Occurrence Forms Of Organic Oxygen And Organic Nitrogen In Xianfeng And Baiyinhua Lignites

Oxygen and nitrogen are the two major heteroatoms in coal. High organic oxygen content is a main reason leading to the low calorific value of lignite, while which provides opportunities for the acquisition of fine chemicals from lignite. Released nitrogen oxides during coal combustion caused a series of pollution problems, such as acid rain, photochemical smog, greenhouse effect and ozone destruction, etc. Therefore, identifying the occurrence forms of organic oxygen and organic nitrogen in lignite is essentical to separate and utilize the oxygen and nitrogen-containing organic compounds in lignite effectively, as well as reveal the structural characteristics of macromolecular in lignite and controll coal-fired pollution.For idenfying the occurrence forms of organic oxygen and organic nitrogen in lignite at the molecular level, it is necessary to dissolve the organic species of lignite using separable and non-destructive methods at mild condition. Two Chinese lignites, Xianfeng(XF) and Baiyinhua(BYH) lignites, were selected as coal samples in this work. Two lignites were sequentially extracted with carbon disulfide(CDS) and acetone(AC) or AC and CDS, and isometric CDS/AC mixed solvent, affording to extracts and extraction residues(ERs). The total extracts yields of XF and BYH in sequential CDS and AC or AC and CDS, and isometric CDS/AC mixed solvent are 4.51, 3.59, 6.54 and 3.02, 2.79, 6.10%, respectively, which indicates there exist obvious synergic effects between CDS and AC during the two lignites extraction process. Afterwards the extraction residues obtained from two lignites extracted in isometric CDS/AC mixed solvent were thermal dissolved sequential in toluene and methanol or methanol and toluene and isometric toluene/methanol mixed solvent in an autoclave reactor at 300 oC, affording soluble portions(SPs) and thermal dissolution residues(TDRs). The SPs yields of ERs from XF and BYH in sequential toluene and methanol or methanol and toluene and isometric toluene/methanol mixed solvent are 14.80, 32.18, 43.94% and 17.91, 19.15, 30.32%, respectively, suggesting that there also exist obvious synergic effects between toluene and methanol during the thermal dissolution of ERs from the two lignites. These results show that the extraction and thermal dissolution of lignites in mixed solvents can dissolve more organic species of lignites than those in sole solvent, which is benefit for thoroughly investigating the composition and structural characteristics of organic oxygen and organic nitrogen compounds in lignites.The raw lignites, ERs and TDRs were characterized with Fourier transform infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy(XPS), thermogravimetry(TG) and direct analysis in real time ionization source coupled to mass spectrometry(DART-MS). There are many peaks in the DART-MS spectra of raw lignites, while much peaks dissapperared in the spectra of ERs and TDRs. In addition, comparing with raw coal, the peak intensities of FT-IR spectra of ERs and TDRs are dramatically weaker than those of their corresponding raw lignites. TG and differential TG(DTG) curves show that peak temperature(Tp) and maximum weight loss rate of ERs are similar with those of raw lignites, while the total weight loss of ERs is slightly lower than that of corresponding raw lignites. Moreover, the weight loss of TDRs is obviously lower than that of corresponding raw lignites, and Tp of TDRs shifed to high temperature region compared with its raw lignites and ERs. XPS analysis results show that there are three forms of organic oxygen in the two lignites, including carbon oxygen single bond class(including ether bond and hydroxyl), carbonyl and carboxyl, and organic nitrogen mainly exist in the forms of pyridine nitrogen(N-6), pyrrole nitrogen(N-5), and quaternary nitrogen(N-Q). Compared with raw lignites, ERs and TDRs contain higher content of C=O, C-O and N-5, but lower content of COO-and N-6. The organic oxygen and organic nitrogen in ERs and TDRs are mainly in the form of C-O and N-5.The composition and structural characteristic of the extracts and SPs were identified with FT-IR, gas chromatography mass spectrometry detector(GC/MS) and atmospheric pressure chemical ionization time of flight mass spectrometry(APCI-TOF/MS). The results show that the oxygen-containing organic compounds(OCOCs) in extracts are dominated with ketones and esters, while those in SPs mainly contain phenols, ketones and esters. Most of the phenols in SPs are the phenol derivatives with isopropyl substituent or different number of methyl substituents, especially the meyhyl phenol. The ketones in their extracts and SPs include ketone and ethyl ketone, and the esters in them are mainly composed of alkanes acid methyl ester continuously distributed from octane to octacosanoic acid methyl ester. The nitrogen-containing compounds(NCOCs) in the extracts and SPs include amine, pyridine and quinolone, etc.