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Influences of SO2, NO, and CO2 Exposure on Pore Morphology of Various Rank Coals: Implications for Coal-Fired Flue Gas Sequestration in Deep Coal Seams Dengfeng Zhang,*,† Jin Zhang,† Peili Huo,† Qianqian Wang,† Haohao Wang,† Wenping Jiang,‡ Jun Tao,† and Li Zhu† †

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China Xi’an Research Institute of China Coal Technology & Engineering Group Corp., Xi’an 710077, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Carbon dioxide (CO2) sequestration in deep coal seams with enhanced coal-bed methane recovery is a promising way to store the main anthropogenic greenhouse gas, CO2, in geologic time. Recently, injection of CO2 mixed with coal-fired flue gas components, i.e., SO2 and NOx, into coal seams has gained attention because it offers great advantages in reducing the cost of CO2 capture, flue gas desulfuration, and denitration. As a preliminary investigation on the feasibility of coal-fired flue gas sequestration in deep coal seams, the influences of SO2, NO, and CO2 exposures on the pore morphology of various rank coals are addressed in this work. Considering the optimum coal reservoir conditions for flue gas sequestration, the interaction of CO2 with coals was studied at a temperature of 45 °C and a pressure of 12 MPa. The results show that both CO2 exposure and SO2 exposure lead to decreases in both the specific surface area and pore volume of micropores of various rank coals. The micropore morphology of both Hulunbuir coal and Shenmu coal after NO exposure exhibits degradation, while the opposite trend is found for Erdos coal and Yangquan coal. The average micropore size of all the coals after contact with CO2, NO, and SO2 decreases. The CO2, NO, and SO2 dependences of the meso- and macropore surface area and volume of coals are complex and strongly related to the coal rank. Fractal analyses show that the pore surfaces of coals after CO2, NO, and SO2 exposures become smooth, as indicated by the surface fractal dimension determined from the Neimark model, which is consistent with the increasing trend of the average meso- and macropore size. Generally, the influences of SO2, NO, and CO2 exposures on pore morphology of various rank coals may play an important role in the diffusion and adsorption performance of fluid within the target coal reservoir. Thus, comprehensive evaluation of the dependence of coal pore morphology on fluid exposure is needed for the practical coal-fired flue gas sequestration in deep coal seams.

1. INTRODUCTION As the main anthropogenic greenhouse gas, carbon dioxide (CO2) is responsible for a series of climate change issues, which are great threats to the ecological system and human society.1 Carbon dioxide capture and sequestration (CCS) has been widely acknowledged as an important option for mitigating worldwide CO2 emissions and meeting the target of limiting global warming to 1.5−2.0 °C over the 21st century.2 CCS first involves separation and capture of CO2 from massive emission sources, and the concentrated CO2 is then transported to the target sequestration site.3 Among the various options for CO2 sequestration, CO2 storage in deep, unminable coal seams with enhanced coal-bed methane recovery (CO2-ECBM) can not only store CO2 in geologic time but also contribute to the enhancement of methane (CH4) recovery. The International Energy Agency’s Greenhouse Gas R&D Programme (IEAGHG) estimated that the implementation of CO2ECBM technology had the potential to sequester approximately 488 billion metric tons of CO2 and recover 50 trillion cubic meters of CH4 worldwide.4 The dominant mechanism of CO2 sequestration in deep coal seams is due to the adsorption ability of the coal matrix, which is abundant in microporosity.5,6 Thus, many works were performed to study CH4 and CO2 adsorption/desorption © 2016 American Chemical Society

behaviors on coals under the optimum coal reservoir conditions, including the concomitant displacement behavior of in situ CH4 adsorbed on coals due to CO2 injection, applying experimental and simulation methods.7−11 The process of CO2 adsorption induces swelling of the coal matrix, which decreases the permeability of the target coal seams and further has a negative effect on fluid transport within the reservoir.12−16 It has also been reported that the main component of coal-fired flue gas, nitrogen (N2), leads to weaker coal matrix swelling compared to CO2 and CH4.17,18 Thus, injection of flue gas composed mainly of CO2 and N2 is demonstrated to have potential to mitigate the permeability reduction of coal seams and enhance CH4 production.18−20 In addition, the injection of flue gas instead of pure CO2 source also offers great advantages by reducing the cost of the CO2 capture process. Recently, injection of coal-fired flue gas including CO2, NOx, and SO2 into coal seams has gained attention,21 as it is particularly helpful to eliminate the capital cost of flue gas desulfuration and denitration. It is known that fluid adsorption on a target sorbent comprises physisorption and chemisorption. PhysReceived: January 28, 2016 Revised: June 5, 2016 Published: June 16, 2016 5911

DOI: 10.1021/acs.energyfuels.6b00220 Energy Fuels 2016, 30, 5911−5921

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Energy & Fuels

The proximate analysis, including ash, fixed carbon, volatile matter, the equilibrium moisture contents and Ro,max, was reported in our previous work.31 As shown in Table 1, HB coal, SM coal, and ED coal

isorption shows a positive correlation with the boiling point of the fluid.22 The boiling points of NO, SO2, and CH4 follow the order of SO2 (−10 °C) > NO (−151 °C) > CH4 (−161.4 °C).23 Thus, the strength of physisorption under the same equilibrium pressure also follows the order of SO2 > NO > CH4. In addition, it is acknowledged that CH4 adsorption on coals is mainly attributable to physisorption.24 However, chemisorption probably also exists between SO2/NO and coals.25 Overall, the adsorption strength of both SO2 and NO is superior to that of CH4 considering both physisorption and chemisorption, which further confirms the feasibility of coalbed CH4 recovery by injection of coal-fired flue gas including SO2, NO, and CO2. Although studies on coal-fired flue gas sequestration in deep coal seams have been initiated, fundamental knowledge about this technology is still limited, such as the influence of SO2 and NOx exposure on the pore morphology of various coals. Hitherto, there are many works focused on the dependence of coal pore morphology on CO2 exposure. A previous study has shown that the optimum depth window for CO2 stored in the coal seams is 800−1000 m,6 where CO2 always exists in the form of a supercritical fluid.26 Hence, complex interactions other than the adsorption phenomenon do exist between CO2 fluid and coal, which will further influence the pore morphology of coal. Gathitu et al.27 applied both adsorption and scanning electron microscopy (SEM) methods to determine the dependence of the pore structure of coals on CO2 at high pressure. They found that CO2 exposure caused an increase in micropore area and volume for dried lignite, as-received lignite, and as-received bituminous coal; this effect was weak for dried bituminous coal. They also reported that the increasing trend was observed for meso- and macropore area of dried and asreceived bituminous coals, while this trend was inverse for both dried and as-received lignite. Kutchko et al.28 concluded that long-time CO2 exposure at a high pressure of 15.30 MPa caused no significant change in the micro- and mesopore area of two highly volatile bituminous coals based on the adsorption and field-emission scanning electron microscopy (FE-SEM) characterizations. Zhang et al.29 found that the shape and pore volume distribution of meso- and macropores of four different rank coals, with the maximum vitrinite reflectance (Ro,max) ranging from 0.47% to 1.35%, were slightly influenced by supercritical CO2 exposure. In comparison with the state-ofthe-art studies of the dependence of coal pore structure on CO2 exposure, investigations on the effect of coal-fired flue gas components, i.e., SO2 and NOx, on pore morphology of coal are very limited. The coal pore morphology plays a dominant role in fluid diffusion and adsorption within the coal seams. Thus, in order to provide basic knowledge of the technology of flue gas sequestration in deep coal seams, the work reported herein was performed to address the influences of SO2 and NOx, as well as CO2, exposures on the pore morphology of various rank coals.

Table 1. Characteristics of Coal Samplesa sample ash (dried basis), wt% fixed carbon (dried basis), wt% volatile matter (dried basis), wt% equilibrium moisture, wt% Ro,max, % a

HB

SM

ED

YQ

13.26 51.35 35.40 18.18 0.77

10.20 58.34 31.47 9.73 0.88

4.31 64.73 30.96 11.93 0.93

19.42 71.68 8.92 5.31 2.62

The data are taken from our previous work.31

are highly volatile bituminous coals. In contrast, YQ coal is classified as anthracite. Prior to the exposures to SO2, NOx and CO2, each coal sample was first crushed as a whole and then sieved to generate particles in a size range of 125−150 μm. 2.2. Interactions of SO2, NOx, and CO2 with Coals. Considering the optimum depth for CO2 sequestration in deep coal seams as previously recommended,6 and the average annual surface temperature and geothermal gradient of our selected coal seams, the interactions of SO2, NOx and CO2 with coals were studied at temperature of 45 °C. The concentrations of both SO2 and NOx present in coal-fired flue gas are much lower than the concentration CO2; thus, the interactions of SO2 and NOx with coals were studied at lower pressures, ranging between 0.4 and 0.7 MPa. It is necessary to point out that NOx emitted from coal-fired air or oxyfuel combustion process is mainly composed of NO and NO2, and NO accounts for >90% of the total NOx emissions.32 Therefore, NO with high purity above 99.99% was chosen to study the exposure of coals to NOx. The interactions of SO2 and NO with coals were conducted in a stainless steel pressure cell which was resistant to fine corrosion by SO2 and NO. The pressure cell was placed in a natural convection air oven with temperature fluctuation of less than ±0.1 °C (UN450 universal oven, Memmert GmbH & Co. KG, Germany). As SO2 and NO can be easily oxidized when in contact with oxygen from the air, the pressure cell was flushed with He and completely evacuated to effectively drive out the air in the cell. The exposure of coal to CO2 at a pressure of 12 MPa was operated in a dynamic model SFE-500 extraction system (Thar Process, Inc., USA). Detailed descriptions of the apparatus and experimental operation procedure can be found in our previous work.31 For each exposure test, 100 ± 0.0005 g of dried coal particles was used, and the duration of the exposure was set to 12 h. 2.3. Micropore Characterization. Nitrogen molecules are commonly not considered to have access to the ultrafine micropores within the coals due to the activated diffusion effect and/or pore shrinkage at the extremely lower temperature of 77 K.5 However, CO2 adsorption at 273.15 or 298.15 K can generate accurate measurements of microporosity of coals due to its superior diffusion ability in the coal pore structure.33 Thus, an adsorption method using CO2 as the molecular probe at 273.15 or 298.15 K was always used to determine the micropore structure of the coals.33,34 Both the large quadrupole moment of CO2 molecules and coal matrix swelling due to CO2 adsorption may show some negative effects on the results of the analysis of the coal micropores when CO2 is used as the molecular probe. However, the maximum equilibrium pressure of CO 2 adsorption is only around 0.10 MPa. Thus, the swelling ratio is extremely low35,36 and has a negligible effect on the analysis of the micropore morphology of the coals. In this work, the ASAP 2020 accelerated surface area and porosimetry system, provided by Micromeritics Instruments, USA, was used for the micropore morphology analysis. The CO2 adsorption temperature was set as 273.15 K. The micropore surface area and volume were obtained by the Dubinin−Radushkevich (D-R) model.27 The average pore size was given by the Dubinin−Astakhov (D-A)

2. EXPERIMENTAL SECTION 2.1. Samples Collection and Preparation. The four coal samples used in this study were gathered from northwest and north China: Hulunbuir coal drilled from Hulunbuir League, Shenmu coal drilled from Yulin city, Erdos coal drilled from Erdos city, and Yangquan coal drilled from Yangquan city. These coal samples are abbreviated as HB coal, SM coal, ED coal, and YQ coal, respectively. Each coal sample was preserved in a sealed plastic container with inert helium to prevent undesired alterations of physicochemical properties due to atmospheric oxidation.30 5912

DOI: 10.1021/acs.energyfuels.6b00220 Energy Fuels 2016, 30, 5911−5921

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Figure 1. CO2 adsorption isotherms at 273.15 K of coal samples: (a) HB, (b) SM, (c) ED, and (d) YQ.

Table 2. Micropore Parameters of Coal Samples sample

state

specific surface area (m2·g−1)

pore volume (cm3·g−1)

average pore size (nm)

HB

raw state after CO2 exposure after NO exposure after SO2 exposure

179.57 176.94 154.32 145.59

0.0720 0.0709 0.0618 0.0583

1.52 1.50 1.49 1.48

SM

raw state after CO2 exposure after NO exposure after SO2 exposure

174.71 171.30 140.21 122.87

0.0700 0.0686 0.0562 0.0492

1.57 1.55 1.52 1.52

ED

raw state after CO2 exposure after NO exposure after SO2 exposure

154.62 153.28 162.92 141.11

0.0620 0.0614 0.0653 0.0565

1.55 1.55 1.39 1.53

YQ

raw state after CO2 exposure after NO exposure after SO2 exposure

208.00 192.24 215.07 173.43

0.0833 0.0770 0.0862 0.0695

1.58 1.58 1.47 1.53

desorption method. N2 adsorption/desorption isotherms at 77 K were collected at relative pressure (P/P0) ranging between 0.005 and 0.990. The specific surface area of each coal sample was calculated using the Brunauer−Emmet−Teller (BET) equation according to the adsorption branch of the isotherm in the P/P0 range of 0.05−0.30.37 The total pore volume was estimated using the Barrett−Joyner−Halenda

model. The pore size distribution (PSD) of the micropores was generated by the Non-Local Density Functional Theory (NLDFT) model.27 2.4. Mesopore and Macropore Characterizations. The mesoand macropore morphology of the coal samples was also determined with the ASAP 2020 system using a low-temperature N2 adsorption/ 5913

DOI: 10.1021/acs.energyfuels.6b00220 Energy Fuels 2016, 30, 5911−5921

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Energy & Fuels (BJH) model according to the desorption branch of the isotherm.22 The meso- and macropore PSD profile of each sample was also determined using the BJH model according to the desorption branch of the isotherm;22 the average pore size of each sample was generated by the total pore volume and the specific surface area. Prior to the micro-, meso-, and macropore analyses, all the coal samples were fully degassed under vacuum conditions to remove residual gas and moisture in the samples. In this work, the vacuum pressure was set at 1 × 10−3 Torr, which is sufficient to meet the requirement of sample degassing. Higher temperatures, especially under the extreme vacuum pressure, may change the physicochemical properties of coals; thus, a mild evacuation temperature of 105 °C was set in this work, which is close to the temperature range of 70−105 °C reported in previous publications.27,28,38 It is necessary to mention that the evacuation time is critical for pore morphology analysis, especially for the coal samples after CO2, NO, and SO2 exposures. Therefore, the dependence of the degassing degree on evacuation time was determined prior to the pore morphology analysis. As shown in Figure S1 in the Supporting Information, no remarkable difference is seen in the CO2 adsorption isotherms of coal samples after highpressure CO2 exposure between degassing times of 12 and 72 h. In addition, it can be seen from both Figures S2 and S3 in the Supporting Information that the weight of each coal sample after NO or SO2 exposure remains constant. This confirms that 12 h is sufficient for degassing operations for all the coal samples before and after CO2, NO, and SO2 exposures. The micropore is critical for adsorption; thus, the repeatability of the micropore analysis of one selected coal sample was tested in this work. As shown in Figure S4 in the Supporting Information, the repeatability of the CO2 adsorption isotherms generated for one coal sample is fairly good. Hitherto, although there are many approaches to classify pore size, this work adopts the standards of classification of the International Union of Pure and Applied Chemistry (IUPAC), i.e., micropore (≤2 nm), mesopore (2−50 nm), and macropore (≥50 nm).

micropore of various rank coals. The effect of CO2 on coal micropore mainly depends on two aspects. On the one hand, the coal matrix swelling induced by high-pressure CO2 occurs which will constrict the pore space and thus decrease the microporosity.27 On the other hand, the CO2 source used for the interaction with coals exists as a supercritical fluid, which possesses physical properties intermediate between gas and liquid and can be used as an effective solvent in the extraction process.41,42 With respect to the coal structure, low-molecularweight organic compounds ( 0.45−0.5; the 5919

DOI: 10.1021/acs.energyfuels.6b00220 Energy Fuels 2016, 30, 5911−5921

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Energy & Fuels consistent with the changing characteristics of coals after fluid exposure as listed in Table 4. The meso- and macropore PSD profiles of the coal samples are shown in Figure 6. It is necessary to mention that the peak corresponding to the pore width of 3.80 nm in each meso- and macropore PSD profile is always deemed as an artificial peak because of the tensile strength effect.5,64 Therefore, this peak is not taken into account for the coals before and after fluid exposure in this work. Both NO and SO2 exposures act different roles on the meso- and macropore PSD profile of coals, and this effect is related to the coal rank. For SM coal and ED coal, both NO exposure and SO2 exposure lead to a decrease in the pore diameter range of 2−3.80 nm but a slight increase in the pore diameter range of 3.80−100 nm. The pore diameters corresponding to 2−3.80 and 3.80−100 nm of HB coal and YQ coal increase after contact with SO2, and NO exposure plays an opposite role in the diameter ranges of 2− 3.80 and 3.80−100 nm. However, no clear trend is seen for the dependence of the meso- and macropore PSD profiles of the coals on CO2 exposure based on Figure 6. The ambiguous role of CO2, NO, and SO2 on the meso- and macropore size is related to the complexity of coal pore system and sample selection. In future work, we plan to increase the number of coal samples to obtain the statistically distinctive differences. 3.3. Implications for Coal-Fired Flue Gas Sequestration in Deep Coal Seams. The classical adsorption theories consider that the micropore plays a dominant role in the adsorption equilibrium, and the meso- and macropore system is of importance in the diffusion process.65 Previous study has confirmed that the adsorption process occurring within the micropore system is the main mechanism of gas storage in coal seams. Based on our study, CO2, NO, and SO2 exposures cause a decrease in the micropore of coals, except for the NO dependence of the micropore morphology of ED coal and YQ coal, which will bring disadvantages to the storage capacity of the target coal seams. For ED coal and YQ coal, it is interesting to notice that NO exposure slightly increases the micropore morphology including specific surface area and pore volume. In addition, our previous study focused on the effect of NO exposure on the chemical structure of coals has pointed out that the amine or amide is formed on various coals after contact with NO molecule. Amine or amide can strengthen the CO2 adsorption on the inorganic or carbon-based adsorption materials based on the electron donor−acceptor interaction mechanism.66−68 Thus, the alterations of both pore structure and surface chemistry property of ED coal and YQ coal due to NO exposure are of great potential to strengthen the CO2 storage capacity of coals, which is beneficial to the simultaneous sequestration of the coal-fired flue gas mainly consisting of CO2 and NO. Future study will be performed to verify the feasibility of this proposal. In general, CO2, NO, and SO2 exposures cause a decrease in roughness while an increase in the average pore size of the meso- and macropores, which is helpful to mitigate the fluid transport and diffusion performance within the complex coal pore system. For the practical flue gas sequestration in the deep coal seams, a comprehensive evaluation work is in need to balance the effect of fluid exposure on the micro-, meso-, and macropores of coals. In addition, it is acknowledged that in nature, coal seams will contain water. The water molecules may show some influences on the interaction process between CO2 or SO2 and coals. Thus, the influences of SO2, NO, and CO2 exposure on pore

morphology of moisture-equilibrated coals will be studied in future work.

4. CONCLUSIONS In this study, the influences of SO2, NO, and CO2 exposures on the pore morphology of various rank coals were investigated to provide preliminary knowledge for coal-fired flue gas sequestration in deep coal seams. The related conclusions are summarized as below. (1) CO2 exposure leads to a decrease in the specific surface area and pore volume of micropores of various rank coals, which is probably related to both supercritical CO2 extraction and coal matrix swelling effects. (2) SO2 exposure also degrades the microporosity of various rank coals. However, the dependence of coal micropore morphology on NO exposure is related to the coal rank. Specifically, the micropore morphologies of HB coal and SM coal after NO exposure exhibit degradation, while the inverse trend is found for both ED coal and YQ coal. (3) CO2, NO, and SO2 exposures have different effects on the meso- and macropore morphology of coals. Nevertheless, according to the pore surface fractal analysis, the pore surfaces of all the test coals after CO2, NO, and SO2 exposures become smooth in general, which is consistent with the increasing trend of the average meso- and macropore size of coals after fluid exposure. (4) Generally, the influences of SO 2 , NO, and CO 2 exposures on the pore morphology of various rank coals play dominant roles in the diffusion and adsorption performance of fluid within the coal seams. Thus, it is necessary to take full consideration of the fluid dependence of the pore morphology of coals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00220. Supplemental Figures S1−S4, showing CO2 adsorption isotherms of coal sample after high-pressure exposure corresponding to different degassing times, effect of degassing time on the weight of coal sample after NO exposure and after SO2 exposure, and repeatability test of adsorption isotherm at 273.15 K using CO2 as molecular probe (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86(871)65920242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is conducted with funding from the project supported by the National Natural Science Foundation of China (Grant No. 41302132), Analysis and Measurement Foundation of Kunming University of Science and Technology (Grant No. 20140826, 20150373), and Training Programmes of Innovation and Entrepreneurship for Undergraduates of Yunnan Province (Grant No. 201510674042). We also appreciate Wei Li, Ph.D., from Department of Earth Science 5920

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and Engineering, Taiyuan University of Technology, for his assistance in coal pore morphology analysis and discussion.

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DOI: 10.1021/acs.energyfuels.6b00220 Energy Fuels 2016, 30, 5911−5921