Method and system for reducing nitrogen oxide and sulfur oxide emissions from carbonaceous fuel combustion flue gases

A method for the reduction of nitrogen oxide and sulfur oxide emissions from carbonaceous fuel combustion flue gases, comprising the steps of:

• (a) injecting a coal water slurry as a reburn fuel into a combustion furnace at a point where the temperature of the flue gases from a carbonaceous fuel burner is in a range from greater than about 2700.degree. F. to about 3200.degree. F., the flue gases including oxides of sulfur and nitrogen, the fuel burner operating under excess air combustion conditions and being fired at a rate to provide for about 70 to about 90 percent of the heat input into the furnace, the coal water slurry comprising about 40 to about 65 weight percent coal being injected as a spray at a rate of about 10 to about 30 percent of the heat input into the furnace to provide a stoichiometric air condition of about 70 to about 90 percent of that needed for the complete combustion of all fuel in the furnace;
• (b) creating a reducing reburn zone in the furnace where the coal water slurry is injected to form reducing gas species that will react with the oxides of nitrogen to form diatomic nitrogen, thereby reducing the concentration of nitrogen oxides in the flue gases in an amount from about 30 to about 80 percent;
• (c) adding an alkali compound to the reducing reburn zone in the furnace at a rate to provide for an alkali to sulfur mole ratio of about 0.5 to about 3, the alkali compound reacting with the oxides of sulfur to form solid reaction products;
• and
• (d) introducing overfire air into the furnace to bring the total stoichiometric air addition up to a level of about 110 to about 130 percent of that required to combust all of the fuel in the furnace, thereby providing for the complete combustion of reburn fuel gases created by the injection of the coal water slurry into the furnace, wherein the overfire air is introduced into the furnace at a point such that the flue gases have a residence time within the reburn zone of about 0.2 to about 1.5 seconds prior to being contacted with the overfire air.

A system for the reduction of nitrogen oxide emissions from carbonaceous fuel combustion flue gases, comprising:

• (a) means for injecting a coal water slurry as a reburn fuel into a combustion furnace at a point where the temperature of the flue gases from a carbonaceous fuel burner is in a range from greater than about 2700.degree. F. to about 3200.degree. F., the flue gases including oxides of nitrogen, the fuel burner operating under excess air combustion conditions and being fired at a rate to provide for about 70 to about 90 percent of the heat input into the furnace, the coal water slurry comprising about 40 to about 65 weight percent coal being injected as a spray at a rate of about 10 to about 30 percent of the heat input into the furnace to provide a stoichiometric air condition of about 70 to about 90 percent of that needed for the complete combustion of all fuel in the furnace, whereby a reducing reburn zone is created in the furnace where the coal water slurry is injected to form reducing gas species that will react with the oxides of nitrogen to form diatomic nitrogen and reduce the concentration of nitrogen oxides in the flue gases;
• and
• (b) means for introducing overfire air into the furnace to bring the total stoichiometric air addition up to a level of about 110 to about 130 percent of that required to combust all of the fuel in the furnace, thereby providing for the complete combustion of reburn fuel gases created by the injection of the coal water slurry into the furnace, wherein the overfire air is introduced into the furnace at a point such that the flue gases have a residence time within the reburn zone of about 0.2 to about 1.5 seconds prior to being contacted with the overfire air.

A system for the reduction of nitrogen oxide and sulfur oxide emissions from carbonaceous fuel combustion flue gases, comprising:

• (a) means for injecting a coal water slurry as a reburn fuel into a combustion furnace at a point where the temperature of the flue gases from a carbonaceous fuel burner is in a range from greater than about 2700.degree. F. to about 3200.degree. F., the flue gases including oxides of sulfur and nitrogen, the fuel burner operating under excess air combustion conditions and being fired at a rate to provide for about 70 to about 90 percent of the heat input into the furnace, the coal water slurry comprising about 40 to about 65 weight percent coal being injected as a spray at a rate of about 10 to about 30 percent of the heat input into the furnace to provide a stoichiometric air condition of about 70 to about 90 percent of that needed for the complete combustion of all fuel in the furnace, whereby a reducing reburn zone is created in the furnace where the coal water slurry is injected to form reducing gas species that will react with the oxides of nitrogen to form diatomic nitrogen and reduce the concentration of nitrogen oxides in the flue gases in an amount from about 30 to about 80 percent;
• (b) means for adding an alkali compound to the reducing reburn zone in the furnace at a rate to provide for an alkali to sulfur mole ratio of about 0.5 to about 3, the alkali compound reacting with the oxides of sulfur to form solid reaction products;
• and
• (c) means for introducing overfire air into the furnace to bring the total stoichiometric air addition up to a level of about 110 to about 130 percent of that required to combust all of the fuel in the furnace, thereby providing for the complete combustion of reburn fuel gases created by the injection of the coal water slurry into the furnace, wherein the overfire air is introduced into the furnace at a point such that the flue gases have a residence time within the reburn zone of about 0.2 to about 1.5 seconds prior to being contacted with the overfire air.

A method for the reduction of nitrogen oxide emissions from carbonaceous fuel combustion flue gases, comprising the steps of:

• (a) injecting a coal water slurry as a reburn fuel with an atomizing media through a dual-fluid atomizer nozzle into a combustion furnace at a point where the temperature of the flue gases from a carbonaceous fuel burner is in a range from about 2400.degree. F. to about 3200.degree. F., the ratio of the atomizing media to the coal water slurry reburn fuel being about 0.05 to about 1 lb/lb, the flue gases including oxides of nitrogen and sulfur, the fuel burner operating under excess air combustion conditions and being fired at a rate to provide for about 70 to about 90 percent of the heat input into the furnace, the coal water slurry comprising about 40 to about 65 weight percent coal being injected as a spray at a rate of about 10 to about 30 percent of the heat input into the furnace to provide a stoichiometric air condition of about 70 to about 90 percent of that needed for the complete combustion of all fuel in the furnace;
• (b) creating a reducing reburn zone in the furnace where the coal water slurry is injected to form reducing gas species that will react with the oxides of nitrogen to form diatomic nitrogen, thereby reducing the concentration of nitrogen oxides in the flue gases;
• and
• (c) introducing overfire air into the furnace to bring the total stoichiometric air addition up to a level of about 110 to about 130 percent of that required to combust all of the fuel in the furnace, thereby providing for the complete combustion of reburn fuel gases created by the injection of the coal water slurry into the furnace, wherein the overfire air is introduced into the furnace at a point such that the flue gases have a residence time within the reburn zone of about 0.2 to about 1.5 seconds prior to being contacted with the overfire air.