EP2273194A2 - Combustion assembly with heat insulation layer on wet slag remover - Google Patents

Abstract

The combustor has wet slag for discharge of combustion residue, and a tray serving as water quench (7) comprising a water surface. The tray is provided for receiving the combustion residue comprising a thermal insulation layer (13) floatable on the water surface. The thermal insulation layer comprises multiple floating bodies (12) moving against each other. The floating bodies are rotated around a rotation axis. An independent claim is also included for a method for the removal of combustion residues.

Description

The present invention relates to a combustion plant comprising a wet slagger with a flexible thermal barrier coating. Furthermore, the invention relates to a method for resource-conserving operation of a combustion plant with wet slagger, in particular with regard to the heat emission from the combustion chamber in the purification bath.

Many incinerators, such as Rotary kiln or grate firing usually consist of a two-stage combustion. In a first stage, especially solids are burned, while in a second stage usually the post-combustion takes place in the gas phase. The substances used in this process are not only disposed of in an environmentally friendly manner, provided that they are residues or wastes, but above all that they are also used for energy purposes, ie. The hot flue gases produced during combustion are used in a waste heat boiler to generate process steam, which can then be fed into the district heating network or converted into electrical energy (electricity).

In order to operate such a combustion system as efficiently as possible and thus to achieve a high energy efficiency, heat losses, especially surface losses due to heat conduction, convection and radiation, must be kept low before entering the waste heat boiler. In the combustion chamber of a combustion plant, the heat loss is reduced by various insulation layers in the refractory lining. The lower the heat loss in the firing area, the higher the energy yield in the form of process steam. The amount of process steam is a major source of revenue for operators of an incinerator.

The wet slagger, which is usually located between the first and second combustion stage of such an incinerator, is a hitherto neglected source of heat loss. About the wet slagger the burned inert residues from the solid combustion (first combustion stage) dry (ash) or molten (slag) are discharged.

For example, in rotary kiln furnaces this discharge is usually molten. In this case, the molten slag falls or drips from the rotary kiln via a chute in a water bath, where the slag is quenched when entering the water bath abruptly. From the water bath of the wet slag the cooled, solidified slag is discharged via a conveyor system as a solid, glassy residue in a collection container and then fed to further treatment processes.

The wet slagger not only offers the possibility of discharging inert solids from the combustion chamber, but at the same time forms the air seal against false air entry from the outside into the combustion chamber. This air seal ensures operation of the incinerator at reduced pressure.

During the combustion process, complex physical and chemical processes take place. In this case, electromagnetic waves in the form of light are emitted by the intermediate and end products of the combustion (eg CO ₂ , CO, hydrocarbons, H ₂ O, soot, ash, etc.) due to their high temperature and energy state. The spectrum of electromagnetic waves ranges from short-wave UV to long-wave IR. If these electromagnetic waves hit the surface of bodies (eg particles, firebox walls, wet slag water), absorption and reflection preloads take place on the surface. Will the radiation from the body absorbs, then increases according to the Kirchhoff radiation law its temperature, which in turn leads to increased emission of temperature / heat radiation.

The emissivity s of a body describes the relationship between the radiation absorbed by the body and the radiation impinging on it. The lower the emissivity ε, the lower the absorption or the higher the reflection of the impinging radiation. If the emissivity ε = 1, then there is an ideal black body which completely absorbs any radiation impinging upon it. The radiation absorbed by the body is converted into heat and then released again in the form of thermal / thermal radiation in all directions of the environment.

Hot combustion chamber walls (ε = approx. 0.8) absorb a large part of this radiation, but also reflect a not inconsiderable proportion back into the interior of the combustion chamber. However, if electromagnetic radiation reaches the dark water surface of a wet slagger (ε = approx. 0.96-0.98), almost all of the incident radiation is absorbed. The water temperature of the wet slagger begins to rise and evaporation on the water surface is favored. The low radiation reflection at the water surface and the relatively cold water vapor escaping from the wet slagger and interfering with the hot combustion gas of the plant, lead to an undesirable lowering of the flue gas temperature, especially at the transition from the rotary kiln to the afterburner. Another disadvantage in this context is the increased consumption of process water.

Especially with rotary kilns with small diameter-length ratios, where due to large surfaces (in relation to the volume) already higher heat losses, or in operating modes with large load fluctuations, such Lowering the temperature at the transition from the rotary kiln into the afterburning chamber lead to rapid and undesired cooling of the molten slag near the rotary kiln discharge. The molten slag begins to solidify on the rotary kiln discharge. A continuous slag discharge can be made more difficult if so-called slag gases are produced by the cooling of the slag at the rotary kiln discharge, which slowly grow out of the rotary kiln. If these slag gases are too big, they will break down due to their gravity and fall into the wet slagger as hot chunks. Break larger slag dry uncontrolled, so the wet slagger and other system components, as a result of violent pressure surges, can be damaged. In extreme cases, these damages can even necessitate an immediate shutdown of the entire incinerator and thereby cause high repair costs.

If the slag solidifies too quickly on the rotary kiln discharge due to a very large temperature gradient, solidification already occurs inside the rotary kiln. By damming up slag near the rotary kiln discharge, the rotary kiln grows slowly. The clear diameter at the rotary kiln discharge shrinks until controlled plant operation is no longer possible. Then the entire system must be driven off immediately and the slag is then mechanically degraded. The problem of slag discharge from the rotary kiln, on the other hand, are applied by the plant operators different methods.

One possibility for facilitating the slag discharge is the use of so-called slag scrapers. With the help of these permanently installed slag scraper, the formation of larger slag gases is avoided, as the outgrowing slag from the combustion chamber scrapes on the slag scrapers and down into the wet slagger falls. Thus, through the use of slag scrapers an additional burden of the wet slagger and the entire combustion system can be avoided. The mechanical and thermal stress of these scrapers is considerable. Instead of slag scrapers, additional scorch burners can be installed near the slag discharge. Due to the permanent or even short-term use of such burners, the temperature (in particular the slag temperature) can be significantly increased at the transition from the rotary kiln into the afterburner. The slag discharge is facilitated because higher temperatures lead to a much more liquid slag with low viscosity, which is cooled more slowly and thus can be discharged more easily from the rotary kiln. The formation of larger slag gases or even the growth of the rotary kiln with slag can be avoided by means of Abschmelzbrennern. Disadvantage here is the design-related effort and the additional consumption of fuel, which increases the operating costs.

The main problem zone for the loss of radiant heat is the direct contact of the water surface of the wet slagger with the combustion chamber. From the prior art, no solutions for reducing the heat losses in the wet slagger are known.

Based on this, it is the object of the invention to provide a wet slag incinerator and a method for discharging combustion residues, which reduce the disadvantages of the prior art.

In particular, this should be used to reduce the heat losses at the wet slag remover of a combustion plant in order to increase plant efficiency. In addition, when using rotary kiln plants by the thermal optimization of the wet slag to improve the slag discharge become. Another object of the invention is to reduce the evaporation of water at the wet slagger. At the same time, however, the entry of combustion residues in the form of solid or liquid slag or ash from the combustion chamber into the water bath of the wet slagger should not be adversely affected.

Furthermore, it is an object of the invention to provide a method with which a combustion plant with wet slag resources can be operated gently as known in the art.

The object is achieved by a combustion plant with wet slag according to the features of claim 1 and a method for discharging combustion residues according to claim 14. Advantageous embodiments are given in the subclaims.

A solution to prevent the loss of radiant heat from the combustion chamber of a combustion plant in the wet slagger, is a cover of the water surface of the wet slagper with a flexible thermal barrier coating. This thermal barrier coating comprises a plurality of floats, which separate the water surface toward the combustion chamber, so that the radiant heat predominantly hits the floats and not the water surface.

The floats are movable against each other. Movable in this context means that the floats can move horizontally on the water surface to create a gap to allow falling combustion residues to pass. Furthermore, the floats can move vertically, which in particular allows a displacement of individual floats between multiple layers.

In a preferred embodiment, the floats have at least one rotational degree of freedom. Rotatory degrees of freedom are movements about one of the three axes of rotation of the float, whereby the center of gravity of the body is not displaced. If combustion residues from the combustion chamber fall on the floats with a rotational degree of freedom, a short-term deflection of the center of gravity takes place, whereupon the floats react with a rotational movement that moves the combustion residues in the direction of the water bath. The rotational movements are not limited in this context to full rotations, tilting movements in which the body after rotation back into the starting position, are included. Consequently, in a particularly preferred embodiment, at least one axis of rotation of the floating body is not parallel to the axis of the gravitational field. The axis of rotation is preferably at an angle to the water surface, which is aligned between 0 ° and 89 °, particularly preferably between 0 ° and 45 °.

Through these features, the floats work as a flexible barrier, so that the combustion residues from the combustion chamber can pass through the consisting of floats thermal barrier layer into the water bath. The floats organize themselves due to their buoyancy, their weight and the water movement in the ingress of slag content to a largely closed layer.

In a preferred embodiment, the floats are made of a material having an emissivity ε which is smaller than that of the water, ie between 0 and 0.96, more preferably between 0.01 and 0.2 (values for polished metal surfaces or metallised surfaces). Thus, it can be ensured that a considerable part of the heat radiation is reflected back into the combustion chamber.

In a further preferred embodiment, the floats should be made of materials that ideally enable a maintenance-free continuous operation. Accordingly, temperature-resistant, preferably refractory materials are required because there are high temperatures in the combustion chamber. Depending on the system concept, fuel and height of the chute, temperatures of approx. 150 ° C - 200 ° C are to be expected above the water surface of a conventional wet slagger without cover. In addition, the falling slag hits the floats even hotter. Accordingly, the buoyant surface requires temperature resistant or refractory materials which have heat resistance at temperatures of at least 200 ° C.

Another aspect is the mechanical resistance of the floats, as the falling combustion residues could damage the floats. Preferred materials in this context are metallic materials, in particular stainless steels, since they also have a high degree of corrosion resistance in addition to the mechanical dimensional stability. Furthermore, metal surfaces have a low emissivity, e.g. polished iron has an emissivity ε between 0.04 and 0.19. Preferably, steel alloys with chromium, nickel, molybdenum, titanium or vanadium are also suitable.

Another preferred material for the floats are ceramic materials. Ceramics are also characterized by a high dimensional stability and mechanical strength. In particular, technical ceramics or engineering ceramics are used. In this connection it is possible to use so-called non-oxide ceramics (for example nitrides, carbides or borides) which are characterized by a mostly gray to dark gray coloration, preferably but the white to yellowish colored and therefore a preferably lower emissivity having oxide ceramics (for example, alumina, titania, zirconia) are used.

As further preferred materials for the floating body temperature-resistant plastics can be used. Polyfluorinated plastic materials such as polytetrafluoroethylene (Teflon®) or polyfluorinated rubber (Viton®) are particularly preferably used. Temperature resistance in this context means a heat resistance at temperatures of at least 200 ° C. According to the manufacturer of Viton®, the heat resistance is 200 ° C and that of Teflon® is 260 ° C

Due to their high specific gravity, metals, ceramics or plastics are usually not buoyant and should preferably be made as a hollow body. Alternatively, the floats may be made of porous material, preferably with the pores closed.

Floating bodies whose surface has a reflective coating which gives the body a particularly low emissivity are particularly preferred. Likewise, an open porosity can be closed by a coating. Preferably, the surface is additionally smoothed or polished.

In a further particularly preferred embodiment, the floats are spherical.

In a particular aspect, the invention relates to the use of a thermal barrier coating for wet slagters in incinerators, comprising a plurality of floats movable relative to one another and preferably rotatable about at least one axis of rotation.

The thermal barrier coating according to the invention is due to their construction with a variety of floats flexibly usable in different incinerators with wet slagger. Existing incinerators can also be retrofitted to the wet slag remover without additional structural measures.

When using the thermal barrier coating according to the invention in incinerators, the operating temperature in the combustion chamber is increased and the heat loss at the wet slagger is lowered. This therefore makes additional energy input unnecessary to compensate for heat losses and / or to liquefy slag constituents. In particular, in incinerators with rotary furnaces the discharge of slag from the combustion system is simplified because the slag does not solidify at the combustion chamber outlet.

Optionally, several layers of floats can be used to maximize coverage of the water surface. For this purpose, floats of different sizes may be used.

Another advantage of the construction of the incineration plant according to the invention is the greatly reduced evaporation of the water in the wet slag remover. In normal operation of a conventional incinerator without thermal barrier coating, the water bath is heated to about 30 ° C to 80 ° C, which represents a significant loss of heat. In addition, there is considerable evaporation at this temperature. The radiant heat impinging on the water surface accelerates the evaporation process. The evaporation of water is an endothermic process; the necessary enthalpy of vaporization is lost to the system and is another source of energy loss in incinerators. Reduce the floats of the insulation layer the contact surface between water bath and gas space (combustion chamber). Thus, in addition, the evaporation of water from the wet slag into the combustion chamber is reduced. A reduced consumption of process water is a further advantage of the invention.

According to the invention, the combustion residues from incinerators with wet slagger are discharged according to the following method: First, a combustion plant is provided with a bath serving as a water bath for receiving combustion residues (wet slagger) comprising a buoyant thermal barrier coating comprising a plurality of mutually movable floats. Subsequently, in the combustion space, the solid fuels such as e.g. Production residues from industry, domestic waste, refuse derived fuels, coal or biomass burned. This can be done both by a grate or a Drehrohrfeuerung but also in coal combustion boilers. The resulting combustion residues (slags, ashes) are discharged in the following process step at the end of the rotary tube or in the lower part of the coal combustion boiler through a chute in the water of the wet slagger, the combustion residues before entering the water bath, the thermal barrier coating penetrate.

Since this water bath is covered according to the invention with a thermal barrier coating of floats, the residues initially fall on the floats, but do not represent a barrier due to their degrees of freedom of movement, but let pass the residues in the water. Here, the floats can move either horizontally or vertically to form a gap.

Preferably, the floats have at least one axis of rotation about which they can rotate. The rotation movement occurs when discharging the combustion residues by the fact that the focus of the floating body is changed by the impinging solids such that in the gravitational field, a rotation or tilting movement is the result, which conveys the combustion residues in the water. This is especially true for spherical floats.

After the combustion residues have passed the thermal insulation layer, the floats spontaneously organize themselves into a closed layer. If individual floats are damaged or made unusable during prolonged operation of the thermal barrier coating, or if floats are lost during removal of the combustion residues from the wet slag remover, new floats can simply be applied to the water surface of the wet sludge slagger.

• Fig. 1 zeigt eine Verbrennungsanlage mit Drehrohrofen und Nassentschlacker aus dem Stand der Technik.
• Fig. 2 zeigt eine schematische Darstellung des Nassentschlacker-Teststandes im Technikumsmaßstab.
• Fig. 3 zeigt einen grafischen Verlauf der Temperatur im Nassentschlacker-Teststand gem. Fig. 2 als Funktion der Höhe über der Wasseroberfläche.

The invention will be explained below with exemplary embodiments and the following figures.

• Fig. 1 shows an incinerator with rotary kiln and wet slag from the prior art.
• Fig. 2 shows a schematic representation of the Nassentschlacker test stand on a pilot plant scale.
• Fig. 3 shows a graph of the temperature in the wet slag test rig gem. Fig. 2 as a function of height above the water surface.

In Fig. 1 By way of example, the structure of a conventional incinerator with first combustion stage 1 and second combustion stage 2 is shown in cross-section. About a chute 3 solid containers are placed in the combustion chamber of the first combustion stage 1 , where they are burned. The slags 4 fall through a chute at the end of combustion 5 in the water bath 7 of the wet slagger 6. The escaping from the first combustion stage 1 hot flue gases enter the gas chamber 8 of the second combustion stage 2. In the second combustion stage 2 (afterburner), the gas phase burn out of partially insufficiently burned flue gases using Nachbrennkammerbrennern. Consequently prevails in this gas chamber 8 is a significant heat radiation, which radiates to the water bath 7 of the wet slagger 6 . The incident on the water bath 7 radiation is largely absorbed.

The in Fig. 2 The pilot scale wet slag test rig has been developed to simulate the basic operations of a wet slagger 6 of an incinerator. This test rig essentially consists of the individual components radiation source 9, water bath 7 and gas space 8 with external insulation 11. The radiation source 9 consisted of 4 × 100 W light emitters, the external insulation 11 of mineral fiber mats / insulating material (about 8 cm thickness). Both in the gas space 8 between radiation source 9 and water bath 7 and in the water, a comprehensive data acquisition system was installed, which includes a plurality of thermocouples 10 and a water level indicator 14 .

By way of example, temperature measurements and water level measurements were carried out on this test stand, which realistically reproduce the temperature distribution at the wet slagger 6 of a combustion plant. The temperature distribution 17-20 was measured as a function of the height above the water surface 16 of the water bath 7 (see Fig. 3 ), wherein the water bath 7 was covered on the one hand without float 12 and on the other hand with glass hollow bodies as floating body 12 of different emissivity.

Changes in the temperature distribution in the water bath 7 and in the gas space 8 as well as the evaporation amount were recorded by a data acquisition system. With the help of a balance the test results could be compared with each other and checked for plausibility.

In Fig. 3 the measured temperature curves 17-20 in the gas space 8 of the test stand are off Fig. 2 above the water surface with and without the use of floats 12 shown. The investigations carried out showed that in comparison to the uncovered water surface alone by the use of floats 12 a significant increase in the average gas temperature 15 above the water surface can be achieved. By using hollow glass spheres with a diameter of 50 mm without coating (emissivity ε = ca, 0.94, temperature profile 18) , the average gas temperature 15 could already be increased by about 15-20%. At the same time, the evaporation / evaporation amount dropped by about 15%.

If the influence of the emissivity is taken into account, the result achieved can be significantly improved. Fig. 3 shows the temperature curves 17-20 in the gas space 8 above the water surface as a function of the emissivity of the float surface (glass bubbles with 50 mm diameter). For this purpose, hollow glass bodies were used, which were treated untreated (emissivity ε = 0.94, temperature curve 18) or their surfaces, eg matte silver painted (emissivity ε = 0.45, temperature curve 19) or metallized (emissivity ε = 0.03, Temperature curve 20) to produce different emissivities with the same feedstock. It can be seen that with decreasing emissivity, the mean gas temperature 15 increases above the water surface. The average gas temperature 15 could be compared with the metal vapor - deposited hollow glass spheres (ε = approx The test stand without thermal barrier coating 13 can be raised by approx. 30-40, while at the same time the evaporation / evaporation amount decreased by up to 35%.

In the case of a large-scale industrial plant with furnace temperatures of 850 - 1200 ° C, it is to be expected that significantly lower temperature increases of approx. 10% (corresponding to a temperature increase of approx. 100 ° C) would be sufficient to considerably facilitate the slag discharge from the rotary kiln. This was the result of the tests on the wet slag tester Fig. 2 a considerable potential for increasing the gas temperatures 15 shown in order to facilitate the slag discharge and to increase the efficiency of the system.

LIST OF REFERENCE NUMBERS

1

First combustion stage

2

Second combustion stage

3

chute

4

slag

5

chute

6

Wet slag

7

water bath

8th

headspace

9

radiation source

10

thermocouples

11

external insulation

12

float

13

thermal barrier

14

Water level indicator

15

Gas temperature [° C]

16

Height above the water surface [mm]

17

Temperature course in the gas space without float

18

Temperature profile in the gas space with floats of an e- missivity ε = 0.94

19

Temperature profile in the gas space with floating body of an ε = 0.45

20

Temperature profile in the gas space with floating body of an ε = 0.03

Claims (16)

Combustion plant with wet slagger (6) for discharging the incineration residues with a tank for receiving the combustion residues serving as a water bath (7) with a water surface comprising a heat-insulating layer (13) which floats on the water surface and comprises a plurality of mutually movable floats (12). Incinerator according to claim 1, wherein the floats (12) are rotatable about at least one axis of rotation. Incinerator according to one of the preceding claims, wherein the floats (12) have a thermal emissivity ε between 0 and 0.96. Incinerator according to one of the preceding claims, wherein the floats (12) consist of refractory material. Incinerator according to one of the preceding claims, wherein the floats (12) are hollow bodies. Incinerator according to one of claims 1 to 5, wherein the floats (12) are made of porous material. Incinerator according to one of claims 1 to 6, wherein the floats (12) are made of metal. The incinerator of claim 7, wherein the floats (12) are stainless steel. The incinerator of any one of claims 1 to 6, wherein the floats (12) are of ceramic. Incinerator according to one of claims 1 to 6, wherein the floats (12) are made of temperature-resistant plastic. The incinerator of any one of claims 1 to 6, wherein the floats (12) are of glass. Combustion plant according to one of the preceding claims, wherein the floating bodies (12) have a reflective coating on the outside. Incinerator according to one of the preceding claims, wherein the floats (12) are spherical. Method for discharging combustion residues from incineration plants into a wet slagger (6), comprising the following method steps: a) providing an incinerator with a water bath (7) with a water surface serving as a receptacle for incineration residues with a floating on the water surface heat-insulating layer (13 ) comprising a plurality of mutually movable floats (12) , b) burning the solids, c) discharging the combustion residues into the wet slagger (6),
wherein the combustion residues before entering the water bath (7 ) penetrate the thermal barrier coating (13) . The method of claim 14, wherein the floats (12) are rotatable about at least one axis of rotation. Method according to one of claims 14 or 15, wherein the floats (12) after penetration through the Combustion residues form again into a closed thermal barrier coating (13) .

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