mitigate the severe attrition problem under the fluidized bed operation conditions. Therefore, the CaO/Ca3Al2O6 as a CO2 sorbent may be promising in the calcium looping technology. Valverde [6] pointed out that while the economic aspect was of course a priority for the use of synthetic sorbents at the industrial level, their studies were still interesting from a fundamental perspective of understanding the physics and chemistry responsible for the enhancement of the CO2 capture performance. In this work, the CaO/Ca3Al2O6 sorbent discharged from the CO2 capture cycles is recycled to remove HCl efficiently, which will further reduce the operation cost. 4. Conclusions
2.4 CaO/Ca3Al2O6 after 0 cycle CaO/Ca3Al2O6 after 5 cycle
2.0
CaO/Ca3Al2O6 after 20 cycle
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-1
-1
Pore size distribution ( mm g nm )
A CaO/Ca3Al2O6 sorbent (mass ratio of CaO/Ca3Al2O6 = 73:27) as a CO2 sorbent in the calcium looping cycles was prepared from carbide slag, aluminum nitrate enneahydrate, and glycerol water solution by the combustion synthetic method. The cycled CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles for CO2 capture was proposed to remove HCl from biomass-fired and RDFs-fired boilers. The XRD
carbide slag after 0 cycle
1.6
carbide slag after 20 cycle
1.2 0.8 0.4 0.0 1
10 Pore size (nm)
100
Fig. 13. Pore size distributions of CaO/Ca3Al2O6 sorbent and carbide slag after various carbonation/calcination cycles.
analysis reveals that the reaction products of the cycled CaO/Ca3Al2O6 sorbent from the carbonation/calcination cycles are CaClOH, CaO, and Ca3Al2O6 after 1 h chlorination. The optimum temperature for HCl absorption by the cycled CaO/Ca3Al2O6 sorbent is 700 °C and the feasible chlorination temperature window (600–800 °C) of the cycled CaO/Ca3Al2O6 sorbent is wider than that of the cycled carbide slag. The HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent rises with the HCl concentration. The presence of CO2 decreases the HCl absorption capacity of the cycled CaO/Ca3Al2O6 sorbent. The number of the carbonation/calcination cycles presents a significant effect on the HCl absorption performance of the CaO/Ca3Al2O6 sorbent. The CaO/Ca3Al2O6 sorbent after 5 cycles exhibits the greatest HCl absorption capacity with the cycle number increasing from 0 to 50. The CaO/Ca3Al2O6 sorbent after 5 cycles shows the largest surface area and pore volume and possesses more pores in 30–100 nm in diameter, which maybe lead to the highest HCl absorption capacity. The CaO/Ca3Al2O6 sorbent appears a promising sorbent for sequential CO2 and HCl removal in the calcium looping. Acknowledgments Financial support from the National Natural Science Foundation of China (51376003) is gratefully appreciated. References [1] Y. Huang, S. Rezvani, D. Mcllveen-Wright, A. Minchener, N. Hewitt, Technoeconomic study of CO2 capture and storage in coal fired oxygen fed entrained flow IGCC power plants, Fuel Process. Technol. 89 (2008) 916–925. [2] J.M. Valverde, A. Perejon, L.A. Perez-Maqueda, Enhancement of fast CO2 capture by a nano-SiO2/CaO composite at Ca-looping conditions, Environ. Sci. Technol. 46 (2012) 6401–6408. [3] M.U.M. Junaidi, C.P. Leo, S.N.M. Kamal, A.L. Ahmad, T.L. Chew, Carbon dioxide removal from methane by using polysulfone/SAPO-44 mixed matrix membranes, Fuel Process. Technol. 112 (2013) 1–6. [4] C.W. Zhao, X.P. Chen, E.J. Anthony, X. Jiang, L.B. Duan, Y. Wu, W. Dong, C.S. Zhao, Capturing CO2 in flue gas from fossil fuel-fired power plants using dry regenerable alkali metal-based sorbent, Prog. Energy Combust. Sci. 39 (2013) 515–534. [5] M. Rydén, P. Ramos, H2 production with CO2 capture by sorption enhanced chemical-looping reforming using NiO as oxygen carrier and CaO as CO2 sorbent, Fuel Process. Technol. 96 (2012) 27–36. [6] J.M. Valverde, Ca-based synthetic materials with enhanced CO2 capture efficiency, J. Mater. Chem. A 1 (2013) 447–468. [7] H.C. Chen, C.S. Zhao, M.L. Chen, Y.J. Li, X.P. Chen, CO2 uptake of modified calciumbased sorbents in a pressurized carbonation-calcination looping, Fuel Process. Technol. 92 (2011) 1144–1151. [8] J.M. Valverde, F.J. Duran, F. Pontiga, H. Moreno, CO2 capture enhancement in a fluidized bed of a modified Geldart C powder, Powder Technol. 224 (2012) 247–252. [9] Y.J. Li, R.Y. Sun, H.L. Liu, C.M. Lu, Cyclic CO2 capture behavior of limestone modified with pyroligneous acid (PA) during calcium looping cycles, Ind. Eng. Chem. Res. 50 (2011) 10222–10228. [10] H.C. Chen, C.S. Zhao, L.B. Duan, C. Liang, D.J. Liu, X.P. Chen, Enhancement of reactivity in surfactant-modified sorbent for CO2 capture in pressurized carbonation, Fuel Process. Technol. 92 (2011) 493–499. [11] C. Luo, Y. Zheng, C.G. Zheng, J.J. Yin, C.L. Qin, B. Feng, Manufacture of calcium-based sorbents for high temperature cyclic CO2 capture via a sol–gel process, Int. J. Greenhouse Gas Control 12 (2013) 193–199. [12] J.M. Valverde, F. Pontiga, C. Soria-Hoyo, M.A.S. Quintanilla, H. Moreno, F.J. Duran, M.J. Espin, Improving the gas–solids contact efficiency in a fluidized bed of CO2 adsorbent fine particles, Phys. Chem. Chem. Phys. 13 (2011) 14906–14909. [13] F.N. Ridha, V. Manovic, A. Macchi, M.A. Anthony, E.J. Anthony, Assessment of limestone treatment with organic acids for CO2 capture in Ca-looping cycles, Fuel Process. Technol. 116 (2013) 284–291. [14] J.M. Valverde, F. Raganati, M.A.S. Quintanilla, J.M.P. Ebri, P. Ammendola, R. Chirone, Enhancement of CO2 capture at Ca-looping conditions by high-intensity acoustic fields, Appl. Energy 111 (2013) 538–549. [15] H.C. Chen, C.S. Zhao, W.W. Yu, Calcium-based sorbent doped with attapulgite for CO2 capture, Appl. Energy 112 (2013) 67–74. [16] A. Coppola, P. Salatino, F. Montagnaro, F. Scala, Hydration-induced reactivation of spent sorbents for fluidized bed calcium looping (double looping), Fuel Process. Technol. 120 (2014) 71–78. [17] Z.S. Li, N.S. Cai, Y.Y. Huang, Effect of preparation temperature on cyclic CO2 capture and multiple carbonation calcination cycles for a new Ca-based CO2 sorbent, Ind. Eng. Chem. Res. 45 (2006) 1911–1917. [18] C.S. Martavaltzi, A.A. Lemionidou, Parametric study of the CaO–Ca12Al14O33 synthesis with respect to high CO2 sorption capacity and stability on operation, Ind. Eng. Chem. Res. 47 (2008) 9537–9543.
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Please cite this article as: X. Xie, et al., HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.028