The thermodynamic effects were researched in hydrate formation experiments for low-concentration coal mine methane gas (CMM , the mole fraction is 30% CH4, 65% N2, and 5% O2) with five kinds of quaternary ammonium salts (QASs) including tetramethyl ammonium fluoride (TMAF), tetramethyl ammonium chloride (TMAC), tetramethyl ammonium bromide (TMAB) and tetrabutyl ammonium fluoride (TBAF), tetrabutyl ammonium chloride (TBAC). The molar concentration of each quaternary ammonium salt was 0.17%, 0.29%, 0.62%, 1.38%, and 3.5%. Under the conditions of phase equilibrium temperature from 282.3 to 304.5 K. Phase equilibrium pressure changed from 0.919 to 8.435 MPa, an isochoric pressure-search method was used to obtain the phase equilibrium data. Comparing the thermodynamic promoting effects of these five kinds of QASs from three aspects: concentration, species of halogen ion and species of alkyl in quaternary ammonium salts. The results showed that these QASs can improve the thermodynamic condition of CMM hydrate formation, the order of the best promoting effects about the three aspects was 3.5% TBAF>0.62% TMAF>3.5% TBAC>1.38% TMAB>1.38% TMAC, fluorine ion quaternary ammonium salts>chloride quaternary ammonium salts or bromide ion quaternary ammonium salts and tetrabutyl quaternary ammonium salts>tetramethyl quaternary ammonium salts.
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