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Analysis of sorption and desorption of unsaturated hydrocarbons: Ethylene, propylene and acetylene on hard coals
Fuel 246 (2019) 232–243
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Full Length Article
Analysis of sorption and desorption of unsaturated hydrocarbons: Ethylene, propylene and acetylene on hard coals
T
Agnieszka Dudzińska Central Mining Institute, Mines Ventilation Department, 40-166 Katowice Plac Gwarków 1, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal Self-heating of coal Sorption Desorption Unsaturated hydrocarbons
Unsaturated hydrocarbons: ethylene, propylene and acetylene are released into the mine atmosphere as a result of the self-heating of coal. Their concentrations in the mine air is one of the indicators for assessing the degree of development of coal self-heating process. The sorption capacity of coals with respect to hydrocarbons may cause their concentrations to decrease in the mine air, which may affect the correct self-heating assessment. This phenomenon is particularly important in the case of coals with a significant sorption capacity. The paper presents the results of studies on sorption of unsaturated hydrocarbons: ethylene, propylene and acetylene carried out on six samples of Polish hard coals taken from the exploited coal seams. High-porous coals with a well-accessible pore system, large specific surface values, low metamorphism and high oxygen content are characterized by the highest sorption capacities. The hydrocarbon sorbed in the largest amount is acetylene, which is due to the smallest size of the acetylene molecule and its high reactivity. The amount of the sorption of ethylene and propylene is lower than that of acetylene and depends on the properties of the coals. Coals with high sorption capacities adsorb larger amounts of propylene than ethylene. For less sorbent coals, the opposite is true. The volume of sorbed hydrocarbons decreases with the increase of the sorption temperature from 298 to 373 K. The temperature has the greatest influence on the volume of sorbed acetylene, the smaller the ethylene. The gas that is the least sensitive to the temperature increase is propylene. On the basis of desorption studies, it has been found that the lowest degree of desorption from coals is displayed by propylene. Higher degrees of desorption were found for acetylene. Ethylene is desorbed the most from coals. The concentration of ethylene in the mine atmosphere is the most frequently used indicator of temperature increase in coal in the self-heating process.
1. Introduction Coal mines are characterized by self-heating of coal. Heat is released a result of the reaction of hard coal with oxygen from the surrounding air. If this heat is accumulated, the self-heating of coal may occur. Uncontrolled development of this process may cause coal self-ignition and, consequently, spontaneous fire. Self-heating of coal is still a current problem in hard coal mining industry [1–6]. Despite the adopted prevention measures, fires resulting in serious financial losses and a threat to the safety of miners are still noted in the mines. The complex nature of the coal self-heating is the cause of intense research that aims to gain a better understanding of this complex process, and search for new solutions to improve preventive actions [7–14]. Activities regarding fire prevention in the mining industry focus on the early recognition of the phenomenon of self-heating, and taking
measures aimed at supressing it. One of the preventive methods used is the monitoring of gas concentrations in the mine air (carbon monoxide, carbon dioxide, hydrogen and unsaturated hydrocarbons: ethylene, propylene and acetylene), which are released into the atmosphere of the mine as the temperature of the heating coal increases [15–18]. On the basis of the concentrations of these gases in the mine atmosphere, fire indicators are determined, and by knowing their values, the temperature of the heating coal seam can be predicted. Proper assessment of the fire hazard situation in the mine is a key tool to ensure work safety and allows the appropriate measures to be taken in order to prevent the further spread of the self-heating, thus, minimizing the risk of a dangerous spontaneous fire. The gases emitted from the source of self-heating migrate through the mine gobs and some of them may undergo sorption on coals, as coal is a porous material exhibiting the ability to adsorb vapours and gases
E-mail address: [email protected]. https://doi.org/10.1016/j.fuel.2019.02.130 Received 13 September 2018; Received in revised form 3 January 2019; Accepted 26 February 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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[19,12]. Conducted sorption and desorption studies of gases accompanying coal self-heating processes indicated that carbon monoxide and hydrogen are subject to very little sorption [20,21], while unsaturated hydrocarbons: ethylene, propylene and acetylene are sorbed by hard coals in significantly greater amounts [22–24]. The phenomenon of hydrocarbon sorption on coals has the effect on reducing the concentrations of these gases in the mine atmosphere. This in turn influences the values of fire indicators, which in many cases may cause some restrictions in their application and negatively affect the accuracy of coal self-heating assessment. The results of sorption and desorption tests will allow for the improvement of the already used method of fire hazard assessment using the concentration of gases emitted from selfheating sources. This paper is a summary of information about the sorption and desorption processes of unsaturated hydrocarbons: ethylene, propylene and acetylene on hard coals. The paper combines the information presented in previous publications with new data to summarize and systematize all the information regarding the sorption and desorption of unsaturated hydrocarbons on hard coals.
Table 2 Porosity, pore volume determined from mercury porosimetry and values of specific surface area determined from nitrogen sorption (77, 5 K) and carbon dioxide sorption (298 K).
2. Experimental part
2.2. Measurement methodology
2.1. Materials
The sorption tests of coals with respect to hydrocarbons: ethylene, propylene and acetylene were carried out by volume method in the pressure range of 0–0.1 MPa using the Micromeritics ASAP 2010 apparatus. Coal samples with a grain size of 0.5–0.7 mm before the sorption measurement were subjected to a degassing process to purify the coal surface from adsorbed vapours and gases. Before degassing, the sample was placed in a helium atmosphere for 24 h. Helium atoms are non-sorbent, and their kinetic energy allows removing the adsorbed gases from the surface of coal. The degassing was carried out under vacuum until the pressure increase over the sample was not higher than 2 ⋅ 10−1 Pa/min. Sorption isotherms were collected at temperatures 298, 323, 348 and 373 K, close to the actual temperature conditions prevailing in coal mines. The degassing temperature was equal to the sorption measurement temperature. The time of determination of the sorption equilibrium at individual measuring points was long (several hours at each measurement point). The hydrocarbon desorption process was carried out at the same temperatures as the sorption process, gradually reducing the gas pressure over the coal surface with the adsorbed gas layer. The expanded uncertainty of the hydrocarbon sorption measurement was ± 4% rel. with the level of confidence of 95% and the coverage factor k = 2.
Coal
1 2 3 4 5 6
Range (5–7500 nm) pore radius Porosity, %
Pore volume, cm3/g
4.05 6.32 3.39 1.88 2.12 2.87
0.031 0.049 0.026 0.016 0.017 0.019
SBET m2/g
SD-A, m2/g
D-A parameter n
D-R micropore volume, cm3/g
1.54 1.54 0.50 0.49 0.55 1.71
225 148 134 139 106 70
1.54 1.94 1.85 1.55 1.99 2.04
0.056 0.057 0.050 0.037 0.043 0.030
(77.5 K) and according to the D-A model based on carbon dioxide sorption isotherms (298 K) and the volume of micropores according to the D-R model are presented in Table 2.
For the sorption and desorption studies, six samples of hard coals were selected which were taken from the exploited coal seams of Polish mines from the Upper Silesian Coal Basin located in the southern part of Poland. Coal samples for testing were collected in accordance with the applicable standard PN-G-04502:2014-11 ‘Hard coal and lignite. Sampling and preparation of samples for laboratory tests. Basic methods’. The collected samples were crushed in a jaw crusher, ground, and a grain size of 0.5–0.7 mm was separated using Fritsch sieves. Samples prepared in this way were stored under nitrogen until the measurements were started. The chemical, technical and petrographic characteristics of the studied coal samples made on the basis of Polish standards is presented in Table 1. The coal samples were also subjected to structural testing. First, measurements of carbon dioxide sorption isotherms at the temperature of 298 K were made to determine the surface area according to Dubinin-Astakhov (D-A) and volume of micropores according to the Dubinin-Radushkevich (D-R) model. Next, the nitrogen sorption isotherms were measured at 77.5 K in order to determine the specific surface area according to the BET model. For coal samples, porosity and pore volume were also determined by means of mercury porosimetry. The tests were carried out using the Pascal 440CE Instruments apparatus in the pressure range of 0.1–150 MPa. The studied range of pore radius was 5–7500 nm, it included meso- and macropores of coals. Data on porosity determined by the mercury porosimetry method, specific surface values determined according to the BET model based on nitrogen sorption isotherms
3. Results and discussion 3.1. Research on the sorption of unsaturated hydrocarbons on hard coals The obtained sorption isotherms of ethylene, propylene and
Table 1 Characteristics of coal samples. Coal
1 2 3 4 5 6
R0, %
0.71 0.68 0.82 0.85 0.93 1.12
Ultimate analysis (wt.% daf)
Proximate analysis (wt.%, dry)
Maceral and mineral (vol. %)
Cdaf
Hdaf
Ndaf
Sdaf
Odaf
Wa
Aa
Vdaf
V
I
L
M
83.20 82.17 84.69 85.14 88.45 86.07
5.07 5.38 4.83 4.60 5.69 5.10
0.96 1.86 1.46 1.40 1.52 1.49
0.58 1.82 0.32 0.22 1.03 0.71
10.45 8.84 8.82 8.83 3.58 6.79
3.08 4.36 1.52 1.22 1.76 1.23
4.23 4.17 4.25 5.26 3.48 27.45
34.31 39.63 34.76 33.37 35.81 24.87
59 84 53 68 75 67
36 11 14 4 22 32
8 5 33 28 8 1
3 0 3 2 2 16
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-G04560:1998; 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. 233
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10 9
volume adsorbed, cm³/g
8 7 6 acetylene
5
propylene
4
ethylene
3
Langmuir
2 1 0 0
0.02
0.04
0.06
0.08
0.1
0.12
pressure, MPa Fig. 1. Isotherms of sorption of unsaturated hydrocarbons on coal No. 3.
Correlation coefficients are similar for all gases, although slightly lower for acetylene (coal No. 2, 4, 6) and for propylene (coal No. 6). In the case of acetylene and propylene, Langmuir isotherms in the pressure range 0.03–0.08 MPa run above the experimental data. However, above the pressure value 0.08 MPa – they run below the experimental data, especially for coals No. 1, 2, 4 and 5. Data analysis indicates that the value of the mean relative error is a better criterion for conforming the fit of the Langmuir model than the correlation coefficient. The volumes adsorbed of hydrocarbons at 298 K (experimental data) read from the sorption isotherms at the pressure of 0.1 MPa are shown in Table 4. Analysing the obtained test results, it was found that the volumes of adsorbed hydrocarbons change depending on the physicochemical properties of the coals and the type of adsorbed gas. The largest amounts of ethylene, propylene and acetylene are adsorbed by the coals No. 1 and 2. These are coals with a well-accessible structure, their surface area and volume of micropores is the highest among the coals studied. It is worth noting that coal No. 1 sorbs slightly higher amounts of hydrocarbons, especially of propylene, than coal No. 2. The coal No. 1 is characterized by an exceptionally high value of surface area according to D-A. The volume of micropores is comparable to coal No. 2, and the porosity determined by mercury porosimetry is slightly lower (Table 2). Both coals (No. 1 and 2) are characterized by a higher moisture content than the others and a low degree of metamorphism, although the vitrinite reflectance of coal No. 1 with a higher sorption capacity is slightly higher (Table 1). It is worth noting that these coals differ in oxygen content. The oxygen content in coal No. 1 is higher than in coal No. 2, due to the fact that there is a slightly higher amount of oxygen polar groups on its surface. This results in stronger
acetylene determined at a temperature of 298 K on the examined coals are characterized by a similar course, they differ only in the amount of adsorbed gas. Fig. 1 presents exemplary hydrocarbon sorption isotherms for one of the coals. Ethylene sorption isotherms determined on coals No. 1, 2 and 5 are presented in the paper [24], and the sorption isotherm of the propylene on coal No. 1 – in the paper [23]. For coal sample No. 6 – in the paper [25]. The Langmuir isotherm equation [26] was used to describe the sorption equilibria of hydrocarbons on hard coals.
a = am
bP 1 + bP
(1)
where: a – amount adsorbed, cm STP/g, am – maximum adsorption capacity, cm3 STP/g, b – Langmuir constant, P – equilibrium pressure, MPa−1. To assess the quality of the adsorption isotherms equations to experimental data, the mean relative error value was used, as defined below: 3
D=
100 k
k
∑ i=1
aiexp − aisym aiexp
(2)
– experimental data, – calculation data, k – number of where, data points. The coefficients of the equation am, b and D values are shown in Table 3. It also provides correlation coefficients R2. The most favourable matching of the Langmuir equation to the experimental data was obtained for ethylene, where D values do not exceed 5.68%. For propylene and acetylene, D values are higher, a maximum of about 9%. aiexp
aisym
Table 3 Langmuir fit parameters for a single component isotherms. Coal
1 2 3 4 5 6
C2H4
C3H6
C2H2
am, cm3/g, STP
b, MPa−1
D, %
R2
am, cm3/g, STP
b, MPa−1
D, %
R2
am, cm3/g, STP
b, MPa−1
D, %
R2
7.77* 7.70* 5.16 3.95 2.16* 3.15*
31.45* 28.90* 27.29 27.22 23.38* 26.42*
5.65* 5.04* 3.55 4.71 5.68* 4.06*
0.989 0.988 0.993 0.988 0.980 0.990
9.68 7.38 5.05 2.78 1.55 2.03*
86.21 64.52 70.72 65.40 60.29 48.72*
7.97 9.54 6.73 7.83 9.42 8.34*
0.986 0.992 0.994 0.991 0.989 0.977
14.02 14.39 11.60 7.09 6.13 6.84*
42.02 37.04 35.92 30.66 26.31 13.17*
7.23 9.42 4.76 6.44 1.99 2.99*
0.986 0.978 0.992 0.984 0.996 0.984
* Data from publications [24,25]. 234
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participate in the sorption process. In addition, coal No. 6 is also characterized by an exceptionally high content of ash and mineral substance (Table 1), which does not affect the increase in sorption capacity of coal. According to Faiz, the mineral substance is characterized by a small internal surface area [27]. The dependence of the sorption capacity of coals on the degree of metamorphism, oxygen content and the volume of micropores of the coals is presented in Fig. 2. The influence of physicochemical properties of coals on their sorptivity on hydrocarbons is described in detail in the papers [22–24]. The decisive role in the process of hydrocarbon sorption on coals has the degree of metamorphism expressed by vitrinite reflectance. According to the information presented in the reference literature, coals with reflectance of vitrinite in the order of 0.5% adsorb large volumes of hydrocarbons [22,24,25]. With the increase in the degree of metamorphism, the amount of adsorbed hydrocarbons decreases due to the decrease in the amount of oxygen reactive groups present on the surface and higher order of coal structure. For coals with reflectance of the vitrinite of the order and above 1% of the amount of adsorbed hydrocarbons, they are significantly smaller compared to low rank coals. The essential element in the sorption of hydrocarbons is also the availability of the internal coal structure, described by the surface and the volume of micropores as well as the polar character of the coal surface. It favours the electrostatic interactions of electrons π within the bonding that occurs between carbon atoms in hydrocarbon molecules with oxygen polar groups present on the surface of hard coals. The effect of the double bond occurring in unsaturated hydrocarbon molecules on the increase in sorption capacity of hard coals was confirmed in the papers [28,29], in which higher sorption was demonstrated for ethylene than ethane, propylene and propane. In Krzyżanowski and Zarębska's paper [30], the higher sorption capacity of the studied coals in relation to unsaturated hept-1-ene as compared to saturated heptane was also confirmed. Among the unsaturated hydrocarbons considered, the tested coals adsorb acetylene in the largest amount. Although the critical temperature of acetylene, amounting to 308 K, is lower than the critical temperature of propylene of 364 K, the sorption capacity of the examined coals with respect to acetylene is higher than that of propylene. The critical temperature of ethylene is the lowest and amounts to 282 K. The acetylene particles are the smallest of the hydrocarbons tested (acetylene kinetic diameter is 0.33 nm, and ethylene and propylene 0.39 and 0.45 nm, respectively) [31], which allows them to penetrate freely into the smallest pores of hard coals not available for ethylene and propylene. The size of the acetylene molecule, therefore, is determinant of its greatest sorption capacity among the molecules of the unsaturated hydrocarbon considered. This is particularly evident for coals with lower porosity and compact structure (No. 3, 4 and 5), for which the largest difference between the volume of adsorbed acetylene and the volume of other adsorbed hydrocarbons was observed. For these coals, the amount of adsorbed acetylene at a temperature of 298 K is 2–3 times higher than the amount of adsorbed ethylene or propylene. In the case of coals with a higher porosity, a less ordered structure (No. 1, 2), the difference between the volumes of adsorbed acetylene, ethylene or propylene is smaller. Acetylene is adsorbed in less than twice as much as ethylene or propylene. At the same time, there is a triple bond between the carbon atoms in the acetylene molecule that is the cause of the greater reactivity of acetylene compared to ethylene or propylene. The probability of occurrence of electrostatic interactions between reactive acetylene molecules and the surface of hard coals on which both electron-donor and electron-acceptor centres are located is larger than in the case of other hydrocarbons studied. As a result, the volume of adsorbed acetylene is greater than the volume of adsorbed ethylene and propylene. Comparing the amounts of adsorbed ethylene and propylene at the temperature of 298 K, it was observed that the coals with high sorption capacity No. 1 and 2, as well as coal No. 3 adsorb propylene rather than
Table 4 The volume of adsorbed hydrocarbons read from the sorption isotherms at a pressure of 0.1 MPa. Volume adsorbed, cm3/g Coal
Temperature, K
Ethylene
Propylene
Acetylene
1
298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373
6.13 4.95 3.89 2.64 5.94 4.91 3.66 2.31 3.87 3.74 3.01 1.93 2.97 2.45 1.88 1.45 1.62 1.47 1.19 1.14 2.37 2.19 1.89 1.60
9.03 5.92 5.45 4.33 6.64 5.19 5.09 4.26 4.66 3.90 3.89 3.13 2.55 1.97 1.90 1.74 1.39 1.53 1.57 1.49 1.89 1.74 1.70 1.65
11.98 9.22 7.43 4.76 11.73 9.02 6.90 4.27 9.39 7.39 5.92 3.79 5.56 4.83 3.97 2.60 4.57 4.55 3.54 2.56 4.08 3.52 2.95 2.23
2
3
4
5
6
interactions with the adsorbed hydrocarbon molecules and thus higher sorption capacity of coal No. 1. Comparing these two coals, it seems that in addition to the availability of a microporous structure, it is the higher content of oxygen in coal No. 1 that largely determines its higher sorption capacity. Smaller amounts of hydrocarbons are adsorbed by coal No. 3. This is a coal with a higher degree of metamorphism than coals No. 1 and 2. Its vitrinite reflectance is 0.82%. At the same time, it is characterized by lower values of porosity, surface determined from nitrogen and carbon dioxide sorption isotherms and the volume of micropores, which results in its lower sorption capacity towards hydrocarbons. Coals No. 4–6 are coals with low sorption capacity. They are characterized by a higher degree of metamorphism compared to others, while coals No. 5 and 6 also have the lowest oxygen content (Table 1). On their surface, there are small amounts of oxygen polar groups responsible for interactions with adsorbed hydrocarbon molecules. The structure availability of coals No. 4, 5 and 6 is small (lower values of porosity micropores compared to other coals). Analysing the results of research for this group of coals, it is difficult to observe an unequivocal correlation between the parameters describing the structure and the sorption capacity of these coals. Among this group, coal No. 5 is characterized by the lowest sorption capacity, despite its structural parameters (porosity, surface area, volume of micropores do not indicate the smallest availability of its structure). Both, the porosity and the pore volume determined by mercury porosimetry and the volume of micropores, determined on the basis of carbon dioxide sorption isotherms for coal No. 5 are higher than those of coals No. 4 and 6. In this case, it seems that the content of oxygen plays a significant role in the description of the impact that physicochemical properties of the coals have on sorption capacity. It is the smallest for coal No. 5 (3.58%) compared to the oxygen content for coals No. 4 and 6 (8.83%, 6.79%). One should pay attention to coal No. 6. Interestingly, it is characterized by an exceptionally high surface area according to BET (1.71 m2/g), but the lowest surface area and volume of micropores of the studied coals, which ultimately results in its low sorption capacity. Probably, this coal has an extensive system of macropores, which are transport routes for the particles of adsorbed hydrocarbons and do not
235
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14 ethylene
volume adsorbed, cm3/g
12
propylene
10
acetylene
8 6 4 2
0 0.5
0.6
0.7
a)
0.8
0.9
1
1.1
1.2
vitrinite reflectance, % 14
volume adsorbed, cm3/g
12 10 8 ethylene 6
propylene acetylene
4 2 0
b)
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0.055
0.06
micropore volume, cm 3/g 14
volume adsorbed, cm3/g
12 10 8 ethylene 6
propylene acetylene
4 2 0
c)
2
4
6
8
10
12
oxygen, %
Fig. 2. Relationship between volume of adsorbed hydrocarbons and factors (a) vitrinite reflectance, (b) micropore volume, (c) oxygen content.
ethylene in a larger amount. The volume of adsorbed propylene by these coals is higher than the volume of adsorbed ethylene by 47, 12 and 20%, respectively. In the case of the discussed coals, the
hydrophilic character of the surface is of great importance. Increased oxygen and moisture content favours the interaction between the oxygen polar groups on the coal surface and the adsorbed propylene 236
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the type of adsorbed gas, the highest impact of temperature was observed on the volume of adsorbed acetylene, the smaller one – ethylene. The sorption capacity of coals in relation to propylene is the least sensitive to temperature increase. Similar observations regarding the low sensitivity of the sorption of propylene to temperature variations are presented in the paper [12] during the flow of gas emitted from the self-heating source by the coal adsorption column. The heat of sorption is the thermodynamic function most commonly used to describe the equilibria in the sorbent – sorbate system. On the basis of the sorption equilibrium isotherms collected at four temperatures, the isosteres in the lnp − 1/T system were determined, and the isosteric heat of sorption was calculated from their inclination according to Clausius-Clapeyron equation.
molecules. Most probably, they are stronger than in the case of ethylene. In addition, these coals are characterized by an open structure, good availability of the sorption system for both gases. Their porosity and pore volume determined by mercury porosimetry are higher than the porosity of the three remaining coals. At the same time, the volume of micropores determined on the basis of carbon dioxide sorption isotherms, that is the basic adsorbing system of hard coals, are higher than the corresponding values for the remaining coals. However, this trend is not confirmed by the value of the surface, especially for coal No. 3. The BET surface area for coal No. 3 is close to the value for other coals with low sorption capacity. At the same time, the area determined according to D-A for this coal is even lower than the surface of coal No. 4 with much lower sorption capacity. For coals with a low sorption capacity, No. 4 ,5 and 6, the volumes of adsorbed propylene at the temperature of 298 K, unlike in the case of coals No. 1–3, are smaller than the volume of adsorbed ethylene by 14, 14 and 20%, respectively. A higher degree of ordering the structure of coals with low sorption capacities makes it difficult to penetrate larger particles of propylene into their pores. Due to the fact the majority of pores in coals is attributed to micro- and sub-micropores of the order of sorbent particles [32–35], coals behave selectively towards the adsorbed gases, showing the properties of the molecular sieve. The micropores with the smallest diameters are therefore not available for larger propylene particles. In summary, among the unsaturated hydrocarbons tested, hard coals adsorb the most acetylene and approximate volumes of ethylene and propylene, depending on the physiochemical properties of coals.
qst = −R
⎛ Δ ln p ⎞ ⎜ Δ1 ⎟ ⎝ T ⎠a
(3)
where: qst – isosteric heat of sorption, kJ/mol, p – equilibrium pressure, Pa, T – temperature of sorption measurement, K, R – gas constant, J/mol⋅K, The obtained heat is a measure of the sorption capacity of the studied coals and allows the assessment of mutual interactions between the sorbate and the adsorbent surface. Fig. 3 shows the obtained heat as a function of the amount of adsorbed gas. The heat values depend on the physicochemical properties of the coals and their sorption capacity. The highest values were obtained for highly-sorbent coals No. 1 and 2, and the lowest values for coals with low sorption capacity, namely No. 5 and 6. Depending on the type of on the sorbed gas, the highest sorption heat is characteristic for acetylene (maximum 33.3 kJ/mol for coal 2). The heat of sorption for ethylene and propylene is comparable (maximum 25.5 and 27.9 kJ/mol for ethylene and propylene respectively), in proportion to the sorption capacity of coals. The exception are coals No. 2 and 3, for which the heats of sorption of ethylene are higher than propylene, contrary to sorption capacity. This is probably due to the shape of the sorption isotherms, which are more steep in the initial section of their course and they run in close range. Together with the increase in the amount of adsorbed gases, a decrease in the sorption heat value is observed. For coals No. 1 and 2 it is small (up to 12%). In the case of coal No. 3, the heat values are reduced by about 20%, for slightly less sorbent coals – by slightly higher values, and for the least sorbent coal – by about 40%. The obtained values of sorption heat for the system: hard coal – hydrocarbons, are not much lower than the value of sorption heat for the system: hard coal – carbon dioxide, most often designated for hard coals [36–37]. In real conditions, the emission of gases from the coal self-heating source takes place with continuous change of environmental parameters, such as: temperature, air flow, inflow of the oxidizing agent and coal mass. These variables are different for each case of coal selfheating and most often their values are not known due to the unavailability of self-heating sources. Due to changes in the temperature of coal in mining conditions, the sorption capacity of coals with respect to hydrocarbons will also change. Generally, as the temperature increases, the amount of adsorbed gases will decrease. The volume of adsorbed propylene will be subjected to the smallest changes. The volume of adsorbed ethylene will decrease to a greater extent, and acetylene will be the largest. For most coals acetylene is the gas adsorbed in the largest amounts, but the effect of temperature results in the fact that volumes of adsorbed acetylene at the highest temperature (373 K) are close to the volume of adsorbed propylene, especially for coals with high sorption capacity.
3.2. Influence of temperature on sorption capacity of coals in relation to unsaturated hydrocarbons In coal self-heating processes in mines, temperature changes are often observed. In this paper, the influence of temperature on the process of hydrocarbon sorption and desorption on the tested hard coals was analysed. The volumes of the adsorbed hydrocarbons read from the sorption isotherms at the pressure of 0.1 MPa for the analysed temperature range are shown in Table 4. With the increase of the sorption temperature in the range of 298–373 K, the volumes of adsorbed hydrocarbons are decreasing. Analysing the test results for the high-sorbent coals (No. 1 and 2) it was found that the influence of temperature on the volume of adsorbed ethylene and acetylene is similar. In both cases, the sorption capacity of these coals decreases by about 60% in the studied temperature range. The decrease in sorption capacity of these coals is relatively even and amounts to about 20% for every 25 K. It is slightly different for propylene, in this case sorption capacity of coals No. 1 and 2 decreases unevenly and to a lesser extent with temperature increase. For coal No. 2, the volume of adsorbed propylene decreases by about 36%, and for coal No. 1 by 52%. For the remaining coals No. 3, 4, 5 and 6, it was observed that the temperature affects the volume of adsorbed acetylene most, while to a lesser extent – ethylene and the lowest – propylene. The volume of adsorbed acetylene decreases by 44–60%, and ethylene by 30–51% in the studied temperature range. In the case of propylene, this decrease is the smallest and amounts to about 32% for coals No. 3 and 4 and 13% for coal No. 6. For coal No. 5 with the lowest sorption capacity, the influence of temperature on the volume of adsorbed propylene is so small that it does not result in the volume of the adsorbed gas reduction. The properties of coal and the type of adsorbed gas are of significant importance in describing the influence of temperature on the volume of adsorbed hydrocarbons. From the point of view of the properties of coals, it should be noted that the temperature has the greatest influence on sorption capacities of coals with high porosity and a well-accessible structure. The influence of temperature on the volume of adsorbed hydrocarbons decreases with the decrease of coal porosity. Considering 237
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35
heat of adsorption, kJ/mol
30 25 20
1 2
15
3 10
4 5
5
6
0
a)
0
0.5
1
1.5
2
volume adsorbed,
2.5
3
cm3/g
35
heat of adsorption, kJ/mol
30 25 1 20
2 3
15
4 10
5 6
5 0 0
b)
0.5
1
1.5
volume adsorbed,
2
2.5
3
cm3/g
35
heat of adsorption, kJ/mol
30 25 1 20
2 3
15
4 5
10
6 5 0
c)
0
0.5
1
1.5
volume adsorbed,
2
2.5
3
cm3/g
Fig. 3. Course of isosteric heats of hydrocarbons adsorption (a) ethylene, (b) propylene, (c) acetylene.
238
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experiment, as a result of lowering the pressure over the coal sample, the lowest degree of desorption from the coal structure is characterized by propylene. Acetylene is desorbed to a greater extent and the most easily desorbed coal is ethylene. Although ethylene molecules are larger than acetylene molecules, they are easier to desorb and are less bonded to the coal surface than acetylene molecules, especially at lower temperatures 298 and 323 K. At higher temperatures of 343 and 373 K, the percentage of desorbed ethylene and acetylene is similar (for coal No. 5 – higher for acetylene). Ethylene is therefore characterized by a greater tendency to desorption from the coals at a lower temperature. This higher tendency of ethylene for desorption is particularly visible for coals with high sorption capacity, e.g. for coal No. 1 and 2 percent of desorbed ethylene at 298 K is higher than for acetylene by 16 and 17, respectively. These coals are characterized by an open and easily accessible structure and a large proportion of micropores, which suggests that a large part of acetylene molecules, which are smaller than ethylene, is located in the micropores and desorption from the smallest pores is more difficult. For the less sorbent coals 4 and 5, the difference between the percentage of desorbed ethylene and acetylene at 298 K is lower and amounts to 6 and 8, respectively. The volumes of desorbed hydrocarbons shown in Table 5 are dependent on the degree of coal metamorphism. With the increase in vitrinite reflectance, the amount of desorbed hydrocarbons is reduced. The exception is coal No. 6, from which more hydrocarbons were desorbed than from coal No. 5 with lower vitrinite reflectance. Analysing the effect of coal structure availability, it was generally found that with the decrease in the availability of the structure, the amount of desorbed gases is generally reduced. From coal No. 1, more hydrocarbons were desorbed than from coal No. 2, although both the porosity, surface area and volume of micropores of coal No. 1 are slightly smaller than coal No. 2. Fewer gases were desorbed from coal No. 5 than it would be indicated by the porosity and volume values in accordance with D-R in comparison to coals No. 4 and 6. No relationship was found between the amount of desorbed hydrocarbons and the surface area determined according to the BET model. A much smaller amount of hydrocarbons was desorbed from coal No. 6 with a surface area equal to 1.71 m2/g, than from coals No. 1 and 2 with a surface area of 1.54 m2/g. In general, it should be emphasized that the BET surface is characteristic only for meso and macropores and does not correlate well with the amount of adsorbed and desorbed hydrocarbons. The largest amount of acetylene was desorbed from the coals studied because of its highest sorption capacity. Comparing the amount of desorbed ethylene and propylene, it was observed that from the coals No. 1–3, a larger amount of ethylene than propylene was desorbed (inversely than in the case of sorption), and from the coals No. 4–6 more ethylene was desorbed than propylene in proportion to the adsorbed amount. The studied coals are characterized by a small percentage of desorbed propylene. Therefore, despite its high sorption capacity, it was desorbed less than ethylene, especially for coals with a low degree of metamorphism. The dependence of the volume of hydrocarbons desorbed from coals on the degree of metamorphism, oxygen content and the volume of micropores of the coals is presented in Fig. 5. Desorption is a process that we observe in mines during the development of the coal self-heating process, when gases, and among them, unsaturated hydrocarbons along with the development of this process are released from the structure of coals going into the atmosphere of the mine. 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. On the basis of the desorption tests carried out, it was observed that the gas among the studied hydrocarbons that is most easily desorbed from coals is ethylene. A high degree of ethylene desorption is already observed at 298 K. For this reason, the content of ethylene in the mine atmosphere is the most frequently used indicator of the increase in the temperature of coal in the self-heating process, and the increase in its concentration is observed at the early stage of self-
Table 5 Volume of hydrocarbons desorbed from coals. Coal
1 2 3 4 5 6
Volume desorbed, cm3/g, T = 298 K Ethylene
Propylene
Acetylene
4.85 4.47 2.99 2.17 0.97 1.54
4.12 3.24 2.28 1.12 0.60 0.97
7.58 6.82 6.75 3.78 2.65 3.54
3.3. Research on desorption of unsaturated hydrocarbons from hard coals Desorption studies of ethylene, propylene and acetylene were carried out to determine the degree of desorption of the gases tested. As a result of the experiment, desorption isotherms were obtained. The course of unsaturated hydrocarbon desorption isotherms was presented in earlier papers [22–24]. In the case of each of the hydrocarbons, desorption isotherms do not combine with the sorption isotherms to form open hysteresis loops. The sorption process is therefore an irreversible process, and the hydrocarbon molecules remaining in the coal structure are more strongly associated with the surface of the coals with the participation of oxygen polar groups [38]. The volumes of desorbed hydrocarbons at 298 K are shown in Table 5. The percentage of desorbed gas, which is the ratio of the volume of gas desorbed to the volume of gas adsorbed at a pressure of 0.1 MPa, was determined on the basis of sorption and desorption isotherms. The percentage of desorbed gas as a function of the desorption temperature is shown in Fig. 4. Analysing the data presented, it was observed that ethylene is a gas which, after being adsorbed, undergoes desorption to a large extent. In the case of coals No. 1, 2 and 3 at 298 K, approximately 75–79% of the absorbed ethylene undergoes desorption, whereas for the remaining coals it is 60–74% of gas. The largest percentage of desorbed ethylene is characteristic for coals with a high porosity and easily accessible structure that adsorbs large amounts of ethylene. With the decrease in the porosity of the coals and, hence, their sorption capacity, the percentage of desorbed ethylene also decreases. With increasing temperature, the percentage of desorbed ethylene increases for highlysorbent coals even up to 92% (coal No. 2). In the case of coals with lower sorption capacity it amounts to 78–88%. Having analysed the results of sorption and desorption studies of propylene at 298 K, it was observed that, for coals of high sorption capacity and for coal No. 3, about 46–49% of sorbed propylene is desorbed. For less sorbent coals it amounts to 43–51%. It is much less than in the case of ethylene. Propylene particles are larger than the ethylene molecules and are more strongly bound to the coal structure, so it is more difficult to pass into the gas phase. The lowest percentage of desorbed propylene was obtained for coals No. 4 and 5. No significant effect of the degree of metamorphism and availability of the coal structure on the percentage of desorbed propylene was found. With increasing temperature, the percentage of desorbed gas increases, reaching about 80% at 373 K for coals with high sorption capacity. For the remaining coals the percentage of desorbed gas at 373 K is 70–75%. Having analysed the results of acetylene sorption and desorption studies, it was found that at 298 K, 58–72% of the adsorbed gas undergoes desorption depending on the properties of the coals. The tendency to release acetylene from the studied coals is to a small extent dependent on their sorption capacity, e.g. coals No. 2 and 5, which adsorb various acetylene volumes, are characterized by the same volume of desorbed gas. As the temperature increases, the percentage of desorbed acetylene increases to a similar value as for ethylene, that is around 85–88%. Summing up the obtained results of the desorption of unsaturated hydrocarbons, it can be concluded that under the conditions of the 239
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100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
a)
0 1
2
3
4
5
6
coal samples 100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
b)
0 1
2
3
4
5
6
coal samples 100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
c)
0 1
2
3
4
5
6
coal samples Fig. 4. Percent of desorbed gas depending on the desorption temperature (a) ethylene, (b) propylene, (c) acetylene.
240
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8 7
ethylene propylene
volume desorbed, cm3/g
6
acetylene
5 4 3 2 1 0 0.5
a)
0.6
0.7
0.8
0.9
1
1.1
1.2
vitrinite reflectance, %
volume desorbed, cm3/g
8 7
ehylene
6
propylene acetylene
5 4 3 2 1 0 0.02
0.03
b)
0.04
0.05
0.06
micropore volume, cm3/g 8
volume desorbed, cm3/g
7 6 5 ethylene
4
propylene
3
acetylene
2 1 0
0
c)
2
4
6
8
10
12
oxygen, %
Fig. 5. Relationship between volume of desorbed hydrocarbons and factors (a) vitrinite reflectance, (b) micropore volume, (c) oxygen content.
241
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as the degree of metamorphism increases and the availability of the coal structure decreases.
heating development. The use of ethylene to assess the degree of development of the self-heating process is confirmed by data presented in the literature [39–42]. In the smallest degree, propylene is released from coals, even at 373 K the percentage of desorbed propylene is lower than ethylene or acetylene. The contents of propylene and acetylene appear in the mine atmosphere only at higher temperatures [41,43] and these gases are used in the assessment of the development of coal self-heating process less frequently than ethylene. Upon searching the scientific literature, a distinct lack of bibliographic entries were found, specifically regarding the use of these hydrocarbons in assessing the phenomenon of coal selfheating.
Acknowledgment This work was supported by the Ministry of Science and Higher Education, Poland [11050517]. References [1] Kaji R, Hishinuma Y, Nakamura Y. Low temperature oxidation of coals: effects of pore structure and coal composition. Fuel 1985;64:297–302. [2] Smith M, Glasser D. Spontaneous combustion of carbonaceous stockpiles. Part I: the relative importance of various intrinsic coal properties and properties of the reaction system. Fuel 2005;84(9):1151–60. [3] Zhang Y, Wu J, Chang L, Wang J, Li Z. Changes in the reaction regime during lowtemperature oxidation of coal in confined spaces. J Loss Prevent Proc 2013;26:1221–9. [4] Song Z, Kuenzer C. Coal fires in China over the last decade: A comprehensive review. Int J Coal Geol 2014;133:72–99. [5] Taraba B, Pavelek Z. Investigation of the spontaneous combustion susceptibility of coal using the pulse flow calorimetric method: 25 years of experience. Fuel 2014;125:101–5. [6] Arisoy A, Beamish B, Yoruk B. Moisture moderation during coal self-heating. Fuel 2017;210:352–8. [7] Xue S, Dickson B, Wu J. Application of 222Rn technique to locate subsurface coal heatings in Australian coal mines. Int. J. Coal Geol. 2008;74:139–44. [8] Choi H, Thiruppathiraja Ch, Kim S, Rhim Y. Moisture readsorption and low temperature oxidation characteristics of upgraded low rank coal. Fuel Process Technol 2011;92:2005–10. [9] Qi X, Xin H, Wang D, Qi G. A rapid method for determining the R70 self-heating rate of coal. Thermochim Acta 2013;571:21–7. [10] Xia T, Zhou F, Gao F, Kang J, Liu J, Wang J. Simulation of coal self-heating processes in underground methane-rich coal seams. Int J Coal Geol 2015;141–142:1–12. [11] Xia T, Wang X, Zhou F, Kang J, Liu J, Gao F. Evolution of coal self-heating processes in longwall gob areas. Int J Heat Mass Transf 2015;86:861–8. [12] Wojtacha-Rychter K, Smoliński A. The interaction between coal and multi-component gas mixtures in the process of coal heating at various temperatures: An experimental study. Fuel 2018;213:150–7. [13] Wojtacha-Rychter K, Smoliński A. Multi component gas mixture transport trough porous structure of coal. Fuel 2018;233:37–44. [14] Wang J, Zhang Y, Xue S, Wu J, Tang Y, Chang L. Assessment of spontaneous combustion status of coal based on relationships between oxygen consumption and gaseous product emission. Fuel Process Technol 2018;179:60–71. [15] Singh AK, Singh RVK, Singh MP, Chandra H, Shukla NK. Mine fire gas indices and their application to Indian underground coal mine fires. Int J Coal Geol 2007;69:192–204. [16] Yuan L, Smith AC. CO and CO2 emissions from spontaneous heating of coal under different ventilation rates. Int J Coal Geol 2011;88:24–30. [17] Baris K, Kizgut S, Didari V. Low-temperature oxidation of some Turkish coals. Fuel 2012;93:423–32. [18] Gurdal G, Hosgormez H, Ozcan D, Li X, Liu H, Song W. The properties of Çan Basin coals (Çanakkale - Turkey): Spontaneous combustion and combustion by-products. Int J Coal Geol 2015;138:1–15. [19] Dudzińska A, Cygankiewicz J. Analysis of adsorption tests of gases emitted in the coal self-heating process. Fuel Process Technol 2015;137:109–16. [20] Cygankiewicz J, Dudzińska A, Żyła M. Sorption and desorption of carbon monoxide in several samples of polish hard coal. Arch Min Sci 2007;52(4):573–85. [21] Cygankiewicz J, Dudzińska A, Żyła M. Examination of sorption and desorption of hydrogen on several samples of polish hard coals. Adsorption 2012;18(3–4):189–98. [22] Dudzińska A. Investigation of adsorption and desorption of acetylene on hard coal samples from Polish mines. Int. J. Coal Geol. 2014;128–129:24–31. [23] Dudzińska A, Howaniec N, Smoliński A. Experimental study on sorption and desorption of propylene on polish hard coals. Energy Fuels 2015;29(8):4850–4. [24] Dudzińska A. Analysis of adsorption and desorption of ethylene on hard coals. Energy Fuels 2018;32:4951–8. [25] Dudzińska A, Cygankiewicz J, Włodarek M. Natural content of gases: Carbon monoxide, carbon dioxide, hydrogen and unsaturated hydrocarbons of ethylene, propylene and acetylene in selected bituminous coal seams. Int J Coal Geol 2017;178:110–21. [26] Langmuir I. The adsorption of gases on plane surfaces of glass, mica and platinum. J Am Chem Soc 1918;40:1362–403. [27] Faiz MM, Aziz NI, Hutton AC, Jones B. Porosity and gas sorption capacity of some eastern Australian coals. In: Beamish B, Gamson P, editors. Proceedings of the Symposium on coalbed methane research and development in Australia, vol. 4. James Cook University of North Queensland, Townsville, Queensland, Australia 1992, p. 9–20. [28] Cygankiewicz J, Żyła M, Dudzińska A. Influence of metamorphism degree of hard coals on sorption and desorption of ethane. Karbo 2012;3:134–44. [29] Dudzińska A, Żyła M, Cygankiewicz J. Influence of the metamorphism grade and porosity of hard coal on sorption and desorption of propane. Arch Min Sci
4. Conclusions The conducted studies of sorption and desorption of ethylene, propylene and acetylene on selected samples of hard coals allowed us to formulate the following conclusions: 1. The volumes of adsorbed unsaturated hydrocarbons depend on the physicochemical properties of coals as well as their porosity, surface area and micropore volume. The largest volumes of hydrocarbons are adsorbed by highly porous coals with high surface and pore volume values, low metamorphism and high oxygen and moisture content. With the increase in the degree of metamorphism and the decrease in porosity and pore volume, sorption capacities of coals with regard to hydrocarbons decrease. 2. For the studied hard coals, acetylene is the hydrocarbon adsorbed in the largest amounts. The volumes of adsorbed acetylene are about 2–3 times higher than the volume of adsorbed ethylene or propylene. Acetylene, due to the triple bond occurring between carbon atoms is characterized by greater availability of electrons of weak bonds π, than ethylene and propylene, hence the electrostatic interactions between acetylene molecules and the surface of coals, on which there are both electron-donor and electron-receptor centres, are stronger than in the other hydrocarbons studied. The acetylene molecules are smaller than the ethylene and propylene particles, which facilitates their penetration into the micropores of the pores of hard coals and results in a larger volume of absorbed acetylene. Comparing the volumes of adsorbed ethylene and propylene, it was found that coals with high sorption capabilities adsorbed lower volumes of ethylene than propylene, while for those less-sorbent coals, ethylene is adsorbed more. 3. The volumes of adsorbed hydrocarbons decrease as the temperature of the sorption process increases from 298 to 373 K. The largest changes with the increase in temperature are connected with the volume of adsorbed acetylene, smaller changes refer to size of sorption of ethylene, and the smallest – propylene. In the case of coal No. 5 with the lowest sorption capacity, the change in temperature does not reduce the volume of adsorbed propylene. The greatest influence of temperature on the volume of adsorbed hydrocarbons was observed for coals with high sorption capacity. With the decrease in sorption capacity of coals, the temperature influence on the volume of adsorbed hydrocarbons also decreases. The determined heat values of hydrocarbon sorption are in the range of maximum 30 kJ/mol. The highest heat values were obtained for acetylene. 4. For the studied coals, the degree of desorption of hydrocarbons from coals has been determined. The highest degree of desorption from coals is characterized by ethylene, slightly lower acetylene, and the gas that is least desorbed from the coals is propylene. The percentage volume of desorbed hydrocarbons from the coal structure depends on the sorption capacity of the coals. The largest percentage of desorbed gas is characterized by coals with high sorption capacity. The volumes of desorbed hydrocarbons depend on the physicochemical properties of coals and for most of them, they decrease 242
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coals. Fuel 2004;83:1085–94. [37] Baran P, Zarębska K, Nodzeński A. Energy aspects of CO2 sorption in the context of sequestration of coal deposits. J Earth Sci 2014;25(4):719–26. [38] Dudzińska A, Żyła M, Cygankiewicz J. Effect of disintegration of bituminous coal on the amount of sorbed ethane. Przem Chem 2014;93(2):206–11. [39] Lu P, Liao GX, Sun JH, Li PD. Experimental research on index gas of the coal spontaneous at low-temperature stage. J Loss Prevent Proc 2004;17:243–7. [40] Dai GL. Study on the gaseous products in coal oxidation at low temperature. Coal Mine Safety 2007;1:1–4. [41] Adamus A, Sancer J, Guranova P, Zubicek V. An investigation of the factors associated with interpretation of mine atmosphere for spontaneous combustion in coal mines. Fuel Process Technol 2011;92:663–70. [42] Xie J, Xue S, Cheng W, Wang G. Early detection of spontaneous combustion of coal in underground coal mines with development of an ethylene enriching system. Int J Coal Geol 2011;85:123–7. [43] Wacławik J, Cygankiewicz J, Branny M. Some aspects of endogenous fires. Kraków: Publishing House School of Underground Mining. PAN; 2000.
2013;58(3):867–79. [30] Krzyżanowski A, Zarębska K. The unpolar liquid vapour sorption on coal with various petrographic compositions. Miner Resour Eng 2007;23(3):175–81. [31] Breck DW. Zeolite Molecular Sieves: Structure, Chemistry and Use. New York, USA: John Wiley & Sons, Inc.; 1974. p. 593–724. [32] Gamson PD, Beamish BB, Johnson DP. Coal microstructure and micropermeability and their effects on natural gas recovery. Fuel 1993;72(1):87–99. [33] Clarkson CR, Bustin RM. The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 1. Isotherms and pore volume distributions. Fuel 1999;78:1333–44. [34] Hemza P, Sivek M, Jirásek J. Factors influencing the methane content of coal beds of the Czech part of the Upper Silesian Coal Basin, Czech Republic. Int J Coal Geol 2009;79:29–39. [35] Cai Y, Liu D, Pan Z, Yao Y, Li J, Qiu Y. Pore structure and its impact on CH4 adsorption capacity and flow capability of bituminous and subbituminous coals from Northeast China. Fuel 2013;103:258–68. [36] Ozdemir E, Morsi BI, Schroeder K. CO2 adsorption capacity of argonne premium
243
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Full Length Article
Analysis of sorption and desorption of unsaturated hydrocarbons: Ethylene, propylene and acetylene on hard coals
T
Agnieszka Dudzińska Central Mining Institute, Mines Ventilation Department, 40-166 Katowice Plac Gwarków 1, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal Self-heating of coal Sorption Desorption Unsaturated hydrocarbons
Unsaturated hydrocarbons: ethylene, propylene and acetylene are released into the mine atmosphere as a result of the self-heating of coal. Their concentrations in the mine air is one of the indicators for assessing the degree of development of coal self-heating process. The sorption capacity of coals with respect to hydrocarbons may cause their concentrations to decrease in the mine air, which may affect the correct self-heating assessment. This phenomenon is particularly important in the case of coals with a significant sorption capacity. The paper presents the results of studies on sorption of unsaturated hydrocarbons: ethylene, propylene and acetylene carried out on six samples of Polish hard coals taken from the exploited coal seams. High-porous coals with a well-accessible pore system, large specific surface values, low metamorphism and high oxygen content are characterized by the highest sorption capacities. The hydrocarbon sorbed in the largest amount is acetylene, which is due to the smallest size of the acetylene molecule and its high reactivity. The amount of the sorption of ethylene and propylene is lower than that of acetylene and depends on the properties of the coals. Coals with high sorption capacities adsorb larger amounts of propylene than ethylene. For less sorbent coals, the opposite is true. The volume of sorbed hydrocarbons decreases with the increase of the sorption temperature from 298 to 373 K. The temperature has the greatest influence on the volume of sorbed acetylene, the smaller the ethylene. The gas that is the least sensitive to the temperature increase is propylene. On the basis of desorption studies, it has been found that the lowest degree of desorption from coals is displayed by propylene. Higher degrees of desorption were found for acetylene. Ethylene is desorbed the most from coals. The concentration of ethylene in the mine atmosphere is the most frequently used indicator of temperature increase in coal in the self-heating process.
1. Introduction Coal mines are characterized by self-heating of coal. Heat is released a result of the reaction of hard coal with oxygen from the surrounding air. If this heat is accumulated, the self-heating of coal may occur. Uncontrolled development of this process may cause coal self-ignition and, consequently, spontaneous fire. Self-heating of coal is still a current problem in hard coal mining industry [1–6]. Despite the adopted prevention measures, fires resulting in serious financial losses and a threat to the safety of miners are still noted in the mines. The complex nature of the coal self-heating is the cause of intense research that aims to gain a better understanding of this complex process, and search for new solutions to improve preventive actions [7–14]. Activities regarding fire prevention in the mining industry focus on the early recognition of the phenomenon of self-heating, and taking
measures aimed at supressing it. One of the preventive methods used is the monitoring of gas concentrations in the mine air (carbon monoxide, carbon dioxide, hydrogen and unsaturated hydrocarbons: ethylene, propylene and acetylene), which are released into the atmosphere of the mine as the temperature of the heating coal increases [15–18]. On the basis of the concentrations of these gases in the mine atmosphere, fire indicators are determined, and by knowing their values, the temperature of the heating coal seam can be predicted. Proper assessment of the fire hazard situation in the mine is a key tool to ensure work safety and allows the appropriate measures to be taken in order to prevent the further spread of the self-heating, thus, minimizing the risk of a dangerous spontaneous fire. The gases emitted from the source of self-heating migrate through the mine gobs and some of them may undergo sorption on coals, as coal is a porous material exhibiting the ability to adsorb vapours and gases
E-mail address: [email protected]. https://doi.org/10.1016/j.fuel.2019.02.130 Received 13 September 2018; Received in revised form 3 January 2019; Accepted 26 February 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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[19,12]. Conducted sorption and desorption studies of gases accompanying coal self-heating processes indicated that carbon monoxide and hydrogen are subject to very little sorption [20,21], while unsaturated hydrocarbons: ethylene, propylene and acetylene are sorbed by hard coals in significantly greater amounts [22–24]. The phenomenon of hydrocarbon sorption on coals has the effect on reducing the concentrations of these gases in the mine atmosphere. This in turn influences the values of fire indicators, which in many cases may cause some restrictions in their application and negatively affect the accuracy of coal self-heating assessment. The results of sorption and desorption tests will allow for the improvement of the already used method of fire hazard assessment using the concentration of gases emitted from selfheating sources. This paper is a summary of information about the sorption and desorption processes of unsaturated hydrocarbons: ethylene, propylene and acetylene on hard coals. The paper combines the information presented in previous publications with new data to summarize and systematize all the information regarding the sorption and desorption of unsaturated hydrocarbons on hard coals.
Table 2 Porosity, pore volume determined from mercury porosimetry and values of specific surface area determined from nitrogen sorption (77, 5 K) and carbon dioxide sorption (298 K).
2. Experimental part
2.2. Measurement methodology
2.1. Materials
The sorption tests of coals with respect to hydrocarbons: ethylene, propylene and acetylene were carried out by volume method in the pressure range of 0–0.1 MPa using the Micromeritics ASAP 2010 apparatus. Coal samples with a grain size of 0.5–0.7 mm before the sorption measurement were subjected to a degassing process to purify the coal surface from adsorbed vapours and gases. Before degassing, the sample was placed in a helium atmosphere for 24 h. Helium atoms are non-sorbent, and their kinetic energy allows removing the adsorbed gases from the surface of coal. The degassing was carried out under vacuum until the pressure increase over the sample was not higher than 2 ⋅ 10−1 Pa/min. Sorption isotherms were collected at temperatures 298, 323, 348 and 373 K, close to the actual temperature conditions prevailing in coal mines. The degassing temperature was equal to the sorption measurement temperature. The time of determination of the sorption equilibrium at individual measuring points was long (several hours at each measurement point). The hydrocarbon desorption process was carried out at the same temperatures as the sorption process, gradually reducing the gas pressure over the coal surface with the adsorbed gas layer. The expanded uncertainty of the hydrocarbon sorption measurement was ± 4% rel. with the level of confidence of 95% and the coverage factor k = 2.
Coal
1 2 3 4 5 6
Range (5–7500 nm) pore radius Porosity, %
Pore volume, cm3/g
4.05 6.32 3.39 1.88 2.12 2.87
0.031 0.049 0.026 0.016 0.017 0.019
SBET m2/g
SD-A, m2/g
D-A parameter n
D-R micropore volume, cm3/g
1.54 1.54 0.50 0.49 0.55 1.71
225 148 134 139 106 70
1.54 1.94 1.85 1.55 1.99 2.04
0.056 0.057 0.050 0.037 0.043 0.030
(77.5 K) and according to the D-A model based on carbon dioxide sorption isotherms (298 K) and the volume of micropores according to the D-R model are presented in Table 2.
For the sorption and desorption studies, six samples of hard coals were selected which were taken from the exploited coal seams of Polish mines from the Upper Silesian Coal Basin located in the southern part of Poland. Coal samples for testing were collected in accordance with the applicable standard PN-G-04502:2014-11 ‘Hard coal and lignite. Sampling and preparation of samples for laboratory tests. Basic methods’. The collected samples were crushed in a jaw crusher, ground, and a grain size of 0.5–0.7 mm was separated using Fritsch sieves. Samples prepared in this way were stored under nitrogen until the measurements were started. The chemical, technical and petrographic characteristics of the studied coal samples made on the basis of Polish standards is presented in Table 1. The coal samples were also subjected to structural testing. First, measurements of carbon dioxide sorption isotherms at the temperature of 298 K were made to determine the surface area according to Dubinin-Astakhov (D-A) and volume of micropores according to the Dubinin-Radushkevich (D-R) model. Next, the nitrogen sorption isotherms were measured at 77.5 K in order to determine the specific surface area according to the BET model. For coal samples, porosity and pore volume were also determined by means of mercury porosimetry. The tests were carried out using the Pascal 440CE Instruments apparatus in the pressure range of 0.1–150 MPa. The studied range of pore radius was 5–7500 nm, it included meso- and macropores of coals. Data on porosity determined by the mercury porosimetry method, specific surface values determined according to the BET model based on nitrogen sorption isotherms
3. Results and discussion 3.1. Research on the sorption of unsaturated hydrocarbons on hard coals The obtained sorption isotherms of ethylene, propylene and
Table 1 Characteristics of coal samples. Coal
1 2 3 4 5 6
R0, %
0.71 0.68 0.82 0.85 0.93 1.12
Ultimate analysis (wt.% daf)
Proximate analysis (wt.%, dry)
Maceral and mineral (vol. %)
Cdaf
Hdaf
Ndaf
Sdaf
Odaf
Wa
Aa
Vdaf
V
I
L
M
83.20 82.17 84.69 85.14 88.45 86.07
5.07 5.38 4.83 4.60 5.69 5.10
0.96 1.86 1.46 1.40 1.52 1.49
0.58 1.82 0.32 0.22 1.03 0.71
10.45 8.84 8.82 8.83 3.58 6.79
3.08 4.36 1.52 1.22 1.76 1.23
4.23 4.17 4.25 5.26 3.48 27.45
34.31 39.63 34.76 33.37 35.81 24.87
59 84 53 68 75 67
36 11 14 4 22 32
8 5 33 28 8 1
3 0 3 2 2 16
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-G04560:1998; 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. 233
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10 9
volume adsorbed, cm³/g
8 7 6 acetylene
5
propylene
4
ethylene
3
Langmuir
2 1 0 0
0.02
0.04
0.06
0.08
0.1
0.12
pressure, MPa Fig. 1. Isotherms of sorption of unsaturated hydrocarbons on coal No. 3.
Correlation coefficients are similar for all gases, although slightly lower for acetylene (coal No. 2, 4, 6) and for propylene (coal No. 6). In the case of acetylene and propylene, Langmuir isotherms in the pressure range 0.03–0.08 MPa run above the experimental data. However, above the pressure value 0.08 MPa – they run below the experimental data, especially for coals No. 1, 2, 4 and 5. Data analysis indicates that the value of the mean relative error is a better criterion for conforming the fit of the Langmuir model than the correlation coefficient. The volumes adsorbed of hydrocarbons at 298 K (experimental data) read from the sorption isotherms at the pressure of 0.1 MPa are shown in Table 4. Analysing the obtained test results, it was found that the volumes of adsorbed hydrocarbons change depending on the physicochemical properties of the coals and the type of adsorbed gas. The largest amounts of ethylene, propylene and acetylene are adsorbed by the coals No. 1 and 2. These are coals with a well-accessible structure, their surface area and volume of micropores is the highest among the coals studied. It is worth noting that coal No. 1 sorbs slightly higher amounts of hydrocarbons, especially of propylene, than coal No. 2. The coal No. 1 is characterized by an exceptionally high value of surface area according to D-A. The volume of micropores is comparable to coal No. 2, and the porosity determined by mercury porosimetry is slightly lower (Table 2). Both coals (No. 1 and 2) are characterized by a higher moisture content than the others and a low degree of metamorphism, although the vitrinite reflectance of coal No. 1 with a higher sorption capacity is slightly higher (Table 1). It is worth noting that these coals differ in oxygen content. The oxygen content in coal No. 1 is higher than in coal No. 2, due to the fact that there is a slightly higher amount of oxygen polar groups on its surface. This results in stronger
acetylene determined at a temperature of 298 K on the examined coals are characterized by a similar course, they differ only in the amount of adsorbed gas. Fig. 1 presents exemplary hydrocarbon sorption isotherms for one of the coals. Ethylene sorption isotherms determined on coals No. 1, 2 and 5 are presented in the paper [24], and the sorption isotherm of the propylene on coal No. 1 – in the paper [23]. For coal sample No. 6 – in the paper [25]. The Langmuir isotherm equation [26] was used to describe the sorption equilibria of hydrocarbons on hard coals.
a = am
bP 1 + bP
(1)
where: a – amount adsorbed, cm STP/g, am – maximum adsorption capacity, cm3 STP/g, b – Langmuir constant, P – equilibrium pressure, MPa−1. To assess the quality of the adsorption isotherms equations to experimental data, the mean relative error value was used, as defined below: 3
D=
100 k
k
∑ i=1
aiexp − aisym aiexp
(2)
– experimental data, – calculation data, k – number of where, data points. The coefficients of the equation am, b and D values are shown in Table 3. It also provides correlation coefficients R2. The most favourable matching of the Langmuir equation to the experimental data was obtained for ethylene, where D values do not exceed 5.68%. For propylene and acetylene, D values are higher, a maximum of about 9%. aiexp
aisym
Table 3 Langmuir fit parameters for a single component isotherms. Coal
1 2 3 4 5 6
C2H4
C3H6
C2H2
am, cm3/g, STP
b, MPa−1
D, %
R2
am, cm3/g, STP
b, MPa−1
D, %
R2
am, cm3/g, STP
b, MPa−1
D, %
R2
7.77* 7.70* 5.16 3.95 2.16* 3.15*
31.45* 28.90* 27.29 27.22 23.38* 26.42*
5.65* 5.04* 3.55 4.71 5.68* 4.06*
0.989 0.988 0.993 0.988 0.980 0.990
9.68 7.38 5.05 2.78 1.55 2.03*
86.21 64.52 70.72 65.40 60.29 48.72*
7.97 9.54 6.73 7.83 9.42 8.34*
0.986 0.992 0.994 0.991 0.989 0.977
14.02 14.39 11.60 7.09 6.13 6.84*
42.02 37.04 35.92 30.66 26.31 13.17*
7.23 9.42 4.76 6.44 1.99 2.99*
0.986 0.978 0.992 0.984 0.996 0.984
* Data from publications [24,25]. 234
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participate in the sorption process. In addition, coal No. 6 is also characterized by an exceptionally high content of ash and mineral substance (Table 1), which does not affect the increase in sorption capacity of coal. According to Faiz, the mineral substance is characterized by a small internal surface area [27]. The dependence of the sorption capacity of coals on the degree of metamorphism, oxygen content and the volume of micropores of the coals is presented in Fig. 2. The influence of physicochemical properties of coals on their sorptivity on hydrocarbons is described in detail in the papers [22–24]. The decisive role in the process of hydrocarbon sorption on coals has the degree of metamorphism expressed by vitrinite reflectance. According to the information presented in the reference literature, coals with reflectance of vitrinite in the order of 0.5% adsorb large volumes of hydrocarbons [22,24,25]. With the increase in the degree of metamorphism, the amount of adsorbed hydrocarbons decreases due to the decrease in the amount of oxygen reactive groups present on the surface and higher order of coal structure. For coals with reflectance of the vitrinite of the order and above 1% of the amount of adsorbed hydrocarbons, they are significantly smaller compared to low rank coals. The essential element in the sorption of hydrocarbons is also the availability of the internal coal structure, described by the surface and the volume of micropores as well as the polar character of the coal surface. It favours the electrostatic interactions of electrons π within the bonding that occurs between carbon atoms in hydrocarbon molecules with oxygen polar groups present on the surface of hard coals. The effect of the double bond occurring in unsaturated hydrocarbon molecules on the increase in sorption capacity of hard coals was confirmed in the papers [28,29], in which higher sorption was demonstrated for ethylene than ethane, propylene and propane. In Krzyżanowski and Zarębska's paper [30], the higher sorption capacity of the studied coals in relation to unsaturated hept-1-ene as compared to saturated heptane was also confirmed. Among the unsaturated hydrocarbons considered, the tested coals adsorb acetylene in the largest amount. Although the critical temperature of acetylene, amounting to 308 K, is lower than the critical temperature of propylene of 364 K, the sorption capacity of the examined coals with respect to acetylene is higher than that of propylene. The critical temperature of ethylene is the lowest and amounts to 282 K. The acetylene particles are the smallest of the hydrocarbons tested (acetylene kinetic diameter is 0.33 nm, and ethylene and propylene 0.39 and 0.45 nm, respectively) [31], which allows them to penetrate freely into the smallest pores of hard coals not available for ethylene and propylene. The size of the acetylene molecule, therefore, is determinant of its greatest sorption capacity among the molecules of the unsaturated hydrocarbon considered. This is particularly evident for coals with lower porosity and compact structure (No. 3, 4 and 5), for which the largest difference between the volume of adsorbed acetylene and the volume of other adsorbed hydrocarbons was observed. For these coals, the amount of adsorbed acetylene at a temperature of 298 K is 2–3 times higher than the amount of adsorbed ethylene or propylene. In the case of coals with a higher porosity, a less ordered structure (No. 1, 2), the difference between the volumes of adsorbed acetylene, ethylene or propylene is smaller. Acetylene is adsorbed in less than twice as much as ethylene or propylene. At the same time, there is a triple bond between the carbon atoms in the acetylene molecule that is the cause of the greater reactivity of acetylene compared to ethylene or propylene. The probability of occurrence of electrostatic interactions between reactive acetylene molecules and the surface of hard coals on which both electron-donor and electron-acceptor centres are located is larger than in the case of other hydrocarbons studied. As a result, the volume of adsorbed acetylene is greater than the volume of adsorbed ethylene and propylene. Comparing the amounts of adsorbed ethylene and propylene at the temperature of 298 K, it was observed that the coals with high sorption capacity No. 1 and 2, as well as coal No. 3 adsorb propylene rather than
Table 4 The volume of adsorbed hydrocarbons read from the sorption isotherms at a pressure of 0.1 MPa. Volume adsorbed, cm3/g Coal
Temperature, K
Ethylene
Propylene
Acetylene
1
298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373 298 323 343 373
6.13 4.95 3.89 2.64 5.94 4.91 3.66 2.31 3.87 3.74 3.01 1.93 2.97 2.45 1.88 1.45 1.62 1.47 1.19 1.14 2.37 2.19 1.89 1.60
9.03 5.92 5.45 4.33 6.64 5.19 5.09 4.26 4.66 3.90 3.89 3.13 2.55 1.97 1.90 1.74 1.39 1.53 1.57 1.49 1.89 1.74 1.70 1.65
11.98 9.22 7.43 4.76 11.73 9.02 6.90 4.27 9.39 7.39 5.92 3.79 5.56 4.83 3.97 2.60 4.57 4.55 3.54 2.56 4.08 3.52 2.95 2.23
2
3
4
5
6
interactions with the adsorbed hydrocarbon molecules and thus higher sorption capacity of coal No. 1. Comparing these two coals, it seems that in addition to the availability of a microporous structure, it is the higher content of oxygen in coal No. 1 that largely determines its higher sorption capacity. Smaller amounts of hydrocarbons are adsorbed by coal No. 3. This is a coal with a higher degree of metamorphism than coals No. 1 and 2. Its vitrinite reflectance is 0.82%. At the same time, it is characterized by lower values of porosity, surface determined from nitrogen and carbon dioxide sorption isotherms and the volume of micropores, which results in its lower sorption capacity towards hydrocarbons. Coals No. 4–6 are coals with low sorption capacity. They are characterized by a higher degree of metamorphism compared to others, while coals No. 5 and 6 also have the lowest oxygen content (Table 1). On their surface, there are small amounts of oxygen polar groups responsible for interactions with adsorbed hydrocarbon molecules. The structure availability of coals No. 4, 5 and 6 is small (lower values of porosity micropores compared to other coals). Analysing the results of research for this group of coals, it is difficult to observe an unequivocal correlation between the parameters describing the structure and the sorption capacity of these coals. Among this group, coal No. 5 is characterized by the lowest sorption capacity, despite its structural parameters (porosity, surface area, volume of micropores do not indicate the smallest availability of its structure). Both, the porosity and the pore volume determined by mercury porosimetry and the volume of micropores, determined on the basis of carbon dioxide sorption isotherms for coal No. 5 are higher than those of coals No. 4 and 6. In this case, it seems that the content of oxygen plays a significant role in the description of the impact that physicochemical properties of the coals have on sorption capacity. It is the smallest for coal No. 5 (3.58%) compared to the oxygen content for coals No. 4 and 6 (8.83%, 6.79%). One should pay attention to coal No. 6. Interestingly, it is characterized by an exceptionally high surface area according to BET (1.71 m2/g), but the lowest surface area and volume of micropores of the studied coals, which ultimately results in its low sorption capacity. Probably, this coal has an extensive system of macropores, which are transport routes for the particles of adsorbed hydrocarbons and do not
235
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14 ethylene
volume adsorbed, cm3/g
12
propylene
10
acetylene
8 6 4 2
0 0.5
0.6
0.7
a)
0.8
0.9
1
1.1
1.2
vitrinite reflectance, % 14
volume adsorbed, cm3/g
12 10 8 ethylene 6
propylene acetylene
4 2 0
b)
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0.055
0.06
micropore volume, cm 3/g 14
volume adsorbed, cm3/g
12 10 8 ethylene 6
propylene acetylene
4 2 0
c)
2
4
6
8
10
12
oxygen, %
Fig. 2. Relationship between volume of adsorbed hydrocarbons and factors (a) vitrinite reflectance, (b) micropore volume, (c) oxygen content.
ethylene in a larger amount. The volume of adsorbed propylene by these coals is higher than the volume of adsorbed ethylene by 47, 12 and 20%, respectively. In the case of the discussed coals, the
hydrophilic character of the surface is of great importance. Increased oxygen and moisture content favours the interaction between the oxygen polar groups on the coal surface and the adsorbed propylene 236
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the type of adsorbed gas, the highest impact of temperature was observed on the volume of adsorbed acetylene, the smaller one – ethylene. The sorption capacity of coals in relation to propylene is the least sensitive to temperature increase. Similar observations regarding the low sensitivity of the sorption of propylene to temperature variations are presented in the paper [12] during the flow of gas emitted from the self-heating source by the coal adsorption column. The heat of sorption is the thermodynamic function most commonly used to describe the equilibria in the sorbent – sorbate system. On the basis of the sorption equilibrium isotherms collected at four temperatures, the isosteres in the lnp − 1/T system were determined, and the isosteric heat of sorption was calculated from their inclination according to Clausius-Clapeyron equation.
molecules. Most probably, they are stronger than in the case of ethylene. In addition, these coals are characterized by an open structure, good availability of the sorption system for both gases. Their porosity and pore volume determined by mercury porosimetry are higher than the porosity of the three remaining coals. At the same time, the volume of micropores determined on the basis of carbon dioxide sorption isotherms, that is the basic adsorbing system of hard coals, are higher than the corresponding values for the remaining coals. However, this trend is not confirmed by the value of the surface, especially for coal No. 3. The BET surface area for coal No. 3 is close to the value for other coals with low sorption capacity. At the same time, the area determined according to D-A for this coal is even lower than the surface of coal No. 4 with much lower sorption capacity. For coals with a low sorption capacity, No. 4 ,5 and 6, the volumes of adsorbed propylene at the temperature of 298 K, unlike in the case of coals No. 1–3, are smaller than the volume of adsorbed ethylene by 14, 14 and 20%, respectively. A higher degree of ordering the structure of coals with low sorption capacities makes it difficult to penetrate larger particles of propylene into their pores. Due to the fact the majority of pores in coals is attributed to micro- and sub-micropores of the order of sorbent particles [32–35], coals behave selectively towards the adsorbed gases, showing the properties of the molecular sieve. The micropores with the smallest diameters are therefore not available for larger propylene particles. In summary, among the unsaturated hydrocarbons tested, hard coals adsorb the most acetylene and approximate volumes of ethylene and propylene, depending on the physiochemical properties of coals.
qst = −R
⎛ Δ ln p ⎞ ⎜ Δ1 ⎟ ⎝ T ⎠a
(3)
where: qst – isosteric heat of sorption, kJ/mol, p – equilibrium pressure, Pa, T – temperature of sorption measurement, K, R – gas constant, J/mol⋅K, The obtained heat is a measure of the sorption capacity of the studied coals and allows the assessment of mutual interactions between the sorbate and the adsorbent surface. Fig. 3 shows the obtained heat as a function of the amount of adsorbed gas. The heat values depend on the physicochemical properties of the coals and their sorption capacity. The highest values were obtained for highly-sorbent coals No. 1 and 2, and the lowest values for coals with low sorption capacity, namely No. 5 and 6. Depending on the type of on the sorbed gas, the highest sorption heat is characteristic for acetylene (maximum 33.3 kJ/mol for coal 2). The heat of sorption for ethylene and propylene is comparable (maximum 25.5 and 27.9 kJ/mol for ethylene and propylene respectively), in proportion to the sorption capacity of coals. The exception are coals No. 2 and 3, for which the heats of sorption of ethylene are higher than propylene, contrary to sorption capacity. This is probably due to the shape of the sorption isotherms, which are more steep in the initial section of their course and they run in close range. Together with the increase in the amount of adsorbed gases, a decrease in the sorption heat value is observed. For coals No. 1 and 2 it is small (up to 12%). In the case of coal No. 3, the heat values are reduced by about 20%, for slightly less sorbent coals – by slightly higher values, and for the least sorbent coal – by about 40%. The obtained values of sorption heat for the system: hard coal – hydrocarbons, are not much lower than the value of sorption heat for the system: hard coal – carbon dioxide, most often designated for hard coals [36–37]. In real conditions, the emission of gases from the coal self-heating source takes place with continuous change of environmental parameters, such as: temperature, air flow, inflow of the oxidizing agent and coal mass. These variables are different for each case of coal selfheating and most often their values are not known due to the unavailability of self-heating sources. Due to changes in the temperature of coal in mining conditions, the sorption capacity of coals with respect to hydrocarbons will also change. Generally, as the temperature increases, the amount of adsorbed gases will decrease. The volume of adsorbed propylene will be subjected to the smallest changes. The volume of adsorbed ethylene will decrease to a greater extent, and acetylene will be the largest. For most coals acetylene is the gas adsorbed in the largest amounts, but the effect of temperature results in the fact that volumes of adsorbed acetylene at the highest temperature (373 K) are close to the volume of adsorbed propylene, especially for coals with high sorption capacity.
3.2. Influence of temperature on sorption capacity of coals in relation to unsaturated hydrocarbons In coal self-heating processes in mines, temperature changes are often observed. In this paper, the influence of temperature on the process of hydrocarbon sorption and desorption on the tested hard coals was analysed. The volumes of the adsorbed hydrocarbons read from the sorption isotherms at the pressure of 0.1 MPa for the analysed temperature range are shown in Table 4. With the increase of the sorption temperature in the range of 298–373 K, the volumes of adsorbed hydrocarbons are decreasing. Analysing the test results for the high-sorbent coals (No. 1 and 2) it was found that the influence of temperature on the volume of adsorbed ethylene and acetylene is similar. In both cases, the sorption capacity of these coals decreases by about 60% in the studied temperature range. The decrease in sorption capacity of these coals is relatively even and amounts to about 20% for every 25 K. It is slightly different for propylene, in this case sorption capacity of coals No. 1 and 2 decreases unevenly and to a lesser extent with temperature increase. For coal No. 2, the volume of adsorbed propylene decreases by about 36%, and for coal No. 1 by 52%. For the remaining coals No. 3, 4, 5 and 6, it was observed that the temperature affects the volume of adsorbed acetylene most, while to a lesser extent – ethylene and the lowest – propylene. The volume of adsorbed acetylene decreases by 44–60%, and ethylene by 30–51% in the studied temperature range. In the case of propylene, this decrease is the smallest and amounts to about 32% for coals No. 3 and 4 and 13% for coal No. 6. For coal No. 5 with the lowest sorption capacity, the influence of temperature on the volume of adsorbed propylene is so small that it does not result in the volume of the adsorbed gas reduction. The properties of coal and the type of adsorbed gas are of significant importance in describing the influence of temperature on the volume of adsorbed hydrocarbons. From the point of view of the properties of coals, it should be noted that the temperature has the greatest influence on sorption capacities of coals with high porosity and a well-accessible structure. The influence of temperature on the volume of adsorbed hydrocarbons decreases with the decrease of coal porosity. Considering 237
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35
heat of adsorption, kJ/mol
30 25 20
1 2
15
3 10
4 5
5
6
0
a)
0
0.5
1
1.5
2
volume adsorbed,
2.5
3
cm3/g
35
heat of adsorption, kJ/mol
30 25 1 20
2 3
15
4 10
5 6
5 0 0
b)
0.5
1
1.5
volume adsorbed,
2
2.5
3
cm3/g
35
heat of adsorption, kJ/mol
30 25 1 20
2 3
15
4 5
10
6 5 0
c)
0
0.5
1
1.5
volume adsorbed,
2
2.5
3
cm3/g
Fig. 3. Course of isosteric heats of hydrocarbons adsorption (a) ethylene, (b) propylene, (c) acetylene.
238
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experiment, as a result of lowering the pressure over the coal sample, the lowest degree of desorption from the coal structure is characterized by propylene. Acetylene is desorbed to a greater extent and the most easily desorbed coal is ethylene. Although ethylene molecules are larger than acetylene molecules, they are easier to desorb and are less bonded to the coal surface than acetylene molecules, especially at lower temperatures 298 and 323 K. At higher temperatures of 343 and 373 K, the percentage of desorbed ethylene and acetylene is similar (for coal No. 5 – higher for acetylene). Ethylene is therefore characterized by a greater tendency to desorption from the coals at a lower temperature. This higher tendency of ethylene for desorption is particularly visible for coals with high sorption capacity, e.g. for coal No. 1 and 2 percent of desorbed ethylene at 298 K is higher than for acetylene by 16 and 17, respectively. These coals are characterized by an open and easily accessible structure and a large proportion of micropores, which suggests that a large part of acetylene molecules, which are smaller than ethylene, is located in the micropores and desorption from the smallest pores is more difficult. For the less sorbent coals 4 and 5, the difference between the percentage of desorbed ethylene and acetylene at 298 K is lower and amounts to 6 and 8, respectively. The volumes of desorbed hydrocarbons shown in Table 5 are dependent on the degree of coal metamorphism. With the increase in vitrinite reflectance, the amount of desorbed hydrocarbons is reduced. The exception is coal No. 6, from which more hydrocarbons were desorbed than from coal No. 5 with lower vitrinite reflectance. Analysing the effect of coal structure availability, it was generally found that with the decrease in the availability of the structure, the amount of desorbed gases is generally reduced. From coal No. 1, more hydrocarbons were desorbed than from coal No. 2, although both the porosity, surface area and volume of micropores of coal No. 1 are slightly smaller than coal No. 2. Fewer gases were desorbed from coal No. 5 than it would be indicated by the porosity and volume values in accordance with D-R in comparison to coals No. 4 and 6. No relationship was found between the amount of desorbed hydrocarbons and the surface area determined according to the BET model. A much smaller amount of hydrocarbons was desorbed from coal No. 6 with a surface area equal to 1.71 m2/g, than from coals No. 1 and 2 with a surface area of 1.54 m2/g. In general, it should be emphasized that the BET surface is characteristic only for meso and macropores and does not correlate well with the amount of adsorbed and desorbed hydrocarbons. The largest amount of acetylene was desorbed from the coals studied because of its highest sorption capacity. Comparing the amount of desorbed ethylene and propylene, it was observed that from the coals No. 1–3, a larger amount of ethylene than propylene was desorbed (inversely than in the case of sorption), and from the coals No. 4–6 more ethylene was desorbed than propylene in proportion to the adsorbed amount. The studied coals are characterized by a small percentage of desorbed propylene. Therefore, despite its high sorption capacity, it was desorbed less than ethylene, especially for coals with a low degree of metamorphism. The dependence of the volume of hydrocarbons desorbed from coals on the degree of metamorphism, oxygen content and the volume of micropores of the coals is presented in Fig. 5. Desorption is a process that we observe in mines during the development of the coal self-heating process, when gases, and among them, unsaturated hydrocarbons along with the development of this process are released from the structure of coals going into the atmosphere of the mine. 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. On the basis of the desorption tests carried out, it was observed that the gas among the studied hydrocarbons that is most easily desorbed from coals is ethylene. A high degree of ethylene desorption is already observed at 298 K. For this reason, the content of ethylene in the mine atmosphere is the most frequently used indicator of the increase in the temperature of coal in the self-heating process, and the increase in its concentration is observed at the early stage of self-
Table 5 Volume of hydrocarbons desorbed from coals. Coal
1 2 3 4 5 6
Volume desorbed, cm3/g, T = 298 K Ethylene
Propylene
Acetylene
4.85 4.47 2.99 2.17 0.97 1.54
4.12 3.24 2.28 1.12 0.60 0.97
7.58 6.82 6.75 3.78 2.65 3.54
3.3. Research on desorption of unsaturated hydrocarbons from hard coals Desorption studies of ethylene, propylene and acetylene were carried out to determine the degree of desorption of the gases tested. As a result of the experiment, desorption isotherms were obtained. The course of unsaturated hydrocarbon desorption isotherms was presented in earlier papers [22–24]. In the case of each of the hydrocarbons, desorption isotherms do not combine with the sorption isotherms to form open hysteresis loops. The sorption process is therefore an irreversible process, and the hydrocarbon molecules remaining in the coal structure are more strongly associated with the surface of the coals with the participation of oxygen polar groups [38]. The volumes of desorbed hydrocarbons at 298 K are shown in Table 5. The percentage of desorbed gas, which is the ratio of the volume of gas desorbed to the volume of gas adsorbed at a pressure of 0.1 MPa, was determined on the basis of sorption and desorption isotherms. The percentage of desorbed gas as a function of the desorption temperature is shown in Fig. 4. Analysing the data presented, it was observed that ethylene is a gas which, after being adsorbed, undergoes desorption to a large extent. In the case of coals No. 1, 2 and 3 at 298 K, approximately 75–79% of the absorbed ethylene undergoes desorption, whereas for the remaining coals it is 60–74% of gas. The largest percentage of desorbed ethylene is characteristic for coals with a high porosity and easily accessible structure that adsorbs large amounts of ethylene. With the decrease in the porosity of the coals and, hence, their sorption capacity, the percentage of desorbed ethylene also decreases. With increasing temperature, the percentage of desorbed ethylene increases for highlysorbent coals even up to 92% (coal No. 2). In the case of coals with lower sorption capacity it amounts to 78–88%. Having analysed the results of sorption and desorption studies of propylene at 298 K, it was observed that, for coals of high sorption capacity and for coal No. 3, about 46–49% of sorbed propylene is desorbed. For less sorbent coals it amounts to 43–51%. It is much less than in the case of ethylene. Propylene particles are larger than the ethylene molecules and are more strongly bound to the coal structure, so it is more difficult to pass into the gas phase. The lowest percentage of desorbed propylene was obtained for coals No. 4 and 5. No significant effect of the degree of metamorphism and availability of the coal structure on the percentage of desorbed propylene was found. With increasing temperature, the percentage of desorbed gas increases, reaching about 80% at 373 K for coals with high sorption capacity. For the remaining coals the percentage of desorbed gas at 373 K is 70–75%. Having analysed the results of acetylene sorption and desorption studies, it was found that at 298 K, 58–72% of the adsorbed gas undergoes desorption depending on the properties of the coals. The tendency to release acetylene from the studied coals is to a small extent dependent on their sorption capacity, e.g. coals No. 2 and 5, which adsorb various acetylene volumes, are characterized by the same volume of desorbed gas. As the temperature increases, the percentage of desorbed acetylene increases to a similar value as for ethylene, that is around 85–88%. Summing up the obtained results of the desorption of unsaturated hydrocarbons, it can be concluded that under the conditions of the 239
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100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
a)
0 1
2
3
4
5
6
coal samples 100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
b)
0 1
2
3
4
5
6
coal samples 100 90
extent of desorption, %
80 70 60 298 K
50
323 K
40
348 K
30
373 K
20 10
c)
0 1
2
3
4
5
6
coal samples Fig. 4. Percent of desorbed gas depending on the desorption temperature (a) ethylene, (b) propylene, (c) acetylene.
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8 7
ethylene propylene
volume desorbed, cm3/g
6
acetylene
5 4 3 2 1 0 0.5
a)
0.6
0.7
0.8
0.9
1
1.1
1.2
vitrinite reflectance, %
volume desorbed, cm3/g
8 7
ehylene
6
propylene acetylene
5 4 3 2 1 0 0.02
0.03
b)
0.04
0.05
0.06
micropore volume, cm3/g 8
volume desorbed, cm3/g
7 6 5 ethylene
4
propylene
3
acetylene
2 1 0
0
c)
2
4
6
8
10
12
oxygen, %
Fig. 5. Relationship between volume of desorbed hydrocarbons and factors (a) vitrinite reflectance, (b) micropore volume, (c) oxygen content.
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as the degree of metamorphism increases and the availability of the coal structure decreases.
heating development. The use of ethylene to assess the degree of development of the self-heating process is confirmed by data presented in the literature [39–42]. In the smallest degree, propylene is released from coals, even at 373 K the percentage of desorbed propylene is lower than ethylene or acetylene. The contents of propylene and acetylene appear in the mine atmosphere only at higher temperatures [41,43] and these gases are used in the assessment of the development of coal self-heating process less frequently than ethylene. Upon searching the scientific literature, a distinct lack of bibliographic entries were found, specifically regarding the use of these hydrocarbons in assessing the phenomenon of coal selfheating.
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4. Conclusions The conducted studies of sorption and desorption of ethylene, propylene and acetylene on selected samples of hard coals allowed us to formulate the following conclusions: 1. The volumes of adsorbed unsaturated hydrocarbons depend on the physicochemical properties of coals as well as their porosity, surface area and micropore volume. The largest volumes of hydrocarbons are adsorbed by highly porous coals with high surface and pore volume values, low metamorphism and high oxygen and moisture content. With the increase in the degree of metamorphism and the decrease in porosity and pore volume, sorption capacities of coals with regard to hydrocarbons decrease. 2. For the studied hard coals, acetylene is the hydrocarbon adsorbed in the largest amounts. The volumes of adsorbed acetylene are about 2–3 times higher than the volume of adsorbed ethylene or propylene. Acetylene, due to the triple bond occurring between carbon atoms is characterized by greater availability of electrons of weak bonds π, than ethylene and propylene, hence the electrostatic interactions between acetylene molecules and the surface of coals, on which there are both electron-donor and electron-receptor centres, are stronger than in the other hydrocarbons studied. The acetylene molecules are smaller than the ethylene and propylene particles, which facilitates their penetration into the micropores of the pores of hard coals and results in a larger volume of absorbed acetylene. Comparing the volumes of adsorbed ethylene and propylene, it was found that coals with high sorption capabilities adsorbed lower volumes of ethylene than propylene, while for those less-sorbent coals, ethylene is adsorbed more. 3. The volumes of adsorbed hydrocarbons decrease as the temperature of the sorption process increases from 298 to 373 K. The largest changes with the increase in temperature are connected with the volume of adsorbed acetylene, smaller changes refer to size of sorption of ethylene, and the smallest – propylene. In the case of coal No. 5 with the lowest sorption capacity, the change in temperature does not reduce the volume of adsorbed propylene. The greatest influence of temperature on the volume of adsorbed hydrocarbons was observed for coals with high sorption capacity. With the decrease in sorption capacity of coals, the temperature influence on the volume of adsorbed hydrocarbons also decreases. The determined heat values of hydrocarbon sorption are in the range of maximum 30 kJ/mol. The highest heat values were obtained for acetylene. 4. For the studied coals, the degree of desorption of hydrocarbons from coals has been determined. The highest degree of desorption from coals is characterized by ethylene, slightly lower acetylene, and the gas that is least desorbed from the coals is propylene. The percentage volume of desorbed hydrocarbons from the coal structure depends on the sorption capacity of the coals. The largest percentage of desorbed gas is characterized by coals with high sorption capacity. The volumes of desorbed hydrocarbons depend on the physicochemical properties of coals and for most of them, they decrease 242
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