A global free radical mechanism for nitrogen release during coal devolatilization based on chemical structure.

摘要

As the pulverized coal combustion industry faces increasingly stringent NOx emission regulations, cost-effective low-NOx strategies like local fuel/air staging are important. Although most coal combustion NO x originates from nitrogen in the coal, the rate of nitrogen release from coal is currently treated empirically when modeling these low-NO x techniques. The objective of this research was to develop a model that relates nitrogen release from coal during devolatilization to the changes in the chemical structure of the char.; Thirty-four rapid pyrolysis tests were performed using coals from around the world ranging in rank from brown coal to low volatile bituminous. Trends in measured tar yields and total volatiles yields as a function of temperature between 900 and 1650 K were distinctly different for different ranks of coal.; Matched tar/char sets from both lignite and bituminous coals pyrolyzed at increasingly severe conditions were analyzed by solid-state 13C NMR. At about 1250 K, tars and chars showed evidence of both ring opening and cluster growth reactions, neither of which appeared to accelerate nitrogen release via ring rupture. Several 13C NMR chemical structural parameters from these experiments and from published studies showed a strong correlation with light gas nitrogen release, suggesting a mechanism other than the pure thermal decomposition seen in pyrolysis of model compounds (i.e. pyridine or pyrrole). Accordingly, a nitrogen model using a three-step free-radical global mechanism was developed to model light gas nitrogen release. This nitrogen model requires only a network devolatilization. model and coal-specific chemical structural input data to adequately predict the nitrogen distribution among pyrolysis products. Because the model is based on char chemical structure it is very robust, accurately describing nitrogen release characteristics even for conditions far different from those used in the model development. The 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 reasonable explanations for the observed release of ring nitrogen at relatively low temperatures and the inherent stability of much of the char nitrogen during pyrolysis; observations not easily explained by a simple thermal decomposition model alone.