Pyrolysis characteristics of low-rank coal under CO-containing

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Cite This: Energy Fuels 2019, 33, 6098−6112

Pyrolysis Characteristics of Low-Rank Coal under a CO-Containing Atmosphere and Properties of the Prepared Coal Chars Cheng Ma, Chong Zou,* Junxue Zhao,* Ruimeng Shi, Xiaoming Li, Jiangyong He, and Xiaorui Zhang

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School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710311, China ABSTRACT: Herein, the pyrolysis characteristics of low-rank coal under a CO-containing atmosphere was studied via thermogravimetry coupled with mass spectrometry and Fourier transform infrared analysis. Further, the pore structure, carbon chemical structure, and combustion reactivity of the prepared coal chars were characterized via N2/CO2 adsorption, Raman spectroscopy, and thermal analysis, respectively. The CO-containing atmosphere suppressed coal devolatilization at the rapid pyrolysis stage. During coal pyrolysis, this atmosphere reduced CH4 evolution by suppressing the formation of free radicals such as CH3• and CH2•; H2 evolution reduced owing to the inhibition of the formation of hydrogen free radicals and the working of the inverse water−gas reaction; the H2O yield was increased through the inverse water−gas reaction but not by the conversion of the hydroxyl group in coal. At 300−700 °C, the disproportionation reaction of CO produced copious CO2 molecules, significantly raising the CO2 emission intensity over the CO2 released during coal pyrolysis. The pore structure of the chars-CO was suppressed via shingling of the carbon particles generated in the disproportionation reaction and the inverse water−gas reaction, which inhibits the release of volatiles in a CO-containing atmosphere. The carbon chemical structure of the chars-CO was ordered by the interaction between the char’s skeleton structure and the newly formed carbon and the contribution of the carbon particles with an ordered structure adhered on the char. The combustion reactivity of chars-CO was not improved by its higher volatile content, because the underdeveloped pore structure and ordered carbon structure degraded the combustion reactivity of chars-CO. Conversely, the pore structure of chars-CO is more developed than that of chars-N2 due to the release of volatiles in the high-temperature segment before the gasification reaction starts so as to improve the gasification reactivity of chars-CO.

1. INTRODUCTION Low-rank coal (LRC) is a conventional fossil fuel with a wealth of reserves distributed around the world. Owing to its relatively low degree of coalification, it has a high moisture content and a high volatile value, but a relatively low calorific value.1,2 The low efficiency and high pollution release of LRC are problematic as LRC is traditionally burned in powergenerating boilers or used for producing civilian energy. Pyrolysis-based technologies that convert LRC to pyrolysis gas, coal tar, and char have rapidly developed in recent years. Pyrolysis gas is a viable alternative for civil and powergeneration fuel. Coal tar is an important raw chemical material that yields fuel oil (via hydrogenation) and other high-value chemical products, whereas char (with low moisture content and volatile matter together with high fixed carbon, calorific value, and reactivity) is primarily used as fuel or for gasifying raw materials.3−5 Therefore, the pyrolysis of LRC and the classification of pyrolysis products promise to improve LRC utilization. Moreover, pyrolysis is an important thermalconversion process and the first step in coking, combustion, gasification, and liquefaction processes.6−8 LRC pyrolysis is a complex process involving physical changes and chemical reactions at elevated temperatures in an oxygen-voided or inert atmosphere. The main physical changes are dehydration and degassing in the initial stage and expansion and condensation at the later stage. Furthermore, the main chemical reactions are bond-breaking reactions and decomposition of the macromolecular organic compounds into smaller fragments, accompanied by polycondensation reactions. During the pyrolysis stage, the organic macromolecular © 2019 American Chemical Society

chains of LRC are gradually broken, releasing numerous gas components. At higher temperatures, the subsequent polycondensation reaction of LRC generates a large amount of gaseous products in the final condensation stage. The pyrolysis process is affected by many factors, including pyrolysis temperature,9 pressure,10 atmosphere,11 heating rate,12 petrographic composition,13 mineral content,14 and catalysts.15 Several pyrolysis technologies have been developed for practical applications in recent years: internal heat pyrolyzers, rotary kilns, and fluidized bed pyrolyzers for low-temperature (550−650 °C) LRC pyrolysis.9 The pyrolysis atmosphere of these processes is normally mixed with N2, CO, H2, CH4, CO2, and other noninert gases. However, owing to its circulatory utilization, pyrolysis gas or coal gasification gas behaves as a pyrolytic heat carrier. Under such pyrolysis conditions, the atmospheric gas can not only participate in bond-breaking reactions, structural evolution, and char formation during devolatilization but also interact with other gases in the reaction atmosphere.11 Consequently, the pyrolysis process is complex, and its atmosphere influences the distribution of the pyrolysis products and properties of char. Thus, the pyrolysis atmosphere is among the most important factors affecting coal pyrolysis.16−18 Many researchers have studied the pyrolysis of coal in different atmospheres, such as H2, CH4, CO, CO2, and mixtures of these gases. Most studies have shown that H2 and Received: March 20, 2019 Revised: June 4, 2019 Published: June 5, 2019 6098

DOI: 10.1021/acs.energyfuels.9b00860 Energy Fuels 2019, 33, 6098−6112

6099

LRC

29

bituminous coal

sub-bituminous coal

11, 28

33

LRC

27

32

bituminous coal

26

sub-bituminous coal LRC

LRC

23−25

31

LRC

18

LRC

LRC

17

30

LRC

coal type

16

reference

450−650

550−900

500−800

300−800

100−1000

750−980

500−800

600

650

500−1000

500−600

850

pyrolysis temperature (°C)

atmospheric pressure atmospheric pressure 3−5 MPa

atmospheric pressure atmospheric pressure atmospheric pressure atmospheric pressure

atmospheric pressure atmospheric pressure atmospheric pressure atmospheric pressure

fixed-bed reactor fast-heating fixed-bed reactor fixed-bed reactor externally heated fluidized bed fixed-bed reactor fixed-bed reactor fluidized bed reactor

fixed-bed reactor

bubbling fluidized bed

fixed-bed reactor

integrated fluidized bed

atmospheric pressure

pressure

fluidized bed

pyrolysis device

N2, simulated pyrolysis gas (H2, CH4, CO, CO2)

synthesis gas (N2, H2, CO, CH4, CO2), H2, N2 N2, H2, steam-containing synthesis (H2O, N2, H2, CO, CO2) CO + N2, CO + H2 + N2

N2, N2 + (H2, CO, CH4, CO2)

N2, N2 + (H2, CO, CH4, CO2)

synthesis gas (N2, H2, CO, CH4, CO2) (N2, H2, CH4, H2 + CO (2:1))

N2, coke oven gas (COG), synthesis gas (H2, CO, CH4), H2

N2, CO

Ar, synthesis gas (SG) (H2, CO, CH4, CO2)

N2 + (H2, CO, CH4, CO2)

atmosphere

effect on yield

not covered

CO increases the yield of char

the char yield is N2 > SG > H2; the tar yield is H2 > COG > N2 H2 and CO increase the yield of tar

the effect of atmosphere change on tar yield is H2 > CH4 > H2 + CO (2:1) > N2 CO increases the yield of tar and char; H2 and CO2 lower the tar and char yields CO increases the yield of tar

the char yield is N2 > COG > SG > H2; the tar yield is H2 > SG > COG > N2 CO increases the yield of tar

CO increases (decreases) the yield of char at low (high) temperature

H2 and CO2 reduce the yield of tar and char. CH4 and CO promote the formation of tar CO in syngas increases the yield of tar and gas

Table 1. Summary of Studies on the Pyrolysis Process of Coal in CO-Containing Atmospheres

CO promotes pyrolysis by providing free radicals before 600 °C CO affects pyrolysis via the disproportionation reaction

not covered

CO affects pyrolysis by providing free radicals, stabilizing the coal pyrolysis of macromolecular fragments not covered

not covered

not covered

not covered

CO and water−gas shift reaction generated during pyrolysis produce highly reactive hydrogen CO promotes (inhibits) the pyrolysis process of coal at low (high) temperature CO inhibits the second decomposition of PCX

not covered

influence on pyrolysis behavior

the char prepared under the SPG atmosphere has lower gasification reactivity

not covered

not covered

not covered

Oxidation reactivity of char made from CO2 increased not covered

not covered

not covered

not covered

not covered

syngas improves char combustion

CO2 promotes oxidation activity of char

influence on char characteristics

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Figure 1. Schematic of the lab-scale fixed-bed pyrolyzer.

Table 2. Proximate and Ultimate Analysis Resultsa proximate analysis (%)

ultimate analysis (%)

sample

Mad

Aad

Vdaf

FCad

Cdaf

Hdaf

O*daf

Ndaf

Sdaf

raw coal char-N2-450 °C char-N2-550 °C char-N2-650 °C char-N2-750 °C char-CO-450 °C char-CO-550 °C char-CO-650 °C char-CO-750 °C

3.83 0.73 0.82 1.31 1.26 0.69 0.70 0.86 0.93

7.53 12.3 14.57 10.41 7.86 6.36 9.69 9.28 6.45

37.06 17.78 12.98 7.86 4.49 22.04 13.69 8.26 4.93

55.97 69.95 72.54 80.42 86.39 70.92 75.86 81.64 87.70

71.97 85.18 87.93 90.00 90.88 73.45 75.47 88.03 88.81

4.38 3.65 2.82 1.97 1.15 3.65 2.85 2.01 1.15

22.51 10.43 8.50 7.49 7.50 22.20 21.07 9.44 9.46

0.93 0.58 0.59 0.39 0.31 0.53 0.44 0.36 0.42

0.2 0.15 0.15 0.14 0.15 0.16 0.16 0.15 0.15

ad, air-dried basis; daf, dry ash-free basis; *, by difference.

a

the structure and properties of char prepared under a CO pyrolysis atmosphere have not been reported. A systematic study of the coal pyrolysis characteristics under a CO atmosphere and the properties of the prepared char would elucidate the role of CO during pyrolysis. Such a study would also help reveal the pyrolysis mechanism under multiple gases and improve the quality of char. In addition, fundamental knowledge can provide a reference for controlling the pyrolysis atmosphere, improving reactor designs, and boosting the production efficiency of industrial-scale pyrolyzers. Thermal analysis coupled with evolved gas analysis during the thermal degradation of fossil fuels is advantageous for the above purposes. Among the various coupled techniques, thermogravimetry coupled with mass spectrometry (TG− MS) is typically used to reveal the devolatilization behavior and identify the volatile components in coal pyrolysis. The advantage of TG−MS is the real-time and sensitive detection of mass losses during coal pyrolysis. Simultaneously, TG−MS analyzes online changes of the evolved gases. The reaction temperature and mode and coal characteristics during pyrolysis (obtained from the evolved gas ion signals in MS) are helpful for determining the pyrolysis mechanism of coal.34−38 This study aims to obtain the characteristic through which CO influences the coal pyrolysis process and char properties. First, pyrolysis characteristics and the gaseous product evolution of LRC in a CO-containing atmosphere are investigated herein via TG−MS, and the varying functional groups of the char products during the devolatilization process

CH4 provide free radicals and combine the unstable intermediate molecular fragments produced during pyrolysis into stable volatiles, thereby lightening the tar and increasing its yield.19 CO2 promotes the breakage of methyl functional groups and the flow of hydrogen free radicals, thus increasing the gas yield.20 H2, CH4, and CO2 atmospheres also impact the carbon chemical structure20 and properties of the char and coal tar.21 CO (10−40%) is another common gas constituent of actual pyrolysis atmospheres.22 Table 1 summarizes the literature involving CO-containing constituent pyrolysis atmospheres and their effects on the behavior of coal pyrolysis. As shown in Table 1, the coal types, pressures, reactor types, and proportions of CO in the atmosphere differ among the various studies. Some researchers believe that CO promotes the yield of tar and char. As regard pyrolysis behavior, Wang33 considers that CO affects coal pyrolysis by driving the disproportionation reaction below 650 °C, whereas Gao18 considers that CO promotes coal pyrolysis by providing free radicals at low temperatures (below 600 °C) and disproportionation above 700 °C. Although the effect of CO atmosphere on coal pyrolysis has been covered in the literature, the mechanism by which CO affects coal pyrolysis has not been clearly investigated.28,33 Furthermore, majority of previous works on coal pyrolysis were conducted in a multicomponent atmosphere with CO as an ingredient, such as synthesis gas (SG) and coke oven gas (COG).23−25,29,33 The coal pyrolysis behavior was thus influenced by all components in the gas, obscuring the influence mechanism of isolated CO. Notably, 6100


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To investigate the effect of atmosphere on the pyrolysis characteristics, char structure, and reactivity, char was prepared by the fixed-bed method under different pyrolysis atmospheres. The experimental apparatus is schematized in Figure 1. Each LRC sample (250 g; grain diameter 2−4 mesh) was placed in a stainless steel reactor (Figures 1−5), suspended from a furnace, and lowered into the reaction zone when the final temperature reached the desired value. Carrier gas [pure N2 or CO-containing N2 (60% N2, 40% CO) atmosphere (99.999%); 50 mL/min] was injected through the bottom pores of the reactor and fully contacted the LRC particles. The pyrolysis temperature was set to 450, 550, 650, or 750 °C, and heat was maintained at the terminal temperature for 30 min. The char samples were then cooled in the furnace in a N2 atmosphere and stored in a desiccator. Table 2 lists the proximate and ultimate analysis results of the char samples prepared from the N2 and COcontaining atmospheres. Raw coal and char prepared in the pyrolysis atmosphere of N2 and char prepared in the pyrolysis atmosphere of CO-containing were designated as LRC, char-N2, and char-CO, respectively. 2.2. TG−MS Analysis. Pyrolysis characteristics of the coal were studied using TG−MS (TG, SETARAM SETSYS Evolution, France; MS, Pfeiffer, Quadrupole mass spectrometer, Germany). The flow rate of the reaction gas [pure N2 or CO-containing N2 (60% N2, 40% CO) atmosphere (99.999%)] was 50 mL/min. The experiments were conducted at the atmospheric pressure of the reaction gas, and the temperature was varied from 25 to 850 °C at a heating rate of 15 °C/ min. Raw coal samples (10 mg, grain diameter less than 75 μm) were tested in each case. Prior to each experiment, the furnace chamber and balance room were vacuum-pumped to remove residual gas from the previous experiment. The contact between the sample and carrier gas in the furnace chamber of the thermal analyzer was set to the twosample placement mode (Figure 3) because, in this setting, pyrolysis behavior can be observed for varying contact degrees between the sample and gas. In mode 1, the sample was placed in a common corundum crucible with an enclosed sidewall. The carrier gas of the crucible mainly contacted the upper surface of the sample. In mode 2, the sidewall of the platinum hanging basket is a net structure with many fine holes that hinder the passage of the sample particles through the wall but admit the carrier gas into the crucible. In this mode, the carrier gas contacts the LRC sample more fully as compared with its contact in mode 1. The gases evolved from the coal pyrolysis were detected using an online mass spectrometer, and the

are examined via Fourier transform infrared (FTIR) analysis. The functional-group changes are then compared with the transformation of pyrolysis gas. Second, the changing pore structures and carbon chemical structures of the char products were characterized using N2 adsorption and Raman spectroscopy, respectively. Finally, the combustion reactivity of the chars is studied using TG analysis.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A sub-bituminous coal with high volatile matter (37.06 wt %) was selected from the Shenmu mine in Shaanxi Province, China. Shenmu coal is characterized by a low degree of coalification, high reactivity, and good thermal stability. The proximate analysis was performed using a proximate analyzer (SDTGA5000, China) following the Chinese Standard GB/T 2122008. The elemental contents of C, H, and S of LRC were determined in an elemental analyzer (vario EL cube, Germany). Table 2 and Figure 2 show the corresponding results and infrared characteristics of

Figure 2. FTIR spectra of LRC. the LRC, respectively. The LRC is rich in various types of hydroxyl groups (3370 cm−1) as well as −CH3 (1438 cm−1) and −CH2 (2925 cm−1) functional groups contributed by aliphatic and aromatic chains. These functional groups can be converted to highly aromatic hydrocarbons during devolatilization. Currently, LRC is converted to tar using the industrialized low-temperature pyrolysis technology, which achieves high tar productivity (>8 wt %).

Figure 3. Schematic of the TG analysis system and sample placement modes. 6101


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Figure 4. TG and DTG results of coal pyrolysis in the two placement modes: (a) mode 1 and (b) mode 2.

Table 3. Pyrolysis Characteristics in Different Pyrolysis Stages Obtained from the TG−DTG Curves of the Coal Samples items mode 1

mode 2

N2 CO difference N2 CO difference

total mass loss (%)

mass loss (I) (%)

mass loss (II) (%)

mass loss (III) (%)

(dw/dt)max (%/°C)

31.14 29.84 −1.30 30.27 28.63 −1.64

1.83 1.27 −0.56 4.36 4.10 −0.26

17.63 17.30 −0.33 16.11 10.05 −6.06

11.68 11.27 −0.41 9.80 14.48 4.68

0.177 0.172 −0.005 0.197 0.093 −0.104

lattice.42 The integrated band areas characterize the intensities of the corresponding peaks, and the band area ratios can precisely characterize the carbon crystal structure. The disorder degree, graphitic-carbon degree, and reactivity of the char samples were characterized by the AD1/AG ratio,43,44 the intensity ratio AG/AAll,39 and the intensity ratio AR/AAll,44 respectively. Here, AG, AD1, and AR represent the integral areas of the G band, the D1 band, and the summed integral areas of the D2, D3, and D4 bands, respectively. The combustion reactivities of the char samples were studied using TG in an air gas flow of 50 mL/min. In each experiment, the char samples (10 mg) were heated from room temperature to 1000 °C at a heating rate of 15 °C/min.

gas molecules [H2 (m/z = 2), H2O (m/z = 18), CH4 (m/z = 16), and CO2 (m/z = 44)] were detected in the pyrolysis stages. The TG and MS instruments were connected by a capillary tube (heated to 200 °C) to realize online detection in real time. 2.3. Characterization. The sample morphologies were observed via scanning electron microscopy (SEM, Hitachi S-3400N, Japan) under 50 kV accelerating voltage. The physical properties (e.g., surface area, pore volume, and average pore size) were determined via N2 adsorption (JW-BK222, JingWei Science and Technology Company, China) and CO2 adsorption (Quantachrome AutosorbIQ2-MP, Quantachrome Instruments). The adsorption medium was N2 (99.999%) and CO2 (99.999%). The pore volume and specific area distribution were calculated using the Barrett−Joyner−Halenda (BJH) and Brunauer−Emmett−Teller (BET) methods and the density functional theory (DFT) method, respectively. The functional-group information of the char samples was provided via FTIR spectroscopy (FTIR, Nicolet iS5, Thermo Fisher Scientific Company). The samples were prepared through KBr compression with a scanning range of 400−4000 cm−1 and a resolution of 4 cm−1. The surface atomic states of the char samples and carbon were analyzed using an X-ray photoelectron spectrometer (XPS) (Thermo Fisher-VG Science, Waltham, MA). The X-ray source (1468.6 eV) was excited by an aluminum target with an operating voltage and power of 40 kV and 300 W, respectively. The carbon structure of the char was characterized using Raman spectra (LabRAM ARAMIS, HORIBA, Jobin Yvon Company, France). The spectral resolution and range were 1 and 800−2000 cm−1, respectively; the testing wavelength was 514 nm, and the data acquisition time was 20 s. Each measurement was performed at three different spots to capture the heterogeneous nature of the char sample. All spectra were processed using linear baseline correction, and the peak analysis in the 800−2000 cm−1 region was completed using Origin 8.0.39,40 To obtain detailed information on the carbon structures, each Raman spectrum was peak-fitted using the Peak Fitting Module, which resolved the curve into four Lorentzian bands and one Gaussian band (designated the D3 band).40 The spectra of the char samples were resolved into five bands, wherein perfect graphite yields a single band at approximately 1580 cm−1 (the G band), which corresponds to the stretching vibration mode of the perfect graphitic lattice.41 In disordered carbons, additional bands around 1350 cm−1 (D1), 1620 cm−1 (D2), 1530 cm−1 (D3), and 1150 cm−1 (D4) were induced by defects in the microcrystalline

3. RESULTS AND DISCUSSION 3.1. Devolatilization Process. Figure 4 shows the TG and differential thermal gravimetric (DTG) curves of LRC pyrolysis in N2 and CO-containing atmospheres. The pyrolysis is divisible into three stages with different mass loss rates. The slow pyrolysis stage (stage I) appears between approximately 200 and 380 °C. In this stage, carboxyl and other unstable functional groups dissociate into CO2 and H2O.45 The rapid pyrolysis stage (stage II) appears between 380 and 550 °C. Many severe pyrolysis reactions occurred in this stage, producing numerous volatile substances containing small molecular gases, such as hydrocarbons, CO2, and CH4.4646 The final stage (stage III), appearing at temperatures above 550 °C, is the polycondensation stage. Heavy hydrocarbons such as aromatic rings crack open at this stage, releasing large amounts of gases such as H2.6,476,47 The mass loss curves of coal pyrolysis in mode 1 slightly differed between the two atmospheres (Figure 4a), being slightly more shifted toward high temperatures in the COcontaining atmosphere as compared with the N2 atmosphere. In addition, the mass loss ratio was lower in the CO-containing pyrolysis than that in N2 pyrolysis at the same temperature. As stated in Table 3, the maximum mass loss rate of LRC (0.177%/°C) is slightly higher in the N2 pyrolysis atmosphere than that in the CO-containing pyrolysis atmosphere (0.172%/ °C). The coal mass during pyrolysis in the CO-containing 6102


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Figure 5. Volatiles and C/H of char samples prepared at different pyrolysis temperatures in different atmospheres.

Figure 6. Evolutions of CH4, H2, H2O, and CO2 in the different pyrolysis modes.

coal pyrolysis behavior was largely influenced by CO when the contact interface between the carrier gas and coal was sufficiently large. Figure 5 shows the volatiles and C/H ratios of the char samples prepared in the lab-scale fixed-bed pyrolyzer under different pyrolysis temperatures and atmospheres. Increasing the pyrolysis temperature of the char decreased the proportion of volatiles and increased the C/H, indicating that higher temperature promoted the transformation from hydrogenous organic components in coal into pyrolysis gas and tar, thereby preserving the carbon-rich structure in char. At pyrolysis temperatures of 450 and 550 °C, the char samples prepared under the CO-containing atmosphere (char-CO) exhibited higher volatile content and lower C/H than those prepared under N2 atmosphere (char-N2). Comparing the TG−DTG curves of the coal samples during the pyrolysis process in mode 2, the temperatures of 450 and 550 °C correspond to the maximum mass loss rate and the end of stage II pyrolysis, respectively. In these stages, the rates of transformation to volatiles and their escape from coal were suppressed by CO, implying that more organic molecules with hydrogencontaining groups (such as −CH3, −OH, and −COOH) were reserved in the char. However, at higher pyrolysis

atmosphere decreased by 0.56, 0.33, and 0.41% in stages I, II, and III, respectively, indicating that CO plays a minor inhibitory role in the mass loss of coal pyrolysis. The TG curves of raw coal pyrolysis in the N2 atmosphere noticeably differed between modes 1 and 2. Specifically, mass losses in stages I and II of the pyrolysis were larger and smaller in mode 2, respectively, than those in mode 1 (4.36 vs 1.83% in stage I, and 9.80 vs 11.68% in stage III). First, because the diffusion resistance of the gas-product evolution from coal particles was lower in mode 2 than that in mode 1, the local concentration of the gaseous pyrolysis product was reduced in mode 2, and the pyrolysis reaction was prompted toward a lower temperature.18 Second, the difference between the mass loss curves of N2 pyrolysis and CO-containing pyrolysis was magnified in mode 2, particularly in the mid and later parts of stages II and III. Furthermore, in mode 2, the influence of the COcontaining atmosphere on the coal pyrolysis in stages II and III is opposed. Under the CO-containing pyrolysis atmosphere, the mass decreased by 6.06% in stage II and increased by 4.68% in stage III (relative to the mass changes in the N2 atmosphere). In addition, the maximum mass loss rate decreased from 0.197%/°C in the N2 pyrolysis to 0.093%/ °C in the CO-containing pyrolysis. These results imply that the 6103


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Energy & Fuels temperatures, the volatiles and C/H ratios of the char samples were similar in both atmospheres. This finding is consistent with the TG−DTG curves in mode 2, namely, the difference between the curves acquired in the two atmospheres reduced in stage III. This occurred because the organic species remained in the char during stage II, but were released in stage III. Overall, CO suppressed coal devolatilization in the rapid pyrolysis stage, but its influence diminished at high pyrolysis temperatures. 3.2. Evolved Gas Analysis. Figure 6 presents the evolution curves of CH4 (m/z = 16), H2 (m/z = 2), H2O (m/z = 18), and CO2 (m/z = 44) during coal pyrolysis. In mode 1, the pyrolysis atmosphere little affected the evolution rate and interval temperatures of CH4, H2O, and H2. However, the CO2 evolution curves remarkably differed between the N2 and CO-containing atmosphere. In mode 2, the release interval was more concentrated, and the releases of gaseous products under the two pyrolysis atmospheres were more widely separated, indicating that the volatile conversion was more sensitive to the CO-containing atmosphere in this mode. Therefore, further analysis of the diversity of curves was focused on in mode 2. In contrast, the curves in mode 1 demonstrate that the contact extent between the carrier gas and solid influences pyrolysis behavior. In the temperature range of 380−610 °C, increasing the pyrolysis temperature of stages II and III rapidly increased and then gradually reduced CH4 evolution (Figure 6b). The CH4 emission intensity was lower in the CO-containing atmosphere than that in the N2 atmosphere, and the initial temperature of CH4 evolution was approximately 30 °C higher in the COcontaining atmosphere. These results indicate that CO decreased the CH4 evolution, which is consistent with the results reported in the literature.16,17 During coal pyrolysis, CH4 was mainly generated during the primary pyrolysis reaction 1 between the methyl functional groups in long aliphatic chains and active hydrogens (H•).49 coal − CH3 (or − CH 2) + H• → coal + CH4

Figure 7. FTIR spectra of the char samples.

the LRC pyrolysis process. Consequently, CH4 production was reduced under this atmosphere. Figure 6 reveals two zones of clear H2O release throughout the LRC pyrolysis. The lower-temperature peak in stage I reflects the moisture released from the sample, including internal and crystal H2O. The second release interval of H2O is generated by the decomposition of various oxygen-containing groups, mainly −OH groups.49 In the CO-containing atmosphere, the H2O release was more intense than that in the N2 atmosphere (Figure 6b), and the second H2O release shifted to the low-temperature region, indicating an enhanced H2O production. The peaks near 3630 and 3727 cm−1 in the infrared spectrum of char (Figure 7) are attributable to −OH stretching vibrations of associative and free-form •OH groups, respectively.49,51 The absorption peaks nearly coincide in the two atmospheres, suggesting that the CO-containing atmosphere exerted minimal effect on the conversion of −OH groups. This result indicates that the H2O production is not directly increased owing to the pyrolysis of LRC, but it may arise from transformations between homogeneous gases. For instance, the reverse water−gas reaction and its side reaction have been reported to yield H2O product in some relevant studies of coal gasification52,53

(1)

Two mechanisms can explain this CH4 reduction. First, CO can inhibit the cleavage of the C−C chemical bonds in long aliphatic chains and aromatic heterocyclic groups, thereby reducing the number of free radicals CH3• and CH2• and lowering the efficiency of the conversion reaction 1. Hence, this mechanism decreases CH4 emission through the chemical action of CO on the pyrolysis reaction. In the second mechanism, the active hydrogens previously participating in the reaction might transform through other routes, such as H2 and hydrocarbon transformations, thus reducing CH 4 production. In the FTIR spectra of the char (Figure 7), the C−H deformation vibration peaks of −CH2 and −CH3 on the alkyl bonds (at ∼1438 cm−1 45) are obviously stronger in the CO-containing atmosphere than that in the N2 atmosphere, indicating that fewer −CH3 and −CH2 bonds were broken in the CO-containing atmosphere. Furthermore, at pyrolysis temperatures of 450 and 550 °C, the absorption intensity of the C−H stretching vibration of alkane and aliphatic compounds (at 2880−3050 cm−1 45,49,50) was higher in chars-CO than that in chars-N2; this again confirms the presence of more −CH2 and −CH3 groups in chars-CO than in chars-N2. Therefore, under the CO-containing atmosphere, the cleavage of the aliphatic C−H bonds was suppressed, and fewer free radicals such as CH3• and CH2• were produced in

CO(g) + H 2(g) → C(s) + H 2O(g)

(2)

CO(g) + 3H 2(g) → CH4(g) + H 2O(g)

(3)

Figure 8 plots the relationship between the Gibbs free energy ΔG of reactions 2 and 3 and the pyrolysis temperature. The threshold temperatures of reactions 2 and 3 were just below 620 and 680 °C, respectively, coincident with the pyrolysis temperature range of the H2O increase. When CO and H2 react according to reaction 3, the evolved CH4 should also increase. However, the release intensity of CH4 was lower in the CO-containing atmosphere than that in the N 2 atmosphere, implying that reaction 3 hardly occurred or proceeded with weak kinetics under the present experimental conditions. Therefore, the enhanced H2O production in the CO-containing pyrolysis atmosphere is more likely caused by reaction 2 than reaction 3. 6104


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higher contact probability between CO and H2. The generated H2O then easily escapes from the basket. In addition, the fresh H2 molecules evolved in the pyrolysis may have higher activity than that in mode 1, further strengthening reaction 2. Consequently, the evolution of H2 was restrained during the rapid reaction 2; the amount of H2O was increased and escaped early in mode 2. CO2 release during the LRC pyrolysis of mode 1 is divisible into three phases. Before 200 °C, the CO2 adsorbed on the surface and crevices of the coal is released. The second emission peak of CO2 at approximately 400−500 °C corresponds to the decarboxylation or decarbonylation reactions of LRC.45 The last CO2 peak at approximately 700 °C derives from the decomposition of carbonates and of thermally stable oxygen-containing heterocycles.46 However, CO2 release is significantly advanced in mode 2, indicating a large role of diffusion in the CO2 release. In mode 1, the COcontaining atmosphere only slightly affected the CO2 release process during stage I; however, in mode 2, it significantly decreased the CO2 intensity during stages I and II (Figure 6). This indicates that CO inhibits the decarboxylation or decarbonylation reactions. CO greatly influenced stages II and III of the pyrolysis in both modes. The release curves were significantly higher under the CO-containing atmosphere than those under the N2 atmosphere. The infrared spectra of char450 °C and char-550 °C (Figure 7) show no obvious absorption peaks at the corresponding wavenumbers of carbonyl and carboxyl groups, indicating that the intensified CO2 release under the CO-containing atmosphere is not caused by the breakage of carbonyl or carboxyl groups. We speculatively attribute the CO2 increase to the disproportionation reaction of carbon (also called the carbon precipitation reaction) in the CO-containing pyrolysis atmosphere55,56

Figure 8. ΔGs of the inverse water−gas reactions as functions of temperature.

The H2 emission around pyrolysis stage II corresponds to the degradation of the hydrogen-rich substrates, and the H2 in pyrolysis stage III is sourced from the condensation of organic matter, resulting in cyclization and aromatization of the hydrocarbons.54 As the temperature increased, the amount of H2 evolution first increased and then decreased. The H2release peak appeared above 720 °C, indicating condensation of aromatic compounds and aromatic hydrogen structures or the decomposition of heterocyclic compounds.46,54 As shown in Figure 6b, the H2 emission intensity of LRC under the COcontaining pyrolysis atmosphere weakened below 720 °C and strengthened above 720 °C. The bands at approximately 808− 877 cm−1 in Figure 7 are assignable to aromatic structures with isolated hydrogen or three adjacent hydrogen atoms per ring.48,50,51 The strong absorption intensity around 808−877 cm−1 in char-CO-450 °C and char-CO-550 °C indicates additional hydrogen atoms on the aromatic rings of these species, suggesting that CO inhibits the formation of H• during stage II of the LRC pyrolysis. However, below 720 °C in stage III of the pyrolysis, the intensity of the H2-release peak was lower in the CO-containing atmosphere than that in the N2 atmosphere, suggesting that reaction 2 occurs in the COcontaining atmosphere. As the reaction consumes part of the H2, the H2-escape intensity simultaneously reduces with H2O increase. It is also inferred that the different H2O release between the two atmospheres relates to the pyrolysis mode, as shown in Figure 9. In mode 1, the H2 generated by coal

2CO(g) → CO2 (g) + C(s)

(4)

Figure 10a plots the change in Gibbs’ free energy ΔG of reaction 4 as a function of temperature. At pyrolysis temperatures below 700 °C, reaction 4 is driven by conductive thermodynamics. To confirm the disproportionation of CO in the pyrolysis process, blank experiments (without sample) were performed in the N2 and CO-containing atmospheres under the same TG−MS conditions as those of the pyrolysis process of LRC. In the N2 atmosphere, the CO2 release curve was nearly constant over the entire temperature range (Figure 10b). However, in the CO-containing atmosphere, the CO2 release peaked at approximately 450 °C and then gradually decreased. As mentioned in Section 2.2, the purity of the carrier gas (CO/N2) was extremely high; hence, the CO2 change cannot originate from external gas. Furthermore, the O2 signal (m/z = 32) was very weak, indicating few oxygen molecules in the carrier gas. Therefore, the increased CO2 content near 450 °C cannot result from the oxidizing reaction between CO and O2. The escaped CO2 above 300 °C probably stemmed from reaction 4 in the chamber of the thermal analyzer. The continued conversion of CO to CO2 above 700 °C in the MS results contradicts the temperature range of CO to CO2 conversion in the thermodynamic analysis. This discrepancy may be explained as follows. The constanttemperature section in the furnace of the thermogravimetric analyzer is 700 °C, whereas the upper part of the furnace does not reach 700 °C. Therefore, reaction 4 will continue at 700 °C, which is consistent with the findings reported in the literature.55,56 In addition, after the pyrolysis process, the

Figure 9. Schematic showing the probable effect of the inverse water−gas reaction on the H2 and H2O evolutions in pyrolysis modes 1 and 2.

pyrolysis escapes from the sample bed and reacts with CO in the carrier gas on the upper side of the crucible, releasing H2O from that part only. In mode 2, the CO molecules in the COcontaining atmosphere can easily pass through the holes of the hanging basket into the interstices of the coal particles, where they immediately react with the H2 evolved from the coal pyrolysis. In this case, the H2O release is enhanced by the 6105


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Figure 10. Demonstration of the disproportionation reaction of carbon: (a) ΔG vs temperature plot of the disproportionation reaction; (b) evolution of CO2 in the blank experiments (from room temperature to 850 °C at a heating rate of 15 °C/min in a N2 atmosphere and a COcontaining atmosphere); (c,d) SEM images of the crucible surfaces under the N2 and CO-containing atmospheres.

crucible surface remained white under the N2 atmosphere but turned black under the CO-containing atmosphere (cf. panels (c) and (d) of Figure 10). SEM observation of the crucible surface in the CO-containing atmosphere revealed that many carbon particles sized 2−10 μm adhered to the surface, further supporting the disproportionation reaction. Therefore, the notably enhanced CO2 emission in Figure 6b is attributable to not only the coal pyrolysis process but also the CO2 generated during reaction 4, which includes the CO2 released by coal pyrolysis itself. Furthermore, nearly all of the volatile matter had been released from the LRC when the pyrolysis temperature reached 850 °C (as confirmed by the near-zero DTG value in Figure 4). The larger mass under the COcontaining atmosphere than that under the N2 atmosphere (1.30% in mode 1; 1.64% in mode 2) is probably due to the carbon structures from reactions 2 and 4 being adsorbed on the crucible surface. Moreover, the intensity of the 1590−1615 cm−1 band was higher in char-CO than that in char-N2, indicating that char-CO contained a higher concentration of aromatic CC groups than char-N2.49,5749,57 This may be because the carbon structure formed in reactions 2 and 4 contains aromatic CC groups. Another potential reason is that the generated graphite-like carbon is converted into char and forms CC functional groups, which promote the ordering of char.49 In general, coal pyrolysis breaks the macromolecular chains and recombines the free radicals. The pyrolysis atmosphere alters the pyrolysis process by participating in the chemical reactions of gas products. As shown in Figure 11, CO inhibits the breakage of methyl and methylene side chains, thereby decreasing the CH4-release intensity. The increased H2O yield under the CO-containing atmosphere is mainly due to the inverse water−gas reaction, not the conversion of the hydroxyl groups in coal. Furthermore, H 2 is reduced through consumption by the reversed water−gas reaction and because CO inhibits the formation of hydrogen free radicals. The intensity of CO2 release was obviously higher under the CO-

Figure 11. Principle illustration of the chemical reaction schemes during the pyrolysis of LRC.

containing atmosphere than that under the N2 atmosphere owing to the disproportionation reaction of CO. The carbon structure generated by the disproportionation reaction improves the ordering of the char structure. This conclusion is also confirmed in the Raman and XPS detection of the charsCO structure in the following section. Although previous pyrolysis experiments obtained less obvious results, they do not negate a significant effect of CO on the pyrolysis process. This may be because of the following reasons. First, the contact between the carrier gas and sample may have been insufficient to detect the effect, as observed in mode 1 of the present experiment. Second, few researchers have studied the effect of CO on the pyrolysis characteristics via MS or FTIR, although the online detection method can accurately and directly characterize the complete pyrolysis process. Furthermore, the carrier gas in majority of the previous studies was mixed gas, which masks the CO effect on pyrolysis. Note that the present experiment did not analyze the influence of the CO-containing atmosphere on the CO 6106


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Figure 12. Specific surface and pore volumes of char samples at different pyrolysis temperatures: (a) N2 adsorption and (b) CO2 adsorption.

Figure 13. Raman analysis of chars: (a) peak fitting of the spectra and (b) characteristic parameters.

The micropores were less developed in the char-CO samples than in their char-N2 counterparts at the same pyrolysis temperature. In the above analysis of Figure 6, we observed that under the CO-containing atmosphere, carbon particles were simultaneously generated by reactions 2 and 4 during pyrolysis. These particles adhered to the char surface and blocked the pores. Consequently, the open pores converted to dead pores, thereby reducing the pore volume and specific area. The N2/CO2-adsorption methods depends on the adsorption capacity of the N2/CO2 molecules for detecting the sample. Therefore, the adhered carbon particles probably decreased the adsorption capacity of the N2/CO2 molecules, thereby reducing the pore volume and specific area. Moreover, the pore blockage increases the diffusion resistance and inhibits the evaporation of volatile matter, further suppressing the development of pores. It is worth noting that the changes of the specific surface area of the char samples calculated by the three methods are consistent, but the specific surface area of the pore structure measured by N2 adsorption is significantly smaller than that measured by CO2 adsorption. Because the CO2 molecule is smaller than the diameter of N2, the CO2 adsorption can be measured to the smaller pores. In addition,

emission of gas products because CO evolution will be strongly covered by the carrier gas during the pyrolysis process; thus, it lacks a reference value. 3.3. Structure of Char. Figure 12 displays the pore volumes and specific areas of the chars at different pyrolysis temperatures, calculated via the BJH, BET, and DFT methods. The specific surface area and pore volume of the char increased with pyrolysis temperature up to 650 °C and declined at higher temperatures. The specific surface area and pore volume of char-CO were lower than those of char-N2, but they exhibited the same temperature trends. During LRC pyrolysis, gas products such as H2O, CnHm, CH4, CO, and CO2 are released from the pores by dissociation of functional groups, forming many new pore structures.28,58 During the subsequent thermoplastic process, the original and formed micropores expand into medium-sized pores at higher temperatures, altering the surface area and pore volume. However, at pyrolysis temperatures above 650 °C, the LRC undergoes a secondary pyrolysis process, and the condensation reaction collapses the pores and shrinks their volume.28,58 This explains the declining properties of the micropores from 650 to 750 °C. 6107


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carbon).61,62 Additional carbon states in the 286−289 eV range caused asymmetry and broadening in the C 1s spectra. These carbon states probably include the 286. 6 eV peak of phenolic or/and ether carbons, the 288.5 eV peak of carbonyl carbon (−CO−), and the 290.5 eV peak of carboxyl carbon (−COOH).61 The peak of carbon particles is intermediate between those of graphitic carbon and char-N2, and the maximum binding energy of C is below 286.6 eV. It was inferred that the carbon states contained graphitic carbon with sp3 and sp2 C−C. Therefore, the carbon particles absorbed on the char particles also contribute to the ordering of the char structure. In general, the char prepared under the CO atmosphere has a more ordered carbon chemical structure than that formed under the N2 atmosphere, not only because the char molecules interact with carbon particles but also because the newly formed carbon adheres with an ordered structure on the char. 3.4. Reactivity of Char. Figure 15 shows the TG−DTG curves of the char samples prepared at different pyrolysis temperatures in different atmospheres. In both chars-CO and chars-N2, increasing the pyrolysis temperature shifted the TG and DTG curves during combustion to the high-temperature region (Figure 15a). The characteristic parameters, namely, the ignition temperature (Ti), the temperature at the maximum combustion rate (Tm), and the burnout temperature (Tf) of the chars, also increased with the pyrolysis temperature (Figure 15b), indicating that the char requires higher temperature to ignite, combust at a rapid rate, and burn out, respectively, at higher pyrolysis temperatures. The combustion performance of the char is quantified by the combustion characteristic index S, which is given as follows63

by comparing Figure 12a,b, it can be seen that the microporous structure of char samples accounts for a large proportion. Overall, the pore structure of char-CO was probably suppressed by the coaction of the shingling of carbon particles generated by the disproportionation reaction and the CO inhibition on volatile release. Raman spectroscopy reveals the chemical structure of carbonaceous materials.28,40,42 Figure 13a shows the Raman spectrum of the char samples at different pyrolysis temperatures. The D1 band is attributed to the presence of large aromatic rings (more than six) and a disordered graphitic lattice.59 The D2 band is related to lattice vibrations of the surface graphene layers.39 The D3 band is sourced from small ring systems (three to five rings) and the amorphous carbon in the char samples.39,60 The D4 band belongs to the mixed sp2− sp3 bonds formed at the periphery of the crystallites and the C−C and CC stretching vibrations of polyene-like structures.41 Both temperature and atmosphere affected the Raman peaks of the present samples. Figure 13b plots the quantitative Raman parameters of the char samples as functions of pyrolysis temperature. Increasing the pyrolysis temperature significantly increased the AG/AAll ratio of charN2, indicating the formation of a more organized char structure. Moreover, the AD1/AG and AR/AAll ratios decreased with increasing pyrolysis temperature, indicating that the amorphous carbon structure and the defect structure were transformed into ordered carbon crystals. Similar temperature trends in the band area ratios were observed in char-CO, which generally exhibited a higher AG/AAll ratio and lower AD1/AG and AR/AAll ratios than char-N2. This means that char-CO possesses a more ordered carbon structure than char-N2 at the same pyrolysis temperature. As mentioned in Section 3.2, charCO has a higher concentration of aromatic CC groups than char-N2, suggesting that carbon transformations through reactions 2 and 4 participated in the polymerization of molecules in the coal pyrolysis. Thus, the CO-containing atmosphere enhances the ordering of char by increasing the lattice size and regularizing the carbon structure. The structural states of the carbon particles adhered to the container walls under the CO atmosphere were identified via XPS (those of char-N2 were also analyzed for comparison). The C 1s spectrum ranging from 280 to 290 eV is shown in Figure 14. The binding energy at 284.65 eV (dashed line in Figure 14) was assigned to sp3 C−C from graphitic carbon.61,62 In char-N2, the peak position shifted to a higher binding energy (285.15 eV), which was assigned to sp2 C−C (the π bonds of the hexagonal aromatic molecules in graphitic

S=

K max ·K mean Ti 2·Tf

(5)

where Kmax and Kmean denote the maximum and average burning rates, respectively. As shown in Figure 16, the combustion characteristic index S of the char decreased with increasing pyrolysis temperature. Therefore, the combustion reactivity of the chars decreased with increasing pyrolysis conversion, as reported in the literature.64,65 As more volatiles exist in char prepared at low temperatures, more pyrolysis gas will be generated during the early stage of char combustion. These gases promote homogeneous ignition and combustion. In turn, homogeneous ignition promotes heterogeneous ignition and accelerates the combustion reaction between the fixed carbon and oxygen by heating the carbon particles. Further, more porous char particles with a high surface area are prepared during the devolatilization stage, benefiting the subsequent fixed-carbon combustion.65 The Ti, Tm, and Tf were higher in char-CO than in char-N2 pyrolyzed at the same temperature (Figure 15). Simultaneously, the combustion characteristic index S was lower in char-CO than in char-N2, implying that char-CO had lower combustion reactivity than char-N2 at the same pyrolysis temperature. In addition to the volatile content, the carbon chemical structure and the pore structure affect the combustion performance of the char.28 A more disordered carbon structure and a better developed pore structure improve the combustion reactivity of the char.28 Although char-CO contains more volatiles than char-N2, which beneficially reduces the characteristic temperature and S of the char, char-CO has a lower specific surface area and pore

Figure 14. Carbon structure in char-N2 and the carbon particles generated in a CO atmosphere, examined via XPS. 6108


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Figure 15. Combustion reactivity of chars: (a) TG−DTG curves and (b) characteristic parameters.

results imply that pore structure and carbon structure are the governing factors of combustion performance, that is, a COcontaining atmosphere affects the pore structure and carbon chemical structure of the char, reducing its combustibility during the pyrolysis process. Figure 17 shows the TG−DTG curves and the characteristic parameters of the char samples. In both chars-CO and charsN2, increasing the pyrolysis temperature shifted the TG and DTG curves during gasification to the high-temperature region (Figure 17a). The characteristic parameters, namely, the gasification temperature of char samples’ conversion to 50% (Tg‑0.5), the temperature at the maximum gasification rate (Tg‑max), and the end of gasification temperature (Tg‑f) of the chars, also show an increased tendency with the pyrolysis temperature (Figure 17b), except that char-CO decreased at 650 °C. The gasification performance of the char is quantified by the gasification activity index Rs, which is given as follows66

Figure 16. Combustion characteristic index S of char samples at different pyrolysis temperatures.

volume than char-N2 and is more ordered than char-N2 at the same pyrolysis temperature. Thus, these three factors influenced the combustion reactivity in two contradictory manners: the reactivity of char-CO is improved by the higher volatile matter as compared with char-N2, but the improvement is more than offset by the underdeveloped pore structures and the ordered carbon structure. The TG−DTG

Rs =

0.5 t0.5

(6)

Figure 17. Gasification reactivity of chars: (a) TG−DTG curves and (b) characteristic parameters. 6109


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Energy & Fuels where t0.5 denotes the time taken to reach the maximum gasification rates. From Figure 18, it can be seen that the gasification activity index Rs of the char increased with increasing pyrolysis temperature from 450 to 650 °C and then decreased.

chars-CO, the structure of the char may have changed to be conducive to the direction of gasification. Figure 19 displays the specific surface area and pore volume of the char-450 °C and char-900 °C (preparation of char-450 °C in pyrolysis of CO2 atmosphere before 900 °C). The specific surface area and pore volume of char-N2-450 °C are higher than those of char-CO-450 °C, and the specific surface area of char-900 °C is obviously increased. The analysis shows that the obvious difference of combustion performance between char-N2 and char-CO is due to the influence of CO pyrolysis atmosphere on the microstructure of char. However, when the char has undergone a new round of pyrolysis before gasification, the volatiles are released from the char, and the specific area of the char increases, then the difference between the specific area of the char-CO-900 °C and the char-N2-900 °C becomes smaller. Because the volatile content of chars-CO is higher than that of chars-N2, the chars-CO will release more volatiles before gasification, which enlarges the pore structure of the chars. Thus, the contact area between char-CO and CO2 is increased during gasification, resulting in an improvement of gasification reactivity. Note that the structure and composition of the coal tar are also helpful for elucidating the pyrolysis characteristic under the CO-containing atmosphere. Analyzing how the interactions between CO and other atmospheres affect the pyrolysis under actual pyrolysis conditions is also worthy of attention. Such research is under way and will be reported in the near future.

Figure 18. Gasification activity index Rs of char samples at different pyrolysis temperatures.

The Tg‑0.5, Tg‑max, and Tg‑f were higher in char-N2 than in char-CO pyrolyzed at the same temperature (Figure 17). Moreover, the gasification activity index Rs was lower in charN2 than in char-CO, implying that char-N2 had lower gasification reactivity than char-CO at the same pyrolysis temperature. The results of Raman analysis of char samples show that the structure of carbon in char is gradually ordered with the increase of pyrolysis temperature, whereas the gasification activity of ordered carbon was lower than that of disordered carbon. Therefore, the gasification activity of char will decrease with the increase of pyrolysis temperature. In addition, the specific surface area and pore volume of the chars (char-N2 and char-CO) increased with pyrolysis temperature up to 650 °C, and declined at higher temperatures. This trend is consistent with that of the gasification activity index Rs. It is considered that the main factors affecting the CO2 gasification reactivity of char are pore structure and carbon structure.67 According to the analysis of the chemical structure of char before, the degree of chemical ordering and regularity of the carbon structure in chars-CO were higher than that in charsN2. Therefore, there is a reasonable suspicion that the pore structure may change before the gasification reactivity (about 900 °C) of char begins. That is to say, before the gasification of

4. CONCLUSIONS (1) As confirmed in the thermal and compositional analyses, the CO atmosphere suppressed the rapid devolatilization of coal between 380 and 550 °C; however, above 550 °C, the devolatilization recovered to that under the N2 atmosphere. (2) During the coal pyrolysis process, the CH4 release was reduced under the CO atmosphere because CO inhibited the breakage of the methyl and methylene side chains (as confirmed via FTIR). The increased H2O yield during coal pyrolysis was due to the inverse water− gas reaction and not by the conversion of the hydroxyl groups in coal. The H2 yield was jointly reduced by the inhibiting effect of CO on the formation of hydrogen free radicals and H2 consumption by the reverse water− gas reaction. The coal pyrolysis behavior was largely

Figure 19. Size distributions of pore structures: (a) Brunauer−Emmett−Teller (BET) surface areas and (b) Barrett−Joyner−Halenda (BJH) pore volumes. 6110


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(5) Morgan, T. J.; Kandiyoti, R. Pyrolysis of coals and biomass: analysis of thermal breakdown and its products. Chem. Rev. 2014, 114, 1547−1607. (6) Arenillas, A.; Rubiera, F.; Pis, J. J.; Cuesta, M. J.; Iglesias, M. J.; Jiménez, A.; et al. Thermal behaviour during the pyrolysis of low rank perhydrous coals. J. Anal. Appl. Pyrolysis 2003, 68−69, 371−385. (7) Zhao, Y.; Hu, H.; Jin, L.; He, X.; Wu, B. Pyrolysis behavior of vitrinite and inertinite from Chinese Pingshuo coal by TG-MS and in a fixed bed reactor. Fuel Process. Technol. 2011, 92, 780−786. (8) Shi, L.; Liu, Q.; Guo, X.; Wu, W.; Liu, Z. Pyrolysis behavior and bonding information of coal-A TGA study. Fuel Process. Technol. 2013, 108, 125−132. (9) Qiu, N.; Li, H.; Jin, Z.; Zhu, Y. Temperature and time effect on the concentrations of free radicals in coal: Evidence from laboratory pyrolysis experiments. Int. J. Coal Geol. 2007, 69, 220−228. (10) Casal, M. D.; Canga, C. S.; Diez, M. A.; Alvarez, R.; Barriocanal, C. Low-temperature pyrolysis of coals with different coking pressure characteristics. J. Anal. Appl. Pyrolysis 2005, 74, 96− 103. (11) Zhong, M.; Zhang, Z.; Zhou, Q.; Yue, J.; Gao, S.; Xu, G. Continuous high-temperature fluidized bed pyrolysis of coal in complex atmospheres: Product distribution and pyrolysis gas. J. Anal. Appl. Pyrolysis 2012, 97, 123−129. (12) Zhang, S.; Zhu, F.; Bai, C.; Wen, L.; Zou, C. Thermal behavior and kinetics of the pyrolysis of the coal used in the COREX process. J. Anal. Appl. Pyrolysis 2013, 104, 660−666. (13) Chang, H.; Deng, H.; Yang, Q.; Du, S.; Hu, F.; Jia, C. Interaction of vitrinite and inertinite of Bulianta coal in pyrolysis. Fuel 2017, 207, 643−649. (14) Karaca, S. Desulfurization of a Turkish lignite at various gas atmospheres by pyrolysis. Effect of mineral matter. Fuel 2003, 82, 1509−1516. (15) Zhao, H.; Song, Q.; Liu, S.; Li, Y.; Wang, X.; Shu, X. Study on catalytic co-pyrolysis of physical mixture/staged pyrolysis characteristics of lignite and straw over a catalytic beds of char and its mechanism. Energy Convers. Manage. 2018, 161, 13−26. (16) Zhong, M.; Ma, F. Analysis of product distribution and quality for continuous pyrolysis of coal in different atmospheres. J. Fuel Chem. Technol. 2013, 41, 1427−1436. (17) Bai, Z.-q.; Li, W.; Yu, C. W.; Bai, J. Low temperature pyrolysis of lignite in the presence of syngas and combustion characteristics of derived char. J. China Univ. Min. Technol. 2011, 40, 726−732. (18) Gao, S. P.; Zhao, J. T.; Wang, Z. Q.; Wang, J. F. Effect of CO on fast pyrolysis behaviors of lignite. J. Fuel Chem. Technol. 2013, 41, 550−557. (19) Zhang, H.; Xiao, R.; Wang, D.; He, G.; Shao, S.; Zhang, J.; et al. Biomass fast pyrolysis in a fluidized bed reactor under N2, CO2, CO, CH4 and H2 atmospheres. Bioresour. Technol. 2011, 102, 4258−4264. (20) Hao, C.-H.; Zhu, S.-H.; Bai, Y.-H.; Li, F. Effect of CO2 on pyrolysis behavior of Shengli lignite. J. Fuel Chem. Technol. 2017, 45, 272−278. (21) Gao, S. P.; Zhao, J. T.; Wang, Z. Q.; Wang, J. F.; Fang, Y. T.; Huang, J. J. Effect of CO2 on pyrolysis behaviors of lignite. J. Fuel Chem. Technol. 2013, 41, 257−264. (22) Guizani, C.; Sanz, F. J. E.; Salvador, S. Effects of CO2 on biomass fast pyrolysis: Reaction rate, gas yields and char reactive properties. Fuel 2014, 116, 310−320. (23) Liao, H.-Q.; Sun, C.-G.; Li, B.-Q. Copyrolysis of coal with coke oven gas: III. Analysis of tar. J. Fuel Chem. Technol. 1998, 26, 7−12. (24) Liao, H. Q.; Sun, C. G.; Li, B. Q. Pyrolysis of coal hydropyrolysis with coke-oven gas. Coal Convers. 1997, 20, 38−43. (25) Liao, H. Q.; Li, B. Q.; Zhang, B. J. Pyrolysis of coal with cokeoven gas: IV. Influence of CH4 and CO on pyrolysis yields. J. Fuel Chem. Technol. 1998, 26, 13−17. (26) Zhang, X. F.; Jin, L.; Xiong, R.; Wang, Y. Effect of reaction atmosphere on tar production from coal pyrolysis in fluidized bed reactor. CIESC J. 2009, 60, 2299−2307.

influenced by CO in the pyrolysis atmosphere as the contact opportunities between the CO molecule and coal particle were increased at the hanging basket mode. (3) The emission intensity of CO2 was significantly higher under the CO-containing atmosphere than that under the N2 atmosphere. This phenomenon is closely related to the disproportionation reaction of CO between 300 and 700 oC, which was verified through the blank MS experiment, morphology observations, and the different final mass losses under the two atmospheres in the TG analysis. (4) Under the CO-containing atmosphere, the pore structure of char-CO is suppressed by the coactions of two phenomena: the shingling of carbon particles generated by the disproportionation reaction and the inhibition of the volatile release in the CO-containing atmosphere. The CO-containing atmosphere promotes an ordered carbon chemical structure through interactions between the char and carbon particles and through the ordered adhesion of many newly formed carbon particles on the char. (5) Char-CO exhibited lower combustion reactivity than char-N2 owing to its underdeveloped pore structures and ordered carbon structures. However, because the charCO with higher volatiles undergoes a secondary pyrolysis stage for releasing the volatiles at high temperature before CO2 gasification, thereby developing the pore structure, the char-CO showed higher gasification reactivity than char-N2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Z.). *E-mail: [email protected] (J.Z.). ORCID

Chong Zou: 0000-0003-4734-361X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation Project of China (grant numbers 51704224 and 51574189), the Natural Science Foundation Research Project, Shaanxi, China (grant number 2016JQ5041), and the Ministry of Education Services Local Scientific Research Program Subsidized Projects, Shaanxi, China (grant number 2017JF012).

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REFERENCES

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Pyrolysis characteristics of low-rank coal under CO-containing

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