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The chemical characteristics of lignin isolated from industrial black liquor were identified by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and two-dimensional (2D) heteronuclear singlequantum coherence (HSQC) nuclear magnetic resonance (NMR), concerning its average molecular weight, distribution of typical interunit linkages, and functional groups. The frequency of β−O−4 linkage was determined to be 17−28/100 C 9 units by 2D NMR, while the content of unit [guaiacol (G), syringol (S), and p-hydroxyphenyl (H)] presents a ratio of 7:2:1.5 for G/S/H. The mass-average molecular weight of lignin was characterized to be 2238 g/mol by GPC analysis. The low polymerization degree of the units in lignin leads to the substantial extent of interunit linkage cleavage at low temperatures. The guaiacol-, syringol-, and phenol-type compounds from fast pyrolysis of lignin in a pyroprobe at 500 °C were notably identified by gas chromatography−mass spectrometry (GC−MS) and presented a ratio of the peak area as 7:2:1. More fragments were observed at higher temperatures from pyrolysis (Py)−GC−MS analysis, because of the commencement of demethoxylation and cracking of side chains. The scheme concerning the cleavage of characterized interunit linkages in lignin was proposed to produce the free radicals. The side chains on the free radicals were preferably to crack on β-site bonds to produce a number of methyl phenolic compounds. The methoxyl group was intensively cracked with the increased temperature because of its high bond dissociation energy (BDE), giving rise to the notable increase of cresol-, phenol-, and catechol-type compounds under high temperatures.

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Freezing and thawing cycles occur with cyclic liquid nitrogen (LN 2 ) injection in coal. The freeze−thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN 2 . We classified freeze−thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN 2 freeze−thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN 2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN 2 freezing time, cyclic freeze−thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze−thaw treatment. It was also found that the number of freeze−thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze−thaw cycles can result in effective fracturing of coal.

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