EP0030396B1 - Process for the thermal treatment of pellets - Google Patents

Abstract

An improved process for heat-treating pellets in a pelletizing machine in which hot gases are generated and passed through a bed of pellets includes charging solid carbonaceous fuel onto the surface of the pellet bed and burning the carbonaceous fuel to generate at least a portion of the hot gases.

Description

The invention relates to a method for the thermal treatment of pellets on a pellet burning machine with the passage of hot gases through the pellet bed, solid carbon-containing fuel being burned to produce at least part of the hot gases, the pellets being cooled by means of cooling gases passed through them, and at least part the heated cooling gases is passed into the thermal treatment stage.

The thermal treatment of pellets, in particular the hard burning of iron ore pellets, mostly takes place on traveling grates with gas hoods, which are referred to as pellet burning machines. The pellet burning machines have different treatment zones in the direction of travel, namely drying zone, thermal treatment zone and cooling zone. These zones can be divided, e.g. Pre-drying and post-drying zone, heating zone, pre-firing zone, main firing zone, post-firing zone, first and second cooling zones. The required process heat is usually brought into the process exclusively or predominantly by hot gases. These hot gases are generated or distributed and collected in gas hoods above the pellet bed by burning liquid, gaseous or dusty solid fuels. Since the exhaust gases are sometimes very hot, various gas recirculation systems are used to utilize the heat.

Such a pellet burning machine is e.g. known from DE-PS 14 33 339. With this machine, the hot cooling gas is led from the first pressure cooling zone in a common gas hood without the interposition of a blower into the thermal treatment zone - which consists of the heating zone, the combustion zone and the afterburning zone. The hot cooling gas is distributed to the individual zones of the thermal treatment zone by means of internals in the common gas hood, these internals leaving channels to the actual combustion chambers of the individual zones. In the combustion chambers of the heating and burning zone, the hot cooling gases are heated to the required temperature by burners. The hot gases are drawn through the bed in wind boxes. Gases from the second cooling zone and exhaust gases from the afterburning and burning zone are directed into the drying zones. It is also described that the hot cooling gases are discharged from a gas hood above the cooling zone through a gas manifold and are distributed through distribution lines into the individual zones of the thermal treatment zone.

From US-PS 36 20 519 a similar pellet burning machine is known, in which, however, the hot cooling gases in the common gas hood above the cooling zone and / or in the transition zone to the thermal treatment stage are heated by means of burners. At least in a part of the transition zone, an installation for shielding the pellets against the hot combustion gases is arranged in the common gas hood above the pellet bed. When covering the entire heat requirement using burners, there is a risk that local overtemperature will occur, causing the fuel ash and / or the dust contained in the process gases to slag, forming deposits in the area of the combustion devices, which could destroy the refractory material through flame deflection and infiltration. As a result, the thermal treatment can also be adversely affected or the output reduced or production must be stopped for repair. In addition, because of the large number of burners on pellet burners, the burner operation is particularly suitable for relatively expensive gaseous and liquid fuels. In the case of coal dust burners, it is advisable to reduce the number of burners because of the necessary transport and distribution facilities for the fuel.

It is also known to incorporate part of the required fuel into the pellets. However, this is only possible to a small extent without adversely affecting the quality of the pellets.

The invention is based on the object of avoiding or substantially reducing the local overheating and the slagging and durability problems which occur in the process. In addition, fuel should be able to be used as cheaply as possible, and the process should also be usable for improvements to existing plants without great effort.

This object is achieved according to the invention in that at least 10% of the fuel supplied to the process from the outside in the form of solid fuel is applied to the surface of the pellet bed, the task of the solid fuel being controlled in such a way that at least part of the thermal fuel Treatment zone with downward flow of the hot gases and / or at least in a part of the cooling zone with upward gas flow, from which the heated cooling gases are led into the thermal treatment zone, solid fuel is present on the bed.

The total heat requirement is covered by the recycled process heat in the recycled gas, plus the fuel heat from any fuel contained in the pellets, plus any reaction heat (e.g. heat of oxidation from Fe ₃ 0 ₄ to Fe ₂ 0 ₃ ), plus fuel supplied to the process from outside. The characteristic “at least 10%” refers to this externally supplied fuel. All types of coal can be used as solid fuel, even those with a high volatile content. The grain size distribution of the solid fuel, its quantity and the choice of the delivery points are set so that the desired in the individual zones and sections of the zones The amount of heat available is such that there is no solid fuel left in the discharge and that local excess temperatures are avoided as far as possible. The reactivity of the solid fuel and its volatile content must be taken into account - under otherwise identical conditions. The solid fuel can be fed in by mechanical or pneumatic loading. The rest of the fuel "supplied from the outside" is supplied in a conventional manner by means of burners which are operated with liquid or gaseous fuels or with coal dust. The solid fuel can be fed in at one or more points only in the thermal treatment zone, the grain size distribution and feed point then being selected such that a portion of the fuel still reaches the cooling zone on the surface of the bed. From a certain grain size, the solid fuel in the cooling zone is entrained by the cooling gas flowing upwards and burns in the cooling gas on the way to the thermal treatment zone. If the entrained solid fuel has not yet burned out in the thermal treatment zone by the time the cooling gas hits the surface of the pellet bed, it will fall back onto the bed. The solid fuel can also be fed into the thermal treatment zone and the cooling zone. In this case too, the solid fuel is entrained in the cooling zone when it has reached a certain grain size due to the combustion. When feeding into the cooling zone, the portion that already has this grain size when it is fed in is immediately carried away. The presence of solid fuel in the thermal treatment zone and in the cooling zone causes a “multi-stage combustion”, since the cooling gas emerging from the pellet bed in the cooling stage is further heated by burning solid fuel in the layer of solid fuel on the pellets , then by burning entrained solid fuel on the way to the thermal treatment zone and finally by passing through the layer of solid fuel on the surface of the bed in this zone. As a result, there are only slight differences in temperature in the gas stream, which significantly reduces the formation of slag from ash and dust and the formation of thermal NO _x .

A preferred embodiment consists in that the heated cooling gases emerging in the cooling zone above the pellet bed are passed under a common gas hood into the thermal treatment zone with the hot gases flowing downwards, and the distribution of the hot gases by controlling the flow resistance of the pellet. Bed. The flow resistance of the pellet bed in the individual sections of the thermal treatment zone is adjusted by regulating the negative pressure in the corresponding sections. As a result, the gas can be distributed in the thermal treatment zone without installations in the gas hood. As a result, a lower overpressure in the cooling zone and a lower underpressure in the thermal treatment zone are required, as a result of which the heat losses due to the escape of hot gases and suction of false air are reduced and the energy consumption of the blowers is also reduced. In addition, the coldest cooling gases - from the last part of the cooling zone - flow under the ceiling of the gas hood and protect them from high temperatures.

A preferred embodiment of the invention consists in that 40 to 80% of the fuel supplied from the outside is applied to the surface of the pellet bed. This area is particularly advantageous if there are no internals in the gas hood in the thermal treatment zone. This results in particularly good operating conditions because a considerable part of the heat is distributed evenly over a larger area of the pellet bed, this part can be generated from cheap fuel, and the remaining heat can be introduced in an easily controllable manner by burners, whereby the burner (s) required to start up can be used anyway.

In existing plants which have internals in the gas hoods of the thermal treatment zone, 10 to 40% of the fuel supplied from the outside is preferably applied to the surface of the pellet bed.

A preferred embodiment is that solid fuel with a high content of volatile constituents is introduced into the thermal treatment zone and the layer thickness and / or grain size of the solid fuel is adjusted so that the flammable volatile constituents expelled predominantly burn in the lower layers of the pellet bed . As a result, the desired firing temperature is also achieved in the lower layers of the bed without the upper layers being overheated. The treatment time can also be shortened. With a higher layer height of the added solid fuel and coarser granulation, more volatile constituents are burned in the lower layers of the pellet bed. In addition, the temperature of the upper layers of the pellet bed can already be reduced by supplying hot gases with a lower temperature in the afterburning zone. In addition, a regulated proportion of fine grain can ensure that part of the solid fuel falls through the gaps into deeper layers and burns out there.

• Abb. 1 ist ein schematischer Längsschnitt durch die Pellets-Brennmaschine mit Einbauten in der thermischen Behandlungszone;
• Abb. 2 ist ein schematischer Längsschnitt durch die Pellets-Brennmaschine ohne Einbauten in der thermischen Behandlungszone.

The invention is explained in more detail with reference to figures and examples.

• Fig. 1 is a schematic longitudinal section through the pellet burning machine with installations in the thermal treatment zone;
• Fig. 2 is a schematic longitudinal section through the pellet internal combustion engine without installations in the thermal treatment zone.

The pellet burner had a reaction area of 430 m ² and a bandwidth of 3.5 m.

Fig. 1:

The unfired pellets 1 are fed onto the traveling grate 3 via a roller grate 2 and dried in the pressure drying zone 4 and the suction drying zone 5 by means of recycled process gases. In the heating zone 6 and in the firing zone 7, heated cooling gases are sucked through the pellet layer. These are fed from the cooling zone 8b via a recuperation line 9 and 38 feed channels 10 to the 38 combustion chambers 11, heated there with the 38 oil burners 12 and fed to the heating and combustion zone via the combustion chamber outlets 13. (For a better overview, only one feed channel 10 with combustion chamber outlet 13 is shown in Fig. 1.) In the afterburning zone 14, heat is transported from the upper to the lower pellet layer by means of hot cooling gases from the cooling zone 8a. The afterburning zone 14 is separated from the firing zone 7 by a separating weir 17, which prevents the cooling gases from the cooling zone 8a and 8b from directly entering the heating or firing zone 6 or 7, and thus the transport of the cooling gases from the cooling zone into the above-mentioned zones allows required pressure drops.

The heating and burning zone are separated from one another by the separating weir 20.

This procedure corresponds to the known prior art.

Embodiment 1

2 tons of coal with a grain size of 0 to 10 mm are fed into the afterburning zone 14 via the feed points 15 and 16. This amount of coal would correspond to an increase in the temperature of the cooling gases of 200 ° C. This increase in temperature takes place in stages, namely by burning the volatile and finest constituents of the fuel content given up at 15, which give off their heat to the cooling air or pellet layer within the pellet layer underneath; by burning the larger fuel particles of the feed point 15, which burn on the surface of the pellet layer in the area of the afterburning zone (with the flow directed downwards); by burning the volatile and finest constituents of the fuel supplied to the feed point 16 within the cooling gas stream directed to the heating zone 14; by burning the larger fuel particles of the feed point 16, which lie on the surface of the pellet layer, burn (with upward flow).

Local partial overheating, which causes the slag formation (from the dust content of the process gases and / or the fuel ash), is also avoided or reduced by this partial combustion (multistage combustion) which takes place at different locations and in which the temperature is raised in several stages. like the formation of thermal nitrogen oxides (NO _x ). This effect is further supported by the uniform allocation of fuel and cooling gas on the pellet layer, since the cooling gas is supplied to the fuel in a very evenly distributed manner due to the resistance of the pellet layer. Larger areas in which unmixed areas and thus excess temperatures occur are thus avoided. The combustion in the layer can therefore also be compared with the combustion of the fuel by means of a large number of small burners.

As the preliminary tests have shown, the larger fuel particles at the feed point 15 (with the downward flow of the cooling gases from FIG. 8a) burn off with weight loss and, if they are not completely burned off at the transition to or within the cooling zone (with the upward flow of the cooling gases) 8a) recirculated to the afterburning zone 14 until they are completely burned out.

• Die Nachbrennzone 14 wird zusätzlich beheizt, ohne dass ein zusätzlicher Anbau von Zuführungskanälen 10 an die Rekuperationsleitung 9, von Brennkammern 11 mit Brennern 12 und das Versetzen des Trennwehres 17 an die Übergangsstelle zwischen der Nachbrennzone 14 und der Kühlzone 8a notwendig ist, da die Aufheizung der Kühlgase 8a direkt über dem Pellets-Bett durch die daraufliegende und teilweise darüber rezirkulierende Brennstoffschicht erfolgt. Der zusätzliche Druckverlust, der bei Anwendung des vorhandenen Beheizungsverfahrens mit Zuführungskanälen 10, Brennkammern 11 und Brennern 12 durch das grössere Gasvolumen am Eintritt in die Rekuperationsleitung entstehen würde, wird vermieden. Damit wird das für den Transport der Kühlgase von der Kühlzone 8b in die Aufheiz- bzw. Brennzone 6 bzw. 7 notwendige Druckgefälle niedriger gehalten und zusätzliche Wärmeverluste durch aus dem Brennofen austretendes Kühlgas 8a (Temperatur 750 °C) oder eintretende Falschluft im Bereich der thermischen Behandlungszonen 6 und 7 entfallen. Der zusätzliche Wärmebedarf für die Beheizung der Nachbrennzone kann durch billigen, verfügbaren festen Brennstoff gedeckt werden. Er beträgt ca. 14% des von aussen zugeführten Brennstoffes.

This configuration has the following advantages:

• The afterburning zone 14 is additionally heated without additional installation of supply channels 10 on the recuperation line 9, of combustion chambers 11 with burners 12 and the relocation of the separating weir 17 to the transition point between the afterburning zone 14 and the cooling zone 8a, since the heating of the Cooling gases 8a take place directly above the pellet bed through the fuel layer lying thereon and partially recirculating above it. The additional pressure loss that would result from using the existing heating method with supply channels 10, combustion chambers 11 and burners 12 due to the larger gas volume at the entrance to the recuperation line is avoided. This keeps the pressure drop necessary for the transport of the cooling gases from the cooling zone 8b into the heating or burning zone 6 or 7 lower and additional heat losses due to cooling gas 8a (temperature 750 ° C) emerging from the furnace or entering false air in the area of the thermal Treatment zones 6 and 7 are omitted. The additional heat required for heating the afterburning zone can be covered by cheap, available solid fuel. It is approximately 14% of the fuel supplied from outside.

Embodiment 2

In addition to the feed of solid fuel via feed points 15 and 16, a further 30% of the fuel fed into the process from outside is fed in in the form of solid fuel via feed points 18, 19, 21 and 22, i.e. a total of 44%. The fuel addition points 18 and 19 are coated with oil to start up the system. After the operating temperature has been reached, these feed points are switched to solid fuel.

The fuel feed point 22 in the suction dryer 5 is designed as a combined burning point, specifically with one for solid fuel and one immediately following for liquid or gaseous fuel for igniting the fuel solid fuel previously charged. This focal point 22 is operated both when starting up the system and in normal operation.

Embodiment 3 (Fig. 2)

The cooling zone 8a, 8b is connected directly to the afterburning zone 14 and the firing zone 7 by means of a common gas hood without internals. The solid fuel is fed through the feed points 15, 16, 18, 21 and 22. The combustion takes place in the manner already described for the area of the afterburning zone in several stages, whereby again the volatile and finest constituents of the coal are burned within the pellet layer, insofar as this occurs in the area of the downward flow of the cooling gases, i.e. at the drop-off points 15, 21 and 22. In this way it is possible to cover up to 100% of the fuel supplied from the outside with solid fuel.

Preferably, however, only 80% are supplied via the feed points 15, 16, 18, 21 and 22 and the remaining 20% are used to regulate the temperature of the cooling gas stream 8b via the fuel feed points 19 and 23, the burn rate of the cooling air also being used at the same time as the temperature of the cooling air solid fuel in the cooling gas flow and on the pellet layer is controlled. To increase the controllability, preferably coal dust, oil or hydrate alcohol is used via the fuel feed points 19 and 23, the liquid fuels being used primarily during start-up operation.

The advantages of the invention are that local overheating on the burners and the associated disadvantages can be largely avoided. Even with a 10% solid fuel feed, the burners can be operated with less load and the disadvantages described can be significantly reduced. The heating of the gases during combustion in the fuel layer takes place very evenly, so that this can be compared to a large number of burners. With a multi-stage heating of the gases in several stages in succession, this leveling out is increased considerably. The thermal NO _x formation is significantly reduced and the use of cheap fuels is possible. The volume of the gases is only partially increased when the fuel layer heats up and the heat transfer within the pellet bed is improved.

Claims (4)

1. A process of heat-treating pellets on a pelletizing machine in which hot gases are passed through a pellet bed, solid carbonaceous fuel is burnt to generate at least part of the hot gases, cooling gases are passed through the pellets to cool them, and at least part of the heated cooling gases is fed to the heat-treating zone, characterized in that at least 10% of the fuel which is supplied to the process from the outside are fed as solid fuel onto the surface of the pellet bed, the feeding of the solid fuel is controlled in such a manner that there is solid fuel on the bed in the heat-treating zone at least in a part thereof in which the hot gases flow downwardly and/or in the cooling zone at least in a part thereof in which the gases flow upwardly and from which the heated cooling gases are supplied to the heat-treating zone.

2. A process according to claim 1, characterized in that the heated cooling gases rising from the pellet bed in the cooling zone are conducted under a common gas hood into the heat-treating zone, in which the hot gases flow downwardly, and the distribution of the hot gases is controlled by a control of the resistance to flow presented by the pellet bed.

3. A process according to claim 2, characterized in that 40 to 80% of the fuel supplied from the outside are fed onto the surface of the pellet bed.

4. A process according to any of claims 1 to 3, characterized in that solid fuel having a high content of volatile constituents is fed to the heat-treating zone and the thickness of the layer of the solid fuel and/or its particle size is so selected that the combustible constituents which have been volatilized burn mainly in lower layers of the pellets bed.

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