Method of reducing the concentration of nitrogen oxides in a gaseous effluent from a thermal plant

Abstract

A method of reducing the concentration of nitrogen oxides in a gaseous effluent from a thermal plant fitted with burners, consisting in the burning of the fuel in two stages under the conditions of oxygen deficiency in a high-temperature zone, the two-stage combustion being accomplished by redistributing the flows of fuel and air inside the burners so that some air mixed with fuel is introduced in a spiral flow at periphery of the passages through the burners, and the rest of the air required for complete combustion of the fuel being introduced in a straight-line flow directed along the axis of the burners.

Description

The present invention relates to the protection of the atmosphere by products of combustion of a natural fuel, predominantly a gaseous one, and more specifically to a method of reducing the concentration of nitrogen oxides in a gaseous effluent from a thermal plant.

Known in the art is a method of reducing the concentration of nitrogen oxides in flue gases of boilers by burning the fuel in two stages. This method consists in that some of the air required for burning the fuel is introduced through burners and the balance required to obtain a complete combustion of the fuel is admitted through special inlets (nozzles) outside the burners, the inlets being arranged as a rule above the top row of burners.

It is known that nitrogen oxides are apt to form in a high-temperature zone and that their concentration increases with the amount of free oxygen present in the high-temperature zone. The known method of burning fuel in two stages prevents the presence of surplus oxygen in the high-temperature zone because the combustion of the fuel at the first stage, which is the high-temperature one, takes place under conditions with a deficiency in the oxidizer.

In the known method, which employs partial admission of air through special inlets outside the burners, a number of disadvantages is encountered such as:

THE NECESSITY IN ADDITIONAL BRANCHING OF WATER TUBES TO GIVE ROOM FOR NOZZLES; THIS REDUCES THE AREA OF HEAT TRANSFER, INVOLVES EXTRA COST AND IMPAIRS THE INTEGRITY OF THE BRICKWORK;

THE NECESSITY IN ADDITIONAL LINES WHICH MUST BE MADE AND INSTALLED IN ORDER TO ADMIT AIR INTO THE NOZZLES;

DIFFICULTIES EXPERIENCED IN CONTROLLING THE REQUISITE RELATIONSHIP BETWEEN THE RATES OF AIR FLOW TO THE BURNERS AND THE NOZZLES; THIS MAKES THE PLANT DIFFICULT TO OPERATE; AND

INABILITY TO CONTROL THE TEMPERATURE OF THE FLAME FROM THE BURNERS SO AS TO MAINTAIN THE TEMPERATURE SPECIFIED FOR SUPERHEATING THE STEAM.

It is the object of the present invention to provide a method of reducing the concentration of nitrogen oxides in the gaseous effluent of thermal plants which eliminates the need for special inlets (nozzles), the additional branching of water tubes, and keeps intact the brickwork of the thermal plant while reducing the concentration of the nitrogen oxides in gaseous effluent.

This object is attained in that a two-stage combustion of the fuel is employed under the conditions of oxygen deficiency in a high-temperature zone, the two-stage combustion being accomplished by redistributing the flow of the fuel and the air inside the burner or burners so that some air mixed with fuel is introduced in a spiral flow at the periphery of a passage through the burner, and the rest of the air, required for complete combustion of the fuel, is introduced in a straight-line flow directed along the axis of the burner.

The inventive method of reducing the concentration of nitrogen oxides is accomplished by directly redistributing the fuel and air flow directly so that the nitrogen oxide content of gaseous effluent is reduced by 30 to 35% without installing special nozzles, disturbing the brickwork of the thermal plant or branching of water tubes. The method also allows to control the parameters of the flame over a wide range, under variable conditions so, that the fuel is utilized in a more effective way and thus a higher operational reliability of the plant is achieved.

The disclosed method has been tested in a laboratory of the Institute of Gas, Ukrainian Academy of Sciences, and in industrial plants. The tests have proved the efficiency of the invention in burning fuel in thermal plants of various designs.

The present invention will be best understood from the following detailed description of a preferred, exemplary embodiment when read in conjunction with the accompanying drawings, in which

FIG. 1 is a schematic longitudinal section of a burner for carrying out the inventive method; and

FIG. 2 is an end view of FIG. 1 in the direction of arrow I.

The method of reducing the concentration of nitrogen oxides in the gaseous effluent from a thermal plant equipped with burners consists in a two-stage combustion of the fuel under the conditions of oxygen deficiency in a high-temperature zone at the first stage. The two-stage combustion is accomplished by redistributing the flow of fuel and air inside the burners so that some of the air mixed with the fuel is introduced in a spiral flow at the periphery of a passage through the burner, and the rest of the air, required for complete combustion of the fuel, is introduced in a straight-line flow directed along the axis of the burner.

The method is performed with the aid of a burner comprising a cylindrical part 1 (FIG. 1) and a tapered part 2. Welded to the cylindrical part 1 there is an annular latter. The gas chamber 3 and the cylindrical part 1 are provided with holes 4 drilled peripherally in preferably two rows so as to admit gaseous fuel from the gas chamber 3 into the inner passage of the cylindrical part 1 of. Welded to the cylindrical part 1 (FIGS. 1 and 2) of the burner there is also a tangential branch 5 serving to admit some of the air supply, the direction of flow being indicated by an arrow. Disposed inside the burner, concentrically with its cylindrical part 1 (FIG. 1), is a cylindrical tube 6 through which the rest of the air, required for the complete combustion of the fuel at the second stage of combustion, is introduced in a straight-line flow along the axis of the burner.

The tapered part 2 of the burner forms a port in a brick-work 7 of the thermal plant (not shown), the port widening towards a fire chamber.

The inventive method of reducing the concentration of nitrogen oxides in the gaseous effluent of the thermal plant is accomplished as follows. Only part of the air required for the combustion of the gaseous fuel is introduced through the tangential branch 5 (FIGS. 1 and 2). The flow of this air spirals around the periphery of the passage through the cylindrical part 1 of the burner.

The gaseous fuel is introduced from the gas chamber 3 through the holes 4 into the spiral flow of some of the air required for combustion. The gaseous fuel mixes with the flow of air spiralling around the periphery. The gas/air mixture (all the fuel and the peripherally spiralling flow of air), being imparted a spiral motion, spreads out into the periphery of the tapered part 2 of the burner and thence to the periphery of the port where partial combustion of the fuel takes place in a cone-shaped flame expanding towards the fire chamber, this place being where a high-temperature zone is formed at the first stage of combustion.

The axial space in the port (formed by the tapered part 2) is not filled with the flame, and introduced into this space is the rest of the air (roughly between 30 and 60% of the total amount required for the complete combustion of the fuel), as it is issued from the axial tube 6 in a straight-line flow. The sucking action of the axial cocurrent flow of air causes the products of incomplete combustion to move inwardly, towards the axis, at a point away from the burner port, where the centrifugal force tending to separate the peripheral flow from the axial one is low enough not to counteract this sucking action.

This is why the rest of the fuel burns up in a straight-line axial flow at a distance from the burner and the first, or high-temperature, zone of combustion, and the second stage of combustion is located at this distance .

Claims

What is claimed is:

1. A method of reducing the concentration of nitrogen oxides in a gaseous effluent of the combustion in a thermal plant fitted with burners having axial passages, the method comprising the steps of burning a fuel in two stages within the same heat-receiving chamber of the thermal plant, under the conditions of oxygen deficiency in a high-temperature zone, the two-stage combustion being accomplished by redistributing the flow of the fuel and air inside the burners so that part of the air is introduced in a first, spiral flow at the periphery of the passages of the burners, and introducing the rest of the air, required for complete combustion of the fuel, in a second, straight-line flow directed along the axis of the burners, all of the fuel being introduced in a peripheral zone surrounding the axial passages, together with the first air flow, with which all fuel is intermixed, thereby causing a cone-shaped flame to emerge from the peripheral zone, constituting a first, high-temperature combustion stage, while the second air flow contributes to a second, lower-temperature combustion stage axially located with respect to but spaced at a distance from the first stage.

2. The method as defined in claim 1, wherein the amount of air allotted to the first, spiral flow is between 40 to 70 per cent of the total air supply while the amount of air constituting the rest, for the second, straight-line flow is 60 to 30 per cent of the supply.