A patent has been awarded to Adnan Eroglu, Jaan Hellat Felix Guethe and Peter Flohr (Assignee: Alstom Technology Ltd.) for a method to operate a gas turbine with sequential combustion to an Integrated Gasification Combined Cycle (IGCC) plant. Alstom gas turbines, such as GT24 and GT 26, feature sequential combustion with reheating by a second combustion chamber. This offers the possibility of operating each of the combustion chambers with different fuels. Such gas turbines with sequential combustion therefore enable new types of starting, operation, and mixed combustion in an IGCC power plant.
An IGCC plant is normally operated with a synthetic gas (syngas or Mbtu), which is produced by gasification of coal, biomass, or other fuels (see, for example. In order to improve the availability of the power plant and to ensure continuous power generation, an auxiliary operation either with natural gas or with a liquid fuel is normally necessary during the maintenance times of the gasification unit.
Such IGCC power plants are customarily started with an auxiliary fuel such as natural gas or oil, and then switched over to syngas after the gasification unit has been put into operation. The switching-over of fuel (fuel switchover, FSWO) from the auxiliary fuel to syngas is a complex changeover process which is generally associated with load fluctuation, pulsations, and the risk of flashback. Furthermore, the risk of a trip of the gas turbine during the FSWO is extremely high.
In specific cases it is possible to individually switch over the fuel for each of the plurality of burner groups in a combustion chamber of the gas turbine, and so to reduce the extent of the load fluctuations or the risk of a trip of the gas turbine. This group-wise operation can also be used for a mixed combustion, where individual burner groups are operated with different fuel compositions, such as syngas/oil or syngas/natural gas.
The main problem with this previously described type of operation is an increased risk of flashback or of pulsations due to different flame shapes and pressure drop coefficients of burners in the same combustion chamber in the case of different fuels.
Instead, in sequential combustion, the two combustion chambers of the gas turbine can be operated at least partially with different fuels, wherein operation with different fuels can also be carried out in a changeover phase, such as in a starting phase.
In another method, for starting the gas turbine, the first combustion chamber is first operated with fuel from the group comprising natural gas and liquid fuel, especially oil, as the auxiliary fuel until the gas turbine has achieved a predetermined part load, such as 33%. In a second step the second combustion chamber is additionally operated with syngas as the second fuel until base load is achieved. In the second step, the first combustion chamber can be switched over to the operation with syngas.
Alternatively to this, with the second combustion chamber having a plurality of groups of burners, the first combustion chamber is first operated with auxiliary fuel until a predetermined part load is achieved, and in a second step the second combustion chamber is additionally operated first with a combination consisting of syngas and an auxiliary fuel. Here the syngas and the auxiliary fuel are combusted in different burner groups, and in a third step the second combustion chamber is finally operated entirely with syngas.
Also, if the first combustion chamber and/or the second combustion chamber have a plurality of burner groups which can be individually supplied with fuel, the burner groups can be selectively connected individually via the fuel distribution device to the fuel feed line or to the outlet of the gasification unit.