Control and treatment of harmful gases

In order to reduce or even eliminate the increasingly serious harm of harmful gases, it is necessary to control and control the emission of harmful gases. Judging from the production process of harmful gases, controlling and preventing air pollution caused by harmful gases is mainly the problem of controlling and treating pollution sources. Since human activities are the main factors that produce harmful gases, the primary problem 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 environmental quality standard

The formulation of atmospheric environmental protection laws and regulations and atmospheric environmental standards is the scientific basis and means to implement atmospheric environmental management, control and treatment of harmful gases.

6.5. 1. 1 Types of atmospheric environmental protection regulations and atmospheric environmental standards

All countries in the world have formulated various atmospheric environmental protection laws and regulations and atmospheric environmental standards according to their own reality. China promulgated the People's Republic of China (PRC) Environmental Protection Law on February 26th 1989 and the People's Republic of China (PRC) Atmospheric Prevention Law on September 5th 1987, which played an important role in protecting the atmospheric environment.

According to their uses, atmospheric environmental standards can be divided into: atmospheric environmental quality standards, concentration standards of harmful substances in the atmosphere, emission standards of atmospheric pollutants and technical standards for air pollution control. According to its scope of application, it can be divided into national standards, local standards and industry standards. China has successively formulated and published the Atmospheric Environmental Quality Standard, the Maximum Allowable Concentration of Hazardous Substances in the Air of Residential Areas, the Maximum Allowable Concentration of Hazardous Substances in the Air of Workplaces, and the Trial Standards for the Discharge of Industrial "Three Wastes".

6.5. 1.2 China atmospheric environment quality standard

China's atmospheric environmental quality standards are divided into the following three grades.

First-class standard: protect natural ecology and people's health, and have no harmful effect on air quality under long-term exposure.

Secondary standard: In order to protect people's health and animals and plants in cities and villages, air quality requirements that will not be harmful in long-term and short-term contact.

Grade III standard: Air quality requires protecting people from acute and chronic poisoning and normal growth of urban animals and plants (except sensitive person).

See Table 6. 10 for the limit value of the third-class standard concentration of air pollutants.

Table 6. 10 Grade III Standard for Air Pollutants (GB3095-82)

6.5. 1.3 air pollution index

Air pollution index is a method to evaluate ambient air quality in many developed countries or regions in the world. It processes a series of complex air quality monitoring data in a certain way and turns it into a form that people can easily understand and master.

According to the unified regulations of the State Environmental Protection Bureau, China's air quality is divided into the following five grades:

When the API value of (1) is 0 ~ 50, the air quality level is Grade I and the air quality is excellent.

(2) When (2) the 2)API value is between 5 1 ~ 100, the air quality grade is Grade II and the air quality is good.

(3) When (3) the 3)API value is 10 1 ~ 200, the air quality level is Grade III, and the air is slightly polluted.

(4) When (4) the 4)API value is 20 1 ~ 300, the air quality level is IV, and the air is moderately polluted. At this time, it has obvious influence on people with sensitive physique, and the general population may also have symptoms such as eye discomfort, asthma, cough and excessive phlegm.

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

Many cities in China publish air quality status in the form of "Air Quality Weekly". For example, from February 27th to March 5th, 1998, the air pollution index in Beijing was 203, the air quality was Grade IV, and the primary pollutant was nitrogen oxides. See table 6. 1 1 for details. Another example is 1998. From March 20th to 26th, the air pollution indexes of major cities in China were: Beijing 138, Changchun 89, Changsha 92, Chongqing 97, Dalian 63, Fuzhou 47, Guangzhou 166, Hangzhou 74, Harbin 74, Hefei 60 and Jinan 103. Shenyang 100, Shenzhen 82, Suzhou 55, Shijiazhuang 192, Tianjin 108, Wuhan 84, Xiamen 46, Zhengzhou 194, Zhuhai 1 12. The air quality in most cities is relatively good this week, which is due to the influence of a strong cold air flow, resulting in a wide range of rain and snow weather. Beijing alone is down 56% from last week's 194. Recently, some cities in China published the daily air quality status in the form of "Air Quality Daily".

Table 6. 1 1 Beijing Air Quality Weekly (1998 February 27th to March 5th)

6.5.2 Main ways to control harmful gas emission

In order to control air pollution sources, it is necessary to solve the problem of harmful gas emission routes. Human production and living activities will inevitably emit harmful gases, and reasonable emission methods can reduce the concentration of harmful gases and reduce air pollution. There are many ways to control the emission of harmful gases, mainly the following.

6.5.2. 1 Overall planning and rational layout

Starting from coordinating the relationship between economic development and environmental protection, comprehensive planning and rational layout of production and living facilities in a region is an important way to control harmful gas emissions. If the industry in a region is too concentrated and the pollutant discharge is too large, the concentration of harmful gases in the region will be too high, which is not easy to dilute and spread quickly and is easy to cause harm. Therefore, it is necessary to rationally lay out industries; The topographical conditions conducive to the diffusion of pollutants should be considered in site selection; Factories should be located in the downwind area of the dominant wind direction in the city, and factories with cooperative relations should be set together to reduce the exhaust volume. There should be a certain distance between the factory and the living area, and greening inside can reduce the harm of harmful gases.

District heating and central heating in 6.5.2.2

Compared with stoves scattered in thousands of households, central heating will greatly improve the heating efficiency and greatly reduce the emission of harmful gases. Generally, it can improve the boiler efficiency by 30%. According to estimates, for the same ton of coal, the smoke and dust generated by residents' scattered use is 1 ~ 2 times more than that generated by industrial centralized use, and the floating dust is 3 ~ 4 times more.

6.5.2.3 has chosen an emission mode that is conducive to the diffusion of harmful gases.

Different emission methods have different diffusion effects of flue gas. Using high chimney emission can make harmful gases spread to a higher and wider range and reduce the concentration of harmful gases on the ground near pollution sources. The other is collective emission, that is, several (usually 2-4) smoke exhaust devices are concentrated in a chimney, which can improve the smoke exhaust speed at the chimney mouth and have good diffusion effect.

6.5.2.4 changed the composition of the combustion.

Replacing coal with clean gas or liquid fuel can significantly reduce the concentration of particulate matter and harmful gases in the atmosphere. Changing fuel composition is a direct solution to the problem of pollutants, so it is an effective way to control and prevent air pollution.

6.5.2.5 reformed its production technology and comprehensively utilized waste gas.

By improving the combustion process, we can improve the combustion efficiency as much as possible and reduce the emission of harmful gases as much as possible. For example, technical transformation of old boilers and combustion equipment and improvement of automobile engines and fuels can reduce exhaust emissions. In the production process, waste gas discharged from one production should be used as raw material for another production as far as possible, so as to achieve the dual benefits of reducing pollutant emission and turning waste into treasure.

6.5.2.6 develops new energy sources.

Solar energy, wind energy, hydro energy, geothermal energy, tidal energy and bioenergy are mostly renewable energy sources, and there will be no environmental problems caused by fossil fuels in the process of utilization. Although there are still some problems in the development and utilization of some new energy sources, this is the direction of future energy development.

6.5.3 Brief introduction of technical measures to control the emission of some harmful gases

Control of sulfur dioxide

At present, there are three main methods to control SO2: flue gas desulfurization, fuel desulfurization and combustion desulfurization.

Desulfurization from flue gas

There are many methods to remove SO2 from flue gas, and the treatment methods and processes are different according to the concentration of SO2 in flue gas. Their principle is to convert sulfur dioxide into useful chemical products or fertilizers after an appropriate reaction.

For flue gas with high concentration of SO2, 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 adopted.

(1) ammonia method. SO2 in tail gas is absorbed by ammonia, and then concentrated sulfuric acid is added for treatment to generate ammonium sulfate and high-concentration SO2. Ammonium sulfate can be used as fertilizer, and high-concentration SO2 can be recycled to sulfuric acid plant to make sulfuric acid again.

(2) Calcium method (lime milk method). This method uses calcium hydroxide or calcium carbonate to absorb SO2 tail gas for desulfurization, and produces by-product gypsum. Because limestone is widely distributed, it is an ancient method and widely used.

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

Fuel desulfurization

Fuel desulfurization is realized in the process of coal washing and conversion. If gravity separation is used, the sulfur content of the separated raw coal can be reduced by 40% ~ 90%. In Japan, Britain, Canada and other countries, in order to control SO2 emission, all thermal coals are washed, and the raw coal washing rate in China is only 17.7%. Direct gasification or liquefaction of coal, that is, decarbonization or hydrogenation of coal changes its original hydrocarbon ratio, which can turn coal into clean secondary fuel.

6.5.3. 1.3 combustion desulfurization

Combustion desulfurization adopts fluidized bed combustion technology. In this method, lime or dolomite flowing medium is injected into the furnace and combusted with coal particles in multi-stage in the furnace, and SO2 is removed in the form of calcium sulfate. This combustion method can not only remove SO2, but also reduce the emission of nitrogen oxides because of the low combustion temperature, so it is paid attention to.

Control of nitrogen oxides in 6.5.3.2

There are many methods to control nitrogen oxides, which can be divided into dry method and wet method. Among the applied methods, it can be divided into catalytic reduction method, liquid absorption method and solid adsorption method.

Catalytic reduction method

Catalytic reduction method is to reduce nitrogen oxides into harmless nitrogen under the action of catalyst, so as to achieve the purpose of decoloring waste gas and eliminating pollution. According to different catalysts, it can be divided into selective catalytic reduction and non-selective catalytic reduction.

Selective catalytic reduction is the selective reduction of nitrogen oxides in waste gas under the catalysis of platinum or copper. For example, ammonia is selected as a reducing agent to selectively react with nitrogen oxides in gas:

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But ammonia will not react with nitrogen in the gas.

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

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Liquid absorption method

Absorb nitrogen oxide waste gas with sodium carbonate, sodium hydroxide, lime milk or ammonia water solution. For example, sodium carbonate is used to absorb nitrogen oxides, and the reaction formula is as follows:

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In the reaction, because NO is insoluble in water, the absorption rate of lye will be very high only when the ratio of NO to NO is appropriate. Generally, when NO∶NO2= 1∶ 1, the rate is the highest.

Solid adsorption method

Use solid adsorbents, such as activated carbon, silica gel, various types of molecular sieves, peat and so on. , nitrogen oxide molecules, water molecules and oxygen molecules are selectively adsorbed, so that they react chemically to generate nitric acid. This not only eliminates pollution, but also recovers nitrogen oxides, which is a better method.

Purification of Automobile Exhaust in 6.5.3.3

Automobile exhaust is one of the main sources of harmful gases, including carbon monoxide, hydrocarbons, nitrogen oxides and sulfur dioxide. The composition of automobile emissions varies greatly with different fuels (gasoline and diesel) and transportation conditions. Taking a gasoline vehicle as an example, its tail gas purification is divided into three aspects: pretreatment (such as unleaded gasoline), internal purification (such as improving fuel system, ignition system and oil supply system) and post-treatment (such as using thermal reactor and catalytic reaction). After this purification process, the contents of carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas can be reduced.

Control of chlorofluorocarbons in 6.5.3.4

Chlorofluorocarbons are mainly discharged during the production and use of aerosol sprays such as refrigerants, foaming agents and solvents, such as refrigerators, air conditioners and fire extinguishers. It can be controlled by the following methods:

(1) Improving the utilization efficiency and reducing the operation loss are the simplest ways to reduce CFCS emissions. In the United States, about two-thirds of CFC- 12 is used in automobile air conditioning, of which 30% is lost due to leakage. Measures such as strengthening seals and valves and reducing the number of joints can be taken to reduce leakage. When reciprocating compressor is used in refrigerator, its CFCS consumption is only 1/3 ~ 1/2 of that of rotary compressor.

(2) Recovery and recycling are also the main methods to reduce CFCS emissions. Most of CFC 1 1 used to make flexible foam is lost by volatilization in the production process, and 50% can be recovered by carbon filter. For CFC- 12 used to make solid foam, similar technology can also reduce emissions by half.

(3) Improve the use of CFCS. Some CFC products pose little or no threat to the ozone layer and can replace CFC- 1 1 and CFC- 12 to some extent. At present, CFC-22 has been rapidly degraded in the atmosphere, and its ozone loss is only 1/5 of CFC- 12. Therefore, CFC-22 can replace CFC- 12 in air conditioners and refrigerators.

(4) Try to find other products to replace or reduce the use of CFCS. For example, the foam insulation layer used in refrigerator and freezer shell is made of CFC- 1 1. At present, there are several advanced thermal insulation materials as substitutes, such as thermal insulation materials made of vacuum slats containing fine powder, vacuum plates made of silica gel, or vacuum systems made of vacuum metal shells.

In addition to the above-mentioned main ways and technical measures to control harmful gas emissions, afforestation and greening the environment are also important measures to purify the atmosphere. Plants not only have the functions of regulating climate, keeping air humidity and preventing soil erosion, but also have the function of purifying air. The main manifestations are as follows: ① Green plants absorb CO2 and release oxygen through photosynthesis to maintain the balance between oxygen and carbon dioxide in the atmosphere. For example, 10 m2 forest can absorb CO2(2 exhaled by a person day and night. (2) Plants can purify many harmful gases in the atmosphere. For example, Cryptomeria fortunei at 1 m2 can absorb 0.07 kg of SO2, and alfalfa at 1 km2 can reduce SO2 in the air by more than 60t every year. (3) Plants, especially trees, have strong ability to block, filter and adsorb dust. For example, the blocking rate of cypress to dust is 12.8%, and that of Robinia pseudoacacia is17.58%; (4) The greening trees have strong sterilization ability, which can effectively reduce the content of pathogenic bacteria in the atmosphere. Therefore, large-scale greening is an important measure to control the harm of harmful gases.