2.2.1. Carbon Atom Analysis
The different forms of carbon in coal and their relative content can be effectively determined by
13CNMR technology.
Figure 2 shows the
13CNMR spectrum of coking coal. The
13CNMR spectrum of coal samples includes three parts: the aliphatic carbon region from 0 to 75 ppm, the aromatic carbon region from 100 to 165 ppm, and the carbonyl carbon region from 165 to 230 ppm.
For the nuclear magnetic resonance (NMR) carbon spectrum of coking coal, data processing was performed by peak fitting to determine the carbon assignment and peak area, as shown in
Table 2. On this basis, the 12 main structural parameters of coking coal were calculated and determined. The specific results are listed in
Table 3.
The ratio of aromatic bridging carbon to peripheral carbon (i.e., the bridging-to-peripheral ratio)
XBP is an important parameter for the molecular structure of coal.
XBP is calculated based on the formula in the table:
From the distribution of different types of carbon atoms in the coal molecular carbon skeleton, it can be seen that in aliphatic carbon atoms, the content of methylene or methyl groups is much higher than that of methyl or methoxy groups. The faH is above 44%, indicating that the large molecular structure of coal is almost entirely composed of protonated aromatic carbon. In this study, the XBP of coking coal is 0.24.
2.2.3. Comprehensive Analysis
A comprehensive analysis was carried out on the aromatic structure, aliphatic carbon structure, and heteroatom structure of coal in combination with the above analyses.
- (1)
Aromatic structure
Aromatic macromolecules are the main structures forming the backbone of coal molecules. The higher the proportion of aromatic macromolecules to cycloalkanes, the higher the degree of condensation.
Table 4 shows the forms of aromatic structures in coking coal molecules. For aromatic structures, the
XBP of benzene, naphthalene, anthracene or phenanthrene, dibenzofuran, and dibenzothiophene is 0, 0.25, 0.4, 0.5, and 0.57, respectively. The
XBP of coking coal is 0.24, which is between benzene and naphthalene and closer to naphthalene, indicating that there is a structure with two bridging carbons, with benzene and naphthalene being the main components of the original coal. The types and quantities of aromatic structures are listed in
Table 4. Actual coal molecular compositions can form aromatic structures with an
XBP of 0.25 when connected to benzene rings, i.e., aromatic structures with the same bridging carbon perimeter ratio as naphthalene.
The mass fraction of C in coking coal is 89.38%. The NMR data analysis reveals that the ratio of aromatic bridging carbon to peripheral carbon in coking coal XBP is 0.24, indicating that the ratio of bridging carbons to peripheral carbons in aromatic compounds with degrees of condensation 1 and 2 is 0 and 0.25, respectively. From this, it can be inferred that the molecular model of coking coal is primarily composed of naphthalene, followed by benzene rings. The bridging-to-peripheral ratio is obtained through the combination calculation of different aromatic structural units. Moreover, the types and quantities of aromatic structural units are determined. At this point, the total number of aromatic ring carbons in the model is 246. Additionally, based on the results of 13CNMR, the proportion of aromatic carbons in coking coal reaches 65.98%, estimating a total of 373 carbon elements in the coal molecule.
- (2)
Aliphatic carbon structure
Ethyl side chains, methyl groups, and cycloalkanes are the primary forms of aliphatic structures in coal. The coal quality is positively correlated with the carbon–hydrogen ratio; the higher the coal quality, the higher the carbon–hydrogen ratio, while the quantities of cycloalkanes and aliphatic side chains decrease accordingly. Aliphatic structures tend to exist in the form of cycloalkanes. With a carbon mass fraction of 89.38%, the average number of atoms in alkyl side chains ranges from 1 to 2, indicating that alkyl side chains in the coal structure should not be too long, with short chains being predominant. For coking coal, falH = 21.29% and fal* = 7.93%, indicating that the relative percentage of methylene, methine, and quaternary carbons is greater than the relative percentage of methyl groups. Since the aromaticity of the coal is 65.98% and the number of aromatic atoms is 246, the number of aliphatic carbon atoms is tentatively fixed at 129.
- (3)
Heteroatom structures
The heteroatom structures in coal are mainly composed of nitrogen and sulfur atoms. Nitrogen atoms exist in the form of pyrrole or pyridine, while sulfur atoms exist in the form of thiols, thioethers, or aromatic compounds.
Table 5 shows the forms of aromatic structures in coking coal molecules. The number of carbon atoms in coking coal molecules is 373. The coal sample contains a certain amount of nitrogen, while the sulfur content is generally very low. Based on the atomic ratios of each element to carbon, the ratio of nitrogen atoms to oxygen atoms to sulfur atoms in coking coal is 8:11:2.
The specific forms and distribution of aromatic carbon, aliphatic carbon, and heteroatoms in coal are determined by calculation and analysis. Based on this and combined with the results of elemental analysis, an initial structural model of the coking coal macromolecule was built with the Chemdraw 19.0. The molecular formula of coking coal is determined to be C
352H
320N
8O
11S
2, as shown in
Figure 4.