钼催化作用下的煤成烷烃气碳同位素演化

为了阐释钼催化作用下的煤成气碳同位素演化特征和机理,采用在原煤中添加单质钼的方式,开展催化生气模拟试验,测定模拟系列气样的单体烷烃气碳同位素组成,并对其演化规律和模式进行探讨.结果表明:无论加钼与否,模拟气甲烷碳同位素组成(δ(13C1))均以镜质组反射率1.1%为界分为演化趋势截然相反的2个阶段,乙烷碳同位素组成(δ(13C2))不存在明显的演化阶段性,但加钼煤样δ(13C2)由在较低成熟度阶段重于原煤样变为在较高成熟度阶段轻于原煤样;加钼条件下,模拟烷烃气呈现出正碳同位素系列,δ(13C1)为(-25~-45)×10-3,δ(13C2)重于-29×10-3,表明钼催化成因气属于有机热成因气范畴.对比模拟烷烃气碳同位素组成与镜质组反射率之间的关系,发现模拟烷烃气碳同位素组成演化规律与前人模式有所不同.加钼煤样的不同单体烷烃气碳同位素组成随有机质成熟度变化速率的不同而不同,指示生气作用在钼催化作用下得到增强,这一效应在裂解气生成过程中更为显著;加钼煤样δ(13C2)在较高成熟度条件下轻于原煤样δ(13C2)的原因可能在于钼催化效应促进了煤中壳质组裂解生气.

Evolution of Carbon Isotope in Coal-derived Alkane Gas Under Catalysis of Molybdenum

In order to understand the mechanism of coal-derived alkane gas under the catalysis of molybdenum(Mo),gas generation simulations from the raw coal sample and Mo-added coal sample were carried out,and the simulated carbon isotopic compositions of monomeric alkane gas were measured,and the evolution pattern of carbon isotope was discussed.The results showed that carbon isotopic composition of methane(δ(13C1)) of simulated gas was evolved in two stages whether or not Mo was added,1.1% of vitrinite reflectance was the demarcation point;there was no obvious evolutionary stage for carbon isotopic composition of ethane(δ(13C2)),but for the Mo-added coal sample,there was a significant change for the δ(13C2),δ(13C2) was heavier than that of raw coal sample in lower maturity stage at first,and then lighter in higher maturity stage;for the Mo-added coal sample,there was a positive carbon isotope series for simulated alkane gas,δ(13C1) was(-25——45)×10-3,δ(13C2) was heavier than-29×10-3,so the Mo-catalytic gas was thermogenic gas from the organic matters in coal.The relationship between carbon isotopic composition and vitrinite reflectance of simulated alkane gas showed that the evolution was different with others’ results.Carbon isotopic compositions of different monomeric alkane gases was different for the Mo-added coal sample with the rate of maturity of organic matter,so gas generation was strengthened under the Mo catalyst and the catalysis was more significant during the pyrolysis gas generation.The reason that δ(13C2) was lighter than that of raw coal sample in higher maturity stage for the Mo-added coal sample,could be that Mo catalytic effect improved the generation of pyrolysis gas generation from exinite group in coal.