Research On Forms Of Organic Oxygen In Lignite And The Transformation Mechanism During Pyrolysis Process

High organic oxygen and water contents and the resulting low calorific value, make lignite difficulty in industrial use, especially for thermoelectricity generation. As a thermochemical conversion technology, pyrolysis has received considerable attention. It is of great significance to understand the formation and release behaviors of oxygen functional groups（OFGs） and their transformation during pyrolysis. Such an approach may lead to the development of efficient lignite upgrading technology.Two Chinese lignites, i.e., Baiyinhua（BYH） and Xilinguole（XLGL）, were selected as coal samples in this work. The occurrence forms of organic oxygen in lignites were characterizated by X-ray Photoelectron Spectroscopy（XPS）, Fourier Transform infrared spectroscopy（FTIR） and ultimate analyzer at the molecular level. OFGs in lignites can be categorized into carboxyl, hydroxyl, carbonyl, active ether functionalities and inactive ether bond. The number of hydroxyl in lignite is thought to play a significant role in the coal-water interactions by providing binding sites for the water molecules. The hydroxyl groups in lignite can be classified into six types, i.e., free OH groups（FHG）, OH-p bonds（PHB）, self-associated OH bonds（SHB）, OH ether hydrogen bonds（EHB）, tightly bound cyclic OH bonds（RHB） and OH-N hydrogen bonds（NHB）, by curve-fitted FTIR spectra of the hydroxyl. Among them, the intensity of RHB and EHB was higher than that of other hydrogen bonds in the region of 3400-3600 cm-1.Fast pyrolysis of BYH and XLGL were conducted in a fixed-bed tube quartz reactor to investigate the effects of pyrolysis temperature and gas resident time（GRT） on the yields of pyrolysis products and the distributions of organic oxygen species（OOSs）, and a series of advanced analysis techniques were introduced to investigate the structure of oxygen. The results indicate that at 600 oC and a GRT of 1.5 s, the tar yield reached the maximum of 15.3% and 12.9% for BYH and XLGL lignite, respectively. CO₂ was found to be the predominant oxygenous gas at 600 oC, and the significant release of CO₂ at temperatures lower than 400 oC should be related to the high carboxyl content in the lignite. The CO yield increased significantly with the raising temperature and reached 46.4 and 44.0 m L/g（daf） for BYH and XLGL, respectively, at 700 oC. The O yield in water was lower than 20% below 200 oC, and sharply increased to 30% at 700 oC because of thermal cracking of volatiles. Above 400 oC, the OFGs in lignite are mainly released as pyrolysis water and gas. The OOSs detected in the tar include phenols, ketones, ethers, alcohols, and dibenzofuran and its substitutes, among which phenols（phenol, cresol, and xylenol） are the main components with yield up to 27.1% and 26.3%（based on tar） for BYH and XLGL, respectively, at 500 oC and GRT of 10 s. The possible oxygen transformation routes for lignite pyrolysis are also discussed in this work. Such an approach may lead to the development of efficient lignite upgrading technology.