Low nitrogen oxides emissions using three stages of fuel oxidation and in-situ furnace flue gas recirculation

A method for reducing NOx emissions from the combustion of carbonaceous fuels using three stages of oxidation and second stage in-situ furnace flue gas recirculation. In the first stage, a partial oxidation combustor is used to partially combust the fuel in the presence of preheated combustion air. The fuel gas produced in the partial oxidation process is passed to a second stage partial oxidation combustor while molten slag is removed and disposed of. Preheated combustion air also is introduced into the second stage of combustion to produce a slightly reducing flue gas and is injected into the furnace in such a way as to create the desired in-situ furnace flue gas recirculation. In the upper part of the furnace a third combustion air is mixed with the flue gas in a third stage of combustion to substantially complete the combustion process. Preheated steam may be added to the first or second stages of combustion. The stoichiometric ratios at each stage of combustion are controlled to minimize overall NOx emissions to acceptable levels. Further, an existing excess air cyclone-fired boiler furnace may be modified to operate in a three-stage combustion mode. This is accomplished by adding a preheated air system that includes two added air injection points into the furnace, with air introduction at a cyclone barrel re-entrant throat that is located at the interface between the cyclone barrel and the furnace, followed by overfire air introduction into the upper part of the furnace.