*2.1. Modification of Lignite*

HLH was low degree of coalification was selected as the experimental sample. HLH sample was ground to 109–180 μm and dried in a vacuum drying chamber at 40 ◦C for 24 h. Take HLH sample in a crucible, added 100 mL distilled water and blended fully. The crucible contained HLH sample was placed in the ultrasonic oscillator to water bath oscillation for 60 s. The crucible was placed in the drying oven for drying treatment with 85 ◦C for 4 h. The crucible containing the HLH sample was placed in the a quartz reaction tube of the microwave reactor, the modification parameters set as 200 W and 60 s, and the microwave activated. Industrial analysis and elemental analysis of lignite samples before and after treatment were carried out and the results are shown in Table 1.

**Table 1.** Proximate and ultimate analyses of HLH lignite.


Note: ad: air-dry basis; daf: dry-and-ash-free basis. M: moisture; A: ash; FC is fixed carbon; V: volatile matter content.
