Experimental study on combustion and NO formation characteristics of semi-coke

Abstract

Pyrolysis condition is vital to physicochemical properties of semi-coke, influencing the subsequent combustion and NO formation. In this study, the combustion behaviors and NO formation characteristics of semi-cokes obtained from various pyrolysis conditions in a fixed-bed reactor were investigated by thermogravimetric analysis and drop tube furnace experiments. Experimental results indicated that the combustion reactivity of semi-coke lowered as pyrolysis temperature, residence time increased and coal particle size decreased. Addition of CO2 or O2 in pyrolysis atmosphere improved the reactivity. NO conversion ratio of semi-coke decreased initially and increased afterwards with the elevation of pyrolysis temperature with a turning point at 800 °C, arose from the fact that higher surface area was helpful for NO reduction during combustion. For mild pyrolysis semi-coke, a critical particle size range was found corresponding to the lowest NO conversion ratio, and it reduced with increasing residence time. O2 addition in pyrolysis atmosphere significantly lowered the NO conversion ratio and burnout ratio, because some combustibles in coal had been oxidized during the foregoing pyrolysis. CO2 addition in pyrolysis atmosphere exhibited relatively slight effect on the burnout or NO conversion, since the gasification between CO2 and char was weak at this temperature. Quantitative relationship was established to reflect major factors affecting NO formation, and the surface area was observed to have the most prominent influence.