2.2.2. FTIR Structural Parameters Analysis

FTIR structural parameters could be obtained by the area of peak with peak fitting [14,15].

(1) Ratio of hydrogen to carbon H/C.

$$\frac{\text{pH}}{\text{C}} = \frac{\text{H}\_{\text{ad}}}{\frac{\text{C}\_{\text{ad}}}{12}} \tag{1}$$

(2) The aromatic carbon ratio far-F: On the premise of ignoring carbonyl carbon, assuming that coal only contains aromatic carbon and aliphatic carbon, the formula for calculating aromatic carbon rate is as follows:

$$\frac{H\_{al}}{H} = \frac{\text{A}(3000-2800)\text{cm}^{-1}}{\text{A}(3000-2800)\text{cm}^{-1} + \text{A}(900-700)\text{cm}^{-1}}\tag{2}$$

$$f\_{dr-F} = 1 - \frac{\mathcal{C}\_{\text{al}}}{\mathcal{C}} = 1 - \frac{\frac{H\_{\text{al}}}{H} \times \frac{H}{\mathcal{C}}}{\frac{H\_{\text{al}}}{\mathcal{C}\_{\text{al}}}} \tag{3}$$

where aromatic hydrogen ratio Har/H: Cal/C is the ratio of aliphatic carbons to the total number of carbons, H/C represents the ratio of hydrogen to carbon atoms, Hal/Cal is 1.8 for all coal samples, and represents the atomic ratio between hydrogen and carbon in aliphatic groups.

(3) Fat carbon ratio *fal-F*:

$$f\_{al-F} = 100 - f\_{ar-F} \tag{4}$$

(4) Lipid chain length and branching degree of coal *I*1: According to the ratio between CH2 and CH3, namely the area ratio of *A*(CH2)/*A*(CH3), the aliphatic group length and the branched chain degree were calculated to determine the aliphatic structural parameters. The intensity ratio of CH2/CH3 was determined by Equation (5):

$$I\_1 = \frac{\text{CH}\_2}{\text{CH}\_3} = \frac{\text{A}\_{2852 \text{cm}^{-1}} + \text{A}\_{2924 \text{cm}^{-1}}}{\text{A}\_{2957 \text{cm}^{-1}}} \tag{5}$$

(5) Alkane branching degree:

$$
\delta\_F > \frac{R\_3CH}{A\_{(3000-2800 \text{cm}^{-1})}} \tag{6}
$$

Compared with the alkane branching degree of the 3 coal samples from Table 6, the δ<sup>F</sup> of HLH without modification is 47.68% which indicate there were much tertiary carbon and quaternary carbon in HLH raw coal. There were many branching structures in HLH raw coal. After microwave modification, the main structure of HLH is methylene carbon with long straight chain.

**Table 6.** FTIR structural parameters of HLH before and after modification.

