RU2802494C1 - Ash and slag waste melting system of an incineration plant - Google Patents

Abstract

FIELD: waste treatment.

SUBSTANCE: thermal processing of municipal solid waste (MSW) at incineration plants (IP). Invention relates to the disposal of ash and slag waste generated during the combustion of MSW, with the production of environmentally friendly chemically inert slag suitable for use in the construction industry. An environmentally safe system for melting ash and slag at the IP using a 3-phase AC electric melting furnace is proposed.

EFFECT: invention provides efficiency of electric furnace up to 100%, mixing of melt with molten charge due to current flow between electrodes, whereas capital costs for creation of 3-phase AC electric furnace are 20% lower compared to DC melting electric furnace.

6 cl, 1 tbl, 3 dwg

Description

The invention relates to the field of thermal processing of municipal solid waste (MSW) at waste incineration plants (WIP), namely to the part of the neutralization of ash and slag waste generated during the combustion of MSW, with the production of environmentally friendly chemically inert slag, suitable for use in the construction industry.

Ash and slag waste generated during thermal neutralization (combustion) of MSW contains 3-6% of the mass of dry fuel, fly ash (dust) and 20-30% of bottom ash (slag). Fly ash (hereinafter referred to as “ash”) is distinguished by a higher alkali content of Na ₂ O + K ₂ O than slag. The composition of ash includes up to 20% sulfates, as well as a large amount of water-soluble microimpurities, such as lead salts, zinc, mercury, cadmium, chlorides and fluorides. Ash belongs to the category of hazardous waste, since its content of heavy metals and dioxins significantly exceeds regulatory levels.

One of the most effective ways to solve the problem of neutralizing ash and slag waste is to melt it, which leads to a significant reduction in the volume of ash and converts it into inert (vitrified) slag.

In world practice, the neutralization of ash and slag waste by smelting is carried out in special autonomous smelting installations.

There are known solutions in which electric arc plasmatrons are used to melt ash and slag waste.

The well-known laboratory plasma melting plant (H.S. Pak. Study of the composition and properties of slag during plasma remelting of ash from incineration plants // Thermophysics and Aeromechanics, 2011, vol. 18, no. 2, p. 325-334) has low productivity, since Ash processing is carried out using a discrete method of 500 g. The installation’s cleaning system does not allow achieving environmental standards for harmful emissions.

In the known ash recycling unit [RU 2502017, 05/10/2012, F23G 5/00; RU 2502018, 05/10/2012, F23G 5/00] electric arc plasma torch has a service life of electrodes of no more than 500 hours.

A known system for melting ash from waste incineration [CN 207709525, 2018-08-10, B01D 46/02; B01D 50/00; B01D 53/04; B01D 53/18; B01D 53/78; B09B 3/00; B09B 5/00; F23G7/06], including an ash pre-treatment system, an ash melting system, and a flue gas purification system.

The ash pre-treatment system includes an ash storage bin and a granulator.

The ash melting system includes a melting furnace with an electric arc plasmatron and a slag water cooling tank. The melting furnace temperature is controlled and maintained at 1300-1700°C. At this temperature, fly ash particles melt, harmful components such as dioxin decompose, and heavy metals dissolve. The melt enters a water cooling tank and is quickly cooled by running cold water to form a vitrified inert slag.

Flue gas purification system includes an afterburner, gas cooler, bag filter, absorption column, carbon filter, induced draft fan, chimney. The gas cooler is used to quickly cool flue gases to 200°C. The bag filter captures secondary fly ash and particulate matter in the flue gases. In the absorption column, water-soluble gases are removed from the flue gases. The carbon filter is used to absorb residual Hg, Pd and other volatile heavy metals and dioxins in flue gases.

Common problems with using electric arc plasma torches for melting ash and slag are:

- high specific energy consumption (about 1 kWh/kg ash),

- limited service life of electrodes (300-500 hours),

- insufficient power of plasma devices (about 500-700 kW),

- low efficiency (70% and below).

A plasma-arc furnace is known for melting ash [JP 2001324124 (A) - 2001-11-22, F23J 1/00; F27B 3/08; F27D 11/08]. The furnace is equipped with two electrodes, the lower one, located on its bottom, and the main one, passing through the furnace lid. At the bottom of the furnace, there is metal material on the bottom electrode, and slag on the metal material. This invention is aimed at solving the problem of extending the service life of electrodes by ensuring more accurate and faster startup of the furnace.

A DC arc furnace is known for plasma melting of fly ash from the combustion of household waste [CN 110142277 (A) - 2019-08-20, B09B 3/00] which contains a housing, a combustion chamber, a graphite cathode and a metal bottom anode. The fly ash entering the combustion chamber is melted by the plasma arc to form molten fly ash, and at least a portion of the plasma arc is contained in the molten fly ash. A layer of conductive metal is used to prevent the combustion chamber refractory material from coming into contact with molten fly ash, which is highly erosive. The fly ash plasma melting furnace of the said invention uses a submerged arc operating mode. Compared to a plasma furnace, open arc operation increases the thermal conductivity to the molten pool by 20-50%.

It should be noted the complexity of the starting system and the high energy intensity of these furnaces, since before starting it is necessary to melt the metal.

A known system for electric arc melting of fly ash from the combustion of MSW [CN 206911916 (U) - 2018-01-23, B09B 3/00; B09B 5/00; C22B 7/00], including a fly ash pre-treatment system, an electric arc melting system and a flue gas purification system.

The fly ash pre-treatment system includes fly ash storage hopper, additive storage hopper, electric rotary valve, electronic screw scale, mixer, bucket elevator and feeder. Fly ash is mixed with additives such as river sand, borax, broken glass and crushed ash and slag in a ratio of 1:0.1-1:0.3. The fly ash storage hopper and additive storage hopper outlet port are equipped with controlled electric rotary valves, which together with electronic scales control the quantitative output of materials. The separately weighed fly ash and additives enter the mixer for complete mixing, and then enter the bucket elevator to be transported to the charging hopper of the electric arc melting furnace.

Electric arc melting system Includes graphite electrode furnace, slag cooling tank, controller and DC power supply. Features of the melting furnace: eccentric installation of the cathode; rotation speed 0.2-1 rpm. Eccentric installation of the cathode allows increasing the contact area between the arc and fly ash during furnace rotation.

The flue gas purification system includes an afterburner, a gas cooler, a bag filter, an absorption column, an alkali washing column, a carbon filter, an induced draft fan and a chimney. Flue gases containing heavy metals, salts and flammable gases from the electric arc melting furnace enter the afterburning chamber. The temperature level in the afterburning chamber is maintained in the range of 1050-1100°C. From the afterburning chamber, the flue gases enter a water-cooled gas cooler, where they are quickly cooled to 200°C. A bag filter catches solid particles from the flue gases, which are sent for re-combustion. In the absorption column, water-soluble gases, such as hydrogen chloride, are removed from the flue gases. After absorption, the flue gases pass alkali washing column, where it is purified from acidic impurities, including carbon dioxide, and a carbon filter. Flue gases processed by such a cleaning system fully comply with emission standards and are discharged into the atmosphere through an induced draft fan and chimney.

The known system of electric arc melting of fly ash from the combustion of MSW with a direct current electric furnace is a rather complex structure. The DC electric furnace used in the system has a number of disadvantages compared to the solution proposed in the application: low efficiency (about 98%), difficulty in scaling the melt productivity of the electric furnace, and high capital costs. The melt is mixed using a magnetic stirrer, which complicates the system and increases energy intensity.

Thus, the known systems do not allow one to effectively solve the problem of neutralizing ash and slag waste from MSZ; therefore, the claimed invention can be classified as a technical solution that does not have a closest analogue.

The goal is to create an effective system for melting ash and slag waste from a waste incineration plant, characterized by simplicity, energy efficiency, environmental safety, and non-waste.

The technical result of the claimed invention is a system that ensures the effective utilization of by-products - fly ash and slag formed during the combustion of MSW at the MSZ, with the production of environmentally friendly, chemically inert vitrified slag, suitable for use in the construction industry.

An effective, environmentally friendly system for melting MSZ ash and slag using a 3-phase AC melting furnace is proposed.

A 3-phase electric melting furnace has the following advantages over a DC electric furnace:

- high efficiency (100%),

- mixing of the melt with the molten charge due to the flow of current between the electrodes,

- ease of scaling the melt productivity of the electric furnace (5 - 10 - 15 t/h).

- low capital costs compared to a DC electric melting furnace (capital costs for creating a 3-phase AC furnace are 20% lower).

The proposed MSZ ash and slag melting system includes:

- charge granulation system,

- system for feeding the charge into the electric furnace,

- electric melting furnace,

- cooling and crystallization system for molten ash and slag,

- ash and slag melt granulation system,

- smoke purification system,

- automated control system.

According to the invention, a 3-phase alternating current electric furnace is used as a melting electric furnace.

According to the invention, the cooling and crystallization system for molten ash and slag includes a ladle, molds, and fans.

According to the invention, the ash and slag melt granulation system includes a crusher and sieves.

According to the invention, the volume of the ladle of the cooling and crystallization system of molten ash and slag is equal to the sum of the volumes of the molds, and the number of fans is equal to the number of molds.

According to the invention, the smoke purification system includes a water-cooled gas duct, a water-cooled gas cooler, a medium-temperature assembly gas duct, a bag filter, a carbon filter, and an exhaust fan.

According to the invention, for cooling the gas cooler of a smoke purification system water jets are used.

According to the invention, the nozzles of the smoke purification system are installed on the top and side of the gas cooler.

According to the invention, the number of nozzles of the smoke purification system and their exact location are determined depending on the performance of the electric furnace and the temperature of the flue gases.

According to the invention, the automated control system includes temperature sensors, air flow sensors, gas analyzer, controller and personal computer with appropriate software.

In fig. Figure 1 shows schematically the MSZ ash and slag melting system, where: 1 - charge granulation system, 2 - charge supply system to the furnace, 3 - three-phase alternating current furnace, 4 - ash and slag melt cooling and crystallization system, 5 - ash and slag melt granulation system, 6 - system smoke purification, 7 - automated control system (ACS).

In fig. Figure 2 shows schematically the system for granulating the charge and the system for feeding the charge into the electric furnace, where: 8 - bunker for storing bottom ash; 9 - bunker for storing fly ash; 10 - hopper for storing lime; 11 - mixer-granulator; 12 - bunkers for charge; 13 - pipe leaks; 14 - electric furnace.

The charge granulation system includes:

- bunker for storing bottom ash;

- bunker for storing fly ash;

- a bunker for storing lime;

- mixer-granulator.

The system for feeding the charge into the electric furnace includes:

- bunkers for charge;

- pipe leaks;

- dispensers (not shown in Fig. 2).

The components of the charge, namely bottom ash (BO) and fly ash (FA), are mixed in a ratio of 7: 3 or 6: 4. In addition, to obtain a vitrified melt of ash and slag, 2% by weight of the charge of slaked lime (calcium hydroxide) is added to the charge Ca(OH) ₂ ).

Taking into account the low bulk density of ash and slag (according to literature data from 1.2 to 1.5 t/m³) it is advisable to pelletize the charge until briquettes, pellets, granules. The advantage of compact material over bulk material:

- reduction of dust levels in production premises,

- reduction in the volume of semi-finished products.

Press materials reduce air content and increase thermal conductivity compared to bulk materials.

One of the most diverse and widely used processes in various industries is granulation. In general, granulation greatly simplifies storage, transportation and dosing. It is aimed at increasing flowability while simultaneously eliminating dust in the workshop and thereby improving working conditions in production and handling. Along with this, the technology is aimed at homogenizing the mixture, increasing heat and mass transfer, and adjusting the structure of the granules. All this contributes to the intensification of processes in which granules are used, increasing labor productivity and production standards.

The weighed mixture is granulated in a mixer-granulator. The turbulent nature of the movement of particles in the granulation (mixing) zone ensures the short duration of the process and the production of relatively homogeneous products even in cases where the quantity and bulk weight of the components differ significantly from each other (for example, slag, ash, lime).

After 15-20 minutes, the granular mixture is fed by a belt conveyor into the hoppers above the electric furnace, and then onto the dispenser into the pipes of the electric furnace.

A 3-phase alternating current electric furnace is used as a melting electric furnace [Electric industrial furnaces. Arc furnaces and special heating installations: Textbook for universities / A.D. Svenchansky and others - M.: Energoizdat, 1981. 296 p.]. The main elements of the electric furnace are the furnace casing, bath lining, three graphite electrodes, electrode holders, flexible current supply, short network, step-down transformer, automatic power regulator. The dimensions and shape of the casing correspond to the shape of the furnace bath. The inner surface of the lining forms the furnace bath and must have the necessary properties to retain metal and slag melts. It is also necessary to reduce heat losses in the bath and ensure the temperature on the furnace casing is acceptable under operating conditions. A round bath for three-electrode furnaces is the easiest to manufacture and operate.

Table 1 shows the differences between a DC electric furnace and a 3-phase AC electric furnace in terms of basic equipment. The technology for molten ash remains the same.

Table 1. Comparison of a DC electric furnace and a 3-phase alternating current electric furnace according to the main equipment. No. Name DC furnace Oven 3-phase AC 1. Melt capacity of electric furnace 5 t 5 t 2. Number of rod type electrodes 1 3 3. Availability of a bottom electrode Yes No 4. Rectifier Yes No 5. Short networks No Yes 6. Melt rotator Yes No 7. Isolation transformer Yes Yes

With the same productivity, capital costs for a 3-phase alternating current electric melting furnace are 20% lower than for a direct current electric furnace.

The process of melting the ash and slag charge begins with an arc discharge along conductive channels (paths) between the electrodes with the formation of electric arc plasma, from which, due to heat transfer, the surrounding charge melts. Then the discharge stops and the melting process continues with ohmic heating of the ash and slag until the melt is drained.

The cooling and crystallization system for molten ash and slag includes: a ladle; molds; fans.

It is well known that if a melt of any material, including ash and slag, is poured into water (into a stationary volume), then after the cessation of all physical and chemical reactions (hardening of the product), granules of various shapes are formed. Large volumes of melt require significant amounts of water and steam generation. It is possible to obtain predetermined granules if the melt of the material is slowly poured into a moving stream of water (stream). So, for a melt mass of 5 tons, many streams will be required to drain and cool the melt in a short time.

It is proposed to pour the entire volume of the melt from the electric furnace (except for the remaining residue - the “swamp”) into a metallurgical ladle, from which the melt is poured in small portions into molds for subsequent cooling and crystallization. The number of molds depends on the volume of molten ash and slag. For example, for 5 tons of molten ash and slag, 12-15 molds will be required. In order to speed up the cooling of the melt, the molds are additionally cooled by fans.

The molten ash and slag granulation system includes a crusher and sieves. The solidified melt is fed to a crusher for grinding into specified fractions. Using a sieve, the required fractions are selected.

Purification of exhaust (flue) gases from the electric furnace for melting ash and slag at the MSZ is one of the critical technological operations that have a significant impact on the environment and human health.

The efficiency of gas cleaning devices largely depends on the physical and chemical properties of the captured ash and the flue gases entering the ash collector.

Features of cleaning waste (flue) gases from the electric furnace for melting ash and slag at MSZ:

- high temperature of flue gases supplied for cleaning (over 1200°C);

- complex composition of gases being purified, including substances of hazard class 1;

- a wide range of fluctuations in the qualitative and quantitative composition of both flue gases and the harmful impurities contained in them;

- high chemical activity of some substances;

- different physical state of harmful impurities, etc.

To purify gases from an electric furnace for melting ash and slag MSZ in terms of operational reliability, ease of operation, increasing the service life of gas cleaning elements and versatility, which consist in a fairly high degree of capture of all without excluding toxic substances present, a “wet” cleaning system is most suitable. Its advantages also include the possibility of simultaneous purification of gases from liquid, solid and gaseous impurities.

Known “wet” cleaning system gases from a plasma-thermal furnace for molten ash and slag [Ariace K., Koga A., Matsuoka Y. et al. Plasma siagging system for incineration of ash // FAPJG, 1995, N 144, p. 120], including an electric furnace, a water-cooled gas duct, a primary gas cooler, a gas duct, a 2nd stage gas cooler, a cartridge filter, a low-temperature gas cooler, a carbon filter, and an exhaust fan. This exhaust gas purification system is complex and requires large volumes of absorbent, and therefore sedimentation tanks for contaminated liquid and sludge after flue gas purification. This is an expensive technical solution.

Since there are no dioxins and carcinogenic substances in the exhaust dust and gas flow of the electric furnace for melting ash and slag at the MSZ (at T ≥ 1200°C), it can be significantly modernized indicated in [Ariace K., Koga A., Matsuoka Y. et al. Plasma siagging system for incineration of ash // FAPJG, 1995, N 144, p. 120] cleaning system gases

In fig. Figure 3 shows a diagram of the proposed smoke purification system, where: 14 - electric furnace; 15 - water-cooled flue; 16 - water-cooled gas cooler (mounted near the furnace); 17 - medium-temperature gas duct layout; 18 - bag filter; 19 - carbon filter; 20 - exhaust fan. In addition, the smoke cleaning system includes nozzles for cooling the gas cooler 16 and may include cyclones for collecting fine particles (not shown in Fig. 3).

The dust and gas flow leaving the electric furnace has a temperature T ≥ 1200°C. Sharp cooling of the exhaust dust and gas flow to a temperature of no more than 200°C (“hardening”) eliminates the possibility of secondary appearance of harmful substances. Sharp cooling of the exhaust dust and gas flow is achieved when it passes through the pre-cooled gas duct 15 and gas cooler 16. In the technological operating mode of the electric furnace, cooling water begins to be supplied to the gas duct and gas cooler until supplying flue gases to the gas cleaning system, that is, the dust and gas flow with a temperature of ~ 1200°C from the furnace enters the already cooled flue and gas cooler, through which it is cooled to a temperature of 200°C and below.

The gas cooler 16 is cooled by water nozzles installed on the top and sides of the gas cooler. The number of nozzles, their exact location and the flow rate of supplied water are determined depending on the productivity of the electric furnace and the temperature of the flue gases.

Cooling the exhaust dust and gas flow at the outlet of the gas cooler to a temperature of no more than 200°C allows you to install a bag filter behind the gas duct to clean the gases from solid particles (maximum concentration - no more than 1 m ³ ). After the bag filter, the flue gases pass through a carbon filter, which absorbs residual volatile heavy metals. The purified gases are released into the atmosphere through a high-pressure fan.

Thus, from the standard scheme of “wet” exhaust gas purification [Ariace K., Koga A., Matsuoka Y. et al. Plasma siagging system for incineration of ash // FAPJG, 1995, N 144, p. 120] exclude the 2nd stage gas cooler, cartridge filter, low-temperature gas cooler.

The automated control system (ACS) includes temperature and air flow sensors; gas analyzer; controller; personal computer (PC) with appropriate software (software).

Automated process control is carried out by periodically taking readings at fixed control points, transmitting readings to a computer equipped with appropriate software, comparing measurement and calculation results with the values of reference characteristics of the process, and transmitting control signals.

ACS provides:

- maintaining a given temperature of exhaust gases by regulating the power of gas coolers by changing the intensity of water cooling of tube bundles;

- activation of the bag filter system;

- maintaining a given vacuum in the path by changing the fan rotation speed;

- control and diagnostic functions.

The effectiveness of the proposed system was confirmed by experimental melting of MSZ ash at the Novokuznetsk Metallurgical Plant.

Claims (6)

1. A system for melting ash and slag waste of a waste incineration plant, including a charge granulation system, a charge supply system to an electric furnace, an electric melting furnace, a cooling and crystallization system for the ash and slag melt, a granulation system for the ash and slag melt, a smoke purification system, a control system, characterized in that a 3-phase alternating current electric furnace is used as a melting electric furnace, the cooling and crystallization system of the ash and slag melt includes a ladle, molds, fans, the ash and slag melt granulation system includes a crusher, sieves, the smoke purification system includes a water-cooled gas duct, a water-cooled gas cooler, a medium-temperature assembly gas duct, a bag filter, carbon filter, exhaust fan, automated control system includes temperature sensors, air flow sensors, gas analyzer, controller, personal computer with appropriate software. 2. The system according to claim 1, characterized in that volume of the ladle of the cooling and crystallization system of molten ash and slag equal to the sum of the volumes of the molds. 3. The system according to claim 1, characterized in that the number of fans for the cooling and crystallization system of molten ash and slag is equal to the number of molds. 4. The system according to claim 1, characterized in that for pre-cooling of the gas cooler of the smoke purification system water jets are used. 5. The system according to claim 4, characterized in that The nozzles are installed on top and on the side of the gas cooler of the smoke purification system. 6. System according to claim 4, characterized in that The number of nozzles of the smoke purification system and their exact location are determined depending on the performance of the electric furnace and the temperature of the flue gases.