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 crosslinking reactions affected the yield of tar. The aromaticity of char increased with an increasing pyrolysis temperature. The abundance of CO 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 C 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.
Chemical changes of Chinese lignite upon drying in superheated steam, microwave, and hot air have been studied in this paper using the Fourier transform infrared (FTIR) spectroscopy technique. The infrared (IR) spectra of raw and dried samples were curve-fitted to a series of bands in aliphatic hydrogen (3000−2800 cm −1 ) and carbonyl absorption (1850− 1500 cm −1 ) zones. It has been found that aliphatic hydrogen absorbance decreased slightly with an increasing temperature during superheated steam drying, while absorption of carboxyl (COOH) and carbonyl (CO) groups decreased drastically, indicative of the loss of oxygen functionalities with an increasing drying temperature. During steam drying, aromatic carbon and aromatic ring stretch absorption remained relatively unchanged up to 250 °C and decreased significantly thereafter because of some pyrolysis reactions that took place at higher drying temperatures. Microwave heating of lignite resulted in a significant decrease in the concentration of oxygen-containing functional groups. Aromatic carbon remained relatively unchanged under microwave drying conditions, while aliphatic hydrogen decreased slightly. The aromaticity of coal calculated from curve-fitted spectra of deconvoluted peaks showed a progressive increase with an increasing drying intensity under both steam and microwave drying conditions. Under air drying conditions, aliphatic hydrogen absorbance decreased drastically at 250 °C, while aromatic carbon remained unchanged. It was observed that oxidation in air mainly took place on aliphatic hydrogen sites, especially on methylene groups. Changes of carboxyl and carbonyl groups during air-dried samples showed a different trend compared to those dried in steam and microwave, increasing gradually up to 150 °C and then a sharp increase at 200 °C. The absorption of these groups decreased significantly at an increased air temperature up to 250 °C.
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