A method for the reduction of nitrogen oxides and carbon monoxide in the furnace chambers of water and steam boilers, especially grate boilers, wherein a process gas being air, flue gas or or a mixture thereof, optionally mixed with a reagent is injected into the furnace chamber through process gas injection lances installed on the front wall of the furnace chamber on one to three levels in the opposite direction to the main, natural direction of the flue gas flow through the furnace chamber, preferably from top to bottom, with a deviation of +/- 15°, the angle being measured between the centre line of the jet and the front wall of the furnace chamber, in a plane parallel to the longitudinal plane of symmetry of the furnace chamber, i.e. in the direction of the temperature rise in the furnace chamber, wherein part of the-stream, up to 20%, injected by the process gas injection lances installed on the front wall of the furnace chamber is injected perpendicularly to the front wall with a deviation of +/- 15°, wherein process gas injection-from process gas injection lances installed on the rear wall of the furnace chamber takes place towards the front wall at an angle of 30° to 60°, and this angle is measured between the stream centre line and the rear wall of the furnace chamber, in the plane parallel to the longitudinal symmetry plane of the furnace chamber, wherein process gas injection from process gas injection lances installed on the upper wall of the furnace chamber occurs on one to four levels and its direction is opposite to the main direction of the flue gas flow in the furnace chamber with a deviation of the jet of +/-60°, the angle being measured between the centre line of the jet and the symmetry axis of the injection lance, which is perpendicular to the grate deck, in a plane parallel to the longitudinal symmetry plane of the furnace chamber, and this deviation, as well as the number of injection points, varies according to the boiler load; wherein process gas injection from process gas injection lances installed on the side walls of the furnace chamber occurs on one to three levels, and the direction of injection is perpendicular to the side walls, i.e. it is perpendicular to the main direction of flue gas flow in the furnace chamber, with a deviation of +/-20°, which means that it goes inside a down swirl; wherein process gas is supplied to the furnace chamber at a speed of 30 to 180 m/s, preferably 135 m/s, in the amount of up to 20% of stoichiometric air demand necessary to burn the fuel supplied to the boiler, preferably 15%, with up to 60% of the process gas being injected through the process gas injection lances installed on the front wall of the furnace chamber, with up to 60% of the process gas being injected through the process gas injection lances installed on the rear wall of the furnace chamber, with up to 25% of the process gas is injected through the process gas injection lances installed on the upper wall of the furnace chamber and with up to 15% of the process gas is injected through the process gas injection lances installed on the side wall of the furnace chamber; wherein reagent is injected to the furnace chamber at a rate of 30 to 180 m/s, preferably 135 m/s, wherein the reagent is injected through injection points located on the front wall of the furnace chamber, on the upper wall of the furnace chamber and on the side walls of the furnace chamber, wherein the number of reagent injection points may be equal to or different from the number of process gas injection points; wherein the injection points on the front wall of the furnace chamber, are up to 0.5 depth of the furnace chamber from the axis of the front wall tubes, preferably 0.1 and at a height from 0.2 to 0.8 of the furnace chamber above the grate deck, preferably 0.5; wherein the injection points on the upper wall of the furnace chamber are at a distance of 0.05 to 0.3 of the furnace chamber depth from the tube axis of the rear wall, preferably 0.15, and at a height of 0.5 to 1.5 of the height of the festoon from the tube axis of the upper wall, preferably 0.9 of the festoon height, wherein the injection points on the side walls of the furnace chamber are located at a distance of 0.05 to 0.5 of the furnace chamber depth from the axis of the tubes of the rear wall and are located on one to three levels, preferably on three levels, with each level located at equal or different distances from the rear wall, preferably, the lowest level in the furnace chamber being located farthest from the rear wall and the injection points are located at a height of 0.1 to 0.7 of the furnace chamber height above the grate deck, and wherein the injection points on the rear wall of the furnace chamber, are located at the bottom edge of the boiler festoon, i.e. at the height of the end of the rear wall with a possible shift in both directions up to 0.6 of the festoon height.