What are the reasons for the excessive levels of nitrogen oxides in coke oven exhaust gases?

This post was last edited by angryant on 2016-8-7 at 12:45. As the content of nitrogen and other nitrogen-containing compounds in gas increases, the amount of nitrogen oxides produced during combustion also increases. As a result, the levels of nitrogen oxides in coke oven exhaust exceed the allowed limits

As the content of nitrogen and other nitrogen-containing compounds in the gas increases, the amount of nitrogen oxides produced during its combustion also increases. As a result, the nitrogen oxide levels in the coke oven exhaust exceed the allowable limits

Lao Niu is absolutely right! Another issue is that, for example, in the denitrification section or equipment, or in certain gas pipelines, it’s not possible to shut them completely, which causes some of the gas to flow directly into subsequent processes!

I personally think it’s mainly due to the excessive ammonia content in the gas

Recommended article for joint study*: The formation mechanism and control of nitrogen oxides (NOx) during the heating and combustion in coke ovens. Abstract: When gas burns in the vertical flues of coke ovens, nitrogen oxides NOx are generated. Based on their formation mechanisms, these can be classified into three types: temperature-thermal type, hydrocarbon fuel rapid type, and nitrogen-containing fuel type. After detailing the formation mechanisms and control methods for the above three types of NOx, the text points out that in coke oven operation, to reduce the generation of NOx, measures such as exhaust gas recycling, staged gas supply, the use of gas with a low nitrogen content, and reducing the temperature of the furnace channels should be adopted. When gas burns in the vertical flue of a coke oven, nitrogen oxides (denoted as NOx) are generated. Based on their formation mechanisms, the nitrogen oxides generated during combustion can be divided into three types, namely ① temperature-thermodynamic type NOx ； ②Rapid NOx from hydrocarbon fuels ； ③Nitrogen-containing component fuel-type NOx. Some sources also refer to the first two types together as temperature-dependent NOx. Studies show that among the NOx produced during combustion, NO accounts for 95%, while NO2 makes up about 5%. In the atmosphere, NO can be slowly converted into NO2; therefore, when studying the formation mechanisms of NOx, the mechanism of NO formation is primarily investigated. I. Mechanism and control of temperature-thermodynamic NO formation 1. Mechanism of temperature-thermodynamic NO formation During combustion, nitrogen brought in with the air is oxidized to NO. The formation of NO is explained by the following series of chain reactions, in which atomic oxygen primarily originates from the dissociation of O2 at high temperatures: O + N2 = NO + N; N + O2 = NO + O. Since reactions between atomic oxygen and nitrogen molecules require a high activation energy, large amounts of NO are not produced in the combustion flame prior to fuel combustion. NO can be generated only in the high-temperature region downstream of the flame (theoretically, it is only downstream of the flame that all the heat energy is accumulated, resulting in the highest temperatures; the areas before and in the middle of the flame are not high-temperature regions). It is there that O2 dissociation occurs, allowing NO to be formed. Regarding the temperature in the high-temperature zone of combustion, according to available data, when α=1.1 and the air is preheated to 1100°C, the theoretical combustion temperature for coke oven gas is 2350°C, while that for blast furnace gas is 2150°C. It is generally believed that the actual combustion temperature is lower than this value, with the actual combustion temperature lying between the theoretical combustion temperature and the measured temperature of the furnace lining. If the measured flue gas temperature is 1300–1350°C (with an average of 1325°C), then the actual combustion temperature of coke oven gas is approximately (2350 + 1325) ÷ 2 ≈ 1840°C, while for blast furnace gas it is approximately (2150 + 1325) ÷ 2 ≈ 1740°C. The book \"Atmospheric Pollution Control Engineering\" discusses the formation mechanisms and control methods of NOx, and includes charts showing the relationship between NOx generation and combustion temperature. The chart shows that the combustion temperature of gaseous fuels ranges from 1700°C to 1850°C (the combustion temperature mentioned here refers to the high-temperature zone of combustion as described earlier, and is different from the upward gas flow temperature used in our calculations for coke ovens). The combustion temperature mentioned here is generally about 150°C higher than the latter; slight variations in the combustion temperature result in relatively large fluctuations in the amount of thermally generated NO. In fact, this relationship is also evident in the relation between NOx concentrations in coke oven exhaust gases and the temperature of the flue: when the flue temperature is between 1300–1350°C, a change of ±10°C in the flue temperature leads to a change of ±30 mg/m3 in NOx levels. Thus, the combustion temperature plays a decisive role in the generation of thermally produced NO. When the combustion temperature is below 1500°C, the amount of NO is very low, but when the temperature exceeds 1500°C, the amount of NO increases rapidly in an exponential manner. The relationship between the combustion temperature shown in this chart and the amount of temperature-thermally generated NO is listed below. Combustion temperature, °C: 1700, 1740–1750, 1800, 1840–1850, 1870. Amount of NO generated, in terms of volume concentration, ppm: >100, >200 (–220), 400, 600–630, 800. When converted to weight concentration in terms of NO2, mg/m3: >200, 220×2.05 ≈450, 400×2.05 =820, 1230–1300, 1640. Of course, the relationships shown in the table above are not specific to coke oven combustion, but they still provide valuable reference for us. Moreover, the relationship shown is close to that in many professional sources as well as in actual situations. As mentioned above, when the coke oven is heated with coke oven gas, the combustion temperature is 1840°C, and the NO generation amount is approximately 600 ppm; in terms of NO2, this is about 1200 mg/m3. When heated with lean gas, the combustion temperature is 1740°C, and the NO generation amount is approximately 220 ppm, which is about 450 mg/m3 when expressed as NO2. This also shows that when heated with low-quality coal, the temperature-dependent NO level does not exceed 500 mg/m3; it is also relatively easy to reduce this level to 170 ppm, which corresponds to 350 mg/m3. 2. Measures to control the amount of thermally generated NO (1) To control the amount of thermally generated NO, the waste gas recycling technology well-known among domestic peers can be employed. The effect of exhaust gas recirculation in reducing NO formation is as follows: ① Exhaust gas recirculation introduces a considerable amount of downward-flowing air into the upward-flowing air, increasing the speed of the upward flow and lowering the temperature of the air flow. ② Exhaust gas recirculation reduces the concentrations of fuel and air to a certain extent, slowing down the combustion reaction. These two effects reduce the combustion temperature. Practice has shown that, due to the lower combustion temperature, when heating with lean gas and keeping the flue temperature at around 1300°C, it is possible to achieve a weight concentration of NOx (expressed as NO2) in the combustion exhaust gases of 350 mg/m3; whereas when using coke oven gas, it is difficult to reach a level of ≤500 mg/m3. (2) Adopt segmented gas supply technology. There are two types of segmented gas supply: in one type, both air and lean gas are supplied in segments, resulting in segmented combustion and thereby reducing the intensity of combustion. Another type has only air segmentation, allowing combustion to take place essentially at a level far from the theoretical air ratio. For the vertical flues of coke ovens with a carbonization chamber height of around 7 meters, air supply is generally divided into three sections. The first section is at the bottom of the flue, while the outlets for the second and third sections are located at an appropriate height in the vertical flue. When air is supplied in segments only, combustion in the first segment at the bottom of the vertical flue can result in an α value of 0.7 to 0.8 (according to available data, when α = 0.8, the amount of NO generated can be reduced by 50% compared to when α = 1.2); thus, 60% to 70% of the air supply should be provided in the first segment (with ≤70% being preferred). The air supply for the second segment should not be excessive, and it should be introduced at a location where the gas flow temperature is lower. It is generally believed that, in the absence of segmented heating, for coke ovens with a height of 4–5 meters, the highest temperature in the upward gas flow flue occurs at a distance of 1000–1500 mm from the bottom of the carbonization chamber, while for ovens with a height of 6–7 meters, this position may be around 1000–1800 mm. During segmented heating, as the combustion intensity at the lower part decreases, the upper limit should be below 1800 mm; therefore, for a 7-m-high coke oven, the location of the gas supply outlet for the second stage should be between 1800 and 2000 mm. The 7.63m coke oven has a width of 2250mm). By the third stage, the α value in the flame channel is brought to around 1.2, allowing both the first and second stages to burn under conditions far from the theoretical air ratio. In the third stage, although α reaches 1.2, the temperature is no longer high, there are few combustible components left, and there is also dilution from the large amounts of exhaust gas from the first and second stages; therefore, the air supplied to the third stage serves primarily to ensure complete combustion of the upward-flowing gases. Theoretically, the lower the air coefficient, the better the control effect on nitrogen oxides. For coke ovens, if the air volume in the first section is too low, there is a risk of low temperatures at the bottom of the carbonization chamber while temperatures are high at the top; therefore, maintaining an α value of 0.7–0.8 for the first section is sufficient. Due to the distributed combustion resulting from segmented gas supply, and since combustion takes place under conditions of insufficient air for each air segment, the combustion temperature is lower compared to when combustion occurs without segmentation. Germany’s Prosper No. 3 coke oven is of the Carl-Still type; it has 6 sections for supplying air and is heated by coke oven gas. The reported NOx level (expressed as NO2) is 390 mg/m3, with a flame temperature of around 1320°C. In terms of combustion temperature, it can be assumed that due to the heating in 6 stages, the combustion intensity is not high in each stage; therefore, the combustion temperature should drop below 1750°C. This is also consistent with the relationship between combustion temperature and NO generation shown in the graphs in the literature, that is, when the combustion temperature

There are two aspects: first, nitrogen oxides are produced by the combustion of cyanides in gas, although their concentration is low; second, under high-temperature combustion conditions, nitrogen and oxygen undergo chemical reactions to form nitrogen oxides. These are the main aspects. Compared to coke ovens with this type of flame channel structure, it is very difficult to reduce the nitrogen oxide levels to below 500 milligrams, whether three-stage heating or exhaust gas recycling is used during the combustion of coke oven gas. In coke ovens that employ combined flame technology – that is, three-stage heating combined with exhaust gas recycling – nitrogen oxide levels can be kept below 500 when using blast furnace gas, thus meeting the control requirements for the general control areas in newly built and existing manufacturing facilities as specified in the latest standard GB16171-2012. Currently, desulfurization and denitrification technologies in China are gradually being applied to coke oven flue gas. The technical concept involves desulfurizing and denitrifying the coke oven flue gas, followed by waste heat recovery. The investment is high.

I envision the optimal process for desulfurization, denitrification, and waste heat recovery to be: a three-stage heating of exhaust gas circulation + waste heat recovery + an integrated tower for desulfurization and denitrification. This combined approach can address the current environmental compliance issues associated with coke ovens. Under the current target of 500 for nitrogen oxides, only exhaust gas recirculation is used to keep this value at 500; wet flue gas desulfurization is then employed to reduce SO2 levels to 50 milligrams. As for denitration, it is activated or not depending on the requirements set by environmental protection standards. In this way, if environmental protection standards regarding nitrogen oxides are further reduced in the future, it will still be possible to meet those environmental requirements without increasing investment in infrastructure.

At present, large-scale coke ovens in China are equipped with segmented heating and exhaust gas recycling systems, but the results are far from satisfactory – emission levels still exceed the allowed limits. The main reason is that the design firms only know that segmented heating is a good approach, but they have not conducted any in-depth research; they are simply imitating others without understanding the underlying principles! The segmented heating technology for design purposes is a highly mature technology abroad, and it can basically achieve source control; as a result, coking enterprises in Europe and the United States almost do not use denitration technologies! !