Method for decreasing nitrogen oxides of a pulverized coal boiler using burners of internal combustion type

Abstract

A method for decreasing nitrogen oxides of a pulverized coal boiler using burners of internal combustion type including during the operation of the boiler, ignition sources in the burners of internal combustion type mounted on side walls of the boiler are always in a working state, and igniting the pulverized coal in the burners in advance; decreasing secondary air amount in a primary combustion zone of the boiler so that the primary combustion zone is in a relatively strong reducing atmosphere and an oxygen-deficient condition for inhibiting generation of NOx is created; and supplying remaining air from an upper part of a furnace of the boiler in a form of over-fire air, so that a deep air staging is carried out in the total furnace. Thus, the NOx generation of combustion can be effectively controlled on the premise of not decreasing efficiency of the boiler.

Claims

1. A method for decreasing nitrogen oxides of a pulverized coal boiler using internal combustion type burners, the method consisting of: keeping ignition sources in the internal combustion type burners mounted on side walls of the boiler in a working state when operating the boiler; igniting pulverized coal in the internal combustion type burners by using the ignition sources; spraying ignited pulverized coal from the internal combustion type burners, into a furnace of the boiler, deferring the time of mixing of the pulverized coal with secondary air, decreasing an amount of secondary air supplied into a primary combustion zone of the boiler, under the condition that the pulverized coal is already ignited when being sprayed from the internal combustion type burners, to form a reducing atmosphere in the primary combustion zone so that the pulverized coal is burnt in an oxygen-deficient state; and supplying over-fire air from an upper part of the furnace of the boiler into the furnace to form an oxidizing atmosphere so as to burn out the pulverized coal which is incompletely burnt in the primary combustion zone of the boiler, wherein only primary air supplies the oxygen amount necessary for the pulverized coal combustion in the burners, such that the excess air coefficient in the burners is lower than 0.4, wherein the burners are interiorly divided into several stages of combustion chambers, and a bent plate is provided at the elbow of the burner, dense/thin separation of the primary air and pulverized coal flow is generated at the bent plate, denser pulverized coal enters the central chamber of the burner, and the remaining thinner pulverized coal enters respective combustion chamber successively stage by stage, and an air flow of the pulverized coal is formed with denseness in the center and thinness in the surrounding in the radial direction of the burner, and the bent plate is arranged near the outside radius of the elbow as a single layer in radial direction of the burner.

2. A method for decreasing nitrogen oxides of a pulverized coal boiler using internal combustion type burners, the method consisting of: keeping ignition sources in the internal combustion type burners mounted on side walls of the boiler in a working state when operating the boiler; igniting pulverized coal in the internal combustion type burners by using the ignition sources; spraying ignited pulverized coal from the internal combustion type burners, into a furnace of the boiler, deferring the time of mixing of the pulverized coal with secondary air, decreasing an amount of secondary air supplied into a primary combustion zone of the boiler, under the condition that the pulverized coal is already ignited when being sprayed from the internal combustion type burners, to form a reducing atmosphere in the primary combustion zone so that the pulverized coal is burnt in an oxygen-deficient state; and supplying over-fire air from an upper part of the furnace of the boiler into the furnace to form an oxidizing atmosphere so as to burn out the pulverized coal which is incompletely burnt in the primary combustion zone of the boiler, adjusting an ignition intensity of the pulverized coal in the burner by changing an energy of the ignition source, which decreases an amount of generated nitrogen oxides, wherein only primary air supplies the oxygen amount necessary for the pulverized coal combustion in the burners, such that the excess air coefficient in the burners is lower than 0.4, wherein the burners are interiorly divided into several stages of combustion chambers, and a bent plate is provided at the elbow of the burner, dense/thin separation of the primary air and pulverized coal flow is generated at the bent plate, denser pulverized coal enters the central chamber of the burner, and the remaining thinner pulverized coal enters respective combustion chamber successively stage by stage, and an air flow of the pulverized coal is formed with denseness in the center and thinness in the surrounding in the radial direction of the burner, and the bent plate is arranged near the outside radius of the elbow as a single layer in radial direction of the burner.

3. The method of claim 1, wherein the burners are a plasma generator or an oil gun adapted as the ignition source; and the burners are straight flow burners or swirl burners; and the boiler is a tangentially-fired boiler or a wall-fired boiler.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of the structure of pulverized coal burner of internal combustion type in which a plasma generator is used as an ignition source according to the present invention.

(2) FIG. 2 is the left view of FIG. 1.

(3) FIG. 3 is a schematic view of a wall-fired pulverized coal boiler in which swirl burners of internal combustion type are applied according to the present invention.

(4) FIG. 4 is a schematic section view of the pulverized coal burner of FIG. 3.

(5) FIG. 5 is a schematic view of a tangentially-fired pulverized coal boiler in which straight flow burners of internal combustion type are applied according to the present invention.

(6) FIG. 6 is a schematic section view of the pulverized coal burner of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) The specific embodiments of the present invention will be described according to the following figures.

(8) FIG. 1 is a schematic view of the structure of a pulverized coal burner of internal combustion type in which a plasma generator is used as an ignition source according to the present invention. As shown in FIG. 1, the burner is divided interiorly into several stages, a bent plate 8 is provided at the elbow of the burner, dense/thin separation of the primary air and pulverized coal flow is generated at the bent plate 8 due to the different inertias between the pulverized coal and air. Denser pulverized coal enters the central chamber 5 of the burner, and the remaining thinner pulverized coal enters respective combustion chamber successively stage by stage. Then the pulverized coal is sprayed into the furnace from a primary air and pulverized coal nozzle 7 of the burner. The pulverized coal in the respective stages of the chambers of burner can be further concentrated through a pulverized coal concentrator 4, so that an air flow of the pulverized coal with denseness in the center and thinness in the surrounding in the radial direction of the burner 2. Thus, a deep fuel staging is formed in the burner 2. Firstly, the dense pulverized coal in the central chamber is fast ignited by the ignite device, and the emitted heat after firing ignites the remaining thinner pulverized coal in the burner stage by stage, so the deep fuel staging is achieved and the fuel is sprayed into the furnace for combustion at the same time.

(9) The plasma generator 1 generates a plasma arc with high temperature and high enthalpy value after starting, which acts on highly concentrated pulverized coal in the central chamber 5 of the burner, causing the pulverized coal particles to burst fast and release volatile constituents, and start to be ignited. A great amount of heat is released from the ignited pulverized coal in the central chamber 5, and this heat further ignites the remaining thinner pulverized coal in the burner 2. During operation, the plasma generator 1 keeps in a working state, that is, makes sure that the pulverized coal is ignited when entering the central chamber 5, all or most of the pulverized coal already starts to be ignited when it is sprayed into the furnace from the nozzle 7 of the burner. The output power of the plasma generator 1 can be adjusted: increasing power can make the amount of the pulverized coal ignited in advance increase to control the ignition degree of the pulverized coal in the burner.

(10) Only the primary air in the burner provides the oxide amount necessary for the combustion of the pulverized coal, the excess air coefficient thereof is lower than 0.4, which is significantly lower than the oxide concentration during the normal ignition of the pulverized coal, and the strong formed reducing combustion environment can effectively decrease the generation of NOx. After the fuel is sprayed into the furnace, since the problems of ignition and stabilized combustion of the pulverized coal have been solved, the time of mixing of the pulverized coal with the secondary air can be deferred properly, the secondary air amount of the primary combustion zone can be decreased, and the excess air coefficient can be maintained at 0.85 or less (the excess air coefficient of the primary combustion zone of the boiler using conventional burners is about 0.850.95), which makes the fuel is in an oxygen-deficient burning state for a long time. Thus, a strong reducing atmosphere is formed inside the burner and in the primary combustion zone, which is beneficial for inhibiting the generation of NOx during combustion process of the pulverized coal.

Embodiment 1

(11) FIGS. 3 and 4 are schematic views of a specific embodiment of a wall-fired pulverized coal boiler in which swirl burners of internal combustion type are applied, in which burners plasma generators are used as the ignition sources. As shown in FIGS. 3 and 4, all of the burners of the boiler are designed or retrofitted as the burners of internal combustion type 21 in which the plasma generators are used as the ignition sources. During the operation of the boiler, the plasma generators 1 show in FIG. 1 keep in a working state, cause the pulverized coal to be ignited stage by stage in the burners 21, the primary air and pulverized coal nozzle 7 of the burner is connected with the primary combustion zone 22 of the furnace, so that all or most of the pulverized coal sprayed into the primary combustion zone 22 of the furnace is in a igniting state. The air amount entering the primary combustion zone 22 from the secondary air nozzle 6 of the burners is controlled so that the oxygen concentration in the primary combustion zone 22 is decreased; the strong reducing atmosphere which is beneficial for inhibiting the generation of NOx is formed. Under the condition of high temperature and oxygen-deficient state, C element in the fuel starts to react in a great deal before it can mix with enough air, and the main products are CO. In a high concentration CO atmosphere, N element in the volatile constituents tends to be converted to reducing substances such as HCN, NHi etc., so that not only the generation of NOx is decreased, but also the generated NOx can be largely reduced in the flame (HCN+NOx.fwdarw.N.sub.2+H.sub.2O+CO, NHi+NOx.fwdarw.N.sub.2+H.sub.2O), and the fuel generation of NOx is decreased finally. Meanwhile, since the excess air coefficient in the primary combustion zone 22 is very low, the pulverized coal is not fully burnt, the temperature is limited, and thus generation of the thermal NOx is controlled.

(12) The remaining air is sprayed into the burnt-out zone 24 of the furnace through the over-fire air nozzle 23 of the upper furnace, and is mixed with the incompletely burnt flue gas coming from the primary combustion zone 22 intensively, and thus a very strong oxidation atmosphere is formed so that the pulverized coal particles in the flue gas are burnt out herein. Since a large amount of low temperature air is sprayed in from the burnt-out air nozzle 23, the temperature in the burnt-out zone 24 of the furnace is not very high, so the amount of NOx generated from the full reaction of pulverized coal is limited. Thus, the generation amount of NOx is decreased without affecting the efficiency of the boiler.

Embodiment 2

(13) FIGS. 5 and 6 are schematic views of a specific embodiment of a tangentially-fired pulverized coal boiler in which straight flow burners of internal combustion type are applied, in which burners plasma generators are used as ignition sources. As shown in FIGS. 5 and 6, the upper three layers of the four layer burners of the boiler are designed or retrofitted as the burners of internal combustion type 32 in which the plasma generators are used as the ignition sources, the lowest layer of burners are still conventional straight flow burners 31.

(14) During the operation of the boiler, the conventional straight flow burners 31 still keep in a normal running state, and a large amount of NOx is generated in the lower of the primary combustion zone 34 of the furnace. The plasma generators 1 shown in FIG. 1 keep in a working state, causing the pulverized coal to be ignited stage by stage in the burner 32. The primary air and pulverized coal nozzle 7 of the burner is connected with the primary combustion zone 34 of the furnace, and thus all or most of the pulverized coal sprayed into the primary combustion zone 34 of the furnace is in an igniting state. The air amount entering the primary combustion zone 34 from the secondary air nozzle 6 of the internal combustion burner 31 is controlled, so that the oxygen concentration in the upper space of the primary combustion zone 32 is decreased, a strong reducing atmosphere which is beneficial for inhibiting the generation of NOx is formed.

(15) Under the condition of high temperature and oxygen-deficient state, C element in the fuel starts to react in a great deal before it can mix with enough air, and the main products are CO. In a high concentration CO atmosphere, N element in the volatile constituent tends to be converted to reducing substances such as HCN, NHi etc., so that not only the generation amount of NOx is decreased, but also the NOx which is produced in the lower space of the primary combustion zone 34 of the furnace is largely reduced in the flame (HCN+NOx.fwdarw.N.sub.2+H.sub.2O+CO, NHi+NOx.fwdarw.N.sub.2+H.sub.2O), and the generation of fuel NOx is decreased finally. Meanwhile, since the excess air coefficient in the upper of the primary combustion zone 34 is very low, the pulverized coal is not fully burnt, the temperature is limited, and the generation of thermal NOx is controlled.

(16) The remaining air is sprayed into the burnt-out zone 35 of the furnace through the over-fire air nozzle 33 in the upper of the furnace, and is mixed intensively with the incompletely burnt flue gas coming from the primary combustion zone 34, a very strong oxidation atmosphere is formed, so that the pulverized coal particles in the flue gas are burnt out herein. Since a large amount of low temperature air is sprayed in from the over-fire air nozzle 33, the temperature level in the burnt-out zone 35 of the furnace is not very high, the amount of NOx generated from the full reaction of the pulverized coal is limited, so that the total generation amount of NOx is effectively controlled. Thus, the generation amount of NOx is decreased without affecting the efficiency of the boiler.