A method for the reduction of nitrogen oxides and carbon monoxide in furnace chambers of water boilers and steam boilers, especially grate boilers, using injection of process gas and reactant into the furnace chamber (4,10) through a system of injection lances (6,7), wherein process gas, the process gas being air, flue gas or a flue gas-air mixture in a proportion from 0 to 100%, and reactant are injected through process gas lances and reactant lances into the furnace chamber in a direction opposite to the main, natural direction of the flue gas flow through the furnace chamber, preferably top to bottom, with a stream deviation of +/-15° in a plane parallel to the longitudinal symmetry plane of the furnace chamber (4,10), that is, in the direction of temperature increase in the furnace chamber, wherein process gas lances and reactant lances are placed on at least one wall of the furnace chamber (4,10) at 0.5 of the depth of the furnace chamber (4,10) from the axis of the front panel wall tubes, wherein for example process gas is injected from an upper, side or front wall of the furnace chamber, wherein process gas is injected through process gas lances from a back wall of the furnace chamber for additional vortex intensification, and is supplied in a horizontal direction, while maintaining a stream deviation of +/-45°, in the plane parallel to the longitudinal symmetry plane of the furnace chamber, the process gas being supplied to the combustion chamber at a speed of 30 to 180 m/s, preferably 135 m/s, in an amount of up to 20% of the air stream necessary to completely burn the fuel while maintaining a minimum Received at EPO via Web-Form on Oct 17, 2021 proportion of oxygen in the flue gas, wherein the injection points of the process gas lances from the back wall of the furnace chamber (4,10) are located at a distance of up to 0.2 of the depth of the furnace chamber from the axis of the back panel wall tubes of the furnace chamber, wherein moreover, the reactant is supplied into the combustion chamber at a speed of 30 to 180 m/s, preferably 135 m/s, wherein process gas and reactant are injected so as to cause an internal recirculation of flue gas in the interior of the furnace chamber (4,10), said internal recirculation being created by a reverse vortex in the plane parallel to the longitudinal symmetry plane of the furnace chamber, the injected process gas and reactant sucking in flue gas from a flue gas stream flowing along the back wall from an area just before a flue gas exit of the furnace chamber (4,10) at an upper end of the back wall, and the mixture of reactant, process gas and flue gas moving along the front wall of the furnace chamber in a direction of temperature increase towards a combustion area of the furnace chamber (4,10). wherein process gas lances and reactant lances are placed on at least one wall of the furnace chamber (4,10) at 0.5 of the depth of the furnace chamber (4,10) from the axis of the front panel wall tubes, wherein for example process gas is injected from an upper, side or front wall of the furnace chamber, wherein process gas is injected through process gas lances from a back wall of the furnace chamber for additional vortex intensification, and is supplied in a horizontal direction, while maintaining a stream deviation of +/-45°, in the plane parallel to the longitudinal symmetry plane of the furnace chamber, the process gas being supplied to the combustion chamber at a speed of 30 to 180 m/s, preferably 135 m/s, in an amount of up to 20% of the air stream necessary to completely burn the fuel while maintaining a minimum Received at EPO via Web-Form on Oct 17, 2021 proportion of oxygen in the flue gas, wherein the injection points of the process gas lances from the back wall of the furnace chamber (4,10) are located at a distance of up to 0.2 of the depth of the furnace chamber from the axis of the back panel wall tubes of the furnace chamber, wherein moreover, the reactant is supplied into the combustion chamber at a speed of 30 to 180 m/s, preferably 135 m/s, wherein process gas and reactant are injected so as to cause an internal recirculation of flue gas in the interior of the furnace chamber (4,10), said internal recirculation being created by a reverse vortex in the plane parallel to the longitudinal symmetry plane of the furnace chamber, the injected process gas and reactant sucking in flue gas from a flue gas stream flowing along the back wall from an area just before a flue gas exit of the furnace chamber (4,10) at an upper end of the back wall, and the mixture of reactant, process gas and flue gas moving along the front wall of the furnace chamber in a direction of temperature increase towards a combustion area of the furnace chamber (4,10).