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Screening of naturally Al/Si-based mineral binders to modify CaO-based pellets for CO2 capture Jian Sun, Cheng Liang, Wenyu Wang, and Wenqiang Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03252 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017
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Screening of naturally Al/Si-based mineral binders to modify CaO-based pellets for CO2 capture Jian Sun*a,b, Cheng Lianga, Wenyu Wangb, Wenqiang Liu*b a
Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control,
School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China b
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,
Huazhong University of Science and Technology, Wuhan 430074, China
*Corresponding author: E-mail:
[email protected];
[email protected]; Postal address: Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, School of Energy and Mechanical Engineering, Nanjing Normal University, 78 Bancang Street, Nanjing 210042, PR CHINA; State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, PR CHINA.
Abstract CO2 removal in the calcium looping process is achieved by circulating the CaO-based sorbents between two interconnected fluidized-bed reactors. During the cyclic CO2 capture process, severe attrition are easily emerged for CaO-based sorbents. Therefore, the preparation of CaO-based pellets with a high anti-attrition ability is important. In this work, a range of inert aluminosilicate binders (i.e., diatomite, rectorite, montmorillonite,
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attapulgite, vermiculite, pumice, and bauxite) were added to the extruded-spheronized CaO-based pellets aiming to improve their attrition-resisting properties. Two modifying modes of binder (dry-mixing and hydration-mixing) were compared in order to confirm the more suitable one for the preparation of binder-bound pellets. It is found that the pumice-bound pellets containing 10 wt % of pumice prepared via hydration-mixing exhibit the highest cumulative CO2 capture capacity of 11.13 g-CO2/g-CaO during 25 cycles, and a promising anti-attrition ability (a weight loss of 0.77% after 3000 rotations). The formed Ca3Al2O6 acting as the skeleton structure can alleviate sorbent sintering, thus resulting in the superior CO2 capture performance. The high anti-attrition ability is mainly attributed to the uniformly distributed pumice particles, the high binding force of hydrate lime and the formation of calcium carbonate during air drying process. Keywords: aluminosilicate binders, CaO-based pellets, modifying mode, anti-attrition ability 1. Introduction CO2 emissions from the large-scale burning of fossil fuels are the most important driving force for the global warming.1 To mitigate the impact of global warming, the concept of CO2 capture and storage (CCS) has been proposed.2, 3 A promising technology for CO2 capture using solid sorbents has been widely studied in both academic and industrial fields. The solid sorbents can be classified according to their working temperatures for CO2 capture, including low-temperature (400 oC).4-7 Among them, CaO-based materials
as
the
high-temperature
sorbents
for
CO2
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capture
have
a
high
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commercialization potential, owing to the high CO2 sorption capacity (theoretically, 0.786 g of CO2/g of CaO) and the low-cost raw materials.8, 9 CaO-based sorbents are commonly used in the calcium looping (CaL) process, which mainly consists of two interconnected fluidized-bed reactors, a carbonator and a calciner.10 To achieve CO2 separation from the flue gas, the CaO-based sorbents are continuously circulated between the carbonator and calciner. However, severe attrition easily occurs for the CaO-based sorbents during circulation process, generating large amounts of fine powders.11, 12 The finely generated powders still possess a certain capability for CO2 capture, which are easily elutriated from the fluidized-bed reactors, causing a decrease of CO2 capture efficiency. Therefore, large amounts of fresh sorbents need to be replenished, but it will greatly increase the operation cost of the CaL.13 In addition, the loss-in-capacity of CaO-based sorbent is another prominent problem encountered during the long-term CO2 capture process.14 Hence, the preparation of CaO-based pellets with high capability for CO2 capture and anti-attrition is an effective way to alleviate the above problem.13, 15 CaO-based pellets are commonly produced via three different granulation methods, such as extrusion9,
13, 16-20
, pelletization21-26 and extrusion-spheronization15,
27, 28
. As
summarized in Table 1, the calcium precursors for the preparation of CaO-based pellets mainly include limestone16, 29, hydrate lime12, 16, 21, 30-32 , acetified limestone33-36, calcium carbonate25, spent CaO-based sorbent19,
23
, carbide slag15 and organometallic calcium
precursors20. To further modify the properties of CaO-based pellets, the Al/Si-based binders (i.e., calcium aluminate cements and naturally occurring aluminosilicate materials) are usually added to the pellets. Calcium aluminate cements with different
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Al2O3 contents (ranging from 39.8 to 80 wt %)
9, 12, 13, 15-21, 23, 26, 29-33, 37-40
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are the most
commonly used binders due to their low costs and excellent refractory properties.16 It is found that calcium aluminate cements can effectively improve the attrition-resisting properties of the CaO-based pellets. Moreover, the formation of Ca-Al-O phases owing the presence of high Al2O3 content can further improve the CO2 capture stability of the cement-bound pellets, as listed in Table S1.9, 26 Some naturally occurring aluminosilicate materials, such as the sodium and calcium bentonites16, clay25 and kaolin34-36, have also been used as binders to modify CaO-based pellets. It is found that the modification effects on CO2 capture performance of CaO-based pellets vary with the compositions and structures of the binders. The naturally aluminosilicate mineral materials are easily available and low-cost, which are potentially used to modify the CaO-based sorbent pellets. However, the naturally occurring aluminosilicate materials that have been studied as the binders for the modification of CaO-based pellets are still limited. Moreover, the modifying mode of binder plays an important role on the cyclic CO2 uptake of the CaO-based sorbents doped with binders.41 It is necessary to investigate the impact of modifying mode on the cyclic CO2 capture performance and anti-attrition property of the binder-bound, CaO-based pellets. Therefore, in this work, seven types of naturally occurring aluminosilicate materials (i.e., diatomite, rectorite, montmorillonite, attapulgite, vermiculite, pumice, and bauxite) were used as the binders, respectively. The binder-bound pellets containing different amounts of binders (0, 5 and 10 wt %) were produced via an extrusion-spheronization method. To find the more suitable modifying mode of binder for the preparation of the
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highly efficient binder-bound pellets, two modifying modes of dry-mixing and hydrationmixing were compared. Moreover, a comprehensive comparison of the raw materials cost, cyclic CO2 capture performance and anti-attrition ability was conducted to identify the optimal parameters for the preparation of binder-bound, CaO-based pellets. 2. Experimental 2.1 Raw Materials The naturally occurring limestone (CaCO3>98 wt %) was purchased from Fude chemical plant located in Tongling of Anhui province in China. After grinding and sieving, the limestone powders with diameters less than 170 um were used as the calcium precursors. The calcined lime was obtained from the calcination of limestone at 900 oC in air for 1 h. The naturally occurring mineral materials purchased from a chemical materials supply Web site (chem.1688.com) were employed as binders for the preparation of CaO-based sorbent pellets, including diatomite (DI), rectorite (RE), montmorillonite (MO), attapulgite (AT), vermiculite (VE), pumice (PU) and bauxite (BA). All the binders were sieved into a size fraction of
