Patent Application
20110030591
The present invention relates to an incinerator which comprises a wet slag remover having a flexible heat insulation layer. The invention further relates to a method for resource-saving operation of an incinerator having a wet slag remover, in particular as regards the heat discharge from the combustion chamber into the deslagging bath.
In general, many incinerators, such as cylindrical rotary furnaces or grate furnaces, consist of a two-stage combustion. In a first stage, predominantly solids are burnt, whilst afterburning in the gaseous phase generally takes place in a second stage. The substances used in this context are not only disposed of in an environmentally friendly manner, where residues or waste materials are involved, but are also predominantly used for energy production, i.e. the hot flue gases resulting from the combustion are used in a heat recovery boiler to produce process steam, which can subsequently be fed into the district heating network or converted into electrical energy (current).
To operate an incinerator of this type as efficiently as possible and thus achieve high energy efficiency, heat losses, above all surface losses due to heat conduction, convection, and radiation, must be kept low before entry into the heat recovery boiler. In the furnace of an incinerator, heat loss is reduced by various insulating layers in the refractory lining. The lower the heat loss in the firing region, the greater the subsequent energy yield in the form of process steam. The amount of process steam is a substantial source of revenue for incinerator operators.
However, the wet slag remover, which is conventionally located between the first and second combustion stages of an incinerator of this type, is a source of heat loss which has been given little consideration until now. The inert, burnt-out residues from the solid combustion (first combustion stage) are discharged via the wet slag remover in a dry (ash) or molten (slag) form.
In cylindrical rotary furnaces, for example, this discharge is generally in a molten form. The molten slag thus falls or drops from the cylindrical rotary kiln into a water bath via a drop chute, the slag being quenched abruptly upon entering the water bath. The cooled, hardened slag is removed from the water bath of the wet slag remover into a collecting vessel via a conveyor system as a solid, vitreous residue and is subsequently supplied to further treatment processes.
The wet slag remover not only offers the possibility of transferring inert solids out of the furnace, but at the same time also forms the air seal preventing secondary air from entering the furnace from outside. This air seal makes it possible to operate the incinerator at a reduced pressure.
Complex physical and chemical processes take place during combustion. Because of the high temperature and energy state thereof, the intermediate and end products of the combustion (such as CO2, CO, hydrocarbons, H2O, soot, ash, etc.) emit electromagnetic waves in the form of light. The spectrum of the electromagnetic waves ranges from the short-wave UV to the long-wave IR range. If these electromagnetic waves impinge on the surface of bodies (such as particles, furnace walls, wet slag remover water), absorption and reflection processes take place on the surface. If the radiation is absorbed by the body, the temperature thereof increases in accordance with Kirchhoff's law of thermal radiation, and this in turn leads to an increased emission of thermal/heat radiation.
The emissivity ε of a body describes the ratio of the radiation absorbed by the body to the radiation incident thereon. The lower the emissivity ε, the lower the absorption and the greater the reflection of the incident radiation. If the emissivity ε=1, this is an ideal black body which completely absorbs any radiation incident thereon. The radiation absorbed by the body is converted into heat and subsequently emitted back into the environment evenly in all directions in the form of heat/thermal radiation.
Hot furnace walls (ε=approx. 0.8) absorb the majority of this radiation, but also reflect a not inconsiderable proportion back into the interior of the furnace. However, if electromagnetic radiation reaches the dark water surface of a wet slag remover (ε=approx. 0.96-0.98), almost all of the incident radiation is absorbed. The water temperature of the wet slag remover begins to rise and evaporation is promoted at the surface of the water. The low radiation reflection at the water surface and the relatively cold water vapour which escapes from the wet slag remover and mixes into the hot combustion gas in the system lead to an undesired reduction in the flue gas temperature, in particular at the transition from the cylindrical rotary kiln into the afterburning chamber. A further disadvantage in this connection is the increased consumption of process water.
In particular in cylindrical rotary systems having small diameter-length ratios, in which there are already relatively high heat losses due to large surface areas (in relation to volume), or in operating modes having large load fluctuations, a temperature reduction of this type at the transition from the cylindrical rotary kiln to the afterburning chamber can lead to rapid and undesired cooling of the molten slag near the kiln discharge. The molten slag will already start to solidify at the kiln discharge. If cooling of the slag at the kiln discharge results in what are known as slag runs, which slowly grow out of the cylindrical rotary kiln, this can make continuous slag discharge difficult. If these slag runs become too large, they break off under the weight thereof and fall into the wet slag remover in hot lumps. If relatively large lumps of slag break off in an uncontrolled manner, the wet slag remover and other system components may be damaged as a result of heavy impacts. In extreme cases this damage may even make immediate shutdown of the whole incinerator necessary and thus lead to high repair costs.
If the slag solidifies too rapidly at the kiln discharge because of an excessively high temperature gradient, there will already be solidification in the inside of the kiln. Build-up of slag near the kiln discharge leads to gradual accretion on the cylindrical rotary kiln. The low diameter at the kiln discharge shrinks until controlled system operation is no longer possible. In this case too, the entire system must be shut down at once and the slag must subsequently be broken down mechanically. However, the system operator may use various methods to address the problem of the slag discharge from the cylindrical rotary kiln.
One possibility for facilitating slag discharge is to use what are known as slag strippers. These permanently installed slag strippers prevent the formation of larger slag runs, since the slag growing out of the combustion chamber is stripped off at the slag strippers and falls downwards into the wet slag remover. Thus, using slag strippers can prevent excess loading of the wet slag remover and of the entire incinerator. There is considerable mechanical and thermal stress on these strippers. Instead of slag strippers, additional tipping torches may also be installed near the slag discharge. Permanent or even just brief use of torches of this type may raise the temperature (in particular the slag temperature) considerably at the transition from the cylindrical rotary kiln to the afterburning chamber. Slag discharge is facilitated, since higher temperatures lead to a substantially more fluid slag having a lower viscosity, which cools more slowly and can therefore be removed more easily from the cylindrical rotary kiln. Tipping torches can prevent the formation of relatively large slag runs or even accretion of slag on the cylindrical rotary kiln. Disadvantages of this are the expense of construction and the increased fuel consumption, which increases the operating costs.
The main problem region for the loss of radiant heat is the direct contact between the water surface of the wet slag remover and the combustion chamber. No solutions for reducing the heat losses at the wet slag remover are known from the prior art.
On this basis, the object of the invention is to provide an incinerator having a wet slag remover and a method for discharging combustion residues which mitigate the disadvantages of the state of the art.
In particular, this is intended to reduce the heat losses at the wet slag remover of an incinerator, so as to increase the system efficiency. Moreover, the slag discharge is to be improved when using cylindrical rotary kilns by thermal optimisation at the wet slag remover. A further aim of the invention is to reduce the evaporation of water at the wet slag remover. At the same time, however, the entry of combustion residues from the combustion chamber into the water bath of the wet slag remover, in the form of solid or liquid slag or ash, should not be impaired.
It is further an object of the invention to propose a method with which an incinerator having a wet slag remover can be operated in a resource-saving manner by comparison with the prior art.
The object is achieved by an incinerator having a wet slag remover in accordance with the features of claim 1 and a method for discharging combustion residues according to claim 14. Advantageous configurations are specified in the subclaims.
A solution for inhibiting the loss of radiant heat from the combustion chamber of an incinerator is to cover the water surface of the wet slag remover with a flexible heat insulation layer. This heat insulation layer comprises a plurality of floating bodies which separate the water surface from the combustion chamber, in such a way that the radiant heat predominantly impinges on the floating bodies and not on the water surface.
The floating bodies are movable relative to one another. In this context, movable means that the floating bodies can move horizontally on the water surface, forming a gap, so as to let falling combustion residues pass. Furthermore, the floating bodies can move vertically, and this in particular makes displacement of individual floating bodies possible between a plurality of layers.
In a preferred embodiment, the floating bodies have at least one rotational degree of freedom. Rotational degrees of freedom are movements about one of the three axes of rotation of the floating body in which the centre of gravity of the body is not displaced. If combustion residues fall from the combustion chamber onto the floating bodies having a rotational degree of freedom, there is a momentary deflection of the centre of gravity, to which the floating bodies react with a rotational movement which moves the combustion residues onwards towards the water bath. The rotational movements in this context are not restricted to complete rotation, but also include tilting movements, in which the body rotates back into the starting position after the rotational movement. Consequently, in a particularly preferred embodiment, at least one axis of rotation of the floating bodies is not parallel to the axis of the gravitational field. The axis of rotation is preferably at an angle of between 0° and 89°, more preferably between 0° and 45°, to the water surface
These features cause the floating bodies to function as a flexible barrier in such a way that the combustion residues from the combustion chamber can pass through the heat insulation layer consisting of floating bodies into the water bath. The floating bodies automatically organise themselves into a generally closed layer because of the buoyancy thereof, the weight thereof and the water movement when slag portions penetrate.
In a preferred embodiment, the floating bodies are manufactured from a material having an emissivity ε which is less than that of the water, i.e. between 0 and 0.96, particularly preferably between 0.01 and 0.2 (values for polished metal surfaces or metallised surfaces). This makes it possible to provide that a considerable proportion of the heat radiation is reflected back into the combustion chamber.
In a further preferred embodiment, the floating bodies are manufactured from materials which in the ideal case make maintenance-free long-term operation possible. Accordingly, temperature-resistant, preferably refractory materials are required, since high temperatures prevail in the combustion chamber. Depending on the system design, the fuel and the height of the drop chute, temperatures of approximately 150° C.-200° C. are to be expected above the water surface of a conventional wet slag remover without a cover. In addition, the falling slag is even hotter when it strikes the floating bodies. Accordingly, temperature-resistant or refractory materials exhibiting heat resistance at temperatures of at least 200° C. are required for the surface of the floating bodies.
A further aspect is the mechanical stress resistance of the floating bodies, since the falling combustion residues might damage the floating bodies. Preferred materials in this context are metal materials, in particular high-grade steels, since these also have a high resistance to corrosion, as well as mechanical dimensional stability. Further, metal surfaces have a low emissivity; for example, polished iron has an emissivity ε of between 0.04 and 0.19. Steel alloys comprising chromium, nickel, molybdenum, titanium or vanadium may preferably be used.
Ceramic materials are a further preferred material for the floating bodies. Ceramic materials are also distinguished by high dimensional stability and mechanical stress resistance. Advanced ceramic materials or engineering ceramic materials are used in particular. In this context what are known as non-oxide ceramic materials (for example nitrides, carbides or borides) may be used, and these are distinguished by a largely grey to dark grey colouring; preferably, however, oxide ceramic materials (for example aluminium oxide, titanium dioxide, zirconium dioxide), which are white to yellow in colour and therefore have a preferred lower emissivity, may be used.
Temperature-resistant plastics materials may be used as further preferred materials for the floating bodies. Polyfluorinated plastics materials such as polytetrafluoroethene (Teflon®) or polyfluorinated rubber (Viton®) are particularly preferably used for this purpose. In this context, temperature-resistance means heat-resistance at temperatures of at least 200° C. According to the manufacturers the heat-resistance of Viton® is 200° C. and that of Teflon® is 260° C.
Because of the high specific densities thereof, metals, ceramic materials and plastics materials generally do not float and should preferably be manufactured as hollow bodies. Alternatively, the floating bodies may be manufactured from porous material, the pores preferably being closed.
Floating bodies of which the surface comprises a reflective coating, which affords the bodies a particularly low emissivity, are preferred. A coating can also seal an open porosity. Advantageously, the surface is additionally smoothed or polished.
In a further preferred embodiment, the floating bodies are spherical.
In a particular aspect, the invention relates to the use of a heat insulation layer for wet slag removers in incinerators, comprising a plurality of floating bodies which are movable relative to one another and preferably rotatable about at least one axis of rotation.
Because of the flexible construction thereof with a plurality of floating bodies, the heat insulation layer according to the invention can be used in various incinerators having wet slag removers. Existing incinerators can also be retrofitted simply without additional constructional measures on the wet slag remover.
When the heat insulation layer according to the invention is used in incinerators, the operating temperature in the combustion chamber rises and the heat loss at the wet slag remover is reduced. As a result, an additional energy input to compensate for heat losses and/or to liquefy slag components is unnecessary. In particular in incinerators having a cylindrical rotary furnace, the discharge of slag from the incinerator is simplified since the slag does not solidify.
Optionally, a plurality of layers of floating bodies may be used, in such a way that the water surface is maximally covered. For this purpose, floating bodies of different sizes may optionally be used.
A further advantage of the construction according to the invention of the incinerator is the greatly reduced evaporation of the water in the wet slag remover. In normal operation of a conventional incinerator without a heat insulation layer, the water bath is heated to approximately 30° C. to 80° C., and this represents a considerable heat loss. Moreover, substantial evaporation takes place at this temperature. The radiant heat incident on the water surface accelerates the evaporation process. The evaporation of water is an endothermic process; the necessary evaporation enthalpy is lost from the system and is a further source of energy loss in incinerators. The floating bodies of the insulating layer reduce the contact area between the water bath and the gas chamber (combustion chamber). In this way, the evaporation of water from the wet slag remover into the combustion chamber is also reduced. Reduced process water consumption is a further advantage of the invention.
According to the invention, the combustion residues from incinerators having wet slag removers are discharged by the following method. Initially, an incinerator is provided with a tank serving as a water bath for receiving combustion residues (wet slag remover), comprising a floating heat insulation layer which is made up of a plurality of floating bodies which are movable relative to one another. Subsequently, the solid combustibles such as production residues from industry, household waste, substitute fuels, coal or biomass are burnt up in the combustion chamber. This may take place in a grate furnace or a cylindrical rotary furnace, but also in coal combustion boilers. In the following method step, the resulting combustion residues (slags, ash) are discharged into the water bath of the wet slag remover at the end of the rotary cylinder or the grating in the lower part of the coal combustion boiler via a drop chute, the combustion residues penetrating the heat insulation layer before entering the water bath.
Since according to the invention this water bath is covered by a heat insulation layer made up of floating bodies, the residues initially fall onto the floating bodies, which because of the degrees of freedom of movement thereof do not, however, form a barrier, but instead allow the residues to pass into the water bath. In this case, the floating bodies may be displaced either horizontally or vertically to form a gap.
The floating bodies preferably have at least one axis of rotation about which they can rotate. The rotational movement comes about when the combustion residues are discharged in that the centre of gravity of the floating bodies is altered by the impacting solids in such a way that a rotational or tilting movement occurs in the gravitational field as a result and conveys the combustion residues into the water bath. This applies in particular to spherical floating bodies.
After the combustion residues have passed the heat insulation layer, the floating bodies spontaneously organise themselves into a closed layer. If individual floating bodies are damaged or made unusable during relatively long operation of the heat insulation layer, or if floating bodies are lost when the combustion residues are transported away from the wet slag remover, new floating bodies can easily be applied to the water surface of the wet slag remover.
In the following, the invention is explained by way of embodiments and the appended drawings.
The pilot scale test setup of a wet slag remover shown in
By way of example, temperature measurements and water level measurements which realistically reproduce the temperature distribution in the wet slag remover 6 of an incinerator were carried out on this test setup. The temperature distribution 17-20 was measured as a function of the height above the water surface 16 of the water bath 7 (see
Changes in the temperature distribution in the water bath 7 and in the gas chamber 8 and the evaporation amount were recorded using a data capture system. By balancing, it was possible to compare the test results with one another and check the plausibility thereof.
If the effect of the emissivity is now taken into account, the result achieved can be considerably improved.
For a large commercial system having furnace temperatures of 850-1200° C., it is to be expected that even substantially lower temperature increases of approximately 10% (corresponding to a temperature increase of approximately 100° C.) would be sufficient to facilitate the slag discharge from the cylindrical rotary kiln considerably. The tests on the wet slag remover test setup of
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 032 760.6 | Jul 2009 | DE | national |
