The Effect of Coal Quality on NO x Emissions: Early Stages of Devolatilization

The devolatilization process of the fuel which occurs near the burner in pulverized coal-fired combustors, and its interaction with the flow, strongly influences both the nitric oxide formation and the stability of the flame. It also bears on the degree of char burnout, as the burning rate of the solids depends in part on the size of the char particle and the pore structure. All are affected by the changes which occur during the devolatilization process. Making use of a flat flame-turbulent-jet device (FFTJ), the coal devolatilization and nitrogen release behaviors during the first 10 ms of the process can be studied. In this technique the coal particles are heated at high heating rates to high temperatures in a turbulent flow regime. The processes and conditions of most practical applications are therefore well simulated. In the present work, eight coals were characterized with respect to their flame stability behavior and NOx formation potential, by means of the FFTJ apparatus. The study formed an integral part of a UK DTI-funded program entitled "The Effects of Coal Quality on NOx Emissions and Carbon Burnout in Pulverised Coal-Fired Utility Boilers." The results provided by the FFTJ experiments led to the correct forecast of the unexpected NOx behavior of certain of the coals when fired in a full scale boiler.