Roles of Ion-Exchangeable Sodium in the Conversion Process of Tar to Soot during Rapid Pyrolysis of Two Brown Coals in a Drop-Tube Reactor.

In this work, two series of brown coals (including acid-washed coal and ion-exchangeable Na-loaded coal) were pyrolyzed in a drop-tube reactor. The experimental results revealed that soot and tar yields of Na-loaded coals were significantly lower than that of acid-washed coals. Gasified Na can reduce the formation of big soot agglomerates. During coal primary pyrolysis, ion-exchangeable Na can reduce the amount and aromaticity of primary tar. Na released with volatiles can catalyze the cracking of aliphatic and aromatic compounds, inhibit the polymerization between aromatic rings, and promote the combination of soot/tar with oxygen-containing substances, resulting in the decrease of graphite crystallite size and the increase of amorphous carbon content. Na can also reduce the organization degree of soot by forming intercalation compounds.

Introduction

Coal pyrolysis includes primary pyrolysis and secondary pyrolysis. Under high-temperature conditions, coal rapidly undergoes primary pyrolysis and releases light gases and primary tar. Some primary tar molecules undergo polymerization reactions during secondary pyrolysis to form soot, and the others may crack and turn into light gases or remain in pyrolysis gas. The light gases can also be converted to soot or tar by addition reactions during secondary pyrolysis. Soot is usually less than 2.5 μm and is designated by USEPA as the main component of PM2.5.[1,2] The surface of soot often adsorbs highly carcinogenic polycyclic aromatic hydrocarbons,[3] and they can be deposited on alveolar cells and enter the blood circulation system.[4,5] Because of the strong radiation heat transfer ability, soot in the atmosphere can affect the Earth’s radiation balance and enhance the atmospheric greenhouse effect.[6] Baltrus et al.[7] suggest that carbon content in carbon concentrates (similar to soot) formed from coal fly ashes is up to 76%. Soot can enhance the heat transfer of pulverized coal particles and reduce the flame temperature by up to 300 °C.[8] In addition, the formation and oxidation of soot can also affect the emission of NO.[9]

Compared to a simple gaseous hydrocarbon flame, the formation of soot during secondary pyrolysis of coal is a much more complicated process and involves many more components.[10] Coal tar generated by coal primary pyrolysis is thought of as the main precursor of coal-derived soot. Nenniger[11] used a drop-tube reactor to analyze the changes in yields of soot and polycyclic aromatic compounds (PACs, main component of coal tar). He reported that with the temperature or residence time increase, the sum of soot and PACs remains unchanged which indicates that PACs are the main precursor of coal-derived soot. Wornat et al.[12] got the same experimental results under similar experimental conditions. They also found that with the increase of the degree of secondary pyrolysis, the stability of PACs increases and the complexity decreases. Ma[13] and Zhang[14] studied the characteristic of coal-derived soot produced in a flue gas environment. Different from the experimental result published by Nenniger[11] and Wornat,[12] soot yield in the flue gas environment is lower than that in the inert atmosphere. This is because tar and even soot react with oxygen-containing substances. Zeng et al.[15] studied secondary reactions of tar produced by different rank coals using an entrained flow reactor and concluded that tar formed from low rank coals have more oxygen-containing functional groups and branched chain structures than high rank coals. They also found that because of addition reactions of light hydrocarbon gas, the sum of soot and tar (soot + tar) yields increases with the temperature increasing when it is above 1400 K.

The roles of inherent mineral matter in the evolution of tar during coal primary pyrolysis has been discovered and reported by many researchers.[16−21] In comparison, the role of metals in coal secondary pyrolysis, especially the role of gasified metals in the conversion of coal tar to soot, is not comprehensive enough.[17,18] Hayashi et al.[22] carried out pyrolysis experiments on raw and acid-washed coal in a drop-tube reactor to reveal the influence of metals in coal on secondary reaction of char and tar. They also found that the amount of soot produced by raw coal is significantly lower than that of acid-washed coal, indicating that mineral matter in coal have an effect on soot formation. However, the focus of Hayashi’s research was steam reforming and gasification of tar and char. The experimental temperature was only 800–900 °C, which is not conducive to the analysis of soot characteristics. The interactions between soot/tar and alkali/alkaline-earth metal has been found in studies on hydrocarbon flame[23,24] and diesel engine.[25] The formation of initial soot particles[26] and the ultrafine particles generated by gasified alkali metal[27] all occur in the boundary layer of pulverized coal. Prior studies have suggested that soot and tar in the coal combustion flame zone interact with inorganic elements.[28,29] Xiao et al.[30] studied the interaction between Na and soot in coal pyrolysis and combustion and analyzed the effect of this interaction on fine particle formation during the initial stage of coal combustion; they concluded that Na can reduce soot yield and change particles size distributions (PSDs) of soot. However limited by the aerosol analysis method, the chemical structure changes of soot and tar caused by Na are still unclear.

We previously studied the effects of different forms of Na (ion-exchangeable state and physical adsorption state) on the formation characteristics of soot during the pyrolysis of Yimin brown coal at 1250 °C, and found that the inhibition of ion-exchangeable Na on soot formation was much stronger than that of physically adsorbed Na.[31] Ion-exchangeable Na could also be released with volatiles in the form of organic bonding,[32] and these organic-bound Na could promote the combination of surrounding carbon atoms with oxygen-containing substances.[33] Therefore, it is meaningful to further study the role of ion-exchangeable Na in the sooting process of tar generated by different coals. Fourier transform infrared spectroscopy (FTIR) can attain the properties of aliphatic-, aromatic-, and graphite-like structures in carbon materials and is sensitive to polar bonds such as oxygen-containing functional groups. Using proper curve fitting to the FTIR spectrum can obtain useful tar/soot chemical structure information,[21] which is mutually confirmed and complementary with the Raman test results.[34]

Based on previous studies, this work carries out two series of pyrolysis comparison experiments between acid-washed coal and ion-exchangeable Na-loaded coal (Baokuang brown coal, abbreviated as BK and Zhundong brown coal, abbreviated as ZD) in a drop-tube reactor at different temperatures. The yields of soot and tar are obtained by dichloromethane extraction and weighing method, and the PSDs of aerosol in pyrolysis gas are measured by an electrical low-pressure impactor (ELPI). In addition, this work also uses FTIR and Raman spectroscopy to analyze the solid phase products of coal secondary pyrolysis (soot and tar) and to explore the effect of ion-exchangeable Na on chemical structure changes of soot and tar during coal pyrolysis.

Results and Discussion

Yields and Size Distribution of Soot and Tar during Coal Pyrolysis

The CHN elemental analysis of aerosol samples from coal pyrolysis given in Table shows that the carbon content of these aerosol are extremely high and the sum of CHN content are all over 85%. According to FTIR test results in Section , there are a lot of organic O (C–O and C=O) in soot and tar. The content of substances that are difficult to be removed by hydrochloric acid washing such as Si and Al is very low in soot generated by coal pyrolysis.[11,15,35,36] Further, ion-exchangeable Na in coal may also be present in tar and soot in the form of combining with the carboxyl group[32] or forming intercalation compound.[37] Therefore, the ash content in aerosol samples is very low, and the particles in aerosol samples are mainly soot and tar.

Table 1

CHN Element Analysis Results of Aerosol Samples in Pyrolysis Gases

sample	C (%)	H (%)	N (%)	CHN (%)
1100 °C BKH	91.86	0.834	0.286	92.98
1250 °C BKH	89.51	0.810	0.271	90.59
1400 °C BKH	87.42	0.785	0.282	88.49
1250 °C BKNa1	88.22	0.883	0.281	89.38
1100 °C BKNa2	86.98	0.842	0.272	88.09
1250 °C BKNa2	85.75	0.841	0.256	86.85
1400 °C BKNa2	84.17	0.808	0.265	85.24
1250 °C BKNa3	84.09	0.759	0.254	85.10
1100 °C ZDH	93.23	1.386	0.559	95.17
1250 °C ZDH	90.89	1.301	0.590	92.78
1400 °C ZDH	88.66	1.197	0.604	90.46
1250 °C ZDNa1	90.24	1.277	0.603	92.12
1250 °C ZDNa2	89.63	1.297	0.584	91.51
1100 °C ZDNa3	89.37	1.425	0.564	91.36
1250 °C ZDNa3	88.31	1.215	0.575	90.10

In this work, the dichloromethane extraction method was used to measure soot and tar yields (the proportion of them in the dry ash free base of coal, see Section for the specific method). Figure shows the yields of soot and tar from various coals under different temperatures. Brown coal have a lower coalification degree and higher oxygen content than bituminous coal, so there are less high-polymerization degree aromatic matter and more oxygen-containing substances in brown coal volatiles, which are not conducive to the formation of soot. Therefore, the yields of soot generated by brown coal in this work are much lower than that of bituminous coal (10–25% of daf coal).[11,12] However, they are similar to the soot yield of Saskatchewan brown coal (less than 5% daf coal) conducted by Zeng et al.[15] Because of low production, the yield of soot and tar generated by ZDNa3 cannot be obtained separately. It can be seen from Figure that although the volatile content is similar (the volatile content of coal is shown in Section ), the yields of the condensed volatile matter (soot + tar) from BK coal are much higher than that of ZD coal, indicating that BK coal volatiles have higher sooting propensities.

Figure 1

Soot and tar yields under different experimental conditions. (a) BKH and BKNa2 at different temperatures. (b) ZDH and ZDNa3 at different temperatures. (c) BK and ZD with different Na contents at 1250 °C.

According to the left parts of Figure a,b, the yields of tar and soot increase with the temperature rising in a 1100–1250 °C range, indicating that in this temperature range, the increase of the pyrolysis temperature promote the addition reaction of light hydrocarbon on soot/tar.[14,15,38] In 1250–1400 °C range, soot and tar yields decrease with temperature rising, this might be because high temperature promote reactions (gasification or oxidation) of tar and soot with oxygen-containing substances in pyrolysis gas.[13,39] The oxygen-containing substances include oxygen-containing molecules such as H2O and CO2 that can undergo reforming reactions with soot/tar and oxygen-containing radicals generated by homolysis of the O covalent bond under high temperature such as O and OH which have high reactivity and play an important role in the oxidation of soot.

Na content dependency of soot and tar yields of BK and ZD is shown in Figure c. Ion-exchangeable Na can significantly reduce the yields of soot and soot + tar, but the change of tar yield with the Na content increase of BK and ZD is different. With Na content in coal increasing, the tar yield of ZD continues to decrease, while the tar yield of BK slowly decreases at the beginning, but eventually increases slightly. Ion-exchangeable Na can affect both primary pyrolysis and secondary pyrolysis of coal, thereby influencing the final yields of soot and tar. Li[40] suggested that alkali metal and alkaline earth metal (AAEM) combined with coal macromolecular networks (organically bound, such as the ion-exchangeable Na in this work) can form cross-linking points to adsorb free radical fragments (precursors of volatiles) and increase their residence time in coal particles. Then, these fragments would crack or convert to char. In particular, Na presented in an organically bound form have a strong catalytic effect on tar cracking[41] and can greatly reduce the release of primary tar. Na released into pyrolysis gas with volatiles (sodium ion, sodium carboxylate, or sodium vapor) can further catalyze tar cracking. In addition, Na can also suppress the sooting process of coal tar, thereby reducing soot yield but increasing the tar yield of BK coal.

According to the right part of Figure a,b, in the low-temperature range (1100–1250 °C), Na decreases tar yield with temperature increase, and the increase of soot yield is also suppressed or even decreased (ZDNa3); in the high-temperature range (1250–1400 °C), Na greatly promotes the decrease of soot and tar yields with temperature increase, indicating that Na in pyrolysis gas has a strong catalytic effect on tar cracking. Na content of ZDNa3 is lower than that of BKNa2, but the soot + tar yield of ZD coal decreases more rapidly when the temperature rises, indicating that ZD coal tar is more susceptible to cracking under the catalysis of Na. BK coal tar has higher sooting propensities, and ZD coal tar is more prone to cracking reactions, this indicates that there should be some differences in the chemical structure of pyrolysis products from two kinds of coal.

The particles collected at all channels of the ELPI were pure black (soot color). In addition, as shown in Figure , the share of tar in the mixture of soot and tar is much smaller than soot, and there would be some tar attached to the surface of soot during the condensation process, which hardly affect the PSD measurement results of pyrolysis aerosol. Therefore, we considered that the particles at all channels of ELPI were mainly soot, which was also proved by the results of the CHN element analysis in Table . The number-size distributions of particles generated by two kinds of coals under different experimental conditions are shown in Figure . The number size distributions have a single peak between 0.1 and 0.2 μm, which move with temperature and Na content. The effect of temperature on PSDs is very obvious. In the low-temperature range, the increase of the pyrolysis temperature has little effect on the particle number-size distribution, but when the pyrolysis temperature is raised from 1250 to 1400 °C, the peak of the particle number-size distribution is obviously move to small particle size direction.

Figure 2

PSDs under different experimental conditions. The curves are vertically displaced in the Y-axis direction. (a) BKH and BKNa2 at different temperatures. (b) ZDH and ZDNa3 at different temperatures. (c) BK and ZD with different Na content.

Agglomeration processes that primary soot particles collide and agglomerate to form soot agglomerates are the main influencing factors of soot PSDs.[42,43] At high temperatures, there is more energy to break chemical bonds and produce more free radicals. The increase of the free radical concentration increases the pyrolysis gas ionization degree, thus changing the agglomeration of soot particles by the ionic mechanism (particle charge exchange and ion neutralization),[44] reducing the formation of big soot agglomerates, and making the peak of the particle number-size distribution move toward small particle size direction. As the content of Na in coal increases, the peak of the soot number concentration curve also moves toward small particle size direction. Because of its low ionization potential, Na can exhibit extensive ionization, increase the level of pyrolysis gas ionization, and reduce agglomeration between soot particles.[23,24]

Evolution of FTIR Spectra of Soot + Tar

FTIR spectra of the mixture of soot and tar form coal pyrolysis under different experimental conditions are displayed in Figure a–c. Spectra were baseline corrected, scaled to 1 mg/cm2, and shifted vertically for clarity. The change between 2300 and 2380 cm–1 is due to CO2 in a spectrometer and should be ignored. As it shown in Section and Figure , the main difference of IR spectra of soot + tar and coal is the absorption peak of minerals such as aluminosilicates[45] and crystal water,[46] which are rare in soot and tar. Therefore, a large amount of research (baseline correction, curve fitting analysis, assignations of absorption peaks, etc.)[47−50] conducted on the IR spectra of coal and char have greatly contributed to this research.

Figure 3

Infrared spectra of soot + tar samples under different experimental conditions and curve fittings of ZDNa3 at 1250 °C in different wavenumber bands as an example. (a) BK and ZD with different Na content at 1250 °C. (b) BKH and BKNa2 at different temperatures. (c) ZDH and ZDNa3 at different temperatures. (d) 2800–2990. (e) 950–1800. (f) 600–950 cm–1.

The absorption peaks in 2800–2990 cm–1 are derived from the stretching vibration of aliphatic hydrogen.[51] It can be seen from Figure a that as Na content in coal increase, the area of absorption peaks in this region decrease; in addition, the absorption peak width of BK coal is much shorter than that of ZD coal. The larger peak width of ZD coal indicates that its aliphatic structures are arranged in a variety of ways or have more connections with other functional groups.[52] The absorption peaks in 1620–1800 cm–1 are caused by a variety of C=O stretching vibrations.[50,51] These peaks are close to the vibration of the aromatic ring (near 1600 cm–1), only the acromion induced by the carboxylic C=O structure appears in some spectra. There are 3–4 absorption bands of the aromatic ring stretching vibration in the range of 1365–1620 cm–1,[51,53] and the absorption peak around 1600 cm–1 is the most obvious. Because of the extensive dehydrogenation condensation of aromatic rings in soot, the graphitization degree of soot is high, while the absorption peak intensity of high-graphitized aromatic ring vibration in the IR spectrum is weakened.[54] Therefore, although the soot + tar samples have a high aromatic ring content, the absorption bands of aromatic ring vibration and aromatic hydrogen out-of-plane deformation vibration (900–700 cm–1)[48,49,51] have a weaker intensity than coal. The absorption peaks near 1420 and 620 cm–1 originate from carboxylate stretching vibration and deformation vibration, respectively.[49,52] Because Na can replace H in the carboxyl group, these two absorption peaks (especially peak near 620 cm–1) are more obvious in the spectrum of aerosol from Na-loaded coals. The absorption peaks in the range of 1020–1300 cm–1 are derived from different C–O stretching vibrations,[49,51,53] and the most obvious two peaks near 1190 and 1120 cm–1 originate from stretching vibrations of phenolic and etheric C–O, respectively. These band intensities increase with increasing temperature and Na content indicating that more oxygen-containing radicals combine with soot and tar.

In order to separate the overlapping absorption peaks in IR spectra and obtain the detailed chemical structure information of soot + tar samples, curve fitting technology was performed on the IR spectrum of various experimental conditions. The positions of each absorbance peaks correspond to the minimum value of spectra second derivative,[55,56] and the function of the absorbance peak adopt a Gaussian + Lorentzian equation.[49,57] According to refs,[21,49,58] this work divides the IR spectrum of these samples into three parts of 2800–2990, 950–1800, and 600–950 cm–1 in light of functional group classification. Figure d–f shows the curve fitting results of ZDNa3 at 1250 °C as an example. By synthesizing the fitting results of all IR spectra, the fitting peak positions and assignments are summarized in Table .

Table 2

Fitted Peaks Appearing during Curve Fitting along with Assignments

no.	center (cm–1)	assignment
1	2951–2958	asymmetric stretching vibration of CH3
2	2918–2925	asymmetric stretching vibration of CH2 in alkanes
3	2890–2906	stretching vibration of CH in alkanes
4	2864–2879	symmetric stretching vibration of CH3
5	2839–2853	symmetric stretching vibration of CH2 in alkanes
6	1761–1787	stretching vibration of C=O in phenolic esters
7	1729–1742	stretching vibration of C=O in conjugated esters
8	1683–1716	stretching vibration of C=O in carboxylic acids
9	1630–1668	stretching vibration of highly conjugated C=O
10	1592–1603	stretching vibration of C=C in aromatic rings
11	1537–1568	stretching vibration of C=C in aromatic rings
12	1492–1525	stretching vibration of C=C in aromatic rings
13	1446–1465	asymmetric deformation vibration of CH3, CH2
14	1438–1440	stretching vibration of C=C in aromatic rings
15	1390–1422	symmetric stretching vibration of COO– in carboxylates
16	1374–1385	symmetric deformation vibration of CH3-aromic
17	1338–1368	symmetric deformation vibration of CH2–C=O
18	1300–1320	stretching vibration of C–OH in carboxylic acids
19	1275–1290	asymmetric stretching vibration of C–O–C in cyclic ethers
20	1239–1267	stretching vibration of C–OH in phenols
21	1219–1234	asymmetric stretching vibration of C–O–C in aromatic ethers
22	1178–1197	stretching vibration of C–OH in phenols
23	1141–1162	stretching vibration of C–O in phenols, ethers
24	1113–1124	stretching vibration of C–O in ethers
25	1065–1087	stretching vibration of C–O sec. alcohols
26	1030–1055	alkyl ethers
27	990–1025	in-plane deformation vibration of =C–H in aromatic structures
28	959–985	in-plane deformation vibration of =C–H in aromatic structures
29	945–951	out-of-plane deformation vibration of C–O–H in carboxylic acids
30	907–931	symmetric stretching vibration of P-(OH)2
31	880–905	out-of-plane deformation vibration of =C–H in aromatic ring (isolated aromatic hydrogens)
32	860–875	out-of-plane deformation vibration of =C–H in aromatic ring (isolated aromatic hydrogens)
33	830–845	out-of-plane deformation vibration of =C–H in aromatic ring (two adjacent hydrogens per ring)
34	775–800	out-of-plane deformation vibration of =C–H in aromatic ring (three adjacent hydrogens per ring)
35	750–765	out-of-plane deformation vibration of =C–H in aromatic ring (four adjacent hydrogens per ring)
36	718–730	rocking vibration of (CH2)n in alkanes, n ≥ 4
37	680–700	out-of-plane deformation vibration of =C–H in aromatic ring (five adjacent hydrogens per ring)
38	627–644	deformation vibration of COO– in carboxylates
39	616–621	twisting vibration of COO– in carboxylates

Solomon and Carangelo[48] proposed a method to calculate the corresponding hydrogen content in coal by dividing the peak area of aliphatic hydrogen and aromatic hydrogen in IR spectra by corresponding coefficients aal and aar (bituminous coal: aal = 746, aar = 686; sub-bituminous coal and brown coal: aal = 710, aar = 541). Because the degree of graphitization of soot and the dilution ratio of KBr pellet preparation are too high, these parameters are not suitable for this work. However, Solomon’s research indicates that if the fitting method is reliable, the absorption peak area of H should be positively correlated with H content in the sample. The relationship between absorption peak area of aliphatic + aromatic hydrogen and mass (mg) of H per gram of the sample is shown in Figure . There is a good linear correlation (r2 = 0.915) between them, which proves that the fitting method adopted in this work is effective.

Figure 4

H content dependence of absorption peak area of hydrogen and linear fit.

Based on the results of curve fitting, nine IR parameters of I1–I6 (I1 = Aaliphatic hydrogen/Atotal, I2 = Aaromatic hydrogen/Atotal, I3 = Aaromatic ring C=C/Atotal, I4 = AC–O/Atotal, I5 = AC=O/Atotal, and I6 = I4 + I5) and R1–R3 (R1 = I1/I2, R2= I2/I3, and R3 = Amethyl hydrogen/Amethylene hydrogen) are calculated to characterize the bonding structure of soot + tar samples with reference to previous studies.[49,59,60]

I1 can characterize the content of aliphatic compounds in the sample. As shown in Figure a, when the pyrolysis temperature is raised from 1100 to 1250 °C, the content of aliphatic compounds in soot + tar samples increase. Cain et al.[51] also found the counter-intuitive increase and they suggested that some aromatic persistent free radicals on the surface of soot can make the aliphatic compound populate on the particle surface to form a shell, thereby increasing the content of the aliphatic compound in the sample. As the temperature continues to increase (1250–1400 °C), the concentration of oxygen-containing radicals in pyrolysis gas increases, which decreases the content of aliphatic compounds by oxidation[14] and decreases I1. The increase of oxygen-containing radicals can also promote the ring rupturing reaction which reduces the content of aromatic hydrogen in the sample. However, the ring rupturing reaction reduces soot formation, which cannot explain the increase of soot yield at the low-temperature section (Figure ). Aromatic ring polymerization can also reduce the content of aromatic hydrogen in the sample. With temperature increasing, the polymerization reaction between aromatic rings become more intense,[61] that is a more reasonable explanation for the continuous decrease of the I2 value of the sample. R1 characterizes the relative content of aliphatic hydrogen and aromatic hydrogen in the sample. When the temperature is raised from 1100 to 1250 °C, R1 of both acid-washed coal and Na-loaded coal increase; when the temperature is raised from 1250 to 1400 °C, because aliphatic compounds are more susceptible to cracking, R1 of acid-washed coal decreases. However, R1 of BKNa2 increases continuously, indicating that Na in pyrolysis gas affected secondary pyrolysis reaction of volatile matter.

Figure 5

I1, I2, and R1 values of soot + tar samples under different experimental conditions. The histogram and the dot–line diagram in the graph are just used for clear display, and there is no difference between them (a) different temperature. (b) Different Na contents at 1250 °C.

As shown in Figure b, both I1 and I2 decrease with the increase of Na content in coal, indicating that Na reduces the content of aliphatic and aromatic substances in soot + tar samples. This means that Na has a catalytic cracking effect on those two substances during coal pyrolysis. With the increase of Na content in coal, R1 of soot + tar samples increase significantly, that is, the relative amount of the aliphatic compound in the condensed volatiles increase. Because the stability of aromatic compounds is higher than that of aliphatic compounds, the reason for this phenomenon is not that Na has a stronger catalytic cracking effect on aromatic compounds, but that Na can form a cross-linking point to keep the aromatic precursor of volatiles in coal during primary pyrolysis, thus reducing the aromaticity of volatile matter.[40] It can also be seen from Figure that the soot + tar samples generated by BK coal and ZD coal have similar aliphatic hydrogen content, but the samples of ZD coal have a much lower aromatic hydrogen content and a higher R1 than that of BK coal, indicating that there are more aliphatic structures (poor sooting propensity and easy to crack) in volatiles released by ZD coal, which is consistent with the previous conclusions on the analysis of soot and tar yield.

I3 can characterize aromatic ring C=C content in the sample; R2 can characterize the average number of hydrogen atoms on each aromatic ring, which is negatively correlated with the proportion of highly substituted aromatic rings in total aromatic compounds. Figure a shows that as the emperature increases, I3 of BK coal increases and I3 of ZD coal decreases. The content of aromatic ring in soot + tar samples is affected by ring rupturing and polymerization,[15] and also relates to the condensation cyclization of aliphatic compounds in volatiles.[62] Volatiles formed by BK coal have a lot of aromatics. Increase of temperature promotes aromatic ring polymerization reaction which increases I3 in the sample of BK coal. In addition, the proportion of highly substituted aromatic rings increases at the same time, resulting in the decrease of R2. The R2 of ZD coal also decreases with the increasing temperature for the same reason. However, a large part of aromatic substances in soot + tar samples of ZD coal are converted from aliphatic compounds, and aliphatic compounds cracking at high temperature indirectly reduces the aromatic ring content in the samples, resulting in the decrease of I3 in the sample of ZD coal. As shown in Figure b, the I3 value of pyrolysis products of two coals decrease with the increase of Na content in coal. Both the promotion of aromatic ring rupturing (cracking) reaction by Na and the inhibition of the aromatic ring polymerization reaction by Na can reduce the I3 value of the sample. Combined with the change of I2 value (aromatic hydrogen content) in Figure b, the effect of Na is dominated by the catalytic cracking reaction. However, R2 of BK coal increases first when Na content increases, indicating that Na in pyrolysis gas also can occupy the reaction position on the benzene ring, and suppress polymerization reaction of aromatic compounds.[63]

Figure 6

I3, R2, and R3 values of soot + tar samples under different experimental conditions. The histogram and the dot–line diagram in the graph are just used for clear display, and there is no difference between them (a) different temperature. (b) Different Na contents at 1250 °C.

The higher the R3 of soot + tar sample, the shorter is the average length of aliphatic chain in the sample. As it shown in Figure a, in the low-temperature section (1100–1250 °C), the temperature rise increases the simple hydrocarbon gas addition reaction and increases the average length of the aliphatic chain; in the high-temperature section (1250–1400 °C), higher temperature promotes the cracking reaction of aliphatic compounds, decreases the aliphatic chain length, and increases the relative content of methyl in the sample, which also causes obvious absorption peaks near 1460 cm–1 in IR spectra of 1400 °C. Because Na can catalyze the cracking reaction of aliphatic compounds, the average length of the aliphatic chain of the soot + tar sample generated by BK and ZD coal pyrolysis also decreases (R3 increases) with the increase of Na content, as shown in Figure b.

As it shown in Figure a, for acid-washed coal, at the low-temperature section, the addition reaction of light hydrocarbons gaseous species reduces the content of oxygen-containing functional groups in soot + tar samples. While at the high-temperature section, more oxygen-containing radicals in pyrolysis gas combine with soot and tar, resulting in a significant increase of the C–O structure (I4). After Na addition, the I4 value is maximum at 1100 °C and then decreased with the increasing temperature. However, Figure b shows that the increase of ion-exchangeable Na content in coal greatly increases the C–O structure content in the mixture of soot and tar, and also increases the content of oxygen-containing functional groups. Quyn et al.[32] found that there are quite a number of carboxylate functional groups such as formate, acetate, and oxalate in brown coal volatiles, indicating that AAEM in the form of carboxylate in coal may remain organic combination to release with volatiles during pyrolysis. However, when temperature increases, more AAEM are gasified by breaking the chemical bond with the organic structure. Na maintaining carboxylate form in volatiles can promote the combination of volatiles with oxygen-containing substances in the pyrolysis gas,[33,64] thereby increasing the content of C–O structures in soot and tar. However, with the increase of temperature, the combination between Na and organic structure is disconnected and Na enters pyrolysis gas in the form of steam. In the vapor state, Na competes with volatiles for oxygen-containing radicals, thereby reducing the contact between oxygen-containing radicals and soot/tar, and reducing the I4 of sample. The decrease in the concentration of oxygen-containing radicals weakens the catalytic cracking effect on aliphatic substances, which results in the increase in R1 of the BKNa2 sample in the high-temperature section with temperature increasing, as shown in Figure a.

Figure 7

I4, I5, and I6 values of soot + tar samples under different experimental conditions.

The influence of temperature and Na content on I5 shows no obvious rule. This is related to that the IR parameters In calculated in this work are proportion in total area of IR spectra, which leads to that I5 value was also affected by the change of the absorption peak areas of C–O structure (I4 value). Perry[65] used 13CNMR to analyze the volatiles and found that the C=O structure content is negatively correlated with the aromatic carbon in cluster. He suggested that the ring rupture reaction causes this phenomenon.

Raman Spectra of Pyrolysis Aerosol

The first-order Raman spectra of carbon materials ranges from 800 to 2000 cm–1. The first-order Raman spectrum of soot produced by BK and ZD coal under different experimental conditions are shown in Figure a,b, respectively. As can be seen from the figure, the Raman spectrum of soot samples are similar in shape, with two distinct peaks at approximately 1350 cm–1 (D1 band) and 1585 cm–1 (G band). G-band is related to the lattice vibration of the ideal graphite crystal.[66,67] The D1 band is related to the carbon atoms at the edge of graphite layer, which is the most obvious band in “defect” bands.[66,68] Previous researchers have used the peak height (amplitude) of G and D1 bands to characterize the properties of carbon materials.[69] However, as shown in Figure a,b, this method is not applicable to soot’s first-order Raman spectrum which has other overlapping bands in it. According to the method (ZDNa3 at 1250 °C, as an example) illustrated in Figure c, the first-order Raman spectrum of soot are divided into five bands, among which the D3 band adopts the Gaussian line shape, and the other bands adopt the Lorentz line shapes.[66] The D2 band is located near 1620 cm–1. Most researchers believe that this band is related to the lattice vibration similar to G band, but originates from the surface graphite layer of graphite crystals.[70] The D3 band (near 1590 cm–1) is derived from amorphous carbon, and its presence means that the measured carbon material is highly disordered.[71] The origin of the D4 band (near 1225 cm–1) is inconclusive, and some researchers believe that it should be classified as a polyene-like structure.[72]

Figure 8

Raman spectra of soot + tar samples under different experimental conditions and spectrum decomposition example: (a) soot + tar samples of BK coal under different experimental conditions, (b) soot + tar samples of ZD coal under different experimental conditions, and (c) spectrum decomposition of ZDNa3 at 1250 °C (a) BK. (b) ZD. (c) 1250 °C, ZDNa3.

La, Rb, and Rc are parameters calculated from Raman spectral decomposition results of each soot sample, and are used to characterize the properties changes of the two kinds of coal-derived soots as the experimental conditions change. La is the size of the graphite crystal in a direction parallel to the graphite layer. Tuinstra and Koenig[69] proposed the formula: La = El/(ID1/IG), and pointed out that when the laser wavelength is 514.5 nm, El = 4.4 (I is the corresponding band intensity). The research of Cançado and his collaborators[73] have shown that the value of El is proportional to the fourth power of laser wavelength. Therefore, according to the wavelength of laser used in this work (532.15 nm), the formula is obtained: La = 5/(AD1/AG). In the formula, the “intensity, I” is represented by the band area A. The formula of Rb is Rb = AD1/(AD1 + AG + AD2), which is often used to characterize the degree of organization of carbon materials.[66,74,75] The parameter Rc = AD3/AG is related to the share of amorphous carbon in carbon material.[76]

The left part of Figure a–c shows the change of three Raman parameters with temperature. The heat treatment experiments of carbon black (appellation of soot-like carbon materials in the industry) in argon of Pawlyta et al.[75] show that as the temperature increases, the La value of carbon black increases, while the Rb and Rc values decrease. Although the coal pyrolysis experiments in this work were performed under a N2 atmosphere because of the presence of oxygen-containing substances in coal pyrolysis gas, the variation of the Raman parameters of soot is not consistent with the experimental conclusions of Pawlyta et al. In the low-temperature section, the increase of the temperature promotes the addition reaction of simple hydrocarbons and the polymerization of aromatic hydrocarbons, which increases the La value of the graphite layer and decreases Rb and Rc values; while in the high-temperature section, the increase of temperature promotes the reaction between soot and oxygen-containing substances, and inhibits the growth of the graphite layer.[77] It can be seen from Section that in the high-temperature section, the content of the C–O structure increases with the increase of temperature, which leads to the increase of defects and disorder degree in soot, thus reducing the values of Rb and Rc.

Figure 9

La, Rb and Rc values of soot + tar samples under different experimental conditions (left: temperature dependence, right: Na content dependence) (a) La = 5/(AD1/AG). (b) Rb = AD1/(AD1 + AG + AD2). (c) Rc = AD3/AG.

The four coals in the left figures of Figure a–c can be roughly arranged in the order of La from large to small, Rb and Rc from small to large: BKH, ZDH, BKNa2, and ZDNa3. This shows that BK coal-derived soot has a higher degree of graphitization, and the loading of Na inhibits the graphitization of soot. The right parts of Figure a–c shows the relationship between three Raman parameters of the two series coal-derived soot and the Na content in coal. According to Figure a, the La value of soot decreases with the increase of Na content in coal. It can be known from Section that Na can catalyze the cracking of tar macromolecules (long-chain aliphatic substances and polycyclic aromatic substances) and can also suppress the polymerization of aromatic compounds, thereby reducing the graphite crystallite size. Na combined with the carboxyl group in soot can promote the binding of their surrounding carbon atoms with O (Figure ), which also can inhibit the growth of graphite crystallites.[78] Na and O as heteroatoms can destroy the structural symmetry and cause the increase of D1 intensity,[79,80] which is an inducement of Rb increasing with the increase of Na content in coal (right part of Figure b). In addition, some Na will enter between the carbon layer to form intercalation compounds during the formation of soot, which can reduce the degree of structure organization of soot and increase the Rb value.[81] Therefore, although the right part of Figure a shows that the content of the C–O structure in the BKNa2 sample decreases with the increase of temperature because of the increase of Na gasification caused by high temperature, the Rb value of BKNa2 soot increases with the increase of temperature in the high-temperature section (the left side of Figure b). The right part of Figure c shows that the Rc value of BK and ZD coal-derived soot increases with Na content increasing, indicating that the content of organic matter such as aliphatic side chains and functional groups (amorphous carbon) in soot increases.[71,76] The increase of these structure contents means that the relative content of aromatic ring decreases, which is consistent with the decrease of the I3 value with the increase of Na content in Figure b.

Conclusions

Acid-washed coal and ion-exchangeable Na-loaded coal with different contents of Na were pyrolyzed in a drop-tube reactor at 1100, 1250, and 1400 °C. Soot and tar yields were obtained by collecting aerosol samples and separating; the number-size distribution of aerosol was measured by ELPI and the chemical structure of the soot + tar samples was analyzed using an FTIR spectrometer and Raman spectroscopy. Compared with acid-washed coal, ion-exchangeable Na-loaded coal has obvious changes in soot and tar physical and chemical properties:

Ion-exchangeable Na can reduce tar formation during primary pyrolysis and catalyze tar cracking during secondary pyrolysis. Therefore, under different pyrolysis temperatures, the loading of ion-exchangeable Na can significantly reduce the yield of soot and soot + tar. The slight increase in tar yield of BKNa3 indicates that Na can also inhibit the conversion of tar to soot.

High temperature and gasified Na can improve the ionization degree of pyrolysis gas, then reduce the agglomeration between soot particles, so the single peak (0.1–0.2 μm) of particle number-size distribution moves to the small particle size direction.

Na can catalyze the cracking reaction of aliphatic compounds, thereby reducing the average length of aliphatic chain. Therefore, as Na content in coal increases, the IR parameter value I1 of soot + tar decreases and R3 increases. Na can promote aromatic ring rupturing reaction and inhibit aromatic ring polymerization reaction, that reduces I3. I2 decreases with Na content rising which indicates that the role of Na is mainly catalytic cracking.

Some of ion-exchangeable Na exist in organically bound in volatiles, which promote the combination of soot and tar with oxygen-containing substances, so the C–O structure content and I4 value increase with ion-exchangeable Na content increase. However, when the temperature reaches 1400 °C, Na breaks the bond with the organic structure, enters pyrolysis gas in steam form and competes with volatiles for oxygen-containing radicals.

In the low-temperature section, temperature rise promotes the formation and graphitization of soot, which increases La and decreases Rb and Rc; in the high-temperature section, temperature rise promotes the reaction of soot with oxygen-containing substance and vaporizes more Na to enter in soot, which decreases La and increases Rb and Rc.

The inhibitory effect of Na on soot formation makes the graphite crystallites smaller. Na combined with carboxyl group and Na entering between the carbon layer of graphite crystallite will reduce the degree of organization of soot. In addition, Na also increases the content of amorphous carbon in soot.

Materials and Methods

Coal Properties

The coal samples used in this study were brown coal from the Baorixile mining area, Inner Mongolia, China (BK) and brown coal from the eastern mining area of the Junggar Basin, Xinjiang, China (ZD). Large coal blocks were ground into pulverized coal using a rod mill. A 115 mesh sieve was used to remove particles larger than 125 μm in pulverized coal, and then a 400 mesh sieve was used to screen pulverized coal several times (no more than 50 g coal each time) to obtain the raw pulverized coal required for the experiment (38–125 μm).

Hydrochloric acid wash can effectively remove ion-exchangeable metal, acid/water-soluble salts in coal and has little influence on the coal organic structure. In this work, 50 g of raw pulverized coal was added in 800 mL of hydrochloric acid (5 mol/L). The mixture of acid and pulverized coal was sealed and heated in a 60 °C water bath for 5 h. During this period, the mixture was continuously stirred to make acid fully contact with pulverized coal. Next, the acid solution was filtered off. The remaining pulverized coal was repeatedly washed by deionized water and then dried to obtain HCl-washed pulverized coal (abbreviated as BKH and ZDH hereafter). Proximate analysis of pulverized coal was conducted in accordance with GB/T212-2008, while ultimate analysis of pulverized coal was performed using a CHN elemental analyzer and infrared sulfur meter. Table summarizes the proximate and ultimate analysis of raw coals and HCl-washed coals. Because the pulverized coal has been fully dried, the data in Table were based on dry-basis.

Table 3

Proximate and Ultimate Analysis of the Coals

	proximate analysisd/%			ultimate analysisd/%
coal	Vda	FCdb	Adc	Cd	Hd	Od	Nd	Sd
BK (raw)	31.24	59.12	9.64	68.99	4.23	16.12	0.90	0.13
BKH	32.78	62.27	4.95	72.74	4.27	17.14	0.77	0.13
ZD (raw)	30.19	65.94	3.87	79.43	4.08	11.57	0.84	0.20
ZDH	32.19	66.74	1.07	79.98	3.92	14.26	0.82	0.16

Vd means volatile, dry-basis.

FCd means fixed carbon, dry-basis.

Ad means ash, dry-basis.

FTIR spectra of ZD before and after acid washing are shown in Figure a. As shown in Figure a, the organic structure of the coal did not change significantly after acid washing. The most obvious change was the increase in the intensity of two peaks labeled (1) and (2) in the figure, which were caused by the stretching vibration of C–O in ether or phenols and C=O in carboxylic acid.[53] As it shown in Table , the oxygen content in ZD coal after acid washing increased, which was consistent with the results in Figure a. Metal in carboxylate was converted to carboxylic acid by H+ displacement during the acid washing process, which resulted in the enhancement of the absorption peak of C=O in carboxylic acid. However, HCl wash did change the content of aliphatic hydrogen and aromatic hydrogen in coal. The absorption peak labeled (3) in Figure a of ZDH, which was caused by CH3-aromatic deformation vibration,[82] was weaker than that of ZD, indicating that acid washing could remove aliphatic branched chains from aromatic compounds in coal. This effect also leaded to the attenuation of the bands in the aliphatic C–H stretching vibration zone and the enhancement of the aromatic C–H out-of-plane deformation vibration zone after acid washing,[51,52] as it shown in Figure a.

Figure 10

Effect of acid washing and Na loading on the properties of ZD and BK: (a) FTIR spectra of ZD, ZDH, and ZDNa3 as an example; (b) content changes of main metal elements in BK before and after acid washing (left) and Na contents of BKH and Na-loaded BKH (right); (c) content changes of main metal elements in ZD before and after acid washing (left) and Na contents of ZDH and Na loaded ZDH (right). (a) FTIR spectra of ZD, ZDH, and ZDNa3. (b) BK. (c) ZD.

Sodium in coal is mainly divided into organically bound Na (i.e., ion-exchangeable Na, mainly carboxylates) and soluble salts (mainly NaCl). Among them, organic bound Na is more prone to gasification and plays a more significant role in coal pyrolysis, which is the research goal of this work. In order to combine Na with the organic structure of coal, such as carboxy and phenols, the loading of Na in coal was realized by ion exchange. Sodium acetate solution with weak alkalinity was selected to provide sodium cations. Fifty gram of HCl-washed coal was added in 500 mL of sodium acetate solution which includes three concentrations of 0.05, 0.1, and 0.2 mol/L. The mixture of sodium acetate solution and HCl-washed coal was sealed and heated in a 60 °C water bath for 18 h. During this period, the mixture was continuously stirred to make solution fully contact with pulverized coal. The Na+ in solution would combine with the carboxyl group or phenols in HCl-washed coal, which completed the loading of ion-exchangeable Na. Next, the solution was filtered off and the remaining pulverized coal was dried to obtain Na-loaded coals (abbreviated as BKNa1, 2, 3 and ZDNa1, 2, 3 hereafter). As shown in Figure a, the formation of carboxylate Na and phenol Na reduced absorption peak intensity of C=O in carboxylic acid and C–OH in phenols in ZDNa3 FTIR spectra, while the intensity of mineral absorption band increased similar to that of raw coal.

Inductively coupled plasma optical emission spectroscopy (ICP–OES) was used to measure the change of main metal contents in coal samples before and after acid washing and the load of Na in the two series of coal. For ICP–OES testing, coal samples needed to be microwave digested: a certain amount of coal (not less than 50 mg) was mixed with an appropriate amount of HNO3, HF, and H2O2 (same amount each time) in a polytetrafluoroethylene crucible; the crucible was sent to a microwave digestion apparatus, and then the temperature was raised to 200 °C. and kept for 1.5 h to completely digest the coal; finally, the digestion solution was diluted to 25 mL by deionized water in a volumetric flask. ICP–OES analysis results of each coal samples are shown in Figure b,c. Except for Na, the content of other elements in acid-washed coal loaded with ion-exchangeable Na was very low and similar to that of acid-washed coal (Table S1 of the Supporting Information). The difference of measurement results (K, Ca, Mg, and Fe content) came from the slight inhomogeneity of pulverized coal and the instrument error. The content of K, Ca, Mg, and Fe in acid-washed coal with different Na content was low and similar, so it can be concluded that the change in the properties of soot and tar came from the ion-exchangeable Na.

Experimental Procedure

The schematic of the drop-tube reactor experimental system is shown in Figure . The experimental system was mainly divided into three parts: feeding system, drop-tube reactor, and sampling system. Pulverized coal particles were fed in the drop-tube reactor using an injection spiral stepping microfeeder at a rate of about 100 mg/min. There was a vibrator on the feeding device to depolymerize the pulverized coal through vibration to make the feeding uniform. In order to prevent the pulverized coal from pyrolysis before entering the furnace, an air cooling device was used to control the temperature of the feed tube. The N2 fed into the furnace was divided into carrier gas and secondary gas. The carrier gas (1 L/min) carried coal particles into the furnace and the secondary gas provided the inert atmosphere. The total gas flow rate was 10 L/min.

Figure 11

Schematic of the experimental system.

The electrically heated section length of the drop-tube reactor was 2000 mm and the internal diameter of center corundum tube was 80 mm. Four groups of U-type silicon-molybdenum heating bars (three bars as a group) were used to heat the corundum tube in four sections, and platinum-rhodium S-type thermocouples were arranged in each heating section to measure and control furnace temperature. Because the temperature in the flame zone of the pulverized coal combustion in a pulverized coal boiler was mostly between 1000 and 1400 °C,[83] and referring to the temperature selected by other researchers, the experimental temperatures of pulverized coal pyrolysis in this work were 1100, 1250, and 1400 °C. At the beginning of the experiment, a calibrated secondary thermocouple was inserted from the bottom of the furnace to correct the error of four platinum rhodium thermocouples under three temperature conditions.

The samples (mixture of soot and tar) were collected from the bottom of the furnace by a 304 stainless steel sampling probe. Because the working temperature of the sampling probe was far higher than the ablation temperature of 304 stainless steel (800 °C), oil cooling device was arranged in the sampling probe for protection. In addition, the oil cooling device could also reduce the temperature inside the sampling tube to further prevent soot denaturation. The sampling gun was inserted into the furnace to a depth of 200 mm, which was located approximately at the thermocouple of the lowermost heating section. The temperature of sampling site did not change during sampling.

Sample Analytical Methods

The pyrolysis gas extracted from the furnace was first passed through a 10 μm cyclone to separate char particles. The inlet flow rate of the cyclone was maintained at 10 L/min during the experiment. As shown in Figure , there were two sampling systems (blue line for ELPI and green line for sampler) for samples in pyrolysis gas. The PSDs of the particulate matter in pyrolysis gas were measured online by ELPI. Because of the limitation of measuring interval of ELPI, at the entrance of the sampling probe, cold-quenched N2 was introduced and pyrolysis gas also needed to be passed through a dilutor and diluted by filtered air before being entered into ELPI. The total dilution ratio was 50. ELPI could divide particles into 12 channels in an aerodynamic diameter range from 30 nm to 10 μm, and measure the particle concentration of each channel.

Before being entered into a large flow sampler, pyrolysis gas was first mixed with clean air which was cooled by the ice water mixture to condense tar. The mixture of soot and tar were then collected by a glass fiber filter in the sampler. The soot and tar mixture needed to be separated to quantify their respective weights. Samples of known weight were added to dichloromethane. Then, the mixture was subjected to ultrasonic vibration for 40 min. The tar in the sample would dissolve in dichloromethane. Soot and tar were separated by passing the solution through a known weight filter. After drying, the filter was weighed again and the original weight of the filter was subtracted to obtain the weight of soot. The total weight of the sample minus the weight of soot was the weight of tar. By combining the soot or tar weights in the sample (msoot/tar) with the sampling time (t), the feed rate (δ), and the ash content in coal (Ad), the soot/tar yields based on dry ash free basis of coal, Y, could be calculated

FTIR was used to analyze the chemical structure of the mixture of soot and tar. A FTIR spectrometer (Nicolet model 5700) was manufactured by Thermo Company of the United States. Spectra were obtained by co-adding 32 scans at a resolution of 4 cm–1. Samples for FTIR testing were prepared by a KBr pellet technique. The mixture of soot and tar was mixed with KBr at a ratio of 1:2000 and the mixture sample was ground in an agate mortar for more than 20 min to ensure uniform mixing. A Hundred and fourty milligram sample was pressed under 10 MPa pressure for 2 min to prepare a 13 mm diameter transparent pellet. At least two pellets (two similar FTIR spectra or the average of three spectra) were prepared for each mixture of soot and tar to make the final results representative. The same sample preparation method was used for the FTIR test of coal, except that the mixing ratio with KBr was 1:400.

A Raman microspectrometer (Horiba Jobin-Yvon LabRAM HR800) was used to get the Raman spectrum of the mixture of soot and tar. The Raman spectrum (600–3500 cm–1 range) spectrum was measured by an argon laser (λ = 532.15 nm) and a ×50 magnification objective. The exposure time of the sample is 12 s and at least 3 spectra were measured for each sample. The laser spot diameter of a Raman microprobe on the sample was about 1000 nm, which was much larger than the diameter of the basic soot particles (30–50 nm). Therefore, a measured Raman spectrum was derived from a large number of basic carbon black particles and had good repeatability.