Editor’s note:
The New Year’s Eve of most areas of North China in 2021 (from the New Year’s Eve of February 11 to the evening of February 13) is accompanied by severe smog. Many people don’t understand: during the Spring Festival, social activities have decreased, and emissions from general industrial enterprises, transportations, constructions and other industries has dropped significantly, but why does pollution still occur frequently? Is it because "fireworks have a great short-term influence" or the weather condition are unfavorable, or are there other deep-rooted reasons？In this regard, the CBCGDF international environment fund put forward problems and suggestions on the advanced treatment of air pollution to the NPC and the CPPCC representatives.
We will continue to sincerely invite people from all walks of life to actively make suggestions for the constructions of the country. Please send your suggestions for the two sessions to this email box: v31@cbcgdf.org. Please indicate the email subject and the file name as: 2021 CBCGDF’s Solicitation of Proposals for Two Sessions – Your Name – Proposal on * *. We are looking forward to your letter.
Problems and Suggestions on Deep Treatment of Air Pollution
Proponent: China Biodiversity Conservation and green development foundation, international environment fund
The severe smog outbreak in 2013 prompted stronger actions to control air pollution, which witnessed goo results. However, air pollution ahs yet been under control and massive smog frequented in northern China (except Beijing) especially in autumn and winter, despite large-scale shutdown and production restriction. To make future air pollution and control more effective, losses, gains, and suggestions are presented below for reference.
1. Problem
Since 2013, governments at all levels have cracked air pollution with iron fists, and three conventional air contaminants, small particles, sulfur dioxide (SO2), and nitrogen oxides (NOx) have all decreased greatly. Indicators all tell an encouraging story, but reality on the ground shows another scene. In spite of lockdown measures during COVID-19, heavy smog didn’t stop.
Pollutant emissions and adverse air diffusion conditions are two main factors for smog to occur. Conventional pollutants and unconventional ones explain the frequent occurrence of smog in autumns and winters in northern China. The former is more likely to cause smog during the peak period (2008-2011). In addition, adverse air diffusion conditions are not only caused by natural factors, but also related to human activities, especially to industrial water vapor emissions.
After repeated research and analysis, we believe that the current technical defects and lack of standards for emission reduction of conventional air pollutants (SO2 and NOx) leads to the emission of a large number of unconventional pollutants, which, if left unregulated, is the root cause of the repeated occurrences of smog. The unfavorable air diffusion conditions caused by a large number of industrial water vapor emissions are an important reason.
2. Defects of treatment Techniques and Lack of Standards
2.1 At the same time of NOx emission reduction, a large amount of ammonia emission is produced
At present, SCR, the mainstream technology of NOx emission reduction uses ammonia as reducing agent. To achieve the ultra-low control index far higher than European and American standards, excessive ammonia injection is very common in enterprises. The excess ammonia forms ammonium salt and other ammonia nitrogen, which is carried out of the flue through fly ash, desulfurization wastewater, droplets etc. Finally, ammonia is formed and discharged to the atmosphere. Many denitration facilities in non-electric industry are set after desulfurization facilities. Excess ammonia is directly discharged into the atmosphere, which is more serious than power plants.
Ammonia emission in industrial process has been ignored. International and domestic studies have shown that industry has long been the main source of ammonia emission in urban areas, especially after the start of ultra-low emission, the consumption of industrial ammonia has increased significantly, and the ammonia content in the air has also increased.
Ammonia is the only high concentration alkaline gas in the air. The ammonia discharged into the atmosphere reacts with acid gases such as nitric acid or sulfuric acid to form secondary fine particles such as sulfate, nitrate and ammonia, which is an important driving force for the transformation of gaseous pollutants in the air into particulate pollutants. In addition, the influence of ammonia (ammonia substances) on smog is also shown in the high humidity environment. Ammonia and nitrate dissolved in droplets as nutrients, which lead to a large number of rapid propagations of some microorganisms, causing the rapid development of smog.
In recent years, the rainfall in North China has changed from local acid rain to comprehensive alkalescence, which also proves from a macro point of view that reducing conventional pollutants leads to excessive ammonia emission. In the case of weak wet deposition capacity of ammonia ammonium salt in low temperature season, excessive ammonia emission will lead to the existence of fine particles in the air for a long time and diffusion distribution in a large range.
2.2 SO2 emission reduction, wet desulfurization process and fine particle emission
At present, most power plants and industrial enterprises use the wet desulfurization process to reduce sulfur dioxide (SO2). Although the desulfurization effect is obvious, since the middle GGH cancelled, the flue gas is discharged at low temperature, low altitude and high humidity. The fine particles in the desulfurization slurry are discharged into the air through saturated wet flue gas entrainment, and the concentration of condensable particles CPM is high and uncontrolled. When GGH is cancelled, under the condition of low temperature season and saturated wet flue gas, the discharged flue gas forms a thick plume, and sulfur dioxide forms sulfite in the droplets in the plume, which is an excellent condition for further formation of sulfuric acid. Such a process of emission change cannot be monitored at all.
According to the test results of 17 units achieving the ultra-low emission standard in Beijing and Shanghai, the average value of CPM is 13.93 mg / m3, exceeding the ultra-low emission particulate matter standard by 10 mg / m3. More seriously, the particle size of CPM is small and the particle number concentration is high. The number of grains per cubic centimeter is tens of millions. The same mass concentration of CPM, the number of grains is far more than the filterable particles, the extinction effect on the air is also far greater than the filterable particulate matters. These CPMS act as condensation nodules in the air. When they condense and absorb moisture, they grow up and produce a secondary compound process in the liquid phase. When the smog breaks out, they appear in the form of PM2.5 mass concentration.
Yong Zhou, a researcher of Qilu University of Technology, confirmed through multiple evidence studies that the sudden increase of PM2.5 particle number concentration after wet desulfurization is the main cause of smog outbreak in Beijing, Tianjin, Hebei and surrounding provinces from 2013 to 2014. The most important factor that caused the increase of PM2.5 particle number concentration is the cancellation of GGH in wet desulfurization process, which is aggravated by the large-scale denitrification at the same time. This also explains why before 2011, when the conventional pollutants such as sulfur dioxide and nitrogen oxides were at the peak, there was no serious smog, but after the increase of control measures (under the stimulation of new air pollutant emission standards, denitration price increase and strict management policies, large-scale construction or transformation of denitration and desulfurization facilities was started in 2012. At the end of 2012, the wet desulfurization system without GGH and in series with denitration facilities was fully implemented and strict online monitoring was implemented), on the contrary, the smog appeared! This phenomenon occurs not only in North China, but also nationwide. Now in winter, from south to north, the sky over major industrial cities are gray for most of the time.
As the CPM of condensable particles is not included in the current particulate matter standard, it is not being regulated. This loophole in the particulate matter index shows why the performances of the environmental protection index did not solve the actual problem of air pollution. Because the indicators only control the filterable solid particulate matter. CPM, which has a more serious impact on smog, has not been regulated. The evaluation of ultra-low emission significantly underestimates the actual emission level of particulate matter. The so-called ultra-low emission is actually about ultra-low on non-key smog-causing indicators, and it is impossible to treat smog properly! In 2018, the pollution control measures commonly known as "dehumidification and dewhitening" were adopted to control the white plume after wet desulfurization, which had a certain effect on CPM control. However, due to various reasons, they were stopped by the competent department! That's why during the epidemic period, only power plants fully realizing ultra-low emissions and other limited industries can carry out emissions, smog still swept most of China.
2.3 Industrial water vapor emission
Humidity is a key factor in the formation of smog. Observation shows that severe smog is accompanied by high humidity weather. During the sensitive stage and smog forming process before the formation of obvious smog in autumn and winter in northern China, the water vapor emitted from coal / natural gas combustion, wet desulfurization and cooling tower contributed about 20% to the atmospheric humidity at that time, and also contributed greatly to the formation of smog.
In the industrial concentration area, the water vapor aerosol formed by water vapor thickens the local cloud layer and forms a "pot cover" over the city, which prevents the free diffusion of pollutants. The air includes all kinds of fine particles, including CPM absorbed humidity and grow in this high humidity environment formed by man-made emission of water vapor（the results show that if the air humidity reaches 95%, the particle size of condensable particles can increase 6-16 times in a short time）. Combined with sulfur oxides, nitrogen oxides, VOC and other pollutants from various pollution sources, they form secondary particles, resulting in the aggravation of smog.
The emission of a large amount of industrial water vapor around the city would also cause the increase of stable and inversion weather. The water vapor aerosol formed by water vapor thickens the local clouds, forms a "pot cover" over the city, weakens the atmospheric fluidity, forms a static and stable environment, and prevents the free diffusion of pollutants; due to the extinction effect of wet water-soluble ionic particles, the atmospheric boundary layer blocks the sunlight to the ground, resulting in low surface temperature, forming an inversion environment. At the same time, these industrial water vapor have heat, which is higher than the ambient temperature. When they are cooled and condensed at high altitude, they release heat, which makes the air temperature at high altitude rise, and also contributes to the formation of inversion weather. The steady and inversion environment leads to the accumulation of pollutants in the atmospheric boundary layer near the surface, forming smog.
Industrial water vapor emission not only affects atmospheric diffusion conditions, but also carries a large number of fine particles. In addition to the fact that the white plume after wet desulfurization contains a large number of condensable particles and soluble salt, in order to save water, the cooling towers of many power plants use reclaimed water as make-up water. Continuous circulation causes that the salt content in water vapor increases greatly. It is estimated that the cooling tower of a million-kilowatt unit can discharge 200-400 kg of dissolved particulate matter into the air per hour, which is 4-8 times of the total flue gas particulate matter emission concentration of the same unit!
3. Analysis of smog formation and outbreak process
Despite years of efforts, conventional pollutants such as sulfur dioxide, SO2, nitrogen oxides, NOx, and particulate matter (dust PM) have been reduced to low levels, however, due to technical defects and lack of standards, the unconventional pollutants (including condensable particulate matter, CPM, ammonia and industrial water vapor) have not been treated and controlled, so that the autumn and winter smog in northern China has not been effectively controlled. The process of its formation and outbreak is as follows:
Under the condition of low temperature, steady and dry weather, the CPM with large number of uncontrolled particles is discharged into the air to form solid or liquid fine particles; wet desulfurization, cooling tower and other industrial steam emissions increase the relative humidity, and dry water-soluble ionic particles in the air absorb moisture and grow up; the residual nitrogen oxides, sulfur oxides, VOCs and ammonia in flue gas, and the dry water-soluble ionic particles after absorbing moisture and grow up and the fine particles in the water vapor are secondary compounded in the air to form secondary particles such as nitrate, sulfate, organic salt and ammonia. Due to the extinction effect of wet water-soluble ionic particles, the atmospheric boundary layer blocks the sunlight to the ground, resulting in low surface temperature and forming an inversion environment. Under the influence of inversion and stable air, these salt particles gather in the lower atmospheric boundary layer and form smog.
In high humidity weather, this process is accelerated and strengthened significantly, and the particle size of wet water-soluble ionic particles increased greatly. The newly discharged water vapor and its fine particles diffuse in the lower atmospheric boundary layer, the air density on the surface of the earth increases, the vertical structure of the atmosphere is relatively stable, the air flow between the top and the bottom is significantly weakened, and the stable and inversion temperature environment is strengthened. Meanwhile, ammonium, nitrate and other nutrients in the water-soluble ionic particles promote the rapid propagation of microorganisms, the mass concentration of PM2.5 increases rapidly, and the smog breaks out, which will last for a period of time until the atmospheric environment changes (such as strong wind, rain, cold air intrusion, etc.).
4. Proposal
4.1 After determining the main causes of smog, carry out targeted treatments to avoid the "shutdown and limited production" measures in autumns and winters
In order to maintain the environmental quality of the northern region, the "Air Pollution Emergency Management and Control Measures" have been implemented since 2017, that is, enterprises are required to "stop work and limit production" in polluted weather. Although this measure alleviates and mitigates the impact of smog pollution, it brings serious harm to the local economy, and also covers up the problems and defects of environmental treatment. From the economic point of view, it is only a 10 billion level investment for a province to control the current air pollution（if energy conservation measures are adopted to reduce emissions, the cost of investment can be recovered）. But the economic loss caused by large-scale shutdown and limited production is 100 billion level, and year after year. According to the previous analysis, the heavy smog is not caused by primary emission, but by the sudden increase of secondary particulate matter after the atmospheric diffusion conditions become worse. The function of "stop work and limit production" is limited. Therefore, we propose to cancel the "Emergency Control and Prevention Measures for Air Pollution" during the"14th Five Year Plan", stop all kinds of "limit production" under various names to ensure the sustainable development of the national economy, especially in the northern region.
4.2 Establishing Smog Control Office of the State Council
Due to the complexity of smog formation and the long-term nature of governance, the strength of environmental protection departments alone is not enough. In recent years, local governments have set up "Air Pollution Prevention and Control Offices" to coordinate the cooperation of environmental protection, energy, transportation, industry, science and technology, urban management and other departments. It is suggested that the State Council should also set up a "smog control office" to integrate resources and comprehensively coordinate the work of air pollution control from the top, rather than the current situation that the environmental protection department is fighting alone, and restricted by the inherent mode of thinking of some interest groups, so as not to carry out real and effective treatment.
4.3 Use waste heat recovery to treat the flue gas discharged after wet desulfurization
The flue gas after wet desulfurization contains not only water vapor and fine particles, but also waste heat. In order to implement the Notice of the State Council on Printing and Distributing the Action Plan for the Prevention and Control of Air Pollution, in 2015, the National Development and Reform Commission issued the implementation plan of waste heat warming project to encourage the recovery of flue gas waste heat, and carried out pilot projects in 150 districts and counties. At present, the technology of waste heat recovery and utilization is fully mature, in order to restrain smog, especially heavy smog, the waste heat and water vapor of flue gas after wet desulfurization should be recovered, letting the enterprise achieve the five benefits of energy saving, water saving, smog reduction, carbon reduction and income increase. This is the most effective way to control smog with the lowest economic cost.
4. Recovery of water vapor and waste heat discharged from industry
According to the industrial layout and environmental capacity of each region, take positive measures to encourage or require the recovery of water vapor and waste heat from cooling, smelting, chemical and other processes in power plants. These measures can reduce or even remove the "pot cover" above the city, greatly improve the atmospheric diffusion conditions, reduce the generation of smog and reduce the salt particles and pollutants discharged with water vapor. At the same time, reuse water, reduce the total amount of industrial water, reduce enterprise costs, waste heat can be recycled, resulting in economic benefits.
4.5 Add the content of ultra-low emission to make up for the lack of emission indicators
Incorporate ammonia (including ammonia gas and ammonium salt) in the control index, and the use of ammonia should be strictly controlled. In addition to the denitration of power and industry, the ammonia emission of urban areas also includes the ammonia emitting from motor vehicles, especially diesel vehicles, and from urea production.
The ultra-low standard of particulate matter must include the condensable particulate matter (CPM). At present, it is difficult to directly detect CPM, which can be monitored by indirect detection methods (such as measuring the conductivity of condensate water).
Translator/Lei Wang
Original Chinese article
https://mp.weixin.qq.com/s/LcsrQRkqnQdIQrCHwxqgig
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