Methane productions from coal pretreated by 1.5 M NaOH for 0 h, 4 h, 8 h, 12 h, and 16 h.

Methane productions from coal pretreated by 1.5 M NaOH for 0 h, 4 h, 8 h, 12 h, and 16 h.</p

The structural parameters of raw coal and coal samples treated with 1.5 M NaOH for 12 h revealed by XRD analysis.

The structural parameters of raw coal and coal samples treated with 1.5 M NaOH for 12 h revealed by XRD analysis.</p

The FTIR spectrograms and curve-fitting peaks of coal samples untreated and treated with 1.5 M NaOH for 12 h.

(a) Original FTIR spectrograms; (b) Curve-fitting peaks of aromatic functional groups; (c) Curve-fitting peaks of oxygen-containing functional groups; (d) Curve-fitting peaks of aliphatic functional groups.</p

XRD diffraction patterns of raw coal and coal treated with 1.5 M NaOH for 12 h.

XRD diffraction patterns of raw coal and coal treated with 1.5 M NaOH for 12 h.</p

The organic composition of filtrate after coal treatment with 1.5 M NaOH for 12 h revealed by GC-MS analysis.

The organic composition of filtrate after coal treatment with 1.5 M NaOH for 12 h revealed by GC-MS analysis.</p

Schematic mechanism of NaOH pretreatment of coal to enhance biomethane production modified from Liu et al (2015) [46].

Schematic mechanism of NaOH pretreatment of coal to enhance biomethane production modified from Liu et al (2015) [46].</p

Methane productions from coal pretreated by 0.1 M, 0.5 M, and 1.5 M NaOH.

Methane productions from coal pretreated by 0.1 M, 0.5 M, and 1.5 M NaOH.</p