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A TG-MS study on the coupled pyrolysis and combustion of oil sludge
Thermochimica Acta 663 (2018) 137–144
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A TG-MS study on the coupled pyrolysis and combustion of oil sludge ⁎
T
Zhentong Wang, Zhiqiang Gong , Zhenbo Wang, Peiwen Fang, Dong Han State Key Laboratory of Heavy Oil, China University of Petroleum (East China), Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil sludge Integrated thermal treatment Nitrogenous gas release Sulfurous gas release TG-MS
Integrated thermal treatment (coupled pyrolysis and combustion) is considered to be an effective method for the utilization of oil sludge (OS). Characteristics of OS pyrolysis, OS combustion and OS char combustion were comprehensively investigated with a thermogravimetry-mass spectrum (TG-MS) system in this work. Small molecule hydrocarbons like CH4 were main components in OS pyrolysis gas. Nitrogen and sulfur containing compounds, H2O, and CO2 were detected out during OS and OS char combustion. The maximum intensities of NH3, HCN and NO all appeared in the OS combustion. H2S was the main precursor of SO2, and the peak of H2S occurred earlier with an increase in the pyrolysis temperature between 500 and 700 °C. Besides, with an increase in the pyrolysis temperature, NO2 and SO2 emissions increased during OS char combustion.
1. Introduction Currently, the implementation of green development has become a common concern around the world with the continuous development of science and technology and the increasingly severe environmental issues [1]. The petrochemical industry, from the upstream of the petroleum exploitation to the downstream of the refining process, has been accompanied by various environmental pollution problems [2]. Oil sludge (OS), which has been a common solid waste in petrochemical industry [3], has been listed as a hazardous waste by law in China. Over one millions tons of OS could be produced every year from a variety of sources [4]. Public and environmental health will be under significant threat unless it is treated properly [5]. OS treatments mainly include solidification [6–8], solvent extraction [9,10], microwave irradiation [11,12], bioremediation [13–15] ultrasonic [16,17], pyrolysis [18–20] and incineration [21–23]. Among these methods, pyrolysis and incineration have attracted a great deal of attention due to their remarkable advantages of energy recovery and significant reduction in waste volume. OS pyrolysis process produces hydrocarbons with lower molecular weight in condensation (i.e., liquid) and/or noncondensable gases. It also generates a solid product, OS char [24]. OS char can be used as fuels for power plants. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment, as showed in Fig. 1. Solid fuel pyrolysis or combustion is quite complex, which involves a variety of physical and chemical changes. To figure out these complicated thermal processes, researchers have done lots of work with
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application of thermal analysis instruments or techniques. Thermogravimetric analyzer (TGA), fourier transformed infra-red spectroscopy (FTIR) and gas chromatography/mass spectrum (GC/MS) or GC, MS have been widely used to analyze thermal process, such as solid fuel pyrolysis and combustion. Moreover, coupled systems, such as TG-GC, TG-FTIR, and TG-MS, have also been widely used to study the variations of products in pyrolysis and combustion process simultaneously. TGA method can only provide the thermogravimetric (TG) and differential thermogravimetric (DTG) results without distinguishing different components of the mixed evolved gases for the thermal process of solid fuels. GC/MS method suffers from the limitation that some polar and/or involatile gas-phase products may lost [25]. Besides, GC/ MS involves the pretreatment and isolation processes, which probably disturb the original compounds of gas-phase products. As for the FTIR method, only a small fraction of nascent products could be distinguished from the absorption bands of the functional groups [26]. Among various coupled techniques, thermogravimetric analysis coupled with mass spectrometry (TG-MS) can achieve real-time and sensitive detection of evolved gases [27–31]. The integration of evolution profiles for all detected species yields quantitative information regarding gas product compositions [27,28]. Thus, TG-MS has been extensively used to examine gaseous products that evolve from the thermal degradation of coal [27,29,30], biomass [31], and municipal solid waste [32]. Han et al. [29] analyzed the thermal decomposition and evolved gas characteristics of five representative lignite samples through TG-MS. Zou et al. [33] investigated the devolatilization characteristics and non-isothermal kinetics of gaseous volatile evolution of bituminous coal through TG-MS. Otero et al. [34] also used TG-MS
Corresponding author. E-mail address: [email protected] (Z. Gong).
https://doi.org/10.1016/j.tca.2018.03.019 Received 10 February 2018; Received in revised form 23 March 2018; Accepted 25 March 2018 Available online 27 March 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
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sizes of 0.5–1.0 mm was tiled at the bottom of an Al2O3 crucible, and the internal atmosphere of the TGA was set to the air atmosphere and Ar atmosphere of 60 ml/min in combustion and pyrolysis experiments, respectively. The initial temperature of the furnace in the STA was set as 50 °C and it would last for 60 min after the sample was fed into the furnace. The purpose was to replace the atmosphere of the whole equipment, including the furnace, and the air inside the balance of the gas chamber, and ensure that the pyrolysis and combustion process were carried out in an inert atmosphere, making the MS fully prepared for the detection of evolved gaseous products. After the initial steady state, the sample was heated with a heating rate of 20 °C/min in the temperature range of 50–1200 °C. The gaseous products were sampled by the mass spectrometer for detection and analysis simultaneously. Main pyrolysis and combustion products are characterized according to previous results [25]. Main gaseous products of OS pyrolysis contain hydrogen (2), methylene (14), methyl (15), methane (16), ammonia (17), moisture (18), acetylene (26), ethylene (28), ethane (30), sulfur (32), sulfur hydrogen ion (33), hydrogen sulfide (34), propene (42) and carbon dioxide (44). Besides, main gaseous products of OS and OS char combustion contain ammonia (17), moisture (18), hydrogen cyanide (27), nitric oxide (30), hydrogen sulfide (34), carbon dioxide (44), nitrogen dioxide (46), carbonyl sulfide (60), sulfur dioxide (64), and carbon disulfide (76).
Fig. 1. The schematic of integrated thermal treatment of OS.
system to study the co-combustion of sewage sludge and coal. It can be seen that most of these aforementioned investigations conducted with TG-MS system were on pyrolysis or combustion of coal, biomass and sewage sludge, while literatures [35] about the integrated thermal treatment (coupled pyrolysis and combustion) of OS with TG-MS method were rare and need to be figured out. The present work focused on the integrated thermal treatment of OS, including OS pyrolysis and OS pyrolysis char combustion with the application of TG-MS. The weight losses and mass loss rate in OS thermal process were obtained by TGA. Besides, mass spectra for the nascent pyrolysis and combustion products and their evolved profiles were investigated and presented, which would be useful for the fundamental study of OS coupled pyrolysis and combustion method.
3. Results and discussion
2. Experimental section
3.1. Proximate and ultimate analysis of OS and OS chars
2.1. Sample preparation OS samples are supplied from an oil tank in Shengli oilfield, located in Dongying, Shandong Province. Simple pyrolysis or combustion method both have their limits, with a relatively low energy utilization efficiency. The grade utilization of OS was proposed based on the composition and structure of OS. OS pyrolysis experiment and OS char preparation both were carried on a horizontal tube furnace under different pyrolysis temperature (500–700 °C) with a heating rate of 5 °C/min. The horizontal tube furnace reactor system was shown in Fig. 2. The internal atmosphere was N2 with a flowrate of 500 ml/min. OS pyrolysis chars were pulverized with particle size less than 74 μm. The samples were dried in an oven at 105 °C for 24 h and stored in a desiccator. The mass of pyrolysis oil and chars can be measured by weighting and the mass of pyrolysis gas was calculated by the difference between the weight of OS sample and the weight of pyrolysis oil and char.
The proximate and ultimate analysis of the OS and OS pyrolysis chars are summarized in Table 1. It can be seen that OS and OS pyrolysis chars differed in their compositions obviously. OS contained a complex mixture of undigested organics such as heavy organic compounds and a large amount of combustible matters [36]. OS pyrolysis chars under different pyrolysis temperatures had higher carbon content and could be used as a solid fuel. The molar ratio of H/C as a parameter for aromaticity and carbonization degree was lower for OS pyrolysis chars from higher temperature pyrolysis [37]. Similarly, the molar ratio of O/C ratio was lower for OS pyrolysis chars from higher temperature pyrolysis which meant that OS pyrolysis chars from higher temperature pyrolysis had lower hydrophilicity with less polar-groups than OS char from lower pyrolysis temperature did [38]. Compared with OS pyrolysis chars, OS had higher molar ratio of H/C and O/C, indicating OS had higher hydrophilicity with more polar-groups. Lower N and S contents of OS pyrolysis chars from higher temperature pyrolysis can reduce NOx and SOx emissions from combustion process.
2.2. TG-MS system analysis
3.2. Analysis of pyrolysis characteristics of OS
Pyrolysis and combustion experiments were carried out with a TGMS system, including a STA449F3 NETZSCH thermogravimetric analyzer (TGA) and a QMS403C Aeolos type quadrupole mass spectrometer. To start an experiment, a sample of about 10 mg with particle
3.2.1. TG and DTG profiles of OS pyrolysis TG and DTG curves during OS pyrolysis with a heating rate of 20 °C/ min are shown in Fig. 3(a). There were three main weight loss groups during OS pyrolysis as marked in the graph. The first weight loss peak Fig. 2. Schematic diagram of horizontal tube furnace reactor system. 1. N2 cylinder; 2. temperature controller; 3. gas flowmeter; 4. horizontal tube reactor; 5. corundum crucible; 6. electric furnace; 7. constant temperature controller; 8. cooling water inlet; 9. cooling water outlet; 10. water-cooling flask; 11. pyrolysis gas outlet; 12. distributor.
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Table 1 Proximate and ultimate analysis of OS and OS pyrolysis chars. Samples
Data OS
Proximate analysis (wt%) 0 Moisturea Volatilea 51.74 Asha 42.82 Fixed carbona,b 5.44 a −1 HHV (MJ kg ) 22.58 Ultimate analysis (wt%) C 44.24 H 5.67 Ob 4.60 N 0.42 S 2.25 H/C 0.128 O/C 0.104
OSS char-500 °C
OS char-600 °C
OS char-700 °C
0 12.37 73.90 13.73 8.84
0 7.75 78.38 13.87 6.65
0 6.62 79.47 13.91 6.13
20.09 1.46 1.30 0.48 2.77 0.073 0.065
17.11 0.60 0.72 0.46 2.73 0.035 0.042
16.62 0.35 0.53 0.42 2.61 0.021 0.032
a Moisture, Volatile, Ash, Fixed carbon and HHV (higher heating value) on an air-dried basis. b O and Fixed carbon, calculated by difference.
Fig. 4. Distributions of char, oil, and gas yields in OS pyrolysis.
the carbonation reaction [18]. The main groups in OS included proteins, fats, and oils, which were easy to break down during the thermal process. The last weight loss (580–680 °C) was attributed to the decomposition of organic residues and inorganic matter [39,40]. There were some heavy metal salts in OS and these salts had complex reactions at high temperature [40]. To guarantee the reproducibility, OS pyrolysis experiments were repeated several times on TG-MS system. Fig. 3(b) presents some typical experimental results for three duplicate runs of OS pyrolysis, and the relative errors were less than 3% of the averaged values. 3.2.2. Pyrolysis products identification of OS The distributions of char, oil, and gas yields in different temperatures of OS pyrolysis are shown in Fig. 4. It can be seen that with the increase of pyrolysis temperature, char yield decreased from 60.60% to 55.56% while gas yield increased from 36.78% to 40.63%. When pyrolysis temperature exceeded 600 °C, the yields of products showed little change. The yields of oil did not demonstrate an obvious decrease when the final temperature exceeded 600 °C, which was consistent with the TG curve of OS pyrolysis as the release of oil components occurred between 200 and 580 °C. As shown in Fig. 4, with the increase of pyrolysis temperature, gas yield increased and char yield decreased. Higher pyrolysis temperature promoted the breakage of groups such as hydroxyl, methyl and methylene, which accelerated the process of release of pyrolysis gas and reduced the yield of char, resulting in increased generation of pyrolysis oil. When the pyrolysis temperature exceeded 600 °C, the oil yield decreased from 5.60% to 3.81%. This was because heavy oil decomposed into small molecular hydrocarbons, resulting in the decrease of pyrolysis oil content [18]. The gaseous products of OS pyrolysis can be separated into several categories, i.e., alkanes (m/z 16 and 30), alkenes (m/z 28 and 42), dienes (m/z 26), small molecular hydrocarbons (m/z 14 and 15), and nitrogen and sulfur containing compounds (m/z 17, 32, 33 and 34). Small amounts of aromatics and phenols, such as C6H6 (m/z 78) and C6H6O (m/z 94), could also be detected out. Fig. 5 shows the relative intensity of integrated values of the major MS signals in OS pyrolysis process, NH3 (m/z 17), H2O (m/z 18), H2 (m/ z 2) and CH4 (m/z 16) were the main products in pyrolysis gas. The relative intensity of H2O was largest, which was attributed to the volatilization of inorganic moisture. NH3 products were produced as precursors of nitrogen containing compounds. Macromolecular organics like C4-6 and C6+ were barely detected out in the released pyrolysis gas. CH4 and H2 products were relatively the main components in pyrolysis gas. In OS pyrolysis, with an increase in pyrolysis temperature, CeH and HeH bonds in the macromolecular hydrocarbons
Fig. 3. TG and DTG profiles of OS pyrolysis.
(< 380 °C) was attributed to the release of light oil components with relatively low boiling points. In the second weight loss (380–580 °C), there was a large weight loss peak, a maximum one, which was caused by mainly two reasons. The first one was the release and decomposition of heavy oil components with higher boiling points, the second one was 139
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Fig. 5. Relative ion intensity of major pyrolysis products of OS.
cracked and recombined to mainly form micromolecular hydrocarbons like CH4 and C2H4. 3.3. Analysis of combustion and emission characteristics of OS and OS pyrolysis chars 3.3.1. TG and DTG profiles of OS and OS pyrolysis char combustion TG and DTG profiles during combustion of OS and OS pyrolysis chars under air atmosphere with a heating rate of 20 °C/min are shown in Figs. 6 and 7, respectively. As shown in Fig. 6(a), OS combustion could be mainly divided into three weight loss regions. The first weight loss (< 380 °C) was due to the removal of some light hydrocarbons with low boiling point. And the second one, found between 380 and 520 °C, was mainly caused by the combustion of large fractions with high boiling point and fixed carbon content. The third weight loss, ranged from 520 to 710 °C, was mainly resulted from the decomposition and combustion of heavy oil components with higher boiling points, and some metal salts decomposition, such as aluminosilicate and calcium aluminate compounds. The second weight loss was much larger with a weight loss of 36% than the first weight loss with a weight loss of 8%. Heavy distillate was evaporated as oil, which was reacted with oxygen in the air, causing rapid weight loss. The heat from combustion of heavy fractions led to the burning of OS char, resulting in a larger weight loss. The total weight loss of OS combustion was 59.6%, which was mainly caused by combustion of the volatile matters and fixed carbon in OS. To guarantee the reproducibility, experiments for OS and OS char combustion were repeated several times on TG-MS system. Fig. 6(b) presents some typical experimental results for three duplicate runs of OS combustion, and the relative errors were less than 3% of the averaged values. Fig. 7 shows the TG and DTG profiles of OS pyrolysis chars with different pyrolysis temperature from 500 °C to 700 °C. The OS char from 500 °C pyrolysis showed higher mass loss than that of the OS char from 600 °C pyrolysis and OS char from 700 °C pyrolysis in TG curves. The TG curves of OS char from 600 °C pyrolysis and OS char from 700 °C pyrolysis were nearly overlapped. The maximum mass loss rate of OS pyrolysis chars was slightly decreased from 2.63 to 2.38%/min, with an increase in the pyrolysis temperature between 500 and 700 °C. This was attributed to the fact that the volatiles flame supplied lots of heat for combustion and decomposition of chars and inorganic matters so that OS char could be ignited at temperature lower than its heterogeneous ignition temperature. 41 With the increase of pyrolysis temperature, the content of volatiles in OS pyrolysis chars decreased, which caused smaller mass loss rate and weight losses. As can be seen in Table 2, OS char decomposition mainly occurred
Fig. 6. TG and DTG profiles of OS combustion.
at 300–650 °C. Ti is the ignition temperature, which is controlled by the ejection of volatile matter from OS and OS chars rather than the O2 concentration. Tb is the burnout temperature which is determined by the chemical oxidation of char [42]. The burnout time of OS chars decreased with the increase of pyrolysis temperature, which was because that higher pyrolysis temperature promoted the formation of porous structure of char and was favor of oxygen adsorption and combustion. Comprehensive combustion characteristic index (S), reflected the combustion performance of the fuels, was defined as follows [43]:
S=
(dW / dt )max (dW / dt )mean Ti2 Tb
(1)
where (dW/dt)max refers to maximum mass loss rate, (dW/dt)mean is average mass loss rate. The combustion performance of the samples was better with a higher value of S. When pyrolysis temperature increased from 500 °C to 700 °C, the S of OS chars decreased from 3.67 × 10−11 to 2.99 × 10−11 min−2 K −3, which indicated that OS chars from pyrolysis under lower temperature exhibited a better combustion performance, and basically unchanged above 600 °C, while the value of S of OS was greater than that of OS chars. 3.3.2. Combustion products identification of OS and OS pyrolysis chars The effect of pyrolysis temperatures on the gaseous products in combustion can be obtained by analyzing the mass spectra. Emission characteristics of several typical products, i.e., nitrogen and sulfur containing compounds, carbon dioxide, and moisture, were demonstrated in Fig. 8. 140
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Fig. 8. Relative ion intensity of major combustion products of OS, OS char500 °C, OS char-600 °C, OS char-700 °C.
carbon dioxide, the signal intensities were relatively stronger with the increase of pyrolysis temperature in combustion, which was attributed that more oxygen adsorbed in OS char from higher pyrolysis temperature and favored the fixed carbon combustion. During OS and OS char combustion process, the signal change of combustion products with the lapse of time could be obtained through the online sampling capability of MID pattern of MS. Figs. 9 and 10 illustrates the evolved profiles of nitrogen and sulfur containing gaseous products during the OS and OS char combustion, respectively. The spectra were recorded continuously for 60 min, with the background noise subtracted. As shown in Fig. 9, NH3, HCN and NO exhibited the maximum peaks in the OS combustion profiles, this was attributed to that large amounts of nitrogen containing compounds were released in pyrolysis, and only small amounts of those with high binding energy preserved in OS chars. HCN and NH3 were considered as the main precursor of NOx during OS combustion [44,45]. Evolved profiles for NH3 of different OS pyrolysis chars shifted right with the increase of pyrolysis temperature, while the relative intensity decreased dramatically at 500–600 °C and showed no change above 600 °C, while the shapes of the HCN release profiles of OS chars were similar. Compared with OS combustion, the generation of nitrogen containing precursor significantly decreased and the temperature frame of gas release expand slightly in OS char combustion, which was due to the formation of nitrogen containing compounds with higher binding energy and stronger stability in OS char. A weak peak of NO profile of OS char from 500 °C pyrolysis appeared at 300 °C. This was caused by the adsorption of OS char, then NO adsorbed was released before fixed carbon combustion [46]. NO2 from combustion of different OS chars approximately appeared and reached the maximum peak at the same time, which indicated that the
Fig. 7. The remaining mass percentages of OS chars.
As shown in Fig. 8, H2O (m/z 18) and HCN (m/z 27) were the main products. OS chars from higher pyrolysis temperature contained smaller amounts of volatiles, thus less H2O products were released in combustion. When the pyrolysis temperature increased from 500 to 700 °C, the signal intensities of sulfur containing compounds increased and reached the maximum at 700 °C during combustion of OS chars. This behavior indicated that higher pyrolysis temperature could promote the decomposition of the inorganic sulfide, which facilitated the generation of these sulfur containing compounds in combustion. In the case of Table 2 TG and DTG summary of OS and OS pyrolysis chars.
S f (10−11) (min−2 K
Samples
OS OS char-500 °C OS char-600 °C OS char-700 °C
Tia (°C )
Tbb (°C )
c Tm (°C )
d DTGmax (%/ min)
M fe (%)
384.3 327.9 339.2 345.6
750.6 641.7 628.7 596.6
461.5 511.1 517.2 531.9
8.24 2.75 2.72 2.53
40.40 77.67 82.94 82.76
a
Ti, ignition temperature. Tb, burnout temperature. c Tm, peak temperature. d DTGmax, maximum mass loss rate. e Mf, residual mass ratio. f S, comprehensive combustion index. b
141
4.61 3.67 3.09 2.99
−3
)
Burnout time (min)
37.53 32.08 31.44 29.83
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Fig. 9. Nitrogen containing compounds release in OS and OS char combustion.
Fig. 10. Sulfur containing compounds release in OS and OS char combustion.
combustion temperature was determined to when the peak of NO2 occurred. With the increase of pyrolysis temperature, the formation time and peak value of products were dramatically promoted, which was mainly attributed to the higher temperature accelerating the
development of porous structure and oxygen adsorption capacity. In Fig. 10, H2S was mainly released between 300 °C and 600 °C, and the peak value in OS combustion was higher than that in OS char combustion. With the increase of pyrolysis temperature, the peak of H2S 142
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Appendix A. Supplementary data
release profiles in OS char combustion occurred earlier. H2S mainly came from the decomposition of pyrite and aliphatic sulfur. In OS pyrolysis, one part of organic sulfur was decomposed and transformed into aliphatic sulfur in OS char at relatively high pyrolysis temperature (500–700 °C), another released in form of gaseous sulfur containing compounds. Thus, H2S was released below 500 °C in OS char combustion. The OS char from higher pyrolysis temperature have more developed porous structure, which can reduce the transmission resistance of H2S. The maximum peak of all the COS products occurred approximately at the same time. The mechanisms of COS are complicated and not fully understood until now [47]. The COS profiles of OS combustion had a higher peak value than those of OS char combustion. In OS char combustion, COS release profiles shifted right and the maximum peak increased with the increase of pyrolysis temperature, which was due to that higher pyrolysis temperature was suitable for the formation of pyrite [48]. The shapes of the SO2 release profiles in OS char combustion were similar. All the SO2 products were released between 250 °C and 600 °C, and the maximum peak was exhibited at the SO2 profile of OS char from 700 °C pyrolysis. SO2 mainly came from the oxidation of aromatic sulfur and pyrite [45]. The ion intensity of SO2 in OS char combustion increased with an increase in pyrolysis temperature, indicating that higher pyrolysis temperature was more conductive to the formation of SO2. Higher pyrolysis temperature could promote O2 adsorption capacity, and more developed porous structure was conductive to the formation and release of SO2. Oil and gas products are recyclable in OS pyrolysis, and considerable solid char from pyrolysis can be used as fuels in power plant. OS pyrolysis chars have developed porous structure with a relatively high carbon contents, which is conducive to combustion. Coupled pyrolysis and combustion method for OS treatment has good foundation and application prospect.
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4. Conclusions Integrated thermal treatment (coupled pyrolysis and combustion) of OS was comprehensively studied with a TG-MS system in this work. OS pyrolysis was classified into three weight losses, which were attributed to the release of light oil components, carbon reaction and the decomposition of heavy oil components. The oil yields did not demonstrate an obvious change when the final temperature exceeded 600 °C, which was consistent with the TG curve of OS pyrolysis as the release of oil components occurred between 200 and 580 °C. CeH and HeH bonds in macromolecular hydrocarbons cracked and recombined to mainly form CH4 and H2 during OS pyrolysis. OS combustion was divided into three weight losses, which were caused by the removal of light hydrocarbons, combustion of large fractions and fixed carbon, and the decomposition of some metal salts. Volatiles flame could supply lots of heat for the combustion of chars and inorganic matters. With an increase in pyrolysis temperature, the S of OS chars decreased from 3.67 × 10−11 to 2.99 × 10−11 min−2 K −3, indicating that OS chars from lower pyrolysis temperature exhibited a better combustion performance. Moreover, higher pyrolysis temperature promoted the development of porous structure and oxygen adsorption capacity of OS chars, which was conductive to NO2 and SO2 conversion.
Acknowledgements The research was supported by the Natural Science Foundation of Shandong Province (No. ZR2017BEE042), the Fundamental Research Funds for the Central Universities (No. 18CX02150A), and the Talent Introduction Project of China University of Petroleum (East China) (No. 2017010068) 143
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A TG-MS study on the coupled pyrolysis and combustion of oil sludge ⁎
T
Zhentong Wang, Zhiqiang Gong , Zhenbo Wang, Peiwen Fang, Dong Han State Key Laboratory of Heavy Oil, China University of Petroleum (East China), Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil sludge Integrated thermal treatment Nitrogenous gas release Sulfurous gas release TG-MS
Integrated thermal treatment (coupled pyrolysis and combustion) is considered to be an effective method for the utilization of oil sludge (OS). Characteristics of OS pyrolysis, OS combustion and OS char combustion were comprehensively investigated with a thermogravimetry-mass spectrum (TG-MS) system in this work. Small molecule hydrocarbons like CH4 were main components in OS pyrolysis gas. Nitrogen and sulfur containing compounds, H2O, and CO2 were detected out during OS and OS char combustion. The maximum intensities of NH3, HCN and NO all appeared in the OS combustion. H2S was the main precursor of SO2, and the peak of H2S occurred earlier with an increase in the pyrolysis temperature between 500 and 700 °C. Besides, with an increase in the pyrolysis temperature, NO2 and SO2 emissions increased during OS char combustion.
1. Introduction Currently, the implementation of green development has become a common concern around the world with the continuous development of science and technology and the increasingly severe environmental issues [1]. The petrochemical industry, from the upstream of the petroleum exploitation to the downstream of the refining process, has been accompanied by various environmental pollution problems [2]. Oil sludge (OS), which has been a common solid waste in petrochemical industry [3], has been listed as a hazardous waste by law in China. Over one millions tons of OS could be produced every year from a variety of sources [4]. Public and environmental health will be under significant threat unless it is treated properly [5]. OS treatments mainly include solidification [6–8], solvent extraction [9,10], microwave irradiation [11,12], bioremediation [13–15] ultrasonic [16,17], pyrolysis [18–20] and incineration [21–23]. Among these methods, pyrolysis and incineration have attracted a great deal of attention due to their remarkable advantages of energy recovery and significant reduction in waste volume. OS pyrolysis process produces hydrocarbons with lower molecular weight in condensation (i.e., liquid) and/or noncondensable gases. It also generates a solid product, OS char [24]. OS char can be used as fuels for power plants. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment, as showed in Fig. 1. Solid fuel pyrolysis or combustion is quite complex, which involves a variety of physical and chemical changes. To figure out these complicated thermal processes, researchers have done lots of work with
⁎
application of thermal analysis instruments or techniques. Thermogravimetric analyzer (TGA), fourier transformed infra-red spectroscopy (FTIR) and gas chromatography/mass spectrum (GC/MS) or GC, MS have been widely used to analyze thermal process, such as solid fuel pyrolysis and combustion. Moreover, coupled systems, such as TG-GC, TG-FTIR, and TG-MS, have also been widely used to study the variations of products in pyrolysis and combustion process simultaneously. TGA method can only provide the thermogravimetric (TG) and differential thermogravimetric (DTG) results without distinguishing different components of the mixed evolved gases for the thermal process of solid fuels. GC/MS method suffers from the limitation that some polar and/or involatile gas-phase products may lost [25]. Besides, GC/ MS involves the pretreatment and isolation processes, which probably disturb the original compounds of gas-phase products. As for the FTIR method, only a small fraction of nascent products could be distinguished from the absorption bands of the functional groups [26]. Among various coupled techniques, thermogravimetric analysis coupled with mass spectrometry (TG-MS) can achieve real-time and sensitive detection of evolved gases [27–31]. The integration of evolution profiles for all detected species yields quantitative information regarding gas product compositions [27,28]. Thus, TG-MS has been extensively used to examine gaseous products that evolve from the thermal degradation of coal [27,29,30], biomass [31], and municipal solid waste [32]. Han et al. [29] analyzed the thermal decomposition and evolved gas characteristics of five representative lignite samples through TG-MS. Zou et al. [33] investigated the devolatilization characteristics and non-isothermal kinetics of gaseous volatile evolution of bituminous coal through TG-MS. Otero et al. [34] also used TG-MS
Corresponding author. E-mail address: [email protected] (Z. Gong).
https://doi.org/10.1016/j.tca.2018.03.019 Received 10 February 2018; Received in revised form 23 March 2018; Accepted 25 March 2018 Available online 27 March 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
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sizes of 0.5–1.0 mm was tiled at the bottom of an Al2O3 crucible, and the internal atmosphere of the TGA was set to the air atmosphere and Ar atmosphere of 60 ml/min in combustion and pyrolysis experiments, respectively. The initial temperature of the furnace in the STA was set as 50 °C and it would last for 60 min after the sample was fed into the furnace. The purpose was to replace the atmosphere of the whole equipment, including the furnace, and the air inside the balance of the gas chamber, and ensure that the pyrolysis and combustion process were carried out in an inert atmosphere, making the MS fully prepared for the detection of evolved gaseous products. After the initial steady state, the sample was heated with a heating rate of 20 °C/min in the temperature range of 50–1200 °C. The gaseous products were sampled by the mass spectrometer for detection and analysis simultaneously. Main pyrolysis and combustion products are characterized according to previous results [25]. Main gaseous products of OS pyrolysis contain hydrogen (2), methylene (14), methyl (15), methane (16), ammonia (17), moisture (18), acetylene (26), ethylene (28), ethane (30), sulfur (32), sulfur hydrogen ion (33), hydrogen sulfide (34), propene (42) and carbon dioxide (44). Besides, main gaseous products of OS and OS char combustion contain ammonia (17), moisture (18), hydrogen cyanide (27), nitric oxide (30), hydrogen sulfide (34), carbon dioxide (44), nitrogen dioxide (46), carbonyl sulfide (60), sulfur dioxide (64), and carbon disulfide (76).
Fig. 1. The schematic of integrated thermal treatment of OS.
system to study the co-combustion of sewage sludge and coal. It can be seen that most of these aforementioned investigations conducted with TG-MS system were on pyrolysis or combustion of coal, biomass and sewage sludge, while literatures [35] about the integrated thermal treatment (coupled pyrolysis and combustion) of OS with TG-MS method were rare and need to be figured out. The present work focused on the integrated thermal treatment of OS, including OS pyrolysis and OS pyrolysis char combustion with the application of TG-MS. The weight losses and mass loss rate in OS thermal process were obtained by TGA. Besides, mass spectra for the nascent pyrolysis and combustion products and their evolved profiles were investigated and presented, which would be useful for the fundamental study of OS coupled pyrolysis and combustion method.
3. Results and discussion
2. Experimental section
3.1. Proximate and ultimate analysis of OS and OS chars
2.1. Sample preparation OS samples are supplied from an oil tank in Shengli oilfield, located in Dongying, Shandong Province. Simple pyrolysis or combustion method both have their limits, with a relatively low energy utilization efficiency. The grade utilization of OS was proposed based on the composition and structure of OS. OS pyrolysis experiment and OS char preparation both were carried on a horizontal tube furnace under different pyrolysis temperature (500–700 °C) with a heating rate of 5 °C/min. The horizontal tube furnace reactor system was shown in Fig. 2. The internal atmosphere was N2 with a flowrate of 500 ml/min. OS pyrolysis chars were pulverized with particle size less than 74 μm. The samples were dried in an oven at 105 °C for 24 h and stored in a desiccator. The mass of pyrolysis oil and chars can be measured by weighting and the mass of pyrolysis gas was calculated by the difference between the weight of OS sample and the weight of pyrolysis oil and char.
The proximate and ultimate analysis of the OS and OS pyrolysis chars are summarized in Table 1. It can be seen that OS and OS pyrolysis chars differed in their compositions obviously. OS contained a complex mixture of undigested organics such as heavy organic compounds and a large amount of combustible matters [36]. OS pyrolysis chars under different pyrolysis temperatures had higher carbon content and could be used as a solid fuel. The molar ratio of H/C as a parameter for aromaticity and carbonization degree was lower for OS pyrolysis chars from higher temperature pyrolysis [37]. Similarly, the molar ratio of O/C ratio was lower for OS pyrolysis chars from higher temperature pyrolysis which meant that OS pyrolysis chars from higher temperature pyrolysis had lower hydrophilicity with less polar-groups than OS char from lower pyrolysis temperature did [38]. Compared with OS pyrolysis chars, OS had higher molar ratio of H/C and O/C, indicating OS had higher hydrophilicity with more polar-groups. Lower N and S contents of OS pyrolysis chars from higher temperature pyrolysis can reduce NOx and SOx emissions from combustion process.
2.2. TG-MS system analysis
3.2. Analysis of pyrolysis characteristics of OS
Pyrolysis and combustion experiments were carried out with a TGMS system, including a STA449F3 NETZSCH thermogravimetric analyzer (TGA) and a QMS403C Aeolos type quadrupole mass spectrometer. To start an experiment, a sample of about 10 mg with particle
3.2.1. TG and DTG profiles of OS pyrolysis TG and DTG curves during OS pyrolysis with a heating rate of 20 °C/ min are shown in Fig. 3(a). There were three main weight loss groups during OS pyrolysis as marked in the graph. The first weight loss peak Fig. 2. Schematic diagram of horizontal tube furnace reactor system. 1. N2 cylinder; 2. temperature controller; 3. gas flowmeter; 4. horizontal tube reactor; 5. corundum crucible; 6. electric furnace; 7. constant temperature controller; 8. cooling water inlet; 9. cooling water outlet; 10. water-cooling flask; 11. pyrolysis gas outlet; 12. distributor.
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Table 1 Proximate and ultimate analysis of OS and OS pyrolysis chars. Samples
Data OS
Proximate analysis (wt%) 0 Moisturea Volatilea 51.74 Asha 42.82 Fixed carbona,b 5.44 a −1 HHV (MJ kg ) 22.58 Ultimate analysis (wt%) C 44.24 H 5.67 Ob 4.60 N 0.42 S 2.25 H/C 0.128 O/C 0.104
OSS char-500 °C
OS char-600 °C
OS char-700 °C
0 12.37 73.90 13.73 8.84
0 7.75 78.38 13.87 6.65
0 6.62 79.47 13.91 6.13
20.09 1.46 1.30 0.48 2.77 0.073 0.065
17.11 0.60 0.72 0.46 2.73 0.035 0.042
16.62 0.35 0.53 0.42 2.61 0.021 0.032
a Moisture, Volatile, Ash, Fixed carbon and HHV (higher heating value) on an air-dried basis. b O and Fixed carbon, calculated by difference.
Fig. 4. Distributions of char, oil, and gas yields in OS pyrolysis.
the carbonation reaction [18]. The main groups in OS included proteins, fats, and oils, which were easy to break down during the thermal process. The last weight loss (580–680 °C) was attributed to the decomposition of organic residues and inorganic matter [39,40]. There were some heavy metal salts in OS and these salts had complex reactions at high temperature [40]. To guarantee the reproducibility, OS pyrolysis experiments were repeated several times on TG-MS system. Fig. 3(b) presents some typical experimental results for three duplicate runs of OS pyrolysis, and the relative errors were less than 3% of the averaged values. 3.2.2. Pyrolysis products identification of OS The distributions of char, oil, and gas yields in different temperatures of OS pyrolysis are shown in Fig. 4. It can be seen that with the increase of pyrolysis temperature, char yield decreased from 60.60% to 55.56% while gas yield increased from 36.78% to 40.63%. When pyrolysis temperature exceeded 600 °C, the yields of products showed little change. The yields of oil did not demonstrate an obvious decrease when the final temperature exceeded 600 °C, which was consistent with the TG curve of OS pyrolysis as the release of oil components occurred between 200 and 580 °C. As shown in Fig. 4, with the increase of pyrolysis temperature, gas yield increased and char yield decreased. Higher pyrolysis temperature promoted the breakage of groups such as hydroxyl, methyl and methylene, which accelerated the process of release of pyrolysis gas and reduced the yield of char, resulting in increased generation of pyrolysis oil. When the pyrolysis temperature exceeded 600 °C, the oil yield decreased from 5.60% to 3.81%. This was because heavy oil decomposed into small molecular hydrocarbons, resulting in the decrease of pyrolysis oil content [18]. The gaseous products of OS pyrolysis can be separated into several categories, i.e., alkanes (m/z 16 and 30), alkenes (m/z 28 and 42), dienes (m/z 26), small molecular hydrocarbons (m/z 14 and 15), and nitrogen and sulfur containing compounds (m/z 17, 32, 33 and 34). Small amounts of aromatics and phenols, such as C6H6 (m/z 78) and C6H6O (m/z 94), could also be detected out. Fig. 5 shows the relative intensity of integrated values of the major MS signals in OS pyrolysis process, NH3 (m/z 17), H2O (m/z 18), H2 (m/ z 2) and CH4 (m/z 16) were the main products in pyrolysis gas. The relative intensity of H2O was largest, which was attributed to the volatilization of inorganic moisture. NH3 products were produced as precursors of nitrogen containing compounds. Macromolecular organics like C4-6 and C6+ were barely detected out in the released pyrolysis gas. CH4 and H2 products were relatively the main components in pyrolysis gas. In OS pyrolysis, with an increase in pyrolysis temperature, CeH and HeH bonds in the macromolecular hydrocarbons
Fig. 3. TG and DTG profiles of OS pyrolysis.
(< 380 °C) was attributed to the release of light oil components with relatively low boiling points. In the second weight loss (380–580 °C), there was a large weight loss peak, a maximum one, which was caused by mainly two reasons. The first one was the release and decomposition of heavy oil components with higher boiling points, the second one was 139
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Fig. 5. Relative ion intensity of major pyrolysis products of OS.
cracked and recombined to mainly form micromolecular hydrocarbons like CH4 and C2H4. 3.3. Analysis of combustion and emission characteristics of OS and OS pyrolysis chars 3.3.1. TG and DTG profiles of OS and OS pyrolysis char combustion TG and DTG profiles during combustion of OS and OS pyrolysis chars under air atmosphere with a heating rate of 20 °C/min are shown in Figs. 6 and 7, respectively. As shown in Fig. 6(a), OS combustion could be mainly divided into three weight loss regions. The first weight loss (< 380 °C) was due to the removal of some light hydrocarbons with low boiling point. And the second one, found between 380 and 520 °C, was mainly caused by the combustion of large fractions with high boiling point and fixed carbon content. The third weight loss, ranged from 520 to 710 °C, was mainly resulted from the decomposition and combustion of heavy oil components with higher boiling points, and some metal salts decomposition, such as aluminosilicate and calcium aluminate compounds. The second weight loss was much larger with a weight loss of 36% than the first weight loss with a weight loss of 8%. Heavy distillate was evaporated as oil, which was reacted with oxygen in the air, causing rapid weight loss. The heat from combustion of heavy fractions led to the burning of OS char, resulting in a larger weight loss. The total weight loss of OS combustion was 59.6%, which was mainly caused by combustion of the volatile matters and fixed carbon in OS. To guarantee the reproducibility, experiments for OS and OS char combustion were repeated several times on TG-MS system. Fig. 6(b) presents some typical experimental results for three duplicate runs of OS combustion, and the relative errors were less than 3% of the averaged values. Fig. 7 shows the TG and DTG profiles of OS pyrolysis chars with different pyrolysis temperature from 500 °C to 700 °C. The OS char from 500 °C pyrolysis showed higher mass loss than that of the OS char from 600 °C pyrolysis and OS char from 700 °C pyrolysis in TG curves. The TG curves of OS char from 600 °C pyrolysis and OS char from 700 °C pyrolysis were nearly overlapped. The maximum mass loss rate of OS pyrolysis chars was slightly decreased from 2.63 to 2.38%/min, with an increase in the pyrolysis temperature between 500 and 700 °C. This was attributed to the fact that the volatiles flame supplied lots of heat for combustion and decomposition of chars and inorganic matters so that OS char could be ignited at temperature lower than its heterogeneous ignition temperature. 41 With the increase of pyrolysis temperature, the content of volatiles in OS pyrolysis chars decreased, which caused smaller mass loss rate and weight losses. As can be seen in Table 2, OS char decomposition mainly occurred
Fig. 6. TG and DTG profiles of OS combustion.
at 300–650 °C. Ti is the ignition temperature, which is controlled by the ejection of volatile matter from OS and OS chars rather than the O2 concentration. Tb is the burnout temperature which is determined by the chemical oxidation of char [42]. The burnout time of OS chars decreased with the increase of pyrolysis temperature, which was because that higher pyrolysis temperature promoted the formation of porous structure of char and was favor of oxygen adsorption and combustion. Comprehensive combustion characteristic index (S), reflected the combustion performance of the fuels, was defined as follows [43]:
S=
(dW / dt )max (dW / dt )mean Ti2 Tb
(1)
where (dW/dt)max refers to maximum mass loss rate, (dW/dt)mean is average mass loss rate. The combustion performance of the samples was better with a higher value of S. When pyrolysis temperature increased from 500 °C to 700 °C, the S of OS chars decreased from 3.67 × 10−11 to 2.99 × 10−11 min−2 K −3, which indicated that OS chars from pyrolysis under lower temperature exhibited a better combustion performance, and basically unchanged above 600 °C, while the value of S of OS was greater than that of OS chars. 3.3.2. Combustion products identification of OS and OS pyrolysis chars The effect of pyrolysis temperatures on the gaseous products in combustion can be obtained by analyzing the mass spectra. Emission characteristics of several typical products, i.e., nitrogen and sulfur containing compounds, carbon dioxide, and moisture, were demonstrated in Fig. 8. 140
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Fig. 8. Relative ion intensity of major combustion products of OS, OS char500 °C, OS char-600 °C, OS char-700 °C.
carbon dioxide, the signal intensities were relatively stronger with the increase of pyrolysis temperature in combustion, which was attributed that more oxygen adsorbed in OS char from higher pyrolysis temperature and favored the fixed carbon combustion. During OS and OS char combustion process, the signal change of combustion products with the lapse of time could be obtained through the online sampling capability of MID pattern of MS. Figs. 9 and 10 illustrates the evolved profiles of nitrogen and sulfur containing gaseous products during the OS and OS char combustion, respectively. The spectra were recorded continuously for 60 min, with the background noise subtracted. As shown in Fig. 9, NH3, HCN and NO exhibited the maximum peaks in the OS combustion profiles, this was attributed to that large amounts of nitrogen containing compounds were released in pyrolysis, and only small amounts of those with high binding energy preserved in OS chars. HCN and NH3 were considered as the main precursor of NOx during OS combustion [44,45]. Evolved profiles for NH3 of different OS pyrolysis chars shifted right with the increase of pyrolysis temperature, while the relative intensity decreased dramatically at 500–600 °C and showed no change above 600 °C, while the shapes of the HCN release profiles of OS chars were similar. Compared with OS combustion, the generation of nitrogen containing precursor significantly decreased and the temperature frame of gas release expand slightly in OS char combustion, which was due to the formation of nitrogen containing compounds with higher binding energy and stronger stability in OS char. A weak peak of NO profile of OS char from 500 °C pyrolysis appeared at 300 °C. This was caused by the adsorption of OS char, then NO adsorbed was released before fixed carbon combustion [46]. NO2 from combustion of different OS chars approximately appeared and reached the maximum peak at the same time, which indicated that the
Fig. 7. The remaining mass percentages of OS chars.
As shown in Fig. 8, H2O (m/z 18) and HCN (m/z 27) were the main products. OS chars from higher pyrolysis temperature contained smaller amounts of volatiles, thus less H2O products were released in combustion. When the pyrolysis temperature increased from 500 to 700 °C, the signal intensities of sulfur containing compounds increased and reached the maximum at 700 °C during combustion of OS chars. This behavior indicated that higher pyrolysis temperature could promote the decomposition of the inorganic sulfide, which facilitated the generation of these sulfur containing compounds in combustion. In the case of Table 2 TG and DTG summary of OS and OS pyrolysis chars.
S f (10−11) (min−2 K
Samples
OS OS char-500 °C OS char-600 °C OS char-700 °C
Tia (°C )
Tbb (°C )
c Tm (°C )
d DTGmax (%/ min)
M fe (%)
384.3 327.9 339.2 345.6
750.6 641.7 628.7 596.6
461.5 511.1 517.2 531.9
8.24 2.75 2.72 2.53
40.40 77.67 82.94 82.76
a
Ti, ignition temperature. Tb, burnout temperature. c Tm, peak temperature. d DTGmax, maximum mass loss rate. e Mf, residual mass ratio. f S, comprehensive combustion index. b
141
4.61 3.67 3.09 2.99
−3
)
Burnout time (min)
37.53 32.08 31.44 29.83
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Fig. 9. Nitrogen containing compounds release in OS and OS char combustion.
Fig. 10. Sulfur containing compounds release in OS and OS char combustion.
combustion temperature was determined to when the peak of NO2 occurred. With the increase of pyrolysis temperature, the formation time and peak value of products were dramatically promoted, which was mainly attributed to the higher temperature accelerating the
development of porous structure and oxygen adsorption capacity. In Fig. 10, H2S was mainly released between 300 °C and 600 °C, and the peak value in OS combustion was higher than that in OS char combustion. With the increase of pyrolysis temperature, the peak of H2S 142
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Appendix A. Supplementary data
release profiles in OS char combustion occurred earlier. H2S mainly came from the decomposition of pyrite and aliphatic sulfur. In OS pyrolysis, one part of organic sulfur was decomposed and transformed into aliphatic sulfur in OS char at relatively high pyrolysis temperature (500–700 °C), another released in form of gaseous sulfur containing compounds. Thus, H2S was released below 500 °C in OS char combustion. The OS char from higher pyrolysis temperature have more developed porous structure, which can reduce the transmission resistance of H2S. The maximum peak of all the COS products occurred approximately at the same time. The mechanisms of COS are complicated and not fully understood until now [47]. The COS profiles of OS combustion had a higher peak value than those of OS char combustion. In OS char combustion, COS release profiles shifted right and the maximum peak increased with the increase of pyrolysis temperature, which was due to that higher pyrolysis temperature was suitable for the formation of pyrite [48]. The shapes of the SO2 release profiles in OS char combustion were similar. All the SO2 products were released between 250 °C and 600 °C, and the maximum peak was exhibited at the SO2 profile of OS char from 700 °C pyrolysis. SO2 mainly came from the oxidation of aromatic sulfur and pyrite [45]. The ion intensity of SO2 in OS char combustion increased with an increase in pyrolysis temperature, indicating that higher pyrolysis temperature was more conductive to the formation of SO2. Higher pyrolysis temperature could promote O2 adsorption capacity, and more developed porous structure was conductive to the formation and release of SO2. Oil and gas products are recyclable in OS pyrolysis, and considerable solid char from pyrolysis can be used as fuels in power plant. OS pyrolysis chars have developed porous structure with a relatively high carbon contents, which is conducive to combustion. Coupled pyrolysis and combustion method for OS treatment has good foundation and application prospect.
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4. Conclusions Integrated thermal treatment (coupled pyrolysis and combustion) of OS was comprehensively studied with a TG-MS system in this work. OS pyrolysis was classified into three weight losses, which were attributed to the release of light oil components, carbon reaction and the decomposition of heavy oil components. The oil yields did not demonstrate an obvious change when the final temperature exceeded 600 °C, which was consistent with the TG curve of OS pyrolysis as the release of oil components occurred between 200 and 580 °C. CeH and HeH bonds in macromolecular hydrocarbons cracked and recombined to mainly form CH4 and H2 during OS pyrolysis. OS combustion was divided into three weight losses, which were caused by the removal of light hydrocarbons, combustion of large fractions and fixed carbon, and the decomposition of some metal salts. Volatiles flame could supply lots of heat for the combustion of chars and inorganic matters. With an increase in pyrolysis temperature, the S of OS chars decreased from 3.67 × 10−11 to 2.99 × 10−11 min−2 K −3, indicating that OS chars from lower pyrolysis temperature exhibited a better combustion performance. Moreover, higher pyrolysis temperature promoted the development of porous structure and oxygen adsorption capacity of OS chars, which was conductive to NO2 and SO2 conversion.
Acknowledgements The research was supported by the Natural Science Foundation of Shandong Province (No. ZR2017BEE042), the Fundamental Research Funds for the Central Universities (No. 18CX02150A), and the Talent Introduction Project of China University of Petroleum (East China) (No. 2017010068) 143
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