Development of super low-level NOx RT burner for annealing furnace TAKAHITO SUZUKI KUNIAKI OKADA

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1 Development of super low-level NOx RT burner for annealing furnace BY TAKAHITO SUZUKI KUNIAKI OKADA SYNOPSIS In the CGL of Fukuyama steelworks, we decided to adapt an only RT (radiant tube) furnace in the annealing furnace. Since RT burner alone does not have capacity whose NOx average value per hour is less than 90ppm (at 11%O2, COG), we decided to develop a super low-level NOx burner method. In order to lower the NOx emission of RT burner, we considered to use a pre-combustion system and exhaust gas recirculating system. As a result, we found out the optimum layout of RT burner and exhaust gas recirculating rate to develop a super low-level NOx RT burner. And this burner have a few heat spots on surface of RT. By using new RT burners in CGL of Fukuyama steelworks, we operate below 90ppm of NOx emission. Keywords: RT, burner, annealing furnace, NOx 1

Background In the Fukuyama area of JFE Steel West Japan Works, we established a new CGL as demand for thin plates increased. The outline of CGL is shown in Fig. 1.

The annealing furnace heats the strip to the target temperature by controlling the combustion amount of many RT (radiant tube) burners and performs annealing.

2 Background In the Fukuyama area of JFE Steel West Japan Works, we established a new CGL as demand for thin plates increased. The outline of CGL is shown in Fig. 1. The CGL has an annealing furnace to anneal the strip. The annealing furnace heats the strip to the target temperature by controlling the combustion amount of many RT (radiant tube) burners and performs annealing. In the new annealing furnace, in order to reduce construction cost, we adopted only RT heating system although we usually adopt the standard combination of direct flame reduction heating system and RT heating system in Fukuyama area. At that time, it was necessary to develop a new super low-level NOx RT burners capable of achieving 90ppm (11% O2 equivalent) or less as the NOx agreement value with the government. Fig.1 The outline of CGL Structure of RT burner and goal of low NOx reduction Fig. 2 shows the outline of the RT heating device. RT heating device consists of a burner, a radiant tube and a recuperator. It uses COG for fuel gas. A burner flame and combustion exhaust gas flow inside the radiant tube 1). This brings the radiant tube into a high temperature state. The radiant heat from the radiant tube in a high temperature state heats the strip indirectly in the annealing furnace. The exhaust gas discharged from the burner recovers the exhaust gas sensible heat through the recuperator. Fig.2 The outline of the RT heating device 2

3 Fig. 3 shows the burner configuration of the annealing furnace. In the existing annealing furnace, in order to burn in unburned state, we combined a direct flame reducing burner which hardly generates thermal NOx and a RT burner which burns with excess air and generates thermal NOx. As a result, we attained the NOx value below the agreement value at the concentric stack. In the new annealing furnace, it was necessary to adopt only RT heating system. Therefore we needed to examine and adopt a new method to achieve the NOx agreement value of 90ppm (11%O2 equivalent) or less. Fig.3 The burner configuration of the annealing furnace How to reduce NOx Fig. 4 shows a method for reducing NOx. An exhaust gas denitration device is one of the methods for lowering NOx emissions of exhaust gas from an annealing furnace. We studied SCR (ammonia denitrification) widely used as an exhaust gas denitration device. In the SCR, NOx in the combustion exhaust gas is decomposed into nitrogen and steam by the reaction with ammonia gas, using a catalyst. However, SCR has a disadvantage of increasing in the running cost as well as construction cost because we need to check and replace the catalyst and to supply ammonia regularly. Therefore, we developed a new RT burner as a method to lower NOx of exhaust gas from the annealing furnace. By changing the burner structure, we aim to reduce NOx below the target agreement value in the same price range as before. 3

Fig.4 Methods for reducing NOx NOx generated by RT burner combustion can be classified

Fuel NOx is generated by oxidizing reaction of nitrogen components such as ammonia

Thermal NOx is generated by oxidizing reaction of nitrogen present in the combustion air.

As for the fuel NOx, it is difficult to decrease NOx since the amount of NOx is

4 Fig.4 Methods for reducing NOx NOx generated by RT burner combustion can be classified into fuel NOx and thermal NOx 2)3). Fuel NOx is generated by oxidizing reaction of nitrogen components such as ammonia existing in the fuel. Thermal NOx is generated by oxidizing reaction of nitrogen present in the combustion air. Fig. 5 shows the ratio of thermal NOx and fuel NOx in the exhaust gas. As for the fuel NOx, it is difficult to decrease NOx since the amount of NOx is determined by the amount of nitrogen contained in the fuel. On the other hand, thermal NOx can suppress the amount of NOx from combustion by changing the combustion state of the burner. Fig.5 The ratio of thermal NOx and fuel NOx in the exhaust gas 4

burner. There are three combustion methods that meet the development condition: the method of multistage combustion, low excess air combustion, and exhaust gas recirculation combustion.

5 A method of suppressing thermal NOx from combustion is summarized in the Fig.6 4). In developing a super low-level NOx RT burner, it is a condition to adopt a combustion method that does not increase the exhaust heat loss without changing the shape of the attachment part of the burner. There are three combustion methods that meet the development condition: the method of multistage combustion, low excess air combustion, and exhaust gas recirculation combustion. The effects by the three methods are shown as follows. 1 In the multi-stage combustion method, the combustion air and apart of the fuel are leanburned in the primary stage combustion region. Then the remaining fuel is combusted with excessive primary combustion gas. This lowers the oxygen concentration and the combustion flame temperature. As a result, NOx is reduced. 2 In the low excess air combustion method, the oxygen concentration in the combustion field is reduced by suppressing excessive air. This lowers the combustion flame temperature. Therefore, NOx is reduced. 3 In the exhaust gas recirculation combustion method, a part of the exhaust gas in a low temperature state is recirculated to the combustion air. After that, the oxygen concentration and the combustion flame temperature decrease. This effect reduces NOx. Fig.6 Methods of suppressing thermal NOx generated The three combustion methods have two effects of lowering the oxygen concentration in the combustion and lowering the combustion flame temperature. The relation of each effect and the change of NOx value in combustion exhaust gas will be illustrated. First, Fig. 7 shows the relation between the oxygen in the combustion field and the theoretical adiabatic flame temperature 5). As you can see in Fig. 7, the theoretical adiabatic flame temperature decreases as the oxygen in the combustion field decreases. NOx is reduced by reducing the amount of oxygen in the combustion field. This is because it suppresses the binding of nitrogen and oxygen in the air. 5

As we can see, the NOx value in the exhaust gas drastically decreases by lowering the combustion flame temperature.

6 Fig.7 The relation between the oxygen in the combustion field and the theoretical adiabatic flame temperature Fig. 8 shows the relation of NOx value in the combustion exhaust gas, combustion time, and combustion temperature. The ordinate shows the NOx value in the exhaust gas in logarithm. As we can see, the NOx value in the exhaust gas drastically decreases by lowering the combustion flame temperature. This is because thermal NOx has a characteristic of being generated easily in a high temperature state. Therefore, in this burner development, we will achieve super low-level NOx (90ppm or less) by reducing the flame temperature in the combustion field. Fig.8 The relation of NOx value in the combustion exhaust gas, combustion time, and combustion temperature Structure of super low-level NOx RT burner and the effect of each combustion method. Fig. 9 shows the structure of the RT heating device incorporating the three combustion methods. 6

7 Therefore one pre-combustion stage is provided to the upstream side of the conventional main burner. After combustion upstream of the primary combustion burner, the oxygen concentration in the combustion air is lowered and slow combustion occurs. In addition, in order to recirculate the exhaust gas, we provided a mixing part of the combustion air heated by the recuperator and the exhaust gas. By providing the burner with air fuel mixture recirculated the exhaust gas, the oxygen concentration in the combustion air becomes lowered and combustion occurs. Furthermore, we set the air ratio at the burner combustion as a low excess ratio and developed a super low-level NOx RT burner. Fig.9 The structure of the RT heating device First, we investigated the influence on the NOx value in the combustion exhaust gas by changing the pre-combustion ratio in the RT burner (the ratio of fuel gas used for precombustion to total fuel gas). Fig. 10 shows the relation between pre-combustion ratio and NOx value in exhaust gas. As a result of the investigation, the NOx value tended to decrease in proportion to the precombustion ratio while the pre-combustion ratio was from 0% to about 15%. However, we were not able to see the effect of decreasing the NOx value proportional to the pre-combustion ratio when the pre-combustion ratio was about 15% or more. It was presumed that the pre-combustion caused a low NOx effect obtained by lowering the oxygen concentration in the combustion air. It was considered that lowering the oxygen concentration in the combustion air suppresses the generation of NOx in the combustion field after the pre combustion. It was also considered that the unburned state inside of the combustion cylinder and the excess combustion air state outside of it suppressed the generation of thermal NOx by lowering the flame temperatures of each side. When the pre-combustion ratio increase 7

to 15% or more, the decrease of NOx value cannot be achieved more than certain level.

stages. Fig.10 The relation between pre-combustion ratio and NOx value in exhaust gas Next, we discussed the effect of exhaust gas recirculation.

This is because slow combustion occurs and the combustion flame temperature decreases when an area is formed where oxygen does not exist in the combustion air. Fig.

8 to 15% or more, the decrease of NOx value cannot be achieved more than certain level. This is presumably because the flame temperature rises relatively at the pre- combustion, offsetting the effects of the primary and secondary combustion in the latter stages. Fig.10 The relation between pre-combustion ratio and NOx value in exhaust gas Next, we discussed the effect of exhaust gas recirculation. In general, it is known that NOx value in exhaust gas decreases by recirculating the exhaust gas. This is because slow combustion occurs and the combustion flame temperature decreases when an area is formed where oxygen does not exist in the combustion air. Fig. 11 shows the relation between the exhaust gas recirculation ratio and the NOx value in the combustion exhaust gas. As Fig.11 shows, it was found that the target NOx value of 90ppm can be achieved by recirculating the exhaust gas of pre-combustion burner at about 23% or higher. Fig.11 The relation between the exhaust gas recirculation ratio and the NOx value 8

Fig. 12 shows the relation between the air ratio when a burner at an exhaust gas recirculation ratio about 23 % or higher and the NOx value in the exhaust gas.

We confirmed that the NOx became below the agreement value of 90ppm (11%O2 equivalent) or less when the air ratio of low excess air combustion was in the range of 1.05 to1.25. As shown in the Fig.

9 Fig. 12 shows the relation between the air ratio when a burner at an exhaust gas recirculation ratio about 23 % or higher and the NOx value in the exhaust gas. As the air ratio at combustion increases, the oxygen concentration in the combustion field increases. Then NOx value tends to increase by the oxidization of nitrogen. We confirmed that the NOx became below the agreement value of 90ppm (11%O2 equivalent) or less when the air ratio of low excess air combustion was in the range of 1.05 to1.25. As shown in the Fig. 12, we also confirmed that the maximum temperature of the combustion tube was below 1000 C which is the heat resistant temperature of general heat resistant steel. Fig.12 The relation between the air ratio when a burner at an exhaust gas recirculation ratio about 23% or higher and the NOx value in the exhaust gas Although 90ppm or less of the NOx agreement value was achieved in this burner structure, localized heat occurred inside of the combustion tube of the RT burner. When localized heat occurred inside of the combustion tube, a very large thermal stress was generated against the strength of the combustion tube. This caused a crack in the tube. Therefore, we need to improve the RT heating device in order not to generate localized heat. As shown in Fig. 13, only one pre-combustion hole was made in the vertical direction from the flowing direction of combustion air. Therefore, we presumed that the pre-combustion flame heated only in one direction and localized heat occurred inside the combustion tube. 9

Fig.13 A localized heat occurs inside of the combustion tube of the RT burner Therefore, we changed the number and arrangement of the

In Type L, three pre-combustion holes were made to flow in the same direction as the combustion air In Type K, six pre-combustion holes were

15 shows the relation of the NOx value in the exhaust gas, the exhaust gas temperature, and the air ratio.

The NOx value in the exhaust gas becomes 90ppm or less in Type E,L and K, but higher than 90 ppm in Type H.

10 Fig.13 A localized heat occurs inside of the combustion tube of the RT burner Therefore, we changed the number and arrangement of the pre-combustion holes. Fig. 14 shows the burner structure studied. In Type H, three pre-combustion holes were made so as to face combustion air. In Type L, three pre-combustion holes were made to flow in the same direction as the combustion air In Type K, six pre-combustion holes were made to flow in the same direction as the combustion air. We compared these four RT heating devices. Fig.14 The burner structure studied Fig. 15 shows the relation of the NOx value in the exhaust gas, the exhaust gas temperature, and the air ratio. It can be seen that the maximum temperature of the combustion tube is 1000 C or less in any Type of burner. The NOx value in the exhaust gas becomes 90ppm or less in Type E,L and K, but higher than 90 ppm in Type H. In Type H, pre-combustion occurs so as to face the combustion air. This raises the combustion air temperature before the primary combustion generally. This is presumably because the combustion flame temperature became high after pre-combustion and the NOx value increased. 10

Fig.15 The relation of the NOx vale in the exhaust gas, the exhaust gas temperature, and the air ratio Fig. 16 shows the temperature distribution on the circumference of each burner in the RT burner.

Therefore, this result was caused by the temperature of the primary combustion. As mentioned earlier, the combustion air became high in Type H.

11 Fig.15 The relation of the NOx vale in the exhaust gas, the exhaust gas temperature, and the air ratio Fig. 16 shows the temperature distribution on the circumference of each burner in the RT burner. Fig. 16 shows the decrease amount of the localized heat in each burner. The heat decreases the most in Type L. Type K comes next ant Type H the third. Therefore, this result was caused by the temperature of the primary combustion. As mentioned earlier, the combustion air became high in Type H. Therefore, we presumed that the flame temperature at primary combustion became high and the localized heat occurs in the circumferential direction of the RT burner. In Type K, the proportion the exhaust gas of the pre-combustion occupies becomes large to the area where combustion air flows. For this reason, it was considered that the temperature of the combustion air became high at primary combustion and a high temperature part occurred locally at the combustion flame temperature of primary combustion. As a result, the localized heat increased in Type K and L. In Type L, the proportion the exhaust gas of the pre-combustion occupies becomes small to the area where combustion air flows. Therefore, compared to the other Types, it was considered that the temperature of the combustion air became low relatively at primary combustion and a high temperature part did not occur locally at the combustion flame temperature of primary combustion. As we found from this, among the burners we studied, Type L can reduce NOx and it is the RT burner with small localized heat. 11

In order to lower the NOx value in the combustion exhaust gas, we provided three pre-combustion sections in the same direction

12 Fig.16 the temperature distribution on the circumference of each burner in the RT burner Fig. 17 shows the burner structure summarizing the results of the study. In order to lower the NOx value in the combustion exhaust gas, we provided three pre-combustion sections in the same direction as the flow of combustion air. We also provided an exhaust gas recirculation section where combustion exhaust gas and combustion air were mixed. Fig.17 The burner structure summarizing the above investigation results 12

Installation result The developed super low-level NOx RT burner was installed into the CGL. Fig. 18 shows the device after the RT burner equipped, and the NOx value in the combustion exhaust gas.

In addition, it can be seen that the low NOx performance is stably maintained even after installation. Fig.

13 Installation result The developed super low-level NOx RT burner was installed into the CGL. Fig. 18 shows the device after the RT burner equipped, and the NOx value in the combustion exhaust gas. As shown in Fig. 18, the new burners were installed into exactly the same place as the old one without changing the design of the CGL. In addition, it can be seen that the low NOx performance is stably maintained even after installation. Fig.18 The device after the RT burner equipped, and the NOx value in the combustion exhaust gas Summary In the Fukuyama area of JFE Steel West Japan Works, we adopted an annealing furnace, using only RT burners. We needed to newly develop a super low-level NOx RT burner to attain the NOx agreement value of 90 ppm or less in the combustion exhaust gas at the concentric stack of the annealing furnace. We were able to reduce the NOx value in the combustion exhaust gas to 90 ppm or less by introducing three methods of multi-stage combustion, exhaust gas recirculation, and lowexcess air combustion to a super low-level NOx RT burner. In addition, we optimized the combustive direction of pre-combustion part and the number of combustion holes and suppressed the localized heat. As a result of equipping the developed super low-level NOx RT burner with the annealing furnace, we have maintained the low NOx performance and the NOx value in the exhaust gas at the concentric stack below the agreement value. References 1)KUNII, T Furnace and combustion equipment. pp )ONO, K New Combustion Engineering in Global Environment. pp )The Japan Society of Mechanical Engineer Combustion engineering handbook. pp

14 4)ARAI. Y, Development and control technology of combustion products. pp )The Japan Society of Mechanical Engineer Technical data Formation Mechanisms and Controls of Pollutants in Combustion System.pp

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