Influence of preparation and combustion parameters of coal-water slurries on gas emission chemistry

The article presents the results of experimental studies performed for the comparative analysis of factors affecting the concentration of the main gas emissions during the combustion of fuel suspensions obtained from coal enrichment waste. The factors influencing the formation of anthropogenic emissions during the flaring of coal-water fuel suspensions were investigated. Experiments were carried out with coal processing waste, both with and without additives in the form of spent mineral oil. The article shows how the concentration of sulfur dioxide, nitrogen dioxide, and hydrocarbons is affected by the parameters of preparation (particle size, solid–liquid phase ratio, additives of used engine oil) and flame combustion (temperature in the combustion chamber, excess air coefficient, and droplet size after spraying) of fuel suspensions. The ranges of parameters were selected at which the best combustion parameters are observed, based on the data obtained: degree of grinding is less than 100 μm, solid part content in the suspension is from 55 to 60%; combustion temperature is in the range from 950 to 1050° C, droplet size when spraying the suspension is less than 300 μm, additive of waste mineral oil is from 3 to 7 wt.%. and air–fuel ratio from 1.2 to 1.3.


Introduction
The demand for high-calorie coals is growing every year. This leads to an increase in the productivity of existing processing plants, the commissioning of new coal processing facilities, and, as a result, an increase in the volume of waste generation. Currently, the amount of accumulated waste from coal processing in Russia is comparable to the reserves of developed deposits, and the annual increase reaches tens of percent of the total production volume. According to their fuel characteristics, such waste is often not inferior to the extracted solid mineral fuel. Today, waste from coal enrichment is usually placed in dumps, which leads to the withdrawal of agricultural, forestry, and other lands for their placement. The fire-hazardous technogenic objects formed in this way, containing fine coal particles, are sources of pollution of the surface layer of atmospheric air, soils, surface and underground waters, as well as the cause of deterioration of public health and increased mortality in certain regions.
An effective solution for the disposal of accumulated and annually generated waste of coal enrichment will reduce the industrial burden on the environment in the zone of impact of coal processing enterprises, lower the loss of energy raw materials, and reduce the costs of enterprises for the production of thermal energy (Bazhin 2016, Borowski andOzga 2020).
Currently, there are a large number of possible ways to dispose of carbon-containing waste-this is briquetting, gasification, dust-coal combustion, etc. However, these methods do not allow to immediately process the resulting waste and require preliminary removal of moisture (Alexandrova and Rasskazovav 2016, . One of the most promising ways to dispose of waste from coal processing today is their use as raw materials for the production of fuel suspensions. This method allows the processing of watered waste immediately after its formation, which eliminates its interaction with the components of the natural environment. This reduces the negative impact on all environmental compartments and simplifies and reduces the cost of fuel preparation due to the lack of costs for primary preparation associated with grinding and drying waste (Silyutin and Epshtein 2020).
The ability to control the process of thermal waste disposal allows to significantly reduce the emissions of pollutants into the air. In this regard, of particular interest is the theory of CWS burning process, as well as the ability to control the combustion process by adjusting the parameters of their preparation and combustion and the possibility of reducing the concentration of pollutants in the exhaust gas.
Most of experiments to determine pollutant emissions during fuel suspension combustion were carried out under static conditions; however, in practice, the combustion of suspension fuel is carried out by a flame combustion (Akhmetshin et al. 2021, Glushkov Dmitrii at al. 2016, Matveeva Vera et al. 2019. Pollutant emissions from fuel suspension combustion under static and dynamic conditions may differ. For this reason, in the research, experiments were carried out in an experimental stand with flame combustion.
The aim of this work is to experimentally study the factors affecting the concentration of the most dangerous components of gaseous emissions during the flame combustion of fuel suspensions obtained from coal enrichment waste in order to establish the best energy and environmental indicators for the thermal disposal of coal enrichment waste.

Experimental setups, methods, and materials
To be able to control the combustion process and obtain the best fuel and environmental characteristics in the thermal disposal of coal enrichment waste, it is necessary to consider the influence of the main parameters of preparation, spraying, and combustion of fuel suspensions.
The following parameters of the preparation of fuel suspensions affect the emissions of pollutants: grinding coarseness, the ratio of the dispersed and dispersion phases of the suspension, and fuel composition. During combustion of the suspension fuel, the composition of waste gases is affected by the following parameters: temperature in the active combustion zone, air-fuel ratio, and droplet size for spray equipment (Shabarov and Nikolaeva 2016).
As a raw material for the preparation of fuel suspensions, slurries from the gravitational enrichment of various coal rank from the Kemerovo region (Russia) were used. The main characteristics of coal processing waste are presented in Table 1.
For the preparation of CWS, the waste of coal enrichment was crushed and mixed with tap water. Homogenization was carried out using overhead stirrers at a rotation speed of 500 rpm for 10 min.
Preparation of CWSP (with the addition of used engine oil) was carried out in two stages. At the first stage, a water-oil emulsion was prepared: for this purpose, waste oil and water were mixed in a laboratory beaker for 5 min. After that, the crushed waste of coal enrichment was added to the tank. The duration of homogenization of the suspension was 10 min (Glushkov et al. 2016).
To conduct experiments on the combustion of fuel suspensions that are close to real conditions, a laboratory stand was designed, which allows conducting tests on the flaring of various types of fuels and a two-phase pneumatic nozzle with external mixing for spraying fuel water-coal suspensions ( Fig. 1).
To determine the component composition of the combustion products, depending on the different characteristics of the preparation and combustion of coal-water fuel obtained from the waste of coal enrichment, the combustion chamber of the stand was heated to a temperature of 850 ± 20 °C. After reaching the temperature required for igniting the fuel suspensions, the blast air was switched to the mode of operation with liquid fuel sprayed by means of a nozzle (Fig. 1).
Determination of droplet size of the suspension during spraying was carried out for each type of fuel suspension. Experimental stand was made to test the parameters of the injector operation (fuel consumption and pressure, compressed air consumption and pressure, and spray pattern), as well as to evaluate fuel suspensions spraying.
To determine the droplet size of the suspension, spraying onto the screen was carried out. The distance from the nozzle to the sheets was 30 cm. As an example, Fig. 2 shows one of these prints. At the lower part of Fig. 2 is a ruler with a step of 1 mm to visualize the obtained spraying parameters.
The processing of the print from the suspension spray was carried out in the ImageJ program. After treatment, the average and median droplet diameters were calculated when the suspension was sprayed. Figure 3 shows the main stages of combustion of coalwater fuel suspensions along the spray axis of flare. The lower part of Fig. 3 depicts the main possible options for the combustion of fuel droplets, depending on the spray parameters, particle size, and characteristics of the combustion of suspensions. Spraying of coal-water fuel is carried out by air in two stages. At the first stage, the suspension jet is crushed due to the kinetic energy of the spraying agent. At the second stage, at a high speed of movement of the sprayed fuel particles, their crushing is carried out due to the resistance forces of the gas-air medium in the boiler furnace. As a result of the resulting dynamic action, the droplets are torn into smaller ones, and the liquid phase with thin carbon particles breaks off from the surface of large coal particles. When a drop of fuel interacts with a heated oxidizer, its temperature increases. This heat is spent on the evaporation of water from the surface layer, as well as on the transition of the organic component of coal to the gaseous state. When the concentration of volatile organic compounds in the vicinity of the drop reaches critical values, their ignition occurs. The heat released during combustion warms up the coke residue, the temperature rises and carbon is ignited. The length of the flame torch in the combustion of water-coal suspensions in comparison with pulverized coal combustion is 20-40% longer due to the addition of water. This contributes to a more uniform course of the combustion process, reduction of the rate of mechanical underburning, as well as reducing the concentration of hydrocarbons in the composition of the exhaust gases.
Determination of the content of pollutants in the exhaust gases from the combustion of CWS and CWSP was carried out after stabilization of the combustion parameters. The content of nitrogen dioxide and sulfur dioxide and the content of hydrocarbons in the exhaust gases was determined using a portable multicomponent gas analyzer "POLAR" with a forced sampling device. The relative measurement error in the determination of nitrogen oxides and sulfur dioxide was less than 5%. In hydrocarbon determination, the relative error was less than 10%.
Sampling of waste gases to determine the content of hydrocarbons was carried out taking into account the recommendations presented in the article (Anichkov et al. 1985). Samples were taken by external filtration on sorption tubes filled with a universal Tenax GR sorbent under isokinetic conditions. Sampling was carried out for 2 min with a flow rate of pumping the gas mixture of 0.5 dm 3 / min. The ambient temperature during the experiments was less than 10 °C; therefore, a rapid decrease in the gas temperature along the intake pipe path was observed. An absorption solution (n-hexane) was placed behind the sorption tube to control the overshoot of hydrocarbons, which was less than 2%. The samples were analyzed for hydrocarbon content using a GCMS-QP2010 Ultra Shimadzu gas chromatography-mass spectrometer with a Unity-2 thermal desorber.
The concentration of hydrocarbons was determined only in those experiments where, according to the literature, a change in the parameter significantly affects the content of organic compounds in the composition of waste gases during fuel combustion. These include the effect of organic compound additives, change of the temperature in the combustion chamber, and the effect of the air-fuel ratio. The obtained values of the concentrations of pollutants after measurements were recalculated for the specific gravity of their formation, referred to as 1 g of burned fuel in terms of dry state (Cheremisina and El'-Salim 2017).
The experiments were conducted at the following parameters: the average temperature in the furnace 900 ± 20 °C; excess air 30%; average particle diameter waste coal 50 ± 10 µm; the mass ratio of waste coal-water equal to 40/60; and average droplet size of the slurry during the spraying of 130 ± 20 µm. When identifying the dependences of the energy and environmental characteristics of thermal utilization, the values of only one of the studied characteristics were changed separately, while the rest were unchanged.

Results and discussion
The results of studies of the effect of each of the parameters of fuel preparation and combustion on the concentration of pollutants separately are presented below.

Particle size distribution
Experiments to determine the effect of the fineness of grinding on the composition of waste gases were carried out for water-coal suspensions obtained from coal enrichment waste with an average particle diameter of 35, 47, 92, 154, and 202 µm. The results of studies of the content of nitrogen and sulfur oxides in the exhaust gases are shown in Fig. 4.
When drops of CWS enter the heated chamber, the temperature of the drops increases rapidly and water vapor and volatile components are released, which lead to the effect of microexplosion (Atal and Levendis 1994). As a result, the diameter of individual droplets decreases, and the specific surface area of the sprayed fuel increases (Fig. 3). When using large particles as raw materials (the average diameter of which is 154 and 202 µm), the effect of microexplosion becomes less noticeable, which means that for the complete combustion of such fuel, a greater amount of time is required to stay in the chamber. With constant combustion parameters, an increase in the fineness of the grinding led to an increase in the combustible losses: from 1.5% for fuel with an average particle diameter of 35 µm up to 11.2% for fuels where particles with an average diameter of 202 µm were used.
An increase in the combustible losses is associated with a decrease in the concentration of pollutants in the exhaust gases with an increase in the average particle diameter. With the selected combustion parameters, suspensions with an average particle size of more than 92 µm are not completely burned out, and sulfur and nitrogen compounds remain in the organic and mineral mass of the raw material. More complete combustion of suspensions increases the energy efficiency of the fuel, reducing the danger of the resulting Fig. 4 The effect of the fineness of by-product coal grinding on the waste gases composition ash and slag waste to the environment. Also, the ash with a low content of organic mass can be used as an additive to asphalt and concrete without preliminary preparation.

Ratios of solid and liquid phases in the suspension composition
Experiments to determine the effect of the proportion of solid and liquid phases on the composition of waste gases were carried out for CWS with a mass ratio of waste coalwater equal to 50/50, 60/40, and 70/30. The results of studies of the content of nitrogen and sulfur oxides in the exhaust gases are shown in Fig. 5.
An increase in the proportion of solid components in the composition of fuel suspensions from coal processing waste from 50 to 70% leads to an increase in the specific mass of the formation of sulfur oxides by 56% and nitrogen oxides by 27%. The pattern of changes in the concentration of sulfur dioxide and nitrogen oxides depending on the solid-liquid phase ratio during flaring and combustion under static conditions is similar .
The presence of a large amount of water in the composition of fuel suspensions lowers the temperature in the zone of active combustion, which leads to a decrease in the amount of formed nitrogen and sulfur oxides. In addition, water vapor under the influence of high temperatures decomposes into oxygen and hydrogen. In this case, the oxygen formed during the reaction intensifies the combustion process, and hydrogen helps to reduce the levels of nitrogen oxides and sulfur in the exhaust gases: (Efstathiou and Olympiou 2017); NO x + CO → N 2 + CO 2 (T ≥ 1000 • C) (Daood et al. 2014, Lee et al. 2016; (Feng et al. 2017); SO x + CO → S + CO 2 (T ≥ 500 • C) (Feng et al. 2017, Bacskay andMackie 2005).
Nevertheless, a decrease in the amount of the combustible component in the composition of fuel suspensions leads to a decrease in its specific heat of combustion, reduces the stability of the flame combustion, and also increases the ignition time of the suspension droplets, which negatively affects the energy characteristics of combustion. It was found that the best combustion parameters are observed with a solid content of 55-60 wt. %.

Waste mineral oil additives
Studies to determine the effect of the addition of spent mineral engine oil to fuel suspensions on the composition of waste gases were carried out at the mass ratio of coal enrichment waste/waste oil/water: 60/0/40, 55/5/40, 50/10/40, and 45/15/40.
Used engine oil has a high specific heat of combustion, 45.8 MJ/kg. When it is used as an additive to fuel suspensions, the total specific heat of combustion increases, the droplet ignition time decreases, and the flame stability increases. When 5% of used engine oil is added to the suspension from coal enrichment wastes, the specific heat of combustion increases by 13%, with the addition of 10% by 25%, at 15% by 37%. Table 2 shows the concentrations of the main groups of hydrocarbons formed during the combustion of CWSP using used engine oil as an additive. The addition of waste oil to coal-water suspensions leads to an increase in the total amount of hydrocarbons in the exhaust gases. The content of unsaturated, oxygencontaining, and polyaromatic hydrocarbons increases, while the content of limiting hydrocarbons decreases. Also, when adding oil in the composition of the exhaust gases, xylenes can be fixed, which were not detected when burning coalwater fuel without additives ( Table 2).
The increase in the content of unsaturated hydrocarbons in the composition of the exhaust gases is presumably associated with an increase in temperature in the zone of active combustion of the flame, which leads to incomplete pyrolysis of saturated hydrocarbons, in addition, the presence of additives in the waste oil containing metals such as Zn, Mo, Cu, etc., can catalyze the reactions of intermolecular dehydrogenation of saturated hydrocarbons upon rapid cooling of a highly heated gas-air mixture (Yan et al. 2020).
The increase in the concentration of oxygenated hydrocarbons is due to the presence of esters in the waste oil. The content of PAHs in the exhaust gases increases due to an increase in the amount of volatiles in the CWSP.
To visualize the obtained data, a graph of the dependence of the specific mass of nitrogen oxides and sulfur oxides and the amount concentration of hydrocarbons on the addition of used engine oil to fuel coal-water suspensions obtained from coal processing waste were plotted (Fig. 6).
The sulfur content of the waste oil is 0.49%. This indicator is slightly higher than for coal enrichment waste (0.32%). The nitrogen content in the waste oil is 0.13%, which is lower than in coal processing waste (1.50%).
An increase in the content of used engine oil in the composition of fuel suspensions during combustion leads to an increase in the concentration of nitrogen oxides in the composition of exhaust gases due to an increase in temperature  in the active combustion zone. Also, the content of sulfur dioxide increases slightly due to an increase in the sulfur content in the combustible mass of the fuel (Poskart and Lech 2007). When even 5% of used engine oil is added to the suspensions, their ignition is greatly simplified due to the relatively low ignition temperature of the oil, which is 226° C. For coal-water suspensions, the ignition temperature is 800-850° C. Also, when using the additive in the form of waste oil, the sedimentation stability of the suspension significantly increased (Samoilik et al. 1990, Routray et al. 2018, Das 2020.
Since the parameters of the coal enrichment waste generated at the enterprise can differ significantly even during one shift, for the stability of combustion and obtaining the required energy parameters, it is possible to control the characteristics of the fuel by adding used oils.

Combustion temperature
Experiments to determine the effect of the combustion temperature on the composition of waste gases were carried out for fuel CWS at temperatures of 800 ± 20 °C, 900 ± 20 °C, 1000 ± 20 °C и 1100 ± 20 °C. Table 3 shows the concentrations of the main groups of hydrocarbons formed during the combustion of fuel suspensions, depending on the temperature in the combustion chamber.
The specific mass of the formation of hydrocarbons at higher temperatures decreased. The ratio of saturated, unsaturated, oxygen-containing, and polyaromatic organic compounds remained practically unchanged with increasing temperature.
The graph of the change in the mass of formation of nitrogen oxides, sulfur dioxide and hydrocarbons with a change in the combustion temperature of CWS obtained from coal wastes is shown in Fig. 7.
As the temperature rises, the concentration of sulfur dioxide increases. At high temperatures, the rate of the oxidation reaction increases, and the process of interaction of fuel sulfur with oxygen intensifies (Matveeva et al. 2019). The pattern of changes in the concentration of sulfur dioxide and nitrogen oxides depending on the temperature during flaring and combustion under static conditions is similar , Glushkov et al. 2016.
Nitrogen oxides during fuel combustion are formed from nitrogen-containing fuel components and molecular nitrogen in the air. During the experiments, it was found that the hightemperature effect in the combustion zone has a decisive  . 7 Influence of the temperature in the combustion chamber on the composition of the exhaust gases influence on the content of nitrogen oxides in the exhaust gases. The formation of thermal nitrogen oxides occurs by the Zeldovich mechanism (Zeldovich et al. 1947, Babiy et al. 1996, in which the blast nitrogen is oxidized in the zone of active flame combustion. Since the activation energy of these reactions is high, the content of nitrogen oxides in the exhaust gases strongly depends on the temperature.

Air-fuel ratio
Experiments to determine the effect of air-fuel ratio during combustion of fuel suspensions obtained from coal processing waste on the composition of waste gases were carried out at air-fuel ratio of 1.1, 1.2, 1.3, and 1.4%. Table 4 shows the values of the concentrations of the main groups of hydrocarbons formed during fuel combustion, depending on the air-fuel ratio. An increase in the excess air, calculated from the stoichiometric equation of combustion of a fuel suspension, from 1.1 to 1.4 leads to a decrease in the specific gravity of the formation of hydrocarbons by almost 2 times. This also reduces the levels of carcinogenic PAHs such as anthracene, pyrene, benzo[a]pyrene, benz[a]anthracene, and naphthalene.
For visualization of the obtained data, a graph of the specific gravity of the formation of nitrogen oxides, sulfur dioxide, and total hydrocarbons from the excess air ratio when burning fuel suspensions obtained from by-product coal was constructed (Fig. 8). Figure 7 shows a significant decrease in the specific mass of the formation of hydrocarbons, but at the same time, the amount of nitrogen oxides increases. The specific gravity of the formation of sulfur dioxide is less dependent on the air-fuel ratio.
With a decrease in the air-fuel ratio, the volumes of the formed thermal nitrogen oxides also decrease. At the same time, an insufficient excess of air in the combustion zone leads to an increase in emissions of hydrocarbons and particulate matter. Also, the regulation of the formation of nitrogen oxides by changing the excess air ratio can lead to high-temperature sulfide corrosion and increase the slagging of the walls of the furnace of the boiler unit. Despite this, the combustion of fuels with a lack of air is one of the most common ways to control nitrogen oxide emissions (Li et al. 2013, Hu et al. 2000.
The lowest amount of pollutants in the exhaust gases during the combustion of fuel suspensions from coal enrichment waste is observed at an air-fuel ratio of 1.2:1.3. Table 4 The concentration of the main hydrocarbons, depending on the air-fuel ratio

Group of hydrocarbons
The concentration of hydrocarbons, depending on air-fuel ratio (mg/g)

Suspension droplet size
Experiments to determine the effect of the degree of suspension spraying on the composition of waste gases were carried out for water-coal suspensions obtained from coal enrichment waste with an average drop diameter of 65, 94, 128, 184, and 253 μm. The results of studies of the content of nitrogen oxides and sulfur in the exhaust gases are shown in Fig. 9. An increase in the average droplet size when spraying the suspension leads to a decrease in the specific mass of nitrogen and sulfur oxides. This is due to the fact that with an increase in the diameter of the drop, more time is required for its heating and ignition. Larger drops during the combustion of suspensions do not have time to completely burn out, as a result of which the combustible losses increases. This is associated with a decrease in emissions of nitrogen oxides and sulfur with an increase in the diameter of the suspension droplets.
The droplets with an average diameter of 65 microns burned more intensively, as a result of which the temperature in the combustion chamber increased and because of this, there was an increase in the concentration of nitrogen oxides and sulfur oxide in the composition of the exhaust gases. The microexplosion effect is less apparent for drops of this diameter (Gaber et al. 2020).
The most optimal solution is to spray CWS to droplets with an average size of 94-128 μm since this requires less energy consumption compared to a suspension dispersed to droplets with an average diameter of 65 μm. The combustible losses when spraying drops with an average diameter of 65, 94, and 128 μm were 2.5-4.9%.

Conclusions
The conducted experiments made it possible to establish the dependence of the formation of sulfur oxides, nitrogen oxides, saturated and unsaturated hydrocarbons, oxygencontaining organic compounds, as well as the main polyaromatic hydrocarbons formed during coal combustion, depending on the preparation parameters and combustion characteristics of coal-water suspensions obtained from coal enrichment waste.
Water in the composition of fuel suspensions reduces the temperature in the active combustion zone, which leads to a decrease in the amount of nitrogen and sulfur oxides formed. The content of the solid component of less than 50% leads to an increase in the ignition time of the droplets of the suspension and reduces the specific heat of combustion. The best combustion parameters are observed at a solid content of 55-60 wt. %.
Reducing the amount of air in the combustion chamber leads to a reduction in the amount of nitrogen and sulfur oxides formed. At the same time, insufficient excess air leads to an increase in hydrocarbon and particulate emissions. The lowest amount of pollutants in the exhaust gases during the combustion of fuel suspensions is observed at an air-fuel ratio of 1.2:1.3.
The concentration of sulfur dioxide and nitrogen oxides in the exhaust gases increases with increasing temperature; however, the amount of hydrocarbons formed is reduced. In the temperature range of 950-1150 °C, the emission hazard was the least.
Adding from 3 to 7 wt.% of used oil in fuel suspensions improves the energy characteristics of the fuel without a Fig. 9 Influence of the suspension spraying degree on the waste gases composition significant increase in the concentration of pollutants in the composition of the exhaust gases.
The particle size distribution of the solid part and the suspension dispersion degree, as studies have shown, are not characteristics that significantly affect the composition of the exhaust gases; however, these characteristics strongly affect the energy efficiency of the fuel. Uniform grinding of particles to sizes less than 100 μm and spraying degree of fuel to sizes less than 300 μm lead to more complete combustion of the fuel.
The research results show possible prospects for the involvement of coal enrichment waste in the fuel and energy cycle. The use of such waste will reduce emissions of pollutants (compared to coal), utilize accumulated and newly generated waste, and reduce the probability of ignition of facilities for the disposal of such waste.
Author contributions All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Ivan Sverchkov, Mariia Chukaeva, and Vera Matveeva. The first draft of the manuscript was written by Ivan Sverchkov, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
-Conceptualization: IS formulated the idea of the research, and VM formulated the goals and aims.
-Methodology: IS designed an experimental stand for flaring liquid fuels and set up an experiment, and MC designed a method for sampling and analyzing samples.
-Formal analysis and investigation: IS and MC used statistical and mathematical methods to analyze the data obtained.
-Writing-original draft preparation: IS wrote the initial draft.
-Writing-review and editing: IS and MC prepared, created, and presented the published work, and VM prepared a critical review of scientific work.
-Supervision: VM carried out supervision and responsibility for the planning and execution of research activities.
Data availability All data generated or analyzed during this study are included in this scientific work (https:// spmi. ru/ sverc hkov-ivan-pavlo vich).

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Ethics approval and consent to participate Not applicable.

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Competing Interests All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.