Liquid heaters and vaporizers – Circulation – Once through
Reexamination Certificate
2001-12-21
2003-03-25
Wilson, Gregory A. (Department: 3749)
Liquid heaters and vaporizers
Circulation
Once through
C122S00600B, C122S406300
Reexamination Certificate
active
06536380
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to a steam generator with a nitrogen removal device for fuel gas and with a combustion chamber for fossil fuel which is followed on the fuel-gas side, via a horizontal gas flue and a vertical gas flue, by the nitrogen removal device for fuel gas.
BACKGROUND OF THE INVENTION
In a power plant with a steam generator, the fuel gas generated during the combustion of a fossil fuel is used for the evaporation of a flow medium in the steam generator. For the evaporation of the flow medium, the steam generator has evaporator tubes, of which the heating by fuel gas leads to an evaporation of the flow medium carried in them. The steam provided by the steam generator may, in turn, be provided, for example, for a connected external process, or for driving a steam turbine. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
The steam generator may in this case be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper “Verdampferkonzepte für Benson-Dampferzeuger” [“Evaporator concepts for Benson Steam Generators”] by J. Franke, W. Köhler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes, provided as evaporator tubes, leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Steam generators are usually designed with a combustion chamber in a vertical form of construction. This means that the combustion chamber is designed for the heating medium or fuel gas to flow through in an approximately vertical direction. In this case, the combustion chamber may be followed, on the fuel-gas side, by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal flow direction taking place at the transition from the combustion chamber into the horizontal gas flue. In general, however, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the production and assembly of the steam generator, this outlay being higher, the greater the overall height of the steam generator is.
A particular problem is the design of the containment wall of the gas flue or combustion chamber of the steam generator with regard to the tube-wall or material temperatures which occur there. In the subcritical pressure range to about 200 bar, the temperature of the containment wall of the combustion chamber may be determined by the height of the saturation temperature of the water. This is achieved, for example, using evaporator tubes which have a surface structure on their inside. Consideration is given, in this respect, to internally ribbed evaporator tubes, of which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
To reduce the nitrogen oxides in the fuel gas of the fossil fuel, the method of selective catalytic reduction, which is known as the SCR method, may be used. In the SCR method, nitrogen oxides (NO
x
) are reduced to nitrogen (N
2
) and water (H
2
O) with the aid of a reducing agent, for example ammonia, and a catalyst.
In a steam generator designed for an SCR method, a nitrogen removal device for fuel gas, with a catalyst, is conventionally arranged downstream of the fuel-gas duct, which is designed as a convection flue and where the fuel gas normally has a temperature of about 320 to 400° C. The catalyst of the nitrogen removal device for fuel gas serves to initiate and/or maintain a reaction between the reducing agent introduced in the fuel gas and the nitrogen oxides of the fuel gas. The reducing agent required for the SCR method is in this case usually injected, together with air as a carrier stream, into the fuel gas flowing through the gas flue. However, as a rule, the nitrogen oxide emission of the steam generator depends on the type of fossil fuel burnt. Therefore, in order to adhere to the legally prescribed limit values, the reducing agent quantity to be injected is normally varied as a function of the fossil fuel used.
However, a nitrogen removal device for fuel gas, arranged downstream of the convection flue on the outlet side, requires a considerable outlay in structural and production terms for the respective steam generator. This is because the nitrogen removal device has to be arranged in the steam generator in a place where it can exert a particularly high purifying effect on the fuel gas in all the operating states of the steam generator. This is normally the case where the fuel gas has a temperature in the range of about 320 to 400° C. Moreover, the outlay in terms of the production of a steam generator increases when the latter has, as well as conventional components, a nitrogen removal device in addition.
SUMMARY OF THE INVENTION
An object on which the invention is based is, therefore, to specify a fossil-fired steam generator of the abovementioned type, which requires a particularly low outlay in structural and production terms and in which a purification of the fuel gas of the fossil fuel is ensured particularly reliably, before these leave the steam generator on the outlet side.
This object is achieved, according to the invention, in that the combustion chamber of the steam generator includes a number of burners arranged level with the horizontal gas flue, the vertical gas flue being designed for an approximately vertical flow of the flue gas from the bottom upward and the nitrogen removal device for fuel gas being designed for an approximately vertical flow of the fuel gas from the top downward.
The invention proceeds from the notion that a steam generator capable of being erected at a particularly low outlay in production and assembly terms should have a suspension structure capable of being produced in a simple manner. A framework, to be erected at a comparatively low technical outlay, for the suspension of the combustion chamber may at the same time be accompanied by a particularly low overall height of the steam generator. A particularly low overall height of the steam generator can be achieved by the combustion chamber being designed in a horizontal form of construction. For this purpose, the burners are arranged, level with the horizontal gas flue, in the combustion chamber wall. Thus, when the steam generator is in operation, the flue gas flows through the combustion chamber in an approximately horizontal direction.
For a particularly reliable purification of the fuel gas of the fossil fuel, the nitrogen removal device for fuel gas may be arranged downstream of the vertical gas flue on the outlet side. To be precise, downstream of the vertical gas flue on the outlet side, the fuel gas has temperatures at which a purification of the fuel gas takes place particularly effectively at a low technical outlay. It must be remembered, in this case, that, for a particularly low overall height of the steam generator, the nitrogen removal device for fuel gas may be designed for an approximately vertical flow of the fuel gas from the top downward. It is thereby possible for the liquid necessary in the SCR method, together with ammonia fractions, to be injected in the main flow direction of the fuel gas, with the result that the nitrogen removal device has a particularly small vertical extent.
However, in a steam generator with a combustion chamber, through which fuel gas can flow in an approximately horizontal main flow direction, the fuel gases, after leaving the horizontal gas flue, flow downward in the vertical g
Kral Rudolf
Pulec Josef
Harness Dickey & Pierce PLC
Siemens Aktiengesellschaft
Wilson Gregory A.
LandOfFree
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