The effects of temperature and molten salt on solar pyrolysis of lignite

Highlights: • Solar driven pyrolysis of lignite has been conducted in molten carbonate salt media. • Products were characterized at various temperatures for revealing formation process. • Molten salt increased gas yield (especially CO and H₂) from 28.3% to 46.1% at 800 °C. • Tar quality was improved by molten salt with 5.8% increase of hydrocarbon contents. • Molten salt char had higher reactivity due to rising active sites and microporosity. -- Abstract: Molten salt pyrolysis driven by concentrated solar radiation is well positioned to utilize solar energy and lignite effectively. This study focused on the effects of temperature (500, 600, 700 and 800 °C) and molten carbonate salt (Li₂CO₃-Na₂CO₃-K₂CO₃) on properties of char obtained from lignite pyrolysis, as well as gas and tar products for revealing their formation mechanism and transformation process. Molten salt pyrolysis of HulunBuir lignite produced more gas products and less char compared to conventional pyrolysis owing to the enhanced heat transfer and catalytic effect of molten salt. The char yield decreased from 58.4% to 43.4%, and the gas yield (especially CO₂, H₂ and CO) increased from 28.3% to 46.1% at 800 °C. CO₂, CO and H₂ production increased about 60.43%, 103.42% and 65.2% at 800 °C, respectively. Additionally, the presence of molten salt improved the tar quality with more hydrocarbon content (maximum increase of 5.8%) and less oxygenated compounds. The structure and reactivity relationship of char was characterized by XRD, BET, SEM, FTIR, Raman spectroscopy and TGA. Molten salt generated char had a higher reactivity due to the increase of disorder, surface area, microporosity (maximum of 71.74%) and active sites.