Nitrogen Isotopic Composition Of Coal And Its Controlling Mechanism

In view of the fact that nitrogen will be transferred to oil and gas,clay minerals and atmospheric nitrogen oxides in the process of coal formation and development as well as utilization,the study on the characteristic and controlling mechanism of nitrogen isotopic composition in coal can not only estimate the contribution of coal combustion to atmospheric nitrogen oxides in different regions,but also enrich and expand the theoretical bases for coal geochemistry and the application of nitrogen isotope geochemistry in oil and gas fields.Based on the selected reliable methods for testing nitrogen isotopes,this thesis clarifies the controlling mechanism of nitrogen isotopic composition in coal,combined with the data of coal chemistry,coal petrology and coal geochemistry.The main understandings are as follows:After comparative study,sealed-quartz-tube combustion method and optimized EA-Conflo ?-IRMS online method can both accurately determine nitrogen isotopic composition in coal,relative to Kjeldahl-Rittenberg method.For coal samples with high ammonium,it is better to directly measure bulk nitrogen isotopic composition by sealed-quartz-tube combustion method or to extract kerogen for determining isotopic composition of organic nitrogen via the optimized EA-Conflo ?-IRMS online method.The ?15N values of Chinese coals measured by sealed-quartz-tube combustion method are between 1.4‰and 5.1‰(n=141),which are similar to the range of ?15N value in foreign coals(0.3‰?5.4‰),and the ?15N value in coal seam profile has heterogeneity,of which the maximum is up to 2.5‰.Statistical results present that average ?15N value shows no obvious relationship with coal-forming age and ash yield but increases first and then decreases along with increased coal rank or sulfur content.However,the obvious variation(?5‰)in value of coals with similar rank or sulfur content implies the coupling control of multiple factors on the nitrogen isotopic composition in coal.The influence of ammonium in clay minerals on nitrogen isotopic composition in coal is clariified by studying nitrogen isotopic compositions of density-grade samples from three bitulinous coal samples,respectively.Relative to the steady increase(?4.8‰)in ?15N values of density-grade samples from lean coal SH15,the sudden decreases in ?15N values of density-grade samples from fat coal TL08(?6.0‰)and coking coal HX10(?3.7‰)with increased ammonium indicate that the nitrogen isotopic composition of ammonium in these coals has obvious heterogeneity(i.e.,ammonium with higher ?15N value than and lower ?15N value than organic nitrogen is concurrent at the same time).With increased coal rank,the heterogeneity gradually decreases,and most of ammonium enriches in heavier nitrogen than organic nitrogen(e.g.ammonium in lean coal SH15).It can be inferred that the degree of coal metamorphism which determines the thermal release of organic nitrogen and the content of clay mineral which determines the ability of coal matrix to fix ammonium,control the content and isotopic composition of ammonium together and further affect nitrogen isotopic composition in coal.Based on the study of nitrogen isotopic composition in coal seal profile,the influence of sedimentary environment on nitrogen isotopic composition in coal is clarified,specially embodied as the direct effects of nitrogen source,nutrient element content,maceral composition and degradation degree of organic nitrogen related to sedimentary environment on the isotopic composition of organic nitrogen in coal.For example,the relationship between the ?15N value and sulfur content in medium-high-sulfur No.8 coal at Sitai Mine in Datong(i.e.,a first increased and then decreased trend in ?15N value with raised sulfur content)mainly reflect the additive effect of nitrogen source and degradation degree of organic nitrogen;and the negative correlation between?15N value and C/N ratio of No.2 superlow sulphur coal at Xinzhi Mine in Huozhou and the negative correlation between ?15N and C/N ratio of No.8 superhigh-organic-sulphur coal at Jiangjiaping Mine in Chenxi chiefly reflect the additive effect of maceral composition,nutrient elements and degradation degree of organic nitrogen.Generally speaking,coals with high nitrate supply from seawater,high nutrient elements and low inertinite content have higher ?15N value in organic nitrogen and as for the influence of degradation degree of organic nitrogen is that ?15N value in organic nitrogen increases first(4.5‰?8.0‰)and then decreases(2.0‰?8.0‰)with increased degradation degree of organic nitrogen.The influence of coal metamorphism on nitrogen isotopic composition in coal is clarified by studying the nitrogen isotopic composition in lignite pyrolysis residues.With increased temperature,?15N value increases first and then decreases where the ranges are both less than 1,which mainly reflects the difference of thermal stability between diverse nitrogen isotopes and occurrence structures in organic matter.Considering the larger variation in the occurrence structure of organic nitrogen,it can be concluded that there is little difference in nitrogen isotopic composition among different occurrence structures of organic nitrogen.Therefore.the influences of the degree of coal metamorphism and controlled occurrence structures of organic nitrogen on isotopic composition of organic nitrogen in coal is minor.In summary,sedimentary environment factors,including nitrogen source,nutrient elements,maceral composition and degradation degree of organic nitrogen,mainly control the isotopic composition of organic nitrogen in coal;the degree of coal metamorphism and clay mineral content chiefly regulate the nitrogen isotopic composition of ammonium in coal;and the contents and isotopic compositions of organic nitrogen and ammonium finally control bulk nitrogen isotopic composition in coal together.