OUTDOOR AIR CONDITIONING

The article is devoted to the topical issue of modern cities concerning purification of outdoor air from harmful contaminants of waste gases of thermal power and industrial enterprises, motor vehicles exhaust gases and other sources of air pollution in the most polluted areas of urban transport arteries. Innovative technical solutions based on the applying cheap and affordable adsorbent reducing the concentration of harmful contaminants in outdoor air by means of installing fixed and mobile street air-conditioners in the places with urban motor-vehicle transport congestions are considered.


INTRODUCTION
Depending on weather conditions, geographic position, the number of motor vehicles, and other factors affecting the atmosphere situations when the concentration of harmful contaminants of exhaust gases in urban atmosphere exceeds health standards in tens and hundreds of times occur in many cities and metropolitan areas. In such cases, the share of motor-vehicle transport polluting emissions ranges from 70% to 90% of total emissions into the atmosphere (the remaining emissions fall to the share of thermal power plants, boiler stations etc.) in large cities. As a result, the composition of the air in large cities and megalopolises contains a significant amount of detrimental impurities that cause serious diseases among urban dwellers. For example, exhaust gases of automobile engines contain more than 200 substances, most of which are toxic. The main components of vehicle exhaust gases are nitrogen (N2), oxygen (O2), water vapour (H2O), carbon oxides (CO2 and CO), sulfur oxides (SOx), nitrogen oxides (NOx), polycyclic (surface-active substances), hydrocarbons, aldehydes, soot, benzapyrene, ozone, etc. How-ever, despite the prohibition of leaded gasoline use, lead content in urban air is still quite high. Moreover, exhaust gases, especially from diesel engines, give unpleasant smell to the outdoor air. The role of road transport in air pollution is caused not only by the amount of emissions but also by the fact that exhaust gases are emitted directly into the surface layer of the atmosphere where the wind speed is insignificant and gases disperse poorly in contrast to the industrial enterprises and power plants waste gasses emitted through high chimney stacks. In addition, heavy gases with the specific gravity more than the one of the air compound the bulk of the exhaust pollutants, which causes them to accumulate in the lower layers of the atmosphere, namely at the level of cars and pedestrians [1, p. 86-90, p. 155]. Along with a high content of harmful contaminants of automobile exhaust gases street air also contains a significant amount of harmful gaseous and suspended particles, the presence of which is due to the activities of urban life support systems and industrial enterprises work. An important factor affecting the life and health of urban population (especially in metropolises) is aeroionic state of urban air. It is known that the comfortable state of air environment is determined not only by its temperature, humidity, fluidity and cleanliness but also by the concentration and composition of aeroions (ionic concentration). Air quality is characterized by the concentration of light aeroions. So, in mountain and resort areas the concentration of light aeroions is up to 2500/cm 3 , whereas in big cities it does not exceed 200-300/cm 3 [2, p. 68-73].

PROTECTION OF URBAN ATMOSPHERE ECOLOGICAL
Protection of urban atmosphere ecological state is a complex of organizational and technological measures. However, these measures do not include outdoor air purification in the places with the high congestion of motor vehicles. A significant obstacle that does not allow carrying out large-scale activities aimed at the purification of outdoor air from harmful contaminants of exhaust gases is that there are no cheap and available reagents for their implementation. To implement measures aimed at the purification of outdoor air from harmful contaminants this paper proposes technical solutions based on the use of granulated blast furnace slags as an adsorbent of harmful contaminants [3][4][5][6]. For these solutions implementation the designs of street air-conditioners for above-ground and underground (fixed and mobile) location have been developed. Granulated blast furnace slag (metallurgical pumice) is a fine-grained material in the form of porous, vitreous or crystalline granules with an average size of 2-8 mm. Slag density depending on its composition is 2.8-3.0 g/cm 3 , hardness of the grains is 5-8 kg/cm2. Slag chemical composition, depending on the composition of the original ore and the type of the produced iron, ranges widely; it may contain 30-49% of CaO; 4.5-20% of Al2O3; 33-44% of SiO2; 0.3-0.8% of Fe2O3; 1.5-15% of MgO; 0.3-3.0% of MnO [06]. The main characteristics of blast furnace slag are porosity, basicity factor, hydraulic properties, and activity. The porosity of blast furnace slag П was determined by formula (1):   1) where ρυ is the true specific gravity of the slag, kg/m 3 ; ρt is the apparent specific gravity of the porous sample, kg/m 3 ; 2) m is the mass of the sample with pores, kg; V is the volume of the sample with pores, m 3 . Slag basicity factor is the characteristic of the activity of metallurgical slags and their stability during lime dissolution. It is determined as the ratio of the content of basic oxides in the slag to the content of acidic ones. When Мо<1, the slags are considered to be acidic and are not affected by lime dissolution; when Мо>1, the slags are considered to be basic and are prone to lime dissolution. The sample volume was determined by measuring [07]. We obtained that the average porosity of the slag used was 60%. The pore size ranged from 1.0mm to 0.1mm, which corresponds to macropores. 9

3)
Since harmful impurities (SOx, COх, NOx) have acidic properties in exhaust gases, blast furnace slag with basicity factor М > 1 should be used in the process of their purification. The hydraulic properties of blast furnace slag are measured using quality factor (K), which is determined by the formula (4, 5): when the content of magnesium oxide is less than 10%:

4)
when the content of magnesium oxide is more than 10%:

5)
where K is the quality factor; CaO, Al2O3, MgO, SiO2, TiO2 are the contents of the oxides in the blast furnace slag in % on weight basis.  The operating principle of street air-conditioner can be considered by the example of a mobile street air-conditioner [05]. The purification of the outdoor air from harmful contaminants of exhaust gases of motor vehicles and dust goes as follows. At first, mobile street air-conditioner (MSAC) is carried on a truck chassis, or on a trailer (movable platform 24), placed at the edge of the traffic way and the pedestrian area of the busiest streets in such a way that the inlet 10 was turned towards the traffic way, and the outlet 13 was turned towards the pedestrian area; MSAC should be connected to all communications (electricity, water supply and sewer system). MSAC may be used being on the movable platform 24 or may be taken from the platform 24, and installed on a special support (not shown in Figure 1). MSAC can be almost completely autonomous: power supply for fans and a pump can be provided by automobile elec-tric generator and water can be brought in water lorries. In this case, only connection to the sewer system (through storm sewer system 25 or sewer manhole) should be provided. Outdoor air is sucked by the forced draft fan 12 through the grille 11 and the inlet pipe 10 into the air washer 8 where its indraft velocity drops rapidly; then the air moves upwards and, in crossflow along the whole height of the air washer 8 it contacts with the particles of water coming from the nozzles of the spraying device 18, where it is humidified and partially cooled. Water, the amount of which is determined by the specified amount of absorbed harmful contaminants from the outdoor air and its moisture content, is fed to the spraying device 18 by the feed pump 23 through the branch pipe 4 and delivery pipe line 17 with the open feedwater valve and closed flushing valve (these valves are not shown in Figure 1).
At the same time with the above described process, in the air washer 8 water captures dust and soot particles and interfacial contact of spray water and air occurs; these result in the absorption of carbon dioxide (CO2) by water, chemical interaction between nitrogen oxides (NOx), sulfur oxides (SOx), ozone (O3), oxygen (O2), water (H2O) in gas and liquid phases, chemisorption of resulting nitrogen dioxide (NO2) and sulfur trioxide (SO3) with water to form nitric (HNO3) and sulfuric (H2SO4) acids. The resulting acidified water saturated with carbon dioxide and mechanical impurities flows into the tray 3 and then together with the dirt through the branch pipe 5 into the storm sewer system 25; after that the mixture of fresh and acidified water is again fed by the feed pump 23 through the branch pipe 4 to the air washer 8. Purified from carbon dioxide and from some portion of oxides of nitrogen and sulfur, dust and soot, humidified and partially cooled outdoor air with acidified water particles goes from the air washer 8 through the port 7 to the purifying compartment 9; then it goes downwards through the detachable perforated baskets 20 filled with the pumice granules 21 made of basic metallurgical slags, the diameters of granules range from 20 mm to 40 mm (the diameter of the granules 21 was specified according to the conditions of providing the minimum aerodynamic drag of the purifying compartment 9 and the standard list of granule sizes for metallurgical pumice). Adsorbed from outside air oxides of nitrogen and sulfur have high reactivity in the pores of the granules 21due to their interaction with the surface of the adsorbent i.e. the surface of slag pumice granules [11, p. 298], and, therefore, they oxidize with oxygen at a rate greater than during the gas phase with the formation of NO 2 and SO3 which are easily soluble in water; NO 2 and SO3, in turn, react with the acidified water particles remaining in the pores of the granules 21 to form the corresponding acids HNO3 and H2SO4. In addition, fine particles (dust, soot etc.) settle on the surface and in the pores of the granules 21; as a result, purified and dried air when passing through the air ionizer 15 is enriched with light air ions and with the help of the induced draft fan 16 is fed through the outlet 13 and distribution grille 14 to the pedestrian side of the street. When the pumice granules 21 in the perforated baskets 20 lose their activity, they are subjected to regeneration directly in the air conditioner, which is carried out when the vehicular flow is the lowest (for example, at night). The process of regeneration involves washing the granules 21 from dust, soot and acidified water containing carbon, sulfur and nitrogen oxides. Washing is carried out by supplying feed water through the washing device 19 into the purifying compartment 9. The amount of wash water and washing time is set up by test. Pumice granules 21 are completely replaced once per season. As a source of water supplied to the air washer 8 to absorb carbon dioxide in large amounts, it is possible to use water from groundwater pumping stations for underground railway (providing that it does not contain harmful impurities) or water brought in water lorries; acidified water discharged through the branch pipe 5 into wastewater disposal system due to the high content of carbon dioxide will facilitate the process of photosynthesis [13, p. 210] on the irrigation fields of municipal wastewater treatment plants.
The height of the street air conditioner is determined by the area of maximum air pollution of the outdoor air and the allowable dimensions of cargoes transported by motor vehicles; the height mark of the inlet 10 and the outlet 13 pipes is at the height of the working zone of a standing person taking into account the angle of the expansion of the air current flowing from the distribution grille 14. The square of crosssection area, the number and capacity size of the perforated baskets 20, the porosity, size and number of the pumice granules 21, the capacity and performance of the fans 12 and 16, the ionizer 15, the feed pump 23, the capacity size of the tray 3, the absorbing capacity of the purifying compartment 9, the amount of water for spraying in the air washer 8 and for the recirculation are determined by air capacity, air pollution and the required degree of its purification. It is recommended to produce street air-conditioner in the form of standard sections (Figure 2 shows one section of a street air-conditioner), each of which can operate independently, which makes it possible to increase or decrease the performance of the entire system depending on the environmental situation in the street.  Thus, the proposed mobile street air-conditioner makes it possible to purify outdoor air from harmful contaminants of exhaust gases of motor-vehicle transport without using expensive and hazardous chemicals, to automate the processes of spraying and washing, and to move the air-conditioner quickly to the most polluted places.

CONCLUSION
A cheap and affordable type of adsorbentgranulated blast furnace slag (metallurgical pumice) is proposed to carry out purification of urban outdoor air from harmful contaminants of exhaust gases of motor vehicles and waste gases of thermal power and industrial enterprises.
The main physical characteristics of granulated blast furnace slags and the experimental data on their absorbing capacity for harmful contaminants (nitrogen oxides, sulfur oxides, carbon oxides) of polluted outdoor air are presented. Innovative engineering solutions (designs of fixed on the ground air-conditioners, underground street air-conditioners, mobile street air-conditioners) for implementing the processes of urban outdoor air purification from harmful contaminants are proposed. The proposed design of street air-conditioners makes it possible to purify outdoor air from harmful contaminants of exhaust gases of motor-vehicle transport and waste gases 1)

4)
of thermal power and industrial enterprises without using expensive and hazardous chemicals, to improve its quality by increasing the content of light aeroions in it and to dispose removed harmful contaminants.