Staged Air Fuel Injection for NOx Reduction-- Modelling and Experiments in a 125MW Boiler

The Limhamn district heating plant consists of one 125 MW coal fired hot water boiler, which delivers heat to the city of Malmo. The boiler was built 1985, and is equipped with low NOx burners and an overfire system, OFA. In order to meet the more stringent environmental requirements of the next decade, a development project has been carried out. The main objective was to reduce the emissions of nitrogen oxides 20-40 %, and sulphur oxides 40 - 60 %.Experiments in the boiler and in an isothermal model, as well as calculations with a mathematical model were used to analyse the results, and to optimise the reburning installation. Model experiments agreed satisfactorily with baseline trials. The model experiments demonstrated that the size of the primary and reburning zones can be efficiently influenced by the direction of the additional air jets. Also their influence on the burnout could be shown, as well as the influence of the stoichiometry on the burnout. These findings were verified in the input/output experiments. The NOx reduction decreased linearly as the stoichiometry of both the primary zone and the reburning zone increased. The reduction increased with the relative reburn fuel input, within the range of available natural gas, but the ratio of increase decreased. The reduction of NOs was accompanied by an increase of CO in the waste gas from the boiler furnace, and also increased the carbon concentration in the flyash. The avalable residence times in the different zones were longer than times reported in literature. It can, therefore be suspected that the char was not completely burned out before the reburning zone. The NOx emission before the retrofit was 240 ppm at 3 per cent 02 in the waste gas. This figure decreased to 113 ppm att full load, -or 53 % reduction - and 118 ppm at 68 per cent load.