Spherical potassium intercalated activated carbon beads for pulverised fuel CO2 post-combustion capture Academic research paper on "Chemical sciences"

Accepted Manuscript

Spherical Potassium Intercalated Activated Carbon Beads for Pulverised Fuel CO2 Post-Combustion Capture

Jingjing Liu, Nannan Sun, Chenggong Sun, Hao Liu, Colin Snape, Kaixi Li, Wei Wei, Yuhan Sun

PII: DOI:

Reference: To appear in:

S0008-6223(15)00551-5 http://dx.doi.org/10.1016/j.carbon.2015.06.036 CARBON 10038

Carbon

Received Date: Revised Date: Accepted Date:

17 March 2015

15 June 2015

16 June 2015

Please cite this article as: Liu, J., Sun, N., Sun, C., Liu, H., Snape, C., Li, K., Wei, W., Sun, Y., Spherical Potassium Intercalated Activated Carbon Beads for Pulverised Fuel CO2 Post-Combustion Capture, Carbon (2015), doi: http:// dx.doi.org/10.1016/j.carbon.2015.06.036

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Spherical Potassium Intercalated Activated Carbon Beads for Pulverised Fuel CO2 PostCombustion Capture

Jingjing Liua, Nannan Sunac, Chenggong Suna*, Hao Liua*, Colin Snapea, Kaixi Lib*, Wei Weic and Yuhan Sunc

a Faculty of Engineering, The Energy Technologies Building, Jubilee Campus, University of Nottingham, Nottingham NG7 2TU, UK

b Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 03001, China c Shanghai Advanced Research Institute, Chinese Academy of Science, No.99 Haike Road Zhangjiang Hi-Tech Park, Pudong, Shanghai, China

3.99 Hai

ABSTRACT

Spherical carbon beads with a uniform diameter of ca. 0.6-0.8 mm and high mechanical strength can be prepared by hydrothermal synthesis. To optimise the performance of these adsorbents for pulverised fuel post-combustion capture, the efficacy of potassium intercalation via a KOH treatment has been investigated, deliberately using nitrogen-free phenolic resin derived activated carbon (AC) beads so that the enhanced CO2 adsorption achieved by potassium intercalation could be delineated from any other effects. At 25 oC and CO2 partial pressure of 0.15 bar, the adsorption capacity of K-intercalated ACs nearly doubled from 0.79 mmol/g for the untreated carbons to 1.51 mmol/g whilst the effect on the morphology and mechanical strength is relatively small. It was found that only slightly more than ca. 1 wt.% of K is required to give the maximum benefit from intercalation that increases the surface polarity and the affinity towards CO2. The notably increased CO2 uptake of the K-AC beads as a result of modest increase in adsorption heat (32-40 kJ/mol compared to 27 kJ/mol for the original AC), coupled with the fast adsorption kinetics, suggest that the overall energy penalty is potentially superior to strongly basic polyethyleneimine and other amine-based solid adsorbent systems for carbon capture.

Corresponding authors. Tel: +44 115 7484577. Email: cheng-gong.sun@nottingham.ac.uk (Chenggong Sun)

Tel: +86 351 4250292. Email: likx@sxicc.ac.cn (Kaixi Li)

Tel: +44 115 7484577. Email: liu.hao@nottingham.ac.uk (Hao Liu)

1. Introduction

The International Energy Agency (IEA) and Intergovernmental Panel on Climate Change (IPCC) have identified carbon capture and storage (CCS) as a critical greenhouse gas reduction solution [16]. However, the successful development and deployment of efficient and cost-effective carbon capture technologies plays a decisive role in determining the viability of CCS as a whole because as it stands now, the cost of carbon capture using the state-of-art energy-intensive CO2 amine scrubbing technology accounts for over 70% of total CCS cost [7, 8]. Consequently, alternative capture technologies have been under intensive development over recent years, including advanced solvent scrubbing, oxyfuel combustion, chemical looping combustion, membrane separation and solid adsorbent looping technologies both at low and high temperatures [9]. Of these capture technologies being developed, low temperature solid adsorbents looping technology (SALT) has widely been recognised as having the potential of being viable for post-combustion CO2 capture (PCC), offering potentially improved process efficiency at significantly reduced energy penalty, lower capital and operational costs and smaller plant footprints. Various solid adsorbents are under investigation such as zeolites, supported/grafted amines, metallic organic frameworks (MOFs), functionalised carbon materials, and calcium or alkali metal based sorbent materials [5, 10-17], and some of these solid adsorbent-based capture systems have been demonstrated at varying scales, the largest being the pilot demonstration of dry carbonate sorbent technology for CO2 capture at Hadong power plant in South Korea [18].

Amongst the most studied materials as candidates for CO2 adsorption [19, 20], carbon-based adsorbents are characterised by the advantages of relatively low cost, ease of regeneration and generally stable cyclic performance. Traditionally, activated carbons are considered as typical physical adsorbents because of their highly developed micro-porosity and large surface area, which leads to the fact that activated carbons can normally only achieve better adsorption capacities at high CO2 partial pressures, limiting their use for post-combustion capture where CO2 partial pressure is usually low [21]. In order to enhance the surface affinity of activated carbons towards the acidic CO2, many investigations have been carried out to modify the chemistry of carbon surface by manipulating either the precursor materials and/or activation methodologies [22-26]. Thermal or thermochemical post-preparation treatment has been investigated as the potential means to introduce different surface functional groups such as basic oxygen functionalities (ketone, pyrone, chromene, etc.), nitrogen functional groups (-NH2-, -CN, pyridinic nitrogen, etc.), other heteroatoms or even ionic liquids [27, 28]. On the other hand, the thermodynamic and kinetic properties of adsorption also play an important role in determining the ultimate overall performance of activated carbons materials for CO2 capture [29]. Liu and Wilcox [30, 31] evaluated how the

realistic surface functional groups effected CO2 adsorption using plan-wave electronic structure calculations and found that the adsorption thermodynamics and kinetics can be effectively improved via re-addressing the surface chemistries of carbon materials. However, limited decisive progress has been achieved so far in this field.

It is only recently that carbon-based materials with enhanced CO2 adsorption capacities at relatively low CO2 pressures (ca. 1.0-1.8 mmol/g at 0.15 bar CO2 and 25 oC) have been reported [32-37]. Most of the adsorbents in these investigations were prepared by involving nitrogen-containing functionalities, and in most cases coupled with chemical activation by potassium hydroxide (KOH) [19, 38-41]. These protocols are effective in enhancing CO2 adsorption capacities, but the samples obtained were typically porous powders with very low bulk densities which will give low CO2 uptakes on a volumetric basis. Further, for practical applications where either moving or fluidized-bed adsorbers are used for CO2 capture, these fine powders need to be agglomerated to form pellets or beads with the aid of binders or other additives, which can dramatically reduce the adsorption capacities and kinetic performance. The use of spherical carbon beads with high physical strength avoids this problem where we have reported on their potential for both pre- and post-combustion capture [42] with CO2 uptakes at 1 bar being similar to those of many other carbons [43-45]. However, attempts to significantly increase CO2 uptakes, both by using nitrogen precursors and post-treatment with ammonia, have met with limited success [46-48].

Although the basis to enhance CO2 uptakes at relatively low partial pressures on carbon-based adsorbents has been established using KOH activation [49, 50], only powdered samples have been reported. In this study the use of potassium (K) intercalation via mild KOH activation as an effective means to boost the adsorption performance of activated carbons for post-combustion CO2 capture has been investigated, using activated-carbon beads with desirable spherical diameters (ca. 0.6 - 0.8 mm) suitable for direct practical applications. It is noteworthy that the KOH/AC mass ratios we used are much lower (from 0.1:1 to 1:1) than those used in typical standard KOH activations (often 2:1 or even higher [41, 51]). The novelty of this study lies in that the potentially vital role of alkaline metal intercalation as a means to enhance the CO2 adsorption capacity and strength of carbons have been investigated and sufficiently strong K-AC beads suitable for use in practical applications are obtained for the first time, with the beneficial effects of K-intercalation being quantified free from potential interference from nitrogen functionalities by using a phenolic resin as the precursor and, finally, the minimum amount of K required to enhance CO2 adsorption with a partial pressure of 0.15 bar CO2 is identified to simulate PCC in pulverised fuel (PF) power plant.

2. Experimental

2.1 Preparation of the raw activated carbon (AC) beads

The AC beads using phenolic resins as precursor were synthesised with a hydrothermal method which has been described elsewhere [42]. Briefly, a solution of hexamethylenetetramine and novolac-type phenolic resins were dissolved in methanol, followed by mixing in aqueous polyvinyl alcohol (PVA), and then the mixture was heated to 130 oC under stirring (400 RPM) in an autoclave for 1 h. After washing with abundant deionised water, the resulted resin beads were dried at 110 oC for overnight before they were carbonised at 830 oC in N2 for 1 h followed by steam activation for another 1 h at the same temperature to obtain the parent raw AC beads (denoted as PR0).

2.2 KOH treatment

5g of the raw AC beads were impregnated with 50 ml aqueous solution of KOH for 24 hours. After drying in a vacuum oven at 70 oC for overnight, which ensures that all the samples were completely evaporated, the samples were heated in a horizontal tube furnace from ambient to a pre-selected treatment temperature at 3 oC /min and maintained at the temperature for an hour. The samples were then washed with deionized water until neutral filtrate was obtained. Different KOH/AC mass ratios for impregnation and various chemical activation temperatures used in the preparation are summarised in Table 1 along with sample designations.

Some of the samples were further treated by exhaustive Soxhlet extraction using de-ionised water to obtain samples with variable contents of intercalated potassium contents (PR3_700_xh, where x stands for the extraction duration in hours). Detailed procedures of Soxhlet extraction with deionised water include: 1) The extraction thimble which contained the carbon bead sample was loaded into the main chamber of Soxhlet extractor; 2) Connect the Soxhlet exactor with distillation flask and reflux condenser; 3) Heat the distillation flask to 100 oC and maintain at this temperature for different periods of extraction. By controlling the extraction time, carbons containing different levels of intercalated potassium can be obtained in order to evaluate the importance of intercalated potassium at different levels.

Table 1 Preparation conditions and designation of the AC bead samples

Number Sample Initial KOH/AC mass ratio

for impregnation

Chemical activation temperature (oC)

10 11 12 13

PR1_600 PR1_700 PR1_800 PR2_600 PR2_700 PR2_800 PR3_600 PR3_700 PR3_800 PR4_600 PR4_700 PR4 800

0 0.1 0.1 0.1 0.3 0.3 0.3 0.5 0.5 0.5 1:1 1:1 1:1

None 600 700 800 600 700

2.2 Characterization of the samples Physical adsorption of N2 at -196 oC was carried out on a Micromeritics ASAP 2420 analyser. Prior to any measurements, all samples were degased at 120 oC for overnight. The apparent surface area (SBET) was calculated according to the method suggested by Parra et al. [52]. The cumulative pore volumes (Vtotal) were calculated from the amount of nitrogen adsorbed at P/P0 of ca. 0.99, and the average pore volume was calculated by 4Vtotal/SBET. The micropore volume (Vmicro) and surface area (Smicro) were determined by the t-plot method.

X-Ray Fluoresc ence (XRF) on selected samples was carried out on a Bruker S8 Tiger Spectrometer. Their morphologies were observed on a FEI Quanta 600 Scanning Electron Microscope (SEM) and JEOL 2100F Transmission Electron Microscope (TEM), respectively. The Energy Dispersive X-ray analysis (EDX) software is Esprit 1.9 by Bruker. X-ray Photoelectron Spectroscopy was measured on a Kratos Analytical Ultra-2008 spectrometer, and X-Ray Diffraction (XRD) was carried out on a Bruker D8 instrument. To determine the quantity of remaining potassium for K-ACs after washing, the samples were ashed using a thermogravimetric analyser (TGA, Q600, TA instruments; samples were first dehydrated at 120 oC for 20 mins, and then heated to 600 oC with a ramping rate of 20 oC /min, followed by an isothermal period for 40 mins.). The resultant ashes were analysed by an Inductive Coupled Plasma Optical Emission Spectrometer (Perkin-Elmer Optima 33-DV ICP-OES, USA).

rmined

2.4 CO2 adsorption measurements

The CO2 adsorption isotherms were measured by the same instrument used for physical adsorption of N2 (Micromeritics ASAP 2420). Similar to the nitrogen adsorption isotherm tests, samples were degased at 120 oC overnight.

CO2 adsorption of the K-AC beads was also investigated by using a thermogravimetric analyser (TGA, Q500, TA instruments). The sample was first dried at 150 oC in pure N2 for 45 minutes to remove any physisorbed moisture and/or CO2, the temperature was then cooled to the adsorption temperature (25 oC). After the temperature stabilized, a flow of 100 mL min-1 of a simulated flue gas containing 15% CO2 in N2 was introduced into the sample chamber at the adsorption temperature for 60 mins and the sample weight was recorded in order to calculate the CO2 uptake. After adsorption, the gas atmosphere was switched to N2 and the temperature was increased to 150 oC at a rate of 30 oC /min to desorb the CO2. Up to 50 adsorption-desorption cycles were performed to evaluate the stability of the adsorbent samples.

^sorption-

capacity of

2.5 Heat of adsorption measurements The heat of adsorption and the specific heat capacity of the K-AC carbon materials were determined using a SENSYS evo TG-DSC instrument (Setaram) under the conditions similar to those used in

adsorption tests using a mixture of CO2 and nitrogen. Since this instrument simultaneously provides mass changes and heat flows, the heat of adsorption can then be deduced in terms of the

heat released per mole of CO2 adsorbed.

gth testi

2.6 Mechanical strength testing

The mechanical strengths of the samples were measured using a DMAQ800 dynamic mechanical analyser (TA instruments, USA). The dynamic force exerted on the sample allows the change in deformation with temperature to be monitored. Each sample was tested ten times during which one carbon bead was randomly selected every time to ensure the accuracy of the results. The average values for the 10 repeat tests for each sample are reported.

3. Results and Discussion

3.1 Characterization of the K-containing activated carbon beads 3.1.1 Chemical composition

Our previous work has already showed that the parent carbon bead sample (PR0) was free from nitrogen (less than 0.01 wt. %) and inorganics, which was expected as the sample was derived from pyrolysis of a phenolic resin containing negligible impurities compared with coal and biomass-derived ACs [42]. Information from XPS, SEM-EDX and XRF on the distribution of the K is

presented in SI1 and SI2. XRF indicates that the K-AC samples activated at 700 oC have concentrations of K in the range of 14-22 wt. %. However, XPS data (SI1) suggests that the surface K concentrations are considerably lower, being slightly less than 10 wt. %. This reveals that most of the K has been effectively intercalated within the carbon beads. It was difficult to precisely link the amount of intercalated K with the KOH/carbon mass ratios used in the preparation method as the formation of some crystalline potassium compound clusters on the outer surface of the carbon beads is evident (see Fig. 1). However, it must be stressed that the materials prepared using the same procedure and conditions are fairly reproducible (SI3).

. ' v . - ! •>

- • \> • V

300.0 um

Fig. 1 SEM image of the PR1_700 sample showing the formation of clustered potassium compounds on the outer surface of the treated carbons during the drying process.

,rope.es

ion iso

3.1.2 Textural prope

The N2 adsorption isotherms are illustrated in Fig. 2 and SI4. Despite the small hysteresis loops observed for some of the samples due to the relatively minor development of mesoporous structures, all isotherms can be generally classified as type I according to IUPAC classification [53],suggesting that the spherical carbons are mainly microporous.

hat the sp

able 2

Table 2 summarises the specific BET surface areas, pore volumes, microporosities for all the samples derived from N2 (77K) adsorption data. The initial AC bead sample (PR0) exhibits a BET surface area of 1128 m2/g, which is typical for phenolic resin based activated carbons [54, 55]. The micropore surface area (Smicro) calculated by the t-plot method is fairly close to the BET surface area, consistent with the dominant microporosity. However, it is interesting to note that despite prolonged washing with water, considerably lower BET surface areas and total pore volumes were observed for most of the K-ACs, particularly the PR3 series. These results are in sharp contrast to those reported in the literature where enhanced porosity of carbons was obtained via KOH

activation [19, 41, 56]. Therefore, it appears that the carbon matrix structures inherited from the earlier steam activation has been further modified leading to intercalation of K species but with the loss of some microporositiy. Previous investigations [19, 57-61] have revealed that KOH activation involves a variety of chemical reactions as detailed below, with the significance of the individual reactions varying with temperatures and KOH/carbon mass ratios. During the activation, KOH acts in three simultaneous/consecutive ways: 1) the catalyst to accelerate the gasification reactions which are shown in reactions (1)-(3) and (5). Otowa et al. [61] proposed the reactions of carbon with CO2 and H2O (steam) produced from KOH decomposition at temperatures from ca.400 oC, as shown in reactions (1)-(3). However, recent work by Linares-Solano and his colleagues [56, 57, 60] has observed that carbon can react directly with KOH as shown in reaction (5) at temperatures from ca. 400 oC as the standard Gibbs free energy of this reaction turns negative from this temperature, and this has been vindicated by the formation of H2 and K2CO3 at temperatures below 700 oC with no appreciable amount of CO and CO2 being produced [56]; 2) the formation of metallic potassium particularly at temperatures higher than 700 oC that can readily be mobilised and intercalated to the carbon matrix during the porosity development, largely due to the high volatility of metallic potassium (the melting and boiling point for metallic K are 63 and 759 oC, respectively), see in reactions (5) and (7) - (9); and 3) the formation of potassium carbonate layer as shown in reactions (4) and (5), which can effectively prevent the carbon from over consumption, although the carbonate can also be partly or wholly consumed in reactions (6) and (9), depending on the KOH/carbon ratios used. It is believed that the intercalated K is most likely associated with the formed oxygen functionalities (as shown in reaction (10)) within the carbon matrix structures via the formation of quasi-chemical bonds, giving rise to the residual potassium species that cannot be readily removed by the routine washing with excessive water.

K2C03 + 2C -> 2K + 3CO K20 + C ^ K +C-O-K

K2C03 -> K20 + C02 K20 +H2 -> 2K + H20 K20 + C -> 2K + CO

2KOH -> K20 +H20 C + H20(steam) -> CO + H2

CO + H20 -> C02 + H2 K20 + C02 -> K2C03

6KOH + 2C -> 2K + 3H2 + 2K2C03

(1) (2)

(8) (9)

The surface area or microporosity of KOH-activated carbons is determined by a combination of activation temperature and the amount of KOH used. As shown in Table 2, however, the considerably lower surface areas obtained for most of the samples from the secondary KOH activation of the original steam-activated carbon bead sample (PRO), which has a considerably higher surface area of 1128 m2 g_1, tend to suggest that the potential extra microporosity developed from the secondary KOH activation was offset by the even larger porosity loss as a consequence of the simultaneous potassium intercalation that can potentially lead to complete or partial occlusion of the micropores. It was found that at the same KOH/AC mass ratios, the surface areas of all the K-AC bead samples increased significantly with increasing activation temperatures, highlighting the temperature-dependent reactivity of carbon with KOH. At the same activation temperature, however, the surface areas of the K-ACs were found to decline first with elevating KOH/AC mass ratios from 0.1:1 to 0.5:1 and then incline with further rise in KOH/AC ratios from 0.5:1 to up to 1:1, indicating the relative significance of porosity development and simultaneous potassium intercalation during the chemical activation process.

Fig. 2 N2 isotherms of the raw parent sample PR0 and KOH-intercalated samples PR1_600,

PR2_600, PR3_600 and PR4_600.

Table 2 Texture properties of the carbon beads

Sample Sbet (m2/g) Vtotal (cm3/g) Average Pore Diameter (nm)

PR0 1128 0.45 1.59

PR1 600 972 0.39 1.60

PR1_700 943 0.38 1.61

PR1_800 1047 0.42 1.61

PR2_600 758 0.31 1.63

PR2_700 902 0.36 1.61

PR2_800 963 0.41 1.70

PR3 600 857 0.35 1.64

PR3 700 826 0.34 1.63

PR3 800 820 0.35 1.69

PR4_600 983 0.40 1.64

PR4_700 1071 0.44 1.65

PR4_800 1171 0.50 1.76

(m2/g)

Vmicro

(cm3/g)

1050 1150

0.43 0.37 0.36 0.40 0.29 .35 .38 0.34 0.32 0.32 0.38 0.42 0.47

3.1.3 Morphology

The morphology of the carbon beads was investigated by both SEM and TEM. Fig. 3 shows some of the images from SEM and more can be found in SI5. Figs. 3a, b and c confirm that the AC

samples have well-developed spherical forms with uniform diameters of 0.6-0.8 mm. More importantly, the macroscopic morphology of the samples shows little change after the incorporation of K; the desired spherical form has been preserved. Specifically, cracks and randomly distributed large holes are present on the outer surface. Figs. 3d, e and f show the cross-sectional images for the carbon beads where the presence of large number of pm-scale pores and some interior hairline channels are evident. TEM images in Fig. 4 and SI5 reveal the amorphous nano-structures with graphite layers being observed. The fact that no crystallized K species were found reflects their superior uniform distribution, which is of great importance to form highly polarized surfaces to enhan ce CO2 capture. This will be further discussed in the following sections.

ice CO2 c

Fig. 3 SEM images of (a) original PR0, (b) PR3_700 (KOH/AC mass ratio of 0.5:1), (c) PR4_700 (KOH/AC mass ratio of 1:1), and their corresponding cross-sectional images (d, e, f).

Fig. 4 High resolution TEM images of the activated carbon beads (sample PR3_700).

3.1.4 Element mapping

The distribution of the K species was obtained from SEM element mapping which is depicted in Fig. 5, where red colour represents the existence of K with the brightness indicating concentration. As can be seen from Fig. 5a, no K was detected for the initial AC, PR0, as expected. As shown in Figs. 5b, c, the K-intercalation treatment led to relatively even distributions of K with no obvious segregation, implying that the incorporated K is highly dispersed throughout the carbon beads.

For KOH activated carbons, it is generally recognised that all the residual K species can be removed by subsequent washing with acid (normally HCl solution), by which the porosity in the carbon samples can be recovered [56, 62-65]. In the present case, however, deionised water was used as a "soft" washing procedure to successively remove first the "free" and then the intercalated K. In other words, we are attempting to enhance the surface polarity of the carbon beads by potassium intercalation into the carbon frameworks as will be discussed later. Therefore, the distribution of potassium and oxygen on the surface in microscopic scale plays an important role in promoting CO2 adsorption capacities (Fig. 5). To further investigate the distributions of K and O in relation to C, TEM mapping was carried out, and the results (Fig. 6) suggest that the C, K and O elements are evenly distributed and appear to be interlinked to each other, which confirms that the surface modification is achieved successfully. The high distribution of potassium species can also be evidenced by the weak diffraction peak in the XRD pattern (SI6).

(a) (b)

'é^Êk - * *■ .. j

400nm 400nm

(C) (d)

1 ■jfo. • r ay -K ... ' . L1, ' • V • ft > • rW i>Vj 1 ■> ■ ® . V ■ « .J* • k I J

400nm 400nm

Fig.6 TEM images of PR3_700, showing the distribution of different elements: (a) original image, (b) carbon mapping, (c) potassium mapping, (d) oxygen mapping.

As mentioned above in 3.1.2, according to the chemical reactions between C and KOH during the chemical activation, metallic K and quasi-chemical bonds of C-O-K as new surface functionalities can be formed at over 700 oC and efficiently intercalated into the carbon framework structures. When washing with excessive water, most of free surface potassium species, such as K2CO3 and K2O formed during the activation at high temperatures, can be readily removed while intercalated potassium species or related functionalities (e.g. the -O-K chemical or quasi chemical bonds) remain in the carbon matrix structures. The element mapping shown in Fig. 6 illustrates a high consistency between the surface distributions of K and O, further suggesting that the intercalated K is most likely associated with the formed oxygen functionalities via the formation of quasi-chemical bonds, giving rise to highly polarized carbon surfaces.

5.2 CO2 a

3.2 CO2 adsorption on the carbon beads 3.2.1 Static adsorption measurement

As the most used method for adsorbent evaluation, the CO2 isotherms of the carbon beads were measured both at 0 and 25 oC by a Sieverts apparatus (volumetric method) to obtain the CO2 adsorption capacities. According to Myers and others [66, 67], absolute loadings are needed for thermodynamic processing such as isotherm fitting and adsorption enthalpy calculation, therefore the obtained excessive CO2 uptakes were converted to absolute uptakes by using Peng-Robinson equation of state (SI7) [68, 69], and some of the results are presented in Fig. 7 while the others are provided in the supporting information (SI8).

"5 E E

u 03 Q. (0 o

•a <

■ (a)0 °C

- ■ PRO Exp

-PRO Fit

* PR1_70Q Exp

-PR W00 Fit

♦ PR2_70QExp

-PR2 700 Fit

is* • PR3_700Exp

¡¡y -PR3 700 Fit

* PR4_700 Exp

PR4 700 Fit

l.l.l i * r i

0.0 0.2 0.4 0.6 Pressure (Bar)

Pressure

Fig. 7 CO2 adsorption isotherms of the carbon beads at 0 oC and 25 oC (symbols represent the measured values and lines are the fitted values with the dual-site Langmuir isotherm equation)

All samples display type I isotherms for CO2 adsorption with a sharp uptake at the early adsorption stage in the low relative pressure region, suggesting that these carbon beads are mainly microporous materials. At atmospheric pressure, sample PR0 adsorbs 4.45 and 2.80 mmol CO2 /g at 0 and 25 oC, respectively, being consistent with those reported elsewhere for phenolic resin derived carbons not modified by nitrogen treatments [42]. For better comparison, the CO2 adsorption capacities measured at different temperatures and partial pressures are summarised in Table 3 while a comparison with those reported in the literatures for KOH activated carbons is presented in Table 4. As expected, all of the KOH-activated carbon bead samples showed substantially higher adsorption capacities at both temperatures, especially at relative low pressures. It was found that at the same activation temperatures, the adsorption capacity of K-ACs ascended with increasing KOH/AC mass ratios used in the chemical activation. However, no obvious correlation was found between the adsorption capacity and the surface area or porosity of the carbon samples derived from different

conditions (Table 2), especially at lower CO2 partial pressures. At 25 oC and a CO2 partial pressure of 0.15 bar, the original steam-activated sample (PR0), which has the highest surface area of 1128 m2/g and micropore volume of 0.43 cm3/g, adsorbed approximately only half of the amount of CO2 adsorbed by the K-intercalated PR3_700 sample (1.51 mmol/g), which has a considerably lower surface area of 826 m2/g and micropore volume of 0.32 cm3/g, highlighting the importance of potassium intercalation for enhanced CO2 adsorption performance of the carbons. Similar phenomenon was also observed for the K-AC samples. For each series of samples obtained from using the same KOH/AC ratios in activation, the adsorption capacity of the activated samples at 800 oC was found to be considerably lower than that of the samples activated at 700 oC, despite the higher surface areas of the activated samples at the higher activation temperatures. For all the KOH/AC mass ratios examined, the K-AC samples activated at 700 oC exhibit the best performance for CO2 uptake.

Although carbon beads derived from a nitrogen-free phenolic resin was deliberately used in this study in order to eliminate the effects of any other heteroatoms in the precursors, a comparison of the K-ACs with the nitrogen-enriched carbons reported in the literature (Table 4) shows that the CO2 adsorption capacities of our K-AC samples are overall comparable to the nitrogen-enriched carbons reported in literature, despite their significantly lower surface areas (hence potentially higher capacities on a volumetric basis) and the absence of nitrogen functionalities (Tables 1, 2 and 4). More importantly, all the K-AC beads were measured as produced with desirable spherical diameters typically varying between 0.6 and 0.8 mm.

arying betwe bribed that al

As already been described that although various AC adsorbents have been reported with high CO2 adsorption capacities, most of the reported carbons were produced in powder forms via processes that often require sophisticated post-treatments, such as HNO3 oxidation and/or NH3 activation, leading to significantly increased production costs and reduced carbon yields. Further, carbons produced in fine powder forms cannot usually be used in moving-bed or fluidized bed reactors and will have to be further engineered via pelletisation or granulation to form different types of shapes with required particle sizes, and this can sharply reduce the adsorption capacity and kinetics of the carbon adsorbents [42].

Table 3 CO2 adsorption capacities of the carbon beads

Sample CO2 capacity at 0 oC (mmol/g)

0.15 bar 1 bar

CO2 capacity at 25 oC (mmol/g)

0.15 bar 1 bar

PR1_600 PR1_700 PR1_800 PR2_600 PR2_700 PR2_800 PR3_600 PR3_700 PR3_800 PR4_600 PR4_700 PR4 800

1.52 1.92 2.07 2.06 2.06 2.19 1.83 2.18 2.33 1.95 2.16 2.35 2.00

4.43 4.45

4.67 4.74 3.95

4.58 4.25 4.29

4.59 4.25

4.68 5.13 5.07

0.79 1.14 1.23 1.26 1.38 1.40 1.12 1.43 1.51 1.23

2.78 3.02 3.17 3.27 2.97

Table 4 Comparison on the adsorption capacities of PR3_700 and other carbon adsorbents

s of PR

.18 3.35 3.06 3.35 3.72 3.47

Materials Precursor ^ -2- Sbet (m /g) Uptake at 25 oC * Ref.

(mmol/g)

0.15bar 1 bar

PC-2 Agaricus 1479 0.88 3.46 [33]

KNC-A-K P-diaminobenzene 614 1.81 4.04 [36]

IBN9-NCI- P-diaminobenzene 890 1.62 4.50 [70]

A VR-93 Vacuum residue 2895 1.02 4.83 [32]

CEM-750 N-doping carbon 3360 0.98 4.38 [34]

RFL-500 Resorcinol, formaldehyde and 467 1.50 3.21 [46]

lysine

CSA-700 Poly (acrylonitrile-co- 1231 2.11 3.80 [37]

acrylamide)

PR3_700 Phenolic resin 826 1.51 3.35 This

* All results are measured by static volumetric method.

As indicated in Table 5, the K intercalation treatment as a means to improve adsorption performance only has a relatively small impact on the mechanical strength of the carbon beads. The addition of 0.1 and 0.3 KOH mass ratio to the carbon beads only reduces the mechanical strength by ca. 10 and 20%, respectively. This reduction is easily tolerated for fluidised bed operation given the high mechanical strength of the initial beads.

Table 5 Mechanical strength of K-ACs (activated at 700 oC)

Sample KOH/ AC mass ratio Maximum Load (N)

PR0 0 3.08

PR1 700 0.1:1 2.86

PR2 700 0.3:1 2.47

PR3 700 0.5:1 2.21

PR4 700 1:1 1.75

One should also bear in mind that while the adsorption capacity on a weight basis is important in terms of the scale-up of the adsorption-based CO2 capture process, the form and density of the activated carbons will also be critical, with the adsorption capacity on a volumetric basis instead ultimately determining the size and process efficiency of the adsorbent bed. Therefore, it is evident that the overall performance of the adsorbents will be a compromise between adsorbent materials having high surface area and micro-porosity while having sufficient density to maximize the volumetric adsorption capacity. To this end, the phenolic resin derived activated carbon beads from the present study are highly advantageous over the reported carbons, given their significantly lower porosity, higher bulk density (0.39 g/cm3) and much easier-to-handle spherical forms with practical diameters. Scaled-up production of these novel carbon bead adsorbents is underway in order to further evaluate their performance for CO2 capture on a volumetric basis using a kg-adsorbent scale fluidized bed reactor [71].

3.2.2 Heat of adsorption

Heat of adsorption is an important thermodynamic parameter for describing the adsorption behaviour of an adsorbent and evaluating the energy performance of a process of adsorption and desorption, such as the pressure swing and temperature swing gas separations. Isosteric heat of adsorption (Qst)

can be u sed to indicate the surface affinity of an adsorbent towards an adsorbate. It is usually estimated by using the Clausius-Clapeyron equation (1):

«- " F4rl. (1)

Where R represents specific gas constant; q stands for the adsorption capacity and T, P is the temperature and pressure, respectively. By fitting the obtained experimental isotherms at different temperatures with isotherms equations, one can acquire the pressure (P) needed to reach the same adsorption capacity (q) at different temperatures (T), and then Qst can be calculated by fitting the Clausius-Clapeyron equation.

In this work, the dual-site Langmuir (DSL) equation was used to fit the adsorption isotherms of all K-AC samples as the one-site Langmuir equation is no longer adoptable due to the surface

heterogeneity of the carbon surface as a result of potassium intercalation [72, 73]. Indeed, recent investigations have proposed that DSL equation is a better model to fit the adsorption isotherms of carbons with high surface heterogeneity [74, 75]. The DSL equation (2) can be expressed as follows:

qSiAXbAXP qs,BXbBXP

q = —--1—---(2)

^ 1+bAXP 1+bBXP

Where qS,A and qS,B are the mono-layer capacity of site A and B, bA and bB are the adsorption equilibrium constants for site A and B. During the fitting of isotherms, one must note [76]:

1) use absolute adsorption capacities;

2) qSA and qS,B should be kept constant at all temperatures to fulfil their physical meanings;

3) bX (X=A or B) usually decreases with increasing adsorption temperature and is related to a temperature-independent constant, b0, x by the Arrhenius equation (3).

b = b0exp(^) (3)

In Eqn. 3, E can be used to indicate the average adsorption heat. Since the fitted isosteric heat of adsorption was obtained as a function of adsorbed CO2 in which case Qst represents the average of all adsorption sites that can be potentially occupied at a certain coverage level [77, 78], the calculated heat of adsorption corresponds to the values at initial coverage (at a CO2 uptake level of 0.01 mmol/g). However, it is noteworthy that the calculated adsorption heat may differ from the isosteric heat (Qst) of adsorption, but the difference should be quite small [78].

After taking account of these criteria, the experimental isotherms were fitted and some of the fitting curves are shown in Fig. 7 while the others are provided in SI8 and SI9. Fig. 8a illustrates the calculated Qst for both the parent and KOH-intercalated carbon beads at a CO2 uptake level of 0.01 mmol/g. Generally, a considerable increase in isosteric adsorption heat was observed for all K-intercalated carbon bead samples comparing with the parent carbon beads (PR0). For instance, the calculated isosteric adsorption heat of K-intercalated PR1 and PR3 series carbons increased by up to 60% to ca. 36-42 kJ/mol CO2 from 26 kJ/mol CO2 for the parent carbon beads (PR0), respectively. This highlights the effectiveness of K intercalation in improving the surface affinity of the carbons for CO2 adsorption particularly at low CO2 partial pressures.

Fig. 8(a) Fitted isosteric adsorption heat at a CO2 uptake level of 0.01 mmol/g for all the carbon beads; (b) Fitted isosteric adsorption heat of PRO and PR3 series as a function of absolute CO2 loading using dual-site Langmuir model

Isosteric adsorption heat is also used to indicate the degree of surface heterogeneity of the adsorbent or different interactions between adsorbent and adsorbate. For energetically heterogeneous surfaces, the isosteric heat of adsorption will decrease with increasing quantities of adsorbed substances due to the occurrence of different types of active adsorption sites. Fig. 8b shows the isosteric adsorption heat both for the parent (PRO) and K-intercalated carbons (PR3 series) at different CO2 uptake levels. The calculated isosteric adsorption heats are by and large close to the upper range of the reported adsorption heats for carbon-based adsorbents [44, 79, 80]. While higher isosteric adsorption heats were obtained for all K-ACs, the variation of isosteric heat of adsorption with the quantities of CO2 adsorbed is much higher for the K-ACs than for the initial carbon beads with the isosteric adsorption heat remaining relatively constant. This demonstrates that the porous surface of

the K-intercalated carbons becomes energetically much more heterogeneous, being indicative of the incorporation of new active adsorption sites (Fig. 6) that led to higher interaction energy of CO2 with the surface.

In order to validate the calculated isosteric heat of adsorption with the dual-site Langmuir model, TGA-DSC has been used to experimentally measure the adsorption heat of the carbon beads. Given that the measured value by TGA-DSC represents the averaged adsorption heat integrated from the DSC heat flow corresponding to the adsorption capacity of a carbon adsorbent, the following equation (4) [81] was used to derive the integral adsorption heat values from the is osteric heat of adsorption. The integral adsorption values correspond to the defined ranges of sorption-phase concentration from 0 to n, where n equals to the equilibrium CO2 adsorption capacity obtained by TGA-DSC in a calorimetric analysis.

Qint = ^i^Qstdn (4)

The calculated adsorption heat was then validated by the adsorption heat experimentally measured by TGA-DSC. As shown in Table 6, for the original steam-activated carbon bead sample, the measured and calculated adsorption heats for the original steam-activated carbon beads are almost identical while for the K-intercalated carbons, the calculated adsorption heat values appears to be slightly lower by 1~6 kJ/mol CO2, compared to the values measured by TGA-DSC. The good agreement obtained between the calculated and measured adsorption heat for most of the carbon bead samples confirms the suitability of the DSL modelling for evaluating the adsorption behaviour of surface-modified carbons.

Table 6 Comparison between analytical integral heat of adsorption by DSL model and

measured DSC heat of adsorption_

Sample _

Analytical integral heat of adsorption by DSL model (kJ/mol)_

Experimental measured heat of adsorption by TG-DSC (kJ/mol)_

PR0 26.0 26.7

PR1 600 34.1 33.9

PR1 700 36.9 39.7

PR1 800 30.6 33.7

PR2 600 35.6 38.2

PR2 700 31.4 35.1

PR2 800 29.6 33.0

PR3 600 33.3 38.8

PR3 700 33.3 35.5

PR3 800 32.4 34.2

PR4 600 33.2 34.9

PR4_700 31.6 34.6

PR4 800 25.3 31.3

3.3 The role of intercalated potassium in CO2 adsorption

The results in the previous sections demonstrate that K intercalation can significantly improve the CO2 adsorption performance of the phenolic resin derived carbon beads. It can be reasonably assumed that the increased CO2 adsorption capacity arose from either increased micro-porosity or modified surface chemistries or a combination of both. As Table 2 indicates, however, a considerable decrease rather than an increase in surface area and microporosity was observed for all the K intercalated carbon samples under the conditions used, despite the washing with excessive quantities of deionised water. This suggests that the improved CO2 adsorption of the K-ACs cannot be accounted for by the surface textual properties but by the modified surface chemistries. It has been well established that KOH-activated carbons are usually enriched in different surface oxygen functionalities [82-84], but the presence of the surface functionalities alone is not sufficient to explain the significant increase in CO2 capacities according to the results reported elsewhere [37, 85]. It is therefore considered that the intercalated K is responsible for the enhanced CO2 adsorption performance of the K-ACs. To provide further insight, an exhaustive Soxhlet extraction at 100 oC with deionised water as the solvent was used to remove the intercalated potassium in the K-AC samples to different levels. This was used to evaluate the variation of CO2 adsorption with different levels of potassium intercalation and hence to determine the minimal content of intercalated potassium that is required to achieve appreciable improvement in CO2 adsorption. One of the K-AC samples, the PR3_700 which exhibited the highest CO2 capacity at 25 oC and a CO2 partial pressure of 0.15 bar, was selected for the exhaustive Soxhlet extraction for different extraction times. The samples produced from different periods of extraction were then subjected to CO2 adsorption tests and other characterisations.

Table 7 summaries the textural properties of the extracted samples and their CO2 adsorption capacities measured at both 0 and 25 oC and a CO2 partial pressure of 0.15 bar. As expected, the amount of K present decreased with increasing extraction time and after 20 hours extraction, the potassium content was reduced from 9.25 % to only 0.57 % on a weight basis. As can be seen from Table 7, the BET surface area of the carbon sample increased considerably with increasing removal of the intercalated potassium from the carbons, particularly at the early stages of Soxhlet extraction. This confirms that the lower surface areas obtained for the K-AC samples (Table 2) was due to the potassium intercalation which led to partial or even complete occlusion of micropores.

Measurements of CO2 adsorption capacity before and after the Soxhlet extraction, presented in Table 7 and Fig. 9, demonstrate that the capacity shows no significant change with decreasing residual potassium content from ca. 9.2 to 2.8 wt. %, but a sharp decrease was observed when

potassium content was further reduced to below 1 wt. %. It appears that there is a critical concentration of potassium above which the excess residual potassium contributes little to CO2 adsorption capacity. It can be reasonably assumed that the hard-to extract residual quantities of K are deeply intercalated into the fine carbon structure in the form of extra-framework K+ cations. Previous investigations [36, 86] showed that stronger CO2 adsorption sites of zeolite, MOF adsorbents reside predominantly around unsaturated framework metal cations. Therefore, it is believed that the significantly improved CO2 adsorption as a result of the intercalation of K cations is due to the involvement of stronger electrostatic forces created by the exposed framework K+ ions on the surface, with the strength of the electrostatic interaction being determined by the surface density and nature of the extra-framework K+ cations, an adsorption mechanism similar to that observed for the MOF-based CO2 adsorbents [36, 86]. Indeed, strong electrostatic properties were observed in handing the K-AC samples which were found to be easily attached to the interior surface of the sample cell and showed appreciable repulsion behaviour between different K-AC carbon beads while similar phenomena were not observed for the parent carbon beads.

It is very likely that the strong electrostatic may lead to the formation of labile

carbonate-like complexes upon CO2 adsorption around the extra-framework cations [87]. As highlighted in the following cyclic adsorption/desorption tests in dry gas conditions, the enhanced CO2 adsorption of the K-ACs, does not require the participation of water or moisture in the adsorption process unlike the K2CO3-based adsorbents where the presence of significant quantities of moisture in the gas stream is essential to the CO2 adsorption [88].

Fig. 9 The influence of K concentration (determined by ICP) on the CO2 capacities for deionised water extracted samples from PR3_700.

Table 7 Characterization results for the extracted samples from PR3_700 (K concentrations determined by ICP)

Sample Sbet (m2 g"1) K wt. %

Sample

PR3_700_1.5h

PR3_700_2.5h

PR3_700_4.0h

PR3_700_20h

9.25 2.79 0.96 0.57 <0.01

3.4 Cyclic adsorption/desorption and stability testing of the K-AC adsorbents Fig. 10 presents the adsorption/desorption characteristics of one of the K/AC samples (PR3_700) obtained from TGA at a temperature of 25 oC and a CO2 partial pressure of 0.15 bar (N2 balance). As shown in Fig.10a, a final equilibrium CO2 adsorption capacity of 1.9 mmol/g is obtained with approximately 90% of the equilibrium capacity achieved in 5 minutes under the TGA test conditions. This highlights the high capacity and fast adsorption kinetics of the K-AC beads, although it takes a slightly longer time for the K-ACs to reach equilibrium adsorption compared to the parent ACs. Fig. 10b shows the cyclic adsorption/desorption testing results for the K-AC carbon bead sample. The cyclic life-time performance testing demonstrates the excellent reversible adsorption performance of the K-intercalated carbon beads, despite the slight decrease in CO2 capacity in the first 5 cycles, which appears to be attributable to the irreversible CO2 adsorption on non-intercalated potassium species (as shown by bright red spots in Fig. 5b) in the dry flue gas conditions.

Apart from overall stability in multi-cycle tests, the regeneration energy needs to be considered in order to evaluate the efficiency and cost-effectiveness of the K-AC samples as CO2 adsorbents. The required regeneration energy of a solid adsorbent is the sum of the sensible heat that is required to heat the adsorbent from the adsorption temperature to the regeneration temperature and the latent heat (heat of adsorption) that is required to overcome the bonding energy to remove CO2 from the adsorbent in the desorption process [89]. It has been reported [90] that amine-functionalised solid adsorbents such as PEI-silica generally require less regeneration energy (in a cyclic temperature swing adsorption/regeneration CO2 capture process) compared with the benchmark MEA scrubbing technology. The facts that the K-AC beads developed in this study have much lower heat of adsorption, and lower specific heat capacity (e.g. 36 kJ/mol, 1.1kJ/kg.K for PR3_700) than PEI/silica adsorbents (60-90 kJ/mol, 1.7kJ/kg.K [71, 91, 92].) and comparable CO2 capacities and temperature swing as well as good regenerability indicate K-AC adsorbents are potentially better alternatives for PEI-silica for PCC in coal-fired PF coal-fired power plants.

. 10 (a) The CO2 adsorption curve of PRO and PR3_700; (b) 50 cycles of the adsorption and orption for PR3_700; both under an atmosphere of 15% CO2 and 85% N2 at 25 oC.

0131013101310112310131010201310100

Conclusions

The use of potassium intercalation via relatively mild KOH activation has been investigated as a novel effective means to boost the adsorption performance of activated carbons for post-combustion carbon capture. A nitrogen-free phenolic resin derived spherical carbon material with desirable spherical diameters (0.6~0.8 mm) which is suitable for direct practical applications without further palletisation or granulation treatment has been used so that the enhanced CO2 adsorption achieved by potassium intercalation could be delineated from any other effects. The results demonstrated that the intercalation of K significantly increased the CO2 capacity of the AC beads by a factor of up to 2 at 0.15 bar CO2 partial pressure while the effects of potassium intercalation on t he mechanical strength and morphological features of the carbon beads were negligible at KOH/AC mass ratios of <0.3.

The results suggest that the enhanced CO2 adsorption of potassium-intercalated carbons, as highlighted by their modestly increased heat of adsorption, is closely related to the formation of carbon framework or extra-framework K+ cations, which can lead to the potential formation of zwitterion-like structures that can boost the electrostatic interaction of the carbon surface with CO2. In addition, there appears to be a low critical K concentration, which is not much more than ca. 1 wt. % in the case of the carbon beads examined, below which the beneficial effect of K intercalation on CO2 adsorption becomes negligible. The results also demonstrate that the dual-site Langmuir model can be used to effectively characterise the adsorption behaviour of the K-intercalated carbons where high surface heterogeneity exists.

The significantly increased CO2 adsorption capacity, modest adsorption heat, fast kinetics and good mechanical strength augurs extremely well that the K-AC adsorbents could provide a sound alternative to the most studied polyethyleneimine-based sorbents for improved energy penalty of post-combustion capture using low temperature solid adsorbent looping technology.

Acknowledgements

The authors wish to acknowledge the financial support from the UK EPSRC (Grant nos: EP/G063176/1 and EP/J020745/1) and the National Natural Science Foundation of China (Grant no: 51061130536, 51172251). The authors also wish to thank Dr Wenbin Zhang from the University of Nottingham for assisting in the measurements of adsorption heat.

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