A boiler having a system to reduce nitrogen oxides (NOx) in flue gas, the boiler comprising: - a combustion chamber; - a passage for flue gas generated in the combustion chamber extending through the boiler and through which the flue gas generated in the combustion chamber may flow; - a source of NOx reducing reagent; - a selective non catalytic reduction (SNCR) system including reagent injection nozzles configured to inject a NOx reducing reagent from said source of NOx reducing reagent into the flue gas, wherein the reagent injection nozzles are mounted to the passage; - one or more convective pass pendant heat exchangers present in the passage downstream of the SNCR system, and through which the flue gas may pass; - a source of pressurized steam; - a mixing device in which steam from the source of pressurized steam and NOx reducing reagent from the source of NOx reducing reagent may mix; - an array of steam injection nozzles mounted on at least one wall of the passage downstream of the SNCR system and downstream of the one or more convective pass pendant heat exchangers, said steam injection nozzles from said array of steam injection nozzles being in fluid communication with the mixing device so as to inject a mixture of steam and the NOx reducing reagent from the mixing device into the flue gas in the passage, wherein the boiler having a system to reduce NOx in flue gas is configured so that the array of steam injection nozzles is arranged to inject the mixture of steam and NOx reducing reagent from the mixing device in the passage at an injection velocity having a Mach number of at least 0.5. - a selective catalytic reduction system downstream of said array of steam injection nozzles which includes a support structure for a catalyst, wherein the support structure is arranged in the passage in the flow path of the flue gas. - a combustion chamber; - a passage for flue gas generated in the combustion chamber extending through the boiler and through which the flue gas generated in the combustion chamber may flow; - a source of NOx reducing reagent; - a selective non catalytic reduction (SNCR) system including reagent injection nozzles configured to inject a NOx reducing reagent from said source of NOx reducing reagent into the flue gas, wherein the reagent injection nozzles are mounted to the passage; - one or more convective pass pendant heat exchangers present in the passage downstream of the SNCR system, and through which the flue gas may pass; - a source of pressurized steam; - a mixing device in which steam from the source of pressurized steam and NOx reducing reagent from the source of NOx reducing reagent may mix; - an array of steam injection nozzles mounted on at least one wall of the passage downstream of the SNCR system and downstream of the one or more convective pass pendant heat exchangers, said steam injection nozzles from said array of steam injection nozzles being in fluid communication with the mixing device so as to inject a mixture of steam and the NOx reducing reagent from the mixing device into the flue gas in the passage, wherein the boiler having a system to reduce NOx in flue gas is configured so that the array of steam injection nozzles is arranged to inject the mixture of steam and NOx reducing reagent from the mixing device in the passage at an injection velocity having a Mach number of at least 0.5. - a selective catalytic reduction system downstream of said array of steam injection nozzles which includes a support structure for a catalyst, wherein the support structure is arranged in the passage in the flow path of the flue gas.