Patent ID: 6325985
Filing Date: 2001-12-04
Classification: B01D,F23C,F23J

Abstract:
A method for reducing NOx emissions in circulating fluidized bed reactors used for combustion of fuel containing large amounts of volatile combustible components, at atmospheric pressure, the circulating fluidized bed reactor including a combustion chamber, having a fluidized bed of particles therein, a particle separator section connected to a flue gas outlet in the combustion chamber and including at least one consecutive particle separator, connected one after the other and each particle separator having an inlet for flue gases and an outlet for cleaned flue gases, a clean gas duct connected to the cleaned flue gas outlet of the last particle separator in the particle separator section, and a convection section connected to the clean gas duct, the convection section having a cleaned flue gas inlet connected to the clean gas duct, for connecting the convection section to the last particle separator, cooling surfaces arranged in a cooling stage in the convection section, and an outlet for cooled flue gases, the method including the following consecutive steps:introducing fuel into the combustion chamber;combusting the fuel at 700 to 1000.degree. C. and transporting gases generated during the combustion process and solid particles as a fast fluidized bed into the upper part of the combustion chamber;discharging flue gases, which contain solid particles, from the combustion chamber into the particle separator section;separating solid particles from the flue gases in the particle separator section;introducing cleaned flue gases from the last particle separator into the clean gas duct;transporting cleaned flue gases through the clean gas duct into the cooling stage of the convection section;cooling the flue gases in the cooling stage;discharging cooled flue gases through the cooled flue gas outlet from the convection section: andinjecting ammonia or its precursor into the flue gases, for reducing NOx therein, wherein the ammonia or its precursor is injected (i) at an injection point into the cleaned flue gases in the clean gas duct connected to the cleaned flue gas outlet of the last particle separator, and (ii) into a constricted portion of the clean gas duct, in which constricted portion, the cross-sectional area A.sub.1 of the duct is one third of the mean cross-sectional area A.sub.2 of the duct.