Method of reducing nox-content in flue gas during heating of coking oven

A method of reducing NO.sub.x -content in a flue gas during heating of a coking oven having a plurality of heating trains cooperating in pairs with a flue gas return passage in each of said pairs at the height of the heating train sole defining a lower combustion stage and an upper combustion stage arranged the lower combustion stage, said method comprising the steps of:

• adjusting a circulating stream rate defined by a volume stream of a returned flue gas divided by a flue gas volume stream without returned flue gas to between 20% and 50%;
• maintaining a stage ratio for a combustion stage number equal at least to 2 defined as an air volume stream of the lower combustion stage divided by a total air volume stream, equal to between 80/I% and 140/I% wherein I is the number of combustion stages;
• arranging the upper combustion stage at (45.+-.10%) .times.(I-1) of the heating train height;
• providing a plurality of poor gas supply points only at the height of the heating train sole for supplying poor gas only at the height of the heating train sole and providing a plurality of primary air supply points at the height of the heating train sole for admixing air to the poor gas;
• and
• providing a plurality of secondary air supply points at the level of the upper combustion stage for supplying only air to the upper combustion stage.