Source: {"pile_set_name": "USPTO Backgrounds"}

The present invention relates to an apparatus for gasifying or combusting solid, carbonaceous material in a circulating fluidized bed reactor. The circulating fluidized bed reactor comprises a separator which is disposed after the reaction chamber and which separates circulating bed material from gas. The separator for circulating material is provided with a gas outlet for discharging gas from said separator and with a duct for returning separated particles preferably to the lower part of the reaction chamber. A separator for fine particulates is also disposed in the gas flow from the separator.
In a circulating fluidized bed reactor, where the flow rate of gas is maintained at such a high level that a considerable portion of solid particles is discharged with gas from the reaction chamber and, after separation of particles, the major part of the solid particles is returned to the fluidized bed, the gasification or combustion of solid carbonaceous material has been recognized to have several advantages over conventional gasification or combustion methods.
Several different methods have been applied in the gasification of solid, carbonaceous fuel, the most important of them being gasifiers based on the fluidized bed concept as described above. The problem with all gasifiers, including (although to a lesser extent) fluidized bed gasifiers, is how to achieve a very high carbon conversion. This problem becomes more acute when fuels with low reactivity, such as coal, are to be gasified. It is also difficult to achieve a high carbon conversion with fuels having a small particle size, such as milled peat.
Poor carbon conversion is principally the result of the comparatively low reaction temperature of fluidized bed gasifiers, which is restricted by the melting temperature of the fuel ashes. Carbon conversion can be significantly improved by increasing the reaction time of the gasification, i.e., by returning the escaped, unreacted fuel to the reactor.
In a circulating fluidized bed gasifier or boiler, the rate of flow of the upwardly directed gas is so high that a substantial amount of solid bed material, entrained with product or flue gases, passes out of the reactor. Most of such outflowing bed material is separated from the gas by separators and returned to the reactor. The finest fraction, however, is discharged with the gas. Circulating material in the reactor comprises ashes, coke and other solid material, such as limestone, possibly introduced in the gasifier, which induces desired reactions such as sulfur capture.
However, separators such as cyclones, which are normally used, have a restricted capacity for separating small particles. Normally, hot cyclones can separate only particles up to the size of 50-100 microns, and finer fractions tend to escape with the gases. Since the unreacted fuel discharged from the reactor with the gas is mainly coke, from which the volatile (reactive) parts have already been discharged, it would, when returned to the reactor, require a longer retention time than the actual "fresh" fuel. However, because the grain size of the returned coke is very small, the returned fine fraction is immediately discharged again from the reaction chamber, and thus the reaction time remains too short and the carbon conversion undesirable low.
The grain size of the coke gradually becomes less during the process, thereby increasing the emission of particulate material from the cyclone, which results in a low carbon conversion.
Even though small coke particles can be separated from the gases with new ceramic filters, additional problems arise. Solid fuels always contain ashes which have to be removed from the system when pure gas is produced. When aiming at a carbon conversion as high as possible, ashes have to be removed so as to avoid discharging large amounts of unreacted carbon with the ashes. The particle size of the ashes, however, always varies within a wide range and fine ashes tend to fly out of the reactor with the fine coke residue.
In order to achieve a high carbon conversion, the following diverse criteria must be reconciled: (1) separation of fine particulates from the gases and return of such to the reactor must be possible, and (2) the carbon contained in the returned particulates has to be made to react, and the ashes have to be separated from the system.
Until now, attempts to reconcile these criteria have been unsuccessful.
It is also common in boiler plants, at fluidizing bed combustion, that unburned coal is easily entrained with the fly ash, especially if poorly reactive fuel is employed or if the boiler plant is under a low load or under an extremely heavy load. Fly ash may contain over 10% coal, sometimes even 20%, which lowers the efficiency of the boiler. It is known that returning the fly ash to the combustion chamber gives a lower carbon content in the fly ash, thus improving the efficiency of the boiler.
Fly ash itself is a problematic produce, however. For example, in the U.S.A., only 20% of the total amount of fly ash can be utilized in the building industry and construction of roads. Final storing causes problems to the power plants. Fly ash is a low density material which means that the residual fly ash requires quite a large storage area. This constitutes a problem in densely populated areas. Furthermore, one has to pay attention to storing of the ashes in such a manner that they do not come into contact with groundwater. Ammonia has recently been introduced into the purification of flue gases, and this has added to the fly ash problem. The fly ash treated with ammonia cannot be used in the concrete industry.
The combustion temperatures in the fluidized bed boilers are substantially lower than, for example, in pulverized combustors and the ash properties are quite different. Ashes produced by combustion at lower temperatures are not stable, but depending on the conditions, there may be gaseous, liquid or dusty emissions.
U.S. Pat. No. 4,315,758 discloses a method and apparatus for solving the problem with the fines recycling. According to this method, the finest particulates separated from the gas are conducted back to the lower part of the reactor while oxygen containing gas is introduced into the same place in the reactor, thereby forming a high temperature zone in which the recovered fine particulates agglomerate with the particles in the fluidized bed. This method introduces an improvement in the so-called "U-gas Process" method.
British Patent No. =GB 2,065,162 discloses a method and apparatus for feeding the fine material separated from gas to the upper part of the fluidized bed in which the fine particulates agglomerate with particles of the fluidized bed when oxygen containing gas is conducted to the same place in the reactor.
The problem with both of these methods is process control. Both methods aim at agglomeration of the separated fine material to the fluidized bed (featuring excellent heat and material transfer properties). It is of major importance that the main process itself can operate at an optimal temperature, and it is easily disturbed when the temperature needed for the agglomeration is not the same as that needed for the main process. Due to the good heat transfer that occurs in the fluidized bed, the temperatures tend to become balanced, which causes new problems. Gas different from the oxygen containing gas used in the actual gasification is needed because of the excess heat. Additionally, because the size of particles contained in the fluidized bed varies considerably, it is difficult to control the agglomeration in the reactor so that production of ash agglomerates of too large a size could be prevented. Ashes stick to large as well as small bed particles and ash agglomerates of too large a size are easily formed, which impede or prevent ash removal, and the gasifying process has consequently to be interrupted. Furthermore, agglomeration in the reactor itself causes local overheating, which in turn leads to abrasion of brickwork.
U.S. Pat. No. 3,847,566 discloses one solution in which high carbon conversion is sought by burning the fine material escaping from the gasifier in a separate combustion device. Coarser, carbonaceous material taken from the fluidized bed is heated with the heat released from combustion. This carbonaceous material is returned to the fluidized bed after the heating. This is how the heat required for the gasification is generated. The gases, flue gas and product gas, released from the combustion and gasification have to be removed from the system in two separate processes both including a separate gas purification system. As can be seen, the arrangement of this method requires quite complicated constructions and results in poorly controlled processes.
The problem with the above-mentioned methods resides in the difficult process conditions where agglomeration conditions have to be controlled. This calls for expensive materials and cooled constructions.
According to the invention, an apparatus for gasification or combustion, by means of which the highest possible carbon conversion is attained without the above-mentioned drawbacks in the process control and without complicated and expensive constructions, is provided. According to the invention, it is also possible to separate the finest carbonaceous particulates from the product or flue gas and return them to the reactor in such a form that the carbon contained in the particulates can be exploited and the ash be separated.
According to the invention, in a circulating fluidized bed reactor, agglomerating means are provided comprising an agglomerating and fluidizing chamber disposed in connection with the return duct for particles. The agglomerating chamber is in communication with a return duct for circulating particles discharged from the separator and with the lower part of the return duct, through which circulating particles are returned to the lower part of the fluidized bed reactor. The bottom of the chamber is provided with means (such as fluidizing gas nozzles) for feeding fluidizing gas to maintain a bubbling fluidized bed in the chamber. The bed material is comprised of circulating particles. The fluidizing gas can also be introduced into the reactor, for example, through a porous bottom plate.
The upper part of the agglomerating chamber is provided with a burner for particulates for heating and for at least partially combusting