Real Time Analysis on Evolved Gas Emissions during High-Temperature Spontaneous Combustion of H₂O-NH₄Ac Extracted Coal

The primary goal of this study is to investigate the effect of inherent calcium on the release of gaseous products during the high-temperature spontaneous combustion of coal. The coal samples were sequentially extracted progressively with water (H₂O), ammonium acetate (NH₄OAc), and hydrochloric acid (HCl) solutions. The amount of Ca in various occurrence modes in coal samples was then evaluated using an inductively coupled plasma equipped atomic emission spectrometer (ICP-MS). The results showed that all of the tested coal samples contained NH₄Ac-soluble Ca, HCl-soluble Ca, and H₂O-soluble Ca. The FTIR of raw and residual coal after H₂O and NH₄OAc extraction indicates that H₂O and NH₄OAc extraction would remove some carboxylate from the coal structure. The high-temperature spontaneous combustion of raw coal and residual coal after H₂O and NH₄OAc extraction was investigated by mass spectrometry and thermogravimetric analysis. H₂O and NH₄OAc extraction resulted in lower initial decomposition and ignition temperature, and no CO₂ was generated during the spontaneous combustion of H₂O and NH₄OAc-treated coal samples at temperatures below 400 °C. Significant CO₂ production was only observed at temperatures above 400 °C. The H₂O-soluble calcium and organically bounded calcium in coal could alter the thermal evolution process of the coal structure, promoting the cracking of aliphatic side chains and therefore increasing the yield of H₂, CH₄, CO, and CO₂. However, for coal with a relatively high H₂O-soluble calcium content, water treatment transforms H₂O-soluble calcium into Ca(OH)₂, which could accelerate the release of gaseous products during spontaneous combustion and increase the emission of H₂, CH₄, and CO. Finally, the mechanism for the effect of inherent calcium on the gaseous product generation was proposed.