JPS6260606B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method of combusting pulverized coal, and particularly to a method of combusting pulverized powder that generates less nitrogen oxides (hereinafter referred to as NOx) during combustion of pulverized coal. [Conventional technology] In addition to components such as carbon and hydrogen, fossil fuels also contain
Contains nitrogen. In particular, coal has a higher nitrogen content than gaseous or liquid fuels.
Therefore, NOx generated during combustion of coal is greater than NOx generated during combustion of gaseous fuel or liquid fuel, and it is desired to reduce this. NOx generated during combustion is classified into thermal NOx and fuel NOx. Thermal NOx
is produced when nitrogen in the combustion air is oxidized by oxygen. Fuel NOx is produced by the oxidation of nitrogen in fuel.
Conventional combustion methods for suppressing the generation of NOx include a multistage combustion method in which combustion air is divided into multiple stages and supplied, and an exhaust gas recirculation method in which combustion exhaust gas with a low oxygen concentration is mixed into the combustion region. As a combustion method in which fuel is supplied in two stages, for example, a plurality of burners are used for one combustion furnace, and fuel is supplied and burned in two stages. This combustion method involves the process of performing main combustion by increasing the flame from the main burner to an air ratio of 1 or more, and the process of supplying fuel from the second stage burner to reduce the NOx generated during this process. This process consists of a step of forming a smaller reduction region, and a step of supplying air from the third stage burner to burn excess fuel in the reduction region. It is well known that NOx can be reduced by this combustion method, but in order to increase the NOx reduction effect, the distance between the first, second, and third stage burners must be increased and each combustion area Since it is necessary to clearly distinguish between the two, the combustion furnace becomes large, which is economically disadvantageous in practical use. In addition, when considering an actual combustion furnace, the cross-sectional area of the combustion furnace is large, and it is necessary to completely mix the main flow from the main burner with the fuel and air ejected from the second and third stage burners installed on the furnace wall. is very difficult. Therefore, non-uniform distribution of fuel and air occurs within the furnace, which in turn causes non-uniform distribution of NOx concentration, reducing the NOx reduction effect. In particular, when the air from the third stage burner is poorly mixed, the amount of unburned matter increases.
This causes a decrease in combustion efficiency. In addition to compensating for these shortcomings, we also
In order to improve the NOx reduction effect, a method has been proposed in which two-stage fuel supply combustion is performed using a single burner. Two-stage feed combustion is performed to generate and reduce NOx within a single flame. Although this method is effective in preventing the combustion furnace from increasing in size, it is still not sufficient in terms of NOx reduction effect. That is, the common principle of conventional low NOx combustion methods is to suppress the reaction between nitrogen and oxygen by lowering the temperature of the combustion flame. Of the two types of NOx mentioned above, thermal NOx can be suppressed from generation by lowering the combustion temperature, and the generation of fuel NOx is less dependent on combustion temperature. Therefore, combustion methods aimed at lowering the flame temperature are effective in reducing NOx from fuels with low nitrogen content, but nearly 80% of the NOx generated is fuel NOx, which is caused by pulverized coal combustion. The effect is small for [Object of the invention] The present invention eliminates such problems and provides effective
The purpose is to enable the reduction of NOx and reduce the NOx concentration contained in the fine powder combustion gas. [Summary of the Invention] The present invention provides a pulverized coal combustion method for supplying coal by dividing it into primary fuel and secondary fuel, in which the air ratio is smaller than 1, which is formed by the combustion of the primary fuel. , a first step of converting the volatile nitrogen compounds in the pulverized coal into reducing nitrogen compounds without oxidizing them to NOx, and burning the secondary fuel with the reducing nitrogen compounds in an air ratio region of 1 or more. The first step uses coal, which tends to generate a large amount of ammonia, as the primary fuel, and combines fuel coal with primary combustion air that also serves as a transport for the fuel coal. This is a process in which the fuel is ejected from a primary fuel nozzle provided in the center of a cylindrical burner at a low air ratio smaller than 1, and is burned at an atmospheric oxygen concentration that tends to generate a large amount of ammonia. The step is a swirling flow in which the fuel coal and the secondary combustion air which also serves to transport the fuel coal are provided around the outer periphery of the primary fuel nozzle so as to be concentric with the fuel nozzle at a high air ratio of 1 or more. This process is characterized by ejecting and burning the secondary fuel from a secondary fuel nozzle equipped with a generator. Combustible components in coal can be roughly divided into volatile components and solid components. In accordance with this unique property of coal, the combustion mechanism of pulverized coal consists of a thermal decomposition process of the pulverized coal in which volatile matter is released, and a combustion process of combustible solid components (hereinafter referred to as char) after the thermal decomposition. The burning rate of volatile components is faster than that of solid components, and volatile components are burned in the initial process of combustion. In addition, during the thermal decomposition process, the nitrogen contained in the coal is divided into two parts: one is volatilized and released like other combustible substances, and the other remains in the coal. Therefore, fuel NOx generated during pulverized coal combustion is divided into NOx from volatile nitrogen and NOx from nitrogen in the coal. Of these two types of fuel NOx, fuel NOx from the air is relatively small in amount, and NOx from volatile components is
accounting for the majority of It is known that volatile nitrogen becomes compounds such as NH ₃ and HCN in the initial stage of combustion and in the oxygen-deficient combustion region. These nitrogen compounds are
In addition to reacting with oxygen to become NOx, the generated NOx
It can also act as a reducing agent that converts NOx to nitrogen by reacting with it. This NOx reduction reaction by nitrogen compounds is
It progresses in a coexisting system with NOx,
In a reaction system where NOx does not coexist, most of the nitrogen compounds are oxidized to NOx. Therefore, an effective method for reducing NOx during pulverized coal combustion is a combustion method in which NOx is caused to coexist with a volatile nitrogen compound having reducing properties, and a reduction reaction is caused to convert NOx to nitrogen. That is, a combustion method that eliminates generated NOx and NOx precursors by using nitrogen compounds, which are precursors of NOx, to reduce NOx is effective for reducing NOx. The present invention has been made based on these principles. For example, the present invention efficiently realizes a combustion method in which coal is dividedly supplied as a primary fuel and a secondary fuel using a single burner, thereby achieving low NOx. Using a pulverized coal combustion burner with an annular secondary fuel injection nozzle installed concentrically around the outer circumference of the primary fuel injection nozzle, the primary fuel is burned at an air ratio of less than 1, and the secondary fuel is heated at an air ratio of less than 1. 1 or more, and by burning the flame formed by the primary fuel as an inner flame and the flame formed by the secondary fuel as an outer flame, the reaction from an air-deficient region can be achieved. This allows for good mixing of the products and the reaction products from the air-excess region, making it possible to achieve low NOx combustion. In addition, by swirling the outer flame to completely surround the outer periphery of the inner flame, radiant heat from the outer flame allows the thermal decomposition reaction region of pulverized coal to proceed within the inner flame for a long period of time. It holds and promotes the reaction. In other words, in the air-deficient region formed by the combustion of primary fuel, volatile nitrogen compounds in pulverized coal are not oxidized to NOx, and NH ₃ , HCN
This reduces NOx generated in the air-excess region where secondary fuel is burned. Coal has many different production processes, so its properties differ greatly depending on the type of coal. Depending on the type of coal, there are two types of coal: low fuel ratio coal, which has a lot of volatile components, and so-called low fuel ratio coal, which has a lot of volatile components. It is roughly divided into high fuel ratio coal, and among low fuel ratio coal, there are those that contain many volatile nitrogen compounds and generate large amounts of NH ₃ etc., which are effective in reducing NOx, during thermal decomposition of coal, and those that do not. It can be divided into things. In other words, when considering more effective low NOx combustion, the type of coal used as the primary fuel does not necessarily mean that any type of coal is possible; it contains many volatile nitrogen compounds and releases NH ₃ etc. during thermal decomposition. This means that it is necessary to use coal that tends to generate a large amount of nitrogen compounds that are effective in reducing NOx, and to burn it under conditions that tend to generate a large amount of nitrogen compounds during thermal decomposition. Figure 1 shows the relationship between the atmospheric oxygen concentration and the conversion rate of volatile N compounds in the coal to NH ₃ for three types of coal, with the horizontal axis representing atmospheric O ₂ (%) and the vertical axis representing the conversion rate (%). %), and A, B, and C are foreign coal (Australia, Blairsall Coal), foreign coal (China, Datong Coal), and domestic coal (Pacific Coal), respectively. This is generated when a given amount of coal is pyrolyzed at a given oxygen concentration.

[Embodiments of the invention]

FIG. 4 is a sectional view of an external flame swirl type fuel supply burner in two stages used to carry out an embodiment of the pulverized coal combustion method of the present invention. This burner has two fuel pulverized coal injection nozzles, a primary fuel nozzle 5 and a secondary fuel nozzle 6 located on the outer periphery of the primary fuel nozzle 5, and a secondary air nozzle 7.
A tertiary air nozzle 8 and an ignition fuel nozzle 9 are provided. Pulverized coal, which is divided into primary fuel and secondary fuel, is ejected from the primary fuel nozzle 5 and secondary fuel nozzle 6, but the secondary fuel nozzle 6 is an annular nozzle that is concentric with the primary fuel nozzle 5. , the outer periphery of the flame formed by the primary fuel is surrounded by the flame formed by the secondary fuel. In addition, each of the secondary fuel nozzle 6, secondary air nozzle 7, and tertiary air nozzle 8 is equipped with an axial flow generator (swirler) as a means for giving a swirling flow to the flame formed by the combustion of the secondary fuel. ) 10, 11, 12 are provided. As schematically shown in FIG. 5, the swirling flow formed by the swirling flow generator 13, which flows in the direction of the arrow, has the effect of lowering the static pressure inside the flame, so the secondary fuel mixture is It is possible to generate a flow in the opposite direction from the flame wake direction where the heat wave starts to decay toward the burner tip to promote mixing of products from the outer flame and the inner flame. Combustion air consists of primary air that carries pulverized coal,
The air is divided into secondary and tertiary air, and the secondary and tertiary air are ejected from a secondary air nozzle 7 and a tertiary air nozzle 8, respectively. Like the secondary combustion nozzle 6, these secondary air nozzles 7 and tertiary air nozzles 8 are annular nozzles installed concentrically with the primary fuel nozzle 5. and the secondary fuel nozzle 6 to control the air ratio of the inner flame, and the tertiary air nozzle 8 is installed around the outer periphery of the secondary fuel nozzle 6 to control the air ratio of the outer flame. It's summery. Figure 6 shows that using this burner and a small combustion furnace with a combustion load of 20 kg of pulverized coal per hour, the air ratio of the inner flame formed by the primary fuel is less than 1, and the secondary fuel The graph shows the results of a low NOx combustion test of pulverized coal, where the air ratio of the outer flame formed by was set to 1 or more. The NOx concentration (ppm) in exhaust gas is shown. D is the result of a two-stage external flame swirl fuel supply test, using domestic coal or foreign coal as both the primary fuel and secondary fuel, and E is the result with foreign coal as the primary fuel and foreign coal as the secondary fuel. This shows the results of similar experiments using domestic coal, foreign coal, or foreign coal. For example, when comparing the values of NOx concentration in exhaust gas when the amount of unburned matter in ash is 5%, in D For 100ppm, E
This has been halved to approximately 50 ppm, indicating that using foreign coal as the primary fuel is extremely effective for low NOx combustion. As described above, according to the example, nitrogen compounds generated when coal is thermally decomposed, especially ammonia, are converted into NOx.
By using it to reduce NOx, NOx can be effectively reduced, and the NOx concentration contained in pulverized coal combustion exhaust gas can be reduced. Further, when implementing this pulverized coal combustion method at a power plant or the like, it is not necessary to install an ammonia generator, and the purpose can be achieved by selecting the coal to be used. [Effects of the Invention] The pulverized coal combustion method of the present invention enables effective reduction of NOx, which is contained in the pulverized combustion exhaust gas.
It makes it possible to reduce NOx concentration,
This has great industrial effects.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between atmospheric oxygen concentration and the conversion rate of volatile N compounds in coal to _NH3 , Figure 2 is a cross-sectional view of a cold model for investigating the effect of external flame operation, Fig. 3 is a characteristic diagram showing the iso-oxygen concentration curve showing the effect of external flame swirl determined by the cold model of Fig. 2, comparing it with the case without external flame swirl. FIG. 5 is a cross-sectional view of an external flame swirl type fuel supply burner used in an embodiment of the combustion method for fine powder; FIG. FIG. 6 is a characteristic diagram showing the effects of the fine powder combustion method of the present invention. 5...Primary fuel nozzle, 6...Secondary fuel nozzle, 7
...Secondary air nozzle, 8...Tertiary air nozzle, 9...Ignition fuel nozzle, 10, 11, 12, 13...Swirling flow generator.

Claims (1)

[Claims] 1 In a pulverized coal combustion method in which coal is divided into a primary fuel and a secondary fuel and supplied, the volatility in the pulverized coal is nitrogen compounds
A first step of converting the secondary fuel into a reducing nitrogen compound without oxidizing it to NOx, and a second step of reducing the NOx generated in the air ratio region of 1 or more when the secondary fuel is combusted by the reducing nitrogen compound. In the first step, coal, which tends to generate a large amount of ammonia, is used as the primary fuel, and the fuel coal and the primary combustion air, which also serves as conveyance of the fuel coal, are mixed at a low air ratio smaller than 1. This is a process in which the fuel is ejected from a primary fuel nozzle provided in the center of a cylindrical burner and burned in an atmosphere with an oxygen concentration that tends to generate a large amount of ammonia. The secondary combustion air which also serves as a secondary combustion air is ejected at a high air ratio of 1 or more from a secondary fuel nozzle provided on the outer periphery of the primary fuel nozzle so as to be concentric with the fuel nozzle and equipped with a swirl flow generating device. A method of combustion of pulverized coal, which is characterized by a process of combustion.