Extraction of sulfur (IV) oxide, nitrogen oxides and carbon oxide from flue gases using a sorbent under conditions of its mechanical activation

The paper substantiates the feasibility of creating an adsorption technology with controlled activation of the adsorbent for the removal of sulfur dioxide, nitrogen oxides, and carbon from the flue gases of coal-fired power plants. In laboratory conditions, the effect of mechanical activation on the adsorption capacity of ash was studied. It is shown that the controlled mechanical activation is able to increase the adsorption capacity of ash relative to sulfur dioxide by 2.7 times, to nitrogen oxides by 2.1 times, to carbon monoxide by 5 times or more.


Introduction
The National Report on the State of the Environment in Ukraine in 2020 [1], shows that 3.8 tons of pollutants were present per square kilometer of the country's territory.The main chemical components entering the atmospheric air from stationary sources are substances in the form of suspended solid particles -248.9 thousand tons (11.1% of the total volume of pollutants), dioxide and other sulfur compounds -782.1 thousand tons (34.9%), methane -429.1 thousand tons (19.1%), nitrogen oxide -3.4 thousand tons (0.15%), carbon monoxide -707.3 thousand tons (31.5%).Almost 37.9 % of pollution from stationary sources comes from the energy sector, mainly thermal power plants.In Ukraine, flue gases from coal-fired power plants, boilers, and heaters are released into the environment without treatment.The problem of atmospheric air pollution in Ukraine with toxicants remains relevant.
Independent researchers [2] also confirm the significant damage that flue gases from coal-fired power plants cause to the environment.According to this report, Ukraine is one of the leaders in Europe in terms of atmospheric air pollution with sulfur and nitrogen emissions emitted together with the flue gases of coal-fired power plants.Among the thirty coal-fired power plants in Europe that pollute SO 2 the most, 12 belong to Ukraine.Ukraine ranks second in Europe in air pollution with nitrogen oxides.Ukraine produces 34% of its electricity consumption from 20 coal-fired power plants built before 1976, none of which have desulfurization equipment.Therefore, the task of cleaning flue gases of coal-fired power plants from sulfur, nitrogen and carbon oxides is extremely relevant in the European context.
There are a lot of various studies aimed at improving coal combustion processes and implementing technological processes that reduce emissions of sulfur oxides, nitrogen, and carbon.Zaporozhets et al [3] provides statistical studies of emissions into the atmosphere by stationary and mobile sources of pollution in Ukraine for 1990-2018.It also indicates the need to reduce emissions of toxicants into the atmosphere.This can be achieved through the introduction of new technological processes at coal-fired power plants.
Chernyavsky et al [4] summarizes the experience of burning coal mixtures at three power plants of Ukraine.The issues of production, grinding, burning of mixtures and ecological aspects of their use are considered.Popov et al [5] shows that burning coal leads to the accumulation of ash and slag waste.Although they are environmental pollutants, under the right conditions these wastes can become a sorbent and be used in technologies for the adsorption of sulfur oxides, nitrogen and carbon from flue gases.Moreover, flue gases are formed in the same combustion processes where these solid ash and slag wastes are formed.
Volchyn et al [6] established an analytical relationship between SO 2 emissions in flue gas at coal-fired thermal power plants and the ash content of Ukrainian thermal coal.Thus, the research data indirectly indicate the possibility of controlling the sulfur oxide content due to technological processes aimed at ash activation.Schmauss and Keppler [7] show that sulfur dioxide is actively adsorbed by volcanic ash, so it can be concluded that ash from burning coal will similarly adsorb SO 2 .Walawska et al [8] proves that the mechanical activation of solid particles significantly improves their characteristics as adsorbents for SO 2 .Wu et al [9] proves that different modes of mechanical activation of coal slag significantly change its physicochemical characteristics and adsorption capacity.That is, mechanical activation of coal ash is potentially able to improve the adsorption of sulfur dioxide and other toxicants from flue gases.Shukor et al [10] studied the adsorption of sulfur dioxide by carbon.Carbon is also contained in ash as a product of incomplete combustion of coal, so the adsorption characteristics of coal ash may partially depend on its carbon content.Articles [11,12] show that carbon can also actively adsorb nitrogen oxides.
The analysis of the literature showed that when burning coal at TPPs, solid wastes are formed -ash and slag, and gaseous wastes -oxides of sulfur, nitrogen, and carbon.At the same time, it is possible to create such a technological process, when the solid emission -ash, will be mechanically activated and will adsorb oxides of sulfur, nitrogen, and carbon.Such a technology, provided it is created, has a real perspective for implementation at coal-fired thermal power plants of Ukraine and Europe.The idea of creating such a technology emerged as a result of own research [13] of various chemical processes, when mechanical and chemical activation of the catalyst significantly accelerated chemical transformations.Adsorption is one of the stages of catalysis, so mechanical activation of MA can affect adsorption as a separate process.Studies of processes using coal as a raw material [14,15] have shown that coal can be mechanically activated.This formed the basis of this work, where ash from coal combustion was activated under controlled conditions.This increased its sorption capacity relative to sulfur dioxide, nitrogen oxides, and carbon monoxide.Research results can become the basis of technology for removing toxicants from flue gases of coal-fired power plants.

Laboratory equipment and methodology of the experiment
The laboratory unit for researching the process of adsorption from flue gases under the conditions of mechanical activation is presented in figure 1.
Adsorption of nitrogen oxides, sulfur oxide (IV), and carbon monoxide was studied.Ash from burning coal and zeolites of various types were used as a sorbents.Before the start of the experiment, the V-1 tank is filled with a mixture of air, sulfur oxide (IV), nitrogen, and carbon, which is an analogue of flue gases produced during the burning of coal at power plants in different modes.The obtained gas mixture enters Ads adsorber from above.The adsorber is filled with sorbent and dispersing material.
The temperature in the adsorber is maintained by an electric heater H, measured by a thermocouple.The activation of raw materials in the adsorber is carried out by forced mechanical oscillations with the help of an vibrating device MA.The temperature in the reaction zone and the intensity of activation are set and maintained by the control unit CU.When the adsorber moves with a certain frequency and amplitude, a vibro-thinned layer of dispersing particles is created and mechanochemical activation of the sorbent surface by them.A metal felt filter is located at the outlet of the adsorber, which prevents the removal of sorbent particles from the adsorption volume.Gases from the adsorber enter the V-2 receiver.Pumping of the gas mixture from V-1 to V-2 is carried out by the compressor GC.The unit has two sampling points of the gas mixture for analysis: the initial mixture and the final products.The content of oxides of nitrogen, sulfur and carbon is determined by a portable gas analyser OKSY-5M-5H with an error of ±5 ppm.The volume of the laboratory adsorber is 8 cm 3 , the capacity for gases is 50 liters.At a gas supply rate of 5-25 l/h, the duration of the experiment is 2-8 hours.Variable parameters during the experimental work were: temperature, flow rate of the gas mixture, concentration of gases in the mixture, vibration frequency of the vibrating device.

Research procedure
The AnC theory allows predicting intensification by mechanical activation of not only chemical transformations, but also other stages of the catalytic process, for example, the adsorption stage.Thus, it is possible to distinguish a separate direction of research -mass exchange processes.The degree of sorption and sorption capacity are highly dependent on temperature, intensity of activation, type of sorbent.The dependence of sorption efficiency and sorption capacity passes through maxima and is very similar to the dependence of the degree of transformation and the speed of chemical reactions on the frequency of mechanochemical activation in processes that take place using aerosol nanocatalysis technology.This indicates that sorbent activation is very  The presence of a maximum of the adsorption capacity at an MA intensity of 1.7 Hz and a tendency to a second maximum at an MA slightly higher than 8.0 Hz is observed.The dependence is described by equations (1).
The capacity achieved with the help of MA adsorption is such that it provides effective purification from sulfur oxides of flue gases from the burning of energy grades of coal with an ash content of 30%.θ = 0.0034f + 0.0034, (0-1.7 Hz); θ = 0.0008f 2 − 0.0076f + 0.0196, (1.7-4.5 Hz); (1) θ = −10 −5 f 3 + 0.0015f 2 − 0.0054f + 0.006, (4.5-8.0Hz); The use of MA is able to increase the adsorption capacity of ash by 2.7 times at a temperature of 423K, i.e. it allows to organize an industrial technology for removing sulfur oxides without cooling flue gases, which will significantly reduce energy costs on the scale of an industrial process.The use of ash from the burning of the same coal as a sorbent will allow reducing material costs on the scale of the industrial process.The process is able to provide 100% removal of SO 2 when its content in flue gases is up to 650 ppm (a typical amount for flue gases from burning low-quality thermal coal with an excess of 1.25 air) at an adsorbent concentration of 60 g/m 3 of flue gases.
In figure 3 shows the effect of MA on the adsorption capacity of the same ash in relation to nitrogen oxides.The values are in the same range as for SO 2 .
The use of MA is able to increase the sorption capacity of ash in relation to nitrogen oxides by 2.1 times at a temperature of 423K and ensure 100% removal of NO x from flue gases from the burning of thermal coal at a concentration of sorbent (ash) at the level of 20 g/m 3 of flue gases, which corresponds to the technological mode of combustion at industrial plants.This cleaning technology does not even require an additional amount of adsorbent.
The presence of a maximum of the adsorption capacity at an intensity of MA near 3.0 Hz and a tendency to a second maximum at an MA above 8.0 Hz is observed.The dependence is described by equations (3).The capacity achieved with the help of MA adsorption is such that it provides effective cleaning of flue gases from the combustion of energy grades of coal with an ash content of 30% and a NO x content of up to 250 ppm from nitrogen oxides.θ = −0.0001x 2 + 0.0007x + 0.0017, (0-4.0Hz); θ = −0.0005x+ 0.0046, (4.0-6.0Hz); (2) θ = 0.001x − 0.0044, (6.0-8.0Hz); In some modes of coal burning at thermal power plants, carbon monoxide formation is possible, so it is advisable to study the ability of ash to adsorb it as well.In figure 4 shows the influence of MA on the adsorption capacity of ash in relation to CO, and the change in adsorption capacity from MA is described by equations (3), (4).θ = 0.0019f, (0-1.2Hz); (3) For carbon monoxide, the influence of MA is particularly noticeable.In the absence of MA, it was not adsorbed at all under the experimental conditions.The use of MA at the level of 1.2 Hz made it possible to increase the adsorption capacity of carbon monoxide by more than 5 times in comparison with other modes of MA.

Conclusions
The development of technological processes capable of removing oxides of sulfur, nitrogen, and carbon from flue gases of coal-fired power plants is an urgent scientific and practical task.Adsorption processes using ash from the burning of the same coal as an adsorbent are promising.
The use of controlled mechanical activation can significantly affect the adsorption capacity of ash relative to oxides of sulfur, nitrogen, and carbon.
It is shown that the controlled mechanical activation is able to increase the adsorption capacity of ash relative to sulfur dioxide by 2.7 times, to nitrogen oxides by 2.1 times, to carbon monoxide by 5 times or more.A mathematical description of the influence of MA intensity on the adsorption capacity of ash relative to oxides of sulfur, nitrogen, and carbon is provided.This makes it possible to optimize the MA regime to achieve the most effective adsorption capacity of ash.
The research results can form the basis for the creation of industrial technology designed for the purification of flue gases of coal-fired power plants from sulfur dioxide, nitrogen oxides and carbon monoxide.But for this, it is still necessary to study the desorption stage and to propose ways of chemical binding of concentrated oxides of sulfur, nitrogen, and carbon into non-toxic products.

Figure 2 .
Figure 2. The effect of MXA on the adsorption capacity of ash in relation to SO 2 at 473K.

Figure 3 .
Figure 3.The influence of MA on the adsorption capacity of ash in relation to NO x at 473 K.

Figure 4 .
Figure 4.The influence of MA on the adsorption capacity of ash in relation to CO at 473 K.