Installation of selective, non-catalytic reduction of nitrogen oxides in grate boilers, equipped with a tank of aqueous reactant solution containing urea or ammonia, lines suppling reactant into the lances, water lines providing water for diluting the reactant solution, air lines for providing air for injecting the reactant solution, electrovalves, lances with nozzles, nitrogen oxide concentration sensors, temperature sensors, and an optical flame detector, characterized in that built-in in both side walls of the furnace chamber of the grate boiler lances with injection nozzles are located on each of these walls in the area of a right-angled triangle, one leg of said triangle is vertical and near the back wall of the furnace chamber and the other leg is horizontal and connects to the upper end of the first leg and runs towards the front wall of the furnace chamber and are placed in the holes in the fins between the screen tubes, so that the major part of the lance with injection nozzles extends from the wall of the grate boiler from its outside and only unsymmetrical two-hole injection nozzles, three-hole symmetric, single-hole injection with hole centrally located are introduced into the furnace chamber of this boiler, wherein lances with injection nozzles are placed on the walls of the furnace chamber of the grate boiler in horizontal rows, which, depending on boiler dimensions and power, as well as injection parameters, ranges from 1 to 10 rows, and each of these rows has less than 15 lances with injection nozzles, while the number of lances with injection nozzles in the lowest row is from 1 to 3 and in each row from the lowest to the highest row, the number of lances with injection nozzles increases by 1-2 lances, wherein in the lowest row lances with injection nozzles have asymmetrical two-hole injection nozzles or single-hole nozzles with center hole and lances with injection nozzles in the highest row there have two-hole unsymmetrical injection nozzles and / or symmetrical three-hole nozzles, while lances with injection nozzles between the highest and the lowest row have two-hole unsymmetrical injection nozzles and / or three-hole symmetric injection nozzles and / or single-hole nozzles with a centrally located hole. a tank of aqueous reactant solution containing urea or ammonia, lines suppling reactant into the lances, water lines providing water for diluting the reactant solution, air lines for providing air for injecting the reactant solution, electrovalves, lances with nozzles, nitrogen oxide concentration sensors, temperature sensors, and an optical flame detector, characterized in that are located on each of these walls in the area of a right-angled triangle, one leg of said triangle is vertical and near the back wall of the furnace chamber and the other leg is horizontal and connects to the upper end of the first leg and runs towards the front wall of the furnace chamber and are placed in the holes in the fins between the screen tubes, so that the major part of the lance with injection nozzles extends from the wall of the grate boiler from its outside and only unsymmetrical two-hole injection nozzles, three-hole symmetric, single-hole injection with hole centrally located are introduced into the furnace chamber of this boiler,