Estimating the Spontaneous Combustion Potential of Coals Using

Feb 4, 2014 - Conversely, low-reactive samples, such as Pocahontas #3, Deep Navigation, and Bulli, showed the highest weight gain and Tpeak values ...
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Estimating the Spontaneous Combustion Potential of Coals Using Thermogravimetric Analysis Claudio Avila,†,‡ Tao Wu,§ and Edward Lester*,† †

Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom § Division of Engineering, University of Nottingham, 315100 Ningbo, People’s Republic of China ABSTRACT: In this paper, the reactive properties of coals associated with low-temperature oxidation were studied using thermogravimetric analysis (TGA). Coal samples were pulverized into a size fraction of 200 >200 177 156

171 80−96, >200 82−110, >200 105, 176 176

155 177 173 160 175 170 166 154 153 191 >200 167 173 166 146 165 127 147

98, 176 167 144 113, 133 89, 160 158 184 149 144 173 73−98, >200 202 155 185 170 132 156 172

142 190

112 171

a Samples were organized by decreasing volatile content. bCrossing between the temperature in the center of the furnace and the center of the sample.23 cCrossing temperature obtained from two adjacent thermocouples at the center of the sample.26 dCoals with two crossing points identified. eCoal prone to spontaneous combustion. fOld sample (more than 3 years old, stored in a small particle size and with some exposure to air).

samples using the heating rates of 3 and 5 °C min−1, and the results are provided in Table 4. For the full set of coals, the average Tstart and Tpeak values were the smallest at the lowest heating rate. The weight increase varied from 0.0 to a maximum of 4.37% (at 3 °C min−1), showing a good correlation between the two heating rates (0.97% R2 with Bulli is excluded as an outlier), with the highest weight gain levels obtained at the lowest heating rate. This could be explained by the increased contact time at 3 °C min−1. The reduction in the heating rate from 5 to 3 °C min−1 resulted in an increase of up to 80% in oxygen adsorption for specific samples. The characteristic temperature values (Tstart and Tpeak) recorded during the experiments were higher than those mentioned in the literature for the theoretical mechanism.6−9 When this behavior is extrapolated to an actual heating rate of a spontaneous combustion event (where an increase of 70 °C in 2 days is equivalent to 0.02 °C min−1), a much lower temperature range can be found if applied directly in TGA. TGA heating rates were therefore selected on the basis of what would be considered reasonable, in terms of time requirements per sample, and what was programmable to still allow for accurate heating control in TGA. In addition, the net adsorption temperature range ΔT is linked to the net weight gain produced by the oxygen adsorption, which

TGspi =

Δ weight loss rate Δ temperature

(% °C−1 min−1)

High values of TGspi were found when the weight loss rates tended to be greatly influenced by a preset temperature change, meaning that the chemical reaction developed faster because of 1767

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Table 3. Proximate Analysisa as received

a

dry and ash-free basis

coal

moisture (wt %)

volatiles (wt %)

fixed carbon (wt %)

ash (wt %)

volatiles (wt %)

fixed carbon (wt %)

carbon/volatiles (wt %)

La Loma Fenosa Hambach Illinois #6 Kaltim Prima Orupka El Cerrejon La Jagua Asfordby Nadins Lea Hall Daw Mill Ironbridge Bentinck Indiana Littleton North Dakota Indo Blue Creek Hunter Valley Zondag 1 Yanowice Goedehoop Kleinkopje Bulli Pocahontas #3 Deep Navigation

3.8 10.9 37.5 3.7 3.5 6.0 3.5 4.3 5.5 7.4 5.5 3.3 4.1 3.6 2.8 5.0 22.0 3.8 3.5 2.2 3.9 5.1 2.3 3.5 0.9 0.6 1.0

45.1 43.3 38.1 38.0 38.0 37.7 36.7 36.2 36.1 35.6 35.2 34.9 34.9 33.9 33.2 31.9 31.2 30.8 28.9 27.3 27.0 26.9 25.7 22.8 22.3 18.8 10.1

39.3 43.8 23.1 43.8 54.5 48.0 58.1 58.4 48.6 50.4 57.4 56.3 52.4 56.8 53.1 51.5 39.6 50.9 59.7 52.3 61.3 60.8 57.9 54.9 67.5 75.8 81.8

11.9 2.0 1.3 14.6 4.0 8.3 1.8 1.2 9.8 6.7 2.0 5.5 8.6 5.7 10.9 11.6 7.2 14.4 7.9 18.2 7.8 7.3 14.1 18.8 9.4 4.9 7.1

53.5 49.7 62.2 46.5 41.1 44.0 38.7 38.3 42.6 41.4 38.0 38.3 40.0 37.4 38.5 38.2 44.1 37.7 32.7 34.3 30.6 30.7 30.7 29.3 24.8 19.8 11

46.5 50.3 37.8 53.5 58.9 56.0 61.3 61.8 57.4 58.6 62.0 61.7 60.0 62.6 61.5 61.8 55.9 62.3 67.3 65.7 69.4 69.3 69.3 70.7 75.2 80.2 89

0.9 1.0 0.6 1.2 1.4 1.3 1.6 1.6 1.3 1.4 1.6 1.6 1.5 1.7 1.6 1.6 1.3 1.7 2.1 1.9 2.3 2.3 2.3 2.4 3.0 4.0 8.1

Samples were organized by decreasing volatile content.

Figure 1. Thermogravimetric profiles of coals obtained using a slow heating ramp in air (3 °C min−1). At low temperatures, samples showed an evident weight increase. (A) British coals, (B) South and North American coals, (C) Australian, South African, and Indonesian coals, and (D) lignite samples. 1768

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Table 4. Maximum Weight Gain and Characteristic Temperatures Obtained for Two Heating Ratesa heating rate 3 °C min−1

a

coal

weight increase (%)

Fenosa Orupka North Dakota Hambach Nadins Daw Mill La Jagua Asfordby Bentinck Yanowice Littleton Lea Hall La Loma Zondag 1 Indiana Indo Hunter Valley Kaltim Prima Ironbridge Goedehoop Kleinkopje Blue Creek Illinois #6 El Cerrejon Bulli Deep Navigation Pocahontas #3

0.00 0.00 0.00 0.00 0.00 0.44 0.73 0.77 0.88 0.93 1.07 1.19 1.24 1.27 1.38 1.50 1.52 1.53 1.81 2.04 2.13 2.25 2.35 2.35 3.22 3.98 4.37

Tstart (°C)

147 124 142 152 131 136 132 131 118 124 112 151 139 121 121 123 153 120 121 105 121 112

5 °C min−1 Tpeak (°C)

ΔTb (°C)

weight increase (%)

Tstart (°C)

Tpeak (°C)

ΔTb (°C)

95 108 92 106 126 114 115 130 122 145 132 137 137 149 134 179 164 147 148 176 185 183

0.00 0.00 0.00 0.00 0.00 0.08 0.61 0.56 0.54 0.65 0.71 0.82 1.14 1.36 1.09 1.58 1.32 1.21 1.50 0.45 1.91 1.95 2.52 2.20 1.91 3.46 4.17

117 169 152 170 145 156 142 122 138 135 126 155 151 122 144 120 183 114 124 138 133 103

248 281 242 267 266 263 252 256 285 282 274 297 284 265 236 295 329 267 266 318 313 326

131 112 90 97 121 107 110 134 147 147 148 142 133 143 92 175 146 153 142 180 180 223

242 232 234 258 257 250 247 261 240 269 244 288 276 270 255 302 317 267 269 281 306 295

Samples were organized by increasing weight gain at 3 °C min−1. bΔT = Tpeak − Tstart.

not showing a weight gain can be associated with a process in which moisture desorption is predominant, but also this might be linked to samples in which the formation of less stable surface oxides are formed and decomposed very rapidly at low temperatures. This type of profile was characteristic of coals prone to spontaneous combustion, such as Fenosa and North Dakota, and high-reactive coals, such as Orukpa. In the second case, a slight weight gain is an indication that the formation of stable surface oxides is partially favored over the formation of less stable complexes, showing desorption and thermal decomposition in a wider temperature range. This profile was characteristic of coals, such as El Cerrejon, Kaltim Prima, and Illinois #6, which are reactive and have the potential to suffer from thermal runaway, under certain conditions. The third type of profile was characterized by a sharp weight increase because of a net oxygen adsorption, as a result of the formation of stable coal−oxygen solids across a wide temperature range over the coal surface. The shape of this curve was characteristic of low-reactive coals, such as Pocahontas # 3, Deep Navigation, and Bulli. A connection was found between these three groups and the historical data about reactivity and incidents of spontaneous combustion of the studied samples. Hence, the type of TGA profile, the total mass gain, and the “starting” and “peak” temperatures are key parameters to classify coals according to their potential to undergo self-oxidation. 3.3.2. Spontaneous Combustion and Ignition. The experimental characteristics of the proposed thermogravimetric tests clearly relate to the particular steps described in the

Figure 2. Relationship between the weight increase and the net adsorption temperature range (ΔT). Coals prone to spontaneous combustion showed nil weight increase.

the high reactivity of the sample. Conversely, low values of TGspi were obtained when the weight loss rates were less influenced by the same preset temperature change, which is a sign of low reactivity. 3.3. Thermogravimetric Characteristics Associated with Self-Oxidation. 3.3.1. Oxygen Adsorption. In the early stages of oxidation, three characteristic weight loss curves can be identified from TGA profiles (Figure 6). In the first case, samples 1769

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Figure 3. Thermogravimetric derivative curves obtained for standard coal samples using different heating rates. At low temperatures, all samples showed a linear segment during the volatilization stage.

Figure 4. Slopes of the derivative curve obtained during the linear volatilization stage, plotted against the heating ramp applied for two representative coals. This second-order profile is unique for each sample.

literature;9,11,44 i.e., “oxygen adsorption” is as the chemisorption step as the “reactivity” is to the burning. The oxygen adsorption test appears to be closely linked to “spontaneous combustion”, while the TGspi method is more closely linked to the ignition phenomenon and subsequent high-temperature combustion, noticed when comparing the crossing point temperatures (shown in Table 2) to the TGspi values obtained. The nature of both reactions is totally different, and the key to obtain a reliable prediction of the self-heating phenomenon is to differentiate between them. The likelihood of overlap between these two reaction steps depends upon a much larger number of variables, such as the size of the coal deposit, oxygen diffusion

through the pile, heat dissipation, and ignition temperature of the coal.10,18 This overlapping also introduces some uncertainties when seeking to combine the results from both TGA tests. For instance, coals with a zero weight increase appear to continue to react faster with air at high temperatures, e.g., Fenosa and North Dakota coals, which is confirmed by the high values of the TGspi index of these samples. Alternatively, coals with a high mass increase, e.g., Pocahontas #3, Goedehope, and Hunter Valley, seem to be more reactive at higher temperatures than coals with smaller weight gains, e.g., Nadins and Daw Mill coals. Consequently, unreactive coals at low temperatures can be very reactive during combustion and be high-risk coals if they 1770

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Table 5. Slope Values Obtained from the Linear Devolatilization Stage at Different Heating Rates and the Corresponding TGspi Index Calculated for Each Coala slope during the linear devolatilization stage (% min−2)

a

coal

3

5

7

10

20

TGspi index (% °C−1 min−1)

r2

North Dakota Orukpa Fenosa Illinois # 6 Kaltim Prima Pocahontas #3 El Cerrejon La Loma Goedehoop Hunter Valley Blue Creek Indo Lea Hall La Jagua Ironbridge Asfordby Bulli Zondag 1 Littleton Nadins Hambach Kleinkopje Daw Mill Indiana Yanowice Bentinck

0.038 0.041 0.029 0.021 0.022 0.033 0.021 0.018 0.020 0.020 0.021 0.020 0.019 0.018 0.018 0.023 0.032 0.021 0.017 0.016 0.008 0.012 0.014 0.012 0.013 0.008

0.132 0.089 0.105 0.063 0.054 0.073 0.060 0.052 0.042 0.050 0.053 0.044 0.046 0.042 0.048 0.053 0.047 0.042 0.041 0.031 0.028 0.032 0.035 0.033 0.032 0.026

0.203 0.195 0.165 0.095 0.102 0.134 0.092 0.082 0.091 0.089 0.094 0.090 0.075 0.073 0.078 0.093 0.088 0.087 0.070 0.067 0.035 0.049 0.060 0.051 0.053 0.038

0.484 0.363 0.373 0.215 0.191 0.237 0.197 0.181 0.169 0.179 0.169 0.149 0.137 0.130 0.125 0.165 0.145 0.127 0.120 0.108 0.128 0.088 0.105 0.093 0.092 0.076

1.831 1.315 1.301 0.710 0.661 0.645 0.582 0.557 0.560 0.528 0.515 0.499 0.426 0.423 0.438 0.417 0.433 0.404 0.374 0.327 0.343 0.288 0.279 0.255 0.261 0.169

0.109 0.078 0.077 0.042 0.039 0.037 0.034 0.033 0.033 0.031 0.03 0.029 0.025 0.025 0.025 0.024 0.024 0.023 0.022 0.019 0.018 0.017 0.016 0.015 0.015 0.010

0.967 0.972 0.974 0.976 0.975 0.992 0.985 0.982 0.979 0.986 0.985 0.979 0.982 0.979 0.973 0.995 0.973 0.981 0.983 0.985 0.973 0.978 0.994 0.991 0.990 0.997

Samples were organized by decreasing TGspi value.

content, which masks the net formation of solid oxides in the surface while appearing highly reactive. For instance, Hambach coal has shown the characteristic profile associated with a reactive coal during the oxygen adsorption test (Figure 1D), losing 40% of the total mass before 100 °C. However, this can be explained by the extreme water content of this coal, ∼37.5%, which masks any oxygen adsorption. However, the TGspi index strongly indicates that this is an unreactive sample. A drying step for these types of coals could reveal the real oxidative tendency, although it is generally accepted that coals with extremely high water contents are naturally less prone to self-oxidation because of the elevated energy requirements to remove the free water required before suffering a thermal runaway. Another limitation of the oxygen adsorption test relates to sample aging. Reactive coals that have been in contact with air for long periods of time “lose” their reactive potential (weathering process).1,2 In such cases, there is a reduction to the number of active sites available to react, producing also more stable coal− oxygen solids that break up at higher temperatures.3,7 This problem can also affect the intrinsic reactivity test, where changes to the TGspi index were observed with sample aging, particularly with reactive coals. For instance, Kleinkopje coal produced an unreactive (or “safe”) TGA profile during the oxygen adsorption test (Figure 1C), and the TGspi index has categorized this coal as low reactive. From historic data, fresh Kleinkopje coal can be highly reactive and there have been several incidents of spontaneous combustion in South Africa.45 The Kleinkopje sample used for the experiment was more than 3 years old, stored in a small particle size with some exposure to air, which explained the results obtained for this sample.40 This highlights the

Figure 5. Comparison of the second-order profile for the full set of samples studied.

have low ignition temperatures. Clearly, the most risky coals are those that are reactive in both circumstances. 3.3.3. Limitations of the Techniques. A particular limitation of the oxygen adsorption test is when coals contain high water 1771

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Figure 6. Characteristic thermogravimetric profiles during the net oxygen adsorption stage. The shape of the thermogravimetric curve relates to the stability of the coal−oxygen complex formed and, consequently, the potential to undergo self-oxidation.

Table 6. Coal Classification test 1, oxygen adsorption group increasing reactivity →

observation

test 2, TGspi index coal

1

weight increase not seen

North Dakota Orukpa Fenosa Nadins Hambach Daw Mill La Jagua Asfordby Bentinck Yanowice

2

slight weight increase (0.1−1.0%)

3

medium weight increase (1.0−2.0%)

4

important weight increase (2.0% and up)

requirement for fresh coal samples to ensure accurate testing. Some practical steps when dealing with the sample preparation could help to minimize the impact of weathering. First, if the age of the coal is unknown, the largest particles must be selected and ground because these are less affected by the oxygen oxidation but will deliver the highest possible reactivity when ground finer. Second, when coal is pulverized, the time between the milling and the TGA tests must be as short as possible, and it is recommended to be less than 24 h. Finally, an adequate storage

Littleton Lea Hall La Loma Zondag 1 Indiana Indo Hunter Valley Kaltim Prima Ironbridge Goedehoop Kleinkopje Blue Creek Illinois #6 El Cerrejon Bulli Deep Navigation Pocahontas #3

observation

coal

high reactive (TGspi > 0.05)

North Dakota Orukpa Fenosa

reactive (TGspi = 0.05−0.03)

Illinois #6 Kaltim Prima Pocahontas #3 El Cerrejon La Loma Goedehoop Hunter Valley Blue Creek Indo Lea Hall La Jagua Ironbridge Asfordby Bulli Zondag 1 Littleton Nadins Hambach Kleinkopje Daw Mill Indiana Yanowice Bentinck

low reactive (TGspi = 0.03−0.02)

non-reactive (TGspi < 0.02)

of the samples after collection prior to testing should be considered to prevent aerial oxidation. A widely used method is keeping the samples frozen or at low temperatures from ∼0 to 4 °C, avoiding oxidation and moisture loss. An alternative might be to keep the samples under non-oxidative conditions (e.g., storage in a N2 atmosphere). 3.4. Coal Classification. Coals were classified into four categories according to the characteristic parameters measured (Table 6). In these groups, increasing values for weight gain (i.e., decreasing reactivity toward self-oxidation) were contrasted with 1772

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TGspi values and ordered in a descending sequence (i.e., decreasing propensity to spontaneously combust). Although trends appear to exist in the data, results confirm the different nature of both tests. From this classification, all samples included in group 1 were wellknown high-risk coals, with the exception of Nadins and Hambach. Group 2 included some reactive samples, such as Kaltim Prima, El Cerrejon, and Illinois #6. Conversely, Kleinkopje coal in group 4 was the only sample found unreactive in both tests, although it must be considered as an old sample. Finally, because the information obtained from these tests is unique in terms of the variables measured, it must be contrasted with alternative tests40,41 to be fully validated.

4. CONCLUSION TGA is a valuable tool for assessing reactive coal properties at low temperatures. Particularly, the detection of the mass increase between 100 and 250 °C, produced by the oxygen adsorption in the coal surface, seems to be a clear indicator of self-oxidation potential. Experimental results agree with the explanation provided by the reaction mechanism of coal self-oxidation described in the literature, which relates to the formation of oxygenated carbon−solid complexes on the coal surface. Subsequently, the stability of these solid complexes can be determined by the magnitude of the mass gained during sample heating. Highly reactive coals prone to spontaneous combustion tend to produce unstable solid complexes, which are thermally decomposed at low temperatures, releasing heat and showing a nil mass increase. Low-reactive coals produce highly stable solid complexes that break up at much higher temperatures, reacting more slowly, producing a visible mass increase. TGA was also useful in determining how the reactivity of coals vary with the temperature. In this case, mass loss changes were associated with the heating rates to calculate a second-order parameter, which indicates the tendency of coals to react across a temperature gradient. The nature of these results appears to be related to ignition and combustion rather than low-temperature oxidation. This procedure was complementary to the oxygen adsorption test. As such, these tests, using TGA, provided a potential new approach for identifying coals prone to self-oxidation.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +44 (0)115 9514974. E-mail: edward.lester@ nottingham.ac.uk. Present Address

‡ Claudio Avila: Hull Research and Technology Centre (HTRC), BP Chemicals Limited, Saltend Lane, Hull HU12 8DS, United Kingdom.

Notes

The authors declare no competing financial interest.

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