Article pubs.acs.org/EF
Examination of Low-Temperature Oxidation of Low-Rank Coals, Aiming at Understanding Their Self-Ignition Tendency Hiroyasu Fujitsuka,† Ryuichi Ashida,† Motoaki Kawase,† and Kouichi Miura*,†,‡ †
Department of Chemical Engineering, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
‡
ABSTRACT: With the aim at understanding the self-ignition mechanism of low-rank coals, low-temperature oxidation behaviors of three coals were investigated by measuring the weight change, gas formation rate, heat generation rate, and change of functional groups during the oxidation at the heating rate of 5 K/min in a helium stream containing 22% oxygen. Detailed examination of the initial stage of oxidation below 200 °C that will be closely related to the self-ignition clarified the following: the reactions occurring are the intake of oxygen into aliphatic carbons as peroxides and the decomposition of the peroxides forming carboxyls and H2O. For two low-rank coals tested, the former process was rapid enough and only the latter decomposition reaction of the peroxides to form carboxyl groups was observed, which resulted in the monotonous weight decrease and the H2O production. For a high-rank coal tested, the former process solely occurred below 140 °C and the former process was faster than the latter process below 250 °C, resulting in the monotonous weight increase. The amount of oxygen involved in these processes was found to be only 1.7−4.6 mol of O/100 mol of C of coal. The amount of heat generated during these processes was large enough to raise the coal from 180 to 320 °C.
1. INTRODUCTION Because low-rank coals, which are abundantly deposited and widely distributed, are important resources in this century, they must be used effectively around the world. Their high selfignition tendency, especially when dewatered, however, has made their transportation and storage extremely difficult. This has limited their use to fuel for power plants near mining sites. It is, therefore, essential to develop the methods suppressing the self-ignition of low-rank coals for using them around the world. To do so, it is necessary to investigate both the mechanism and kinetics of self-ignition. Many studies have been carried out to clarify the mechanism of low-temperature oxidation of coals.1−4 Fourier transform infrared spectroscopy (FTIR)5,6 and X-ray photoelectron spectroscopy (XPS)7,8 have been used to observe the change in functional groups in coal during oxidation, and gas chromatography has been employed to quantify gaseous products. It is generally accepted that the aliphatic carbon moieties are main reactants of the oxidation at low temperatures. The aliphatic carbons are first oxidized to form peroxides,9,10 and then the peroxides are further oxidized to form oxygen-containing functional groups, such as aldehyde, carboxyl, esters, and anhydrides, with simultaneous formation of H2O and CO2. The aliphatic carbon moieties are finally converted to CO2, H2O, and CO gases. Oxidation of the aliphatic carbon moieties will generate heat, raise the coal temperature, accelerate the oxidation reaction, and finally ignite coal, leading to self-ignition. Quantitative and kinetic examinations of the low-temperature oxidation process are necessary to relate the oxidation mechanism to self-ignition. In other words, it is requested to estimate the extent, heat of reaction, and rate of the early stage of oxidation reactions that will take place at low temperatures. However, only a few quantitative works have been carried out, except for the works performed by Kelemen’s group to estimate the reaction rate8 and © 2014 American Chemical Society
performed by Kaji’s group to measure the heat of reaction per oxygen.11 This may come from experimental difficulties to examine quantitatively the oxidation reactions that proceed too slowly at the ambient temperature. Then, one of the solutions to overcome the difficulty is to investigate the oxidation behavior at the temperatures high enough to observe it accurately. Then, this study intended to examine the low-temperature oxidation quantitatively. To do so, the weight change, gas formation rate, and heat generation rate during the oxidation of coal were measured at a constant heating rate using a sensitive thermobalance, a micro gas chromatograph (GC), and a differential scanning calorimeter, respectively. Furthermore, the fate of functional groups in coal was observed using an in situ FTIR spectrometer. When all of the results obtained are combined, the mechanism of low-temperature oxidation was examined in more detail. Then, the extent and heat of reaction of the oxidation reactions at the initial stage were estimated.
2. EXPERIMENTAL SECTION 2.1. Coals Used. A brown coal from Australia, Loy Yang, a lignite from U.S.A., Powder River Basin (PRB), and a sub-bituminous coal from U.S.A., Alabama, were used. Each coal was ground and sieved to the particle size less than 0.2 mm and served to experiments without drying. The analyses of the coals, including Brunauer−Emmett−Teller (BET) surface areas, estimated from CO2 adsorption isotherms at 25 °C are given in Table 1. 2.2. Weight Change and Gas Formation Rate. A sensitive thermobalance (Shimadzu, TGA-50) directly connected to a micro GC (Varian, CP-4900) was employed to measure the weight change and gas formation rate simultaneously. To avoid the heat accumulation in the sample, only about 2 mg of sample was placed on a Pt sample pan Received: December 17, 2013 Revised: February 26, 2014 Published: March 31, 2014 2402
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Table 1. Properties of the Samples Used elemental composition (wt %, dafa) Loy Yang PRB Alabama a
proximate analysis (wt %, dbb)
C
H
N
O
volatile matter
fixed carbon
ash
moisture (wt %, arc)
BET surface area (m2/g)
68.5 73.2 81.4
6.5 5.0 5.7
0.5 1.0 1.6
24.5 20.7 11.3
51.9 38.9 40.4
46.6 58.0 55.9
1.6 3.1 3.8
54.1 17.7 1.6
174.1 237.2 74.0
daf = dry and ash-free basis. bdb = dry basis. car = as-received basis.
(6 mm inner diameter and 3 mm height) in the thermobalance and the sample was heated to 110 °C in a helium stream and kept for 30 min to remove the remaining water. Then, the sample was cooled to 40 °C and heated to 700 °C in a helium stream containing 22% oxygen at the heating rate of 5 K/min. The flow rate of gas passing through the thermobalance was 50 mL/min. Gaseous products were analyzed every 80 s to quantify CO using the micro GC with a MS5A column and CO2 and H2O using the micro GC with a PPQ column. It was confirmed that the formation of other gases is negligible from preliminary experiments. 2.3. Changes of Functional Groups in Coal. Changes of functional groups in coal through oxidation were observed using a FTIR spectrometer (JEOL, JIR-WINSPEC50) with a hot stage (JEOL, CS-30) and a microscope (JEOL, MAV 204). The hot stage was attached to the microscope stage for the in situ measurement. About 0.5 mg of sample was leveled and pressed on a sample holder in the hot stage using a spatula. The sample was heated at the rate of 5 K/min from 110 to 500 °C in a 22% oxygen-containing helium stream. The diffuse reflectance spectra were collected by acquisition of 64 scans at 4 cm−1 of resolution at 20 °C of intervals. The infrared (IR) spectra were converted to the Kubelka−Munk (K−M) function. 2.4. Heat Generation Rate. The heat generation rates during the oxidation of the coal were measured using a differential scanning calorimeter (Shimadzu, DSC-600). An aluminum crucible with a lid having a dozen pinholes was used as a sample holder. About 4 mg of coal was placed in the crucible. After the sample in a helium stream was dried at 110 °C for 30 min, the sample in the crucible was heated at the heating rate of 5 K/min to 450 °C in a helium stream containing 22% oxygen. An empty aluminum crucible with a lid was used as a reference. Melting points of indium and zinc were used for calibrations of temperature and heat flow of the differential scanning calorimeter.
Figure 2. Weight change, gas formation rates, and heat generation rate during the oxidation at the rate of 5 K/min for PRB.
3. RESULTS AND DISCUSSION 3.1. Oxidation Behaviors of Coal at the Initial Stage of Oxidation. Figures 1, 2, and 3 show the weight change, gas formation rate, and heat generation rate measured at the heating rate of 5 K/min during the oxidation of Loy Yang, PRB, and Figure 3. Weight change, gas formation rates, and heat generation rate during the oxidation at the rate of 5 K/min for Alabama.
Alabama, respectively. Formations of tar and gases, except for CO, CO2, and H2O, were negligible during the oxidation for all of the coals, as stated above. It is clearly seen that the oxidation behaviors change at around 350 °C for all of the coals. As described later, it was found that most of the functional groups disappeared above 350 °C from the FTIR measurement. Thus, oxidation above 350 °C is the gasification of the so-called char. Because the self-ignition will be closely related to the oxidation at low temperatures, we focused on the oxidation behaviors below 200 °C. To do so, the closeup inset, showing the oxidation behaviors below 250 °C, is given in each figure. Here, we call the oxidation below 200 °C the initial stage of oxidation for simplicity. First, we examined the weight change and gas formation behaviors at the initial stage of oxidation. For Loy Yang, the weight started to decrease at around 120 °C, simultaneously producing
Figure 1. Weight change, gas formation rates, and heat generation rate during the oxidation at the rate of 5 K/min for Loy Yang. 2403
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only H2O, and both the weight decreasing and the H2O formation rates increased monotonously up to 250 °C. At around 240 °C, the formation of CO2 started. For PRB, the weight was kept almost constant up to 150 °C; however, the formation of H2O started at around 120 °C, and H2O was the sole gaseous product below 200 °C. For Alabama, the weight monotonously increased up to 250 °C and H2O started to form above 160 °C. These results suggest that the main reaction occurring at the initial stage was oxidation, producing H2O for Loy Yang and PRB and the oxygen intake for Alabama. Next, we focused on the heat generation rate. The heat generation rates were negative as a result of the sensible heat of the coals, but they turned to positive at around 150 °C and increased monotonously with an increasing temperature up to 250 °C for all of the coals, indicating that the heat generations are closely related to the weight change and formation of H2O below 250 °C. 3.2. Extent of Reaction and Heat of Reaction of the Early Stage of Oxidation. It was found that the reactions below 200 °C, the initial stage of oxidation, are the intake of oxygen and formation of H2O and that the reactions generate heats as stated above. Because the reactions are caused by gaseous oxygen supplied, we then examined the distribution of oxygen consumed quantitatively at the initial stage of oxidation. In other words, we examined how much oxygen was consumed to produce H2O and to be taken in by the coal. This can be performed from the elemental balance using the weight change profile and the H2O formation rate shown in Figures 1−3 with the elemental compositions of the raw coals. First, the amount of oxygen consumed is the sum of the amount of oxygen taken in by the coal and the amount of oxygen involved in the gaseous products formed, H2O, CO2, and CO. Then, the amount of oxygen consumed per 100 mol of carbon involved in the raw coal (daf basis), nO,consum, is expressed as nO,consum =
Figures 4, 5, and 6 show the estimated values of nO,intake, nH2O, nCO2, and nCO against nO,consum for Loy Yang, PRB, and Alabama, respectively. The accumulated amounts of heat generated, which are estimated by integrating the heat generation rate shown in Figures 1−3, are also shown in the figures. Figures 4 and 5 clearly show that all of the oxygen consumed was used to form H2O for
Figure 4. Accumulation of heat generation and oxygen distribution plotted against oxygen consumption below 250 °C for Loy Yang.
100MC (Wcoal + wH2O + wCO2 + wCO) − Wcoal,0 MO Wcoal,0xC (1)
where Wcoal,0 is the weight of raw coal, Wcoal is the weight of coal at a selected temperature, wH2O, wCO2, and wCO are the accumulated weights of H2O, CO2, and CO produced by the temperature, respectively, xC is the weight fraction of carbon in the raw coal, and MO and MC are the atomic weights of oxygen and carbon, respectively. The amount of oxygen involved in the gaseous products formed per 100 mol of carbon involved in the raw coal, nO,gas, is calculated by
Figure 5. Accumulation of heat generation and oxygen distribution plotted against oxygen consumption below 250 °C for PRB.
nO,gas = nO,H2O + nO,CO2 + nO,CO =
2MO M ⎞ M 100 MC ⎛⎜ wH2O O + wCO2 + wCO O ⎟⎟ ⎜ MCO ⎠ MCO2 M H2O Wcoal,0xC MO ⎝ (2)
where nH2O, nCO2, and nCO are the amounts of oxygen involved in H2O, CO2, and CO formed, respectively, and MH2O, MCO2, and MCO are the molecular weights of H2O, CO2, and CO, respectively. Next, the amount of oxygen taken in by the coal per 100 mol of carbon involved in the raw coal, nO,intake, is estimated by eq 3. nO,intake = nO,consum − nO,gas
Figure 6. Accumulation of heat generation and oxygen distribution plotted against oxygen consumption below 250 °C for Alabama.
(3) 2404
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Table 2. Oxygen and Hydrogen Consumptions, Heat of Reaction, and Adiabatic Temperature Increase Estimated from the Reactions Occurring below 200 °C oxygen consumption, nO,consum (mol of O/100 mol of C)
hydrogen consumption in coal (mol of H/100 mol of C)
enthalpy of reaction, ΔHO (MJ/mol of O)
adiabatic temperature increase, ΔTad (°C)
4.6 2.1 1.7
9.2 4.2 0.9
−145 −165 −185
317 181 182
Loy Yang PRB Alabama
both Loy Yang and PRB. The amount of oxygen consumed below 200 °C, at the initial stage of oxidation, was estimated to be 4.6 and 2.1 mol of O/100 mol of C for Loy Yang and PRB, respectively. These values indicated that 9.2 mol of H/100 mol of C, 8% of hydrogen, in Loy Yang and 4.2 mol of H/100 mol of C, 5% of hydrogen, in PRB were converted to H2O below 200 °C, as listed in Table 2. The slope of the accumulated heat generation curve represents the heat of the reaction per 1 atom of oxygen. The heats of the reaction generated by the formation of H2O were also estimated to be 145 and 165 kJ/mol of O for Loy Yang and PRB, respectively. For Alabama coal, on the other hand, the amount of oxygen consumed below 200 °C was estimated to be 1.7 mol of O/100 mol of C and a quarter of the oxygen consumed was taken in by the coal. The heat of the reaction was estimated to be 185 kJ/mol of O. These results suggest that the heat of reaction for H2O production is smaller than the heat generated by the intake of oxygen. If all of the heat generated below 200 °C is completely used for the temperature increase of the coal, the adiabatic temperature increase, ΔTad, is obtained by the equation Cp,coal ΔTad =
xC nO,consum( −ΔHO) 100MC
(4)
where Cp, coal is the heat capacity of coal and ΔHO is the enthalpy of reaction per 1 atom of oxygen. Using this equation under the assumption of 1.26 J g−1 K−1 of heat capacity of each coal, the temperature increase was calculated to be 317 °C for Loy Yang, 181 °C for PRB, and 182 °C for Alabama, suggesting that the reaction or intake of several molecules of oxygen against 100 molecules of carbon of coal may well produce heat large enough to lead self-ignition. 3.3. Mechanism of the Low-Temperature Oxidation. Because the initial stage of oxidation is associated with the functional groups in coal, the changes of functional groups through the initial stage of oxidation were examined to clarify the mechanism of the formation of H2O and oxygen intake by in situ FTIR. Figures 7, 8, and 9 show the in situ FTIR spectra ranging from 1500 to 4000 cm−1 measured during the oxidation for Loy Yang, PRB, and Alabama, respectively. The spectra were acquired from 120 to 380 °C at every 20 °C interval. These figures clearly show how functional groups are changed with the progress of oxidation and finally completely consumed below 380 °C. To examine the changes quantitatively, the spectra ranging from 1500 to 1900 cm−1 and those ranging from 2200 to 3700 cm−1 were curve-resolved to nine peaks by the curve-fitting method.12 The changes of the amount of functional groups with an increasing temperature are shown in Figures 10, 11, and 12 for Loy Yang, PRB, and Alabama, respectively. Although we are focusing on the initial stage of oxidation, we briefly introduce how the functional groups change with the progress of the oxidation. The intensities of aliphatic carbons, corresponding to the peaks at around 2850− 3050 cm−1, decreased gradually with the progress of the oxidation reaction. Corresponding to the changes in the aliphatic carbons, the intensity of the carboxyl group, peak at 1700 cm−1, increased from 140 to 240 °C, the intensity of ester groups, peak at
Figure 7. FTIR spectra measured during oxidation at the rate of 5 K/min for Loy Yang.
1760 cm−1, increased from 180 to 260 °C, the intensity of anhydride, peak at 1850 cm−1, increased from 240 to 320 °C, and finally, all peaks disappeared below 380 °C, indicating that all functional groups were oxidized to form H2O, CO2, and CO. These changes in functional groups are well-associated with the behaviors of gas formation and heat generation below 350 °C shown in Figures 1−3. In comparison of the weight change and fate of functional groups for Alabama coal, it was found that aliphatic carbons were decreased from 140 °C, although the weight was increased below 140 °C, suggesting that there was an intermediate between aliphatic carbons and carboxyl groups. This intermediate was probably peroxides, which are difficult to detect directly but are commonly believed to be formed.9,10 Then, the whole oxidation process of the functional groups may be summarized, as shown in Figure 13. Now, we focus on the change of the functional groups at the initial stage of oxidation. When the above discussion is summarized, the reactions occurring below 200 °C are the intake of oxygen onto aliphatic carbons as peroxides and the decomposition of the peroxides forming carboxyls and H2O. For Alabama, the former process solely occurred below 140 °C and the former process was faster than the latter process below 200 °C and, hence, the weight monotonously increased below 200 °C. For Loy Yang and PRB, on the other hand, the former process was rapid enough and only the latter decomposition reaction of the peroxides to form carboxyl groups was observed, which resulted in the monotonous weight decrease and the H2O 2405
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Figure 10. Changes in the functional groups in Loy Yang during oxidation at the rate of 5 K/min. Figure 8. FTIR spectra measured during oxidation at the rate of 5 K/min for PRB.
Figure 11. Changes in the functional groups in PRB during oxidation at the rate of 5 K/min.
production below 200 °C. These reaction processes occurring below 200 °C will be closely related to the self-ignition of coal, as stated earlier.
4. CONCLUSION Aiming at understanding the self-ignition mechanism of low-rank coals, low-temperature oxidation behaviors of three coals were
Figure 9. FTIR spectra measured during oxidation at the rate of 5 K/min for Alabama. 2406
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Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 12. Changes in the functional groups in Alabama during oxidation at the rate of 5 K/min.
Figure 13. Schematic figure of the mechanism of low-temperature oxidation.
investigated by measuring the weight change, gas formation rate, heat generation rate, and change of functional groups during the oxidation in a helium stream containing 22% oxygen at the heating rate of 5 K/min. Detailed examination of the initial stage of oxidation below 200 °C that will be closely related to the selfignition clarified the following: the reactions occurring are the intake of oxygen into aliphatic carbons as peroxides and the decomposition of the peroxides forming carboxyls and H2O. For the two low-rank coals tested, the former process was rapid enough and only the latter decomposition reaction of the peroxides to form carboxyl groups was observed, which resulted in the monotonous weight decrease and H2O production. For the high-rank coal tested, the former process solely occurred below 140 °C and the former process was faster than the latter process below 250 °C, resulting in the monotonous weight increase. The amount of oxygen involved in these processes was found to be only 4 or 5 molecules per 100 molecules of carbon of coal. The amount of heat generated during these processes was large enough to raise the coal from 180 to 320 °C. These examinations will be useful to clarify the mechanism of the selfignition of coal.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +81-774-38-3420. Fax: +81-774-38-3426. E-mail:
[email protected]. 2407
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