Furnaces – Combined – With boiler
Reexamination Certificate
1998-02-17
2001-01-30
Lazarus, Ira S. (Department: 3743)
Furnaces
Combined
With boiler
C110S238000, C110S313000, C110S336000, C122S00600B, C422S185000
Reexamination Certificate
active
06178898
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a recovery boiler for burning spent liquor of a pulp mill, the recovery boiler comprising a combustion chamber surrounded by heat transfer surfaces constituted by walls formed of tubes, into which combustion chamber the spent liquor to be burned and combustion air are fed, the upper part of the combustion chamber comprising superheaters for recovering the heat generated by the combustion and for cooling flue gases.
The spent liquor of a pulp mill is fed into a recovery boiler to be burned therein so that, firstly, heat energy and, secondly, chemicals are recovered therefrom, which can be recycled to the process of the pulp mill. The heat energy generated at the combustion in the combustion chamber is recovered by utilizing heat transfer surfaces, in which is circulating either water or steam, depending on the location Part of the heat transfer surfaces are tight walls formed of tubes by welding and surrounding the combustion chamber and part of the heat transfer surfaces are different kinds of heat recovery units formed of tubes, which units are known as superheaters and are positioned as screen tubes in the upper part of the recovery boiler, in the upper part of the very combustion chamber, and from there forward in a flue gas channel and possibly across the combustion chamber. The different heat transfer surfaces are mutually connected to a water and steam circulation in such a way that the cold water to be fed into the recovery boiler warms up and then vaporizes so that high-pressure steam superheated in the heat transfer surfaces serving as superheaters is generated for suitable purposes.
For making the recovery boiler corrosion-resistant enough and, on the other hand, for recovering the heat energy of the flue gases as well as possible, the flue gases shall be cooled to a sufficiently low temperature. The present recovery boilers are planned according to the above principle.
BACKGROUND OF THE INVENTION
For the operation of the recovery boiler, it is very important that gaseous bodies in the combustion chamber, such as the fed combustion air and the fuel, can be mixed and the fuel can be burned as well as possible so that combustion reactions are mainly over before the flue gases arrive at the upper part of the combustion chamber, between densely located heat transfer surfaces, i.e. superheaters. The fact is that the mixing of gases is prevented or retarded between densely located heat transfer surfaces and the vicinity of surfaces colder than gases stops or retards reactions, though the temperature of the gases otherwise were sufficient for the implementation of the reactions. On account of this, a predetermined retention time in the combustion chamber is necessary, within which time the reacting substances can be evenly mixed. This retention time can generally be caused by increasing the height of the combustion chamber. A problem with big recovery boilers is that even mixing is insufficient within the whole cross-section of the combustion chamber, and it is difficult to adjust the operation conditions of the recovery boiler and the feed of various substances and gases needed for the process. A further problem is that if the flow rate of the gases in the boiler is high, fuel drops try to follow the gas flow and, accordingly, the combustion increases in the upper part of the boiler and causes both corrosion and scaling of the heat transfer surfaces.
With growing effect of the boiler and with accordingly growing size of the combustion chamber, it is necessary to increase the number of cooling surfaces, because the flue gases have to be cooled to a predetermined temperature. From this follows that the bottom area of the boiler and the volume of the combustion chamber increase remarkably when the height grows.
Moreover, the growing cross-section and the accordingly growing height of boilers require higher buildings, and so the construction costs increase in this respect considerably. In addition, because the recovery boiler is suspended from its brackets, a big and heavy char bed on the bottom of the boiler causes a remarkable additional weight load, which still complicates the dimensioning and constructing of the boiler. Further, it is inconvenient and complicated to feed a sufficient amount of air in such a way that the combustion conditions can be made as efficient as possible.
BRIEF SUMMARY OF THE INVENTION
The object of this invention is to provide a recovery boiler, which is easier to control as the known recovery boiler construction and which makes the recovery of heat energy more efficient, so that it is not necessary to increase the height of the boiler significantly in spite of growing effect. The recovery boiler according to the invention is characterised in that the combustion chamber comprises at least one vertical heat transfer surface formed of tubes, by means of which surface heat generated by combustion is recovered, and which surface divides the combustion chamber in its transverse direction, so that the breadth of the combustion space at said heat transfer surface between this surface and the wall of the combustion chamber is smaller than the distance between the walls of the combustion chamber of the recovery boiler in the same direction.
The essential idea of the invention is that heat transfer surfaces mounted in the combustion chamber of a recovery boiler divide the combustion chamber into combustion spaces having a breadth smaller than the distance between the walls of the boiler, into which combustion spaces air and fuel can easily be fed in a controlled manner and in which spaces the substances participating in the process can be mixed and thus burned sufficiently well and efficiently in the lower part of the recovery boiler already, before the gases arrive at the densely located heat transfer surfaces, such as superheaters. An advantage of the invention is that, the combustion spaces being narrower than the whole breadth of the combustion chamber of the recovery boiler, both the feed of fuel, i.e. spent liquor, and the air feed and phasing of the air feed are easier to carry out in such a way that the substances participating in the combustion process can be well intermixed. By means of these heat transfer surfaces, the heat generated at the first end of the combustion process is also easy to recover, whereby the gases cool faster and the retention time in the boiler can be kept shorter than in a boiler constituted by a combustion chamber having an integral cross-section. Moreover, the cooling medium of these heat transfer surfaces can be kept as an entirely separate circulation, if desired, which makes it possible to use other than water as cooling medium, though it is also possible to connect the surfaces to the rest of the water circulation of the heat transfer surfaces.
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Ruohola Tuomo
Salmela Matti
Ciric Ljiljana V.
Kvaerner Pulping Oy
Lazarus Ira S.
Rockey Milnamow & Katz Ltd.
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