Hydrogen gas turbines
Hydrogen is suitable as a storage medium for renewable energies when they produce excess power (e.g. strong wind on a public holiday) Hydrogen-containing gases lead to combustion temperatures that are too high for conventional gas turbines For this reason, these turbines still have to be developed.
Turbines that can burn hydrogen-rich gases are also required in the combined gas and steam turbine process, which is used downstream of a coal gasification process (IGCC). The synthesis gas present after the gasifier and the downstream shift converter mainly consists of CO2 and Hydrogen (H2). CO2 capture can be achieved in a relatively simple manner using a physical absorption process, with the result that almost pure hydrogen will be available as a combustion gas for a gas turbine. For this reason, one important focus of research is the development of gas turbines that are reliable, offer fuel flexibility and are also low in emissions for this type of H2-rich synthesis gas.
In order to reduce the number of laborious combustion tests, researchers are currently using calculation programmes to simulate behaviour in the combustion chamber with ever-improving prediction reliability based on numerical procedures.
- Increasing fuel flexibility (burner systems for a large spectrum of fuels – natural gas / hydrogen-rich synthesis gas / alternative fuels, data and models for reaction kinetics)
- Improving the combustion system of the gas turbine for hydrogen use based on the latest natural gas turbine technology, with the goal of improving efficiency, increasing reliability and reducing nitrogen oxide emissions. A further goal is the optional use of natural gas, synthesis gas or – if possible – methanol.
- Development of new gas turbine combustion systems. Diffusion burners are currently being used to burn low-calorific gases. Turbine inlet temperatures can be increased by using pre-mix burners. One possible option here is the FLOX® combustion approach, which avoids temperature peaks in the combustion chamber and the formation of prohibited nitrogen oxide concentrations by pre-mixing exhaust gases and the fuel/air mixture.
- Development of innovative gas turbines for hydrogen-rich synthesis gas and higher ISO inlet temperatures (high efficiency potential). The basis for this is the ongoing development of gas turbines for standard operation with natural gas. For example, the H class turbine developed for the Irsching gas and steam power plant is to be ready for service in IGCC plants too by 2020.
The capacity of current gas turbines that run successfully on synthesis gas is around 200 MWel. An IGCC plant output of 350 MWel can be achieved in this way in combination with a steam turbine. Current high-performance gasifiers can supply raw gas for a 500 MWel plant. This would require that the gas turbine output be increased to 300 MWel. At the Buggenum and Puertollano IGCC plants, Siemens gas turbines with respective outputs of 190 and 200 MW, inlet temperatures of around 1,160 and 1,230 °C, and efficiencies of around 36-39% are in use. With the support of DOE, Siemens is working on the development of an H class gas turbine suitable for synthesis gas and hydrogen, which is to be ready for service by 2017. Siemens has already developed the H class with an output of 340 MW for the Irsching gas and steam power plant, which is fuelled by natural gas. The operating temperatures here are up to 1,500 °C. The efficiency of gas and steam plants that have already been built is to be increased by 2 percentage points using this new gas turbine.
The gas turbine burners currently used for the combustion of low-calorific gases operate in diffusion mode, i.e. fuel and air are mixed in the combustion chamber and not earlier. The development of new combustion systems with pre-mixing will allow turbine inlet temperatures to be increased relative to the gas turbines with diffusion burners that are currently in service and will thus also increase efficiencies. Researchers expect an increase in inlet temperature from 1,250 to 1,400 °C to lead to an increase in efficiency of around 2 percentage points.
The EU-FP6 ENCAP project is the first wide-ranging project aimed at developing large-scale, certified stationary turbines to be run on hydrogen-rich fuel. These investigations are being supplemented by national work on the fuel-flexible use of gas turbines – e.g. as part of AG Turbo. Even after these projects have been successfully concluded, it is likely that further research work will be necessary to deal with important issues that remain. Gas turbines for standard operation with natural gas are being developed on an ongoing basis. The latest available gas turbine technology with pre-mix burners is being used in each case as the basis for the development of combustion systems for the use of hydrogen-rich fuels. This will ensure that the progress made by these projects can also be applied in modern IGCC power plants with CO2 capture.
The gas turbine burners currently used in the combustion of synthesis gas operate in diffusion mode, in contrast with natural gas burners. Fuel and air are mixed in the combustion chamber and not earlier. In order to adhere to future NOx limit values, hydrogen and water will have to be added and the combustion temperature reduced. This leads to a significant reduction in efficiency as compared to pre-mixing operation, where fuel and air are mixed in a duct before the actual burner.
Equipment with pre-mix burners allow for further increases in turbine inlet temperatures as compared to equipment with diffusion burners that is currently in use. Higher efficiencies and improved reliability can then be achieved. The still-to-be-developed combustion system with a pre-mix burner will produce low emissions of less than 15 ppm NOx with minimal dilution, even when operated with H2-rich fuel. H2O and excess nitrogen from the air separation unit are available as dilution media.
The low NOx value is to be achieved even in the case of operation with a dry secondary fuel (natural gas). Diluting the fuel with inert media leads to additional investment and operating costs. These costs have to be minimised. The mixture between gas and air has to be optimised in order to achieve low emissions and reliable operation with minimal dilution requirements. Only in this way can the increased volumetric flows and increased reactivity relative to natural gas be accommodated.
4 current research projects
Industrial gas turbine burners for alternative combustion gases (IGAB)
Organisation carrying out research: German Aerospace Center (DLR) - Institute of Combustion Technology (EN-VT)
Project number: 0327718B