Furnaces – Process – Burning pulverized fuel
Reexamination Certificate
1998-03-24
2001-03-20
Lazarus, Ira S. (Department: 3743)
Furnaces
Process
Burning pulverized fuel
C110S348000, C110S264000, C110S265000, C110S211000
Reexamination Certificate
active
06202578
ABSTRACT:
The object of the invention is a method for processing, particularly for flame combustion, substances having a wide particle size distribution, in which method the fuel is blown, with the aid of a current of air, tangentially into a swirl chamber containing a burning mass thus creating a vortex, from the centre of which the flow of substances is led out of the swirl chamber. The invention is also concerned with a reactor for carrying out the method. Though the substances to be burned are generally granular solids, a low-grade liquid fuel may also be used. The reactor is usually part of the burner, but simple carburation may also be involved, for example in an industrial process.
Traditionally, the flame combustion of pulverized materials has been based on grinding the substance to be burned into a fine-particle dust, which is burned in a flame formed outside the actual burner. The greatest delay permitted by flame combustion, i.e. the time, in which a particle must be able to burn completely, is clearly less than 1 second, and is usually only a few tenths of a second. To stabilize the flame and maintain a reasonable length of flame, the substance to be burned must be ground to a size of less than 0.1 mm. Grinding has taken place in a separate mechanical mill. It is much more problematic to grind organic fuels to a particle size suitable for flame combustion than it is to grind coal, for example. Grinding organic fuels to suit dust combustion consumes 1-5 MJ/kg of energy, which is much too great to be economical. Milled peat must also be ground before it becomes suitable for flame combustion. Especially in small units, dust combustion that requires effective grinding has been economically uncompetitive, while in such units it has been also necessary to apply fluid bed combustion, which is quite expensive. It is even possible to say that the lack of a cheap solution to the combustion technique has been one factor weakening the economic competitiveness of peat and organic fuels.
A problem in dust combustion is the shortness of the reaction time of the particles, due to which the material to be burned must be very finely ground. Stability conditions in dust burners are strict, due to the very small mass of the ignition zone. Stable combustion also requires that the dust to be burned be adequately and evenly dry. For reasons of safety, dust burners must usually be safeguarded with a support flame using oil or gas. A dust combustion system thus requires the drying and grinding of the material to be burned, as well as stabilization burners and the dust burners proper. In small units, a system of this kind is not economically competitive.
One known combustion method that has also previously been quite widely applied in practice is so-called melt cyclone combustion. In cyclone burners, all the combustion air is brought to the cyclone and these generally operate at such a high temperature that the ash forming in the cyclone is removed in a molten state. Problems with cyclone burners have included the control of the temperature. Too low a temperature has led to the uncontrolled accumulation of a layer of slag on the walls of the cyclone, while at a high temperature the life of the protective lining of the cyclone has been too short. Due to the high temperature, emissions of nitrogenous gases from the cyclone burners have also been large and have exceeded permitted emission limits. For these reasons, cyclone combustion is scarcely ever used now.
Solutions to these costs and technical problems in dust and cyclone combustion have been sought in fluid-bed technology, among other things. Unfortunately, however, despite the development of fluid-bed technology, it seems that its economic competitiveness will remain poor in small units. Fluid-bed technology is divided into the so-called bubbling fluid bed (BFB) and circulating mass fluid bed (CFB) techniques. In the latter, a large flow of solids travels through the combustion chamber, and is separated in the cyclone and then returned to the lower section of the vertical combustion chamber. CFB combustion requires complicated equipment, which includes an air division chamber with a nozzle base, a vertical reaction chamber, a cyclone, and solids return equipment. CFB combustion is used to create a selective delay of coarse particles. In CFB combustion, the delay of the particles is primarily determined by the conditions in the vertical chamber (riser). The essential difference between CFB combustion and traditional cyclone combustion is in the quantity of solids stored in the system. In CFB combustion, the quantity of solids is much greater than in cyclone combustion, and almost without exception other solids besides the fuel are used in CFB combustion, these forming the greater part of the solids in the system. The large amount of solids is intended to improve the stability of the system and in some applications to reduce emissions too. CFB combustion will clearly not operate above the sintering temperatures of ash. In cyclone combustion, the quantity of solids is very small and the burners generally operate in the molten ash range. Both combustion techniques have obvious advantages and certain similarities. The question naturally arises whether it would be possible to combine the advantages of the simplicity of the equipment of cyclone combustion with the process-technical advantages of CFB combustion. This invention concerns a method in which the principal advantages of a CFB reactor are achieved in a simple swirl chamber. Low-grade liquid fuels are also a problem in combustion techniques. This invention is intended to solve these problems in the known technology.
Using the method according to the invention, it is possible to burn coarse particle solid substances and low-grade liquid fuels efficiently and at low cost. In the method according to the invention, a swirl chamber is used for the simultaneous chemical and physical processing of the material being processed. Later, the name Chemi Mechanical Reactor (CMR), which depicts its operation, will be used for the invention, which operates as follows. Coarse-particle fuel, together with a gas containing oxygen, is fed to the CMR. The ratio of oxygen and fuel is regulated, generally to a clearly sub-stoichiometric level, so that the temperature of the cylindrical jacket of the CMR settles to a most suitable level of 450-650° C. Air is most typically used as the gas containing oxygen, in which case the amount of the flow of air to be directed to the CMR is, for a dry fuel, 30-50% of the amount of air required for complete combustion. In a quite restricted area in the centre of the reactor, the temperature becomes much higher than in the cylindrical jacket, but this is not a problem. The oxidizing gas is brought to the CMR tangentially, so that a vortex is created in the CMR, preventing the coarse particles from escaping from the CMR. Because of this, solid material collects in the CMR and circulates as a band along the jacket of the CMR. Due to the internal turbulence in the CMR, the particles made friable by the temperature collide with one another and with the walls of the CMR and are thus ground into fine particles. In addition, pyrolysis and evaporation contribute to this phenomenon. When the particles go below a limit size that depends on their physical state, they leave the CMR. The vortex in the CMR is adjusted so that it permits the passage of particles whose reaction time in flame combustion is sufficiently small. In the CMR, the delay of large particles becomes great, whereas fine particulate material leaves the CMR quickly. A selective delay is obviously advantageous to combustion.
From the above, the following advantages are achieved in the CMR:
1. Due to the full adequate circulation of solids and to sub-stoichiometry, the temperature can be easily and precisely regulated.
2. The vortex of solids formed inside the CMR evens out circumferential differences in temperature and protect the structure from overheating.
3. The consumption of energy required for the grinding of the combust
Miettinen Markku
Oksanen Mauno
Ruottu Seppo
Ciric Ljiljana V.
Kilpatrick & Stockton
Lazarus Ira S.
Vapo Oy
LandOfFree
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