引用本文：程晨,赵峰华,任德贻,苗雪娜.2018.中国煤中氮同位素组成特征初步研究[J].地质学报,92(9):1959-1969. CHENG Chen,ZHAO Fenghua,REN Deyi,MIAO Xuena.2018.Preliminary Study on the Characteristics of Nitrogen Isotopic Compositions in Chinese Coals[J].Acta Geologica Sinica,92(9):1959-1969.
Abstract:In this study, nitrogen isotopic compositions (δ15N) of a series of Chinese coals were determined using Dumas combustion method to preliminarily figure out the factors affecting nitrogen isotopic compositions in coals, combined with previous results and geological background of coal deposits. The study shows that (1) The δ15N values of Chinese coals range from +1.4‰ to +5.1‰, within the range of δ15N values (+0.3‰~+5.4‰) of coals from other regions in the world. The δ15N values within the same coal profiles display heterogeneous characteristic, with the maximum of 2.5‰. (2) The δ15N values in coals are controlled by multiple effects including coal metamorphism and sedimentary environment, in which the former involves the effects of original coal grades, metamorphic grades and types, and the latter contains the effects of nitrogen sources of coal-forming plants as well as physical-chemical conditions and microbial activities of swamp medium. (3) In terms of coal metamorphism, the δ15N values of Chinese coals increase with increasing coal grade (about 1‰) but decrease at anthracite, which reflects the difference in the stability of two nitrogen isotopes. Before transforming into high rank bituminous coal, the δ15N values increase along with the preferential removal of 14N. Between high rank bituminous coal and low rank anthracite, the δ15N values decrease first and keep stable to high rank anthracite, due to the preferential removal of unstable 15 N in this stage but synchronous elimination of 14 N and 15 N after this stage. Regardless of N-containing geological fluid, hypozonal metamorphism and contact metamorphism should have the same effect on the δ15 N values in coals. (4) As for sedimentary environment, the coals from transition facies with medium high and high inorganic sulfur content show higher δ15N values than the coals from lacustrine and fluvial facies with ultralow and low sulfur content, and the lowest coals δ15 N values are showed in coals from carbonate platform facies with superhigh organic sulfur content. The trend reflects the differences in nitrogen sources and degradation degrees of plant organic matter during peatification. When the microbial degradation of coal-forming precursor is weak (ultralow-sulfur and low-sulfur coals), nitrogen content in coals would be high and δ15 N would increase. When the microbial degradation of coal forming precursor becomes stronger (superhigh- organic- sulfur coals), protein would be degraded severely to lead to a decrease in nitrogen content and δ15 N again. Moreover, δ15 N in coals are also related to inertinite content, which has a relatively low δ15 N due to the loss of massive nitrogen during fusainisation. When the microbial degradation of coal- forming precursor is weak (#12 ultralow-sulfur coal bed in Sitai Mine), the control of inertinite content on δ15 N is more significant. (5) In the case of coal forming age, Late Paleozoic and Mesozoic Chinese coals show a similar average δ15 N value, which is higher than Cenozoic coals. The difference in δ15 N values is not caused by distinct plants in different coal-forming ages, but the low N loss of Cenozoic lignite without undergoing coal metamorphism.