Analysis of Adsorption and Desorption of Ethylene on Hard Coals

Subscriber access provided by Queen Mary, University of London

Fossil Fuels

Analysis of Adsorption and Desorption of Ethylene on Hard Coals Agnieszka Dudzi#ska Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00308 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Author: Agnieszka Dudzińska a

Ph. D., Central Institute of Mining, Mines Ventilation Department, 40-166 Katowice Plac

Gwarków 1, tel. tel. (32)259-21-27, fax: (32)259-21-82, e-mail: [email protected]

1 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analysis of Adsorption and Desorption of Ethylene on Hard Coals Key words: coal, self-heating of coal, sorption, desorption, ethylene

Abstract Ethylene is an unsaturated hydrocarbon that is released into the atmosphere of a mine as a result of an increase in the temperature of coal caused by the self-heating process. The content of ethylene in mine air is one of the indicators for assessing the degree of the coal self-heating process. Sorption capacities of hard coals with respect to ethylene may be the reason for the decrease in its concentration in mine air, which may affect the accuracy of selfheating process assessment. This phenomenon may be particularly important in the case of coals with significant sorption capacities. This paper presents the results of ethylene sorption studies carried out on six samples of bituminous coals collected from currently operational coal seams in Polish mines. The coals under analysis are characterized by varying sorption capacity with regard to ethylene. The highest sorption capacities were reported for low rank, high-porosity coals with easily accessible pore structure, high specific surface area values and high oxygen content. The interactions between the electron-donor (and electron-acceptor) centres of coal surface and π electrons of double bonds in gas-sorbed molecules are significant in the process of ethylene sorption. Coals with low oxygen content and more nonpolar surface structure, low porosity, and compact internal structures are characterized by low ethylene sorption capacity. On the basis of desorption studies, ethylene is a gas which once adsorbed mostly desorbs from coals. The percentage of desorbed gas increases with increasing temperature and it is higher for coals with high sorption capacities. Some of the undesorbed ethylene


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

molecules remain in the coal structure. They are permanently bonded to the surface of coals, and their number depends on the type of coals and their sorption capacities.

1. Introduction

Hard coal mines are characterized by the process of coal oxidation which takes place due to the oxygen contained in the mine air. As a result of this reaction, heat is released, which results in the self-heating of coal. A consequence of the above may be the self-ignition of coal, and as a result spontaneous fire1-5. The development of coal self-heating is accompanied by the release of gases into the mine atmosphere, including unsaturated hydrocarbons: ethylene, propylene and acetylene6-10. The values of their concentrations in the mine air are used to determine the temperature of self-heating coal and to assess the degree of the development of the self-heating process11. Early recognition of coal self-heating allows action to be taken to inhibit its further spread, thus minimizing the risk of fire. Unsaturated hydrocarbons: ethylene, propylene and acetylene, once separated from the source of coal selfheating, migrate through the mine's gobs undergoing sorption in coal, which causes their concentration in the atmosphere to decrease and may cause the incorrect assessment of the coal self-heating process. The sorption capacities of coals in relation to propylene and acetylene is described in the papers12-14. The aim of the research presented in this paper is to assess sorption capacities of bituminous coals with respect to ethylene, the hydrocarbon most often used in the assessment of coal self-heating. It is important to determine the influence of coal’s physicochemical properties and their structure on the volume of the sorbed gas. Ethylene is the simplest unsaturated hydrocarbon which contains a double bond between two carbon atoms. One of the bonds is a permanent bond of type σ, which is formed by the overlap of three hybridized sp2 orbitals, while the second bond of type π is formed by 3 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the side overlap of unhybridized p orbitals. Bonding π involves two electron density areas located on both sides of the line connecting the nucleus of two carbon atoms. π is a weak bond, so it can easily break and the resulting system has the characteristics of a weak dipole compound15. Due to the chemical and energetic character of the surface of hard coals, there may be electrostatic interactions between dipole ethylene molecules and energy centres, both positive and negative, on the surface of coals. Analysis of literature confirms that bituminous coals are characterized by higher sorption capacities for ethylene than for ethane, which has a single bond between carbon atoms16. The kinetic diameter of an ethylene molecule is 0.39 nm and it displays similar characteristics to the kinetic diameter of methane and carbon monoxide particles (0.38 nm), and is wider than the kinetic diameter of carbon dioxide and acetylene (0.33 nm)17. The size of the ethylene molecule determines its penetration into the smallest pores of coals. The critical temperature of ethylene (282 K) is slightly lower than the critical temperature of carbon dioxide (304 K) and acetylene (308 K), yet it is significantly different from the critical temperature of nitrogen and methane (126 K and 191 K, respectively). The physical properties of ethylene under discussion influence its sorption in hard coals.

2. Experimental part Six samples of hard coals collected from active Polish coal mines located in the Upper Silesian Coal Basin in the southern part of Poland were selected for sorption and desorption studies. Samples for testing were collected in accordance with the applicable standard, PN-G04502: 2014-11 ‘Hard coal and lignite. Collection and preparation of samples for laboratory tests. Basic methods’. Samples were crushed in a jaw crusher, ground and sieved by means of Fritsch sieves, in order to obtain a coal grain size in the range of 0.5 to 0.7 mm. The prepared samples were stored in a nitrogen atmosphere until the sorption measurements began. 4 ACS Paragon Plus Environment

Page 4 of 30

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The chemical and technical characteristics as well as petrographic analysis of the tested samples were performed on the basis of Polish standards. The results of these analyses are summarized in Table 1. The porosity and the pore volume of the tested coal samples were determined using mercury porosimetry. The tests were performed using the Pascal 440CE Instruments with a pressure range from 0.1 - 400 MPa. The test results are shown in Table 2. For the samples tested, surface areas were also calculated according to the DubininRadushkevich model based on the carbon dioxide sorption isotherms determined at a temperature of 298 K and specific surface areas calculated according to the BET model, based on the nitrogen sorption isotherms determined at a temperature of 77.5 K. Isotherms of carbon dioxide and nitrogen sorption were determined by the volumetric method using the Micromeritics ASAP 2010. The results are presented in Table 2. Table 2 also contains the micropore volumes calculated according to the Dubinin- Radushkevich model based on carbon dioxide sorption isotherms. Sorption-desorption measurements were carried out on crushed coals with a grain size of 0.5-0.7 mm using the volumetric method with a pressure range from 0 - 0.1 MPa using the Micromeritics ASAP 2010. Before the measurements were taken, coal samples were subjected to vacuum degassing and rinsed several times with helium in order to remove impurities from the surface of coals18. The sorption isotherms were measured at temperatures of 298, 323, 343 and 373 K, close to the actual temperature conditions prevailing in coal mines. The ethylene desorption process was carried out at the same temperatures, gradually reducing the gas pressure over the coals’ surface with the desorbed gas layer.

3. Results and discussion

3.1.

Research on the sorption of ethylene on hard coals 5 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

The obtained ethylene sorption isotherms determined for the studied C-1 - C-6 coals at the temperature of 298 K are presented in Fig. 1. The time taken for the establishment of sorption equilibrium at individual measurement points was long (several hours at each measurement point), as the process of ethylene sorption on coals is slow. The isotherms are characterized by their regular shape. The volume of sorbed ethylene increases significantly in the initial range of pressure increase. For pressure up to 0.05 MPa, about 75% of the total adsorbed volume of ethylene is adsorbed. The Langmuir isotherm equation19 was used to describe the equilibrium of ethylene sorption on hard coals.

a = am

bP 1 + bP

(1)

where: a – amount adsorbed, cm3 STP/g, am – maximum adsorption capacity, cm3 STP/g, b – Langmuir constant, P – equilibrium pressure, MPa. The coefficients of equation am and b are presented in Table 3. To assess the quality of the equations of adsorption isotherms’ adjustment to the experimental data, the value of the mean relative error was used. This is defined as:

D=

100 k

k

∑ i =1

aiexp − aisym aiexp

(2)

where, aiexp – experimental data, aisym – calculation data, k – number of data points


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The D values, also summarized in Table 3, are in the range of 3.5 - 6.5%. The coal with the worst fit to the theoretical isotherm was C-1 coal, for which experimental points in the pressure range 0.02 - 0.08 MPa are below the theoretical isotherm, and within the pressure range of 0.08 - 0.1 MPa - slightly above this isotherm. For the remaining coal types, better compliance between experimental and theoretical data was obtained. The volumes of sorbed ethylene are clearly dependent on the properties of the given coal. Ethylene is sorbed by C-1 coal to the greatest degree, as the volume of sorbed ethylene at the temperature of 298 K is about 9 cm3/g at a pressure of 0.1 MPa. High sorption capacity of this coal are the result of its specific structure. It is low rank coal (vitrinite reflectance is 0.51%, and the content of the carbon element is 77.69%), characterized by high porosity (13.55%). Surface values were determined according to the BET model and DubininRadushkevich model and the pore volume of this coal is the largest of all the coals tested (Table 2). Ethylene sorption, in this case, occurs both in the pore surface systems and in the microporous internal coal structure. C-1 coal is characterized by its low degree of structure ordering, small clusters of aromatic compounds and a large amount of surface reactive oxygen groups, which account for 2/3 of the total oxygen content, constituting 15.18%. In the case of coal with such a total oxygen content, these are mainly hydroxyl groups, about 8%, in smaller amounts carbonyl groups - about 2% and carboxylic groups, about 0.7% 20-22. The above are responsible for the electrostatic interactions between the electron-donor and electron-acceptor centres present on the surface of coals with electrons π of double bond ethylene molecules12, 23

.

Slightly less ethylene, when compared to C-1 coal, is sorbed by coals C-2 and C-3, for which the volumes of sorbed ethylene at the temperature of 298 K are about 6 cm3/g, at a pressure of 0.1 MPa. In comparison to C-1 coal, these are higher rank coals, their vitrinite reflectance is 0.68 - 0.71%, and carbon content 82.17 - 83.20%. They are characterized by lower porosity,


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

within the range of 4.05 - 6.32%, and lower surface values according to BET, which is the reason for the limited availability in their pore system for particles of the penetrating adsorbate. The surface area according to Dubinin-Radushkevich is lower than for C-1 coal. At the same time, lower oxygen content which is within 8.84 - 10.45%, (including a much lower content of reactive oxygen groups of about 2 - 3%, of which about 1% is of the hydroxyl group, 0.2% is of the carboxyl group and 0.5% is of the carbonyl group21, indicates a more apolar character of the coal surface. This is the cause of weaker interactions between the oxygen reactive groups present on the surface of these coals and the sorbed gas. The next band of ethylene sorption isotherms in Fig. 1 form coals with even lower sorption capacities, i.e. C-4 and C-5 coals, for which the volume of sorbed ethylene at the temperature of 298 K is about 3 - 4 cm3/g. The vitrinite reflectance of these coals is 0.70 0.89%, and carbon content amounts to 84.07 - 84.39%. Despite the fact that vitrinite reflectance of C-4 coal is comparable to the vitrinite reflectance of C-2 and C-3 coals, its sorption capacity is significantly lower. The main importance in this case will be attributed to the compact and poorly accessible structure of these coals. Their porosity is within 1.86 3.47%, and the specific surface area, determined using the BET model, is small (0.48 – 0.75m2/g) in comparison with the volume of meso- and macropores (0.014 - 0.028 cm3/g). The transport pore system of these coals is less developed. Ethylene sorption occurs mainly in the surface pores of the coal, and the compact structure of these coals causes the blockage of transport pores reducing the flow of sorbed gases to the micropore system. At this point, it is worth emphasizing the great importance of the porosity and accessibility of coal pores in the process of ethylene sorption. The lowest sorption capacity for ethylene is in C-6 coal, for which the volume of sorbed ethylene is about 1.6 cm3/g, at a pressure of 0.1 MPa. This is coal which has easily accessible pores (higher porosity, higher surface values) than C-5 coal, yet it sorbs less


Page 8 of 30

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

ethylene than C-5 coal. The large degree of structure ordering of this coal is the cause of its low sorption capacities. This coal is also characterized by having the smallest oxygen content (3.58%) of the tested samples, which indicates a more apolar character of its surface with a small amount of oxygen reactive groups (less than 0.5%, mainly carboxylic and carbonyl) being responsible for interactions with the sorbed ethylene molecules. The remaining part of the total oxygen content is unreactive oxygen20. Through the analysis of the collected data, it can be concluded that the volumes of sorbed ethylene depend on the physicochemical properties of coals and their structure. Coals with higher porosity (over 3.5%), with pore volume determined by mercury porosimetry being above 0.028 cm3/g and high specific surface area according to the BET model (over 1.5 m2/g) and a complex micropore system, definitely sorb greater ethylene volumes. The structures of C-1, C-2 and C-3 coals are easily accessible for molecules that penetrate the adsorbate. The remaining, C-4, C-5 and C-6 coals adsorb lower ethylene volumes. In the sorption process, in addition to the micropore system, the transport pore system of meso- and macropores is also important as it allows the transport of penetrating gas molecules into the micropore system24. It is worth focusing on C-4 coal, as large volume values (0.064 cm3/g) and micropore surfaces (160.51 m2/g), determined according to the Dubinin- Radushkevich model, testify to the complex main adsorptive system and suggest large sorption capacities of this coal. However, the transport pore system does not allow the free penetration of sorbed ethylene particles into the micropores, which results in the lower than anticipated sorption capacities of this coal. This is evidenced by its very low specific surface area compared to the surface values of C-2 and C-3 coals, according to BET, amounting to 0.75 m2/g. The sorption capacity of hard coal is related to coal rank25-26. As a measure of the coal rank, the ability to reflect light from the surface of vitrinite was adopted, i.e. vitrinite reflectance due to the fact that vitrinite shows the most regular change of its properties with a change in


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the rank coal. The reflectance of the vitrinite in the studied coals is from 0.51 - 0.93% and with its increase the volume of sorbed ethylene clearly decreases. Coals with a higher coal rank, greater structure ordering and fewer cases of low-molecular bonding, whose vitrinite reflectance is within 0.70 - 0.93% (C-4, C-5, C-6), are characterised by significantly lower sorption capacity. Coals with low reflectance of vitrinite adsorb the largest amounts of ethylene. The influence of the moisture content in coal on the volume of sorbed ethylene should be highlighted. Moisture contained in coal (analytical moisture) is a consequence of the hydrophilicity of its surface27-28. C-5 and C-6 coals with low humidity, from 1.19 - 1.76% and the same low polarity of surface, adsorb small volumes of ethylene. Coals with higher sorption capacity have a moisture content above 3%. This confirms the effect of the polarity of the coal surface on their sorption capacities with regard to ethylene. The presence of oxygen in coal, which is found in oxygen reactive groups (carboxyl, hydroxyl and carbonyl) is also significant in the process of ethylene sorption on hard coals. A larger number of these groups affect the possibility of interaction between them and the sorbed ethylene molecules of dipole nature, which results in an increased volume of the sorbed gas. The largest amount of reactive oxygen groups is found in coals with a low degree of metamorphism and with a total oxygen content above 13% - C-1 coal (15.18%)28-29. This oxygen content mainly consists of hydroxyl groups (making up 9%), and to a lesser extent carboxyl (up to 1.5%) and carbonyl (up to 3%) groups. For the remaining coals, with total oxygen content between 8.84 - 10.45%, the amount of reactive oxygen groups is much smaller and amounts to 3% in total20-22. In the case of C-6 coal, with total oxygen content of 3.58%, the number of reactive oxygen groups is negligible. When analysing the influence of coal properties on their sorption capacities, the petrographic composition of coals should also be mentioned. In the scientific literature there 10 ACS Paragon Plus Environment

Page 10 of 30

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

is information about the significant impact of maceral groups on sorption, mainly the impact of methane and carbon dioxide30. For the studied coals, no clear relationship was found between their petrographic composition and the volume of sorbed ethylene. To illustrate this fact, C-2 and C-3 coals with a similar rank, similar structure (similar surface values and volume of micropores), and similar sorption capacity towards ethylene are characterized by different maceral composition. Moreover, C-2 and C-6 coals with significantly different sorption capacity are characterized by a similar maceral composition. Additionally, it was found that there ash content in coal has no significant effect on the volume of sorbed ethylene. On the basis of studies of methane sorption in hard coals, it was shown that an increase in the ash content in coal causes a decrease in their sorption capacities31-33. The sorbed gas is mainly retained in the organic matter, which is characterized by a high content of micropores, whereas in the structure of the mineral substance there are usually meso- and macropores present34. C-1 coal with its exceptionally high ash content (14.45%) sorbs the largest volumes of ethylene from the tested types of coal, while C-6 coal which sorbs the smallest volume of ethylene is not characterized by increased ash content. The decrease in coal sorptivity in relation to ethylene does not correlate with an increase in ash content in coal. The influence of ash content on the sorption capacities of the studied coals in relation to ethylene is not conclusive. To summarize, it was found that the investigated hard coals sorb various ethylene volumes. These volumes depend on the availability of the pore structure and coal rank. The largest volumes of gas are sorbed by low rank coals which have increased porosity, easily accessible internal structure of their pores and a large number of oxygen reactive groups present on their surface.

3.2.

The influence of temperature on the volume of sorbed ethylene


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In order to analyse the influence of temperature on the process of the sorption of ethylene in the studied coals, sorption measurements at higher temperatures were carried out. The proposed temperatures of 323, 343 and 373 K correspond to the temperature conditions prevailing in situ. The obtained values of sorption, read at the pressure of 0.1 MPa are summarized in Table 4. Through analysing the obtained data it can be seen that the sorption capacities of coal with respect to ethylene decrease with the increase in the sorption temperature. The decrease in the volume of the sorbed gas depends on the properties of the coals and their sorption capacity. In the case of C-1 coal, the volume of the sorbed gas decreases by approximately 72% across the studied temperature range, 298 - 373 K. For coals with considerable sorption capacities such as C-2 and C-3, it has been observed that the volumes of sorbed ethylene decrease, as temperature increases from 298 to 373 K, by 57 62%, i.e. by a lower value than in the case of C-1 coal. In coals with average sorption capacities, such as C-4 and C-5, smaller differences, than in the case of coals with considerable sorption capacities, can be observed between the volume of sorbed ethylene at the temperatures of 298 K and 373 K. The volume of sorbed ethylene decreases in the entire temperature range by about 45 - 51%. In the case of the least-sorbing coal C-6, the temperature influence on the sorption capacities for ethylene is very small. The volume of sorbed ethylene decreases by 30% across the studied temperature range. The results of studies on the influence of temperature on the sorption capacities of coal in relation to ethylene, allow the conclusion to be made that with the increase in temperature, in the range of 298 - 373 K, the volume of sorbed ethylene decreases. The influence of temperature depends, however, on the properties of coals. Sorption capacities of coals characterized by considerable sorption capacities decrease to the greatest extent.

3.3.

Studies on desorption of ethylene from hard coals


Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The desorption of ethylene from coals is a process during which the molecules of adsorbed gas are released from the adsorbent surface to the gas phase. Ethylene desorption tests were carried out to determine the degree of desorption of the test gas from coals. The ethylene desorption process was carried out at temperatures of 298 K, 323, 343 and 373 K, gradually reducing the gas pressure over the coal surface with the adsorbed gas layer. The time taken to establish the desorption equilibrium ranged from 30 minutes to several hours at individual measurement points. As a result of the experiment, desorption isotherms were obtained. Systems of sorption - desorption isotherms for the tested samples of hard coals at the temperature of 298 K are shown in Fig. 2. The course of the ethylene desorption isotherms is dependent on the properties of coals and their sorption capacity. In each case, ethylene desorption isotherms do not combine with sorption isotherms to form open hysteresis loops. Better sorbing coals: C-1, C-2 and C-3 are characterized by narrow hysteresis loops. Desorption isotherms run slightly above the sorption isotherms (Figure 2 a-c). For coals with lower sorption capacity, C-4 and C-5, wider hysteresis loops are characteristic and desorption isotherms run significantly above the sorption isotherms and assume a more flat shape, especially in the initial section of their course (Figure 2 d-e). The volumes of desorbed gas are smaller. In the case of coal with the lowest sorption capacity, C-6, the desorption isotherm also forms a wide hysteresis loop, and the initial isotherm section, in the pressure range 0.04 - 0.1 MPa, is parallel to the axis of abscissas (Figure 2f). Open hysteresis loops suggest a high proportion of micropores in the porous coal structure. The sorption process is an irreversible process and in each coal sample there is a certain amount of non-desorbed ethylene. The ethylene molecules which remain in the coal structure are bonded to the surface of coals through the donor-acceptor centres present in it23.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The obtained hydrocarbon desorption isotherms when juxtaposed with sorption isotherms allow us to estimate the volume of the desorbed gas, as well as to determine the volume of gas remaining in the coal structure. Table 5 summarizes the adsorbed ethylene volumes for all coal types tested in four temperature ranges, as well as the percentage of the gas that has been desorbed. The data in Table 5 shows that ethylene is a gas which after being adsorbed is largely desorbed. In the case of coals with high sorption capacities (coals C-1, C-2 and C-3) over 75% of the adsorbed ethylene undergoes desorption. For medium and less sorbent coals (C-4, C-5 and C-6) over 66% is desorbed. As the temperature rises, the percentage of desorbed gas increases, for highly-sorbing coals up to 92% at the temperature of 373 K, and for less sorbing coals up to 88%. It can therefore be assumed that ethylene relatively easily desorbs from coals. The percentage volumes of ethylene desorbed from the coal structure depend on the sorption capacity of coals. Coals with a higher sorption capacity for ethylene are characterized by a higher percentage of desorbed gas. Similar observations have been made for the desorption of methane from coal. It was found that the higher the rank of the coal, the smaller the desorption of methane was35. Desorption is a process which can be observed in mines during the development of the coal self-heating process, when gases and the hydrocarbons among them are released from the structure of coals, as this process develops, and pass into the mine atmosphere. The amount of gases released under real conditions depends on many factors, i.e. the properties of coal, its fragmentation, oxygen supply, coal temperature, the degree of self-heating development and others. Under the conditions of the experiment, it was observed that ethylene is largely desorbed from coals. In addition, the degree of ethylene desorption depends on the properties of coals and their sorption capacities. The content of ethylene in the mine atmosphere is the most frequently used indicator of temperature increase in the coal self-heating process with


Page 14 of 30

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

regard to hydrocarbons. The increase in this gas concentration is observed at the early stage of self-heating development8, 36. The use of ethylene to assess the degree of development of the self-heating process is confirmed by data presented in the literature8, 37.

4. The use of ethylene for the assessment of coal self-heating A series of fire indicators is used to determine the temperature of self-heating coal and to assess the degree of the development of the self-heating process. Some of these indicators are based on the content of hydrocarbons in the mine air, including ethylene and acetylene, e.g. the C2H4/C2H2 indicator used in Ukrainian mines, the C2H4/H2 - Noack indicator38 or indicators: (C2H4+ C2H2)/H2, (C2H4+ C2H2)/C2H2 used in the Polish mining industry39. Due to the significant sorption capacity of coals in relation to hydrocarbons, including ethylene and acetylene, it can be assumed that these gases, while being separated from the self-heating source and migrating through the gobs, may undergo sorption in coal, which may result in their lower concentration being recorded at measurement points, which in effect will cause erroneous values of the above-mentioned indicators. Thus, the estimated temperature of the self-heating coal will not correspond to the actual value occurring at the place of danger. The volume of the adsorbed gas is influenced by the properties of the coals and the distance of the self-heating source from the gas concentration measurement point. At greater distances, there is a higher probability of the sorption of hydrocarbons on coal and, consequently, the reduction of their content in the mine atmosphere. Among the fire indicators based on the content of hydrocarbons, the indicator C2H4/C2H2, calculated as the ratio of ethylene to acetylene concentration measured in mine air, is often used. The decrease in the concentration of ethylene and acetylene caused by the sorption phenomenon in the mine's measurement stations will cause a deviation of this indicator value compared to the standard index value determined during the heat treatment of 15 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a coal sample in laboratory conditions where the sorption process does not occur. The degree of the deviation of the index value from the actual value will depend on the difference in the extent of the ethylene and acetylene sorption. Most often, the volume of sorbed acetylene is about two to three times higher than the volume of sorbed ethylene40, which causes the overestimation of coal heating temperature. In real mining conditions, however, it is difficult to locate unequivocal self-heating sources, it is also difficult to determine the volume of sorbed ethylene due to the variable temperature, coal weight and many other factors, which further hinder the control of this process. Therefore, assessment of self-heating process development based only on ethylene concentrations may be insufficient. This is especially true in cases of coals with high sorption capacities. Self-heating process assessment should be supplemented with indicators based on other gases emitted during the development of self-heating, such as carbon monoxide and hydrogen, e.g. CO/H2 indicator. Sorption capacity of carbon monoxide and hydrogen is very low40-42, so the sorption process does not lower the value of their concentrations in relation to the real values emitted from the self-heating source. The results obtained from sorption measurements can be used to improve the assessment methods of the coal self-heating process.

5. Conclusions 1. The sorption capacities of hard coals in relation to ethylene are varied. The largest volumes of ethylene are sorbed by low rank coals which have high porosity, large surface values and pore volume values, and significant oxygen content. With the increase in the rank of coals, there is a decrease in porosity and pore volume and the sorption capacities of coals in relation to ethylene decreases. Coals with low accessibility of pores sorb low volumes of ethylene. 16 ACS Paragon Plus Environment

Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2. With the temperature increase from 298 - 373 K, the volume of sorbed ethylene decreases. The decrease in the sorption capacities of coals, relative to ethylene, with temperature is highest for coals with high sorption capacity and amounts to 72%. The influence of temperature on the volume of sorbed ethylene on low-sorbing coals is small, the decrease in sorption capacity for C-6 coal is 30%. 3. The process of the sorption of ethylene on hard coals is irreversible, plotted desorption isotherms run above the adsorption isotherms forming unclosed hysteresis loops. The shape of desorption isotherms depends on the rank of the coal and its sorption capacities. 4. The majority of ethylene adsorbed on coal undergoes desorption. The percentage of desorbed gas increases with increasing temperature and it is dependent on the physicochemical properties of coals and their sorption capacity. For high-sorbing coals, it is above 75% and for lower sorbing coals above 66%. 5. The process of the sorption of ethylene on hard coals leads to a decreases in its content in the mine atmosphere. One consequence of this is deviations of the fire indicators from the actual values, determined on the basis of concentrations of ethylene in the mine air, which in turn affects the adequacy of the assessment of the coal self-heating development. This problem is particularly important in the case of low-carbon coals, which in comparison to highly coalified coals have higher sorption capacity for ethylene and a larger percentage of desorbed gas. If these coals experience temperature increase, greater ethylene desorption will occur.

Literature 1. Kim, A.G. Int. J. Coal Geol. 2004, 59, 49-62.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2. Zhou, F. B.; Ren, W.X.; Wang, D.M.; Song, T.L.; Li, X.; Zhang, Y.L. Int. J. Coal Geol. 2006, 67, 95-100. 3. Pone, J.D.N.; Hein, K.A.A.; Stracher, G.B.; Annegarn, H. J.; Finkleman, R.B.; Blake, D.R.; McCormack, J.K; Achroeder, P. Int. J. Coal Geol. 2007, 72, 124140. 4. Kuenzer, C.; Stracher, G.B. Geomorphology 2012, 138, 209-222. 5. Lin, Q.; Wang, S.; Liang, Y.; Song, S.; Ren, T. Fuel Process. Technol. 2017, 159, 38-47. 6. Singh, R.N. J. Coal Qual. 1986, 5, 108-113. 7. Dai, G. L. Coal Mine Safety 2007, 1, 1-4. 8. Adamus, A.; Šancer, J.; Guřanová, P.; Zubicek, P. Fuel Process. Technol. 2011, 92, 663-670. 9. Xie, J.; Xue, S.; Cheng, W.; Wang, G. Int. J. Coal Geol. 2011, 85, 123-127. 10. Lu, W.; Cao, Y.J.; Tien, J.C. Int. J. Min. Sci. Technol. 2017, 27, 839-846. 11. Yuan, L.; Smith, A.C. J. Loss Prev. Process Ind. 2012, 25, 131-137. 12. Dudzińska, A. Int. J. Coal Geol. 2014, 128-129, 24-31. 13. Dudzińska, A.; Howaniec, N.; Smoliński, A. Energy & Fuels 2015, 29, 8, 48504854. 14. Dudzińska, A.; Howaniec, N.; Smoliński, A. Energies 2017, 10, 1919, doi:10.3390/en10111919 15. McMurry, J., Organic Chemistry, 8th edition 2000. 16. Cygankiewicz, J.; Żyła, M.; Dudzińska, A. Karbo 2012, 3, 134-144. 17. Breck D.W., Zeolite Molecular Sieves: Structure, Chemistry and Use, John Wiley & Sons, Inc. New York, USA 1974, 593-724.


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

18. Saha, S.; Sharma, B.K.; Kumar, S.; Sahu, G.; Badhe, Y.P.; Tambe, S.S. Kulkarni, B.D. Fuel 2007, 86 1594-1600. 19. Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1362-1403. 20. Bloom, L.; Edelhausen, L.; van Krevelen, D.W. Fuel 1957, 36, 135-153. 21. Ihnatowicz, A. GIG Announcement 1952, 125. 22. Ogunsola, O.I. Fuel 1992, 71, 775-777. 23. Dudzińska, A.; Żyła, M.; Cygankiewicz, J. Przemysł Chemiczny 2014, 93 (2), 206211. 24. Clarkson, C.R.; Bustin, R.M. Fuel 1999, 78, 1333-1344. 25. Karacan, C.O.; Mitchell, G.D. Int. J. Coal Geol. 2003, 53, 201-217. 26. Ozdemir, E.; Morsi, B.I.; Schroeder, K. Fuel 2004, 53, 1085-1094. 27. Stachurski, J.; Żyła, M. Arch. Min. Sci. 1995, 40, 317-327. 28. Żyła, M.; Kreiner, K. Arch. Min. Sci. 1993, 38, 41-50. 29. Franco, D.V.; Gelan, J.M.; Martens, H.J.; Vanderzande D.J.M. Fuel 1992, 71, 553557. 30. Ceglarska-Stefańska, G. Arch. Min. Sci. 1998, 43, 277-289. 31. Faiz M,; Saghafi, A.; Sherwood, N.; Wang, L. Int. J. Coal Geol. 2007, 70, 193208. 32. Laxminarayan, C.; Crosdale, P.J. Int. J. Coal Geol. 1999, 40, 309-325. 33. Schwarzer, R.R.; Byrer, B. Variation in quality of methane adsorbed by selected coals as a function of coal petrology and coal chemistry: final draft report. U.S. Department of Energy Contract 1983, No. DE-AC21-80MC14219. 34. M. M. Faiz, N.I. Aziz, A.C. Hutton, B. Jones, Porosity and gas sorption capacity of some eastern Australian coals. In: Beamish, B., Gamson, P. (Eds.), Proceedings of the Symposium on coalbed methane research and development in Australia, vol. 4.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

James Cook University of North Queensland, Townsville, Queensland, Australia 1992, 9-20. 35. Wang, B.; Qin, Y.; Shen, J.; Zhang, Q.; Wang, B. J. Pet.Sci. Eng. 2018, 165, 1-12. 36. Wacławik, J.; Cygankiewicz, J.; Branny, M. Publishing House School of Underground Mining, Polish Academy of Sciences (PAN), Kraków, Poland 2000. 37. Lu, P.; Liao, G.X.; Sun, J.H.; Li, P.D. J. Loss Prevent. Proc. 2004, 17, 243-247. 38. Noack, K.; Eicker, H. Gliickauf-Forschung 1992, 53, 9-27. 39. The Decree of the Minister of Economy of the Republic of Poland on Health and Safety, Operational Management and Specialized Fire Protection in Underground Mining Plants, Journal of Laws of 28 June 2002 No. 139, 1169. 40. Dudzińska, A.; Cygankiewicz, J. Fuel Process. Technol. 2015, 137, 109-116. 41. Cygankiewicz, J.; Dudzińska, A.; Żyła, M. Arch. Min. Sci. 2007, 52, 573-585. 42. Cygankiewicz, J.; Dudzińska, A.; Żyła, M. Adsorption 2012, 18, 189-198.


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure captions

Fig. 1 Ethylene sorption isotherms for the coal samples tested Fig. 2 Hysteresis for ethylene sorption in coal samples: a) C-1 b) C-2 c) C-3 d) C-4 e) C-5 f) C-6. Table 1 Characteristics of coal samples Table 2 Porosity, pore volume determined from mercury porosimetry and values of specific surface area determined from nitrogen sorption (77, 5K) and carbon dioxide sorption (298 K) and micropore volume Table 3 Langmuir fit parameters for the ethylene isotherms Table 4 The volume of sorbed ethylene on C-1 - C-6 hard coal samples depending on the temperature of the sorption measurement Table 5 Comparison of the sorption and desorption potential of coal with respect to ethylene


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 1


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig 2a

Fig 2b


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 2c

Fig 2d


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig 2e

Fig 2f


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

Table 1 Characteristics of coal samples

Coal

Ultimate analysis

Proximate analysis

(wt.% daf)

(wt.%, dry)

R0 , %

Maceral and mineral (vol. %)

Cdaf

Hdaf

Ndaf

Sdaf

Odaf

Wa

Aa

Vdaf

V

I

L

M

C-1

0,51

77,69

4,53

1,17

1,48

15,18

11,11

14,45

38,14

67

28

5

11

C-2

0,71

83,20

5,07

0,96

0,58

10,45

3,08

4,23

31,80

59

36

8

3

C-3

0,68

82,17

5,38

1,86

1,82

8,84

4,36

4,17

39,63

84

11

5

0

C-4

0,70

84,39

5,29

1,14

0,37

9,01

3,39

2,65

37,91

60

30

10

1

C-5

0,89

84,07

4,44

1,40

0,32

9,99

1,19

7,69

32,94

67

31

2

4

C-6

0,93

88,45

5,69

1,52

1,03

3,58

1,76

3,48

35,81

75

22

8

2

Parameters determined according to the following Polish standards: C, H, N, O, PN-G-04571:1998; volatile matter, PN-G-04516:1998; ash, moisture, PN-G-04560:1998; vitrinite, inertinite, liptinite, mineral PN-ISO 7404-3:2001, vitrinite reflectance, PN-ISO 7404-5:2002. R0 – vitrinite reflectance, C - carbon content; H - hydrogen content; N - nitrogen content; O – oxygen content, V – vitrinite, I – inertynite, L – liptinite, M - mineral


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2 Porosity, pore volume determined from mercury porosimetry and values of specific surface area determined from nitrogen sorption (77, 5K) and carbon dioxide sorption (298 K)

Range (5 – 7500 nm) Porosity, %

Pore volume, cm3/g

SBET m2/g

SD-R, m2/g

D-R micropore volume, cm3/g

C-1

13,55

0,115

18,90

170,1

0,068

C-2

4,05

0,031

1,54

140,43

0,056

C-3

6,32

0,049

1,54

141,34

0,057

C-4

3,47

0,028

0,75

160,51

0,064

C-5

1,86

0,014

0,48

90,44

0,036

C-6

2,12

0,017

0,55

108,52

0,043

Sample


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3 Langmuir fit parameters for the ethylene isotherms am,

b,

D,

cm3/g, STP

MPa

%

C-1

11,29

32,81

6,47

C-2

7,77

31,39

5,14

C-3

7,82

28,42

4,69

C-4

4,93

28,53

3,78

C-5

4,04

26,07

4,61

C-6

2,16

23,37

5,25


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 4 The volume of sorbed ethylene on C-1 - C-6 hard coal samples depending on the temperature of the sorption measurement Coal

Volume of sorbed ethylene, cm3/g at a pressure of 0.1 MPa 298 K

323 K

348 K

373 K

C-1

9,01

5,28

4,05

2,54

C-2

6,13

4,95

3,89

2,64

C-3

5,94

4,91

3,66

2,31

C-4

3,78

3,39

2,70

2,02

C-5

2,99

2,47

1,86

1,47

C-6

1,62

1,47

1,19

1,14


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 5

Page 30 of 30

Comparison of the sorption and desorption potential of coal with respect to ethylene

298

Temperature, K 323 343

373

C-1

V, cm3/g Vmin, cm3/g Vdes, %

9,01 1,80 80

5,28 0,62 88

4,05 0,35 91

2,54 0,21 92

C-2


6,13 1,28 79

4,95 0,76 85

3,89 0,45 88

2,64 0,35 87

C-3


5,94 1,47 75

4,91 0,80 84

3,66 0,42 88

2,31 0,19 92

C-4


3,78 1,28 66

3,39 0,78 77

2,70 0,72 73

2,02 0,41 80

C-5


2,99 0,80 73

2,47 0,46 81

1,86 0,28 85

1,47 0,17 88

V, cm3/g 1,62 1,47 1,19 1,14 Vmin, cm3/g 0,55 0,38 0,31 0,29 Vdes, % 66 74 74 75 V – volume of the sorbed gas at a pressure of 0.1 MPa, Vmin – volume of non-desorbed gas; Vdes – ratio of the volume of desorbed gas to the volume of sorbed gas C-6


Analysis of Adsorption and Desorption of Ethylene on Hard Coals

Recommend Documents