FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 6. Predicting the Evolution of Fuel Nitrogen from Various Coals

Abstract

The release of fuel nitrogen during the primary devolatilization of any bituminous coal involves only two mechanisms, the shuttling of nitrogen as an element in tar molecules and the conversion of char-N into HCN. This modeling study characterizes these processes for heating rates from 5 to 10 4 K/s, temperatures to 1550 K, and pressures from vacuum to atmospheric. Evaluations against a database compiled from the behavior of 40 coals of rank from lignite through lowvolatile bituminous demonstrates that predicted evolution histories of tar-N, HCN, and char-N and the nitrogen contents of tar are within experimental uncertainty throughout this domain. Under conditions of rapid heating, tar shuttling is the only mechanism for nitrogen release as long as tar is being expelled, but for slower heating conditions it is overlapped by HCN production from char-N. With FLASHCHAIN, no additional parameters or rate expressions are required to quantitatively predict the contributions from tar shuttling from any bituminous coal at any operating conditions, including the nitrogen contents of tar. The data evaluations also show that tar shuttling imparts the familiar dependences of tar evolution on coal rank, heating rate, and pressure onto the evolution of the nitrogen species distributions. Conversion of char-N into HCN is analogous to the release of H 2 during the graphitization of char at high temperatures. Only one reaction rate expression is needed to predict the yields and evolution rates of HCN during the primary devolatilization of any coal, provided that it is based on an extremely broad distribution of activation energies. The nominal rates of HCN production must also be reduced for coals of progressively higher rank in proportion to their O/N ratios