On the gains and losses of ultra-low emission of nitrogen oxides in cement plants

Date: 2019-06-20 Views:

Preface


With the State Council publicly releasing the "Three-Year Action Plan to Win the Blue Sky Defence War", it is required to "by three years of efforts, significantly reduce the total emissions of major air pollutants, synergistically reduce greenhouse gas emissions, and further significantly reduce PM2.5 concentration; to 2020 In the year, the total emissions of sulfur dioxide and nitrogen oxides decreased by more than 15% compared with 2015; the concentration of cities with PM2.5 not reaching the prefecture level and above decreased by more than 18% compared with 2015.” Locally for steel, building materials, coking, foundry, colored High-emissions industries such as chemical and chemical industries have separately formulated peak-peak production plans and implemented differentiated management measures. Some local governments require that by the end of 2020, the production of cement clinker production lines will adopt technologies such as the process of suppressing nitrogen oxides and raw fuels. Upgrade and reform measures to reduce the concentration of nitrogen oxides emissions, strictly controlled below 160 mg / m3, to achieve ultra-low emissions in the cement industry. The ultra-low emission without increasing energy consumption and other pollutants is definitely an improvement in the quality of the atmospheric environment. However, under the current conditions of relatively mature denitration technology, it is necessary to increase the use of reducing agent (NH3) in order to achieve ultra-low emission. It will cause an increase in ammonia slip, an increase in clinker energy consumption, and a transfer of pollutants. In the implementation process, it is necessary to increase investment in research to control the production of nitrogen oxides at the front end instead of focusing on end treatment. The implementation of ultra-low emission of nitrogen oxides in cement plants should consider the increase in PM2.5 and the transfer of pollutants due to ammonia slip.

1 ammonia escape leads to an increase in PM2.5


At present, most of the denitrification of cement production line uses ammonia reducing agent to remove nitrogen. The ammonia reducing agent mostly uses ammonia water and urea. The ammonia water has lower cost and is widely used, especially selective non-catalytic reduction technology. Therefore, ammonia escaping is an inevitable problem in the actual production process, and there are ammonia escaping in different degrees in the transportation, storage and denitration of ammonia.

Ammonia is an aqueous solution of gaseous ammonia. Ammonia is highly volatile and overflows. It has a strong pungent odor, has a burning and explosion hazard, and has occupational health hazards. The reaction of ammonia with an acid produces an ammonium salt, ammonium sulfate, ammonium nitrate, etc., to form haze. The chemical composition of PM2.5 consists of a water-soluble component, a carbon-containing component and an inorganic component, and the latter two components are relatively small. The water-soluble component is one of the important components of PM2.5, generally accounting for 20% to 60% of the mass of PM2.5. The secondary ions such as SO42-, N03-, NH4+ are the main water-soluble ions in PM2.5. Its concentration accounts for about one-third of the mass of PM2.5. It is mainly derived from the secondary conversion of gaseous precursors SO2, NOx and NH3. The conversion rate is affected by temperature, humidity, atmospheric photochemical reaction and regional transport. Impact [1]. Therefore, ammonia escape has a greater contribution to PM2.5.

According to Xue Wenbo [2] and other papers "The Impact of Ammonia Emissions on PM2.5 Pollution in China", based on the WRF-CMAQ meteorological chemical coupled air quality model, the impact of ammonia emissions on PM2.5 concentration in urban cities was quantitatively simulated. The results show that the contribution rate of ammonia emission to the annual average concentration of sulfate, nitrate, ammonium salt and PM2.5 in cities nationwide is 4.2%, 99.8%, 99.7% and 29.8%, respectively. Controlling ammonia emissions will effectively reduce PM2.5 concentration. In particular, nitrate and ammonium salt contamination can be significantly reduced.

According to the formation mechanism of secondary ammonium nitrate formation in the atmosphere, there are mainly two ways [3]: in the case of sufficient N H3, N O2 is oxidized by ·OH (hydroxyl radical) to form HNO3, and nitric acid gas or liquid and NH3 occur. The reaction produces ammonia nitrate; in the case of insufficient NH3, N2O5 on the surface of the particles undergoes heterogeneous hydrolysis to form HNO3, and then reacts with NH3 to form ammonia nitrate. Its main reaction formula is:

NH3 (gas) + HNO3 (gas) → NH4NO3 (solid or solution) 17 80

The main reaction formula for the formation of ammonium sulfate is:

2NH3 (gas) + H2SO4 (gas) → (NH4)2SO4 (solid or solution) 34 132

It can be seen from the above formula that 1 g of ammonia can produce 80/17 g of ammonium nitrate or 132/34 g of ammonium sulfate, that is, escaping 1 mg of ammonia produces 4.7 mg of ammonium nitrate or 3.9 mg of ammonium sulfate. The formed PM2.5 will also grow by 4.7 times or 3.9 times.

2 Ammonia Escapes and Pollutant Emissions from Cement Plants


Under the current cement burning technology conditions, it is difficult to achieve the nitrogen oxide emission standard of 400 mg/m3 by the front-end control technology. Most cement plants still rely on the back-end treatment to meet the emission requirements, and the cement factory with better firing control. The amount of ammonia per tonne of clinker (calculated as 25% ammonia) is 1.5 kg/t clinker, the higher ammonia content is even 5 kg/t clinker, and the ammonia content is generally 3.5 kg/t clinker. The exhaust gas ammonia escape is generally 5-8 mg/Nm3. The ultra-low emission of nitrogen oxides without breaking the existing process technology will inevitably increase the amount of ammonia used and cause more ammonia to escape.

According to the operation of the building materials industry announced by the National Development and Reform Commission in 2017, the national cement output in 2017 was 231.625 million tons, and the clinker production was 70% of the cement output. The general level of emission standards was estimated, and the ammonia escaped 5 mg/Nm3. When the clinker discharge capacity is 2.6 Nm3/kg, the annual ammonia slip of the cement plant in China is: 2 316 250 000 × 0.7 × 2 600 × 0.000 000 005 = 21 078 (t / a) The ammonium nitrate-based PM2.5 emissions can reach 99 067 t/a. The author points out the advantages and disadvantages of denitrification in selective non-catalytic reduction technology in cement plants [4], using selective non-catalytic reduction technology. In the case of denitrification, the reducing agent will have an occupational health hazard, and the corresponding harmful exhaust gas will be transferred and discharged during the production process of the reducing agent for detailed analysis. Taking the commonly used 5 000 t/d cement clinker production line in China as an example, using liquid ammonia (or ammonia) denitrification, energy consumption per tlinker increased by 1.90 kgce/t, increased SO2 emissions by 45.6 g/t, increased CO2 emissions by 4 668.3 g/t, increased NOx emissions by 19.19 g/t; denitration with urea, tons Clinker energy consumption increased by 2.40 kgce/t, increasing SO2 emissions by 57.6 g/ t, increase CO2 emissions by 7 025.2 g/t, and increase NOx emissions by 24.24 g/t.

According to the amount of ammonia (25%) of clinker clinker (3.5%), the annual amount of ammonia water is: 2 316 250 000 × 0.7 × 0.003 5 = 5 674 813 (t / a) according to the comprehensive energy consumption limit of synthetic ammonia unit products 2 200 kgce / t, the production of ammonia (25%) consumption of standard coal is: 5 674 813 × 2.2 × 25% = 3 121 147 (t / a)

According to the conch's 5 000 t/d production line, the denitration needs to increase the standard coal consumption by 150 kg/h. The standard coal consumption of clinker is 0.72 kg/t clinker due to the injection of ammonia water to evaporate and warm the water. The increase in coal consumption caused by cement burning is: 2 316 250 000 × 0.7 × 0.000 72 = 1 167 390 (t / a) Thus, the annual denitration consumption of selective non-catalytic reduction technology adopted by China's cement industry to meet current emission standards The standard coal reaches 3 121 147+1 167 390=4 288 537 t. According to the CO2 emission of 1 t standard coal burning 2 457 kg/t, the annual CO2 emission from denitrification transfer reaches 19.53 million tons. The calculation of sulfur dioxide emissions of 1 t standard coal is 24 kg/t, and the annual SO2 emission from denitrification and discharge is 100,000 tons; the annual denitration and discharge is calculated according to the emission of 1 t standard coal NOx emissions of 10.70 kg/t. The NOx emissions amount to 50,000 tons. If ultra-low emissions of nitrogen oxides are carried out under existing process conditions, ammonia slip and pollutant transfer emissions will increase exponentially.

3 Analysis of China's and foreign NOx emission standards


China's "Cement Industry Air Pollutant Emission Standards" (GB4915-2013) has an emission limit of 400 mg/m3 for nitrogen oxides (in terms of NO2) and 320 mg/m3 for special areas. The US Portland Cement Plant Emission Standard (EPA 40 CFR 60 Subpart F) emits no more than 1.50 pounds of NOx per ton of clinker, approximately 340 mg/m3. The European Union's Industrial Emissions Directive (Integrated Pollution Prevention and Control) (2010/75/EU) has an emission limit of 450 mg/m3 for nitrogen oxides (in terms of NO2). The German “Air Quality Control Technical Directive” (TA luft) has a nitrogen oxide emission standard of 0.50 g/m3. Japan's "Air Pollution Prevention and Control Act" controls the emission of nitrogen oxides from fixed sources to 123-820 mg/m3 according to different industry requirements. It can be seen that China's cement industry has stricter requirements for nitrogen oxide emission limits. Denitrification in cement production line is a complex system engineering. Firstly, it is affected by the composition of various raw materials. The equipment configuration and technical equipment level of each production line are also different. The technical parameters and control indicators of production operation vary widely to meet the requirements of nitrogen oxides. The requirements of emission standards require in-depth and meticulous analysis of the firing process, thorough research on the characteristics of various raw materials, and the establishment of efficient and low-cost denitration systems from the solution of nitrogen oxides. Put on how to backend governance. The cement emission standards are formulated according to the current technical level and management methods. With the advancement and development of technology, the nitrogen oxide emission standards will become more and more strict, but for the moment, the ultra-low emission of nitrogen oxides is proposed. It is too early, at least at the same time as the ultra-low emission of nitrogen oxides, the issue of ammonia slip and emissions transfer.

4 Online automatic monitoring of cement plants


According to the “Technical Guidelines for Self-Monitoring of Polluting Units” (HJ848-2017), specific regulations have been made on the monitoring points, indicators and frequency of organized exhaust emissions in cement plants. The items requiring automatic monitoring are: particulate matter, nitrogen oxides. And sulfur dioxide, and the monitoring of ammonia only requires a quarterly monitoring, ammonia emissions are not included in the online automatic monitoring, failed to cause sufficient attention of the environmental protection department. Compared with nitrogen oxides, the reason why ammonia gas is not valued may be that the previous calculation model underestimated the influence of ammonia gas. From the above analysis, the contribution of ammonia gas to atmospheric pollutants such as PM2.5 is still

It should not be overlooked, at least the level of emphasis should be mentioned relative to the level of nitrogen oxides. At present, the statistics on ammonia emissions of atmospheric pollutants in cement plants are still relatively scarce. It is recommended that when the relevant emission standards are revised, the cement plants using ammonia denitration technology should be required to conduct automatic online monitoring of ammonia slip. The standard should also deal with ammonia in waste gas. The bottom concentration is treated differently from the ammonia slip concentration.


5 Conclusion


In recent years, the impact of nitrogen oxides on atmospheric smog in cement plants has been recognized. As a result, various ultra-low emission policy measures for nitrogen oxides have been issued in various places, and even calls for control below 50 mg/m3 have been proposed, resulting in denitrification of cement kiln. Ammonia water volume has increased sharply. In the absence of breakthroughs in key technologies for denitrification, ammonia slip is an unavoidable problem, and its contribution to the formation of haze weather needs to be further studied. This paper summarizes the chemical reaction, formation conditions, emission status, monitoring technology and related standards of ammonia slip formation PM2.5, and provides theoretical reference for future in-depth research. At the same time, it is recommended to add ammonia escape online when the emission standard is revised. Monitoring and requirements. At the same time, it is hoped that all research institutions will increase investment in scientific research, and make breakthroughs in the future cement production technology, and try to control the generation of nitrogen oxides from the source, so that the pollution of PM2.5 in China can be fully controlled and improved, and the cement industry will become A model for cleaner production.

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