Influence of potassium loading at different reaction temperatures on the NOx reduction process by potassium-containing coal pellets☆

The activity of potassium-containing coal pellets and the corresponding free-metal char for NOx reduction in an oxygen-rich environment has been investigated by temperature-programmed reactions (TPRs) up to 750 °C, 2 h isothermal reactions in the range of 250–475 °C and lifetime tests, (until the samples were completely consumed), for selected samples and temperatures. An interesting ‘reactivity window’, where NOx reduction is observed, but carbon conversion is negligible, was found from TPRs experiment for a high potassium content sample, at moderate temperatures. This interval was not observed for the char. The catalytic effect of potassium is more dramatic at high temperatures, therefore, metal loading and reaction temperature are very much interrelated. Lifetime tests provide a very valuable information (average selectivity, profitable use of samples for NOx reduction, etc.), allowing us to check the whole efficiency of the samples. The progressive addition of potassium to the pellets makes samples more effective in terms of: (i) capacity to reduce higher NOx amounts; (ii) maximum NOx conversion values and (iii) higher values of average selectivity. In general, the samples studied, exhibit a maximum temperature, very much dependent on their potassium contents, that must not be exceeded with a view to practical applications.