The formation of nitrogen oxides in a pulverised coal boiler

Abstract

The oxides of nitrogen (NOx) are airborne pollutants that result from the combustion of pulverised coal. The aim of this project is to identify operational methods that reduce the NOx emissions from a coal fired boiler whilst maintaining satisfactory performance. This project describes important combustion properties and the processes occurring during the combustion of pulverised coal. Detail is provided on the pulverisation plant, draught plant and steam system of a large utility boiler. The dominant NOx formation mechanisms in coal fired boilers are discussed and NOx reduction strategies applicable to these boilers are described. Engineering models are developed to describe initial flame temperature, furnace residence times, furnace heat pickup and thermal NOx formation. A series of tests were designed and undertaken to measure and assess the effect on NOx formation and boiler performance to variations in:- - The distribution of secondary air to each windbox, and - The level of excess oxygen measure at the boiler exit. During each test the following was undertaken:- - Detailed temperature survey of the furnace region - Coal sampling for laboratory analysis - Fly Ash sampling to determine loss of ignition - Logs of relevant data to determine plant performance. Difficulties and shortcomings regarding the predictive models are discussed and the performance of the boiler under each test is compared.