RU2769293C1 - Method for igniting and stabilizing the combustion of water-coal fuel in installations for the disposal of high-moisture waste using low-temperature non-equilibrium plasma and a device for its implementation - Google Patents

Abstract

FIELD: thermal disinfection.

SUBSTANCE: invention relates to the field of thermal disinfection and waste disposal, and in particular to the processes of ignition and stabilization of fuel combustion in waste heat boilers operating on coal-water fuel (CWF) in the mode of production waste disposal. A method for igniting and stabilizing the combustion of water-coal fuel in installations for the disposal of high-moisture waste using low-temperature non-equilibrium plasma, characterized in that in the starting mode, pulverized coal fuel in the form of pulverized coal aero mixture is fed into a gas-discharge electric spark chamber adjacent at an angle to the pre-furnace, while ignition of the pulverized coal aero mixture is carried out using an electric spark discharge of alternating current, which generates a low-temperature non-equilibrium plasma, and the ignited pulverized coal mixture from the gas discharge chamber is fed into the pre-furnace and then into the vortex adiabatic furnace, where it is heated to a temperature of 860-900°C, the pulverized coal supply in the operating mode is carried out in two streams - the base enters the water-coal fuel preparation unit with subsequent supply fuel to a water-coal burner located coaxially with the pre-furnace, auxiliary - in the form of a pulverized coal mixture is fed into a gas-discharge electric spark chamber, and the ignited pulverized-coal air mixture from the gas-discharge electric spark chamber is fed into the pre-furnace, then into the vortex adiabatic furnace, where the combustion of the water-coal flame is stabilized.

EFFECT: reduction of energy consumption for ignition and stabilization of CWF combustion in a boiler unit operating in the waste disposal mode, reduction of electrode wear due to the use of an electric spark discharge instead of an electric arc discharge during ignition and combustion of pulverized coal fuel in a gas-discharge electric spark chamber.

8 cl, 4 dwg

Description

The invention relates to the field of thermal disinfection and waste disposal, and in particular to the processes of ignition and stabilization of combustion (illumination) of fuel in waste heat boilers operating on water-coal fuel (VUT) and can be used in various sectors of the economy associated with the thermal method of disposal and disinfection of high-moisture waste.

Prerequisite for the creation of the invention, analogues of the invention

The use of coal-water fuel prepared on the basis of liquid production waste and coal enrichment waste can significantly reduce emissions of harmful substances and dangerous microorganisms into the biosphere [1]. The use of coal-water fuel leads to a significant increase in the efficiency of the boiler unit and a sharp decrease in emissions of harmful substances formed during combustion (heat supply systems for domestic and agricultural consumers, systems for decontamination and waste disposal with a capacity of up to 5 MW are considered), [2].

The developed method for thermal disinfection and disposal of infected organ-containing waste in various aggregate states (RF patent 2718563, publ. 04/08/2020) allows for complex disinfection and disposal of biological waste in the power units of the boiler house, including the most significant in terms of impact on the biosphere - waste in the liquid state of aggregation.

One of the main problems of combustion of water-coal fuel and high-moisture (W>75%) wastes in small-capacity boilers, identified when testing this method of utilization at the SFSC RAS, is the need to use an initiating highly reactive fuel (fuel oil, diesel fuel) to stabilize the combustion of water-coal suspension, due to a temperature drop in the furnace when fuel is supplied, which negatively affects the process of fuel gasification, thereby making it difficult to start the boiler and maintain stable combustion of fuel at low loads.

The standard firing mode for a coal-water boiler is as follows [2]: draft devices are turned on, an initiating liquid fuel burner (fuel oil, diesel fuel) is connected to the boiler pre-furnace (prechamber), which heats the furnace up to 600 ... 650 gr. Celsius. Upon reaching the specified temperature, a water-coal burner is connected and a joint mode of combustion of water-coal and initiating fuels is provided. Upon reaching a temperature of 850 ... 900 gr. Celsius, the initiating burner is turned off and switched to the mode of using coal-water fuel. In the case of low-quality fuel, the pilot burner is used to continuously illuminate the flame.

Expensive liquid fuel is used to ignite and illuminate the torch. The use of a coal-water boiler for thermal utilization of high-moisture waste significantly increases the consumption of high-quality liquid fuel. At present, in order to reduce costs, it is proposed to use various electrotechnological methods instead of high-quality fuel to ensure the stabilization of flame combustion, in particular, an electric arc discharge.

There is a method (RF patent No. 2210700) where the ignition of a pulverized coal flame is carried out by generating a low-temperature plasma jet in a unipolar current plasma torch with a grounded electrode connected to a converter that gives a pulsating or intermittent current of the main arc, the converter is connected to the main network, an auxiliary low-current DC arc with its power supply from a separate source. A variable power plasma jet is fed into the burner fuel thermochemical preparation chamber, a pulverized-coal aero mixture is fed into the same chamber, and the fuel mixture ignited in the chamber is fed from the burner into the boiler furnace. The disadvantage of this method is the use of a DC arc, which requires the use of powerful power equipment equipment (power supply, rectifier), rapid wear of the electrodes and significant power consumption during an electric arc discharge.

A method is known (RF patent No. 2498159 publ. 10.11.2013, bull. No. 3) of burning pulverized coal fuel, which consists in the fact that its ignition is produced by an electric arc discharge, the flame is stabilized and intensified by influencing the flame formation zone with an alternating electric current of high frequency, forming in a diffuse electric discharge in the flame formation zone. The technical result achieved by the proposed technical solution is to ignite, stabilize and enhance the combustion process at a lower cost of electrical energy. The disadvantage of this method is the use of an electric arc discharge at the stage of ignition, with the disadvantages inherent in this method (rapid wear and corrosion of the electrodes, high power consumption).

At present, works have appeared on the theory of combustion, which justifies the use of low-temperature (the average electron temperature is less than 10 eV), weakly ionized (the degree of ionization n/N<<1, where n and N are the densities of charged and neutral particles), non-equilibrium (electrons in a cold plasma are superheated, by about 100 times, with respect to ions, as well as to atoms and molecules of the plasma-forming gas) plasma as a source of free radicals that determine the kinetics of combustion processes. In the plasma gas, intense ultraviolet radiation occurs, as well as physically active particles (metastable atoms and molecules, radicals, ozone and other reactive compounds) and vibrationally excited nitrogen molecules that promote fuel combustion at relatively low specific energy consumption [3] - [8].

Gas-discharge methods for creating a non-equilibrium plasma in air have been developed, in particular, using a barrier discharge (a streamer discharge of alternating current, in which the conduction current in the interelectrode gap is provided by a bias current through a dielectric barrier). To create a non-equilibrium plasma inside the electrode system, a sinusoidal or repetitively pulsed voltage is applied to the electrodes with a frequency of 50 to 10 ⁵ Hz with an amplitude, as a rule, up to 10 kV. At an AC voltage frequency of 50 Hz, the characteristic level of the time- and space-averaged specific Joule power released in the interelectrode gap of the barrier discharge is 1–3 W/cm ³ . The average power density of the barrier discharge (BR), reduced to 1 cm ^{2 of} the surface of the electrodes, is low and corresponds to 0.1-1 W/cm ² . The typical speeds of pumping the processed gas through the gap of the barrier discharge are also small: V=0.1-1.0 m/s [3]. A significant disadvantage of BR is its sensitivity to the moisture content in the processed air flow, which prevents the use of BR for ignition and initialization of a water-coal suspension of high-moisture waste.

The MIPT report [9] notes that the arc discharge plasma is a thermally equilibrium plasma, in which about half of the entire discharge energy is spent on heating the gas, which in turn leads to significant erosion of the electrodes. Moreover, since only one plasma channel is formed during an arc discharge, the energy input of the discharge per unit volume significantly exceeds the required ignition energy of the fuel-air mixture. Low-temperature plasma is non-equilibrium; at a relatively low, almost room temperature of ions and molecules, it has an electron temperature comparable to the temperature in an arc discharge. Such high-energy electrons, colliding with fuel and oxygen molecules, form chemically active radicals, which lead to the initiation and development of combustion chain reactions.

Low-temperature nonequilibrium plasma has a number of advantages, including low electrode erosion due to low ambient gas temperature, increased discharge volume due to the formation of many discharge channels, and a relatively low (but sufficient for ignition) energy input per unit volume. The paper substantiates that in order to accelerate the ignition of fuel mixtures below the self-ignition threshold, it is desirable to organize the energy input in different ranges of the reduced electric (electromagnetic) field in order to achieve the production of not only atoms and radicals, but also vibrationally excited nitrogen molecules. In this case, a synergistic effect can play an important role in the acceleration of ignition, when chain reactions begin and continue due to the atoms and radicals produced in the discharge, and chain termination reactions are inhibited due to the presence of vibrationally excited nitrogen molecules [7].

There is a known method for kindling and maintaining stable combustion in boilers using water-coal fuel (patent RU 2505748, publ. 01/27/2014), adopted as a prototype, in which, to maintain combustion at low loads, water-coal fuel is supplied to different places of the fuel mixture preparation module, and namely, to the zone near the torch formed by a high-temperature heat source, and to the zone of connection of the specified module with the furnace, while from 10 to 50% of the throughput volume of the fuel line is supplied to the zone near the torch, and from 0 to 100% - from 0 to 100% to the zone of connection of the module with the furnace capacity of the fuel line. Due to the supply of CWF in a small volume to the region of the plasma torch, reliable gasification of the supplied fuel is ensured, all the supplied fuel passes into a gaseous state and begins to ignite easily. The disadvantage of this method of kindling and maintaining stable combustion of water-coal fuel is the use of an electric arc discharge generated in the plasma torch, which leads to significant energy costs in the stabilization mode of combustion of water-coal fuel.

It is advisable in our case to use not an arc discharge, which forms the main contribution in the infrared part of the radiation spectrum, accompanied by high energy costs, but an electric spark discharge, which forms a low-temperature nonequilibrium plasma, providing low electrode erosion, an increased discharge volume due to the formation of many discharge channels, and a relatively low energy input. per unit volume by generating atoms, radicals and vibrationally excited nitrogen molecules. When the fuel is ignited, chain reactions in this case begin and continue due to the atoms and radicals produced in the discharge, and chain termination reactions are inhibited due to the presence of vibrationally excited nitrogen molecules.

The technical effect of the invention is: increasing the stability of ignition and stability of combustion of water-coal fuel during thermal recycling of high-moisture waste.

The technical results of the invention are:

- reduction of energy consumption for ignition and stabilization of combustion of water-coal fuel in a boiler unit operating in the waste disposal mode by using low-temperature non-equilibrium plasma for ignition of pulverized coal fuel and stabilization of combustion of water-coal fuel in the boiler furnace;

- reduction of electrode wear due to the use of an electric spark discharge on electrodes of a special shape instead of an electric arc discharge.

These effects are achieved by the fact that, according to the invention, the pulverized coal fuel from the mill initially enters the pulverized coal air mixture preparation unit (PUA) and is fed into the electric spark gas discharge chamber by the pulverized coal air mixture transportation system. PUA entering the electrospark chamber is ignited by an electric spark discharge. The combustion regime is maintained by the formation of a low-temperature plasma in the electric spark gap. The use of low-temperature plasma ensures low electrode erosion due to the low temperature of the surrounding gas, the arcuate shape of the electrodes, the increased discharge volume due to the formation of many discharge channels, and the relatively low energy input per unit volume due to the generation of atoms, radicals, and vibrationally excited nitrogen molecules. When the fuel is ignited, chain reactions in this case begin and continue due to the atoms and radicals produced in the discharge, and chain termination reactions are inhibited due to the presence of vibrationally excited nitrogen molecules. Combustion products formed in the electric spark chamber enter the prechamber and, further, into the furnace space, providing the required temperature in the boiler furnace for ignition and stabilization of combustion of water-coal fuel and waste.

When switching to the operating mode, the main part of the pulverized coal fuel (up to 80 ... 90% by weight) is used for the preparation of water-coal fuel and subsequent combustion in the boiler furnace (after reaching a temperature of 850 ... 900 ° C). The remaining fraction of the fuel (from 10 to 20%) continues to flow into the electrospark chamber, providing the necessary illumination of the combustion flame for water-coal fuel and incoming waste, stabilizing the combustion process.

A block diagram of the proposed method for igniting and stabilizing the combustion of water-coal fuel in installations for the disposal of high-moisture waste is shown in the figure (Fig. 1).

The following sequence of operations is implemented:

When starting, the pulverized coal fuel from the mill (1) enters the pulverized coal flow distribution and control unit (2), directing the flow of pulverized coal fuel to the pulverized coal aero mixture preparation unit (3) and transportation by means of the pulverized coal aero mixture transportation unit (4) using compressor compressed air ( 11), into a gas-discharge electric spark chamber (5) connected to a high-voltage power supply unit (6).

The electrode system of the electric spark gas discharge is made multiphase, according to the number of phases of the supply electrical network, and the electric spark discharge is carried out on several groups of electrodes, each group of which consists of phase and zero electrodes connected to different phases of an alternating electric current with a frequency of 20 kHz. The electrodes are arc-shaped (Fig. 2) with a variable distance between the phase and zero electrodes along the arc, which ensures the occurrence of several discharges from different points of the electrode, minimizing wear on the electrodes.

The minimum gap between the phase and neutral electrodes is 4 mm at an idle supply voltage of the high voltage source of 12 kV. The calculated reduced electric field strength in the spark gap on arc-shaped electrodes is 25-100 Td. With the specified configuration of the electrodes, their geometric dimensions and the specified values of the electric field strength, an electric spark discharge is formed, which creates a plurality of plasma channels generating a low-temperature non-equilibrium plasma, which ensures the ignition of the pulverized-coal aero mixture at ambient temperature. In the experiment, the ignition of the pulverized-coal mixture occurred without preliminary thermochemical preparation of the fuel at a temperature of the pulverized-coal mixture of 2-10°C. A possible explanation of the effect obtained is as follows: high-energy electrons generated during an electric spark discharge, at the specified values of the electric field strength and a given geometry of the electrodes, colliding with fuel and oxygen molecules, form chemically active radicals that lead to the occurrence and development of combustion chain reactions. Low-temperature non-equilibrium plasma has an increased discharge volume due to the formation of many discharge channels and a relatively low energy input per unit volume, providing reliable ignition of a pulverized-coal aero mixture at low energy costs compared to an arc discharge. The specified range of values of the reduced electric field ensures the production of not only atoms and radicals, but also vibrationally excited nitrogen molecules, which ensure the inhibition of the termination reaction of combustion chain reactions.

When the temperature in the boiler furnace reaches 800-850°C, the system switches from starting to operating mode. The block for distributing and regulating the flows of pulverized coal (2) switches to the operating mode - 10 ... 7), where the liquid part of the waste coming from the liquid waste supply unit (14) is used as the water component of the CWF. Further transportation is carried out using the coal-water fuel transportation unit (8), to the coal-water burner (9), which adjoins the pre-furnace (10) of the vortex adiabatic furnace (12), which forms a pulverized coal torch using air supplied from the compressor (11).

Stabilization of flame combustion is carried out due to the high temperature in the vortex adiabatic furnace and additional illumination of the combustion products coming from the gas-discharge electric spark chamber (5).

The separation of pulverized coal fuel into two streams makes it possible to obtain water-coal fuel for burning high-moisture waste and pulverized coal fuel for ignition and stabilization of combustion of water-coal fuel in the boiler furnace. The use of high-quality (liquid) fuel or arc electric discharge for ignition and illumination is excluded, which reduces the cost of ignition and stabilization of fuel combustion. Replacing an electric arc electric discharge with an electric spark discharge provides a reduction in power consumption and electrode wear. Ignition and combustion of pulverized coal fuel is carried out by generating a low-temperature non-equilibrium plasma during an electric spark discharge, accompanied by the production of free radicals, and providing ignition of pulverized coal fuel without preliminary thermochemical fuel preparation. The combustion of a pulverized coal mixture in an electric spark gas-discharge chamber on an experimental stand is shown in Fig. 3-4.

Used sources

1. Patent 2718563 Russian Federation,) IPC F23G 5/02 (2006.01);

The method of thermal disinfection and disposal of infected organ-containing wastes in various aggregate states / Delyagin V.N.; applicant and patentee Siberian Federal Center for Agricultural Biotechnology RAS, Appl. 09/28/19; publ. 04/08/20, Bull. No. 10.

2. Delyagin V.N., Murko V.I., Ivanov N.M., Revyakin E.L. The use of coal-water fuel in the energy supply of the agro-industrial complex. M.: FGBNU "Rosinformagrotech", 2013. - 92 p.

3. Akishev Yu.S. Low-temperature plasma at atmospheric pressure and its potential for applications. Izv. universities. Chemistry and chem. 60 technology. 2019. V. 62. Issue. 8. P. 26 For citation: Akishev Yu.S. Nonthermal plasma at atmospheric pressure and its opportunities for applications. Izv. Vyssh. Studybn. Zaved. 60-Khim. Khim. Tekhnol. 2019. V. 62. N 8. P. 26.

4. M.F. Zhukov, E.I. Karpenko, V.S. Peregudov and others - Novosibirsk: Science. Siberian Publishing Company RAS, 1995. - 304 p. Low temperature plasma. T. 16, p. 102-178, see Appendix No. 1.

5. Non-equilibrium air plasmas at atmospheric pressureio Edio by K.H. Becker, U. Kogelschatz, K.H. Schoenbach, R.J. barker. Institute of Physics Publishing. 2005. 682 p.

6. Non-Thermal Plasma Techniques for Pollution Control. Ed. by B.M. Renetrante, S.E. Schultheis. Berlin Heidellerg: Springer-Verlag. 1993. NATO ASI Series. Ser. G: Ecolog. sci. V. 34. Parts A and B.

7. Friedman A., Kennedy L.A. Plasma Physics and Engineering. New York, London: Taylor & Francis. 2004. 853 p.

8. Andrzej G., Dors M., Grosch H., Holub M., Pawelec A., Brandenburg R., Barankova H., Bardos L., Jogi I., Laan M., Mizeraczyk J., Stamate E. Plasma -Based Depollution of Exhausts: Principles, State of the Art and Future Prospects. Open Access Books. Published: 23 August 2011 by InTech in Monitoring, Control and Effects of Air Pollution. DOI: 10.5772/20351.

9. Report on the research work Plasma-stimulated combustion of ultra-lean fuel-air mixtures to increase the efficiency of power devices code: 2011-1.9-519-008-064.

Claims (8)

1. A method for igniting and stabilizing the combustion of water-coal fuel in installations for the disposal of high-moisture waste using low-temperature non-equilibrium plasma, characterized in that, in the starting mode, pulverized coal fuel in the form of a pulverized coal mixture is fed into a gas-discharge electric spark chamber adjacent at an angle to the pre-furnace, while igniting the pulverized coal aeromixture is carried out using an electric spark discharge of alternating current, which generates a low-temperature non-equilibrium plasma, and the ignited pulverized coal aeromixture from the gas discharge chamber is fed into the pre-furnace and then into the vortex adiabatic furnace, where it is heated to a temperature of 860-900 ° C, the pulverized coal fuel is supplied in the operating mode two streams - the main one enters the unit for the preparation of water-coal fuel, followed by the supply of fuel to the water-coal burner located coaxially with the pre-furnace, the auxiliary one - in the form of a pulverized coal aero mixture is fed into the gas discharge the ignited pulverized coal mixture from the gas-discharge electric spark chamber is fed into the pre-furnace, then into the vortex adiabatic furnace, where the combustion of the water-coal flame is stabilized. 2. The method according to p. 1, characterized in that the pulverized coal mixture is fed into the gas-discharge electric spark chamber without prior thermochemical preparation. 3. A device for igniting and stabilizing the combustion of a coal-water flame in a vortex adiabatic furnace using low-temperature non-equilibrium plasma, including a device for grinding coal, a coal-water fuel transportation unit, a compressor, a pre-furnace, a burner, a coal-water fuel preparation unit, a liquid waste supply unit, a pulverized-coal aero mixture preparation unit , a unit for transporting pulverized-coal aeromixture to a gas-discharge electric spark chamber, a power supply unit, a gas-discharge electric spark chamber with a system of electrodes associated with a pre-furnace, characterized in that the device for switching from starting to operating mode contains a unit for distributing and regulating the flows of pulverized coal between the unit for preparing water-coal fuel and unit for preparing pulverized coal mixture. 4. The device according to claim 3, characterized in that the system of electrodes of the electric spark gas discharge chamber is multi-phase in terms of the number of phases of the supply network, and the electric spark discharge is carried out on several groups of electrodes, each group of which consists of phase and zero electrodes connected to different phases alternating electric current with a frequency of 20 kHz. 5. The device according to claim 3, characterized in that the electrodes of the electric spark gas-discharge chamber are made of an arcuate shape with a distance between the phase and zero electrodes that is variable along the arcs. 6. The device according to claim 3, characterized in that the electrodes of the electric spark gas-discharge chamber are oriented parallel to the flow of the pulverized-coal aero mixture. 7. The device according to claim 3, characterized in that the angle of contact of the electric spark gas-discharge chamber to the pre-furnace is 15 ... 20 degrees. 8. The device according to claim 3, characterized in that the minimum gap between the phase and zero electrodes is 4 mm when the supply voltage of the high voltage source at idle is 12 kV.

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