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The reburning thermal characteristics of residual structure of lignite pyrolysis
Fuel 259 (2020) 116226
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Full Length Article
The reburning thermal characteristics of residual structure of lignite pyrolysis
T
Haihui Xina,b,c,1, Hetang Wanga,b,c,1, Wenjie Kanga,b,c, Cuicui Dia,b, Xuyao Qia,b,c, ⁎ Xiaoxing Zhonga,b, , Deming Wanga,b, Fangming Liud a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, No. 1 University Road, Xuzhou, Jiangsu 221116, China Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China c International Research Center for Underground Coal Gasification, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China d School of Marxism, Shanghai University of Finance and Economics, Shanghai 200000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignite Pyrolysis residue Reburning Thermal characteristics
The reburning of pyrolysis residual is very important in coal fire control and preventing fire resurgence. This paper researched the pyrolysis and reoxidation characteristics of lignite using thermogravimetric and infrared analysis. The macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues were analyzed. The critical temperature for lignite pyrolysis and pyrolysis residues reburning. The results show that a significant turning point before 450 °C such as the variation of pyrolysis residual weight, the variation of constant temperature residual weight, and the pyrolysis residual combustible component weight. The aliphatic hydrocarbons are gradually reduced due to the side chain cracking in the coal, and the aromatic hydrocarbons increase with the degree of aromatization of the coal structure in lignite pyrolysis. The pyrolysis residue at ~400 °C is the easiest to reburning, and the pyrolysis residue at ~450 °C had the strongest reburning intensity.
1. Introduction Lignite is rich in reserves, accounting for 13% of the total coal resources in China. Lignite is a very important chemical industry-used coal. However, lignite is the coal with the lowest rank of coal mineralization. Lignite has large internal pores, many oxygen-containing functional groups. Lignite is the easiest spontaneous combustion of coal compare compared with other rank coals. Thus, Lignite mining faced the serious problem of coal spontaneous combustion and reburning in coal fire zone, which greatly restricts the safe and efficient mining of lignite resources in China [1–3]. Along with the increasing of coal chemical industry, the demand for lignite is also growing. The problem of safe production caused by the spontaneous combustion of lignite and reburning in fire zones is increasingly prominent [4–6]. At present, many researches have been done on the mechanism and characteristics of coal spontaneous combustion [7–12], especially the lignite which is easiest spontaneous combustion coal. However, there was few researches on the reburning characteristics of coal in coal fire zone. The reburning problem in coal fire area has not been solved. The
coal fire reburning is most common in lignite mining zones. Reburning of residual in coal fire area is mainly caused by the thermal decomposition reaction of coal around the coal fire source. The internal structure and composition of coal have undergone major changes under the pyrolysis. The macromolecular structure is broken, resulting in more easily oxidized small molecular structures, more active sites and flammable materials such as tar and volatiles were in pyrolysis residual which may be easier oxidation than raw coal. The reburning of fire zone will be occurred easily. Thus, it is necessary to research the structural residual and reburning characteristics of lignite pyrolysis in different temperature. Thermogravimetric analyzer is the most commonly methods to study the processing characteristics of coal pyrolysis and combustion [13–21] controlled. Such as Chen et al. [22] made the char samples burned in a thermogravinietric analyzer to study the intrinsic reactivity of char samples. He et al. [23,24] tested nine char samples in thermogravimetric analyzers with continuously rising temperatures and a global one-step kinetic reaction model was used to describe the char combustion. Jiang et al. [25] studied the chemical properties, pyrolysis
⁎
Corresponding authors at: State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, No. 1 University Road, Xuzhou, Jiangsu 221116, China. E-mail addresses: [email protected] (H. Xin), [email protected] (X. Zhong). 1 Co-first author: These authors contributed equally to this work. https://doi.org/10.1016/j.fuel.2019.116226 Received 23 April 2019; Received in revised form 15 August 2019; Accepted 17 September 2019 Available online 24 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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350 °C, 400 °C, 450 °C and 500 °C respectively at the heating rate of 10 °C/min under nitrogen (100 ml/min). All thermoanalysis experiments keep 30 min at outlet temperature until the mass is nearly constant. Based on the thermoanalysis results, the pyrolysis characteristic parameters was analyzed and compared with the changing of functional groups in coal to investigate the pyrolysis process characteristic and residual structural feature. Then the reburning thermogravimetric experiments of coal pyrolysis residual were carried out.
behaviors of Shenmu (SM) coal using the thermal analysis method. Based on the applications of thermal analysis methods in numerous researches, it can measure the whole process of coal pyrolysis and combustion very well. It is very suitable to measure the residual characteristics of coal pyrolysis and prepare pyrolysis residue samples. The functional groups distribution characteristics of coal and pyrolysis char were analyzed by analytical chemical techniques such as nuclear magnetic resonance (NMR) [26,27] and Fourier transform infrared spectroscopy (FTIR) [28–32]. Due to the simple and affordable technique of FTIR, especially diffuse reflectance FTIR (DRIFT) [33–35], many researches using FTIR study the changes of functional groups in coal pyrolysis and combustion [36–43]. Such as Niu et al. [44] investigated the evolution of six main functional groups in two coking coals with different ranks by FTIR. Thus, the FTIR technology can give a rapid and affordable results of functional groups for coal pyrolysis residue. Based on the functional groups changes, the content of aromatic hydrocarbon and aliphatic groups could be described to explore the structural transformation in coal pyrolysis including the producing of semicoke and tar. Some researchers have analyzed the relationship of functional groups changes and tar product during coal pyrolysis. Such as Song et al. [45] studied the importance of aromatic structures in tar to the destruction of tar itself during the volatile-char interactions, and found the aromatic structures in tar are more reactive with char than the non-aromatic structures. These analysis process and results can help analyzed the functional groups distribution characteristics of coal pyrolysis residue. The thermal analysis and FTIR method can provide the feasible testing technology for exploring the pyrolysis and reburning characteristics of lignite. The Shengli coalfield (SL) is one of the major lignite production areas in China, which is the important raw coal resource of coal chemical industry distributed in Inner Mongolia, Shanxi of China. Thus, this paper focused on SL lignite coal. The lignite coal samples would be subjected to constant temperature pyrolysis thermogravimetric analysis at 350 °C, 400 °C, 450 °C and 500 °C. The macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues would be analyzed. Simultaneously, the reburning characteristics of coal pyrolysis residual structure would be tested. The results can help to reveal the characteristics of the coal fire reburning during lignite mining. It is great significance for preventing and controlling the lignite reburning after spontaneous combustion effectively, and ensuring the safe and efficient mining of lignite.
After coal pyrolysis at 350 °C for 30 min, the residue is cooled to room temperature under nitrogen. The oxidation thermal analysis experiment of residue coal is from 30 °C to 900 °C at the heating rate of 5 °C/min under air (100 ml/min). The same experiments with step (1) for lignite pyrolysis to 400 °C, 450 °C and 500 °C. Thus, the reoxidation characteristics of lignite pyrolysis residue at different temperatures were obtained. As a contrast, the pyrolysis of lignite raw coal were tested at the heating rate of 10 °C/min under nitrogen (100 ml/min) from 30 °C to 900 °C. The oxidation of lignite raw coal were tested at the heating rate of 5 °C/min under air (100 ml/min) from 30 °C to 900 °C. 2.3. Infrared spectrum experiments In order to analyzed the structural properties of lignite at different pyrolysis temperatures and revealed the evolution mechanism of the functional groups in the pyrolysis stage of SL lignite, the infrared spectrum experiments were carried out. The functional groups changes under different pyrolysis reaction temperatures could be obtained. Thus, the pyrolysis residue of SL lignite from 350 to 500 °C with 50 °C intervals were performed in a Nicolet 6700 FTIR instrument (Thermofisher Scientific, Unit States) using the diffuse reflection facility. The wave number is from 650 cm−1 to 4000 cm−1, with a resolution of 4 cm−1 and scanning number of 64 scans. Pure ground KBr was used to obtain the reference spectrum before testing the coal sample. About 4 mg of the powdered coal sample, with 0.038 mm to 0.074 mm particle size, was packed into the sample holder, and then placed in the diffuse reflectance chamber. The pyrolysis residue samples are from the thermoanalysis experiments. 3. Discussion and results 3.1. Pyrolysis characteristics and residual parameters
2. Experimental methods The SL lignite pyrolysis experiment program was used to heat lignite to the set temperature, then thermostat for 30 min until the mass change rate is less than 0.01%/min. The residual structure of lignite pyrolysis at set temperature is obtained. Fig. 1 shows the TG-DTG curves of SL lignite at different temperatures for constant temperature pyrolysis experiments and temperature programmed oxidation experiments. Through the thermogravimetric curve of lignite coal samples and its characteristic parameters, it can be seen that the pyrolysis process curve of lignite coal samples with the same tendency at different pyrolysis temperatures. With the increase of pyrolysis temperature, the residual weight of coal samples at constant temperature and the residual weight of the pyrolysis end point were gradually decreased. This is mainly because the pyrolysis temperature increases and provides more reaction energy. Gradually activate the macromolecular structure in the coal to react. The amount of volatiles that are precipitated increases, and the residual weight of the coal sample decreases. The characteristic parameters of coal pyrolysis and constant temperature reaction were obtained by analyzing the lignite thermogravimetric curve in Fig. 1. As shown in Table 2 below:
2.1. Coal sample The lignite coal found in Inner Mongolia Autonomous Region of China is the main lignite coal source of China. However, the spontaneous combustion of lignite coal is very common. In this paper, a lignite coal sample was selected, as it is the most important variety of coal employed for liquefaction and chemical industry. The sample was taken from east two strip mine of Shengli coalfield (SL) in Xilinhaote of Inner Mongolia Autonomous Region in China. The surface layers of the coal were removed and the inner core of the coal sample was crushed in an oxygen-free glove box. The coal sample was then ground into small grains (between 0.038 and 0.074 mm in size) which were used for the experimental investigations. The coal particles were kept under an inert and dry atmosphere before experimental testing. The results of the proximate and ultimate analyses are listed in Table 1. 2.2. The thermoanalysis of coal pyrolysis and residue reoxidation Differential thermal analysis (TG, DTG and DSC) was performed using the TA-Q600 simultaneous thermal analyzer (TA Instruments, Unit States). About 5 mg of each of the coal samples was heated to
Initial separation residue of volatile components weight Wv-%. Refers to the coal sample entering the thermal decomposition stage, 2
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Table 1 Ages, mines, origins, vitrinite reflectance, rank, proximate and ultimate analyses for Shengli lignite coal 0.038–0.074 mm in size. Component
Content
Component
Coal information Coalfield Origins Ages vitrinite reflectance (%) Coal rank
Shengli Inner Mongolia cretaceous 0.31 Lignite
Content
Component
Content a
Proximate analysis (wt%, air dry basis)
Ultimate analysis (wt%, dmmf basis)
Moisture Ash Volatile matter Fixed Carbon Calorific value, kJ/g
Carbon Hydrogen Oxygen Sulfur (organic) Nitrogen
23.05 22.69 28.56 25.7 15.17
45.86 5.365 47.59 0.986 0.195
The wt% is weight percent. The dmmfa is dry mineral matter free.
Constant temperature starting point of coal sample residual value Wc-%, That is, the residual weight of the coal sample when the constant temperature pyrolysis is started; Residual weight value at the end of pyrolysis Wp-%, That is, the constant temperature is completed, and the residual weight of the coal sample at the end of the pyrolysis experiment; Residual weight value at the end of pyrolysis oxidation Wo-%, That is, when the temperature is programmed to 900 °C, the residual material of the coal sample. At this time, the coal sample has been completely oxidized, and the residue components are all incombustible substances such as ash. From Table 2, the maximum release rate of volatile lignite is slowly increased between 350 and 450 °C. It is reached the maximum Vmax value at 450 °C, and is almost constant between 450 °C and 500 °C. The maximum release rate of volatiles in coal samples at 350 °C has reached 0.513%/min. It is indicated that the coal sample is in the first section of the thermal decomposition stage at 350 °C completely, that is, a large amount of volatile gas is generated at this time, and at the same time, the production of tar leads to the formation of colloidal coal. Between 450 °C and 500 °C, after the coal sample reaches the maximum Vmax value, it enters the second interval of the thermal decomposition stage. The colloidal body begins to further decompose into semi-coke, which is still accompanied by the generation of light hydrocarbons such as gas. The critical temperature is the maximum precipitation temperature of 429.58 °C at 500 °C pyrolysis. The critical temperature is the maximum precipitation temperature of 429.58 °C at 500 °C constant temperature pyrolysis. In order to further study the stage characteristics of coal thermal decomposition process, analyze its reaction content. The characteristic parameters at different pyrolysis temperatures were analyzed such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo of SL lignite coal samples. Investigate the law of change, find out the pyrolysis stage of the pyrolysis residue, and analyze the reaction content and residue composition. Defining the variation of pyrolysis residual weight ΔWvp-%, that is ΔWvp = Wv−Wp, which indicates the amount of change in the weight of the coal sample during thermal decomposition. The variation of constant temperature residual weight ΔWcp-%, that is ΔWcp = Wc−Wp, Indicates the weight loss of coal sample during constant temperature pyrolysis. The pyrolysis residual combustible component weight ΔWpo,
Fig. 1. Thermal gravity analysis and derivative thermogravimetric analysis differential thermal gravity curve of pyrolysis (a) and oxidation (b) of lignite.
the weight of the coal sample at the beginning of weight loss, generally the temperature point at which the TG curve begins to decrease constantly. dw Volatile maximum separation rate Vmax= dt -%/min: The larger
( )
max
the value, the stronger the volatile release intensity. That is, the peak of the thermal decomposition peak of the DTG curve;
Table 2 Pyrolysis eigenvalue of coal at different temperatures. Coal type
Constant temperature, °C
Volatile maximum separation rate Vmax, %/min
Initial separation residue of volatile components weight W v, %
Constant temperature starting point of coal sample residual value Wc, %
Residual value of pyrolysis end coal sample Wp, %
Residual value of Oxidation end coal sample Wo, %
SL lignite
350 400 450 500
0.513 0.7997 1.025 1.031
78.73
76.14 72.56 68.52 63.79
72.77 67.58 62.75 60.37
11.15
3
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higher temperature of the constant temperature before 450 °C, the more reactions occur in the constant temperature stage, the production of small molecular structure products increases, and the variation of constant temperature residual weight increases; At 450 °C, the thermostatic decomposition residue reaches its peak value, and the macromolecular structure has the highest decomposition rate. When the constant temperature is further increased to 500 °C, the macromolecular structure has been largely decomposed between 450 °C and 500 °C. After the constant temperature is started at 500 °C, the reactants which precipitate the volatile matter become residual heavy hydrocarbons such as tar. Its content is relatively small, so the variation of constant temperature residual weight is reduced at this time. The pyrolysis residual combustible component weight ΔWpo of SL lignite coal samples overall low. This is mainly due to the large water content of the lignite, and the residual weight of the coal sample is greatly reduced when the moisture is removed during the dry degassing phase of the pyrolysis reaction. As the temperature increases, the ΔWpo value decreases gradually, and an inflection point appears at 450 °C. This is due to the 350 °C to 450 °C at the early stage of the thermal decomposition, mainly produce gas, tar and other volatile matter, formation of colloidal coal. With the increase of temperature, more and more macromolecular structures participate in the reaction inside the coal body, and the amount of volatile matter is increased, resulting in a decrease in the weight of the coal sample. After 450 °C, the coal body enters the late stage of thermal decomposition reaction. At this time, although the decomposition reaction is still carried out in the coal sample, at the same time, the coal colloidal body begins to decompose rapidly and solidifies into coke. Compared with the early stage of thermal decomposition, the degree of separation of volatiles decreases slightly. Therefore, the reduction degree of residual weight is relatively reduced. In conclusion, lignite coal samples temperature range from 350 °C to 500 °C across the early and late stages of thermal decomposition, the thermal decomposition reaction content changes, and pyrolysis residue composition is different. The critical point temperature at about 450 °C. So the characteristic parameter curve shows a significant turning point before 450 °C such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention.
Fig. 2. Characteristic parameters of SL lignite pyrolysis (a) the variation of volatile maximum separation rate Vmax, pyrolysis residual weight ΔWvp, and the variation of constant temperature residual weight ΔWcp, (b) variation of the pyrolysis residual combustible component weight ΔWpo.
that is ΔWpo = (Wp−Wo)-%. Indicates the amount of combustible material present in the residual structure of the coal sample after the end of pyrolysis. Fig. 2 shows pyrolysis reaction parameters change rule of the SL lignite constant temperature pyrolysis in 350 °C, 400 °C, 450 °C, 500 °C. The thermal decomposition stage of coal can be further divided into two stages which the maximum release rate of volatile matter as the limit. The first stage begins at the primary volatile point and ends at the fastest volatile point. This stage is mainly release of gas and the production of tar leads to the formation of colloidal coal. At the most separation point of the volatiles, the tar yield reaches the maximum. The second stage from the fastest precipitation point to the end of the thermal decomposition stage. This stage is mainly produces the gas and solidifies the colloidal into semi-coke. According to the TG-DTG curve, the variation of pyrolysis residual weight ΔWvp of the coal sample increases linearly between 350 °C and 500 °C, but the inflection point occurs at 450 °C, and the growth rate decreases. The analysis shows that in the early stage of pyrolysis, the weight loss rate of coal sample increases rapidly. Reflect on the thermal decomposition of residual weight value namely ΔWvp values increase rapidly. After 450 °C, when the coal sample reaches the maximum weight loss rate, it mainly solidifies the colloid into semi-coke. Although the volatiles were still continuously released, the growth rate decreased obviously. The curve of the variation of constant temperature residual weight ΔWcp showed a parabolic trend between 350 °C and 500 °C, Increase first and then decrease, the peak occurs at 450 °C. This indicates that in the early stage of pyrolysis, the temperature rise activates a larger number of macromolecular structures to participate in the reaction. The
3.2. Functional group distribution characteristics of pyrolysis residues This section carries out infrared spectrum analysis. Obtaining changes in the content of functional groups in the coal sample under different pyrolysis reaction temperatures, thus analyzed the structural properties of lignite at different pyrolysis temperatures, revealed the evolution mechanism of the functional groups in the pyrolysis stage of SL lignite. Fig. 3 is an infrared test spectrum of five samples of SL lignite raw coal, 350 °C, 400 °C, 450 °C, and 500 °C pyrolysis residues. The bands at 3200–3600, 2920, 2855, and 2965 cm−1 in the range of 200–4000 cm−1 represent the tensile vibration of OeH and CeH in the benzene ring, methylene and methyl groups. The band at 1700 and 1655 cm−1 in the 650–2000 cm−1 band represents the tensile vibration of C]O in the carboxyl group (eCOOH) and the ketone carbonyl group (eC]O). The carbon–carbon double bond (C]C) tensile vibration on the benzene ring corresponds to a band of 1495–1615 cm−1. The peak for each functional group cannot be confirmed clearly in the raw coal spectra. Therefore, Fourier deconvolution of the spectra is used to separate these groups and especially the shoulder peak clearly. The curve-fitting method is applied to obtain the experimental spectral intensity of each functional group. The peaks of each functional group were integrated to obtain the original peak area quantitatively. The 4
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Fig. 3. Pyrolytic infrared spectra of coal samples. Fig. 4. Changes of lignite functional groups in pyrolysis.
extinction coefficient differs of different functional groups varies greatly. In order to obtain the accurate distribution characteristics of functional groups from coal infrared spectra, the original peak area of the functional group was corrected by using the correction relationship between the absorbance of different functional groups and the peak area, as shown in Table 3. The correction relationship [46] is the ratios of experimental infrared intensities and unit absorption intensities f. The vibration strength f of each functional group varies greatly, so there is a big gap between the corrected peak area A(v)/f and the original peak area A(v). In the original spectrum, the infrared intensity of the hydroxyl group is much larger than other groups, but it also has a large vibration intensity, the difference between the corrected infrared intensity and other groups is greatly reduced. The theoretical infrared vibration intensity obtained by quantum chemical calculation eliminates the influence of extinction coefficient on the quantitative calculation of experimental infrared spectrum and corrects the peak area ratio of each functional group. According to the data in Table 3, the contents of each group in raw coal and pyrolysis residue at different temperatures were obtained, as shown in Fig. 4. Fig. 4 shows that the content of aromatic hydrocarbon is up to more than 50% in raw coal structure, the sum of aromatic structure, aliphatic hydrocarbon and associative hydroxyl content accounts for more than 90% of the total functional group content, and the content of oxygencontaining functional group is relatively less. With the increase of pyrolysis temperature, the content of associative hydroxyl, aliphatic hydrocarbon and oxygen-containing functional group in coal gradually decreases, while the content of aromatic skeleton structure in coal gradually increases. The associative hydroxyl groups are very sensitive to the pyrolysis reaction. At 350 °C, the associative hydroxyl groups in the coal samples
have been reduced from 12.34% to 1.9%, the difference is 5.49 times. This is mainly due to the large amount of water loss in the coal caused by the initial dry degassing stage of the pyrolysis reaction, resulting in a large reduction in the water-associated hydroxyl groups in the coal. In the pyrolysis reaction stage, the hydroxyl content continues to decrease, but the rate of reduction is very small. This is mainly due to the loss of a large amount of hydroxyl groups in the initial drying stage. After entering the pyrolysis stage, the content of water-associated hydroxyl groups in the coal has been very small. At this time, due to the influence of high temperature exotherm, the polyol phenol associated hydroxyl in coal begins to decompose thermally, because its content is much less than the water-associated hydroxyl group, the overall reduction in hydroxyl content is small. The content of aliphatic hydrocarbons also decreased with the increase of pyrolysis temperature, and it was very sensitive to pyrolysis reaction. The content of aliphatic hydrocarbons in SL lignite before pyrolysis was 25.4%, decreased to 15.27% at 350 °C. By the time the pyrolysis proceeds to 500 °C, the aliphatic hydrocarbon content in the coal structure is only 8.46%, which is one third of the aliphatic hydrocarbon content of the raw coal. This situation is mainly caused by high temperature causing weak chemical bond breakage in coal, side chain thermal cracking, formation of light volatile matter and tar, resulting in a decrease in aliphatic side chains and a decrease in aliphatic hydrocarbon content in coal structure. The content of aromatic hydrocarbons in SL lignite is greatly affected by pyrolysis reaction. The aromatic hydrocarbon content of raw coal is 54.46%. The aromatic structure content increases to 74.47% at 350 °C pyrolysis. When the temperature reaches 500 °C, the aromatic
Table 3 Original peak areas and corrected values of functional groups in coal. Functional group −1
Band (cm
)
OeH
eCH3
eCH2-
eCOOH
eC]O
Benz
ene (C]C)
3200–3600
2965.00
2924.00
2854.00
1700.00
1660.00
1620–1490
Raw coal
A(v) A(v)/f
14063.94 22.25
905.95 11.73
2277.36 34.08
683.11
1204.45 4.90
1437.01 9.16
4229.94 98.19
350 °C Pyrolysis residue
A(v) A(v)/f
63.47 0.10
13.72 0.18
33.72 0.63
20.90
71.31 0.29
23.75 0.15
169.36 3.93
400 °C Pyrolysis residue
A(v) A(v)/f
1755.96 2.78
302.47 3.91
1938.93 28.59
544.54
812.49 3.30
1063.40 6.78
7983.90 185.33
450 °C Pyrolysis residue
A(v) A(v)/f
948.80 1.50
316.77 4.10
1531.40 23.22
485.78
698.27 2.84
864.71 5.51
8736.04 202.79
500 °C Pyrolysis residue
A(v) A(v)/f
869.23 1.38
333.34 4.31
1169.55 18.62
448.42
578.57 2.35
886.91 5.65
10291.78 238.90
5
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Table 4 Eigenvalue of coal oxidation. Characteristic parameters
Raw coal
350 °C
400 °C
450 °C
500 °C
Ignition temperature Ti, °C Burnout temperature Th, °C Maximum speed of weightlessness dWmax Maximum weightlessness temperature TWmax, °C Combustion half peak width ΔT 1 -°C
237.2 588.09 0.6399
235.11 584.85 0.8601
232.24 576.67 0.9364
234.72 577.56 0.9818
236.9 581.47 0.948
388.38
385.88
387.87
385.31
384.47
81.53
77.04
69.83
66.71
67.9
2
pyrolysis is relatively reduced, indicating that the combustion degree is more concentrated. However, the TG-DTG curves of different residual structures of coal sample pyrolysis appear to overlap with each other during the entire oxidation process. The influence of pyrolysis temperature on the oxidation characteristics of residual structures cannot be quantitatively analyzed. Therefore, the coal combustion characteristic parameters are used as reference objects to quantitatively analyze the influence of pyrolysis temperature. Five different feature point values were selected to characterize the thermal analysis characteristics of TG-DTG. Table 4 shows the characteristic parameters of re-oxidation of lignite raw coal and residual structure at different temperatures. Ignition point Ti -°C, The temperature point at which the coal sample changes from slow oxidation to intense oxidation. There are many methods to determine the temperature of the ignition point, including TG-DTG method (extrapolation start method), TGA method, DTGA method, TGA curve demarcation method, DTGA curve demarcation method and so on. The TG-DTG method is selected to determine the ignition point temperature. That is, the peak point of the DTG curve is made as a vertical line, and then the tangent line of the intersection point of the vertical line and the TG curve is made, and the intersection temperature of the tangent line and the parallel line passing through the TG curve weight loss starting point is the ignition point temperature, then do the tangent of the intersection of the perpendicular and the TG curve. The junction temperature between the tangent and the parallel line passing through the starting point of the weight loss of the TG curve is the ignition temperature. Burnout temperature Th-°C, The temperature at which the oxidation reaction ends. The determination methods include TG-DTG, TG, DTG and fixed percentage of weight loss. The TG-DTG method is still selected here to determine. That is, the intersection temperature of the parallel line passing the end point of weightlessness of TG curve and the tangent line made before. This temperature indicates that the combustible material (volatile components, fixed carbon, etc.) in coal has been consumed, and the whole combustion process is ended. dw Maximum speed of weightlessness dWmax = dT -%/°C,
Fig. 5. Thermogravimetric curve of different coal oxidation (a) TG results and (b) DTG results.
hydrocarbon structure content is even as high as 88.08%. This is mainly because the reaction has been in the late stage of thermal decomposition. At this time, the colloidal coal are transformed from the initial formation to the thermal decomposition, the weak chemical bonds such as oxygen-containing groups are thermally cracked, the molecular structure is condensed, the degree of aromatization of the coal structure is increased, and the aromatic hydrocarbon content is increased.
3.3. Pyrolysis residue reoxidation
( )
Fig. 5 shows the results of TG-DTG of SL lignite raw coal and pyrolysis residual structure at different temperatures. As can be seen in Fig. 5, SL lignite has no obvious oxygen-absorbing weight gain process in the whole oxidation reaction. Because the lignite coal rank is low, the degree of structural polymerization is small, and the group also has high activity, the weight consumed by the oxidation reaction exceeds the weight increased by oxygen absorption, resulting in an insignificant process of oxygen-absorbing weight gain. In addition, observing the oxidation curve of the raw coal and pyrolysis residue of lignite, it can be clearly seen that the pyrolysis process changes the oxidation characteristics of the coal sample. Coal sample after pyrolysis can be clearly seen the process of drying degasification and oxygen absorption gaining weight in the process of reoxidation. At the same time, the combustion intensity of pyrolysis residual structure is obviously higher than that of raw coal. The combustion concentration degree is similar to the combustion intensity, which is greatly affected by pyrolysis. The semi-peak width of coal sample combustion after
max
Maximum weightlessness temperature TWmax -°C. That is, the speed at which the oxidation reaction is most intense and the corresponding temperature point reflect the combustion intensity of the coal sample, which is reflected as the peak point of the weight loss peak on the DTG curve. Combustion half peak width ΔT 1 −° C . That is, the half peak width 2 of coal sample DTG curve combustion peak, representing the concentration degree of coal sample combustion. The value is smaller, the concentration degree of coal sample combustion is stronger. As shown in Table 4, with the increase of residual temperature, the ignition temperature and the burnout temperature of SL lignite firstly decrease and then increase, and reach the lowest value at 400 °C. The highest temperature variation between the ignition points is 5 °C, and the highest temperature variation at the burnout point is 12 °C. Before
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spontaneous combustion effectively, and ensuring the safe and efficient mining of lignite.
400 °C, it is believed that generated more small molecular structure fragments which are easy to oxidize before the thermal decomposition of coal. With the increase of temperature, leading to the advance of the ignition temperature. 400–450 °C, although the amount of small molecular structure fragments is increasing, the structure which is easily oxidized further decomposed and lost, resulting in a temperature rise of the ignition point. Burnout temperature changes show that 400 °C is the best temperature in key segments of aromatic structure bridge fracture. The bridge bond rupture completely, aromatic structure has not been polymerization, lead to burnout point the lowest temperature, below or above 400 °C, aromatic skeleton structure are greater than 400 °C. The maximum weight loss rate of SL lignite combustion increases with the increase of residual pyrolysis temperature, and reached the maximum at 450 °C, then began to decrease, while the maximum weight loss rate point temperature change was small. This shows that the maximum combustion intensity is 60% higher than that of raw coal due to the thermal decomposition. At the same time, the combustion half peak width is reduced by 18%. It shows that the burning intensity of coal rise with the increase of residual temperature of pyrolysis. And reach maximum at 450 °C, consistent with the critical transition temperature of coal thermal decomposition. The analysis suggests that the smaller molecule fragments formed by the early pyrolysis, the more favorable the concentrated combustion reaction of the structure. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention.
Acknowledgments This work was supported by Natural Science Foundation of China (51704284), the Independent research project of State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM19X0013), the Natural Science Foundation of Jiangsu Province (BK20161183) and Natural Science Foundation of China (51774275), the Research Funds of the International Research Center for Underground Coal Gasification, CUMT (2018KJZX05). References [1] Querol X, Zhuang X, Font O, Izquierdo M, Alastuey A, Castro I, et al. Influence of soil cover on reducing the environmental impact of spontaneous coal combustion in coal waste gobs: a review and new experimental data. Int J Coal Geol 2011;85(1):2–22. [2] Wang HH, Dlugogorski BZ, Kennedy EM. Pathways for production of CO2 and CO in low-temperature oxidation of coal. Energy Fuels 2003;17(1):150–8. [3] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Model predictions and experimental results on self-heating prevention of stockpiled coals. Fuel 2001;80(1):125–34. [4] Shao ZL, Jia XY, Zhong XX, Wang DM, Wei J, Wang YM, et al. Detection, extinguishing, and monitoring of a coal fire in Xinjiang China. Environ Sci Pollut Res 2018;25(26):26603–16. [5] Zhong XX, Wang MM, Dou GL, Wang DM, Chen Y, Mo YM, et al. Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction. J Energy Inst 2015;88(4):398–405. [6] Zhong XX, Dou GL, Xin HH, Wang DM. Study on low-temperature oxidation process of low rank coal by in-situ FTIR. Stafa-Zurich: Trans Tech Publications Ltd; 2013. p. 871–6. [7] Qi Xuyao, Wang Deming, Xin Haihui, Qi Guansheng. In situ FTIR study of real-time changes of active groups during oxygen-free reaction of coal. Energy Fuels 2013;27(6):3130–6. [8] Qi Xuyao, Xin Haihui, Wang Deming, Qi Guansheng. A rapid method for determining the R-70 self-heating rate of coal. Thermochim Acta 2013;571:21–7. [9] Wang De-ming, Xin Hai-hui, Qi Xu-yao, Dou Guo-lan, Qi Guan-sheng, Ma Li-yang. Reaction pathway of coal oxidation at low temperatures: a model of cyclic chain reactions and kinetic characteristics. Combust Flame 2016;163:447–60. [10] Wang HH, Dlugogorski BZ, Kennedy EM. Thermal decomposition of solid oxygenated complexes formed by coal oxidation at low temperatures. Fuel 2002;81(15):1913–23. [11] Wang HH, Dlugogorski BZ, Kennedy EM. Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Prog Energy Combust Sci 2003;29(6):487–513. [12] Xin Hai-hui, Wang De-ming, Qi Xu-yao, Zhong Xiao-xing, Ma Li-yang, Dou Guo-lan, et al. Oxygen consumption and chemisorption in low-temperature oxidation of subbituminous pulverized coal. Spectrosc Lett 2018;51(2):104–11. [13] Zhou Q, Ding DQ, Zhang YF, Zhang YM, Dong T, Chen XY, et al. Effect of semi-char content on oxygen-enriched co-combustion behaviors of lignite and semi-char. J Energy Eng 2018;144(4):9. [14] Zhang GJ, Su AT, Zhang YF, Wang P. Shrinkage character in the process of semicoke formation. Chiang Mai J Sci 2015;42(2):401–6. [15] Yoruk CR, Meriste T, Sener S, Kuusik R, Trikkel A. Thermogravimetric analysis and process simulation of oxy-fuel combustion of blended fuels including oil shale, semicoke, and biomass. Int J Energy Res 2018;42(6):2213–24. [16] Wu Q, Zhu ZZ, Shi GJ, Wang F, Wang ZL, Xie YY. Improving quality of coke made from chinese xinjiang gas coal with high strength modifier. Proceedings of the 3rd Pan American Materials Congress. 2017. p. 529–38. [17] Wang X, Zeng WL, Guo QJ, Geng QJ, Yan YM, Hu XD. The further activation and functionalization of semicoke for CO2 capture from flue gases. RSC Adv 2018;8(62):35521–7. [18] Tran QA, Stanger R, Xie W, Smith N, Lucas J, Wall T. Linking thermoplastic development and swelling with molecular weight changes of a coking coal and its pyrolysis products. Energy Fuels 2016;30(5):3906–16. [19] Sun M, Ning X, Zhang J, Li K, Tang Q, Liu Z, et al. Combustion kinetics and structural features of bituminous coal before and after modification process. J Therm Anal Calorim 2018;131(2):983–92. [20] Shao ZL, Wang DM, Wang YM, Zhong XX, Tang XF, Hu XM. Controlling coal fires using the three-phase foam and water mist techniques in the anjialing open pit mine, China. Nat Hazards 2015;75(2):1833–52. [21] Mochizuki Y, Ono Y, Tsubouchi N. Evolution profile of gases during coal carbonization and relationship between their amounts and the fluidity or coke strength. Fuel 2019;237:735–44. [22] Chen Q, He R, Xu XC, Liang ZG, Chen CH. Experimental study on pore structure and apparent kinetic parameters of char combustion in kinetics-controlled regime. Energy Fuels 2004;18(5):1562–8. [23] He R, Sato J, Chen CH. Modeling char combustion with fractal pore effects. Combust Sci Technol 2002;174(4):19–37.
4. Conclusions This paper selected lignite coal sample from Shengli coalfield of Inner Mongolia for pyrolysis and reburning experiments. Using thermogravimetric analysis and infrared analysis to analyze the macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues. The lignite coal samples were subjected to constant temperature pyrolysis thermogravimetric analysis at 350 °C, 400 °C, 450 °C and 500 °C. The reburning characteristics of coal pyrolysis residual structure were analyzed. The results can help to understand the thermogravimetric characteristics of the reburning in the surrounding pyrolysis area affected by coal combustion in coal fir. It is great significance for the prevention and control of coalfield fire and safe unsealing of the closed working face in coal mine. It is found that the coal sample has the same trend of pyrolysis process curves at different pyrolysis temperatures. As the pyrolysis temperature increases, the residual temperature of the coal sample and the residual weight of the pyrolysis end point gradually decrease. A significant turning point before 450 °C such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo. With the increase of pyrolysis temperature, the water-associated hydroxyl group is greatly reduced due to the dehydration and drying of the coal sample. The aliphatic hydrocarbons are gradually reduced due to the side chain cracking in the coal, and the aromatic hydrocarbons increase with the degree of aromatization of the coal structure. For the oxidation characteristics of lignite pyrolysis residue, the ignition temperature and the burnout temperature of SL lignite firstly decrease and then increase with the increase of residual temperature, and reach the lowest value at 400 °C. The maximum weight loss rate of SL lignite combustion increases with the increase of residual pyrolysis temperature, and reached the maximum at 450 °C. However, the combustion half peak width decreased and reached the minimum at 450 °C. The pyrolysis residue at ~400 °C is the easiest to reburning, and the pyrolysis residue at ~450 °C had the strongest reburning intensity. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention. It is great significance for preventing and controlling the lignite reburning after 7
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Full Length Article
The reburning thermal characteristics of residual structure of lignite pyrolysis
T
Haihui Xina,b,c,1, Hetang Wanga,b,c,1, Wenjie Kanga,b,c, Cuicui Dia,b, Xuyao Qia,b,c, ⁎ Xiaoxing Zhonga,b, , Deming Wanga,b, Fangming Liud a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, No. 1 University Road, Xuzhou, Jiangsu 221116, China Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China c International Research Center for Underground Coal Gasification, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China d School of Marxism, Shanghai University of Finance and Economics, Shanghai 200000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignite Pyrolysis residue Reburning Thermal characteristics
The reburning of pyrolysis residual is very important in coal fire control and preventing fire resurgence. This paper researched the pyrolysis and reoxidation characteristics of lignite using thermogravimetric and infrared analysis. The macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues were analyzed. The critical temperature for lignite pyrolysis and pyrolysis residues reburning. The results show that a significant turning point before 450 °C such as the variation of pyrolysis residual weight, the variation of constant temperature residual weight, and the pyrolysis residual combustible component weight. The aliphatic hydrocarbons are gradually reduced due to the side chain cracking in the coal, and the aromatic hydrocarbons increase with the degree of aromatization of the coal structure in lignite pyrolysis. The pyrolysis residue at ~400 °C is the easiest to reburning, and the pyrolysis residue at ~450 °C had the strongest reburning intensity.
1. Introduction Lignite is rich in reserves, accounting for 13% of the total coal resources in China. Lignite is a very important chemical industry-used coal. However, lignite is the coal with the lowest rank of coal mineralization. Lignite has large internal pores, many oxygen-containing functional groups. Lignite is the easiest spontaneous combustion of coal compare compared with other rank coals. Thus, Lignite mining faced the serious problem of coal spontaneous combustion and reburning in coal fire zone, which greatly restricts the safe and efficient mining of lignite resources in China [1–3]. Along with the increasing of coal chemical industry, the demand for lignite is also growing. The problem of safe production caused by the spontaneous combustion of lignite and reburning in fire zones is increasingly prominent [4–6]. At present, many researches have been done on the mechanism and characteristics of coal spontaneous combustion [7–12], especially the lignite which is easiest spontaneous combustion coal. However, there was few researches on the reburning characteristics of coal in coal fire zone. The reburning problem in coal fire area has not been solved. The
coal fire reburning is most common in lignite mining zones. Reburning of residual in coal fire area is mainly caused by the thermal decomposition reaction of coal around the coal fire source. The internal structure and composition of coal have undergone major changes under the pyrolysis. The macromolecular structure is broken, resulting in more easily oxidized small molecular structures, more active sites and flammable materials such as tar and volatiles were in pyrolysis residual which may be easier oxidation than raw coal. The reburning of fire zone will be occurred easily. Thus, it is necessary to research the structural residual and reburning characteristics of lignite pyrolysis in different temperature. Thermogravimetric analyzer is the most commonly methods to study the processing characteristics of coal pyrolysis and combustion [13–21] controlled. Such as Chen et al. [22] made the char samples burned in a thermogravinietric analyzer to study the intrinsic reactivity of char samples. He et al. [23,24] tested nine char samples in thermogravimetric analyzers with continuously rising temperatures and a global one-step kinetic reaction model was used to describe the char combustion. Jiang et al. [25] studied the chemical properties, pyrolysis
⁎
Corresponding authors at: State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, No. 1 University Road, Xuzhou, Jiangsu 221116, China. E-mail addresses: [email protected] (H. Xin), [email protected] (X. Zhong). 1 Co-first author: These authors contributed equally to this work. https://doi.org/10.1016/j.fuel.2019.116226 Received 23 April 2019; Received in revised form 15 August 2019; Accepted 17 September 2019 Available online 24 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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350 °C, 400 °C, 450 °C and 500 °C respectively at the heating rate of 10 °C/min under nitrogen (100 ml/min). All thermoanalysis experiments keep 30 min at outlet temperature until the mass is nearly constant. Based on the thermoanalysis results, the pyrolysis characteristic parameters was analyzed and compared with the changing of functional groups in coal to investigate the pyrolysis process characteristic and residual structural feature. Then the reburning thermogravimetric experiments of coal pyrolysis residual were carried out.
behaviors of Shenmu (SM) coal using the thermal analysis method. Based on the applications of thermal analysis methods in numerous researches, it can measure the whole process of coal pyrolysis and combustion very well. It is very suitable to measure the residual characteristics of coal pyrolysis and prepare pyrolysis residue samples. The functional groups distribution characteristics of coal and pyrolysis char were analyzed by analytical chemical techniques such as nuclear magnetic resonance (NMR) [26,27] and Fourier transform infrared spectroscopy (FTIR) [28–32]. Due to the simple and affordable technique of FTIR, especially diffuse reflectance FTIR (DRIFT) [33–35], many researches using FTIR study the changes of functional groups in coal pyrolysis and combustion [36–43]. Such as Niu et al. [44] investigated the evolution of six main functional groups in two coking coals with different ranks by FTIR. Thus, the FTIR technology can give a rapid and affordable results of functional groups for coal pyrolysis residue. Based on the functional groups changes, the content of aromatic hydrocarbon and aliphatic groups could be described to explore the structural transformation in coal pyrolysis including the producing of semicoke and tar. Some researchers have analyzed the relationship of functional groups changes and tar product during coal pyrolysis. Such as Song et al. [45] studied the importance of aromatic structures in tar to the destruction of tar itself during the volatile-char interactions, and found the aromatic structures in tar are more reactive with char than the non-aromatic structures. These analysis process and results can help analyzed the functional groups distribution characteristics of coal pyrolysis residue. The thermal analysis and FTIR method can provide the feasible testing technology for exploring the pyrolysis and reburning characteristics of lignite. The Shengli coalfield (SL) is one of the major lignite production areas in China, which is the important raw coal resource of coal chemical industry distributed in Inner Mongolia, Shanxi of China. Thus, this paper focused on SL lignite coal. The lignite coal samples would be subjected to constant temperature pyrolysis thermogravimetric analysis at 350 °C, 400 °C, 450 °C and 500 °C. The macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues would be analyzed. Simultaneously, the reburning characteristics of coal pyrolysis residual structure would be tested. The results can help to reveal the characteristics of the coal fire reburning during lignite mining. It is great significance for preventing and controlling the lignite reburning after spontaneous combustion effectively, and ensuring the safe and efficient mining of lignite.
After coal pyrolysis at 350 °C for 30 min, the residue is cooled to room temperature under nitrogen. The oxidation thermal analysis experiment of residue coal is from 30 °C to 900 °C at the heating rate of 5 °C/min under air (100 ml/min). The same experiments with step (1) for lignite pyrolysis to 400 °C, 450 °C and 500 °C. Thus, the reoxidation characteristics of lignite pyrolysis residue at different temperatures were obtained. As a contrast, the pyrolysis of lignite raw coal were tested at the heating rate of 10 °C/min under nitrogen (100 ml/min) from 30 °C to 900 °C. The oxidation of lignite raw coal were tested at the heating rate of 5 °C/min under air (100 ml/min) from 30 °C to 900 °C. 2.3. Infrared spectrum experiments In order to analyzed the structural properties of lignite at different pyrolysis temperatures and revealed the evolution mechanism of the functional groups in the pyrolysis stage of SL lignite, the infrared spectrum experiments were carried out. The functional groups changes under different pyrolysis reaction temperatures could be obtained. Thus, the pyrolysis residue of SL lignite from 350 to 500 °C with 50 °C intervals were performed in a Nicolet 6700 FTIR instrument (Thermofisher Scientific, Unit States) using the diffuse reflection facility. The wave number is from 650 cm−1 to 4000 cm−1, with a resolution of 4 cm−1 and scanning number of 64 scans. Pure ground KBr was used to obtain the reference spectrum before testing the coal sample. About 4 mg of the powdered coal sample, with 0.038 mm to 0.074 mm particle size, was packed into the sample holder, and then placed in the diffuse reflectance chamber. The pyrolysis residue samples are from the thermoanalysis experiments. 3. Discussion and results 3.1. Pyrolysis characteristics and residual parameters
2. Experimental methods The SL lignite pyrolysis experiment program was used to heat lignite to the set temperature, then thermostat for 30 min until the mass change rate is less than 0.01%/min. The residual structure of lignite pyrolysis at set temperature is obtained. Fig. 1 shows the TG-DTG curves of SL lignite at different temperatures for constant temperature pyrolysis experiments and temperature programmed oxidation experiments. Through the thermogravimetric curve of lignite coal samples and its characteristic parameters, it can be seen that the pyrolysis process curve of lignite coal samples with the same tendency at different pyrolysis temperatures. With the increase of pyrolysis temperature, the residual weight of coal samples at constant temperature and the residual weight of the pyrolysis end point were gradually decreased. This is mainly because the pyrolysis temperature increases and provides more reaction energy. Gradually activate the macromolecular structure in the coal to react. The amount of volatiles that are precipitated increases, and the residual weight of the coal sample decreases. The characteristic parameters of coal pyrolysis and constant temperature reaction were obtained by analyzing the lignite thermogravimetric curve in Fig. 1. As shown in Table 2 below:
2.1. Coal sample The lignite coal found in Inner Mongolia Autonomous Region of China is the main lignite coal source of China. However, the spontaneous combustion of lignite coal is very common. In this paper, a lignite coal sample was selected, as it is the most important variety of coal employed for liquefaction and chemical industry. The sample was taken from east two strip mine of Shengli coalfield (SL) in Xilinhaote of Inner Mongolia Autonomous Region in China. The surface layers of the coal were removed and the inner core of the coal sample was crushed in an oxygen-free glove box. The coal sample was then ground into small grains (between 0.038 and 0.074 mm in size) which were used for the experimental investigations. The coal particles were kept under an inert and dry atmosphere before experimental testing. The results of the proximate and ultimate analyses are listed in Table 1. 2.2. The thermoanalysis of coal pyrolysis and residue reoxidation Differential thermal analysis (TG, DTG and DSC) was performed using the TA-Q600 simultaneous thermal analyzer (TA Instruments, Unit States). About 5 mg of each of the coal samples was heated to
Initial separation residue of volatile components weight Wv-%. Refers to the coal sample entering the thermal decomposition stage, 2
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Table 1 Ages, mines, origins, vitrinite reflectance, rank, proximate and ultimate analyses for Shengli lignite coal 0.038–0.074 mm in size. Component
Content
Component
Coal information Coalfield Origins Ages vitrinite reflectance (%) Coal rank
Shengli Inner Mongolia cretaceous 0.31 Lignite
Content
Component
Content a
Proximate analysis (wt%, air dry basis)
Ultimate analysis (wt%, dmmf basis)
Moisture Ash Volatile matter Fixed Carbon Calorific value, kJ/g
Carbon Hydrogen Oxygen Sulfur (organic) Nitrogen
23.05 22.69 28.56 25.7 15.17
45.86 5.365 47.59 0.986 0.195
The wt% is weight percent. The dmmfa is dry mineral matter free.
Constant temperature starting point of coal sample residual value Wc-%, That is, the residual weight of the coal sample when the constant temperature pyrolysis is started; Residual weight value at the end of pyrolysis Wp-%, That is, the constant temperature is completed, and the residual weight of the coal sample at the end of the pyrolysis experiment; Residual weight value at the end of pyrolysis oxidation Wo-%, That is, when the temperature is programmed to 900 °C, the residual material of the coal sample. At this time, the coal sample has been completely oxidized, and the residue components are all incombustible substances such as ash. From Table 2, the maximum release rate of volatile lignite is slowly increased between 350 and 450 °C. It is reached the maximum Vmax value at 450 °C, and is almost constant between 450 °C and 500 °C. The maximum release rate of volatiles in coal samples at 350 °C has reached 0.513%/min. It is indicated that the coal sample is in the first section of the thermal decomposition stage at 350 °C completely, that is, a large amount of volatile gas is generated at this time, and at the same time, the production of tar leads to the formation of colloidal coal. Between 450 °C and 500 °C, after the coal sample reaches the maximum Vmax value, it enters the second interval of the thermal decomposition stage. The colloidal body begins to further decompose into semi-coke, which is still accompanied by the generation of light hydrocarbons such as gas. The critical temperature is the maximum precipitation temperature of 429.58 °C at 500 °C pyrolysis. The critical temperature is the maximum precipitation temperature of 429.58 °C at 500 °C constant temperature pyrolysis. In order to further study the stage characteristics of coal thermal decomposition process, analyze its reaction content. The characteristic parameters at different pyrolysis temperatures were analyzed such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo of SL lignite coal samples. Investigate the law of change, find out the pyrolysis stage of the pyrolysis residue, and analyze the reaction content and residue composition. Defining the variation of pyrolysis residual weight ΔWvp-%, that is ΔWvp = Wv−Wp, which indicates the amount of change in the weight of the coal sample during thermal decomposition. The variation of constant temperature residual weight ΔWcp-%, that is ΔWcp = Wc−Wp, Indicates the weight loss of coal sample during constant temperature pyrolysis. The pyrolysis residual combustible component weight ΔWpo,
Fig. 1. Thermal gravity analysis and derivative thermogravimetric analysis differential thermal gravity curve of pyrolysis (a) and oxidation (b) of lignite.
the weight of the coal sample at the beginning of weight loss, generally the temperature point at which the TG curve begins to decrease constantly. dw Volatile maximum separation rate Vmax= dt -%/min: The larger
( )
max
the value, the stronger the volatile release intensity. That is, the peak of the thermal decomposition peak of the DTG curve;
Table 2 Pyrolysis eigenvalue of coal at different temperatures. Coal type
Constant temperature, °C
Volatile maximum separation rate Vmax, %/min
Initial separation residue of volatile components weight W v, %
Constant temperature starting point of coal sample residual value Wc, %
Residual value of pyrolysis end coal sample Wp, %
Residual value of Oxidation end coal sample Wo, %
SL lignite
350 400 450 500
0.513 0.7997 1.025 1.031
78.73
76.14 72.56 68.52 63.79
72.77 67.58 62.75 60.37
11.15
3
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higher temperature of the constant temperature before 450 °C, the more reactions occur in the constant temperature stage, the production of small molecular structure products increases, and the variation of constant temperature residual weight increases; At 450 °C, the thermostatic decomposition residue reaches its peak value, and the macromolecular structure has the highest decomposition rate. When the constant temperature is further increased to 500 °C, the macromolecular structure has been largely decomposed between 450 °C and 500 °C. After the constant temperature is started at 500 °C, the reactants which precipitate the volatile matter become residual heavy hydrocarbons such as tar. Its content is relatively small, so the variation of constant temperature residual weight is reduced at this time. The pyrolysis residual combustible component weight ΔWpo of SL lignite coal samples overall low. This is mainly due to the large water content of the lignite, and the residual weight of the coal sample is greatly reduced when the moisture is removed during the dry degassing phase of the pyrolysis reaction. As the temperature increases, the ΔWpo value decreases gradually, and an inflection point appears at 450 °C. This is due to the 350 °C to 450 °C at the early stage of the thermal decomposition, mainly produce gas, tar and other volatile matter, formation of colloidal coal. With the increase of temperature, more and more macromolecular structures participate in the reaction inside the coal body, and the amount of volatile matter is increased, resulting in a decrease in the weight of the coal sample. After 450 °C, the coal body enters the late stage of thermal decomposition reaction. At this time, although the decomposition reaction is still carried out in the coal sample, at the same time, the coal colloidal body begins to decompose rapidly and solidifies into coke. Compared with the early stage of thermal decomposition, the degree of separation of volatiles decreases slightly. Therefore, the reduction degree of residual weight is relatively reduced. In conclusion, lignite coal samples temperature range from 350 °C to 500 °C across the early and late stages of thermal decomposition, the thermal decomposition reaction content changes, and pyrolysis residue composition is different. The critical point temperature at about 450 °C. So the characteristic parameter curve shows a significant turning point before 450 °C such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention.
Fig. 2. Characteristic parameters of SL lignite pyrolysis (a) the variation of volatile maximum separation rate Vmax, pyrolysis residual weight ΔWvp, and the variation of constant temperature residual weight ΔWcp, (b) variation of the pyrolysis residual combustible component weight ΔWpo.
that is ΔWpo = (Wp−Wo)-%. Indicates the amount of combustible material present in the residual structure of the coal sample after the end of pyrolysis. Fig. 2 shows pyrolysis reaction parameters change rule of the SL lignite constant temperature pyrolysis in 350 °C, 400 °C, 450 °C, 500 °C. The thermal decomposition stage of coal can be further divided into two stages which the maximum release rate of volatile matter as the limit. The first stage begins at the primary volatile point and ends at the fastest volatile point. This stage is mainly release of gas and the production of tar leads to the formation of colloidal coal. At the most separation point of the volatiles, the tar yield reaches the maximum. The second stage from the fastest precipitation point to the end of the thermal decomposition stage. This stage is mainly produces the gas and solidifies the colloidal into semi-coke. According to the TG-DTG curve, the variation of pyrolysis residual weight ΔWvp of the coal sample increases linearly between 350 °C and 500 °C, but the inflection point occurs at 450 °C, and the growth rate decreases. The analysis shows that in the early stage of pyrolysis, the weight loss rate of coal sample increases rapidly. Reflect on the thermal decomposition of residual weight value namely ΔWvp values increase rapidly. After 450 °C, when the coal sample reaches the maximum weight loss rate, it mainly solidifies the colloid into semi-coke. Although the volatiles were still continuously released, the growth rate decreased obviously. The curve of the variation of constant temperature residual weight ΔWcp showed a parabolic trend between 350 °C and 500 °C, Increase first and then decrease, the peak occurs at 450 °C. This indicates that in the early stage of pyrolysis, the temperature rise activates a larger number of macromolecular structures to participate in the reaction. The
3.2. Functional group distribution characteristics of pyrolysis residues This section carries out infrared spectrum analysis. Obtaining changes in the content of functional groups in the coal sample under different pyrolysis reaction temperatures, thus analyzed the structural properties of lignite at different pyrolysis temperatures, revealed the evolution mechanism of the functional groups in the pyrolysis stage of SL lignite. Fig. 3 is an infrared test spectrum of five samples of SL lignite raw coal, 350 °C, 400 °C, 450 °C, and 500 °C pyrolysis residues. The bands at 3200–3600, 2920, 2855, and 2965 cm−1 in the range of 200–4000 cm−1 represent the tensile vibration of OeH and CeH in the benzene ring, methylene and methyl groups. The band at 1700 and 1655 cm−1 in the 650–2000 cm−1 band represents the tensile vibration of C]O in the carboxyl group (eCOOH) and the ketone carbonyl group (eC]O). The carbon–carbon double bond (C]C) tensile vibration on the benzene ring corresponds to a band of 1495–1615 cm−1. The peak for each functional group cannot be confirmed clearly in the raw coal spectra. Therefore, Fourier deconvolution of the spectra is used to separate these groups and especially the shoulder peak clearly. The curve-fitting method is applied to obtain the experimental spectral intensity of each functional group. The peaks of each functional group were integrated to obtain the original peak area quantitatively. The 4
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Fig. 3. Pyrolytic infrared spectra of coal samples. Fig. 4. Changes of lignite functional groups in pyrolysis.
extinction coefficient differs of different functional groups varies greatly. In order to obtain the accurate distribution characteristics of functional groups from coal infrared spectra, the original peak area of the functional group was corrected by using the correction relationship between the absorbance of different functional groups and the peak area, as shown in Table 3. The correction relationship [46] is the ratios of experimental infrared intensities and unit absorption intensities f. The vibration strength f of each functional group varies greatly, so there is a big gap between the corrected peak area A(v)/f and the original peak area A(v). In the original spectrum, the infrared intensity of the hydroxyl group is much larger than other groups, but it also has a large vibration intensity, the difference between the corrected infrared intensity and other groups is greatly reduced. The theoretical infrared vibration intensity obtained by quantum chemical calculation eliminates the influence of extinction coefficient on the quantitative calculation of experimental infrared spectrum and corrects the peak area ratio of each functional group. According to the data in Table 3, the contents of each group in raw coal and pyrolysis residue at different temperatures were obtained, as shown in Fig. 4. Fig. 4 shows that the content of aromatic hydrocarbon is up to more than 50% in raw coal structure, the sum of aromatic structure, aliphatic hydrocarbon and associative hydroxyl content accounts for more than 90% of the total functional group content, and the content of oxygencontaining functional group is relatively less. With the increase of pyrolysis temperature, the content of associative hydroxyl, aliphatic hydrocarbon and oxygen-containing functional group in coal gradually decreases, while the content of aromatic skeleton structure in coal gradually increases. The associative hydroxyl groups are very sensitive to the pyrolysis reaction. At 350 °C, the associative hydroxyl groups in the coal samples
have been reduced from 12.34% to 1.9%, the difference is 5.49 times. This is mainly due to the large amount of water loss in the coal caused by the initial dry degassing stage of the pyrolysis reaction, resulting in a large reduction in the water-associated hydroxyl groups in the coal. In the pyrolysis reaction stage, the hydroxyl content continues to decrease, but the rate of reduction is very small. This is mainly due to the loss of a large amount of hydroxyl groups in the initial drying stage. After entering the pyrolysis stage, the content of water-associated hydroxyl groups in the coal has been very small. At this time, due to the influence of high temperature exotherm, the polyol phenol associated hydroxyl in coal begins to decompose thermally, because its content is much less than the water-associated hydroxyl group, the overall reduction in hydroxyl content is small. The content of aliphatic hydrocarbons also decreased with the increase of pyrolysis temperature, and it was very sensitive to pyrolysis reaction. The content of aliphatic hydrocarbons in SL lignite before pyrolysis was 25.4%, decreased to 15.27% at 350 °C. By the time the pyrolysis proceeds to 500 °C, the aliphatic hydrocarbon content in the coal structure is only 8.46%, which is one third of the aliphatic hydrocarbon content of the raw coal. This situation is mainly caused by high temperature causing weak chemical bond breakage in coal, side chain thermal cracking, formation of light volatile matter and tar, resulting in a decrease in aliphatic side chains and a decrease in aliphatic hydrocarbon content in coal structure. The content of aromatic hydrocarbons in SL lignite is greatly affected by pyrolysis reaction. The aromatic hydrocarbon content of raw coal is 54.46%. The aromatic structure content increases to 74.47% at 350 °C pyrolysis. When the temperature reaches 500 °C, the aromatic
Table 3 Original peak areas and corrected values of functional groups in coal. Functional group −1
Band (cm
)
OeH
eCH3
eCH2-
eCOOH
eC]O
Benz
ene (C]C)
3200–3600
2965.00
2924.00
2854.00
1700.00
1660.00
1620–1490
Raw coal
A(v) A(v)/f
14063.94 22.25
905.95 11.73
2277.36 34.08
683.11
1204.45 4.90
1437.01 9.16
4229.94 98.19
350 °C Pyrolysis residue
A(v) A(v)/f
63.47 0.10
13.72 0.18
33.72 0.63
20.90
71.31 0.29
23.75 0.15
169.36 3.93
400 °C Pyrolysis residue
A(v) A(v)/f
1755.96 2.78
302.47 3.91
1938.93 28.59
544.54
812.49 3.30
1063.40 6.78
7983.90 185.33
450 °C Pyrolysis residue
A(v) A(v)/f
948.80 1.50
316.77 4.10
1531.40 23.22
485.78
698.27 2.84
864.71 5.51
8736.04 202.79
500 °C Pyrolysis residue
A(v) A(v)/f
869.23 1.38
333.34 4.31
1169.55 18.62
448.42
578.57 2.35
886.91 5.65
10291.78 238.90
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Table 4 Eigenvalue of coal oxidation. Characteristic parameters
Raw coal
350 °C
400 °C
450 °C
500 °C
Ignition temperature Ti, °C Burnout temperature Th, °C Maximum speed of weightlessness dWmax Maximum weightlessness temperature TWmax, °C Combustion half peak width ΔT 1 -°C
237.2 588.09 0.6399
235.11 584.85 0.8601
232.24 576.67 0.9364
234.72 577.56 0.9818
236.9 581.47 0.948
388.38
385.88
387.87
385.31
384.47
81.53
77.04
69.83
66.71
67.9
2
pyrolysis is relatively reduced, indicating that the combustion degree is more concentrated. However, the TG-DTG curves of different residual structures of coal sample pyrolysis appear to overlap with each other during the entire oxidation process. The influence of pyrolysis temperature on the oxidation characteristics of residual structures cannot be quantitatively analyzed. Therefore, the coal combustion characteristic parameters are used as reference objects to quantitatively analyze the influence of pyrolysis temperature. Five different feature point values were selected to characterize the thermal analysis characteristics of TG-DTG. Table 4 shows the characteristic parameters of re-oxidation of lignite raw coal and residual structure at different temperatures. Ignition point Ti -°C, The temperature point at which the coal sample changes from slow oxidation to intense oxidation. There are many methods to determine the temperature of the ignition point, including TG-DTG method (extrapolation start method), TGA method, DTGA method, TGA curve demarcation method, DTGA curve demarcation method and so on. The TG-DTG method is selected to determine the ignition point temperature. That is, the peak point of the DTG curve is made as a vertical line, and then the tangent line of the intersection point of the vertical line and the TG curve is made, and the intersection temperature of the tangent line and the parallel line passing through the TG curve weight loss starting point is the ignition point temperature, then do the tangent of the intersection of the perpendicular and the TG curve. The junction temperature between the tangent and the parallel line passing through the starting point of the weight loss of the TG curve is the ignition temperature. Burnout temperature Th-°C, The temperature at which the oxidation reaction ends. The determination methods include TG-DTG, TG, DTG and fixed percentage of weight loss. The TG-DTG method is still selected here to determine. That is, the intersection temperature of the parallel line passing the end point of weightlessness of TG curve and the tangent line made before. This temperature indicates that the combustible material (volatile components, fixed carbon, etc.) in coal has been consumed, and the whole combustion process is ended. dw Maximum speed of weightlessness dWmax = dT -%/°C,
Fig. 5. Thermogravimetric curve of different coal oxidation (a) TG results and (b) DTG results.
hydrocarbon structure content is even as high as 88.08%. This is mainly because the reaction has been in the late stage of thermal decomposition. At this time, the colloidal coal are transformed from the initial formation to the thermal decomposition, the weak chemical bonds such as oxygen-containing groups are thermally cracked, the molecular structure is condensed, the degree of aromatization of the coal structure is increased, and the aromatic hydrocarbon content is increased.
3.3. Pyrolysis residue reoxidation
( )
Fig. 5 shows the results of TG-DTG of SL lignite raw coal and pyrolysis residual structure at different temperatures. As can be seen in Fig. 5, SL lignite has no obvious oxygen-absorbing weight gain process in the whole oxidation reaction. Because the lignite coal rank is low, the degree of structural polymerization is small, and the group also has high activity, the weight consumed by the oxidation reaction exceeds the weight increased by oxygen absorption, resulting in an insignificant process of oxygen-absorbing weight gain. In addition, observing the oxidation curve of the raw coal and pyrolysis residue of lignite, it can be clearly seen that the pyrolysis process changes the oxidation characteristics of the coal sample. Coal sample after pyrolysis can be clearly seen the process of drying degasification and oxygen absorption gaining weight in the process of reoxidation. At the same time, the combustion intensity of pyrolysis residual structure is obviously higher than that of raw coal. The combustion concentration degree is similar to the combustion intensity, which is greatly affected by pyrolysis. The semi-peak width of coal sample combustion after
max
Maximum weightlessness temperature TWmax -°C. That is, the speed at which the oxidation reaction is most intense and the corresponding temperature point reflect the combustion intensity of the coal sample, which is reflected as the peak point of the weight loss peak on the DTG curve. Combustion half peak width ΔT 1 −° C . That is, the half peak width 2 of coal sample DTG curve combustion peak, representing the concentration degree of coal sample combustion. The value is smaller, the concentration degree of coal sample combustion is stronger. As shown in Table 4, with the increase of residual temperature, the ignition temperature and the burnout temperature of SL lignite firstly decrease and then increase, and reach the lowest value at 400 °C. The highest temperature variation between the ignition points is 5 °C, and the highest temperature variation at the burnout point is 12 °C. Before
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spontaneous combustion effectively, and ensuring the safe and efficient mining of lignite.
400 °C, it is believed that generated more small molecular structure fragments which are easy to oxidize before the thermal decomposition of coal. With the increase of temperature, leading to the advance of the ignition temperature. 400–450 °C, although the amount of small molecular structure fragments is increasing, the structure which is easily oxidized further decomposed and lost, resulting in a temperature rise of the ignition point. Burnout temperature changes show that 400 °C is the best temperature in key segments of aromatic structure bridge fracture. The bridge bond rupture completely, aromatic structure has not been polymerization, lead to burnout point the lowest temperature, below or above 400 °C, aromatic skeleton structure are greater than 400 °C. The maximum weight loss rate of SL lignite combustion increases with the increase of residual pyrolysis temperature, and reached the maximum at 450 °C, then began to decrease, while the maximum weight loss rate point temperature change was small. This shows that the maximum combustion intensity is 60% higher than that of raw coal due to the thermal decomposition. At the same time, the combustion half peak width is reduced by 18%. It shows that the burning intensity of coal rise with the increase of residual temperature of pyrolysis. And reach maximum at 450 °C, consistent with the critical transition temperature of coal thermal decomposition. The analysis suggests that the smaller molecule fragments formed by the early pyrolysis, the more favorable the concentrated combustion reaction of the structure. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention.
Acknowledgments This work was supported by Natural Science Foundation of China (51704284), the Independent research project of State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM19X0013), the Natural Science Foundation of Jiangsu Province (BK20161183) and Natural Science Foundation of China (51774275), the Research Funds of the International Research Center for Underground Coal Gasification, CUMT (2018KJZX05). References [1] Querol X, Zhuang X, Font O, Izquierdo M, Alastuey A, Castro I, et al. Influence of soil cover on reducing the environmental impact of spontaneous coal combustion in coal waste gobs: a review and new experimental data. Int J Coal Geol 2011;85(1):2–22. [2] Wang HH, Dlugogorski BZ, Kennedy EM. Pathways for production of CO2 and CO in low-temperature oxidation of coal. Energy Fuels 2003;17(1):150–8. [3] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Model predictions and experimental results on self-heating prevention of stockpiled coals. Fuel 2001;80(1):125–34. [4] Shao ZL, Jia XY, Zhong XX, Wang DM, Wei J, Wang YM, et al. Detection, extinguishing, and monitoring of a coal fire in Xinjiang China. Environ Sci Pollut Res 2018;25(26):26603–16. [5] Zhong XX, Wang MM, Dou GL, Wang DM, Chen Y, Mo YM, et al. Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction. J Energy Inst 2015;88(4):398–405. [6] Zhong XX, Dou GL, Xin HH, Wang DM. Study on low-temperature oxidation process of low rank coal by in-situ FTIR. Stafa-Zurich: Trans Tech Publications Ltd; 2013. p. 871–6. [7] Qi Xuyao, Wang Deming, Xin Haihui, Qi Guansheng. In situ FTIR study of real-time changes of active groups during oxygen-free reaction of coal. Energy Fuels 2013;27(6):3130–6. [8] Qi Xuyao, Xin Haihui, Wang Deming, Qi Guansheng. A rapid method for determining the R-70 self-heating rate of coal. Thermochim Acta 2013;571:21–7. [9] Wang De-ming, Xin Hai-hui, Qi Xu-yao, Dou Guo-lan, Qi Guan-sheng, Ma Li-yang. Reaction pathway of coal oxidation at low temperatures: a model of cyclic chain reactions and kinetic characteristics. Combust Flame 2016;163:447–60. [10] Wang HH, Dlugogorski BZ, Kennedy EM. Thermal decomposition of solid oxygenated complexes formed by coal oxidation at low temperatures. Fuel 2002;81(15):1913–23. [11] Wang HH, Dlugogorski BZ, Kennedy EM. Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Prog Energy Combust Sci 2003;29(6):487–513. [12] Xin Hai-hui, Wang De-ming, Qi Xu-yao, Zhong Xiao-xing, Ma Li-yang, Dou Guo-lan, et al. Oxygen consumption and chemisorption in low-temperature oxidation of subbituminous pulverized coal. Spectrosc Lett 2018;51(2):104–11. [13] Zhou Q, Ding DQ, Zhang YF, Zhang YM, Dong T, Chen XY, et al. Effect of semi-char content on oxygen-enriched co-combustion behaviors of lignite and semi-char. J Energy Eng 2018;144(4):9. [14] Zhang GJ, Su AT, Zhang YF, Wang P. Shrinkage character in the process of semicoke formation. Chiang Mai J Sci 2015;42(2):401–6. [15] Yoruk CR, Meriste T, Sener S, Kuusik R, Trikkel A. Thermogravimetric analysis and process simulation of oxy-fuel combustion of blended fuels including oil shale, semicoke, and biomass. Int J Energy Res 2018;42(6):2213–24. [16] Wu Q, Zhu ZZ, Shi GJ, Wang F, Wang ZL, Xie YY. Improving quality of coke made from chinese xinjiang gas coal with high strength modifier. Proceedings of the 3rd Pan American Materials Congress. 2017. p. 529–38. [17] Wang X, Zeng WL, Guo QJ, Geng QJ, Yan YM, Hu XD. The further activation and functionalization of semicoke for CO2 capture from flue gases. RSC Adv 2018;8(62):35521–7. [18] Tran QA, Stanger R, Xie W, Smith N, Lucas J, Wall T. Linking thermoplastic development and swelling with molecular weight changes of a coking coal and its pyrolysis products. Energy Fuels 2016;30(5):3906–16. [19] Sun M, Ning X, Zhang J, Li K, Tang Q, Liu Z, et al. Combustion kinetics and structural features of bituminous coal before and after modification process. J Therm Anal Calorim 2018;131(2):983–92. [20] Shao ZL, Wang DM, Wang YM, Zhong XX, Tang XF, Hu XM. Controlling coal fires using the three-phase foam and water mist techniques in the anjialing open pit mine, China. Nat Hazards 2015;75(2):1833–52. [21] Mochizuki Y, Ono Y, Tsubouchi N. Evolution profile of gases during coal carbonization and relationship between their amounts and the fluidity or coke strength. Fuel 2019;237:735–44. [22] Chen Q, He R, Xu XC, Liang ZG, Chen CH. Experimental study on pore structure and apparent kinetic parameters of char combustion in kinetics-controlled regime. Energy Fuels 2004;18(5):1562–8. [23] He R, Sato J, Chen CH. Modeling char combustion with fractal pore effects. Combust Sci Technol 2002;174(4):19–37.
4. Conclusions This paper selected lignite coal sample from Shengli coalfield of Inner Mongolia for pyrolysis and reburning experiments. Using thermogravimetric analysis and infrared analysis to analyze the macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues. The lignite coal samples were subjected to constant temperature pyrolysis thermogravimetric analysis at 350 °C, 400 °C, 450 °C and 500 °C. The reburning characteristics of coal pyrolysis residual structure were analyzed. The results can help to understand the thermogravimetric characteristics of the reburning in the surrounding pyrolysis area affected by coal combustion in coal fir. It is great significance for the prevention and control of coalfield fire and safe unsealing of the closed working face in coal mine. It is found that the coal sample has the same trend of pyrolysis process curves at different pyrolysis temperatures. As the pyrolysis temperature increases, the residual temperature of the coal sample and the residual weight of the pyrolysis end point gradually decrease. A significant turning point before 450 °C such as the variation of pyrolysis residual weight ΔWvp, the variation of constant temperature residual weight ΔWcp, and the pyrolysis residual combustible component weight ΔWpo. With the increase of pyrolysis temperature, the water-associated hydroxyl group is greatly reduced due to the dehydration and drying of the coal sample. The aliphatic hydrocarbons are gradually reduced due to the side chain cracking in the coal, and the aromatic hydrocarbons increase with the degree of aromatization of the coal structure. For the oxidation characteristics of lignite pyrolysis residue, the ignition temperature and the burnout temperature of SL lignite firstly decrease and then increase with the increase of residual temperature, and reach the lowest value at 400 °C. The maximum weight loss rate of SL lignite combustion increases with the increase of residual pyrolysis temperature, and reached the maximum at 450 °C. However, the combustion half peak width decreased and reached the minimum at 450 °C. The pyrolysis residue at ~400 °C is the easiest to reburning, and the pyrolysis residue at ~450 °C had the strongest reburning intensity. These results can help determine the reburning risk of residual coal in the spontaneous combustion area of lignite, which is the basis for further determining effective measures of fire prevention. It is great significance for preventing and controlling the lignite reburning after 7
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