EP0122846A1 - High energy fluid and its use in the feeding of a reaction zone with combustible material - Google Patents

Abstract

An energetic fluid product containing finely divided solid combustible matter suspended in at least one liquid phase and capable of being made to circulate in a line for supplying a treatment chamber, and a device for the preparation and a particular application of this product. The solid particles (13) are dispersed homogeneously within a stable foam produced by mixing a gas phase with the liquid phase to which stabilizing and emulsifying products have been added, and the liquid phase consists solely of fine film (12) connecting the solid particles (13) together and confining gas bubbles (11) which occupy the spaces between the solid particles (13). The invention applies in particular to the supply of combustible matter to a combustion chamber or to a reactor for coal gasification.

Description

The subject of the invention is a fluid product with an energy vocation containing a finely divided combustible material, a device for preparing such a product and a particular application of the product to the supply of combustible material to a reaction chamber.

Various combustion or gasification processes are known using a reaction chamber supplied with solid combustible material such as finely divided coal.

The fuel must normally be introduced continuously into the reaction chamber and it is useful for this purpose to prepare it in the form of a fluid product capable of being conveyed in supply lines by simple means. One can use for example a pneumatic transport, the particles being suspended in a stream of air, but this results in a risk of self-ignition and explosion. If inexpensive inert gas is not available, it is generally preferred to disperse the particles in a liquid phase to form a mixture having the consistency of a mud and which can be conveyed for example by means of a positive displacement pump. Generally, the coal is suspended in water but, in this case, the combustion reactions are delayed, which forces the combustion chambers to be increased and the thermal efficiency of the reaction is greatly reduced since a large part the energy supplied only serves to vaporize the water. It is therefore sought to reduce as much as possible the proportion of water relative to the proportion of solid matter. However, as the solid concentration increases the viscosity of the product also increases and more energy must be used to pump the product. The viscosity of the mixture for a minimum proportion of water can be reduced by means of chemical additives, but these are expensive and moreover can be corrosive.

It has also been envisaged to play on the particle size of the solid particles because it is known that a denser mixture is obtained by using particles belonging to at least two dimensional intervals. It is then necessary to add to the particles normally employed other, finer particles, in determined proportions, but the constitution of such mixtures with multimodal particle size is expensive and in any case, the quantity of solid matter contained cannot exceed 70 to 75% depending on the nature of the solid.

However, the recovery of the fuel is all the more efficient the finer its particle size since the time necessary for the reaction is a function of the grain size. It is therefore interesting to seek to use fuels with a particle size as fine as possible, which prohibits the use of filters and leads to reducing the liquid content as much as possible without however reaching a compactness which would prevent pumping for the purpose of introduction. of the mixture in the reactor.

The problem of introducing the combustible material into a reactor is further complicated by the fact that the reaction yield is better when the reactor is operating under pressure, which means that the product must also be pressurized if one wishes to '' introduce continuously into the reactor. The difficulties of introducing a low humidity mixture are then increased.

It follows from these difficulties that, generally, the fluid products produced up to now for supplying the reactor with combustible material contain a proportion of liquid of at least 30%. Of course, the energy drawbacks of the use of such a proportion of liquid phase if it consists of a liquid fuel but, in this case, the economic advantage of the process is also reduced.

The subject of the invention is a new fluid product with an energy vocation in which the proportion by weight of liquid phase relative to that of solid particles is significantly reduced compared to the known product and which also makes it possible to use very fine particles.

The fluid product according to the invention consists of a stable foam produced by mixing a gaseous phase with the liquid phase supplemented with stabilizing and emulsifying products and inside which the solid particles are dispersed in a homogeneous manner, the liquid phase being constituted only by thin films connecting together the solid particles and limiting gas bubbles which occupy the spaces between the solid particles.

In such a product according to the invention, the solid particles can have any particle size distribution and the proportion by weight of solid phase can exceed 75 X.

According to a characteristic of the invention, the gas phase can be a neutral gas, an oxidizing gas or else a combustible gas.

The invention also relates to a device for preparing a fluid product according to the invention comprising a means of preparation a stable foam by incorporating a gaseous phase into a liquid phase containing emulsifiers and stabilizers and a means for homogeneous dispersion of the particles of combustible material inside the foam thus prepared.

In a particularly advantageous embodiment, the means for homogeneous dispersion of the solid particles in the foam comprises a double screw mixer comprising, inside an elongated cylindrical sheath, an agitator in the form of a helical ribbon driven in rotation around the axis in a direction determining the advance from upstream to downstream and on the periphery of the sheath of the foam introduced by an orifice placed at the upstream end, the helical ribbon surrounding an axial free space in the upstream part of which penetrates a screw feeder for the introduction of a determined flow of solid particles.

In a particular application, the invention also relates to a process for introducing into a reaction chamber a solid combustible material finely pulverized and homogeneously dispersed in a stable foam in which the liquid phase consists solely of the films bonding between they solid particles and limiting gas bubbles which occupy the spaces between the particles, said foam then being circulated in a supply line of the reaction chamber.

To prepare the fluid product according to the invention, preferably, the foam is first produced by vigorous stirring of the liquid phase in the presence of the gas phase and then the solid particles are dispersed in the foam thus prepared, that -this can be stored in advance for later use.

According to an advantageous characteristic, before its introduction into the chamber, the fluid product can be subjected to an increase in pressure leading to an increase in the proportion of solid material per unit of volume, up to a pressure below the limit pressure from of which the volume of the compressed product remains constant.

• La figure 1 est un schéma représentant à grande échelle un échantillon du produit.
• La figure 2 est un schéma du produit après compression jusqu'à la pression limite.
• La figure 3 représente schématiquement une installation de préparation et d'utilisation du produit.
• La figure 4 représente schématiquement un autre exemple d'installation de préparation et d'utilisation du produit.

The following description of a particular embodiment given by way of example and shown in the accompanying drawings, will allow a better understanding of the invention.

• Figure 1 is a diagram showing on a large scale a sample of the product.
• Figure 2 is a diagram of the product after compression to the limit pressure.
• FIG. 3 schematically represents an installation for preparing and using the product.
• FIG. 4 schematically represents another example of installation for preparing and using the product.

In FIG. 1 is shown schematically a sample of the product 1 which consists of a foam formed of bubbles 11 limited by liquid particles 12 in the form of a meniscus and inside which are homogeneously dispersed solid particles 13 connected together by liquid films 12. In the drawing the solid particles 13 have been symbolized by spheres but can obviously have any shape. The average grain width is around 50 microns but can even drop below 20 microns. The gas bubbles 11 can have dimensions of the order of a millimeter but can also be smaller, the use of fine bubbles making it possible to increase the proportion of solid particles incorporated in the foam.

It can be seen that, in such a product, the liquid phase consists solely of the films 12 for bonding between the particles 13 and that, consequently, the proportion by weight of the liquid in the product can be very reduced. However, since the particles are separated from each other by the bubbles, the mixture is pumpable and can be transported inside pipes by any known means and it behaves like a compressible fluid.

In Figure 3, there is given by way of example a preparation scheme for such a product. The liquid phase, for example water, is introduced into a tank 2 in which it is vigorously stirred with a thickening product 21, introduced by a metering device and which makes it possible to produce a kind of gel 20. This is then led into a second tank 23 into which water is also introduced via an inlet 24 and an emulsifier of the surfactant type via a line 25. The whole is vigorously stirred until all the liquid phase is emulsified with the gas present in the enclosure.

The foam 10 thus produced is directed to a third tank 4 into which the solid particles 13 are poured by means of a dosing hopper 44. The whole is vigorously stirred to disperse the solid particles evenly inside the product 1 which is then shown schematically in the form shown in FIG. 1.

The thickening product 21 introduced into the tank 2 stabilizes the emulsion by eliminating spontaneous ruptures of the films of liquid ensuring the partitioning of the foam and thus ensuring the suspension of solid particles. Non-volatile hygroscopic soluble products can be used such as, for example, carboxymethylcellulose, glycerol, dodecane, or else polyvinyl alcohol.

The emulsifier 25 is a surfactant which emulsifies the gas in water. One can use an alkylarylsulfonate or another known foaming agent, for example a saponified fatty acid, an amine, quaternary ammonium, an alkylpolyethoxyetherphosphate etc ...

By way of example, a product was produced in which the liquid phase prepared before the incorporation of the gas contained 1% of surfactant and 0.2% of stabilizer.

Since the spacing of the solid particles ensuring the fluidity of the product is obtained by the presence of gaseous bubbles of negligible weight, it is indeed possible to incorporate into the product a very large proportion by weight of solid phase without being obliged to associate particles with different particle sizes.

Thus it was possible to prepare a fluid product containing 75% by weight of pulverized coal whose particles were 80% of a diameter less than 80 microns.

It should also be noted that, in the form of gas bubbles, the presence of air in the mixer, even in significant proportion, is not dangerous, since the particles of carbon are coated in the liquid films limiting the bubbles and that these reduce the risks of oxidation of the particles which can cause a rise in temperature.

To use the foam, it can be circulated by means of a pump 27 in a closed circuit 28 from which the necessary foam is withdrawn by metering pumps 29. It is also possible to put the circuit 28 under pressure and replace the metering pumps by simple inlet valves making it possible for example to supply the burners to a reaction chamber 6.

Indeed, according to an advantageous characteristic, the fluid product constituted by the stabilized foam can undergo an increase in pressure which determines a reduction in the size of the bubbles and the approximation of the particles and consequently an increase in the density of the product.

When the product is at atmospheric pressure, the particles of carbon, randomly dispersed in the foam are practically without contact between them. The mixture is then easily pumpable.

If the pressure P applied to the product is increased, the particles approach each other, the bubbles decreasing in diameter. However, the product remains pumpable as long as the particles are sufficiently spaced from each other.

There is therefore a limit pressure above which, as shown in Figure 2, the carbon particles are arranged in a compact stack. In this case, the mixture is no longer pumpable, the friction between the particles opposing their movements.

As a result, the increase in pressure applied to the fluid product must remain below the limit from which the specific volume of the product no longer decreases when the pressure is increased.

This limit depends on the concentration of solid particles and can be determined either empirically, by a series of tests, or by calculation.

It is known, for example, that a compact stack of unimodal particle size distribution comprising 80% of particles of dimension less than 80 microns corresponds to a porosity of about 0.4. This means that, in a compact stack, the volume occupied by the solid particles is 60% of the whole.

Furthermore, it has been determined experimentally that, for a proportion of solid particles of 75% by weight, one kilogram of product occupies a volume of approximately two liters. By assimilating gas to an ideal gas and applying Mariotte's law, we can therefore, from these data, determine, starting from atmospheric pressure, the limit pressure increase from which the volume occupied by a determined mass of product no longer varies. For example, in the case of a product based on carbon, air and water and containing 75% of carbon by weight, the limit pressure is of the order of 9 bars. Care will therefore be taken, in the operating circuit 27, 28 and 29, not to reach this pressure for which the final obstruction of the pipes would be determined.

This pressure may prove to be too low for the supply of a gasification reactor. However, it was calculated for a product prepared at atmospheric pressure and at room temperature. However, it can be shown that if the product is prepared at an absolute pressure higher than atmospheric pressure, for example 5 bars, the limit pressure can reach 45 bars. On the other hand, the limit pressure is also a function of the ratio between the operating temperature and the temperature of preparation. Thus, in the case where the product is prepared at 20 ° C, the limit pressure can be increased by 10% if, at the time of use, the temperature is raised to 50 ° C as far as obviously, where the foam remains stable at such a temperature.

In Figure 4, there is shown by way of example an installation for the preparation under pressure of a fluid product based on coal, air and water.

The aqueous solution prepared, as in the case of FIG. 3, by mixing water with a thickener then an emulsifier, is brought by a pump 31 into a device 3 for preparing the foam consisting of a tube in the middle of which a venturi 30 is placed. The air is blown by a compressor 32 into the venturi and is thus incorporated into the aqueous solution to form a foam which is directed towards the mixer 4. The latter, which may include several parts in line , consists essentially of an agitator in the form of a helical ribbon 41 driven in rotation about its axis by a motor 42 inside a cylindrical sheath 4 provided with an orifice 43 for introducing the foam, placed at the upstream end in the direction of flow of the product resulting from the rotation of the helical ribbon 41. The latter limits on the other hand a free axial space into which a screw metering device of known type enters, comprising a hopper provided at its base with a screw driven in rotation at around its axis and which extends into a tube 45 opening into the free space delimited by the helical strip 41 in the axis of the sleeve 4.

In this way, the foam prepared in the device 3 and introduced through the orifice 43 in the sheath 4 is driven downstream along the internal periphery of the sheath by the rotation of the ribbon 41 and takes care of the finely divided coal , poured into the hopper 44 and which is therefore incorporated inside the foam with a flow rate regulated by the rotation of the screw. It is thus possible to obtain a perfectly homogeneous dispersion of the solid particles inside the foam.

Such a mixer can operate at a low pressure below the limit pressure, for example from three to six bars and thus supply a main tank 5 maintained at the desired pressure.

The product thus prepared can therefore be stored in advance in a tank maintained under low pressure.

If the product has to be injected into the reactor under a higher pressure, it is advantageous to pass through a buffer tank 51 being under a pressure higher than the working pressure. station, for example ten bars. The buffer tank 5I is supplied from the main tank 5 by a positive displacement pump fitted with a booster. Indeed, the large proportion of gas contained in the product stored in the tank 5 could cause discontinuous and random discharge. The booster device could be constituted, very simply, by an archimedean screw placed in the bottom of the storage tank 5 and supplying a positive displacement pump 52.

From the buffer tank 51, it is possible to introduce the fluid product directly into the reactor 6 by means of a supply line 61 provided with a rolling valve 62 ensuring the expansion of the product to the pressure wanted.

If the buffer tank 51 is at a pressure higher than the operating pressure, the expansion of the compressed gas contained in the bubbles at the time of injection into the reactor promotes the spraying of the fluid product and very effectively disperses the coal particles inside the reactor. Given the fineness of the particles which can be achieved by the process according to the invention, a real atomization of the combustible material in the reactor is obtained.

Moreover, in general, the possibility of using, thanks to the invention, coal with a particularly fine particle size represents an important advantage, the recovery of the fuel being all the more effective as its particle size is fine since the time required for the reaction is a function of the grain size.

It will also be noted that the three-phase constitution of the product, with a high proportion of gas, reduces the probabilities of impact of the particles against the walls, since the particles are kept inside the bubbles constituting the foam and consequently reduce the erosion of the organs injection.

Furthermore, the possibility of manufacturing the product in advance in a stable, storable and directly usable form is an important advantage because it makes it possible to separate the installation for manufacturing the product from the users, who only have to provide devices for pressurizing and injecting into the reactor a fluid product already prepared.

In addition, for the same weight content of solid filler, the use of a product in the form of foam reduces the pressure drop in the transport lines. It could therefore be interesting to prepare the combustible product in a place possibly very far from the place of use, for example near a place of production or importation of coal and to transport the sparkling product in a pipe to the combustion plant which could even be distant of several hundreds of km. Thanks to the low pressure drop, energy consumption for transport would be reduced.

By way of example, a comparison has been made, for different mass flow rates, of the pressure losses resulting from the transport in the same pipe of a fluid product according to the invention and of a conventional coal-water mixture containing the same proportion of coal ( made up of 80% of particles with a diameter of less than 80 microns).

In this example, the product according to the invention had the following composition (by weight):

the pressure drop increases relatively little with the mass flow rate since, in a pipe with a diameter of 25.5 mm, it varies from 0.12 to 0.20 Bar / m when the flow rate changes from 0.10 to 0.25 kg / s.

On the other hand, for a conventional coal-water mixture containing 70% of coal particles, the pressure drop increases very rapidly with the flow rate and goes from 0.20 to 1.10 Bar / m for the same flow rates.

This reduction in pressure drop therefore makes it possible to transport over a long distance a highly concentrated product which can be directly used in the reactor.

Of course, the arrangements which have just been described have only been given by way of example and other devices could be used which make it possible to produce a foam and to incorporate solid particles into it.

Furthermore, if the product has been described in the case of a coal-water-air mixture, it is obvious that a different solid fuel could be used with a liquid phase and a gaseous phase of other nature. It will be noted that, generally, the proportion of liquid phase must be as low as possible since the use of water decreases the energy yield and that a combustible liquid is more expensive. On the other hand, the gas phase is generally useful for the reaction and one could for example use as gas phase either an oxidizing gas or a combustible gas.

Claims (12)

1. Fluid product for energy use containing a finely divided solid combustible material suspended in at least one liquid phase, and capable of being circulated in a supply line of a treatment chamber, characterized in that the solid particles (13) are dispersed homogeneously inside a stable foam produced by mixing a gaseous phase with the liquid phase supplemented with stabilizing and emulsifying products, and that the liquid phase consists solely of fines films (12) connecting the solid particles (13) together and limiting gas bubbles (11) which occupy the spaces between the solid particles (13). 2. energetic fluid product according to claim 1, characterized in that the proportion by weight of solid phase can reach at least 75%. 3. energetic fluid product according to one of claims 1, or 2, characterized in that the solid combustible material is a pulverized coal. 4. energetic fluid product according to one of claims 1 to 3 characterized in that the gas phase is an oxidizing gas. 5. energetic fluid product according to one of claims 1 to 4, characterized in that the gas phase is a combustible gas. 6. energetic fluid product according to claim 1, characterized in that the stabilizing product is a carboxymethylcellulose and the emulsifying product a surfactant of the kind alkylarylsulfonate. 7. Device for preparing a fluid product containing a high proportion of finely divided solid combustible material, characterized in that it comprises a means (2,3) for preparing a stable foam by incorporating a gaseous phase in a liquid phase containing emulsifying and stabilizing products and a means (4) for homogeneous dispersion of the particles of combustible material inside the foam thus prepared. 8. Device for preparing a fluid product according to claim 7 characterized in that the means (4) for dispersing solid particles in the foam comprises a twin screw mixer comprising, inside an elongated cylindrical sheath (40) an agitator (41) in the form of a helical ribbon driven in rotation about the axis in a direction determining the advance from upstream to downstream, on the periphery of the sleeve (40) of the foam introduced by an orifice (43) placed at the end upstream, the helical ribbon (41) surrounding an axial free space in the upstream part of which penetrates a screw metering device (45) for the introduction of a determined flow of solid particles. 9. Method for supplying a reaction chamber (6) with a finely divided solid combustible material suspended in at least one liquid phase to form a fluid product capable of being circulated in a supply line ( 61) opening into the reaction chamber (6), characterized in that the particles (13) of solid material are homogeneously dispersed in a stable foam (1) produced by mixing a gas phase with the liquid phase added with a stabilizing product and an emulsifying product and containing only the quantity of liquid phase necessary for the formation of films (12) connecting together the solid particles (13) and limiting gaseous bubbles (11) occupying the spaces between the particles (13), said foam (1) then being circulated in the supply line of the reaction chamber. 10. Method according to claim 9 for supplying combustible material, characterized in that a foam is first produced by vigorous mixing of the liquid phase with the gaseous phase and that the solid particles are then dispersed in the foam thus prepared, which can be stored in advance for later use. 11. Method according to one of claims 9 and 10 for supplying combustible material to a reaction chamber 6, characterized in that before its introduction into the chamber (6), the fluid product (1) is subjected to an increase in pressure capable of increasing the proportion of solid material (13) per unit of volume up to a pressure lower than the limit pressure from which the volume of the compressed product remains constant. 12. Method according to one of claims 9, 10, 11, for supplying a reaction chamber (6) with combustible material, characterized in that the fluid product is prepared in a place remote from the place of 'use, the reaction chamber being connected to the preparation device by a long supply circuit.

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