Characteristics of chars from low-temperature pyrolysis of lignite

Fanrui Meng, Jianglong Yu, Arash Tahmasebi, Yanna Han, Huan Zhao, John Lucas, Terry Wall

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114 Citations (Scopus)

Abstract

Low-temperature pyrolysis offers a potential way of upgrading lignite and producing chars to replace thermal or pulverized coal injection (PCI) coals in combustion or being used as inert components in a blend for coking. In this study, the characteristics of chars from low-temperature pyrolysis of two lignite coals have been investigated. The changes in char morphology and chemical structures were investigated using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The combustion reactivity of chars was analyzed in a thermogravimetric analyer (TGA) using non-isothermal techniques. The results show that chars from low-temperature pyrolysis of lignite coal below 450 C were more reactive than higher temperature chars. Higher reactivity of low-temperature chars was attributed to the higher concentration of active sites and lower degree of structural order compared to that of high-temperature chars. Indonesian (YN) lignite showed a higher weight loss rate compared to Hulunbeier (HL) coal, which was attributed to a higher concentration of liptinite and vitrinite in YN coal. FTIR analysis indicated that the aliphatic structures and oxygen-containing functional groups decreased with an increasing pyrolysis temperature. The intensity of tightly bound cyclic OH tetramers and OH-ether O hydrogen bonds were higher than other hydrogen bonds in the 3700-3600 cm-1 region of infrared (IR) spectra. The density of alkyl chains and cross-linking reactions affected the yield of tar. The aromaticity of char increased with an increasing pyrolysis temperature. The abundance of Cî - O and COOH structures decreased drastically with increasing temperature. A lower concentration of active sites on high-temperature chars resulted in lower combustion reactivity compared to low-temperature chars. The C-O and Cî - C groups decreased as the temperature increased possibly because of the aromatic condensation. The extent of aromatic substitution decreased up to 650 C. At temperatures above 650 C, the degree of aromaticity was strengthened and larger condensed aromatic nuclei were formed. Brunauer-Emmett-Teller (BET) surface area analysis revealed that high-temperature chars have significantly higher surface area compared to chars produced at low temperatures. However, the concentration of active sites was lower in high-temperature chars. Therefore, it can be concluded that diffusion was the main reaction mechanism in high-temperature chars.

Original languageEnglish
Pages (from-to)275-284
Number of pages10
JournalEnergy and Fuels
Volume28
Issue number1
DOIs
Publication statusPublished - 16 Jan 2014
Externally publishedYes

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