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Abstract

Содержание

Introduction

Atmospheric pollution leads to environmental problems. For example, when burning fossil fuels, they get into the atmosphere sulfur dioxide (SO2) and nitrogen oxides. When these compounds come into contact with water, oxygen and other substances, sulfuric acid is formed and nitric acid, which increase the acidity of rain. Acid rain contributes to the degradation of reservoirs and soil, damages plants, destroys buildings, infrastructure, sculptures. Air pollution also increases the intensity of eutrophication — saturation of water with elements such as nitrogen and phosphorus. This leads to an increase in biological activity in the water: reproduction of blue-green algae or cyanobacteria, some of them emit dangerous to human and animal health cyanotoxins.

Emissions that lead to high concentrations of SO2, as a rule, also lead to the formation of other SOx. The largest sources of SO2 emissions are the burning of fossil fuels at power plants and other industrial facilities

1. Theme urgency

The problem of environmental pollution is becoming more and more acute every year and is gaining global proportions. Sulfur dioxide emissions in the world amount to 100-150 million tons/year. Atmospheric pollution with sulfur oxides increases the level of morbidity and mortality of the population due to cardiovascular diseases. Sulfur dioxide, entering the atmosphere, causes the precipitation of "acid rain". The most promising way to protect the environment from sulfur dioxide is the purification of industrial waste gases.

2. Goal and tasks of the research

The aim of the work is to develop sulfur dioxide adsorbents based on alkaline earth metal compounds.

The main objectives of the study:

  1. To study methods of purification of gases from sulfur dioxide
  2. Consider liquid and catalytic methods of sulfur dioxide absorption.
  3. To analyze solid-phase methods of sulfur dioxide absorption.
  4. Development of sulfur dioxide adsorbents based on alkaline earth metal compounds.

3. Methods of gas purification from sulfur dioxide

3.1 Formation of sulfur dioxide

Sulfur dioxide is obtained by the combustion of hydrogen sulfide, sulfur, as well as by heating various sulfides in a stream of air or oxygen. Under normal conditions, sulfur dioxide is a colorless gas with a strong smell of burning sulfur. Sulfur is almost 2,3 times heavier than air. It does not burn and does not support gorenje. The sulfur dioxide molecule is polar. It is an isosceles triangle with a sulfur atom at the top.

The density of sulfur dioxide at 0 °C is 2.926 kg/m3. Sulfur dioxide is easily converted into a liquid at atmospheric pressure and cooled to 10.5 °C. At 72.5 °C, SO2 freezes. The vapor pressure of sulfur dioxide over the liquid phase is 1329.3 kN/m2 at 20 °C and 851.13 kN/m2 at 50°C.

Sulfur dioxide is soluble in water, oleum and sulfuric acid. In one volume of water at 20 ° C, about 40 volumes of PO2 are dissolved, while heat is released in the amount of 34,4 kDzh/mol.

Describing the properties of sulfur dioxide as a substance that pollutes the air, it should be noted its ability to oxidize to SO3. Sulfur trioxide in humid air can turn into sulfuric acid. Sunlight contributes to the course of this reaction in the air, ozone, as well as catalyzing substances. It should be borne in mind that at low concentrations of SO2, small amounts of sulfuric acid aerosol vapors may be present in the air together with it, which exacerbates air pollution.[1].

3.2 The entry of sulfur dioxide into the atmospheric air

Technogenic sources of sulfur oxides entering the atmosphere are:

- fuel energy, is 55%;

- metallurgical industry, is 25%;

- purification and processing of oil and coal, is 10%;

- chemical industry, transport and other types of human economic activity account for 10%.

Mainly, atmospheric pollution with sulfur oxides occurs during the burning of oil, coal, natural gas, and wood. Sulfur in the composition of the fuel is not the main component. The amount of sulfur-containing compounds in coal and oil can vary from fractions to 5-6%, and also depends on its type and on the place of extraction. Sulfur dioxide is a product of fuel combustion.

The metallurgical industry and the processing of polymetallic ores are also important sources of sulfur dioxide. Metals in ores are mainly in the form of sulfides (sphalerite, pyrite, zinc blende, galena), much less of them are in the form of iron, magnesium, and calcium sulfates. Sulfur dioxide predominates among other gaseous sulfur compounds of technogenic origin, according to various sources, this excess ranges from 1.5 to 8 times. Waste from some plants contains 4-10% sulfur dioxide.

According to various sources, the planetary technogenic intake of sulfur dioxide into the atmosphere averages 140-290 m t per year. The main part of it is concentrated in the soil and in the biota, about 1/3 is carried out into the oceans. It is assumed that in the XXI century, the emission of sulfur dioxide will increase by 3-5 times. 94 % of SO2 emissions occur in the northern hemisphere, where mainly global industry is concentrated. In Europe, its main sources are the industrial complexes of the Ruhr basin of Great Britain and Germany [2].

Anthropogenic emissions of sulfur and nitrogen oxides exceed natural emissions. This is evidenced by the numerous indicative estimates that various authors have received. The absolute indicators in them are not always the same, but they contain the same pattern.

Technogenic emissions of sulfur dioxide affect not only the environment with highly developed industry, but also neighboring countries due to cross-border transport. The range of gases in the atmospheric air is 300-400 km, and in some cases it can reach 2000 km. In many European countries, up to 50% of the total amount of sulfur compounds comes from neighboring countries. For example, the deposition of sulfur dioxide in the Netherlands, Luxembourg, Switzerland due to transboundary transport reaches up to 78% of their total amount. In the Scandinavian countries, their income due to cross-border transfer is up to 63%. The intake of sulfur into the atmosphere of Russia from neighboring Western countries is at least 40% of the total anthropogenic load [3].

The intake of harmful substances from atmospheric air to the earth's surface, including substances of an acidic nature, occurs as a result of the processes of dry and wet precipitation. Wet deposition of acid precipitation is the main way of precipitation of anthropogenic acid products from the atmosphere. With a shortage of precipitation, the precipitation of gaseous and solid precipitation in the form of dry aerosol deposition prevails. The ratio of the contribution of the influence of dry and wet precipitation of acidic products may be different. For example,in high-altitude European regions, up to 90% of the intake of acidic substances is due to wet precipitation of sulfate ions.

The fallout of technogenic sulfur in industrially developed countries is huge. In most of the European territory of the Russian Federation, annual sulfur precipitation is up to 1.0 g/m2. In the eastern part of the Russian Federation, they are not higher than 0.5 g/m2, and in industrial centers they are more than 2 g/m2.

Background levels of sulfur dioxide in the atmosphere are 5-10 micrograms/m3. The PC of a single intake of SO2in the air is 500 micrograms /m3, the average daily MPC level is 50 micrograms/ m3. Only at an altitude of 3-4 km in the atmosphere the concentration of sulfur dioxide is leveled. In all major cities, due to local sources of pollution, this level of sulfur dioxide in the atmosphere is exceeded.

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Figure 1 - The entry of sulfur dioxide into the atmospheric air

3.3 The effect of sulfur dioxide on the human body

Due to the increase in deaths and diseases associated with atmospheric pollution, the health authorities of various states have begun to show special interest in the harmful effects of atmospheric pollution on the human body. Little data has been accumulated on the harmful effects of low concentrations of sulfur dioxide on humans. The conducted studies are based on the concept that the concentration of sulfur dioxide in the inhaled air and the duration of this play an important role. Almost any impurity in the air begins to show its harmful effects when its content is above a certain norm [3].

The question of the harmful effect of any substance in the atmospheric air essentially boils down to the question of its maximum permissible concentration.

The influence of thermal power plant emissions on the health of people living in areas with different intensity of atmospheric air pollution was studied. As a result, it was found that there was an adverse effect of emissions on sanitary conditions and public health in the smoke zone at a maximum concentration of SO2 in the atmosphere of 3.3-4.0 mg/m3 and dust 2.5-4.6 mg/m3. The total number of complaints and the frequency of upper respiratory tract diseases in some forms in this zone was twice as high as in a relatively clean area. In the main group of schoolchildren (smoke zone), most children had a low hemoglobin content, the presence of SO2 in the blood (from traces to 0.02 mg), a high incidence of conjunctivitis (13.3% compared to 3.8% in the control area).

In particular, the harmful effect of SO2 increases with increasing humidity and dustiness. Studies of many medical institutions have shown the dependence of morbidity on the pollution of the air basin. Respiratory diseases were predominant. Signs of chronic intoxication with sulfur dioxide have been established.

4. Methods and techniques of the experiment

Methods of cleaning gas emissions from SO2 are usually divided into "wet" and "dry". The absorption of sulfur dioxide using the liquid phase is carried out at relatively low temperatures.

Processes based on the interaction of gas with the solid phase usually occur at high temperatures. To purify flue gases from sulfur dioxide, solid chemosorbents are used by introducing them in a pulverized state into the furnace or flues of thermal power units. Limestone, dolomite or lime can be used as chemosorbents. To increase the activity of chemosorbents, special additives are introduced in the form of inorganic salts. The dry methods include the absorption of sulfur dioxide by carbon sinks (active coals and semi-coxes) at a temperature of (110 - 150) ° C.

Among the gaseous substances polluting the atmospheric air, one of the main places is occupied by sulfur dioxide. Under normal conditions, it is a colorless gas with a sharp irritating odor.

Sulfur oxides with a low content cause irritation of the respiratory tract, with a content of 0.01 %, poisoning of people occurs in a few minutes. A mixture of SO2 with other gaseous impurities causes a violation of the genetic function of the body during prolonged exposure. It is advisable to divide all processes into two main classes, differing in the physical state of the main reagent for the extraction of sulfur dioxide from gases. The first class includes processes using the liquid phase and is therefore carried out at relatively low temperatures. The second class includes processes and methods based on the interaction between the gas and the solid phase (dry cleaning processes). There are no restrictions on the temperature of the main reaction, which is typical for "wet" cleaning processes associated with the evaporation temperature of the liquid phase. Each of these classes can in principle be divided into three main groups, differing in the characteristics of the use of sulfur dioxide.

The first group includes processes whose main purpose is only the purification of gases without taking into account the possibilities of utilization of sulfur dioxide. The products of the main interaction of sulfuric anhydride with certain reagents are waste here and are thrown away. This group includes the first attempt to actually carry out flue gas desulfurization - the process of cleaning by flushing gases with water.

In this process, the main, largest part of SO2 is directly dissolved in water, and a certain amount, which forms sulfurous acid, reacts chemically with salts (HCO3)2 and Mg(NCO3)2 contained in water, with the formation of CaCO3, MgCO3 and carbon dioxide. The main difficulties of water purification of gases from SO2 are the large consumption of the necessary water (5000-6000 m3 at 50 ° C per 1 ton of captured sulfur), which after flushing the gas acquires an acid reaction. Descent the release of acidic water into reservoirs is no less dangerous from a sanitary point of view than the release of sulfur dioxide into the atmosphere.[4].

The second group includes the so-called cyclic processes and methods for the production of commercial sulfur dioxide or elemental sulfur. Here, sulfur dioxide is extracted from gases using reagents, which are regenerated and returned to the cycle for further use.

Various solutions are used as absorbers in liquid cyclic processes: ammonia, suspensions of metal oxides (MgO, CaO, ZnO) and others. The regeneration of absorbers in these processes is carried out by heating the solution and distilling SO2 in a current of water vapor or calcination of filtered slightly soluble sulfuric acid salts in furnaces.[5].

The third group of gas purification processes from sulfur dioxide includes processes in which the extraction of sulfur dioxide is carried out in conjunction with its use to obtain new chemicals. This group of processes is characterized by the fact that the absorber does not return to the process, but is used for reaction with sulfur dioxide. Of the liquid processes of the third group, the most well-known are the methods in which ammonia is used as an absorber, as a result of which the production of ammonia fertilizers can be carried out.

Oxidative methods of purification from sulfur dioxide are distinguished in a separate group. In these methods, sulfur dioxide is oxidized by oxygen, which is contained in the flue gases to sulfur anhydride in the presence of a catalyst. In the future, sulfuric anhydride interacts with water vapor to produce sulfuric acid. The process ends with the production of either sulfuric acid as a finished product, or a sulfuric acid salt (for example, ammonium sulfate).[6]

Adsorbents are an effective means for the recovery and neutralization of sulfur dioxide, and their potential in this direction has not yet been fully identified. Carbon porous substances are mainly used to capture SO2. If the capture is made from a gas stream, the absorption of sulfur dioxide occurs according to the laws of physical adsorption and during desorption, the activity of the adsorbent is completely restored. However, real process and ventilation gases in the vast majority of cases have oxygen.

The adsorbate contains three categories of substances: physically adsorbed sulfur dioxide, irreversibly adsorbed sulfuric acid, sulfur compounds strongly bound to carbon, which are not removed by washing with water, but can be extracted with hydrogen peroxide. The ratio between reversibly and irreversibly adsorbed sulfur dioxide depends on the adsorption temperature.

At high temperatures, the rate of conversion of sulfur dioxide into sulfuric acid increases, as a result of which the nature of the bond of the adsorbate molecules with the adsorbent is predominantly chemical, that is, irreversible. The rate of sulfuric acid extraction depends on the structure of the active carbons. Excessive development of coal makes it necessary to increase the extraction period and the consumption of washing water.[7]

Conclusion

One of the essential pollutants of the atmosphere is sulfur dioxide or sulfur dioxide - SO2. Pollution of the atmosphere with sulfur oxides increases the incidence rate and, getting into the atmosphere, causes the precipitation of "acid rain". Due to the high activity, the main part of SO2 is deposited in the soil and in the biota, about 1/3 is carried out to the oceans.

The development and implementation of fundamentally new technological processes and purification systems is now the main direction of technological progress and the formation of the transition to waste-free production and waste-free technologies. Despite the large number of works devoted to the problems of gas purification from SO2, the search for new sorbents is very relevant.

When writing this abstract, the master's work has not yet been completed. Final completion: June 2023. The full text of the work and materials on the topic can be obtained from the author or his supervisor after the specified date.

References

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