Modeling nitrogen evolution during coal pyrolysis based on a global free-radical mechanism

A global free-radical mechanism and an associated rate equation for the evo lution of light gas nitrogen during coal devolatilization are presented. Th e light gas nitrogen rate equation was used to modify an existing nitrogen model so that only a network devolatilization model and coal-specific chemi cal structural input data are required to adequately predict light gas nitr ogen release at a variety of pyrolysis conditions. Model predictions show g ood agreement with nitrogen release data from pyrolysis of coals of rank fr om lignite to low volatile bituminous at heating rates from 0.5 to 10(5) Ws , temperatures as high as 2300 K, and residence times from 18 ms to 30 min. Good agreement is also achieved with measurements of the nitrogen per arom atic site in partially devolatilized chars. The modified nitrogen model is the first to describe the rank dependence of nitrogen release as light gas without the use of correlations. The model is also the first to offer reaso nable explanations for the observed release of ring nitrogen at relatively low temperatures and the inherent stability of much of the char nitrogen du ring pyrolysis.