Dry hydrated potassium carbonate for effective CO2 capture

Suying Wanga, Zhengwen Liua, Andrew T. Smithbc, Yanxian Zenga, Luyi Sun*bcd and Weixing Wang*a
aKey Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China. E-mail: cewxwang@scut.edu.cn
bPolymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA. E-mail: luyi.sun@uconn.edu
cDepartment of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA
dDepartment of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA

Received 8th May 2019 , Accepted 30th June 2019

First published on 1st July 2019


Dry hydrated potassium carbonate (DHPC) is a free-flowing powder prepared by uniformly mixing hydrated K2CO3 and hydrophobic nanosilica. We demonstrated that DHPCs can absorb CO2 rapidly because of their high surface-to-volume ratio. Their CO2 capture performance is superior to that of 30 wt% monoethanolamine (MEA), an industry standard. DHPC-75 (containing 75 wt% K2CO3) has a CO2 uptake capacity of 233 mg g−1 (90% saturation uptake within 13 min), higher than the widely used 30 wt% MEA aqueous solution (111 mg g−1, 90% saturation uptake within 25 min). DHPC-75 also exhibits an excellent cycling performance, thus becoming a promising candidate for practical application.


1. Introduction

Nowadays climate change has become a growing concern. The Paris Agreement aims at keeping a global temperature rise this century well below 2 °C above pre-industrial levels.1 Carbon dioxide (CO2), a major greenhouse gas, has the largest contribution to climate change. CO2 is primarily produced by the combustion of fossil fuels used in power generation facilities, manufacturing industries, and transportation vehicles. The Intergovernmental Panel on Climate Change (IPCC) estimates that CO2 emissions to the atmosphere could be reduced by 80–90% with a conventional power plant equipped with CO2 capture and storage technology.2,3

Several processes have been developed for CO2 capture from a power plant flue gas, including absorption with solvents, adsorption using porous materials, membrane separation, and cryogenic fractionation.3–17 However, these technologies face challenges in terms of cost and energy consumption. Thus, an alkali metal carbonate-based absorbent (mainly K2CO3) has been investigated as an alternative approach for CO2 capture from the flue gas, and is expected to be more cost-effective and energy-efficient.9,18–22 However, the main issue of using pure K2CO3 is its slow reaction kinetics. To address such a problem, researchers developed K2CO3-based sorbents on various supports, such as activated carbon, ZrO2, Al2O3, TiO2, etc.23–30 However, the optimum loading ratio of K2CO3 (the weight ratio of loaded K2CO3 to support) was usually ca. 30%; excess K2CO3 will block the pores or channels of the support, resulting in the reduction of CO2 absorption capacity and reaction kinetics.23,25,26,28

Recently, Cooper and co-workers made a breakthrough to develop ‘DryK2CO3’ formed by the high-speed mixing of an aqueous solution of K2CO3 with hydrophobic nanosilica for CO2 capture.31 The finely dispersed K2CO3 aqueous solution droplets in gas-permeable hydrophobic nanosilica coating increased the gas–liquid interface area significantly, as a result of which the kinetics of CO2 absorption increased dramatically. However, the capacity was unsatisfactory because of the relatively low concentration of K2CO3 aqueous solution (the maximum concentration of K2CO3 in ‘DryK2CO3’ is ca. 45 wt%). Herein, we modify this technology to significantly increase the CO2 uptake capacity and reaction kinetics by means of encapsulating hydrated K2CO3 in gas-permeable hydrophobic nanosilica coating to form dry hydrated potassium carbonate (DHPC) (with a K2CO3 concentration of 70 wt% or higher, and the corresponding loading ratio of K2CO3 is 700% or higher) for effective CO2 capture.

2. Experimental

2.1 Materials

K2CO3 (99%) was purchased from Aladdin Co. Ltd, China. Na2CO3 (99.8%) was purchased from Tianjin Qilun Chemical Technology Co. Ltd, China. Hydrophobic nanosilica (H18, containing quasi-spherical particles with a primary diameter of ca. 20 nm) was purchased from Wacker-Chemie, Germany. CO2 (99.9%) and a gas mixture (15 vol% CO2 in N2) were purchased from Zhuozheng Gas Co. Ltd, China. Monoethanolamine (MEA, 99%) was purchased from Jiangsu Yonghua Chemical Technology Co. Ltd, China.

2.2 Preparation of DHPCs

A typical sample of DHPC was formed by rapidly mixing a pre-determined amount of K2CO3, water, and hydrophobic nanosilica (H18) in a conventional kitchen blender (Philips, HR2168, 2-liter, China) at 18[thin space (1/6-em)]000 rpm for 60 seconds. Table 1 shows the detailed compositions of various DHPC samples.
Table 1 Composition of DHPCs
Sample K2CO3 (g) Hydrophobic nanosilica (H18, g) Water (g)
DHPC-70 70.0 10.0 20.0
DHPC-72 72.0 10.0 18.0
DHPC-75 75.0 10.0 15.0
DHPC-77 77.0 10.0 13.0
DHPC-80 80.0 10.0 10.0


2.3 Characterization

The microstructures of DHPCs were characterized using a low vacuum environment scanning electron microscope (ESEM; Quanta 200, FEI, The Netherlands). The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 diffractometer (Bruker, Germany) with Bragg–Brentano θ−2θ geometry (20 kV and 5 mA), using a graphite monochromator with Cu Kα radiation. Thermogravimetric analysis (TGA) was conducted using a TG 209F3 thermogravimetric analyzer (Netzsch, Germany) at a heating rate of 10 K min−1 under a N2 atmosphere.

2.4 CO2 uptake experiments

To measure the CO2 capture capacity of the DHPC samples, 5.0 g of DHPC was charged into a 50 mL container, which was exposed to CO2 using a balloon containing a sufficient amount of CO2 gas (ca. 5 L with a pressure of ca. 1.05 bar). The amount of CO2 captured by each DHPC sample was measured using a balance.

2.5 Regeneration of DHPCs

In order to evaluate the cycling performance of the DHPCs, after the CO2 absorption experiment, a tube furnace (Shanghai Guier Machinery and Equipment Co. Ltd, China) was used to regenerate the DHPC samples by heating them for 2 h at 200 °C, followed by absorbing water from the air until the original weight was reached (ca. 3–5 hours under ambient conditions with a relative humidity of 50–70%).

3. Results and discussion

Fig. 1 shows the CO2 uptake kinetic curves using various DHPCs as absorbents at 30 °C. It was found that DHPC-70, DHPC-72, and DHPC-75 exhibited higher CO2 uptake and faster absorption rates among the tested samples. For DHPC-70, ca. 90% of the absorption occurred within 5 min (t90 = 5 min), and reached a saturation CO2 uptake of 216 mg g−1 after 60 min. The CO2 uptake capacity increased to 224 mg g−1 when the mass fraction of K2CO3 was increased to 72 wt% and t90 was ca. 6 min. DHPC-75 has an even higher CO2 uptake capacity of 233 mg g−1 compared with DHPC-70 and DHPC-72, and its t90 was ca. 13 min. Although DHPC-77 achieved the highest capacity (240 mg g−1), its CO2 absorption rate was relatively slow, with a t90 of ca. 25 min. However, the absorption capacity did not further increase with increasing K2CO3 concentration. For DHPC-80, a very low CO2 absorption capacity was observed (18 mg g−1). When the concentration of K2CO3 in DHPCs exceeds 80 wt%, none can absorb CO2 appreciably, and their overall performance is similar to DHPC-80 (data are not shown).
image file: c9dt01909j-f1.tif
Fig. 1 CO2 uptake kinetics of various DHPCs and 30 wt% MEA aqueous solution at 30 °C.

The amine-based CO2 capture system is a proven technology that is already commercialized.4 In order to prevent excessive corrosion, typically 30 wt% monoethanolamine (MEA) aqueous solution is used.4 The MEA-based capture system shows similar CO2 uptake kinetics to DHPCs initially, but the overall absorption capacity is relatively low (111 mg g−1 versus 233 mg g−1 for DHPC-75), and its t90 was 25 min.

The main reason for the rapid reaction rate and high CO2 uptake capacity of DHPCs is their higher surface area-to-volume ratio. As shown in Fig. 2(a), DHPC-75 is a free-flowing powder composed of hydrated K2CO3 and hydrophobic nanosilica (H18). Because of the hydrophobicity of H18 and the hydrophilicity of hydrated K2CO3, after high-speed mixing in a blender, the hydrated K2CO3 particles were surrounded by a network of hydrophobic nanosilica, allowing for gas diffusion.31–34 Fig. 2(b) shows an SEM image of DHPC-75, which is composed of irregular spherical particles of size ranging from 10 to 40 μm. Besides the effect of surface area-to-volume ratio, we demonstrated that the CO2 uptake capacity and reaction kinetics of the DHPCs also depend on their water content. As for DHPC-80, the molar ratio of K2CO3 to H2O is ca. 1[thin space (1/6-em)]:[thin space (1/6-em)]1. Theoretically, DHPC-80 should have a higher CO2 uptake capacity, but it exhibits an extremely low CO2 uptake capacity (18 mg g−1 versus 233 mg g−1 for DHPC-75). This suggests that the extra water present in DHPCs plays a significant role in the CO2 uptake capacity and reaction kinetics. DHPC-70 has a relatively lower CO2 uptake capacity compared with DHPC-75 (216 mg g−1 versus 233 mg g−1), but its t90 is only 5 min (versus 13 min for DHPC-75).


image file: c9dt01909j-f2.tif
Fig. 2 (a) Free-flowing DHPC-75 from a glass funnel; (b) SEM image of DHPC-75.

To further investigate the CO2 uptake capacity of DHPC-75, the XRD patterns of fresh DHPC-75 and its reaction products after 2, 5, 10, and 60 min of absorption reaction at 30 °C were recorded and are shown in Fig. 3. The XRD pattern of DHPC-75 was very close to the standard pattern of K2CO3·1.5H2O. With the reaction of DHPC-75 and CO2, characteristic peaks of K2CO3·1.5H2O disappeared, and an intermediate structure, K4H2(CO3)3·1.5H2O, appeared after 2 min of reaction, according to the patterns. Eventually, virtually pure KHCO3 formed after 60 min of reaction.


image file: c9dt01909j-f3.tif
Fig. 3 XRD patterns of fresh DHPC-75 and its reaction products after 2, 5, 10, and 60 min of CO2 absorption reaction at 30 °C.

Considering that temperature has a profound influence on the kinetics, we specifically studied the effect of temperature on the CO2 uptake capacity of DHPCs. As shown in Fig. 4, when the temperature is below 30 °C, the CO2 uptake capacity of DHPC-75 is almost the same. Further increasing the temperature led to a slight decrease in CO2 uptake probably because of the evaporation of water in DHPC-75.


image file: c9dt01909j-f4.tif
Fig. 4 CO2 uptake kinetics of DHPC-75 at different temperatures.

To further confirm the CO2 uptake in DHPC-75, TGA was carried out by heating up the fresh DHPC-75 and CO2-loaded DHPC-75 from 30 to 500 °C. Fig. 5 shows the weight change of the fresh DHPC-75 and CO2-loaded DHPC-75 during the heating process. The weight of the fresh DHPC-75 remained virtually constant up to 65 °C, after which the curve dropped quickly because of the evaporation of water. As expected, DHPC-75 was thermally stable up to 300 °C and the weight loss of the fresh DHPC-75 was about 15.0%, consistent with the composition of DHPC-75. The weight of the CO2-loaded DHPC-75 changed sharply after 120 °C because of the decomposition of KHCO3. In addition, it was found that the remaining weight was ca. 69.6% at 300 °C, which was roughly the theoretical content of K2CO3 and H18 in the CO2-loaded DHPC-75, suggesting that it is thermally stable up to 300 °C. Thus, DHPC-75 can be easily regenerated by heating it to a certain temperature (such as 200 °C) to allow for a complete release of water and CO2. DHPC-75 also exhibited excellent recyclability with only marginal deterioration in the CO2 uptake capacity after regeneration, as shown in Fig. 6.


image file: c9dt01909j-f5.tif
Fig. 5 TGA thermograms of fresh DHPC-75 and CO2-loaded DHPC-75.

image file: c9dt01909j-f6.tif
Fig. 6 Recycling performance of DHPC-75 after regeneration at 200 °C.

For practical CO2 capture in the post-combustion process, the concentration of CO2 is low, typically ∼15% in the flue gas. As shown in Fig. 7, DHPC-75 works well in such a low partial pressure of CO2. It has a relatively higher CO2 uptake capacity compared with 30 wt% MEA aqueous solution (124 mg g−1 vs. 79 mg g−1).


image file: c9dt01909j-f7.tif
Fig. 7 CO2 uptake kinetics of DHPC-75 and 30 wt% MEA aqueous solution (15 vol% CO2 in N2 at 30 °C).

For large-scale industrial CO2 capture applications, the use of Na2CO3 will be more cost-competitive. As shown in Fig. 8, DHSC-60 (dry hydrated sodium carbonate with 60 wt% Na2CO3, 10 wt% H18 and 30 wt% water) showed a slightly higher CO2 uptake capacity compared with DHPC-75 (244 mg g−1 vs. 233 mg g−1). We also studied the CO2 uptake of dry water as a control, which exhibits a CO2 uptake capacity of ca. 1.6 mg g−1 (Fig. 8).


image file: c9dt01909j-f8.tif
Fig. 8 CO2 uptake kinetics of DHPC-75, DHSC-60, and dry water at 30 °C.

In addition to hydrophobic nanosilica, we believe that other hydrophobic nanomaterials may serve a similar function to prepare free-flowing “powder stuff”, and have many potential applications, including catalysis, energy storage, sensors, and anti-corrosion.35–43

4. Conclusions

DHPCs exhibit an excellent performance in terms of both CO2 absorption capacity and reaction kinetics, especially DHPC-75, which exhibits an absorption capacity of 233 mg of CO2 per g of absorbent at 30 °C with a t90 of 13 min. DHPCs can also be easily regenerated and recycled multiple times. Considering their high CO2 uptake capacity, quick reaction kinetics, and low cost, these DHPCs are promising for practical CO2 capture applications. In addition, other than K2CO3, other dry hydrated metal carbonates such as Na2CO3 can also be prepared using the same strategy to further lower the cost and/or to improve performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

W. W. acknowledges the support from the National Natural Science Foundation of China (21676107 and 21176093). The authors thank Yuanhao Cai, Liang Li, and Pinzhen Lin for their assistance.

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