Effect of calcium precursors on pelletized property and cyclic CO 2 capture performance

. CaO is widely used for capturing CO 2 in large scale application however the work on the manufacture of CaO based pellets for the application of CO 2 capture is relatively limited. In this research investigated the effect of calcium precursors (Ca(OH) 2 , Ca(NO 3 ) 2 and Ca(CH 3 COO) 2 ) and their synthesis method (sol-mixing and wet-mixing) on pelletization. Adsorbent synthesized from Ca(CH 3 COO) 2 as a precursor using sol-mixing pelletize to porous spherical provide the best mechanical strength and CO 2 adsorption capacity 0.51 and 0.41 g CO 2 / g CaO at first cycle and tenth cycle respectively. The outstanding cyclic performance of Ca(CH 3 COO) 2 sol-mixing can be ascribed to the presence of inert support material i.e. Ca 9 Al 6 O 18 and Ca 12 Al 14 O 33 with suitable pore volume to prevent sintering of CaO and structure collapsing at high temperature.


Introduction
A dramatic increase of carbon dioxide, a known greenhouse gas, in atmosphere is a worldwide environmentally concern.Calcium looping is one of potential technologies for the separation of CO2 from flue gas mixture.Moreover, it can be applied as a sorbent in sorption enhanced reforming for H2 production [1].The CO2 capture reaction for CaO adsorbent is occurred by the reversible reaction between CaO and CO2, named carbonation-calcination reaction [2] (s) CaCO (g) CO CaO(s) Such a calcium looping process involves in multicycles carbonation-calcination; however, the makeup flow of fresh sorbent in calcium looping is still required due to the deactivation of it lead to higher operating cost.Consequently, several technologies have been explored for enhancing the CO2 capture activity of the sorbents, such as steam or water hydration [3][4], incorporation of metal ion using wet mixing method , sol-gel process [5], flame spray pyrolysis technology [6], coprecipitation [7,8] It is worth to note that, no matter what method was employed, the type of calcium precursor used plays a significant role in the carbonation-calcination performance.The dependent on the type of calcium precursor used is one of interested in this research.
On the other side, because CaO particles are soft and are able to erode quickly in fluidized bed, mass loss is therefore another problem when using a calcination reactor operating in fluidization mode [9].Also the pelletization has outgrowth in reducing pressure drop more than the powder sorbent when applied in fixed bed adsorpter, which is of more interest for the use in industry; however the work on the manufacture of CaO based pellets for the application of high-temperature CO2 capture is relatively limited.
It has been reported that the pellet sorbent can increase the mechanical strength for sorbent [10][11].Moreover, Wu et al. [12] evaluated the use of Ca(OH)2 as a precursor of Ca-base pellet sorbent for the application of CO2 capture.Experimental results show that Ca(OH)2 precursor provides better adsorption capacity than CaO precursor and CaCO3 precursor, of which 10.7 mol CO2/kg of adsorbent can be obtained at temperature of 650°C and at CO2 partial pressure of 0.4 atm.This is due to the decomposition of Ca(OH)2 which release water creates more surface area.Ridha  studied the effect of pellet size on CO2 uptake capacity.Natural kaolin and Al(OH)3 obtained from acid leaching of kaolin were employed as binders.They found the smaller pellets have higher ability to capture about 20% more CO2 than the larger pellets in the first cycles.However, after a large number of cycles, the effect becomes insignificant.In addition, the pellet with Al(OH)3 binder has higher thermal stability than that without binder [13].
In this study, a dependent of calcium precursor on pelletization and their carbonation/calcination performance are investigated.

Preparation of sorbents
Two techniques of mixing method were used to prepare sorbents: sol-mixing and wet-mixing.The synthesis procedure for sol-mixing technique, briefly, calcium precursor was firstly decomposed to pure CaO by calcination in the furnace; at 850°C for calcium acetate precursor and at 900°C for calcium hydroxide precursor.Then CaO was mixed with aluminium precursor in the solution of propanol and stirred for 1 h at 75°C.The reaction was aged for overnight and then recovered by drying in an oven at 120°C for 12 h followed by calcination at 900°C for 1.5 h.In the case of wet-mixing, the solution of calcium and aluminium were mixed and stirred at 75°C for 1 h.The solution was then dried in an oven at 120°C for 12 h, dry white powder was obtained.The obtained material was calcined at 900°C for 1.5 h to form CaO. The ratio of calcium to aluminium was calculated to be 75/25 wt% for all samples.In this work, we denoted the synthetic materials prepared from wet-mixing as -w and from sol-mixing as -s, for example, the adsorbent prepared from wet-mixing method using calcium hydroxide as a precursor was denoted as Ca(OH)2-w.Pellet-type adsorbents were prepared by mixing water to the powder of CaO-based sorbent with the ratio of 1.5 g/g of sorbent powder.Pellet was prepared by gradually dropping water in powder sorbent and stirred until a gel was obtained.Then the gel was subjected to molding and pressed until a stiff consistency is reached.After shaping, the samples were dried at room temperature for 24 h.

Adsorbent characterization
Crystallinity of synthetic adsorbents were characterized by powder X-ray diffraction (XRD).The spectra were recorded on a Rigaku, Miniflex II desktop X-ray diffractometer using Cu Kα radiation in the 2θ range of 10-80°C at room temperature.Structure and morphologies of the adsorbents were imaged by scanning electron microscopy (SEM, JEOL JSM-5800 LV).The surface properties, including BET surface area, average pore size, and pore size distribution, were determined by N2 adsorption/desorption at 77 K, provided by BEL SORP mini II.

CO2 adsorption test
CO2 adsorption capacity was performed by introducing the CaO pellets in a vertical alumina tube placed in the furnace at constant carbonation temperature of 450°C.CO2 gas was flowed into the reactor at 10 bar, the weight and pressure changes were monitored until it became constant.Regeneration of CaO was performed by heating the sorbent to calcination temperature of 900°C for 1.5 h.

Effect of calcium precursor on sorbent characteristic
The synthetic adsorbents were characterized by XRD as shown in Fig. 1.All the samples are composed of CaO, Ca12Al14O33, and Ca9Al6O18 with different compositions depend upon calcium precursor and synthesis method.Fig. 2 presents SEM images of the adsorbents CaObased alumina.The adsorbents obtained by the use of Ca(CH3COO)2 as calcium precursor has the smallest size of crystal where the others aggregated to larger particle.Moreover comparing between wet-mixing and solmixing of Ca(CH3COO)2 precursor, the wet-mixing exhibit comparatively lower particle size.N2 adsorption/desorption isotherm measurement was conducted to determine surface properties texture of the sorbents as summarized in Table 1.Two sorbents, Ca(OH)2-s and Ca(NO3)2-w have low surface area and are similar to that of commercial CaO, whereas, Ca(CH3COO)2-w and Ca(CH3COO)2-s offer distinctive high surface area than others.The significantly increased of specific surface area is in consistent with the results of SEM as smallest particle of the sorbent obtained from calcium acetate precursor via wet-mixing method (Fig. 2b).In addition, using wet-mixing in Ca(CH3COO)2-w increased their pore volume remarkably comparing to the sol-mixing of Ca(CH3COO)2-s (Table 1).Thermal stability of the sorbents in pellet form has been examined before performing high temperature CO2 adsorption test.Thermal stability test has been conducted at 900°C similar to the temperature where calcination reaction takes place.Fig. 3 shows thermal resistance of spherical pellets of different preparation techniques.The results show that CaO-based alumina in spherical shape, which was derived from Ca(OH)2 cracked (Fig. 3d) while others can retain their shape and structure (Fig. 3ac)

CO2 adsorption performance
Since the mechanical property of Ca(OH)2-s was unsuitable, the CO2 adsorption performance test of Ca(OH)2-s was neglected in this section.The value of CO2 sorption capacity of different pellet-type sorbent are shown in Fig. 4. Ca(CH3COO)2-w and Ca(CH3COO)2-s exhibit higher CO2 sorption capacity than that of commercial CaO (the values are 0.58, 0.51 and 0.42 g CO2/g CaO, rescpectively), while Ca(NO3)2 had the lowest sorption capacity (0.22 g CO2 / g CaO).Although the understanding is not straightforward, the much lower in total pore volume of Ca(NO3)2-w might be a reason of a much lower CO2 sorption capacity while the surface area also a dominant parameters of the sorption capacity as observed from the tendency of others sorbent.The cyclic carbonation/calcination performance are further tested for three sorbents except Ca(NO3)2-w.

Conclusion
A screening of potential calcium precursors and their synthesis method for pelletization of CaO-based sorbents for CO2 capture was conducted in this work.Among calcium precursors including Ca(OH)2, Ca(NO3)2 and Ca(CH3COO)2), CaO derived from Ca(CH3COO)2 precursors using both wet-and solmixing methods exhibit higher CO2 sorption capacity than that of commercial CaO.However, the pelletized CaO from Ca(CH3COO)2 wet-mixing and commercial CaO cannot retain their structure after carbonation/calcination for 3 cycles lead to a drastic loss in sorption capacity.Ca(CH3COO)2 sol-mixing exhibit the most carbonation/calcination stability with 0.51 and 0.41 g CO2/ g CaO at first cycle and tenth cycle respectively.The presence of Ca9Al6O18 and Ca12Al14O33 with suitable pore volume might be key parameters to prevent sintering of CaO and structure collapsing at high temperature.

Fig 4 :Fig 5 :
Fig 4: CO2 sorption capacity of pellet sphere sorbents in the first cycle.As shown in Fig.5, although Ca(CH3COO)2-w can exhibit the highest sorption capacity in the first cycle, the capacity drastically decreased in the 2 nd and 3 rd cycle.Generally, the presence of Ca9Al6O18 and Ca12Al14O33 inert support material effectively prevents sintering of CaO to some extent during regeneration of sorbent at high temperature.Although, Ca(CH3COO)2-w consists of Ca9Al6O18 inert materials acting as structural supports, a very large pore volume of Ca(CH3COO)2-w are hypothesized as a reason for structure collapse after only three cycle carbonation/calcination.This undesirable loss of pellet strength during carbonation/calcination cycles can be explained due to the presence of cracks