Calorimetric investigation of chemical additives affecting oxidation of coal at low temperatures

Interactions in the system “coal–chemical additive–oxygen” were studied at the temperature 30 °C with the aim of recognising inhibitors of the coal oxidation process that are able to modify the coal surface in a chemical way. Three coals of different ranks were tested. As possible inhibitors, 14 aqueous solutions (10 wt.%) of both inorganic (chlorides, sulphates, nitrates, phosphates, and sulphites) and organic (formates, acetates, urea, and thiourea) substances were examined. The action of additives on coal was evaluated from the heat effect during immersion of the coal in the additive solution. It was found that most of the studied substances proved immersing heats quite comparable with that of pure water. However, rather a chemical action was ascertained for sodium sulphite, phenol, urea and/or thiourea with heat effects several times exceeding heats for pure water. To quantify oxyreactivity of original coals as well as additive-treated ones, oxidation heats were measured by the pulse flow calorimetric method. Urea was found to be the chemically acting additive with the most significant inhibiting efficiency (up to 70%) of coal oxidation at low temperatures.