Molecular nitrogen from coal pyrolysis: Kinetic modelling

Abstract

It has been shown that molecular nitrogen (N 2) in natural gas accumulations can result from thermal maturation of coal and other forms of sedimentary organic matter. The present paper investigates the loss of coal nitrogen in the late stage of coalification and the genesis of N 2 from coal nitrogen in studying N 2 evolution during programmed pyrolysis of a coal series ranking until meta-anthracite. The work confirms the decrease of nitrogen content already observed in the late stage of coalification. This late decrease is correlated with a relative increase of the N 2 content in the total gas emitted by pyrolysis. N 2 emission occurs above 600°C in two broad peaks. Assuming that the rate of denitrogenation is described by single first-order reactions with a single pre-exponential factor, a discrete distribution of activation energies is computed by data processing of the first nitrogen peak of a representative semi-anthracite coal (R o 2.3%). Activation energies are distributed between 65 and 80 kcal/mol with a mode at 72 kcal/mol and a pre-exponential factor of 7.71 × 10 12 s −1. Similar values have been obtained by other authors for the thermal decomposition of pyridine. These kinetic parameters are used to predict the N 2 genesis in the Centre Broad Fourteens Basin (southern North Sea). The model indicates that Carboniferous anthracites in this basin could have released up to 0.575 m 3 of N 2 per ton of carbon.