Effect of adjusting coal properties on HulunBuir lignite pyrolysis

- Cross-linking reactions during lignite pyrolysis were suppressed by pretreatment.
- More pores, surface defects, and carboxyl groups formed after acid-washing.
- The tar quality improved after thermal pretreatment.

It is necessary to develop an effective method to suppress cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis. The effect of thermal pretreatment, acid-washing, and acid-washing combined with thermal pretreatment on lignite pyrolysis at a low temperature with a fixed-bed reactor under N2 atmosphere was investigated. The chemical and physical characteristics of lignite with and without pretreatment were analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction. The molecular weight distributions of tars were estimated by gel permeation chromatography and the composition of gas products was characterized by using gas chromatography. The relation between the structure change and the pyrolysis performance is the key topic for discussion. The results show that there are more pores and surface cracks in coal after acid-washing, and the carboxyl groups increased. The yields of tar and gas were improved while the yields of char and pyrolysis water were reduced after pretreatment. The tar yields increased significantly, namely from 6.88 wt% for raw lignite to 7.23 wt% after thermal pretreatment, 8.57 wt% after acid-washing, and 9.27 wt% after acid-washing combined with thermal pretreatment, consequently. Meanwhile, the hexane soluble of tar increased from 75.57 wt% to 77.75 wt% after thermal pretreatment, while it decreased to 67.21 wt% or 69.70 wt% after acid-washing.

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