ScienceDirect Assessing Impure CO2 Adsorption

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 51 (2014) 308 – 315

7th Trondheim CCS Conference, TCCS-7, June5-6 2013, Trondheim, Norway

Assessing Impure CO2 Adsorption Capacity on selected South African Coals: Comparative Study using low and high concentrated Simulated Flue Gases Major Mabuzaa,*, Kasturie Premlalla a

Tshwane University of Technology, Department of Chemical & Metallurgical Engineering, Private Bag X680, Pretoria, 0001, Republic of South Africa

Abstract

The effects of coal rank, coal composition, and pressure on the CO2 adsorption capacity of five South African coal samples were evaluated. Adsorption isotherms of the flue gases on coal were measured at 35 ºC and up to a maximum pressure of 88 bar using the volumetric method. The flue gases used were simulated industrial flue gases of a coal fired plant. Adsorption tests with flue gases were conducted to study the degree of coal preferential sorption of the individual components. It was observed that adsorption capacity of CO2 onto coal is considerably reduced by the addition of other gases. © 2013 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of SINTEF Energi AS. Selection and peer-review under responsibility of SINTEF Energi AS Keywords: Coal; flue gases; adsorption isotherms; adsorption capacity

1. Introduction In the past decade the continuous release of greenhouse gases, including CO2 and CH4, has become a more serious form of environmental pollution. Greenhouse gases make the global air temperature rise continuously. Estimates of economic growth and associated emissions from the usage of fossil fuels as a provider of primary

* Corresponding author. Tel.: +27(0)-76-146-4651; fax: +27(0)-86-645-2478. E-mail address:[email protected]

1876-6102 © 2013 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of SINTEF Energi AS doi:10.1016/j.egypro.2014.07.037

Major Mabuza and Kasturie Premlall / Energy Procedia 51 (2014) 308 – 315

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energy, suggest that the concentration of CO2 in the atmosphere will continue to grow during this century unless significant steps are taken to reduce the release of CO2 into the atmosphere. So there is an increasing interest in the utilization, management and optimization of deep coal as an economic resource for CO2 sequestration [1, 2]. Given the increasingly intensive global focus on climate change, there is a growing realisation that South Africa is going to need to adopt a lower-carbon intensive energy trajectory, if it hopes to avoid the financial and social penalties that now look inevitable, not only for the developed world, but also for advanced developing countries like South Africa [3]. South Africa is a Non-Annex I Party to the Kyoto Protocol, and is one of the largest developing country emitters. It recognizes the need to move towards a low-carbon society and, in December 2009, committed at Copenhagen to reduce GHG emissions by 34% by 2020 and 42% by 2025 on condition that it received the necessary finance, technology and support from the international community [4]. The South African Centre for Carbon Capture and Storage (SACCCS) was launched in 2009 under the funding of partnerships between government and private companies with the aim of performing a feasibility study of carbon capture and storage in South Africa (based on a desktop and technical assessment of South African geology) [5]. The centre recently launched a Carbon Dioxide Storage Atlas to identify potential storage sites and their respective storage capacities for South Africa [6]. Proposals for CO2 storage have included plans to sequester CO2 in deep, unmineable coal seams. Coal seam CO2 sequestration involves adsorption of CO2 to the coal surface in large quantities [7], reducing the risk of CO2 migration to the surface and meanwhile enhancing coalbed methane recovery (ECBM) [8]. Before embarking on an ambitious carbon capture and storage campaign in South Africa, it was important to ascertain its in-country potential. To that end, The Department of Minerals and Energy commissioned an investigation from the CSIR, the results of which were released during the year 2004 and indicated that such potential did exist [9]. According to a geological survey conducted by Viljoen et al. [10] a total estimated CO2 storage capacity in the South African coal fields is 1, 271.9 Mt, however, this is based on techno-economics and the future of mining these coalbeds must also be taken into consideration. In the context of the geological storage of CO2 a few projects consider the direct injection of flue gases from power plants or other flue gas emitting industries [11]. The sorption behaviour of flue gases into coal seams is not yet well understood, in particular, for South African coals. Therefore, the paper reveals how the addition of impurities in a CO2 stream influences the CO2 adsorption capacity of selected South African coals; hence the use of an industrial flue gases instead of pure CO2. The experiments were carried out at 35 ºC and at high pressures of up 88 bar under dry equilibrated conditions. The experiments included sorption of two different flue gases on coals, which contained 96.2 mole % CO2 and 12 mole % CO2, respectively. Of particular interest in the current study is comparativeness of CO2 adsorption capacity on coals from high and low concentrated simulated flue gases under insitu conditions which provides a predefined experimental base from which predictive assessment of CO2 sequestration could be conducted.

2. Experimental 2.1. Experimental Set-up A volumetric sorption apparatus was designed in order to perform experiments to evaluate and estimate the sorption capacities of the coal samples that were studied. A schematic diagram of High Pressure CO2 Volumetric Adsorption System (HPCVAS) is shown in Fig. 1. The setup consists of a reservoir cell, a sorption cell, a sample drying vessel and a digital control system for temperature and pressure control. The reservoir and sorption cells were made from stainless steel and have volumes of 467.2 cm3 and 64.89 cm3, respectively. The pressure in the reservoir and adsorption cells was controlled by a

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digital control system via accurate pressure sensors. Each cell can be controlled individually with an error less than 2.5 kPa. Temperature Sensor Pressure Sensor

Temperature 35.00 ºC Meter

Temperature Sensor

GC

CO2 N2 O2 NO2 SO2

Reactor Cell Reference Cell

PC

Isothermal Oven Gas Cylinder

Data Logging System Fig. 1. Schematic Diagram of the Adsorption Apparatus

An SRI instruments Gas Chromatograph (GC – Model 8610C) supplied by Chromspec was attached to the HPCVAS setup and it was used to measure the gas composition before and after the multicomponent adsorption tests. The data logging system was used to collect the temperature and pressure data every 0.3 seconds.

2.2. Sample description and preparation The five South African coals involved in this study originate from four coalfields which are located in the main Karoo basin; Witbank, Highveld, KwaZulu-Natal, and Ermelo. Coal B and Coal D coals originate from Witbank coalfield, Coal C is from Highveld coalfield, Coal E is from Ermelo coalfield, and Coal A is from KwaZulu-Natal Coalfield. Each sample was identified by location and was characterized by coal rank (vitrinite reflectance (vitr. ref.)), maceral composition (petrographic analysis), and chemical composition (proximate analysis). The proximate and petrographic properties of all the six coals are presented in Table 1. Particle grain size range of -5 mm + 4.75 mm was used in this research. Table 1. Proximate and Petrographic Analysis of the Coals Used Proximate Analysis (wt%, adb)

Petrographic Analysis (vol%, inc. mm)

Rank

Sample ID

Fixed C

Moisture

Vol. Matter

Ash

Vitrinite

Inertinite

Liptinite

Vitr. Ref.

Coal A

84.6

1.4

5.2

8.8

32

63.4

0

3.27

High Rank C

Coal B

56.3

2.0

23.3

18.4

45

42.4

2.2

0.89

Medium Rank C

Coal C

51.4

5.0

24.3

19.3

18.2

67.2

2.2

0.71

Medium Rank C

Coal D

53.9

3.4

32.3

10.3

45.6

39

4.2

0.75

Medium Rank C

Coal E

27.8

4.5

59.8

17.9

12.8

74.2

3.3

0.64

Medium Rank C

adb – air dried basis inc. mm – including mineral matter


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2.3. Experimental procedure Each run involved three sequential procedures: degassing the sample being studied; filling the reactor cell with the coal, placing the reactor cell under vacuum prior to gas injection; determining the void volume (Vvoid) of the reactor cell filled with a coal sample, and; running the adsorption tests. Since the adsorption capacity and other properties such as the surface area, pore size, density, and porosity for coals could be affected by the presence of moisture within the coal sample [12], although other explanations cannot be ruled out completely, residual moisture appears to play a dominate role in affecting the measured adsorption isotherms of CO2 on dried coals [13]. Each coal sample was dried before the adsorption measurements could take place. Each sample was subjected to vacuum at a pressure of -0.7 bar and at a temperature of 130 ºC for 2 hours so as to degas the sample. The void volume, Vvoid, in the equilibrium cell was determined by injecting a known quantity of helium. Since helium is not adsorbed, the void volume can be determined from measured values of the temperature, pressure and amount of helium injected into the cell as described by Sudibandriyo [14]. After the void volume determination, the helium was evacuated from the system and was replaced by an appropriate adsorbate (flue gas). The adsorbate was charged into the system, nine pressure steps were chosen for these adsorption tests and they were increased accordingly from atmospheric pressure to 88 bar. In the initial tests of up to 24 hrs, it was found that 90 min was sufficient for the adsorption of flue gases involved to reach equilibrium. The captured data of temperature and pressure was used determine the amount of gas adsorbed in millimoles per gram of coal tested.

2.3.1. Adsorption of multicomponent gases The mixed gas adsorption tests of CO2/O2/N2 (MG1) and CO2/O2/N2/SO2/NO2 (MG2) on dry South African coals at 35 ºC and pressures up to 88 bar were conducted at 96.2/1.5/2.3 and 12/5.5/82/0.38/0.12 mole % feed composition, respectively. An assumption was made that the composition of the flue gases is homogeneous in the cylinder throughout the experiments; hence the adsorption measurements were conducted at one feed composition for each test. Initially, the gas mixture was sampled from its respective cylinder using a 5 cm3 syringe and analyzed using the Gas Chromatograph (GC) so as to verify a good homogeneity of the gas composition in the cylinder. After equilibrium pressure was reached for each pressure step, the gas remaining in the sample cell was sampled via sampling valve connected between the sample cell and the GC. This was done so as to determine the unadsorbed amounts of individual gas components. 3. Results and Discussion 3.1. MG1 adsorption isotherms Fig. 2 presents the experimental data for MG1 adsorption on all five coals under investigation at nominal molar feed composition of 96.2% CO2. It is observed that in all five coals CO2 was the most adsorbed gas than the other two gases involved in the flue gas. This is mainly because CO2 concentration in MG1 is very high, meaning high partial pressure for adsorption. Again, CO2 has got a very high affinity for coal; with the CO2 partial pressure being high in the MG1 it made the affinity of CO2 on coal to increase as well as the pressure steps increased; hence O2 and N2 stood no chance of being adsorbed more than CO2 since they both have very low affinity for coal. Carbon dioxide molecules are small, have low activation energy, and can easily penetrate pores inaccessible to other gases. The CO2 molecule has a non-polar nature; hence it does not have any permanent dipole moment and has a linear structure [15]. This makes CO2 the most preferred gas to be adsorbed by coal from the MG1. On average the adsorption capacities of O2 and N2 are about 20 and 7 times, respectively, less than that of CO2.

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(a)

(c)

(b)

(d)

(e)

Fig. 2. Gibbs Adsorption of MG1 on: (a) Coal A, (b) Coal B, (c) Coal C, (d) Coal D, and (e) Coal E.

3.2. MG2 adsorption isotherms Fig. 3 presents the experimental data for MG2 adsorption on all five coals under investigation at nominal molar feed composition of 12% CO2. Despite the fact that CO2 had a small concentration (12 mole %), in all five coals CO2 was still the most adsorbed gas than the other four gases involved in the flue gas. Through the gases competition for adsorption site factor of CO2 having a high affinity for coal played its role; hence this behaviour is observed.


(a)

(c)

(b)

(d)

(e)

Fig. 3. Gibbs Adsorption of MG2 on: (a) Coal A, (b) Coal B, (c) Coal C, (d) Coal D, and (e) Coal E.

The adsorption of multicomponent gas mixtures on coals is typically a competitive adsorption with a strong interaction between the components. A high preferential adsorption of CO2 was observed for high ranked Coal A, while low rank coals (medium rank C coals) showed a lower preferential adsorption of CO2 over the whole pressure range. The results obtained in sections 3.1 and 3.2 align with the work of other authors [11, 16-20], they (authors) all found that CO2 is preferentially adsorbed on coal over other gases. However, most of the time authors conduct

313

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adsorption studies on flue gases that contain CO2, CH4, N2, and SO2. Moreover, on average the adsorption capacities of O2, N2, SO2, and NO2 are about 1.19, 1.24, 5 and 6 times, respectively, less than that of CO2. This major reduction in adsorption capacity ratios is mainly due to the addition of other two gases, making adsorption more competitive than in MG1 and also it is due to the low partial pressure of CO2 making it be adsorbed less by the coal.

3.3. Comparison of MG1 and MG2 excess sorption capacities on coals Looking and examining Fig. 4, the adsorption capacity of all coals is much higher for MG1 than for MG2. As mentioned earlier, this is due to the fact that the addition of extra components reduces the sorption capacity, mainly by decreasing the partial pressure of the high coal affinity component, CO2. As mentioned by Hildenbrand et al. [21] that in general, the sorption capacity increases with increasing pressure until a certain saturation pressure is reached, this also explains the low adsorption of CO2 in MG2 since its partial pressure would be very low at high pressures compared to MG1; hence a maximum of 0.225 mmol of CO2/g-coal was reported compared to the 1.62 mmol of CO2/g-coal. On average the amount of reduction of CO2 adsorption capacity from MG1 to MG2 is 81%.

Fig. 4. Comparison of CO2 adsorption capacities (in mmol of CO2/g-coal) for MG1 and MG2 gases.

From the reduction percentage figures mentioned above it is unmistakable that there is a huge drop in adsorption capacities due to the addition of other components in the CO2 stream. The aim is to make sure that CO2 is adsorbed and stored underground as much of it as possible in specific coals seams. Now if there is reduction in CO 2 adsorption capacity (as established in this research) it means purifying the CO2 stream before storage, even though this (purifying) might come with its costs. However, the costs will be worth halting the CO2 emissions from dispersing into the atmosphere.

4. Conclusions The addition of impurities (N2, O2, SO2, & NO2) in the CO2 stream significantly reduces the adsorption capacity of CO2 on coals. This phenomenon is due to the fact that the addition of extra components reduces the adsorption capacity, mainly by decreasing the partial pressure of CO2; hence increasing the competition for sorption sites. This further implies that CO2 streams need to be purified before storage in South African coal seams. All coals


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investigated showed a high sorption preferential for CO2 than other gases (N2, O2, SO2, & NO2) involved in the flue gas mixtures (MG1 and MG2), this is mainly because CO2 has got very high affinity for coal compared to the other gases. SO2 and NO2 hardly adsorbed to the coal.

Acknowledgements The authors acknowledge the National Research Foundation (NRF) — South Africa for the financial support provided to undertake this research successfully. The findings and conclusions expressed in this publication are solely those of the authors and do not necessarily reflect the views of the Foundation. Many thanks to Nicola Wagner, Associate Professor, School of Chemical and Metallurgical Engineering, University of the Witswaterand for conducting the petrographic analysis on the samples used for this research.

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