Control and treatment of harmful gases

In order to reduce or even eliminate the increasingly serious harm of harmful gases, the emission of harmful gases must be controlled and managed. From the perspective of the generation process of harmful gases, controlling and preventing air pollution caused by harmful gases is mainly a matter of controlling and treating pollution sources. Since human activities are the main factor in the generation of harmful gases, the primary issue is to formulate and implement various policies and regulations for environmental protection and strengthen the protection and management of air quality.

6.5.1 Atmospheric environment quality standards

The formulation of atmospheric environment protection regulations and atmospheric environment standards is the scientific basis and means for implementing atmospheric environment management, control and control of harmful gases.

6.5.1.1 Types of atmospheric environment protection regulations and atmospheric environment standards

Countries around the world have formulated various atmospheric environment protection regulations and atmospheric environment standards based on their actual conditions. Our country promulgated the "Environmental Protection Law of the People's Republic of China" on December 26, 1989, and the "Atmospheric Prevention and Control Law of the People's Republic of China" on September 5, 1987, which played an important role in protecting the atmospheric environment. effect.

Atmospheric environment standards can be divided into: atmospheric environment quality standards, concentration standards of harmful substances in the atmosphere, air pollutant emission standards, air pollution control technical standards, etc. according to their uses. According to their scope of application, they can be divided into: national standards, local standards and industry standards. Our country has successively formulated and promulgated the "Atmospheric Environmental Quality Standards", "The Maximum Allowable Concentration of Hazardous Substances in the Atmosphere in Residential Areas", "The Maximum Allowable Concentration of Hazardous Substances in the Air in Workshops", "Trial Standards for Industrial "Three Wastes" Emissions", etc.

6.5.1.2 my country’s atmospheric environment quality standards

my country’s atmospheric environment quality standards are divided into the following three levels.

First-level standard: In order to protect the natural ecology and human health, air quality requirements that will not cause any harmful effects under long-term exposure.

Second level standard: To protect human health and urban and rural animals and plants, air quality requirements that do not cause harm under long-term and short-term exposure conditions.

Level 3 standards: air quality requirements to protect people from acute and chronic poisoning and normal growth of urban animals and plants (except sensitive ones).

The three-level standard concentration limits of air pollutants are listed in Table 6.10.

Table 6.10 Level 3 Air Pollutant Standard (GB3095-82)

6.5.1.3 Air Pollution Index

Air Pollution Index (API) ) is currently a way for many developed countries or regions in the world to assess the ambient air quality status of the region. It converts a series of complex air quality monitoring data into a form that is easy for people to understand and grasp after being processed according to a certain method.

According to the unified regulations of the State Environmental Protection Administration, my country’s air quality is divided into the following five levels:

(1) When the API value is between 0 and 50, the air quality level is Level I. The air quality For excellence.

(2) When the API value is between 51 and 100, the air quality level is Level II, and the air quality is good.

(3) When the API value is 101 to 200, the air quality level is Level III, and the air is lightly polluted.

(4) When the API value is 201 to 300, the air quality level is Level IV, and the air is moderately polluted. At this time, it will have a significant impact on people with sensitive constitutions, and the general population may also experience symptoms such as eye discomfort, asthma, cough, and excessive phlegm.

(5) When the API value is greater than 300, the air quality level is Level V, which is severe pollution. At this time, healthy people will also have obvious symptoms, their exercise tolerance will be reduced, and some diseases may appear in advance, so they should avoid outdoor activities.

Many cities in my country publish air quality status in the form of "Air Quality Weekly Report". For example, from February 27 to March 5, 1998, the air pollution index in Beijing was 203, the air quality was level four, and the primary pollutant was nitrogen oxides. See Table 6.11 for details. Another example is the air pollution index in major cities across the country from March 20 to 26, 1998: Beijing 138, Changchun 89, Changsha 92, Chongqing 97, Dalian 63, Fuzhou 47, Guangzhou 166, Hangzhou 74, Harbin 74, Hefei 60, Jinan 103, Nanjing 56, Nanning 55, Qingdao 113, Shanghai 90, Shenyang 100, Shenzhen 82, Suzhou 55, Shijiazhuang 192, Tianjin 108, Wuhan 84, Xiamen 46, Zhengzhou 194, Zhuhai 112. The air quality in most cities has been relatively good this week. This is due to the influence of a strong cold airflow, which has led to widespread snow and rain. Beijing alone dropped by 56 from 194 last week. Recently, some cities in my country have published daily air quality status in the form of "Air Quality Daily".

Table 6.11 Beijing Air Quality Weekly Report (February 27 to March 5, 1998)

6.5.2 Main ways to control harmful gas emissions

In order to control the sources of air pollution, it is necessary to solve the problem of emission pathways of harmful gases. Human production and living activities inevitably emit harmful gases, and reasonable emission pathways can reduce the concentration of harmful gases and reduce air pollution.

There are many ways to control harmful gas emissions, the main ones are as follows.

6.5.2.1 Comprehensive planning and rational layout

Starting from the coordination of the relationship between economic development and environmental protection, comprehensive planning and rational layout of production and living facilities in a region are carried out. It is an important way to control harmful gas emissions. If the industry in an area is too concentrated and the amount of pollutant emissions is too large, the concentration of harmful gases in the area will be too large, which will not be diluted and diffused quickly, thus easily causing harm. Therefore, industries should be reasonably dispersed; factory site selection should consider terrain conditions that are conducive to the diffusion of pollutants; factories should be located in downwind areas of the city's dominant wind direction, and factories that have a cooperative relationship with each other should be located together to reduce the emission of exhaust gases. Emission quantity. There should be a certain distance between the factory area and the living area, and greening and afforestation can reduce the harm of harmful gases.

6.5.2.2 District heating and central heating

Compared with the heating efficiency of stoves scattered in thousands of households, central heating and heating will greatly improve the heating efficiency, and also greatly reduce the harmful effects. Gas emissions. Generally, boiler efficiency can be increased by 30%. It is estimated that for the same ton of coal, scattered use by residents produces 1 to 2 times more smoke and 3 to 4 times more dust than concentrated use by industry.

6.5.2.3 Choose an emission method that is conducive to the diffusion of harmful gases

Different emission methods have different diffusion effects of smoke. The use of high chimney emissions can spread harmful gases to a higher and wider range, reducing the concentration of harmful gases on the ground near the pollution source. The other is collective emission, that is, several (usually 2 to 4) smoke exhaust equipment are concentrated into one chimney to discharge them. This can increase the smoke exhaust speed at the chimney mouth and have a good diffusion effect.

6.5.2.4 Change the composition of combustion

Replacing coal with clean gas or liquid fuel can significantly reduce the concentration of particulates and harmful gases in the atmosphere. Changing the fuel composition directly solves the problem of pollutant generation and is therefore an effective way to control and prevent air pollution.

6.5.2.5 Reform the production process and comprehensively utilize waste gas

By improving the combustion process, the combustion efficiency can be increased as much as possible and the emission of harmful gases can be reduced as much as possible. For example, technological transformation of old boilers and combustion equipment, and improvements to automobile engines and fuels can all reduce exhaust emissions. In the production process, we strive to use the waste gas emitted in one production as raw materials in another production, so as to achieve the dual benefits of reducing pollutant emissions and turning waste into treasure.

6.5.2.6 Develop new energy

Solar energy, wind energy, hydropower, geothermal energy, tides, biomass, etc. are mostly renewable energy sources, and no fossil fuels are produced during the utilization process. environmental problems caused. Although there are still problems in the development and utilization of some new energy sources, they are the direction of future energy development.

6.5.3 Introduction to technical measures to control certain harmful gas emissions

6.5.3.1 Control of sulfur dioxide

There are currently three main ways to control SO2: Removal of sulfur from flue gases, desulfurization of fuels and desulfurization in combustion.

6.5.3.1.1 Removing sulfur from flue gas

There are many ways to remove SO2 from flue gas. Depending on the concentration of SO2 in the flue gas, the treatment method and The process is also different. Their principle is to turn SO2 into useful chemical products or fertilizers after appropriate reactions.

For high-concentration SO2 flue gas, SO2 is generally catalyzed to generate sulfuric acid for recycling:

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For low-concentration SO2 flue gas, The following methods can be used to deal with it.

(1) Ammonia method. Ammonia is used to absorb SO2 in the tail gas, and then concentrated sulfuric acid is added for treatment, thereby generating ammonium sulfate and high-concentration SO2. Ammonium sulfate can be used as fertilizer, and high-concentration SO2 can be recycled to the sulfuric acid plant to be reused to produce sulfuric acid.

(2) Calcium method (milk of lime method). This method uses calcium hydroxide or calcium carbonate to absorb SO2 tail gas for desulfurization and produce gypsum as a by-product. Because limestone is widely distributed, this is an ancient method that is used more frequently.

(3) Activated carbon method. Using the activity and large specific surface area of ??activated carbon, SO2 in the flue gas reacts with oxygen and water vapor on the surface of the activated carbon to generate sulfuric acid and be absorbed. Then, hot reducing gases, such as carbon monoxide and hydrogen, are introduced to desorb SO2.

6.5.3.1.2 Fuel Desulfurization

Fuel desulfurization is achieved during the process of coal washing and conversion. For example, using gravity separation method, the sulfur content of raw coal can be reduced by 40% to 90% after separation. In order to control SO2 emissions, countries such as Japan, the United Kingdom, and Canada wash all thermal coal. The washing rate of raw coal in my country is only 17.7%. Direct gasification or liquefaction of coal, that is, decarbonization or hydrogenation of coal to change its original carbon-to-hydrogen ratio, can turn coal into clean secondary fuel.

6.5.3.1.3 Combustion desulfurization

Combustion desulfurization is carried out using fluidized bed combustion technology. This method is to spray lime or dolomite flowing medium into the furnace, and perform multi-stage combustion with coal particles in the furnace. SO2 can be removed in the form of calcium sulfate.

This combustion method can not only remove SO2, but also reduce nitrogen oxide emissions due to its low combustion temperature, so it has attracted attention.

6.5.3.2 Control of nitrogen oxides

There are many methods for controlling nitrogen oxides, which can be divided into two categories: dry methods and wet methods. Among the methods that have been applied Among them, it can be divided into three types: catalytic reduction method, liquid absorption method and solid adsorption method.

6.5.3.2.1 Catalytic reduction method

The catalytic reduction method reduces nitrogen oxides to harmless nitrogen under the action of a catalyst, thereby decolorizing the exhaust gas and eliminating pollution. Purpose. According to different catalysts, it can be divided into two types: selective catalytic reduction and non-selective catalytic reduction.

The selective catalytic reduction method selectively reduces nitrogen oxides in exhaust gas under the catalytic action of platinum or copper. For example, ammonia is used as a reducing agent to selectively react with NOx in the gas:

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But ammonia does not react with nitrogen in the gas.

The non-selective catalytic reduction method uses precious metals as catalysts and reacts with reducing agents to reduce nitrogen oxides and oxygen in the exhaust gas together to generate nitrogen, water, and carbon dioxide. For example, using methane as the reducing agent and platinum as the catalyst, the reaction is as follows:

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6.5.3.2.2 Liquid absorption method

Use sodium carbonate, sodium hydroxide, lime milk or ammonia solution to absorb nitrogen oxide exhaust gas. For example, when sodium carbonate is used to absorb nitrogen oxides, the reaction formula is as follows:

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In the reaction, since NO is difficult to dissolve in water, only when it is appropriately combined with NO2 When the ratio is high, the rate of absorption by the alkali solution will be high. Generally, the rate is highest when NO:NO2=1:1.

6.5.3.2.3 Solid adsorption method

Use solid adsorbents, such as activated carbon, silica gel, various types of molecular sieves, peat, etc., to absorb nitrogen oxide molecules and water molecules. It selectively adsorbs oxygen molecules and reacts chemically to generate nitric acid. This eliminates pollution and allows nitrogen oxides to be recycled, so it is a better method.

6.5.3.3 Purification of automobile exhaust

Automobile exhaust is one of the main sources of harmful gases. The harmful gases it emits include: carbon monoxide, hydrocarbons, and nitrogen oxides. and sulfur dioxide, etc. The composition of automobile emissions varies greatly depending on the fuel (gasoline, diesel) and transportation conditions. Taking gasoline vehicles as an example, exhaust purification is divided into pre-treatment (such as unleaded gasoline), in-machine purification (such as improving the fuel system, ignition system, fuel supply system, etc.) and post-processing (such as using thermal reactors, catalytic Reaction), after this purification process, the content of carbon monoxide, hydrocarbons, nitrogen oxides and other components in the exhaust gas can be reduced.

6.5.3.4 Control of chlorofluorocarbons

Chlorofluorocarbons are mainly used in the production and use of aerosol sprays such as refrigerants, foaming agents, and solvents. Such substances are used in products that emit emissions, such as freezers, refrigerators, air conditioners, fire extinguishers, etc. It can be controlled through the following methods:

(1) Improving utilization efficiency and reducing operating losses are the simplest ways to reduce CFCS emissions. In the United States, about 2/3 of the total amount of CFC-12 is used in automobile air conditioners, of which 30% is lost through leakage. Measures such as strengthening seals and valves, reducing the number of joints, etc. can be used to reduce leakage; it is used in refrigerators to reciprocate The CFCS consumption of a rotary compressor is only 1/3 to 1/2 of that of a rotary compressor.

(2) Recovery and recycling are also the main methods to reduce CFCS emissions. Most of the CFC11 used to make flexible foam is lost due to volatilization during the production process. 50% of it can be recovered through carbon filters. For CFC-12 used to make solid foam, similar technology can also reduce emissions by half. quantity.

(3) Improve the use of CFCS products. Some CFCS products pose little or no threat to the ozone layer and can replace the use of CFC-11 and CFC-12 to a certain extent. The currently used CFC-22 degrades quickly in the atmosphere, and its ozone loss is only 1/5 of CFC-12. Therefore, CFC-22 can be used in air conditioners and refrigerators instead of CFC-12.

(4) Find other products to replace or reduce the use of CFCS as much as possible. For example, the foam insulation layer used in the outer shell of refrigerators and freezers is made of CFC-11. Currently, there are several types of advanced insulation materials as substitutes. For example, insulation materials composed of evacuated slats containing fine powder are made of CFC-11. Vacuum plates made of silica gel, or vacuum systems composed of evacuated metal shells, etc.

In addition to the above-mentioned main ways to control harmful gas emissions and treatment technical measures, afforestation and greening the environment are also important measures to purify the atmosphere. Plants not only regulate climate, maintain air humidity, and prevent soil erosion, but also purify the air.

The main manifestations are: ① Green plants absorb CO2 and release oxygen through photosynthesis, and maintain the balance of oxygen and carbon dioxide in the atmosphere. For example, a 10 m2 forest can absorb the CO2 exhaled by a person throughout the day and night; ② Plants are responsible for many harmful gases in the atmosphere. It has the ability to purify. For example, 1 m2 of cedar trees can absorb 0.07 kg of SO2 per year, and 1 km2 of alfalfa can reduce SO2 in the air by more than 60 tons per year; ③Plants, especially trees, have a strong barrier to dust. Filtration and absorption capabilities, for example, the dust blocking rate of cypress is 12.8% and that of acacia is 17.58%; ④ Greening trees have strong bactericidal and sterilizing abilities and can effectively reduce the content of pathogenic bacteria in the atmosphere. Therefore, large-area greening is an important measure to control harmful gas hazards.