Changes in caking properties of caking bituminous coals during low-temperature pyrolysis process

Abstract

We proposed a low-temperature pyrolysis treatment (LTPT) to destroy the caking properties of a fat coal (FC) and a 1/3 coking coal (CC). To reveal the factors affecting caking property, the changes in the chemical properties of pyrolysis products were analyzed using Fourier transform infrared spectrometry, X-ray diffractometer, and gas chromatograph/mass spectrometer (GC/MS). The in situ morphological changes in the liquid crystals produced by the two coal samples during LTPT process were also recorded using a hot-stage microscope. Of particular interest was a microscopic description of the factors affecting caking property. The results showed that the caking index (G) of FC decreased with increasing temperature and became zero at about 500 °C. Hot-stage microscope showed FC produced liquid crystals at 395 °C, the size of which continuously increased, with an accompanying of the generation of new liquid crystals. GC/MS showed large amounts of aliphatic hydrocarbons with the number of carbon of 6–25 (AlHsc6-25) and aromatic hydrocarbons with 2–4 rings (ArHsr2-4) were removed from FC. The aromaticity (fa) of FC increased only substantially from 395 to 494 °C. Meanwhile, the G of FC decreased only significantly in the same temperature range. A similar result was observed for CC. The removal of AlHsc6-25 leads to insufficient supply of hydrogen radicals; therefore, relatively high-molecular weight aromatics quickly cross-linked into semicoke. Additionally, the evolution of ArHsr2-4 and the increase in fa restrain the originating of crystal liquids. As a result, the loss of G was observed.