中国煤中氮同位素组成特征初步研究

Preliminary Study on the Characteristics of Nitrogen Isotopic Compositions in Chinese Coals

本文采用Dumas燃烧法测定了部分中国煤样的氮同位素组成(δ^(15 )N),并结合文献数据和煤样品的地质背景初步探讨了影响煤中氮同位素组成的因素。研究表明:(1)中国煤的δ^(15 )N值介于+1.4‰^+5.1‰之间,与世界其他地区煤的δ^(15 )N值范围(+0.3‰^+5.4‰)相吻合;同一煤层剖面分层煤样(煤级相似)的δ^(15 )N值具有非均质性特征,最大可达2.5‰。(2)煤的δ^(15 )N值是煤变质作用、沉积环境等多种因素耦合作用的结果。变质作用对煤的δ^(15 )N值的影响主要包括原始煤级、变质程度(煤级)和煤变质作用类型;沉积环境对煤的δ^(15 )N值的影响包括成煤植物的氮源以及沼泽介质的物理化学条件和微生物活动性等因素。(3)就煤变质作用(煤级)而言,中国煤的δ^(15 )N值随煤级升高呈现增高的趋势,到无烟煤又有所降低,这是煤中氮同位素稳定性差异的结果。在高级烟煤阶段(贫瘦煤)之前,随煤级升高稳定性较差的14 N优先脱除,δ^(15 )N值增高,增高幅度约1‰。在高级烟煤至初级无烟煤阶段,煤中剩余的14 N已趋于稳定,随煤级升高,部分不稳定15 N优先脱除,δ^(15 )N值降低。在无烟煤阶段,随煤级升高,14 N和15 N同步脱除,δ^(15 )N值几乎不变。在不考虑含氮地质流体影响的情况下,深成变质作用和接触变质作用对煤中δ^(15 )N值的影响应相似。(4)就沉积环境而言,形成于海陆过渡相的中高硫煤/高硫煤的δ^(15 )N值最高,形成于陆相的特低硫煤和低硫煤的δ^(15 )N值次之,而形成于碳酸盐岩台地相的超高有机硫煤的δ^(15 )N值最低,这主要与沉积环境中成煤植物的氮源以及泥炭化作用阶段植物有机质降解程度的差异有关。一般以富集15 N的海水硝酸盐为氮源的成煤植物形成的煤(海陆过渡相中高硫煤/高硫煤)较以相对亏损15 N的大气氮为氮源的成煤植物形成的煤(陆相特低硫煤和低硫煤)要富集15 N。当成煤母质在泥炭化作用阶段受到微生物降解作用较弱时(陆相特低硫煤和低硫煤),形成的煤氮含量较高,δ^(15 )N会有所上升;当成煤母质在泥炭化作用阶段受到强烈的微生物降解作用时(碳酸盐岩台地相超高有机硫煤)成煤植物蛋白质(富15 N)被降解损失的较多,形成煤的氮含量较低,δ^(15 )N值又会有所降低。此外,煤的δ^(15 )N值还与惰质组含量有关,因为在丝炭化过程中大量损失氮使得惰质组的δ^(15 )N值偏低,当成煤母质遭受的降解作用较弱时(四台煤矿12号特低硫煤),惰质组含量变化对煤δ^(15 )N值的控制作用尤为明显。(5)就成煤时代而言,中国的晚古生代煤与中生代煤的δ^(15 )N值相近,都高于新生代煤的δ^(15 )N值。δ^(15 )N值的这种差异并不是因为不同成煤时代的成煤植物不同造成的,而是因为新生代煤样为尚未经历煤变质作用的褐煤,其氮损失较少,所以新生代褐煤δ^(15 )N值较低。

In this study,nitrogen isotopic compositions（δ15 N）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 δ15 N values of Chinese coals range from ＋1.4‰ to ＋5.1‰,within the range of δ15 N values（＋0.3‰ ＋5.4‰）of coals from other regions in the world.The δ15 N values within the same coal profiles display heterogeneous characteristic,with the maximum of 2.5‰.（2）The δ15 N 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 δ15 N 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 δ15 N values increase along with the preferential removal of 14 N.Between high rank bituminous coal and low rank anthracite,the δ15 N 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 higherδ15 N values than the coals from lacustrine and fluvial facies with ultralow and low sulfur content,and the lowest coalsδ15 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 coalforming 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.