Enhancement of low‐temperature lignite pyrolysis by recycled carbocoal for high‐quality tar in fixed‐bed reactor

Abstract

Lignite could be converted to high value‐added char and tar through a pyrolysis process. To improve the tar yield as well as the quality of tar/char during a low temperature (550 °C) lignite pyrolysis (LT‐LP) process, the influence of mixing inexpensive and accessible recycled carbocoal with three kinds of lignite was investigated in a fixed‐bed reactor. The LT‐LP processes with and without recycled carbocoal were investigated using a TGA and were compared in detail. The results indicated that the addition of the recycled carbocoal into the three lignite optimized the light tar yield from 4.63, 3.33, and 2.09 g/g to 10.50, 6.10, and 5.01 g/g, and improved the gasification reactivity of the chars up to 2.88, 2.58, and 2.88 h−1, respectively. To identify the role of recycled carbocoal during the LT‐LP process, the obtained products, such as tar and char, were characterized by chromatography, thermal gravimetric analyzer, chromatograph‐mass spectrometer, elemental analyzer, Fourier transform infrared spectroscopy, and scanning electron microscope. It was demonstrated that the existence of the recycled carbocoal could impede the self‐polymerization of small molecule hydrogen‐containing (SM‐HC) free radicals and cut down the reactions between SM‐HC and hydroxyl radicals at the weak‐bonds breaking zone (300–450 °C), thereby supplying sufficient free radical stabilizers for the tar intermediates at the molecular skeleton region (450–550 °C). The high‐valued tar and char were obtained. In summary, the employment of recycled carbocoal during the LT‐LP process could not only promote the economic benefit of LT‐LP products but also reduce the cost of lignite value‐added utilization.