Mechanism Underlying the Emission of Gases during the Low-Temperature Oxidation of Bituminous and Lignite Coal Piles: The Involvement of Radicals

Coal is one of the major fuels for power generation, and it will continue in this capacity for the next several decades. Two types of coal are mainly used: lignite and bituminous coals. When exposed to air, post-mining, the coal surface undergoes LTO (low-temperature oxidation) at RT-150 °C according to the atmospheric oxygen level. The LTO process decreases the calorific value of the coal, and consequently, different gases are released [mainly carbon oxides (CO, CO₂), water vapor, hydrogen (H₂), and also some low molecular-weight organic gases (C_1-5)]. Some of these gases are toxic and flammable. In extreme cases, fires erupt. The mechanism by which the molecular oxygen oxidizes the coal macromolecule at the temperature range of 30-150 °C (LTO process) is complex and also involves a chain of radical reactions that take place; however, the exact underlying mechanism is not yet clear. The LTO process was studied in detail by simulating the processes occurring in the coal piles by using two coal types: an American Bailey coal, used in Israeli coal-fired utilities and a German Hambach lignite, used in German utilities. The mechanism underlying the LTO process and the radical reactions that are involved are discussed in detail.