Nitric oxide reduction by chars derived from high ash inertinite-rich discard coals

Abstract

An investigation was performed to determine the kinetics of nitric oxide (NO) reduction by six different chars derived from inertinite-rich low grade coals. The experimentation involving the reduction of NO was confined to conditions used in fluidised bed combustion with aspirations to provide information for the fluidised bed technology using high ash and inertinite rich South African coals. A detailed characterisation of six different chars was executed with respect to physical structural, chemical, mineralogical and petrographic properties. The coal samples had ash content ranging from 37% to 48.7% (% by mass, dry basis) and the respective chars had 49% to 63.7% ash (dry basis). The organic constituents of the parent coals were inertinite-rich, with inertinite content ranging from 25% to 66% (% volume, mmb). The charring process, conducted at 900 C for 60 minutes, significantly shifted the total reflectance scans of the parent coals (1.09-1.46% Rsc) to higher values in the corresponding chars (4.84-5.45% Rsc). The chars were characterised by carbon forms which marked the macerals transformation during char formation. These char carbon forms constituted of dense chars, isotropic coke, char networks and partially reacted macerals. The reactivity experiments were carried out using 1 mm char particles in a thermogravimetric analyser (TGA) under isothermal conditions at temperatures in the range of 750 to 900 C at atmospheric pressure using varying dilute mixtures (1.25-5.0%) of nitric oxide (NO) in nitrogen (N2). The effects of temperature and gas composition on reactivity were established to conform to published trends. The kinetic parameters were determined by using the random pore model (RPM), the model was based on the random pore model without pore diffusion (chemical reaction controlled). Validation of the model against experimental results displayed a good correlation for all the six chars. Due to the complex nature of the model which consists of numerous unknown parameters, a step-wise regression was applied with success. Correlation of reactivity with coal attributes was undertaken but only the modified reactive maceral index (RMI*) gave a meaningful relation.