Synergistic Effect of the Moisture Content and pH Value of Aqueous Solutions on Oxidation Characteristics of Coal

In order to study the synergistic effect of the moisture content and pH value of aqueous solutions on coal oxidation, coal samples with different moistures were prepared by using aqueous solutions with different pH values (3, 5, 7, and 8). The CO, C₂H₄ production, oxygen consumption rate, and crossing point temperature parameters in the process of low-temperature oxidation of the prepared coal and raw coal samples were studied by using a low-temperature oxidation device. Thermogravimetry-derivative thermogravimetry (TG-DTG) curves of coal samples with different moistures were obtained on the basis of thermogravimetric analysis of coal by using a synchronous thermal analyzer. According to the characteristic temperature, the process of coal spontaneous combustion (CSC) can be divided into the water evaporation and desorption stage, oxygen absorption and weight gain stage, thermal decomposition and combustion stage, and burnout stage. The Coats-Redfern integral method was used to select the appropriate reaction mechanism function to calculate the apparent activation energy at different stages of the coal reaction process. The results show that the activation energy of the oxidation reaction of the coal sample with a moisture of 15% is the smallest, and the oxidation reaction is the most easy to occur. When the coal samples with a moisture of 15% were oxidized at low temperature, the CO and C₂H₄ emissions and oxygen consumption rates were the highest. The pH value of the aqueous solution has dual effects on CSC. When the moisture is 15%, the higher the pH value of the aqueous solution, the weaker the promotion effect on the coal oxidation process is and the lower the pH value of the aqueous solution is, the higher the production of CO and C₂H₄ and the oxygen consumption rate is and the earlier the crossing point temperature is reached during low-temperature oxidation of the coal sample.