EP2275517A1

EP2275517A1 - Plant for converting solid waste into fuel gas

Info

Publication number: EP2275517A1
Application number: EP08736749A
Authority: EP
Inventors: Pere Escriba Nogues
Original assignee: Energia Natural De Mora Sl
Current assignee: Energia Natural De Mora Sl
Priority date: 2008-03-31
Filing date: 2008-03-31
Publication date: 2011-01-19
Also published as: CA2719956A1; WO2009121982A1

Abstract

The present invention relates to a plant for converting solid waste, specifically biomass, into fuel gas, using a fluidized bed reactor in which the material used therein has been improved. The plant has a compensated vacuum-pressure system which controls the pressure throughout the circuit through which the gas flows, an air-conditioning system, the purpose of which is to cool the air that leaves the reactor at an extremely high temperature and a system for the dry filtration of the gas obtained including a bag filter, a filter cleaning device, an ash extraction device and a thermal ash cleaner.

Description

Object of the Invention

The present invention relates to a plant for converting solid waste into fuel gas and to the improvements made therein.

Background of the Invention

Patent document ES2199612 describes a plant for converting solid waste into a fuel gas, of the type of those comprising:

a fluidized bed reactor provided with a diffuser grate and with air outlets formed by injectors which allow a uniform fluidization, a worm screw at the height of the grate which analyzes the behavior of the inert material; a compensated vacuum-pressure system which maintains a stable vacuum inside the gasification circuit; an air-conditioning system, the purpose of which is to cool the air that leaves the reactor; and a cleaning device for cleaning the gas obtained.

The injectors of the air outlets are formed by a steel cylinder provided with an inner orifice and with radial outlets, by a cap closing the upper end of the cylinder and forcing the air to leave through the radial outlets and by a concentric diffuser ring redirecting the air upwards.
The compensated vacuum-pressure system includes a compressor introducing air through the lower part of the reactor and a compressor located at the end of the equipment, which sucks in the fuel gas obtained in the reactor, making it leave through the upper area of such reactor.
The air-conditioning system includes exchangers cooled by the air which serves to fluidize and react with the fuel. These exchangers are formed by an outer body, inside which there are several tubes with a very small diameter.
In the present invention the reactor is a fluidized bed reactor.
In a fluidized bed, the upward air stream is used to supply the oxygen necessary for the reactions and to maintain the solid particles in suspension.
The surface speed starts increasing progressively, and the pressure drop starts increasing until the apparent weight of the bed is balanced with the air passing through. If the speed continues to increase, the pressure drop remains approximately constant and the particles are separated from one another, causing an increase in the height of the bed. When this occurs, the bed behaves like a fluid, which allows taking advantage of these characteristics to cause the conversion and reactions of the fuel solid which is introduced in order to convert it into gas.
In this type of bed, materials in heat-stable and non-abrasive particles are used.

Description of the Invention

The present invention describes a plant for converting solid waste, specifically biomass, into fuel gas. In said plant a fluidized bed reactor is used in which the material used therein has been improved.
In the conversion, air which is introduced in the reactor together with the waste to be gasified is used, which air converts the solid waste, biomass, into gas.
The waste is introduced through a feeding system.
It is necessary for the plant to have a compensated vacuum-pressure system which controls the pressure throughout the circuit through which the gas flows, since the gas which is obtained in this conversion is toxic.
The plant has an air-conditioning system, the purpose of which is to cool the air that leaves the reactor at an extremely high temperature.
The plant also has a system for the dry filtration of the gas obtained including a bag filter, a filter cleaning device, an ash extraction device and a thermal ash cleaner.
The problem arising in fluidized beds is that high temperatures are reached in the reactor, and the ash compounds sinter with the material of the bed, causing the defluidization and the agglomeration thereof.
In the present invention, the problem which arises is solved by means of using a dolomite-based material in the fluidized bed. The material of the bed is specifically sintered dolomite.
Only the use of dolomite or calcined dolomite in beds as an additive, and not as a base of the fluidized bed, was known up until now.
The material used in the fluidized bed is dolomite which is previously combusted at a temperature between 1600°C-2000°C, such that magnesium oxide and calcium oxide crystals are formed, the crystals at this temperature sinter and agglomerate. These sintered and agglomerated crystals are broken into small pieces until obtaining a grain size between 0-1 mm.
The sintering of dolomite at this temperature assures the preservation and use of sintered dolomite inside the reactor in the extreme reaction conditions.
In a first aspect of the invention the latter relates to a fluidized bed reactor for the conversion of biomass into gas in which the material of the base of the bed is sintered dolomite.
Preferably, the dolomite used as a starting material has more than 30% magnesium oxide and less than 1 % impurities.
Multiple advantages are obtained by using the dolomite-based product described in this invention. The hardness of the material upon being sintered assures the durability and also the heat transfer which is needed in these reactions. Due to its specific formulation it reacts catalytically and accelerates the dissociation of hydrocarbons. Another advantage is that it reduces the melting point of the ashes and binds them such that they can be easily separated from the inert material, preventing the agglomeration of the bed.
In the plant of the invention, a compressor introducing air through the lower part of the reactor and a compressor sucking in the fuel gas obtained in the reactor are included to control the pressure.
A by-pass regulated by a valve is installed in both compressors and the excess gas or air enters a gasification circuit.
A second option is to install in both compressors frequency changers which maintain the suitable pressure by means of the Hz increase or decrease.
The fluidized bed reactor is provided with a diffuser grate and with air outlets formed by injectors which allow a uniform fluidization; at the height of the grate there is a worm screw for the recovery of material for the purpose of analyzing the behavior of the inert material.
The grate has the function of support of the inert material and of the injectors. These injectors are designed to allow the exit of air and prevent the entrance of the inert material.
The injectors are steel cylinders provided with an inner orifice and with radial outlets. The injector has a cap closing the upper end of the cylinder, forcing the air to leave through the radial outlets, and has a concentric diffuser ring redirecting the air upwards.
The cylinder has a diameter suitable for being introduced by simple pressure in the grate.
The diffuser ring which projects from the perpendicular of the cap and which is an independent part can be filled with small stones which are mixed with the fuel and make the function thereof difficult. To that end, it is provided that the cylinder has the same diameter as the cap and that in the upper part it is machined with a conical shape to direct the air in the suitable direction.
The radial outlets in the steel cylinder will be at least four in number and will be arranged perpendicularly in orifices traversing the cylinder until finding the general orifice and separated horizontally and in parallel in the plane.
The orifices of the diffuser grate will have two different diameters, at the lower part the measurement calculated to allow the sufficient entrance of air, keeping the pressure equal in all of them, and at the upper part the diameter allowing the injector to enter under pressure for its securing.
Some biomasses contain a considerable amount of stones, even of several centimeters. Under these circumstances, which would drastically limit the operation time due to the blocking of the air outlets, a new grate with holes is superimposed.
Since there are holes throughout the new grate the inert material will pass therethrough and will be deposited around the injectors, having the supporting grate as a base.
The inert material which is deposited between the two grates is not detrimental to the fluidization at all, on the contrary it helps to maintain the injectors in a good condition and without wear, since no exothermic reactions occur between the two grates as biomass cannot enter either.
To remove the stones the plant has a worm screw for extracting stones on top of the grate.
Taking into account that this type of reactor cannot have exit or entrance of air or gases because it would affect the operation, two valves are provided so that they automatically open and close sequentially to prevent this problem.
As a result of the new material forming the bed, the ashes do not exit to the outside together with the gas, but rather they remain in the bed being bound to one another without affecting the particles of material and forming ash balls. To that end at least one hermetic valve is provided which opens or closes depending on the material inlet worm screw being activated. The material that leaves is sieved.
The fixed carbon resulting from the biomass which at that time reacts in the gasifier to produce gas after the volatiles and the water have been released in its decomposition remains in the upper part. This charcoal can be subsequently introduced in the gasifier.
The ashes in spherical form leave through the middle part.
The material used in the bed leaves in the lower part, which material is again introduced in the gasifier, by means of a worm screw, pneumatic transport or by depositing it in the feed itself of the biomass.
In order to achieve a gasification system that is as sustainable as possible and eliminate the production of contaminated water of the plant which generates an ecologically undesirable drawback, the plant has a cooler-condenser including tubes through the inside of which water is passed and through the outside of which the gas obtained is passed. The tubes are flanged for the purpose of obtaining a larger surface of contact with the gas.
When the gas releases the hydrocarbons between the flanges, it also deposits the excess of water carried by the fuel and which has not reacted. This water washes the flanges and entrains the hydrocarbon to the lower part.
Once the gas has left the dry filter, it is introduced through the lower part of the condenser. Although it is virtually free of particles, the efficiency of the filter is not total therefore the soot which has passed with the gas is adhered together with the condensable elements. This paste is gradually deposited in the flanges making their efficiency difficult.
Other groups of smooth tubes are added below the flanged tubes. The purpose is to maintain the flanges of the upper part cleaner and that they can perform their function in better conditions.
Indeed, when the gas that has left the filter condenses, a paste is formed between the liquid hydrocarbons and the solid soot of the gas and is adhered in the smooth tubes.
Due to their design, when the drops fall from the flanged tubes which are in the upper part, the smooth ones are cleaned since they have a surface without obstructions.
The efficiency of the exchanger as well as its maintenance is thus increased.
In order to have an easy option in the event of having to disassemble them, each exchanger is made up of modules, each formed by four pipelines so that they can be easily cleaned if necessary. Likewise these modules are screwed for the same reason.
The unburnt residues resulting from the gasification can have different alternatives:

o introducing them again in the same gasifier
o taking them to a combustion boiler
o treating them in the thermal cleaner

As has been mentioned, the plant has a system for the dry filtration of the gas obtained including a bag filter, a filter cleaning device, an ash extraction device and a thermal ash cleaner.
The ash extraction device is formed as a collecting worm screw for collecting ashes and unburnt residues located below the dry filter, it deposits them in a worm screw which is provided with several intermediate discharges, in each discharge there is a valve for facilitating the discharge in the provided outlet.
Each worm screw outlet is connected to an inlet belonging to a tank into which the ash falls.
This tank the base of which is made of refractory material serves as a storage for the slow oxidation of the ashes.
In order for this to occur, it requires the material to be in continuous movement so that the air entering through an orifice provided for that purpose and forced by the suction of a centrifugal fan causes the controlled thermal oxidation reactions, and to that end it has an arm moved by a geared motor in the lower central part of the tank. This arm rotates at a determined speed and in the direction marked by an inclined cut that it has along it. Thus, due to the rising movement that it forces the material to perform the air finds the surface for reacting.
In the lower part of the tank and at an end, there is an opening controlled by a valve so that when it is to be emptied, and once open, the arm itself feeds the emptying thereof to deposit it in the collecting worm screw.
For safety reasons the temperature must not exceed 300°C since gases are formed, and when the unburnt residues enter the tank, since they are so fine, they explode.
The gas generated by this process has some fly ashes incorporated therein, for this reason it is passed through a cyclone and, to increase the efficiency and quality of the air, also through a bag filter.

Description of the Drawings

The present specification is complemented with a set of drawings which illustrate the preferred embodiment but never limit the invention.

Figure 1 shows a diagram of the air injectors.
Figure 2 shows a diagram of the ash storage tank.
Figure 3 shows a diagram of the attachment of the reactor with the air injectors.

Preferred Embodiment of the Invention

In an embodiment the plant for converting solid waste into fuel gas of the invention consists of a fluidized bed reactor provided with a diffuser grate and with air outlets formed by injectors which allow a uniform fluidization; a compensated vacuum-pressure system which maintains a stable vacuum inside the gasification circuit; an air-conditioning system, the purpose of which is to cool the air that leaves the reactor; and a cleaning device for cleaning the gas obtained.
Figure 1 shows a diagram of the air injectors (1) in which it is observed that the size in the diameter (1.1) of the injector at the lower end decreases and that the radius of the cap (1.2), is equal to the radius of the body of the injector.
The gas obtained in the plant of the invention is filtered and the ashes obtained from filtering the gas are deposited in a container, which container (2) is shown in Figure 2. The tank has the following elements: an unburnt residue inlet (2.1), a suction gas outlet (2.2), and an air inlet (2.3). In order for the reaction to be carried out, the stirring arm (2.4) controlled by a geared motor (2.5) is necessary. Finally, the tank has an inspection opening (2.6), an indicator of the temperature and the ash outlet (2.7).
As is shown in Figure 3, to facilitate the disassembly of the body of the reactor (3) with the air inlet cone and in order for the diffuser grate with the injectors to be in the middle, it is provided that the lower part of the cylinder of the reactor has incorporated therein a flat bar (4).
This flat bar (4) will be welded throughout the reactor (3) but with the particularity that it will project through the two ends of the reactor (3).
The flat bar (4) in the inner part will project enough to serve as support for the insulation and refractory material from which the reactor (3) is manufactured. The flat bar (4) projecting through the outer part has through holes for attachment screws.
To prevent the leakage of air in the cone through the grate which will be seated on top of the cone (3), there is a gasket to act as a seal.
When the grate with holes containing the injectors is placed between the air inlet cone and the lower part of the cylinder of the reactor (3) it can cause important leakages of gas and destabilize the correct operation of the fluidized bed.
To prevent this drawback, a ceramic gasket of a sufficient size has been arranged, so that upon being tightened it serves as a seal between the metal parts.
Since the place in which the gasket is seated is smooth and receives a pressure force from the reactor precisely through where the air of the injectors leaves, such gasket can move and allow material to leave the bed.
It must be taken into account that there is a very high temperature and it causes expansions.
To solve this problem two concentric circles (5) in which the sealing bead (6) can be placed are added to the part of the upper plate of the grate.
These circles (5) will have a height lower than the bead and a larger width so that upon tightening it has space for expanding.
To complete this mission in the part of the reactor which must be superimposed on the gasket two concentric circles (7) are added which will coincide at the outside of those existing in the grate, thus creating a safe space to prevent the movement of the gasket and prevent exits of gases or entrances of air.
Independently of the air inlet incorporated in the grate, there is an independent inlet for when it is necessary to heat the bed for the first time and to place a burner.
As the power of the capacities of the reactors can vary it may be necessary for the design to incorporate four support legs for those having a smaller size or more for a larger size.
Since it is necessary to remove the lower part of the reactor which is where the grate of the injectors is located, it is provided that when the reactor has four support legs, two of them are sufficiently separated, although they are not symmetric, to allow the exit and entrance of the grate of injectors.
When the reactor has six legs, four are placed in the same way and the other two vertically in the direction of the exit of the grate, but with the particularity that the one that is between the two which maintain the distances to extract the grate can be disassembled. To that end there is provided in this support leg and at the height which allows the exit of the grate of injectors a cut with two flat bars with holes for the attachment thereof.
Variations in materials, shape, size and arrangement of the component elements, described in a non-limiting manner, do not alter the essence of this invention, the latter being enough for its reproduction by a skilled person.

Claims

Fluidized bed reactor for the conversion of biomass into gas, characterized in that the material of the base of the bed is sintered dolomite.
Reactor according to claim 1, characterized in that the sintered dolomite has a grain size comprised between 0-1 mm.
Fluidized bed reactor according to claim 1, characterized in that it has a diffuser grate and injectors inserted in the grate in the form of cylinders with an inner orifice and radial outlets, the injector has an upper cap and a concentric diffuser ring.
Reactor according to claim 3, characterized in that a second grate with holes is superimposed on the diffuser grate.
Reactor according to claim 3, characterized in that the reactor has a flat bar (4) for assembling the body of the reactor with an air inlet cone, and in order for the diffuser grate with the injectors to be in the middle, the flat bar (4) is welded to the reactor and projects through the two ends thereof.
Reactor according to claim 5, characterized by a gasket for preventing the leakage of air in the cone through the grate which will be seated on top of the cone.
Reactor according to claim 5, characterized in that two concentric circles (5) in which the sealing bead (6) can be placed are installed in the part of the upper plate of the grate, these circles (5) will have a height lower than the bead and a larger width, in the part of the reactor which must be superimposed on the gasket two concentric circles (7) are added which will coincide at the outside of those existing in the grate.
Reactor according to claim 3, characterized in that it has six legs, wherein two of them are placed vertically in the direction of the exit of the grate.
Plant for converting solid waste into gas including the reactor according to any of the previous claims, from among those including a compensated vacuum-pressure system which maintains a stable vacuum inside the gasification circuit; an air-conditioning system, the purpose of which is to cool the air that leaves the reactor; and a cleaning device, characterized in that the vacuum-pressure system has a compressor in the reactor inlet and a compressor which sucks in the fuel gas obtained in the reactor and wherein both compressors have a by-pass regulated by a valve and the excess gas or air enters a gasification circuit.
Plant according to claim 9, characterized in that frequency changers are installed in both compressors, which frequency changers maintain the suitable pressure by means of the Hz increase or decrease.
Plant for converting solid waste into gas according to claim 9, characterized in that the air-conditioning system has flanged tubes through the inside of which water is passed and through the outside of which the gas obtained is passed and a second group of smooth tubes to maintain the flanged ones cleaner.
Plant for converting solid waste into gas according to claim 9, characterized in that the cleaning device has a collecting worm screw for collecting ashes and unburnt residues located below a dry filter, which deposits them in a worm screw which is provided with several intermediate discharges, in each discharge there is a valve for facilitating the discharge in the provided outlet, each worm screw outlet is connected to an inlet belonging to a tank into which the ash falls, and it is stored for its oxidation.
Plant for converting solid waste into gas according to claim 11, characterized in that the ash storage tank has at least one air inlet, and an arm moved by a geared motor.