EP0081624B1 - Method and apparatus for combustion of humid bituminous slates with low calorific power - Google Patents

Description

The present invention relates to a process for the combustion of wet bituminous shales with low calorific value in a combustion chamber, in which the shales are ground in the presence of a hot drying gas, the relatively large ground grains are screened in order to separate and recycle them. the coarsest fraction, then introduced in suspension in primary air into the combustion chamber.

It also extends to a device for implementing this method.

Oil shales of this kind have a calorific value and a composition of the order of:

They notably contain carbonates and silicates.

The combustion of oil shales has already been carried out in mechanical grate boilers. Such boilers are only acceptable for the combustion of relatively small hourly tonnages.

One could think of transposing to the combustion of such shales processes used for the combustion of poor quality coals or lignites. However, the large proportion of abrasive ashes, rich in silica, contained in these shales would lead to very high energy consumption in the mills and to excessive wear thereof.

In addition, because of their low calorific value, they require a very large gas flow rate to ensure their drying and their transport after their grinding. Air cannot be used for this purpose, which would then be used as oxidant in the combustion chamber, since the combustion would be of poor quality and incomplete.

The object of the present invention is to remedy these difficulties, and to provide a process which ensures the grinding of wet bituminous shales with relatively low energy consumption and wear of the grinders, and easy separation and recovery of the most of the grinding product, as well as satisfactory combustion in the combustion chamber.

The process according to the invention is characterized in that after grinding the hot gas and the ground grains are introduced into an expansion chamber from which the finest grains are entrained by the drying gas towards filtration means, then combined with the relatively larger grains downstream of their screening, and in that the drying gas consists of the mixture, in proportions dosed as a function of the humidity level of the shales, of hot gases sampled in the combustion chamber and cooled flue gases taken off the combustion chamber to the atmosphere.

• - Le broyage est effectué par simple attrition des grains les uns contre les autres.
• - Le broyage n'est effectué que jusqu'à l'obtention d'une grosseur moyenne de grains comprise entre environ 0,1 et 1 mm.
• - Les grains relativement plus gros sont criblés de façon à n'admettre à l'alimentation de la chambre de combustion que des particules de grosseur inférieure à 0,8 mm.
• - Les particules broyées et criblées sont introduites dans la chambre de combustion en forte concentration dans un débit d'air primaire relativement faible par rapport à une injection en parallèle d'air secondaire.

It also preferably meets at least one of the following characteristics:

• - The grinding is carried out by simple attrition of the grains against each other.
• - Grinding is only carried out until an average grain size of between approximately 0.1 and 1 mm is obtained.
• - The relatively larger grains are screened so as to admit to the combustion chamber supply only particles of size less than 0.8 mm.
• - The ground and screened particles are introduced into the combustion chamber in high concentration in a relatively low primary air flow compared to a parallel injection of secondary air.

The device for implementing the method of the invention, comprising means for grinding the wet bituminous shales in the presence of a hot drying gas, means for screening the relatively larger crushed grains in order to separate the coarsest fraction thereof. , means for introducing the grains screened into a combustion chamber suspended in primary air, is characterized in that it further comprises an expansion chamber downstream of the grinding means, means for driving the finest grains separated in the expansion chamber towards filtration means, means for joining the finest grains stopped by the filtration means with relatively larger grains downstream of the screening of the latter, and means for mixing proportions proportioned as a function of the humidity level of the shales to be burned, of hot gases sampled in the combustion chamber and of cooled fumes sampled at the evacuation of the combustion chamber towards the mosphere, to form said hot drying gas.

• - les moyens de broyage sont constitués par des broyeurs autogènes
• - les moyens de filtration des grains les plus fins sont constitués par des filtres à manches
• - les moyens de criblage sont constitués par des tamis vibrants
• - la chambre de combustion est un foyer à double voûte.

This device more preferably corresponds to at least one of the following characteristics:

• - the grinding means are constituted by autogenous grinders
• - the finest grain filtration means consist of bag filters
• - the screening means consist of vibrating screens
• - the combustion chamber is a double vault fireplace.

It is described below, by way of example and with reference to the figure of the accompanying drawing, a device for grinding, transporting and burning wet bituminous shales with low calorific value according to the method of the invention.

The wet bituminous shales to be treated, in blocks of average size 0 to 300 mm, have a higher calorific value of 1000 Cal / Kg, and water contents of approximately 15% by weight, in organic matter of approximately 20% by weight (of which approximately 80% of volatile matter), and in ash of approximately 65% by weight. From the feed hopper 1, they are brought by the belt 2 to the feed box 3. The latter is supplied with hot drying gas, consisting of a mixture of combustion gases taken off at around 900 ° C. in the the combustion chamber and recycled through line 4, and smoke at about 160 ° C taken from the bottom of the boiler chimney, arriving through line 5, in proportions adjusted so as to obtain a drying gas at 650 ° C. about. The blocks to be treated are introduced with the drying gas into a highly shielded autogenous crusher 6, formed by a relatively flat cylinder carried by two bearings rotating at a speed of about 12 to 20 revolutions / minute and driven by the motor 7.

The blocks crush themselves by mutual shocks inside the mill, while drying. The roughly ground product leaves through the evacuation pin 7A and enters the expansion chamber 8. If the shales to be treated had a relatively high hardness, or if it was desired to obtain a product in fairly fine grains, it could be added to the grinder 6 of the balls. It would then be necessary to feed it in blocks of smaller dimensions (0 to 25 mm for example).

In the expansion chamber 8, the finest particles, formed by grains of average size less than 100 microns, and constituting approximately 15% of the treated product, are entrained by the drying gas in the vertical pipe 9, which brings them to a set of bag filters 10. These particles are stopped by the filters, while the drying gas sucked in by the fan 11 is discharged into the atmosphere by the piping 12. The separated particles are returned by conduits such as 13 to the feed hoppers to the combustion chamber burners, where they are joined to the larger screened grains, as will be described later.

The largest grains gathered in the bottom of the expansion chamber 8 are brought by the bucket elevator 14 to a main distributor 15, then to the secondary distributors 16A and 16B, from where they fall into screens such as 17A, 17B. These consist of vibrating sieves of 0.8 mm mesh. The larger grains form a refusal which is returned by the inclined conduits 18A and 18B, then 19, to the feed box 3 of the mill, to be reprocessed there.

The screened grains of dimension less than 0.8 mm are brought by conveyors 20A, 20B, then a distributor belt 21, to a hopper 21A for supplying the combustion chamber. The distribution belt is also supplied with fine particles separated in the bag filters, by conduits such as 13.

From the hopper 21A, the grains are transported at a flow rate dosed by the conduits such as 22, then entrained by air in airfoils with a slight slope (10 ° to 15 °) such as 23, to the supply of the burners. They are suspended, analogously to what is already done for other granular fuels, in primary air, but at a relatively high concentration, of the order of one kg of shale grains. for 0.5 kg of air, to facilitate their ignition.

They are then blown into the combustion chamber 24 with a double arch (this introduction being represented diagrammatically by the orifices such as 25A). In the usual way, secondary air arriving through ducts such as 26A is also blown into the burners around the grains in suspension in the primary air, the proportion of the latter in the total combustion air being approximately 40%.

A supply station (not shown) similar to that which has been described above, also supplies a set of burners disposed on the other side of the combustion chamber, represented by the orifices such as 25B.

In the combustion chamber 24, the flames follow U-shaped paths such as 27, 28 before rising in the upper part (arrow 29) towards the water circulation tubes to be heated and vaporized. A fraction of the hot gases from the combustion chamber, at around 900 ° C., is withdrawn therefrom via line 4, and combined with fumes taken from the bottom of the exhaust chimney to the atmosphere, arriving via the conduit 5, as indicated above.

In the usual way in the combustion chambers, the bottom ash is collected at the bottom thereof on a grid, cooled and evacuated. The soot entrained by the combustion gases is captured by natural or electrostatic filtration means and also evacuated.

Although the device which has just been described with reference to the figure appears to be the preferable embodiment of the invention, it will be understood that various modifications can be made to it without departing from its scope. In particular, the bag filters for recovering dust from grinding can if necessary be replaced by cyclone separators, the performance of which is however less good. We could then if necessary reinject into the combustion chamber the gas charged with residual dust.

Claims (11)

1. A method of burning humid bituminous slates with low calorific power in a combustion chamber (24), in which the slates are crushed (6) in the presence of a hot drying gas, the relatively large crushed grains are screened (17A, 17B) in order to separate and recycle (19) the largest size fraction, then introduced in suspension in primary air (25A, 25B) into the combustion chamber, characterized in that after the crushing, the hot gas and the crushed grains are introduced into an expansion chamber (8) from where the finest grains are transferred by the drying gas (9) towards filter means (10), then reunited (13) downstream of their screening with the relatively larger size grains, and in that the drying gas is constituted by the mixture, in dosed proportions according to the humidity rate of the slates, of hot gases withdrawn from the combustion chamber (4) and cooled smokes withdrawn from the evacuation duct between the combustion chamber and the atmosphere (5).

2. A method according to claim 1, characterized in that the crushing is obtained by simple attrition of the grains against one another.

3. A method according to claims 1 or 2, characterized in that the crushing is only carried out until the average grain size lies between 0,1 and 1 mm.

4. A method according to claim 3, characterized in that the relatively larger grains are screened so as to admit for the supply of the combustion chamber only particles of a size less than 0,8 mm.

5. A method according to one of claims 1 to 4, characterized in that the crushed and screened particles are introduced into the combustion chamber at a high concentration rate in a primary air flow the rate of which is relatively small with respect to an injection in parallel of secondary air.

6. A method according to claim 5, characterized in that the combustion is carried out by making the gas, which is charged with grains, follow a U-shaped trajectory (26, 27) during combustion.

7. A device for carrying out the method according to claim 1, comprising crushing means (6) for the humid bituminous slates in the presence of a hot drying gas, screening means (17A, 17B) for those crushed grains which are relatively larger in size in order to separate from them the largest size fraction, means for introducing screened grains into a combustion chamber in suspension in primary air (25A, 25B), characterized in that it further comprises an expansion chamber (8) downstream of the crushing means, means (9) for dragging the finest grains separated in the expansion chamber towards the filter means (10), means (13) for reuniting the finest grains stopped by the filter means, with the relatively large size grains downstream of the screening of the latters, and means (4, 5) for mixing, in proportions dosed with respect to the humidity rate of the slates to be burned, hot gases taken from the combustion chamber and cooled smoke taken at the evacuation of the combustion chamber towards the atmosphere, for forming said hot drying gas.

8. A device according to claim 8, characterized in that the crushing means are constituted by autogenous crushers.

9. A device according to claims 7 or 8, characterized in that the filtering means of the finest grains are sleeve filters.

10. A device according to one of the claims 7 to 9, characterized in that the screening means are constituted by vibrating screens.

11. A device according to one of the claims 7 to 10, characterized in that the combustion chamber (24) is a double arched hearth.

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