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Types and characteristics of NOx
in cement plant

NOx emissions from the cement industry are mainly thermal NOx and fuel NOx. Thermal NOx mainly generates directly from oxygen and nitrogen in the rotary kiln. Fuel NOx are mainly converted from nitrogen-containing compounds in the calciner. At present, Chinese cement enterprises produce not only thermal NOx and fuel NOx, but also the new NOx source – raw material NOx (feeding NOx). In this paper, the characteristics of thermal NOx, fuel NOx and feeding NOx are introduced.
NOx (nitrogen oxides) are one of the gas pollutants from the cement industry. With the improvement of environmental protection standards, many scholars are studying the generation of NOx [1] and the technology for reducing emissions of NOx from the cement industry [2-7]. The authors introduce the characteristics of different NOx from the cement industry, and the NOx emission of the Chinese cement industry.

1 NOx emission in the Chinese cement industry

According to the investigation of more than 100 Chinese cement clinker plants without any denitration pretreatment, the NOx emission concentration of cement plants ranges from 400-1200 mg/m3 (note: all NOx concentrations in this paper are Chinese national standard emission concentration, not ppm), and the average value is nearly 800 mg/m3.

In 2011, the total NOx emission of the cement industry in China was 2.3 million t. In the same year, China began to control the NOx emission from the cement industry. In the emission standard of air pollutants issued by China in 2013, the NOx emission concentration is limited to below 400 mg/m3. In 2017, the total NOx emission of the Chinese cement industry was reduced to about 1 million t. Since 2019, many Chinese provinces have generally issued ultra-low emission standards, and the NOx emission limit is 100 mg/m3. Today most Chinese cement plants are equipped with intelligent SNCR [8] for catering to limit and more plants have begun to build SCR systems. It can be predicted that the total NOx emission of China’s cement industry will be reduced to 500 000 t in 2025.

 

2 NOx in cement plants

The three kinds of NOx generated in cement plants are as follows: thermal NOx, fuel NOx, and feeding NOx. Conventional wisdom holds that NOx from the cement industry are mainly thermal NOx and fuel NOx. However, feeding NOx generates from nitrate in raw materials. There is actually another NOx which generates from co-disposal of wastes. Due to the variety of wastes, NOx from co-disposal of wastes are too complex to estimate and are still in research and therefore not discussed in this paper. Most thermal NOx is generated over 1400 °C in the rotary kiln. Theoretically the fuel NOx is generated in the rotary kiln and the preheater. But the fuel NOx in the kiln are generally ignored.

 

3 Thermal NOx

3.1 Survey of thermal NOx

Thermal NOx are generally considered to generate directly from nitrogen and oxygen at high temperature. The amount of thermal NOx can be determined by measuring the composition of the flue gas at the riser. However, the working temperature at the riser is almost 1000-1200 °C, and the dust concentration is as high as 50-100 g/m3. The working environment is so harsch that the on-line flue gas analyzer’s life is short. Therefore, the reliability of the measured value is low and its accuracy is also questioned. In order to obtain the on-line NOx concentration, it can only be inferred indirectly from the perspective of the cement production process, such as camera flame brightness, free calcium content, kiln current fluctuation, etc. With the rise of image processing, artificial intelligence and other technologies, the technology of computer fire viewing is going to mature. Many cement enterprises in China, such as hknf3, have begun to test this technology. Furthermore, an online free calcium analyzer has been applied in hknf3 to provide parameters for predicting thermal NOx concentration.

The method of using a quartz tube plus a portable flue gas analyzer to manually measure thermal NOx has been popularized in China. The measurement results have high accuracy and low systematic error, so that they can be used as an effective intermittent measurement method.

 

3.2 Influencing factors of thermal NOx

From the perspective of chemical reaction, the main factors affecting the formation of thermal NOx are temperature and oxygen concentration.

 

1) Influence of volatile

As mentioned earlier, the flame temperature of the rotary kiln cannot be measured accurately. But the combustion characteristics of pulverized coal, especially the volatile content, can affect the flame temperature and indirectly affect the generation of thermal NOx. The combustion rate of high volatile coal is higher than that of low volatile coal, and the flame temperature in the rotary kiln of high volatile coal is lower than that of low volatile coal. Therefore, in theory the thermal NOx concentration of volatile coal in the cement production line is lower than that of burning low volatile coal.

Figure 4 shows the company’s numerical simulation of the combustion of bituminous coal and anthracite in a 5000 t/d rotary kiln designed by the authors. The highest NOx concentration of a bituminous coal-fired rotary kiln is 2500 mg/m3, and the average NOx concentration at the outlet of rotary kiln is 1200 mg/m3. The highest concentration of NOx of anthracite coal-fired rotary kiln is 3200 mg/m3 and the average concentration of NOx at the outlet of rotary kiln is 1600 mg/m3.

According to the investigation of more than 40 cement plants in China, the relationship between volatile matter of pulverized coal and NOx in the riser is as follows.

Figure 5 shows that there is an obvious corresponding relationship between the volatile content of pulverized coal and the NOx content. When the volatile content is less than 20%, the thermal NOx content decreases with the increase of volatile content. When the volatile content is higher than 20%, the thermal NOx content is hardly affected by the volatile content.

 

2) Effect of oxygen concentration

The formation of thermal NOx is related to the oxygen concentration in the main combustion area, so the oxygen concentration in the riser can also indirectly reflect the thermal NOx concentration. Figure 6 shows the continuous test conducted in an anthracite production line in Hunan province. There is a good corresponding relationship between riser oxygen concentration and thermal NOx concentration in the riser. The higher the oxygen concentration, the higher the thermal NOx concentration. When the oxygen concentration is close to 0, the thermal NOx concentration is close to 0. The test results of other production lines are similar to this line.

According to the above discussion, we give a corollary that the relationship between riser NOx concentration and volatile/riser oxygen concentration is inferred as follows:

 

NOx (mg/m3, riser) = 1.15 ∙ volatile^2 - 75 ∙ volatile + 1800 + 500 ∙ (oxygen content - 1.5)

 

If the volatile content of pulverized coal is 20 %, volatile = 20; the oxygen concentration is 2.5 %, and the oxygen content = 2.5.

 

4 Fuel NOx

4.1 Survey of fuel NOx

Fuel NOx comes from the conversion of nitrogen-containing fuel coal in the calciner of the cement kiln. According to our investigation, fuel nitrogen accounts for about 30-60 % of the NOx emission of the cement industry if no denitration measures are installed. At present, the formation mechanism of fuel NOx is not completely clear. It is generally considered that one part is the transformation of nitrogen-containing substances in volatile matter, and the other part is in coal coke. The conversion ratio of nitrogen-containing substances to NOx in volatile matter is higher than that in coke, because coke has a certain reduction effect on NOx.

 

4.2 Influencing factors of fuel NOx

In the cement industry, it can be clearly observed that the influencing factors of fuel NOx are mainly oxygen concentration and nitrogen content of the pulverized coal.

 

1) Effect of oxygen concentration (off-line calciner and on-line calciner)

The oxygen concentrationin the combustion area of off-line or on-line calciners is different.

According to the investigation of more than 20 production lines, the average NOx emission concentration of an off-line calciner is 15-20 % than that of an on-line calciner. This is because the tertiary air in the off-line calciner is pure high-temperature air, and the oxygen concentration in the main combustion area is the same as that of the air, while the on-line calciner’s volume is a mixed flue gas with low oxygen concentration.

When the oxygen concentration of the preheater is insufficient because of excessive production increase (referring to the oxygen concentration at the outlet of the calciner is lower than 1 % or the CO concentration is higher than 2000 ppm), a situation similar to staging-combustion will occur. This will lead to a decrease of NOx concentration at the outlet of the calciner. It not only greatly reduces fuel NOx but also thermal NOx. The author once conducted a test in a 5000 t/d production line in the Henan province. When the oxygen concentration at the outlet of the calciner is stable below 1 %, the NOx emission concentration of the chimney is reduced to about 100-200 mg/m3 without any denitration pretreatment. However, the injection of aqueous ammonia cannot continue to reduce the NOx emission in this plant. The reason is that the reaction between ammonia and NOx requires the participation of oxygen free radicals. An insufficient oxygen concentration cannot provide enough oxygen free radicals, which results in a poor SNCR effect. The NOx emission cannot be stabilized below 100 mg/m3 and cannot meet the ultra-low emission limit. Although the initial NOx emission concentration can be reduced by hypoxia of the preheater, it is not an effective denitration method.

 

2) Effect of nitrogen content in pulverized coal

The nitrogen content in fuel will also affect the formation of fuel NOx. The nitrogen content in bituminous coal was analyzed in more than 10 cement plants of a cement group in Guangxi Province. Figure 7 shows that there is a strong corresponding relationship between the nitrogen content and the NOx emission. Therefore, the selection of coal with low nitrogen content is conducive to reducing NOx emission.

 

3) Effect of thermal NOx

Does thermal NOx affect the formation of fuel NOx? Experiments were conducted in a cement plant in Hunan province for two weeks. The clinker production is 5000 t/d, and the normal NOx emission concentration is 900 mg/m3. While keeping the total volume in the chimney unchanged, it was ensured that the total flue gas volume in the calciner remained unchanged. Through adjusting the proportional relationship between secondary air and tertiary air, the generation of thermal NOx in the rotary kiln was changed. So, the relationship between the NOx emission and the thermal NOx could be observed. Figure 8 shows the result.

There is a linear relationship between the NOx emission concentration in the riser and the NOx emission concentration in the chimney. When the concentration of NOx in the riser is high, the emission concentration from the chimney is also high.

In order to find out the relationship between NOx in the riser and fuel NOx, the following calculation was made. The oxygen concentration was used to calculate the excess air coefficient in the riser, and the coal feeding quantity in the kiln was used to calculate the theoretical flue gas volume. Then the total flue gas volume in the kiln was calculated. The NOx concentration (mg/m3) and oxygen concentration in the riser was used to calculate the original NOx concentration (ppm). Furthermore the total thermal NOx in the kiln was calculated. The total NOx emission was calculated by using the NOx emission concentration, oxygen concentration and flue gas volume of the chimney. The total amount of fuel NOx was obtained by total NOx emission minus total thermal NOx.

Figure 9 shows that the amount of NOx per unit clinker fuel is calculated according to the amount of thermal NOx per unit clinker. According to the correlation analysis, the correlation coefficient between the thermal nitrogen oxide production per unit clinker and the nitrogen oxide production per unit clinker fuel is 0.1367, less than 0.3, so it can be considered that there is no statistical relationship between them. The authors therefore infer that fuel NOx generation is not affected by thermal NOx.

 

5 Nitrogen oxide of raw material (feeding NOx)

The feeding NOx comes from the decomposition of some nitrates in the preheater. Normally the feeding NOx is little. However, due to the increasingly complex source of cement raw materials, many raw materials often contain some nitrates. Nitrates begin to decompose at 120 °C. After entering the cement process, NOx will be released from the preheater. And this part of NOx cannot be removed by conventional methods, so it is very easy to exceed the standard.

The following takes a cement plant in Guangdong province as an example to illustrate the characteristics of the feeding NOx. One of the characteristics is that NOx concentration will be inverted in the preheater. Figure 10 shows that NOx concentration at the outlet of the calciner is the lowest, and NOx concentration will gradually increase with the flow of flue gas, especially in C1 and C2. The NOx emission almost equals the concentration in C1 because the decomposition in the mill is slow due to low temperature.

The second characteristic of feeding NOx is that the final NOx emission has an obvious linear relationship with the raw material feed quantity. In this case with the increase of 10 t/h feeding amount, the final NOx emission will increase by about 35 mg/m3. On the premise of the maximum amount of ammonia in the SNCR system, the emission concentration cannot meet the environmental protection standard.

The third characteristic of feeding NOx is the ingredients’ affect. We found that the NOx source in the raw material of the plant is a kind of clay as aluminum raw material. The clay is a by-product of chemical fertilizer produced by a nitrate factory. If we use this clay as raw material, the NOx emission will exceed the standard immediately. After the replacement of aluminum raw materials, the NOx emission no longer changes with the raw meal feeding, and decreases significantly.

According to the above analysis, the following expression can be deduced:

 

ΔNOxfeeding = K ∙ Δ feeding rateraw material ∙ Xproportion of nitrate-containing substances in raw meal ∙Xproportion of nitrate content in nitrate containing substances

 

K is a constant. ΔNOxfeeding is the NOx emission change caused by the feeding rate.

Therefore, feeding NOx has nothing to do with thermal NOx and fuel NOx. The elimination of feeding NOx cannot be solved by the commonly used SNCR technology and staged combustion technology. In order to meet the environmental protection standards, the cement plant should strictly control the raw materials quality.

 

6 Summary

The authors introduce the current NOx emission situation in China’s cement industry, and study the main characteristics and influencing factors of NOx. The main research results are as follows:

Thermal NOx is affected by the volatile matter of pulverized coal and oxygen concentration in the kiln.

The production of fuel NOx is not affected by thermal NOx, but mainly by oxygen concentration and nitrogen content of coal.

The production of feeding NOx has nothing to do with thermal NOx and fuel NOx. It is affected by raw material batching and raw material feeding amount.

[1] Qingquan, Xu; Xiaochen, Hao; Xin, Shi; Zhipeng, Zhang; Quanwei, Sun; Yinlu, Di: Control of denitration system in cement calcination process: A Novel method of Deep Neural Network Model Predictive Control, Journal of Cleaner Production. Available online 5 December 2021, in Press, Journal Pre-proof
[2] Huixing, Wu; Jun, Cai; Qiangqiang, Ren; Chaoting, Shi; Angran, Zhao; Qinggang, Lyu: A thermal and chemical fuel pretreatment process for NOx reduction from cement kiln. Fuel Processing Technology, 210 (2020), pp. 1-12
[3] Jun, Cai; Huixing, Wu; Qiangqiang, Ren; Li, Lin; Tuo, Zhou; Qinggang, Lyu: Innovative NOx reduction from cement kiln and pilot-scale experimental verification. Fuel Processing Technology,1 99(2020), pp 1-12
[4] Yunhui, Gua; Haihua, Cao; Wei, Liu; Xiaohu, Lin; Tao, Zheng; Wei, Cheng; Juwen, Huang; Jingcheng, Xu: Impact of co-processing sewage sludge on cement kiln NOx emissions reduction. Journal of Environmental Chemical Engineering, 9 (2021), pp. 1-8
[5] Ruifeng Dong; Hongfang, Lu; Yunsong, Yu; Zaoxiao, Zhang: A feasible process for simultaneous removal of CO2, SO2 and NOx in the cement industry by NH3 scrubbing. Applied Energy, 97 (2012), pp. 185-191
[6] Shubham, Kinekar; Shital, Mone; Adnan, Taqi; Pravin, Mane; Bajirao, Gawali; Vivek, Vitankar: NOx reduction in calciner using air staging and raw meal split technology. Materials Today: Proceedings 45 (2021), pp. 3091-3096
[7] Yasin Orooji; Mohammad Javadi; Hassan Karimi-Maleh; Alireza Zamani Aghaie; Kazem Shayan; Afsaneh L. Sanati; Rozhin Darabi: Numerical and experimental investigation of natural gas injection effects on NOx reburning at the rotary cement kiln exhaust. Process Safety and Environmental Protection, 151 (2021), pp. 290-298
[8] Tang Xinyu: A cement SNCR intelligent method and system. 2021, CN114067933A
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