Effects of drying method on self-heating behavior of lignite during low-temperature oxidation

Pore structure changes during pre-drying of lignite affect its low-temperature oxidation and increase the susceptibility to spontaneous combustion. In this study, the effects of drying methods (i.e., vacuum drying and N₂ drying) on self-heating of Indonesian lignite during oxidation were investigated using a dual fixed-bed quartz reactor. The variation of coal temperatures was recorded and the release of CO₂ and CO was measured by a gas chromatography. The pore volume and surface area of dried samples were measured using Brunauer-Emmett-Teller (BET) method. Mesopores in lignite initially increased and collapsed with further increasing drying intensity during drying in N₂, resulting in a rapid self-heating rate of lignite within a critical moisture content range of 6-13%. However, vacuum drying caused a gradual increase in mesopores, which lead to a monotonic increase in self-heating rate with decreasing residue moisture content in lignite. The experimental results indicated that the production rates of both CO₂ and CO during oxidation of raw lignite increased with reducing particle size and increasing gas flow rate, but decreased at lower moisture contents. Typically, the variation of production rates of both CO₂ and CO as a function of particle size and gas flow rate followed a similar trend to that of raw lignite when the lignite was completely dried by the vacuum drying method. The impacts of lignite particle size and gas flow rate on the yields of CO₂ and CO was limited due to less diffusion of O₂ into small pores, suggesting that the oxidation reaction between lignite and oxygen has been shifted from diffusion controlled to kinetic controlled reactions.