Power plants – Combustion products used as motive fluid – With addition of steam and/or water
Reexamination Certificate
2000-04-14
2001-05-15
Thorpe, Timothy S. (Department: 3746)
Power plants
Combustion products used as motive fluid
With addition of steam and/or water
Reexamination Certificate
active
06230482
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and an appliance for operating a gas turbine installation combustion chamber with liquid fuel.
BACKGROUND OF THE INVENTION
In newer gas turbine installation combustion chambers, weak premixing burners are employed which exhibit particularly low pollutant emissions when operated with gaseous fuels, such as natural gas or methane. Such a burner, designated a double-cone burner, is known from EP-B1 03 21 809. This double-cone burner for operating with either gaseous or liquid fuels has separate fuel lines for the two types of fuel. An atomizing nozzle is also necessary for the liquid fuel. Because it is designed for both types of fuel, the double-cone burner is therefore of larger configuration and is equipped with more components than would be necessary for operation with only one type of fuel.
In the previously known methods and appliances for supplying fuel to a combustion chamber operated with liquid fuels, such as diesel oil or extra-light heating oil, the evaporation of the fuel and the mixing of fuel vapor and combustion air takes place within the burners or the combustion chamber. The evaporating fuel droplets lead to low temperatures on the droplet and in the vicinity of droplets. In addition, inhomogeneous temperature and concentration fields occur in the combustion space because of the evaporating fuel. A wide distribution, which is difficult to control, of the local combustion temperatures and air ratios therefore occurs in the combustion space and this cannot be prevented even with adjustment to average stoichiometry of the combustion pairs. Because, however, the formation of oxides of nitrogen takes place very rapidly at high temperatures and, in contrast, the decomposition reactions of oxides of nitrogen are very much slower, inhomogeneous temperature and concentration fields lead to an increased formation of oxides of nitrogen. Summarizing, it can therefore be stated that the combustion of liquid fuels often results in NO
X
figures that are not satisfactorily low because parts of the evaporation and mixing processes take place in parallel, both spatially and in time.
In order to correct this disadvantage, tubes of small diameter are fitted as separate oil evaporation elements in the exhaust gas path of an engine, as described in Förster, S.: “Umweltfreundlicher Öldampf-Motor (Environmentally-friendly oil vapor engine)”, in “Berichte des Forschungszentrums Jülich (Reports of the Jülich Research Center)”, No. 2564, ISSN 0366-0885, pp. 1-4. This solution, however, requires very large additions of water (in the range of between 2 and 5 kg of water per kg of oil) in order to prevent deposits on the inside walls of the tubes and therefore blockage of them. Because of the high level of water addition and the associated efficiency losses, however, it is not possible to use such evaporation elements for the combustion of liquid fuels in gas turbine installations.
SUMMARY OF THE INVENTION
The invention attempts to avoid all these disadvantages. Its object is based on producing a method for operating a gas turbine installation combustion chamber with liquid fuel, by means of which the NO
X
figures obtained are similar to those for gaseous fuels even if cheap liquid fuel is employed. The size of the burners is then also reduced, despite the possibility of employing liquid and gaseous fuels as alternatives. It is also an object of the invention to provide a corresponding appliance for carrying out the method.
This is achieved in accordance with the invention in that, the liquid fuel is evaporated in at least two steps in a separate evaporative reactor located upstream of the combustion chamber. In a first step, the liquid fuel is atomized to a fuel vapor/liquid fuel mixture by direct heat exchange with a first heat exchange medium and, in this process, a part of the liquid fuel is instantaneously evaporated. The second step contains the evaporation of the remaining residue of the liquid fuel by indirect heat exchange with a second heat exchange medium.
Because of its evaporation in the evaporative reactor provided upstream, the original liquid fuel is already gaseous on entering the burners or the combustion chamber and it is therefore possible to dispense with the necessity for fuel evaporation at this location. On entry to the burners, therefore, it is only necessary to effect the mixing of the fuel with the combustion air and, for this reason, the actual combustion of the premixed fuel vapor/air mixture in the combustion chamber can take place substantially more rapidly and with a smaller energy requirement. In addition, the excess air number during the combustion of the prepared liquid fuel vapor can be varied within a range of between 2 and 3.5 because of the high flame extinction limit. In consequence, combustion takes place at a high level of excess air so that the whole of the liquid fuel is burnt. In this way, low pollutant emissions similar to those achieved in the combustion of natural gas can also be achieved when liquid fuels, such as extra-light heating oil or other low boiling point fractions of mineral oil, are used.
It is particularly advantageous to adjust to a temperature equal to or greater than 250° C. during the indirect heat exchange and also to add oxygen. In this way, thermal cracking of the liquid fuel into lighter hydrocarbon fractions and a conversion of saturated into unsaturated hydrocarbons takes place in addition to the evaporation. Furthermore, the indirect heat exchange to the remaining residue of the liquid fuel can take place in the presence of a catalyzer, using, in particular, a catalyzer based on nickel. For this purpose, the displacement unit of the evaporative reactor either possesses a catalytically effective surface layer or is manufactured completely from a catalytically effective material. The evaporation and the cracking of the liquid fuel are therefore supported by catalytic partial cracking.
Because the evaporation of the liquid fuel, the cracking of the hydrocarbons and also the conversion of saturated to unsaturated hydrocarbons are endo-thermic reactions, the energy required for this purpose is extracted from the second heat exchange medium. This energy is released again in the subsequent combustion in the combustion chamber and the result is therefore an improved gas turbine efficiency. In addition, the reaction rate of the fuel vapor/air mixture is increased because of the cracked and unsaturated hydro-carbons and therefore permits, as compared with natural gas, a wider operating range extending to higher excess air ratios.
It is particularly useful for the oxygen to be introduced into the first heat exchange medium in the form of previously compressed, hot compressor air even before the direct heat exchange, i.e. upstream of the evaporative reactor. The air quantity employed is up to 10%, preferably between 1 and 5%, however, of the quantity required for a stoichiometric combustion reaction. The addition of such a relatively small quantity of air leads to exothermic partial reactions in the fuel vapor/liquid fuel mixture so that its temperature level can be increased by additional heat release. Because of the supply of air which has already taken place upstream of the evaporative reactor, there is a uniform air distribution so that the cracking reactions also take place uniformly in the evaporative reactor.
The quantity of air supplied can be controlled relatively rapidly, as a function of the loading condition of the gas turbine installation, by actuating a control valve provided in a corresponding air line. Apart from improved evaporation of the residual liquid fuel in the evaporative reactor, this also substantially reduces the condensation tendency of the fuel vapor which is produced. Soot formation is practically eliminated because of the presence of the steam. The evaporative reactor is insulated in order to minimize the heat losses.
In order to bring the method into effect, an evaporative reactor connected to the mixing section of the combustion chamber is v
Dobbeling Klaus
Reh Lothar
Wang Yunhong
ABB Research Ltd.
Burns Doane Swecker & Mathis L.L.P.
Gartenberg Ehud
Thorpe Timothy S.
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