110 / 2023-06-15 19:22:32

Study on the effect and mechanism of carboxylate on coal spontaneous combustion

Coal spontaneous combustion,Thermal decomposition,Acid-base treatment,Ion exchange,CM-Na structure

It is well known that coal with low metamorphism contains a large number of stable primary oxygen-containing functional groups, which form a large number of active sites with the generation of CO and CO₂ gas products during thermal decomposition[1]. Among them, the process of CO₂ generation with the decomposition of carboxyl groups played a dominant role in the generation of active sites[2]. In addition to the influence of organic structures, there are a large number of inorganic salt structures in coal[3]. At present, the effect of inorganic salts on coal spontaneous combustion has been studied for decades and relatively mature research results have been obtained. Previous studies generally believed that the inorganic salts compounds of K, Na, Ca, Mg in coal are easy to absorb external moisture and has an inhibitory effect on coal spontaneous combustion[4]. However, the transition metals such as Fe²⁺, Co²⁺, Ni²⁺, can reduce the activation energy of the oxidation reaction in the form of catalysts, and promote the oxidation reaction of coal[5]. At present, numbers of investigators focused on the functional study of inorganic metals, however, ignoring the organic metal structures in coal. Usually, the organic metals exist mainly in the form of carboxylates[6], and the effects of these structures on coal spontaneous combustion are rarely discussed, which is worth an in-depth study. The coal sample with high content of oxygen functional groups and organic metal ions was selected. Acid and alkali pre-treatment were conduct to get carboxylate coal samples with different content. The content of carboxylate was seen through the SEM-EDX experiments. The pyrolysis-room temperature oxidation of pre-treated coals and thermogravimetric experiments were executed to study the effect of carboxylate on coal spontaneous combustion. Thermodynamics, XPS and ESR experiments were conducted to investigate the mechanism of carboxylates on coal spontaneous combustion. It was found that the carboxylate structures had a great effect on the coal oxidation process. Alkali treatment can convert the carboxylic acid structures into carboxylate structures, thus promoted the generation of active sites and the oxidation process in the low-temperature stage. But The presence of carboxylate structures in coal can lead to the formation of CM-Na structures, which had weak stability in low-temperature stage and can decompose to produce active sites. With the increment of reaction temperature, the bond energy will gradually strengthen, thereby inhibiting the coals combustion process in the high-temperature stage. Therefore alkali-treated coals (including coals soaked in alkaline mine water and coals extinguished using alkaline extinguishing agents) and low-rank coals that rich of AAEM require special attention to prevent coal spontaneous combustion. In practical, alkaline inhibitors are often used in the extinguishing of high-temperature fires due to the high-temperature inhibition effect of CM-Na. However, special attention should be paid to the possible risk of re-ignition. Acid treatment to convert carboxylate structures into carboxylic acid structures can observably inhibit the production of active sites. However, hydrochloric acid cannot be used in preventing coal spontaneous combustion due to its corrosion of equipment. Therefore, other acid inhibitors can replace hydrochloric acid for efficient inhibition, such as citric acid, phytic acid, ethylenediaminetetraacetic acid, etc.