Investigation of the polystyrene combustion completeness increasing method in the environmentally safe waste disposal installation

. The article presents the study results of the solid waste components (on the example of polystyrene) combustion optimizing based on traditional solutions used in the furnaces organization (stratified combustion, combustion in a vortex flow, combustion products recirculation), as well as due to the electrostatic field impact. Relative changes in the CO and NO emissions values were obtained for various methods of air supply to the combustion chamber and electrostatic field parameters. The use of an electrostatic field with a potential difference of 3 – 4 kV made it possible to increase the combustion efficiency, which led to a decrease in the formation of CO by up to 40%, and NO to almost zero.


Introduction
Currently, the problem of industrial and domestic waste disposal is acute: landfills are overcrowded, and the share of waste recycling in the Russian Federation does not exceed 30% [1].In this regard, the simplest and most effective disposal option is the municipal solid waste incineration [2].At the same time, this method has a number of significant drawbacks: the waste itself does not burn well; when burned, a large amount of toxic substances are released into the atmosphere.
In the field of waste incineration, flaring, vortex method and fluidized bed are more often used [3].In this work, a combined combustion method is used, which incorporates the positive qualities of combustion in a dense layer and in a volume with vortex swirl and recirculation.Regardless of the combustion method, an increase in the combustion efficiency is achieved by preliminary fuel preparation, as well as by selecting the optimal oxidizer supply modes [4].
For additional combustion optimization, it is useful to use the physical fields action, in particular, an electric field.The authors in [5] note a decrease in the soot proportion and particle size under the field influence, which indicates the soot formation controlling possibility.The work [6] shows the possibility of controlling the NO and CO formation.The influence of the configuration and field strength on these substances concentrations in the combustion products is noted.In [7], the possibility of controlling the polymeric materials combustion completeness by applying an electric field to the combustion zone was shown.
The work aim was to compare the effectiveness of various combustion control methods in an incineration plant, estimated from the change in CO and NO emissions.

Research objects and methods
The study of the municipal solid waste combustion completeness was carried out on an experimental stand, which allows organizing combustion in various aerodynamic conditions, as well as creating an electrostatic field in the combustion zone.The general scheme of a waste incineration plant laboratory sample is shown in Fig. 1.Basic installation elements: 1) a double coaxial combustion chamber with zones for solid waste loading and combustion products removing; 2) air supply tract; 3) electric field system.
The combustion chamber consists of two parts: internal, in which solid waste is burned; external, necessary for the combustion products recycling organizing.
The air supply tract is organized with two directions: 1) air blowing from below, through the grate; 2) axial blowing into the internal combustion chamber (to organize swirl).
The electric field system includes a high-voltage source and two electrodes placed in the combustion zone.One electrode (plus) is located under the grate (this is an air injector isolated from other installation parts), and the other electrode (minus) is located above the combusted fuel, it has the rod appearance made of heat-resistant steel and is placed in a ceramic insulator.
Combustion products sampling is carried out at the upper end of the chimney.To measure the combustion products (NO, CO) concentration, an OPTIMA-7 gas analyzer was used, which made it possible to obtain data on emissions in a continuous mode.
The setup, as well as the results processing procedure, are described in more detail in [8].
Polystyrene was used as fuel, it is one of the most common components of household waste (disposable tableware, food packaging, children's toys, medical equipment, household appliances cases, etc.).The combustion efficiency was determined by the CO formation amount, and the NO concentration was additionally measured.

Results and discussion
In the first experiments series, the effect of air supply methods on CO emissions was considered as combustion efficiency indicator.The results are given in relative terms (β), with the initial emission values taken at an air flow rate of 10 l/min with air supply through the grate.Fig. 2 shows the data for different air flow rates and supply modes.The horizontal axis shows the total flow rate of air supplied to the combustion chamber.Air supply through the axial channel (tangential blowing) is carried out with a minimum blowing through the grate (10 l/min).When air is supplied from below, through a grate, for polystyrene, with an increase in air flow, the CO concentration decreases, there are local minima: 40 and 70 l/min (Figure 2).For NO, there is an air flow rate at which the emission concentration is minimal -20 l/min, after 50 l/min the NO concentration increases.
The creation of a swirling oxidizer flow in the combustion chamber leads to a monotonous increase in NO emission.CO emissions initially (at 20-30 l/min) increase slightly and then decrease, with a minimum at an air flow of 80 l/min.
In the second series of experiments, changes in the polystyrene combustion parameters under the electrostatic field influence were studied.To compare the results, air supply conditions were chosen when CO emissions in the first series of experiments were minimal (air flow through the grate 60 l/min).The calculation of relative values was carried out to values without a field.The results are shown in Fig. 3.The electrostatic field creation in the combustion chamber leads to a decrease in carbon monoxide in the exhaust gases.The CO content is reduced by up to 40% at a potential difference of 3-4 kV, relative to the values without a field, and the NO formation is reduced to almost zero.
When organizing the electric field combustion control method, it is necessary to take into account the field influence on excess charges (positive and negative), as a result of which the flame shape changes and the combustion temperature rises.An increase in temperature occurs due to an increase in the combustion completeness, resulting in the gasification products composition changes [9].This is manifested in the incomplete combustion products (CO) concentration decrease.

Conclusion
Thus, various ways of increasing the polystyrene combustion completeness in an incinerator are presented.It can be seen that the most effective reduction in emissions is realized with a combined selection of the optimal air flow, the method of its supply and the electric field system presence.The electric field effect can further increase the completeness and environmental friendliness of combustion.The use of an electrostatic field with a 3-4 kV potential difference made it possible to reduce the CO formation rate in combustion products by up to 40%, and NO to almost zero.

Fig. 2 .
Fig. 2. Relative change in the CO and NO formation at various air flow rates and supply methods.

Fig. 3 .
Fig. 3. Dependence of the CO formation rate in emissions on the voltage between the electrodes.