Study on the inhibition effect of citric acid on coal spontaneous combustion

Transition metals ions in coal can accelerate the coal spontaneous combustion by reducing the activation energy of the coal-oxygen reaction during coal oxidation. The catalytic action of metal ions may be decreased by adding metal chelators to coal. In this article, an environment-friendly metal chelating agent, citric acid, was proposed as an inhibitor of coal spontaneous combustion. Experiments were carried out to investigate the inhibitory effect of citric acid on coal spontaneous combustion. The results showed that the generation of CO, CO2 and the consumption of O2 in the oxidation of inhibited coal samples were much lower than those of raw coal. Compared to raw coal, the crossing point temperature of inhibited samples increased by 9–12 °C, and the advent time of the crossing point temperature was delayed by 30 to 40 min. In addition, the temperature of the characteristic points of the experimental coal samples after inhibition was higher than that of raw coal. The reaction enthalpy of inhibited coal samples was smaller than raw coal when experimental samples entered the rapid combustion stage. Through Fourier Transform infrared spectroscopy (FTIR) experiments, it was found that the amount of -CH3 and -CH2- groups in coal samples increased after inhibition, but the amount of -COOH and C=O groups decreased. Scanning electron microscope (SEM) analysis showed that the surface of the coal sample became smoother and the pore size decreased after citric acid treatment. In conclusion, citric acid has a good inhibitory effect on different types of coal.