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Investigation on the effect of transition metal chloride on anthracite combustion
Fuel 264 (2020) 116796
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Full Length Article
Investigation on the effect of transition metal chloride on anthracite combustion
T
Bo Tan , Hongyi Wei, Bin Xu, Hongru Zhao, Zhuangzhuang Shao ⁎
School of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Transition metal chlorides Burn off Heat release efficiency Free radical
In recent years, the international community has paid more attention to environmental pollution and climate change caused by the use of fossil fuels. How to strengthen the efficient use of coal has become a problem that must be solved. This work aims to study the effect of transition metal chlorides (CuCl2, FeCl3 and ZnCl2) on the promotion of “burn off” reaction and the improvement of anthracite heat release efficiency by using TG-DSC, FTIR and EPR. The effect of adding three transition metal chlorides on oxygen absorption and heat release of anthracite was studied by TG-DSC. Results show that the addition of CuCl2, FeCl3 and ZnCl2 can promote the “burn off” reaction, thereby increasing the heat release efficiency of anthracite. Specifically, the addition of CuCl2 increases the heat release of the anthracite by 15%. FTIR was used to study the structural changes during the heating process of raw coal and coal with transition metal chlorides. Results show that CuCl2 can effectively promote the direct pyrolysis of aliphatic functional groups and reduce the formation of oxygen-containing functional groups. In addition, the mechanism of CuCl2 promoting the “burn off” reaction was studied by ESR spectra analyses. It is found that the free radical concentration of the coal with CuCl2 is higher than that of the raw coal, which indicating that the “burn off” reaction is an oxidation reaction dominated by free radical. This work may have a guiding significance for the efficient utilization of anthracite.
1. Introduction When coal is used as a fuel, heat release is an important aspect for assessing its combustion performance. The oxygen adsorption reaction sequence and the “burn off” reaction sequence are two independent reaction sequences of coal oxidation process at the low-medium temperature, which have some influences on the heat release of coal [1]. The “burn off” reaction, which is similar to the direct combustion reaction of a solid fuel, can react rapidly with oxygen and generate gas products. Although many scholars have demonstrated the existence of the “burn-off” reaction by using kinetic analysis [2–5], the “burn off” reaction is often difficult to observe for many reasons. The mechanism of this reaction and its relationship with the oxygen adsorption reaction sequence are both worthy of discussion. There are many free radicals both on surfaces of the coal particles and the new crack of coal, which can create conditions for the coal oxidation [6–8]. Jing et al. [9] concluded that free radical concentration was a key factor in controlling the “burn-off” reaction. Many scholars have used electron paramagnetic resonance spectroscopy (ESR/EPR) to directly test free radical concentration in coal [10,11]. In addition, some scholars have found that some transition metal ions ⁎
have catalytic effects, so they can promote the formation of free radicals in coal structures as initiators [12,13], which could affect the combustion characteristics of coal. Cui et al. [14] studied the effect of a series of Ni-Co ternary molten salt crystals on the catalytic pyrolysis mechanism of Datong coal by using thermal gravimetric analyzer (TGA) and a reactive kinetic model. The experimental results showed that NiCo ternary molten salt catalysts had the capability to bring down activation energy required by pyrolytic reactions at its initial phase. Also, the catalysts exerted a preferable catalytic action on macromolecular structure decomposition and free radical polycondensation reactions. In order to further reduce the NOx emission from power plants, the catalytic effects of main metals in coal ash (Na, K, Fe, Ca) on advanced reburning of pulverized coal was reported by Qiu et al. [15]. The study found that FeCl3 had a certain catalytic effect on NO reduction and some types of metals can improve the NO reduction in reburning via increasing the concentration of CH4 and CO during the reactions. In addition, Kodama et al. [16,17] demonstrated Metal-oxide-catalyzed CO2 gasification of coal in small packed-bed and fluidized-bed reactors using a solar furnace simulator, for the purpose of converting solar high-temperature heat to chemical fuels. Results showed that in the packed-bed reactor, ZnO much improved the chemical coal conversion
Corresponding author. E-mail address: [email protected] (B. Tan).
https://doi.org/10.1016/j.fuel.2019.116796 Received 30 July 2019; Received in revised form 20 November 2019; Accepted 29 November 2019 Available online 13 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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by about 2–3 times at the catalyst loading of 30 wt%-Zn in the coalmetal-oxide mixture at temperatures around 1000–1400 K. At present, scholars have not found an effective way to make the “burn off” reaction occur significantly. The recognition that the “burn off” is the reaction which dominated by free radical comes from the speculation of other scholars' experimental results, not the detection of the free radical concentration. In addition, coal is expected to remain indispensable component of global energy system in the coming decades [18–20]. Therefore, under the call of promoting energy conservation and protecting the environment, it is of practical significance to explore the efficient use of coal. In past research works, the catalytic action of transition metals is mostly used in the fields of coal pyrolysis and combustion and removing nitrogen oxides and desulfurization in the chemical industry [21–24]. Chiu et al. [22] examined the influences of V, Cu and Mn oxide impregnation on the multipollutant Hg/SO2/NO control using a SCR catalyst. The study found that Hg0 oxidation, SO2 removal, and NO reduction of the SCR catalyst can be enhanced after the metal oxide impregnation. CuOx-impregnated catalysts had not only excellent Hg0 oxidation but also great NO reduction. Yadav et al. [24] studied the effect of ZnCl2 on the thermal conversion of coal by changing the ZnCl2 concentration and carbonization time. Results showed that ZnCl2 catalyst enhanced the burn-off rate at low temperature and was economically beneficial at 7% concentration and 602 °C carbonization temperature. However, there are very few scholars who focus on the effect and mechanism of transition metals on improving the heat release efficiency of coal combustion. CuCl2, FeCl3 and ZnCl2 are three typical and low-cost transition metal chlorides which are often used in catalytic experiments [25–28]. Therefore, CuCl2, FeCl3 and ZnCl2 are selected to explore their effects on prompting “burn off” reaction and heat release of anthracite using synchronous thermal analyses (TG-DSC) and Fourier transform infrared spectroscopy (FTIR). Specifically, the mechanism and key factor of the “burn off” reaction are further discussed by using electron paramagnetic resonance spectroscopy (EPR), which could provide a new way for efficient utilization of anthracite.
Table 2 The composition of the samples. No.
Mass of coal (g)
Additive
Mass of additive (g)
Purity of additive
1 2 3 4
5 5 5 5
None CuCl2 FeCl3 ZnCl2
0 0.15 0.15 0.15
None Analytical pure Analytical pure Analytical pure
flow rate was 100 mL/min, and the temperature was elevated from 30 °C to about 800 °C at heating rate 5, 10, 15 °C/min, respectively. Combined with an infrared spectrometer (TENSOR27), gas products of the samples during the oxidation process were detected in real time. Samples were subjected to infrared spectrum analysis using the infrared spectrometer (TENSOR27). First, the samples were placed in a synchronous thermal analyzer for constant temperature oxidation. Then they were mixed with KBr powder at a ratio of 1:150 (i.e., sample was 1 mg, KBr powder was 150 mg) and ground. The well-ground powder was placed in a tablet machine for tableting. Then the sheet was placed in a sample chamber of the infrared spectrometer for testing. The wave number ranged from 400 to 4000 cm−1, the resolution was 4.0 cm−1, the number of scans was 32 times and the time was 1 min. The experimental conditions of the sample preparation process and the test process were the same for each test. The JES-FA200 spin resonance spectrometer was used to test the changes of free radical concentration of raw coal and coal with CuCl2 at low-medium temperature. The microwave frequency was 9442.465 MHz, the microwave power was 0.998 mW, the central magnetic field was 337.291 mT, the scanning width was 7.5 mT, the modulation frequency was 100 kHz, the modulation width was 1 mT and the time constant was 0.03 s. To obtain information on free radical concentration, g-factor and line width, Tempo was used as the standard sample. 3. Results and discussions
2. Experimental
3.1. TG & DSC curves analyses
2.1. Samples preparation
Some scholars have used TG-DSC curves to study the characteristics of low-medium temperature oxidation processes of coal at different temperature ranges [30,31]. As shown in Fig. 1, a set of TG/DSC curves of raw coal at 5 °C/min are cited as an example, because low heating rate can eliminate thermal hysteresis of samples. The oxidation process
The experimental coal sample was anthracite, selected from Yangquan Coal Mine in Shanxi Province, China. Proximate analysis and ultimate analysis of coal are shown in Table 1. First, coal samples of mixed particle size were selected and dried in a drying oven to a constant mass. The additives were also separately dried to remove water. To prevent sample contamination caused by moisture in the air, CuCl2, FeCl3 and ZnCl2 were thoroughly ground and pulverized, and then blended with the raw coal for about 10 min under an infrared lamp. The prepared samples were sealed and stored in sample bags. Some scholars have found that adding 3 wt% CuCl2 can significantly promote the decomposition of coal [29], so the composition of the test samples are shown in Table 2. 2.2. Experimental scheme The STA449F3 synchronous thermogravimetric analyzer was used to observe the mass and heat changes of four samples in Table 2 during the heating process. Approximately 10 mg of sample was weighed per test. The atmosphere was N2 (80 mL/min) and O2 (20 mL/min), the gas Table 1 Proximate and ultimate analysis for the coal used in the current study. Proximate analysis (wt%) Mad 3.9
Aad 10.14
Vad 8.58
Ultimate analysis (wt%) FCad 78.11
Cad 73.13
Had 1.001
Oad 4.13
Nad 0.92
Sad 0.236
Fig. 1. Four stages of coal oxidation up to 500 °C determined by a set of TG/ DSC curves at 5 °C/min. 2
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Table 3 Some parameters of the samples during the heating process. No. 1 2 3 4
sample C C&CuCl2 C&FeCl3 C&ZnCl2
Tc1 (℃) 130 187 229 204
Tc2 (℃) 370 303 320 347
Tc3 (℃)
ΔG (%)
440 318 392 405
3.36 1.25 1.04 1.75
of coal (0–500 °C) is divided into four stages: stage I – dehydration stage (T < Tc1), Tc1 is the temperature corresponding to the mass that reached a minimum value at the end of dehydration stage on the TG curve; Stage II – oxygen absorption and mass gain stage (Tc1 < T < Tc2), Tc2 is the temperature corresponding to the maximum mass on the TG curve; Stage III – a significant pyrolysis stage (Tc2 < T < Tc3), Tc3 is the temperature corresponding to a distinct peak on the DDSC curve (differential of the DSC curve), and the rate of change of the heat flow rate rapidly decreases to the minimum from this peak; Stage IV – the oxidation reaction stage dominated by the “burnoff” reaction (T > Tc3). Tc1, Tc2, and Tc3 of the four samples are shown in Table 3. The TG and DSC curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 are compared in Fig. 2. It can be seen from Fig. 2(a) that the change trend of TG curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 are basically the same: first, samples undergo the dehydration stage, and the mass shows a downward trend. Because the transition metal chlorides have water absorption, dehydration stages of C&CuCl2, C&FeCl3 and C&ZnCl2 continue to about 200° C and their Tc1 lag significantly behind that of the raw coal (130 ℃). After that, during the oxygen absorption and mass gain stage, the mass of samples slowly increases to a maximum value. As shown in Table 3, Tc2 of C&CuCl2, C& FeCl3 and C&ZnCl2 are earlier than that of the raw coal (370 °C). C& CuCl2 reaches the oxygen absorption peak earliest, about 70 °C ahead of the raw coal, followed by FeCl3, and finally ZnCl2. Oxygen absorption quantity ΔG of C&FeCl3 is the lowest, only about one third of that of the raw coal. As the temperature increases, the samples entered the significant pyrolysis stage, and the mass decreases again. Finally, at oxidation reaction stage dominated by the “burn-off” reaction, the mass of the samples all drop sharply. At 500 °C, the mass of C&CuCl2, C&FeCl3 and C&ZnCl2 are 9.53%, 64.42% and 53.17%, respectively, which are less than that of the raw coal (80.31%), indicating that the addition of transition metal chlorides can accelerate the oxidation and decomposition of coal. When four samples are burned out, the residual masses
ΔH5 (J/g) 2.06 2.59 2.08 2.37
× × × ×
ΔH10 (J/g) 4
10 104 104 104
2.27 2.70 2.29 2.54
× × × ×
4
10 104 104 104
ΔH15 (J/g) 2.05 2.41 2.02 2.38
× × × ×
104 104 104 104
of the samples with the transition metal chlorides are slightly higher than that of the raw coal, which may be because the transition metal chlorides do not completely react. It can be seen from Fig. 2(b) that the DSC curves first change from endothermic to exothermic after the dehydration stage. At the oxygen absorption and mass gain stage, the heat flow rate drops sharply, then maintains a slight decline at the significant pyrolysis stage. Finally, at the “burn-off” reaction, the heat flow rate drops sharply again. Comparing the DSC curves of the four samples, it can be seen that the temperatures at which “burn-off” reaction appeared after adding the transition metal chlorides are 318 °C, 392 °C and 405 °C, respectively, which are all lower than that of the raw coal (440 ℃). It indicates that the addition of transition metal chlorides promotes the occurrence of “burn-off” reaction and burning rate. In addition, multiple DSC peaks indicate that oxidation reaction catalyzed by metal chlorides was more complicated. By integrating the DSC curves separately, the heat release ΔH of samples during the entire heating process are obtained, as shown in Table 3. ΔH5, ΔH10 and ΔH15 represent the heat release of the samples at the heating rate of 5, 10 and 15 °C/min, respectively. It is found that the total heat release of C&CuCl2 and C&ZnCl2 are higher than that of raw coal. ΔH of C&CuCl2 at 5 °C/min reaches 2.59 × 104 J/g, which is 15% higher than that of raw coal, indicating that the heat release capacity of anthracite is enhanced. 3.2. FTIR analyses of gas products Fig. 3 shows an infrared spectrum of gas products of four samples during oxidation process. It can be seen from Fig. 3 that the absorption peak intensity of CO2 (2650–2200, 850–400 cm−1) is much larger than CO (2200–1900 cm−1) and H2O (3700–3625, 1650–1350 cm−1), which means CO2 is the main gas product [32,33]. For raw coal, peak position of the CO2 absorption peak appears later (about 566 °C), while the CO2 absorption peak intensity of the coal with additives reach their respective the peak position of the absorption peak earlier. It can be
Fig. 2. TG and DSC curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 5 °C/min. 3
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Fig. 3. Gas products of samples during oxidation process.
seen from Fig. 3 that there are two peaks on curves after adding CuCl2, FeCl3 and ZnCl2, which indicates that the production of gas in the oxidation process includes two stages. Some scholars have studied the gas products of coal with different nickel content and find that there are also the same trends [34], but their specific mechanisms need further study. For raw coal, the maximum value of absorption peak intensity of CO2 is about 0.32% at 566 °C. After the addition of transition metal chlorides, the CO2 production increases. Specifically, the increase of CO2 produced by C&CuCl2 is the most obvious. When the temperature is 401 °C, the maximum value of absorption peak intensity of CO2 of C& CuCl2 is close to 0.58%, which is 1.8 times that of raw coal. In addition, after adding the transition metal chloride, the content of incomplete combustion product such as CO is reduced. The absorption peak intensity of CO of C&CuCl2 and C&ZnCl2 are both about 0.02%, which are only two-fifths of the raw coal (0.05%). It indicates that the addition of transition metal chlorides can effectively promote the full combustion of anthracite, thereby improving the heat release efficiency. CuCl2, FeCl3 and ZnCl2 have water absorption, so the absorption peaks intensity of H2O of C&CuCl2, C&FeCl3 and C&ZnCl2 are about 0.03%, 0.02% and 0.03%, respectively, which are higher than that of the raw coal (0.01%).
Fig. 4. Changes in the functional groups of raw coal with temperature.
which corresponds to the removal of moisture in the system. As the temperature continues to increase, the absorption peak intensity of the aliphatic functional groups gradually weakens, while that of the oxygen-containing functional groups gradually increase. This is because aliphatic hydrocarbons undergo an oxygen adsorption reaction and are oxidized to oxidation products such as aldehydes, carboxylic acids and esters. When the temperature is 440 °C, for one thing, the absorption peak intensity of aliphatic hydrocarbons continue to decrease, while the increase rate of absorption peak intensity of the oxygen-containing functional groups become slow, for another, the absorption peak
3.3. FTIR analyses of coal In order to compare the functional group changes of four samples at low-medium temperature oxidation process, FTIR analyses are of samples performed at 30 °C, 130 °C, 370 °C, 440 °C, and 500 °C. 130 °C, 370 °C, and 440 °C correspond to the Tc1, Tc2 and Tc3 of the raw coal, respectively. The infrared spectrums of the raw coal at different temperatures are shown in Fig. 4. It can be seen that the absorption peak intensity of the hydroxyl significantly weakens from 30 °C to 130 °C, 4
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which are consistent with the results of other scholars [35–38]. Since anthracite is a high metamorphic coal, there are few aliphatic structures. As mentioned above, 130–370 °C, 440–500 °C correspond to the oxygen absorption and mass gain stage and the “burn-off” reaction stage of the raw coal, respectively. Peak areas of the raw coal and C& CuCl2 in the two stages are shown in Fig. 7. It can be seen that the peak areas of –CH, –CH3 and –CH2 in the raw coal increase sequentially, and –CH2 and –CH account for 68–90% of the total amount of aliphatic hydrocarbons, indicating that there are more alkyl side chains in the coal. At 30–440 °C, the peak areas of aliphatic structures of the raw coal show a downward trend with the temperature. This is mainly because the aliphatic functional groups react with oxygen during the heating process to produce oxygen-containing functional groups such as carboxylic acids and aldehydes. However, the peak areas of aliphatic structures increase when the raw coal enters the “burn-off” reaction (440–500 °C). This may be because after entering the “burn-off” reaction, the benzene ring skeleton decomposes, resulting in an increase in the peak areas of methyl and methylene. The peak areas of the aliphatic structures of C&CuCl2 are less than that of the raw coal at the same temperature. As can be seen from Table 3, C&CuCl2 carry out the “burnoff” reaction at 318 °C. Therefore, when the raw coal enters the “burnoff” reaction at 440 °C, the aliphatic structure of C&CuCl2 is much smaller than that of the raw coal. It indicates that the addition of CuCl2 promotes the occurrence of a “burn-off” reaction, thereby promoting the consumption of aliphatic structures.
Fig. 5. Functional groups of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 440 °C.
3.3.2. Curve-fitting analysis of oxygen-containing functional groups 1000–1800 cm−1 belongs to the absorption vibration zone of oxygen-containing functional groups. As shown in Fig. 8, the results of the curve-fitting analysis of the raw coal at different temperatures include 10–11 peaks, which are consistent with the results of other scholars [35,37,38]. Peak areas of the functional groups of this region at 130–370 °C and 440–500 °C are shown in Fig. 9. It can be seen that the conjugated C]O (1660 cm−1) exists at the low temperature, while disappears at a high temperature. The carboxylic acid C]O (1701 cm−1) shows a different trend. This is because some of the highly reactive alkyl side chains are easily oxidized to carboxyl groups during the oxidation process of coal. The carboxylic acid C]O of the raw coal appears at 370 °C, while that of C&CuCl2 could be observed at 130 °C, indicating that the addition of CuCl2 accelerates the oxidation process of the coal. At 370–500 °C, the C]O peak area of the C&CuCl2 is always lower than that of raw coal at the corresponding temperature, indicating that the addition of CuCl2 effectively inhibits the formation of carboxylic acid oxygen-containing functional groups. In addition, both the aryl ether (1100 cm−1) and the alkyl ether (1025 cm−1) show increase trends after adding CuCl2. Some scholars believe that the addition of CuCl2 can promote the conversion of O–H bond to ether bond.
Fig. 6. Curve-fitting analysis of aliphatic functional groups of raw coal at 440 °C.
intensity of the aromatic structures decrease sharply, indicating that the sample enters the “burn-off” reaction. Fig. 5 is infrared spectrums of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 440 °C. It can be seen from the above analysis results that the addition of CuCl2 is most effective for improving the heat release efficiency of anthracite. Therefore, changes of functional groups of raw coal and C&CuCl2 at different temperatures will be the focus of research. Because the length of this paper is limited, only raw coal is used as an example, its curve-fitting analysis results of aliphatic structures, oxygen-containing functional groups and aromatic structures at 440 °C are shown in Figs. 6, 8, and 10. Peak areas of functional groups at different temperatures of the raw coal and C&CuCl2 are shown in Figs. 7, 9, 11.
3.3.3. Curve-fitting analysis of aromatic structures 900–700 cm−1 belongs to the absorption vibration zone of aromatic structures. As shown in Fig. 10, the results of the curve-fitting analysis of the raw coal at different temperatures include 3–6 peaks, which are consistent with the results of other scholars [35,37,38]. Peak areas of the functional groups of this region at 130–370 °C and 440–500 °C are shown in Fig. 11. It can be seen that when the raw coal enters the “burn-off” reaction (440–500 °C), the aromatic structures decompose rapidly. At 30 °C, the peak areas of each aromatic functional group of C &CuCl2 are similar to that of the raw coal. However, the aromatic structure peak areas of C&CuCl2 are smaller than that of the raw coal with temperature. At 440 °C, isolated aromatic hydrogens and four adjacent aromatic hydrogens per ring of C&CuCl2 are completely decomposed. It indicates that the addition of CuCl2 can promote the decomposition of the stable benzene ring, thereby producing more CO2 and improving the heat release efficiency.
3.3.1. Curve-fitting analysis of aliphatic functional groups 2800–3000 cm−1 belongs to the absorption vibration zone of aliphatic hydrocarbons. As shown in Fig. 6, the results of the curve-fitting analysis of the raw coal at different temperatures include 5–6 peaks, 5
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Fig. 7. Peak areas of aliphatic functional groups of raw coal and C&CuCl2 at different temperatures.
the above analysis results that the addition of CuCl2 is most effective for prompting the “burn-off” reaction of anthracite. Therefore, the electron paramagnetic resonance spectrometer is used to analyze the free radical concentration changes of raw coal and C&CuCl2 at different temperatures. The electron paramagnetic resonance spectrums of the raw coal at different temperatures are shown in Fig. 12(a). It can be seen that the Lande factor g of the raw coal basically maintains at about 2.002 and the line width ΔH also has no significant change with temperatures. This is consistent with the results of EPR spectra analyses obtained by other scholars during the heating process [39]. Fig. 12(b) shows the EPR spectra of raw coal and C&CuCl2 at 440 °C. As shown in Fig. 12(b), the Lande factor g of C&CuCl2 slightly deviates from that of the raw coal. Some scholars believe that unpaired electrons in transition metal ions and their compounds contribute a lot to the orbital magnetic moment, which leads to differences in g between the two samples [40,41]. In addition, the ΔH of C&CuCl2 is wider than that of raw coal, which means the relaxation time is shorter. Changes of free radical concentration of raw coal and C&CuCl2 at different temperatures are shown in Fig. 13. It can be seen that, for the raw coal, the sample undergoes the dehydration stage and the oxygen absorption and mass gain stage from 30 °C to 370 °C, and the free radical concentration gradually increases. Some scholars have pointed out that proper temperature can activate free radicals [10]. As the temperature continues to rise, the raw coal goes to a significant pyrolysis stage so the free radicals are consumed heavily, reaching a minimum of 1.2898 1017/g at 440 °C. At the “burn-off” reaction stage, the free
Fig. 8. Curve-fitting analysis of oxygen-containing functional groups of raw coal at 440 °C.
3.4. EPR spectra analyses As the combustion of other organic matter is a chain reaction initiated by free radicals [38], the free radical concentration must be the key factor affecting the “burn-off” reaction of coal. It can be seen from
Fig. 9. Peak areas of oxygen-containing functional groups of raw coal and C&CuCl2 at different temperatures. 6
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and can inhibit the oxygen adsorption sequence. Since the oxygen absorption reaction sequence of the coal is dominant at the low temperature, the functional groups of coal are difficult to oxidize after being substituted by Cl-. In addition, at this time, the concentration of free radicals is small, so it is not enough to support the “burn off” reaction. Chloride inhibitors (such as CaCl2) mainly affect the oxygen adsorption reaction sequence. However, transition metal ions such as Cu2+, Fe3 +and Zn2+ are special, and they have catalytic abilities. Although the substitution reaction of Cl- inhibits the oxygen adsorption reaction sequence, the catalytic ability of Cu2+ is strong, which can catalyze the production of free radicals, thereby promoting the “burn off” sequence to appear and accelerating anthracite combustion. On the one hand, compared with many reactive sites that can be catalyzed in coal, the number Cl- is very small, so its retardant effect is limited although Cl- has the substitution reaction. On the other hand, Cu2+ has a strong catalytic ability, so the radicals are bound to increase. 4. Conclusions (1) The mass loss and heat release during the oxidation process of raw coal and samples with CuCl2, FeCl3 and ZnCl2 at 30–500 °C are observed by TG-DSC. And the three temperature points Tc1, Tc2, Tc3 are selected to divide the oxidation process of the coal into four stages. Results show that the addition of transition metal chlorides can effectively promote the “burn-off” reaction, thereby improving the anthracite heat release efficiency. Specifically, heat release of C &CuCl2 is 15% higher than that of raw coal. (2) By analyzing gas products in the heating process, it is found that the content of incomplete combustion product such as CO decreases after adding the transition metal chlorides, while the contents of H2O and CO2 both increase. CO2 produced by C&CuCl2 is about 1.8 times that of the raw coal, which shows that the addition of transition metal chlorides can promote the full combustion of the anthracite by strengthening the “burn-off” reaction. (3) The structural changes during the heating process of samples are analyzed by FTIR. For raw coal, the C–H absorption peak intensity continues to decrease with temperatures, while the C]O bond absorption peak intensity has a little increase or even decreases. It indicates that the C–H bond in the system no longer participates in the oxygen adsorption reaction sequence, but a “burn-off” reaction sequence is carried out to directly release gas products. The addition of CuCl2 greatly promotes the pyrolysis reaction of oxidation products such as carboxylic acids. At the same time, the benzene ring skeleton which is the main structure of the coal decomposes at high temperature. In addition, the oxidation reaction of a large amount of the benzene ring does not prevent the disappearance of
Fig. 10. Curve-fitting analysis of aromatic structures of raw coal at 440 °C.
radical concentration of the raw coal increases again. It indicates that the “burn-off” reaction can produce a large number of free radicals. The free radical concentration of C&CuCl2 is always higher than that of the raw coal during the heating process. Through the theoretical analysis, Jing et al. [29] also speculated that the addition of CuCl2 can greatly increase the concentration of free radicals in anthracite. Since the addition of CuCl2 accelerates the oxidation and decomposition of anthracite, the radical concentration of C&CuCl2 reaches a minimum at around 370 °C, which is consistent with the TG-FTIR analyses above. CuCl2 can promote the “burn-off” reaction just by increasing the free radical concentration in the system, indicating that the free radical concentration is the key factor in controlling the “burn-off” reaction. Some scholars believe that water-absorbing salt inhibitors (such as CaCl2) have retardant effects, mainly due to the substitution and complexation reaction of chloride ions, which affect the activation energy of chemical reaction of coal and the stability of coal molecular structure, thereby slowing down the oxidation process of coal. While the experimental results in this paper prove that the transition metal chlorides such as CuCl2 can promote the full combustion and improve the heat release efficiency of anthracite. This may be due to the existence of two sequences in the low-medium temperature oxidation process of coal. The substitution reaction of Cl- exists for all chlorides
Fig. 11. Peak areas of aromatic structures of raw coal and C&CuCl2 at different temperatures. 7
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Fig.12. Electron paramagnetic resonance spectra of raw coal and C&CuCl2.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Funding: This work was supported by the National Key Research and Development Program of China (2016YFC0801800) and the financial support of the National Nature Science Foundation of China (51774291, 51864045). References [1] Petit JC. Calorimetric evidence for a dual mechanism in the low temperature oxidation of coal. J Therm Anal 1991;37(8):1719–26. [2] Wang HH, Dlugogorski BZ, Kennedy EM. Coal oxidation at low temperatures: Oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Chemin 2004;29(6):487–513. [3] Wang H, Dlugogorski BZ, Kennedy EM. Kinetic modeling of low-temperature oxidation of coal. Combust Flame 2002;131(4):452–64. [4] Kam AY, Hixson AN, Perlmutter DD. The oxidation of bituminous coal-I Development of a mathematical model. Chem Eng Sci 1976;31(9):815–9. [5] Kam AY, Hixson AN, Perlmutter DD. The oxidation of bituminous COAL-II experimental kinetics and interpretation. Chem Eng Sci 1976;31(9):821–34. [6] Taub T, Ruthstein S, Cohen H. The involvement of carbon-centered radicals in the aging process of coals under atmospheric conditions: an EPR study. Phys Chem Chem Phys 2018;20(42). [7] Carr RM, Kumagai H, Peake BM, Robinson BH, Clemens AH, Matheson TW. Formation of free radicals during drying and oxidation of a lignite and a bituminous coal. Fuel 1995;74(3):389–94. [8] Wu DJ, Song ZY, Schmidt M, Zhang Q, Qian XM. Theoretical and numerical study on ignition behaviour of coal dust layers on a hot surface with corrected kinetic parameters. J Hazard Mater 2019;368:156–62. [9] Zhan J, Wang HH, Zhu F, Song SN. Analysis on the governing reactions in coal oxidation at temperatures up to 400 °C. Int J Clean Coal Energy 2014;03(2):19–28. [10] Liu JX, Jiang XM, Shen J, Zhang H. Influences of particle size, ultraviolet irradiation and pyrolysis temperature on stable free radicals in coal. Powder Technol 2015;272:64–74. [11] Liu MX, Yang JL, Yang Y, Liu ZY, Shi L, He WJ, et al. The radical and bond cleavage behaviors of 14 coals during pyrolysis with 9,10-dihydrophenanthrene. Fuel 2016;182:480–6. [12] Chen Y, Liu G, Wang L, Kang Y, Yang J. Occurrence and fate of some trace elements during pyrolysis of Yima Coal, China. Energy Fuels 2008;22(6):3877–82. [13] Pehlivan E, Richardson A, Zuman P. electrochemical investigation of binding of heavy metal ions to Turkish lignites. Electroanalysis 2010;16(16):1292–8. [14] Cui X, Qi C, Li L, Li Y, Li S. Effect of Ni–Co ternary molten salt catalysts on coal catalytic pyrolysis process. Int J Thermophys 2017;38(8):116. [15] Qiu PH, Huang H, Zhang JQ, Liu L, Chen YQ. Catalytic effects of main metals in coal ash on advanced reburning of pulverized coal. Energy Fuels 2010;24(3):4919–24. [16] Kodama T, Funatoh A, Shimizu T, Kitayama Y. Metal-oxide-catalyzed CO2 gasification of coal using a solar furnace simulator. Energy Fuels 2000;14(6):1323–30. [17] Kodama T, Funatoh A, Shimizu K, Kitayama Y. Kinetics of metal oxide-catalyzed CO2 gasification of coal in a fluidized-bed reactor for solar thermochemical process. Energy Fuels 2001;15(5):1200–6. [18] Diana C, Sorin M, Mirela I, Laura M, Sabina I. Creating competitive advantage in coal mining industry in Romania: a new challenge. Proced Econ Financ
Fig. 13. Changes of free radical concentration of raw coal and C&CuCl2 during heating process.
the C]O, indicating that the benzene ring structure does not produce solid phase oxidation products, but instead enters a fast “burnoff” reaction. It is an important reason for the improvement of anthracite heat release. (4) Results of EPR find that temperatures have little effect on the Lande factor g and the line width ΔH of the anthracite, while the free radical concentration fluctuates with temperatures. The free radicals in the sample increase rapidly after entering the “burn off” reaction. Since the transition metal chlorides have catalytic effects, they can promote the formation of free radicals in anthracite, thereby providing a key material basis for initiating the “burn off” reaction at a low temperature. Author contributions Bo Tan and Hongyi Wei designed the study and wrote the paper. Hongyi Wei and Zhuangzhuang Shao performed the experiments. Hongyi Wei and Bin Xu processed the experimental data. Bo Tan, Bin Xu and Hongru Zhao reviewed and edited the manuscript. All authors read and approved the manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial 8
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thermogravimetry/mass spectrometry. Energy Fuels 2014;28(5):3024–35. [31] Chen P, Zhang L, Huang K. Kinetic modeling of coal thermal decomposition under air atmosphere. Energy Fuels 2016;30(6). acs.energyfuels.6b00902. [32] Song ZY, Fan HR, Jiang JJ, Li CX. Insight into effects of pore diffusion on smoldering kinetics of coal using a 4-step chemical reaction model. J Loss Prev Process Ind 2017;48:312–9. [33] Song ZY, Wu DJ, Jiang JC, Pan XH. Thermo-solutal buoyancy driven air flow through thermally decomposed thin porous media in a U-shaped channel: towards understanding persistent underground coal fires. Appl Therm Eng 2019;159:113948. [34] Shirai M, Arai M, Murakami K. Structure and catalysis of ion-exchanged nickel species during pyrolysis of Loy Yang Coal. Energy Fuels 1999;13(2):465–70. [35] Li KJ, Khanna R, Zhang JL, Barati M, Sahajwalla V. Comprehensive investigation of various structural features of bituminous coals using advanced analytical techniques. Energy Fuels 2015;29(11). acs.energyfuels.5b02064. [36] Zhang YL, Wang JF, Xue S, Wu JM, Chang LP, Li ZF. Kinetic study on changes in methyl and methylene groups during low-temperature oxidation of coal via in-situ FTIR. Int J Coal Geol 2016;154–155:155–64. [37] Song HJ, Liu GR, Zhang JZ, Wu JH. Pyrolysis characteristics and kinetics of low rank coals by TG-FTIR method. Fuel Process Technol 2017;156:454–60. [38] Li F, Zhao GY, Zhao YY, Zhao MS, Tang JW. Construction of the molecular structure model of the Shengli lignite using TG-GC/MS and FTIR spectrometry data. Fuel 2017;203. S0016236117305379. [39] Li ZH, Wei AZ, Yang YL. Research on free radical reactions in spontaneous combustion of coal using an electron spin resonance. J China Univ Min Technol 2006;35(5):576–80. [40] Singh VP, Singh S, Singh DP, Tiwari K, Mishra M. Synthesis, spectroscopic (electronic, IR, NMR and ESR) and theoretical studies of transition metal complexes with some unsymmetrical Schiff bases. J Mol Struct 2014;1058(8):71–8. [41] Ji M, Tan LK, Jen-Jacobson L, Saxena S. Insights into copper coordination in the EcoRI-DNA complex by ESR spectroscopy. Mol Phys 2014;112(24):3173–82.
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Full Length Article
Investigation on the effect of transition metal chloride on anthracite combustion
T
Bo Tan , Hongyi Wei, Bin Xu, Hongru Zhao, Zhuangzhuang Shao ⁎
School of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Transition metal chlorides Burn off Heat release efficiency Free radical
In recent years, the international community has paid more attention to environmental pollution and climate change caused by the use of fossil fuels. How to strengthen the efficient use of coal has become a problem that must be solved. This work aims to study the effect of transition metal chlorides (CuCl2, FeCl3 and ZnCl2) on the promotion of “burn off” reaction and the improvement of anthracite heat release efficiency by using TG-DSC, FTIR and EPR. The effect of adding three transition metal chlorides on oxygen absorption and heat release of anthracite was studied by TG-DSC. Results show that the addition of CuCl2, FeCl3 and ZnCl2 can promote the “burn off” reaction, thereby increasing the heat release efficiency of anthracite. Specifically, the addition of CuCl2 increases the heat release of the anthracite by 15%. FTIR was used to study the structural changes during the heating process of raw coal and coal with transition metal chlorides. Results show that CuCl2 can effectively promote the direct pyrolysis of aliphatic functional groups and reduce the formation of oxygen-containing functional groups. In addition, the mechanism of CuCl2 promoting the “burn off” reaction was studied by ESR spectra analyses. It is found that the free radical concentration of the coal with CuCl2 is higher than that of the raw coal, which indicating that the “burn off” reaction is an oxidation reaction dominated by free radical. This work may have a guiding significance for the efficient utilization of anthracite.
1. Introduction When coal is used as a fuel, heat release is an important aspect for assessing its combustion performance. The oxygen adsorption reaction sequence and the “burn off” reaction sequence are two independent reaction sequences of coal oxidation process at the low-medium temperature, which have some influences on the heat release of coal [1]. The “burn off” reaction, which is similar to the direct combustion reaction of a solid fuel, can react rapidly with oxygen and generate gas products. Although many scholars have demonstrated the existence of the “burn-off” reaction by using kinetic analysis [2–5], the “burn off” reaction is often difficult to observe for many reasons. The mechanism of this reaction and its relationship with the oxygen adsorption reaction sequence are both worthy of discussion. There are many free radicals both on surfaces of the coal particles and the new crack of coal, which can create conditions for the coal oxidation [6–8]. Jing et al. [9] concluded that free radical concentration was a key factor in controlling the “burn-off” reaction. Many scholars have used electron paramagnetic resonance spectroscopy (ESR/EPR) to directly test free radical concentration in coal [10,11]. In addition, some scholars have found that some transition metal ions ⁎
have catalytic effects, so they can promote the formation of free radicals in coal structures as initiators [12,13], which could affect the combustion characteristics of coal. Cui et al. [14] studied the effect of a series of Ni-Co ternary molten salt crystals on the catalytic pyrolysis mechanism of Datong coal by using thermal gravimetric analyzer (TGA) and a reactive kinetic model. The experimental results showed that NiCo ternary molten salt catalysts had the capability to bring down activation energy required by pyrolytic reactions at its initial phase. Also, the catalysts exerted a preferable catalytic action on macromolecular structure decomposition and free radical polycondensation reactions. In order to further reduce the NOx emission from power plants, the catalytic effects of main metals in coal ash (Na, K, Fe, Ca) on advanced reburning of pulverized coal was reported by Qiu et al. [15]. The study found that FeCl3 had a certain catalytic effect on NO reduction and some types of metals can improve the NO reduction in reburning via increasing the concentration of CH4 and CO during the reactions. In addition, Kodama et al. [16,17] demonstrated Metal-oxide-catalyzed CO2 gasification of coal in small packed-bed and fluidized-bed reactors using a solar furnace simulator, for the purpose of converting solar high-temperature heat to chemical fuels. Results showed that in the packed-bed reactor, ZnO much improved the chemical coal conversion
Corresponding author. E-mail address: [email protected] (B. Tan).
https://doi.org/10.1016/j.fuel.2019.116796 Received 30 July 2019; Received in revised form 20 November 2019; Accepted 29 November 2019 Available online 13 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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by about 2–3 times at the catalyst loading of 30 wt%-Zn in the coalmetal-oxide mixture at temperatures around 1000–1400 K. At present, scholars have not found an effective way to make the “burn off” reaction occur significantly. The recognition that the “burn off” is the reaction which dominated by free radical comes from the speculation of other scholars' experimental results, not the detection of the free radical concentration. In addition, coal is expected to remain indispensable component of global energy system in the coming decades [18–20]. Therefore, under the call of promoting energy conservation and protecting the environment, it is of practical significance to explore the efficient use of coal. In past research works, the catalytic action of transition metals is mostly used in the fields of coal pyrolysis and combustion and removing nitrogen oxides and desulfurization in the chemical industry [21–24]. Chiu et al. [22] examined the influences of V, Cu and Mn oxide impregnation on the multipollutant Hg/SO2/NO control using a SCR catalyst. The study found that Hg0 oxidation, SO2 removal, and NO reduction of the SCR catalyst can be enhanced after the metal oxide impregnation. CuOx-impregnated catalysts had not only excellent Hg0 oxidation but also great NO reduction. Yadav et al. [24] studied the effect of ZnCl2 on the thermal conversion of coal by changing the ZnCl2 concentration and carbonization time. Results showed that ZnCl2 catalyst enhanced the burn-off rate at low temperature and was economically beneficial at 7% concentration and 602 °C carbonization temperature. However, there are very few scholars who focus on the effect and mechanism of transition metals on improving the heat release efficiency of coal combustion. CuCl2, FeCl3 and ZnCl2 are three typical and low-cost transition metal chlorides which are often used in catalytic experiments [25–28]. Therefore, CuCl2, FeCl3 and ZnCl2 are selected to explore their effects on prompting “burn off” reaction and heat release of anthracite using synchronous thermal analyses (TG-DSC) and Fourier transform infrared spectroscopy (FTIR). Specifically, the mechanism and key factor of the “burn off” reaction are further discussed by using electron paramagnetic resonance spectroscopy (EPR), which could provide a new way for efficient utilization of anthracite.
Table 2 The composition of the samples. No.
Mass of coal (g)
Additive
Mass of additive (g)
Purity of additive
1 2 3 4
5 5 5 5
None CuCl2 FeCl3 ZnCl2
0 0.15 0.15 0.15
None Analytical pure Analytical pure Analytical pure
flow rate was 100 mL/min, and the temperature was elevated from 30 °C to about 800 °C at heating rate 5, 10, 15 °C/min, respectively. Combined with an infrared spectrometer (TENSOR27), gas products of the samples during the oxidation process were detected in real time. Samples were subjected to infrared spectrum analysis using the infrared spectrometer (TENSOR27). First, the samples were placed in a synchronous thermal analyzer for constant temperature oxidation. Then they were mixed with KBr powder at a ratio of 1:150 (i.e., sample was 1 mg, KBr powder was 150 mg) and ground. The well-ground powder was placed in a tablet machine for tableting. Then the sheet was placed in a sample chamber of the infrared spectrometer for testing. The wave number ranged from 400 to 4000 cm−1, the resolution was 4.0 cm−1, the number of scans was 32 times and the time was 1 min. The experimental conditions of the sample preparation process and the test process were the same for each test. The JES-FA200 spin resonance spectrometer was used to test the changes of free radical concentration of raw coal and coal with CuCl2 at low-medium temperature. The microwave frequency was 9442.465 MHz, the microwave power was 0.998 mW, the central magnetic field was 337.291 mT, the scanning width was 7.5 mT, the modulation frequency was 100 kHz, the modulation width was 1 mT and the time constant was 0.03 s. To obtain information on free radical concentration, g-factor and line width, Tempo was used as the standard sample. 3. Results and discussions
2. Experimental
3.1. TG & DSC curves analyses
2.1. Samples preparation
Some scholars have used TG-DSC curves to study the characteristics of low-medium temperature oxidation processes of coal at different temperature ranges [30,31]. As shown in Fig. 1, a set of TG/DSC curves of raw coal at 5 °C/min are cited as an example, because low heating rate can eliminate thermal hysteresis of samples. The oxidation process
The experimental coal sample was anthracite, selected from Yangquan Coal Mine in Shanxi Province, China. Proximate analysis and ultimate analysis of coal are shown in Table 1. First, coal samples of mixed particle size were selected and dried in a drying oven to a constant mass. The additives were also separately dried to remove water. To prevent sample contamination caused by moisture in the air, CuCl2, FeCl3 and ZnCl2 were thoroughly ground and pulverized, and then blended with the raw coal for about 10 min under an infrared lamp. The prepared samples were sealed and stored in sample bags. Some scholars have found that adding 3 wt% CuCl2 can significantly promote the decomposition of coal [29], so the composition of the test samples are shown in Table 2. 2.2. Experimental scheme The STA449F3 synchronous thermogravimetric analyzer was used to observe the mass and heat changes of four samples in Table 2 during the heating process. Approximately 10 mg of sample was weighed per test. The atmosphere was N2 (80 mL/min) and O2 (20 mL/min), the gas Table 1 Proximate and ultimate analysis for the coal used in the current study. Proximate analysis (wt%) Mad 3.9
Aad 10.14
Vad 8.58
Ultimate analysis (wt%) FCad 78.11
Cad 73.13
Had 1.001
Oad 4.13
Nad 0.92
Sad 0.236
Fig. 1. Four stages of coal oxidation up to 500 °C determined by a set of TG/ DSC curves at 5 °C/min. 2
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Table 3 Some parameters of the samples during the heating process. No. 1 2 3 4
sample C C&CuCl2 C&FeCl3 C&ZnCl2
Tc1 (℃) 130 187 229 204
Tc2 (℃) 370 303 320 347
Tc3 (℃)
ΔG (%)
440 318 392 405
3.36 1.25 1.04 1.75
of coal (0–500 °C) is divided into four stages: stage I – dehydration stage (T < Tc1), Tc1 is the temperature corresponding to the mass that reached a minimum value at the end of dehydration stage on the TG curve; Stage II – oxygen absorption and mass gain stage (Tc1 < T < Tc2), Tc2 is the temperature corresponding to the maximum mass on the TG curve; Stage III – a significant pyrolysis stage (Tc2 < T < Tc3), Tc3 is the temperature corresponding to a distinct peak on the DDSC curve (differential of the DSC curve), and the rate of change of the heat flow rate rapidly decreases to the minimum from this peak; Stage IV – the oxidation reaction stage dominated by the “burnoff” reaction (T > Tc3). Tc1, Tc2, and Tc3 of the four samples are shown in Table 3. The TG and DSC curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 are compared in Fig. 2. It can be seen from Fig. 2(a) that the change trend of TG curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 are basically the same: first, samples undergo the dehydration stage, and the mass shows a downward trend. Because the transition metal chlorides have water absorption, dehydration stages of C&CuCl2, C&FeCl3 and C&ZnCl2 continue to about 200° C and their Tc1 lag significantly behind that of the raw coal (130 ℃). After that, during the oxygen absorption and mass gain stage, the mass of samples slowly increases to a maximum value. As shown in Table 3, Tc2 of C&CuCl2, C& FeCl3 and C&ZnCl2 are earlier than that of the raw coal (370 °C). C& CuCl2 reaches the oxygen absorption peak earliest, about 70 °C ahead of the raw coal, followed by FeCl3, and finally ZnCl2. Oxygen absorption quantity ΔG of C&FeCl3 is the lowest, only about one third of that of the raw coal. As the temperature increases, the samples entered the significant pyrolysis stage, and the mass decreases again. Finally, at oxidation reaction stage dominated by the “burn-off” reaction, the mass of the samples all drop sharply. At 500 °C, the mass of C&CuCl2, C&FeCl3 and C&ZnCl2 are 9.53%, 64.42% and 53.17%, respectively, which are less than that of the raw coal (80.31%), indicating that the addition of transition metal chlorides can accelerate the oxidation and decomposition of coal. When four samples are burned out, the residual masses
ΔH5 (J/g) 2.06 2.59 2.08 2.37
× × × ×
ΔH10 (J/g) 4
10 104 104 104
2.27 2.70 2.29 2.54
× × × ×
4
10 104 104 104
ΔH15 (J/g) 2.05 2.41 2.02 2.38
× × × ×
104 104 104 104
of the samples with the transition metal chlorides are slightly higher than that of the raw coal, which may be because the transition metal chlorides do not completely react. It can be seen from Fig. 2(b) that the DSC curves first change from endothermic to exothermic after the dehydration stage. At the oxygen absorption and mass gain stage, the heat flow rate drops sharply, then maintains a slight decline at the significant pyrolysis stage. Finally, at the “burn-off” reaction, the heat flow rate drops sharply again. Comparing the DSC curves of the four samples, it can be seen that the temperatures at which “burn-off” reaction appeared after adding the transition metal chlorides are 318 °C, 392 °C and 405 °C, respectively, which are all lower than that of the raw coal (440 ℃). It indicates that the addition of transition metal chlorides promotes the occurrence of “burn-off” reaction and burning rate. In addition, multiple DSC peaks indicate that oxidation reaction catalyzed by metal chlorides was more complicated. By integrating the DSC curves separately, the heat release ΔH of samples during the entire heating process are obtained, as shown in Table 3. ΔH5, ΔH10 and ΔH15 represent the heat release of the samples at the heating rate of 5, 10 and 15 °C/min, respectively. It is found that the total heat release of C&CuCl2 and C&ZnCl2 are higher than that of raw coal. ΔH of C&CuCl2 at 5 °C/min reaches 2.59 × 104 J/g, which is 15% higher than that of raw coal, indicating that the heat release capacity of anthracite is enhanced. 3.2. FTIR analyses of gas products Fig. 3 shows an infrared spectrum of gas products of four samples during oxidation process. It can be seen from Fig. 3 that the absorption peak intensity of CO2 (2650–2200, 850–400 cm−1) is much larger than CO (2200–1900 cm−1) and H2O (3700–3625, 1650–1350 cm−1), which means CO2 is the main gas product [32,33]. For raw coal, peak position of the CO2 absorption peak appears later (about 566 °C), while the CO2 absorption peak intensity of the coal with additives reach their respective the peak position of the absorption peak earlier. It can be
Fig. 2. TG and DSC curves of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 5 °C/min. 3
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Fig. 3. Gas products of samples during oxidation process.
seen from Fig. 3 that there are two peaks on curves after adding CuCl2, FeCl3 and ZnCl2, which indicates that the production of gas in the oxidation process includes two stages. Some scholars have studied the gas products of coal with different nickel content and find that there are also the same trends [34], but their specific mechanisms need further study. For raw coal, the maximum value of absorption peak intensity of CO2 is about 0.32% at 566 °C. After the addition of transition metal chlorides, the CO2 production increases. Specifically, the increase of CO2 produced by C&CuCl2 is the most obvious. When the temperature is 401 °C, the maximum value of absorption peak intensity of CO2 of C& CuCl2 is close to 0.58%, which is 1.8 times that of raw coal. In addition, after adding the transition metal chloride, the content of incomplete combustion product such as CO is reduced. The absorption peak intensity of CO of C&CuCl2 and C&ZnCl2 are both about 0.02%, which are only two-fifths of the raw coal (0.05%). It indicates that the addition of transition metal chlorides can effectively promote the full combustion of anthracite, thereby improving the heat release efficiency. CuCl2, FeCl3 and ZnCl2 have water absorption, so the absorption peaks intensity of H2O of C&CuCl2, C&FeCl3 and C&ZnCl2 are about 0.03%, 0.02% and 0.03%, respectively, which are higher than that of the raw coal (0.01%).
Fig. 4. Changes in the functional groups of raw coal with temperature.
which corresponds to the removal of moisture in the system. As the temperature continues to increase, the absorption peak intensity of the aliphatic functional groups gradually weakens, while that of the oxygen-containing functional groups gradually increase. This is because aliphatic hydrocarbons undergo an oxygen adsorption reaction and are oxidized to oxidation products such as aldehydes, carboxylic acids and esters. When the temperature is 440 °C, for one thing, the absorption peak intensity of aliphatic hydrocarbons continue to decrease, while the increase rate of absorption peak intensity of the oxygen-containing functional groups become slow, for another, the absorption peak
3.3. FTIR analyses of coal In order to compare the functional group changes of four samples at low-medium temperature oxidation process, FTIR analyses are of samples performed at 30 °C, 130 °C, 370 °C, 440 °C, and 500 °C. 130 °C, 370 °C, and 440 °C correspond to the Tc1, Tc2 and Tc3 of the raw coal, respectively. The infrared spectrums of the raw coal at different temperatures are shown in Fig. 4. It can be seen that the absorption peak intensity of the hydroxyl significantly weakens from 30 °C to 130 °C, 4
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which are consistent with the results of other scholars [35–38]. Since anthracite is a high metamorphic coal, there are few aliphatic structures. As mentioned above, 130–370 °C, 440–500 °C correspond to the oxygen absorption and mass gain stage and the “burn-off” reaction stage of the raw coal, respectively. Peak areas of the raw coal and C& CuCl2 in the two stages are shown in Fig. 7. It can be seen that the peak areas of –CH, –CH3 and –CH2 in the raw coal increase sequentially, and –CH2 and –CH account for 68–90% of the total amount of aliphatic hydrocarbons, indicating that there are more alkyl side chains in the coal. At 30–440 °C, the peak areas of aliphatic structures of the raw coal show a downward trend with the temperature. This is mainly because the aliphatic functional groups react with oxygen during the heating process to produce oxygen-containing functional groups such as carboxylic acids and aldehydes. However, the peak areas of aliphatic structures increase when the raw coal enters the “burn-off” reaction (440–500 °C). This may be because after entering the “burn-off” reaction, the benzene ring skeleton decomposes, resulting in an increase in the peak areas of methyl and methylene. The peak areas of the aliphatic structures of C&CuCl2 are less than that of the raw coal at the same temperature. As can be seen from Table 3, C&CuCl2 carry out the “burnoff” reaction at 318 °C. Therefore, when the raw coal enters the “burnoff” reaction at 440 °C, the aliphatic structure of C&CuCl2 is much smaller than that of the raw coal. It indicates that the addition of CuCl2 promotes the occurrence of a “burn-off” reaction, thereby promoting the consumption of aliphatic structures.
Fig. 5. Functional groups of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 440 °C.
3.3.2. Curve-fitting analysis of oxygen-containing functional groups 1000–1800 cm−1 belongs to the absorption vibration zone of oxygen-containing functional groups. As shown in Fig. 8, the results of the curve-fitting analysis of the raw coal at different temperatures include 10–11 peaks, which are consistent with the results of other scholars [35,37,38]. Peak areas of the functional groups of this region at 130–370 °C and 440–500 °C are shown in Fig. 9. It can be seen that the conjugated C]O (1660 cm−1) exists at the low temperature, while disappears at a high temperature. The carboxylic acid C]O (1701 cm−1) shows a different trend. This is because some of the highly reactive alkyl side chains are easily oxidized to carboxyl groups during the oxidation process of coal. The carboxylic acid C]O of the raw coal appears at 370 °C, while that of C&CuCl2 could be observed at 130 °C, indicating that the addition of CuCl2 accelerates the oxidation process of the coal. At 370–500 °C, the C]O peak area of the C&CuCl2 is always lower than that of raw coal at the corresponding temperature, indicating that the addition of CuCl2 effectively inhibits the formation of carboxylic acid oxygen-containing functional groups. In addition, both the aryl ether (1100 cm−1) and the alkyl ether (1025 cm−1) show increase trends after adding CuCl2. Some scholars believe that the addition of CuCl2 can promote the conversion of O–H bond to ether bond.
Fig. 6. Curve-fitting analysis of aliphatic functional groups of raw coal at 440 °C.
intensity of the aromatic structures decrease sharply, indicating that the sample enters the “burn-off” reaction. Fig. 5 is infrared spectrums of raw coal and coal with CuCl2, FeCl3 and ZnCl2 at 440 °C. It can be seen from the above analysis results that the addition of CuCl2 is most effective for improving the heat release efficiency of anthracite. Therefore, changes of functional groups of raw coal and C&CuCl2 at different temperatures will be the focus of research. Because the length of this paper is limited, only raw coal is used as an example, its curve-fitting analysis results of aliphatic structures, oxygen-containing functional groups and aromatic structures at 440 °C are shown in Figs. 6, 8, and 10. Peak areas of functional groups at different temperatures of the raw coal and C&CuCl2 are shown in Figs. 7, 9, 11.
3.3.3. Curve-fitting analysis of aromatic structures 900–700 cm−1 belongs to the absorption vibration zone of aromatic structures. As shown in Fig. 10, the results of the curve-fitting analysis of the raw coal at different temperatures include 3–6 peaks, which are consistent with the results of other scholars [35,37,38]. Peak areas of the functional groups of this region at 130–370 °C and 440–500 °C are shown in Fig. 11. It can be seen that when the raw coal enters the “burn-off” reaction (440–500 °C), the aromatic structures decompose rapidly. At 30 °C, the peak areas of each aromatic functional group of C &CuCl2 are similar to that of the raw coal. However, the aromatic structure peak areas of C&CuCl2 are smaller than that of the raw coal with temperature. At 440 °C, isolated aromatic hydrogens and four adjacent aromatic hydrogens per ring of C&CuCl2 are completely decomposed. It indicates that the addition of CuCl2 can promote the decomposition of the stable benzene ring, thereby producing more CO2 and improving the heat release efficiency.
3.3.1. Curve-fitting analysis of aliphatic functional groups 2800–3000 cm−1 belongs to the absorption vibration zone of aliphatic hydrocarbons. As shown in Fig. 6, the results of the curve-fitting analysis of the raw coal at different temperatures include 5–6 peaks, 5
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Fig. 7. Peak areas of aliphatic functional groups of raw coal and C&CuCl2 at different temperatures.
the above analysis results that the addition of CuCl2 is most effective for prompting the “burn-off” reaction of anthracite. Therefore, the electron paramagnetic resonance spectrometer is used to analyze the free radical concentration changes of raw coal and C&CuCl2 at different temperatures. The electron paramagnetic resonance spectrums of the raw coal at different temperatures are shown in Fig. 12(a). It can be seen that the Lande factor g of the raw coal basically maintains at about 2.002 and the line width ΔH also has no significant change with temperatures. This is consistent with the results of EPR spectra analyses obtained by other scholars during the heating process [39]. Fig. 12(b) shows the EPR spectra of raw coal and C&CuCl2 at 440 °C. As shown in Fig. 12(b), the Lande factor g of C&CuCl2 slightly deviates from that of the raw coal. Some scholars believe that unpaired electrons in transition metal ions and their compounds contribute a lot to the orbital magnetic moment, which leads to differences in g between the two samples [40,41]. In addition, the ΔH of C&CuCl2 is wider than that of raw coal, which means the relaxation time is shorter. Changes of free radical concentration of raw coal and C&CuCl2 at different temperatures are shown in Fig. 13. It can be seen that, for the raw coal, the sample undergoes the dehydration stage and the oxygen absorption and mass gain stage from 30 °C to 370 °C, and the free radical concentration gradually increases. Some scholars have pointed out that proper temperature can activate free radicals [10]. As the temperature continues to rise, the raw coal goes to a significant pyrolysis stage so the free radicals are consumed heavily, reaching a minimum of 1.2898 1017/g at 440 °C. At the “burn-off” reaction stage, the free
Fig. 8. Curve-fitting analysis of oxygen-containing functional groups of raw coal at 440 °C.
3.4. EPR spectra analyses As the combustion of other organic matter is a chain reaction initiated by free radicals [38], the free radical concentration must be the key factor affecting the “burn-off” reaction of coal. It can be seen from
Fig. 9. Peak areas of oxygen-containing functional groups of raw coal and C&CuCl2 at different temperatures. 6
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and can inhibit the oxygen adsorption sequence. Since the oxygen absorption reaction sequence of the coal is dominant at the low temperature, the functional groups of coal are difficult to oxidize after being substituted by Cl-. In addition, at this time, the concentration of free radicals is small, so it is not enough to support the “burn off” reaction. Chloride inhibitors (such as CaCl2) mainly affect the oxygen adsorption reaction sequence. However, transition metal ions such as Cu2+, Fe3 +and Zn2+ are special, and they have catalytic abilities. Although the substitution reaction of Cl- inhibits the oxygen adsorption reaction sequence, the catalytic ability of Cu2+ is strong, which can catalyze the production of free radicals, thereby promoting the “burn off” sequence to appear and accelerating anthracite combustion. On the one hand, compared with many reactive sites that can be catalyzed in coal, the number Cl- is very small, so its retardant effect is limited although Cl- has the substitution reaction. On the other hand, Cu2+ has a strong catalytic ability, so the radicals are bound to increase. 4. Conclusions (1) The mass loss and heat release during the oxidation process of raw coal and samples with CuCl2, FeCl3 and ZnCl2 at 30–500 °C are observed by TG-DSC. And the three temperature points Tc1, Tc2, Tc3 are selected to divide the oxidation process of the coal into four stages. Results show that the addition of transition metal chlorides can effectively promote the “burn-off” reaction, thereby improving the anthracite heat release efficiency. Specifically, heat release of C &CuCl2 is 15% higher than that of raw coal. (2) By analyzing gas products in the heating process, it is found that the content of incomplete combustion product such as CO decreases after adding the transition metal chlorides, while the contents of H2O and CO2 both increase. CO2 produced by C&CuCl2 is about 1.8 times that of the raw coal, which shows that the addition of transition metal chlorides can promote the full combustion of the anthracite by strengthening the “burn-off” reaction. (3) The structural changes during the heating process of samples are analyzed by FTIR. For raw coal, the C–H absorption peak intensity continues to decrease with temperatures, while the C]O bond absorption peak intensity has a little increase or even decreases. It indicates that the C–H bond in the system no longer participates in the oxygen adsorption reaction sequence, but a “burn-off” reaction sequence is carried out to directly release gas products. The addition of CuCl2 greatly promotes the pyrolysis reaction of oxidation products such as carboxylic acids. At the same time, the benzene ring skeleton which is the main structure of the coal decomposes at high temperature. In addition, the oxidation reaction of a large amount of the benzene ring does not prevent the disappearance of
Fig. 10. Curve-fitting analysis of aromatic structures of raw coal at 440 °C.
radical concentration of the raw coal increases again. It indicates that the “burn-off” reaction can produce a large number of free radicals. The free radical concentration of C&CuCl2 is always higher than that of the raw coal during the heating process. Through the theoretical analysis, Jing et al. [29] also speculated that the addition of CuCl2 can greatly increase the concentration of free radicals in anthracite. Since the addition of CuCl2 accelerates the oxidation and decomposition of anthracite, the radical concentration of C&CuCl2 reaches a minimum at around 370 °C, which is consistent with the TG-FTIR analyses above. CuCl2 can promote the “burn-off” reaction just by increasing the free radical concentration in the system, indicating that the free radical concentration is the key factor in controlling the “burn-off” reaction. Some scholars believe that water-absorbing salt inhibitors (such as CaCl2) have retardant effects, mainly due to the substitution and complexation reaction of chloride ions, which affect the activation energy of chemical reaction of coal and the stability of coal molecular structure, thereby slowing down the oxidation process of coal. While the experimental results in this paper prove that the transition metal chlorides such as CuCl2 can promote the full combustion and improve the heat release efficiency of anthracite. This may be due to the existence of two sequences in the low-medium temperature oxidation process of coal. The substitution reaction of Cl- exists for all chlorides
Fig. 11. Peak areas of aromatic structures of raw coal and C&CuCl2 at different temperatures. 7
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Fig.12. Electron paramagnetic resonance spectra of raw coal and C&CuCl2.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Funding: This work was supported by the National Key Research and Development Program of China (2016YFC0801800) and the financial support of the National Nature Science Foundation of China (51774291, 51864045). References [1] Petit JC. Calorimetric evidence for a dual mechanism in the low temperature oxidation of coal. J Therm Anal 1991;37(8):1719–26. [2] Wang HH, Dlugogorski BZ, Kennedy EM. Coal oxidation at low temperatures: Oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Chemin 2004;29(6):487–513. [3] Wang H, Dlugogorski BZ, Kennedy EM. Kinetic modeling of low-temperature oxidation of coal. Combust Flame 2002;131(4):452–64. [4] Kam AY, Hixson AN, Perlmutter DD. The oxidation of bituminous coal-I Development of a mathematical model. Chem Eng Sci 1976;31(9):815–9. [5] Kam AY, Hixson AN, Perlmutter DD. The oxidation of bituminous COAL-II experimental kinetics and interpretation. Chem Eng Sci 1976;31(9):821–34. [6] Taub T, Ruthstein S, Cohen H. The involvement of carbon-centered radicals in the aging process of coals under atmospheric conditions: an EPR study. Phys Chem Chem Phys 2018;20(42). [7] Carr RM, Kumagai H, Peake BM, Robinson BH, Clemens AH, Matheson TW. Formation of free radicals during drying and oxidation of a lignite and a bituminous coal. Fuel 1995;74(3):389–94. [8] Wu DJ, Song ZY, Schmidt M, Zhang Q, Qian XM. Theoretical and numerical study on ignition behaviour of coal dust layers on a hot surface with corrected kinetic parameters. J Hazard Mater 2019;368:156–62. [9] Zhan J, Wang HH, Zhu F, Song SN. Analysis on the governing reactions in coal oxidation at temperatures up to 400 °C. Int J Clean Coal Energy 2014;03(2):19–28. [10] Liu JX, Jiang XM, Shen J, Zhang H. Influences of particle size, ultraviolet irradiation and pyrolysis temperature on stable free radicals in coal. Powder Technol 2015;272:64–74. [11] Liu MX, Yang JL, Yang Y, Liu ZY, Shi L, He WJ, et al. The radical and bond cleavage behaviors of 14 coals during pyrolysis with 9,10-dihydrophenanthrene. Fuel 2016;182:480–6. [12] Chen Y, Liu G, Wang L, Kang Y, Yang J. Occurrence and fate of some trace elements during pyrolysis of Yima Coal, China. Energy Fuels 2008;22(6):3877–82. [13] Pehlivan E, Richardson A, Zuman P. electrochemical investigation of binding of heavy metal ions to Turkish lignites. Electroanalysis 2010;16(16):1292–8. [14] Cui X, Qi C, Li L, Li Y, Li S. Effect of Ni–Co ternary molten salt catalysts on coal catalytic pyrolysis process. Int J Thermophys 2017;38(8):116. [15] Qiu PH, Huang H, Zhang JQ, Liu L, Chen YQ. Catalytic effects of main metals in coal ash on advanced reburning of pulverized coal. Energy Fuels 2010;24(3):4919–24. [16] Kodama T, Funatoh A, Shimizu T, Kitayama Y. Metal-oxide-catalyzed CO2 gasification of coal using a solar furnace simulator. Energy Fuels 2000;14(6):1323–30. [17] Kodama T, Funatoh A, Shimizu K, Kitayama Y. Kinetics of metal oxide-catalyzed CO2 gasification of coal in a fluidized-bed reactor for solar thermochemical process. Energy Fuels 2001;15(5):1200–6. [18] Diana C, Sorin M, Mirela I, Laura M, Sabina I. Creating competitive advantage in coal mining industry in Romania: a new challenge. Proced Econ Financ
Fig. 13. Changes of free radical concentration of raw coal and C&CuCl2 during heating process.
the C]O, indicating that the benzene ring structure does not produce solid phase oxidation products, but instead enters a fast “burnoff” reaction. It is an important reason for the improvement of anthracite heat release. (4) Results of EPR find that temperatures have little effect on the Lande factor g and the line width ΔH of the anthracite, while the free radical concentration fluctuates with temperatures. The free radicals in the sample increase rapidly after entering the “burn off” reaction. Since the transition metal chlorides have catalytic effects, they can promote the formation of free radicals in anthracite, thereby providing a key material basis for initiating the “burn off” reaction at a low temperature. Author contributions Bo Tan and Hongyi Wei designed the study and wrote the paper. Hongyi Wei and Zhuangzhuang Shao performed the experiments. Hongyi Wei and Bin Xu processed the experimental data. Bo Tan, Bin Xu and Hongru Zhao reviewed and edited the manuscript. All authors read and approved the manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial 8
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