Models To Predict Kinetics of NOx Reduction by Chars as a Function of Coal Rank

Abstract

During coal combustion, NOₓ reduction occurs by two possible routes: homogeneous reduction by hydrocarbons and heterogeneous reduction by char formed during coal devolatilization. This paper investigates the latter route, which also has potential as the basis for post-combustion NOₓ clean-up processes, including reburning. The purpose of this investigation is to develop a kinetic model for the reduction of NOₓ by char during coal combustion and to understand the role of coal rank and char surface area on the resulting char reactivity. This investigation reports original kinetic data for nine char samples including graphite, coconut char, and five coal chars ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3). An empirical kinetic model with six universal (not char-specific) parameters reproduces the experimental data for all chars. The investigation also presents an alternative and simpler model with only two parameters that differ for each char type. Correlations for the two model parameters were then developed as a function of two char surface areas: (1) active mineral matter surface area measured using CO₂ titration after high-temperature exposure and (2) total sample surface area measured using CO₂ at room temperature and Dubinin–Polanyi theory. Predictions of the rate constant values over a wide range of temperature using this universal approach with only the surface areas differing among the six chars generally fit the experimental data within ±50%.