Session: 03-07 Methanol

Paper Number: 120971

120971 - Utilisation of Evaporated Ethanol in Natural Gas Fired Lean-Premix Combustors of Power Generation Gas Turbines 

With the transition to low carbon power generation there will be a continuing need for dispatchable power when intermittent renewable power sources are not sufficient to supply demand. Gas turbines are a flexible source of dispatchable power, but for maximum environmental benefit need to be fired with low carbon fuels. A large fleet of power generation gas turbines exists that have significant remaining life and will need to be re-purposed to fire low-carbon fuels. For distillate fuel fired gas turbines HVO may be a viable “drop-in” alternative and Uniper have demonstrated its viability.  

There is a risk that natural gas fired machines may become isolated as natural gas pipelines are re-purposed to convey hydrogen.  Conversion to hydrogen firing may not be economically viable due to limited operating hours in grid support roles. Thus, alternative fuels are needed to realise the full life of these units.

It has been shown that micro-gas turbines with lean premix natural gas combustors can successfully fire a wide range of evaporated liquid fuels without modification and also fire blends of evaporated liquids and natural gas. It is possible larger power generation gas turbines will also be able to utilise evaporated liquid fuels without significant modification.  Ethanol and methanol were considered but methanol was eliminated because of its significantly lower heating value impacting both storage capacity and fuel volume flows. The heating value and effective Wobbe index for ethanol are lower than for typical natural gas but are close to the range of experience for lean premix combustors justifying further investigation.

This preliminary assessment considers the potential for power generation gas turbines to burn evaporated ethanol in lean premix burners.  A Siemens SGT5-2000E gas turbine as operated in Uniper’s Killingholme plant is used as an example.

Impacts on key combustion performance parameters; emissions, combustion dynamics, flashback and blow-off have been considered. These parameters are affected by both chemical properties of the fuels and the effect of their physical properties on burner aerodynamics, fuel placement and mixing quality.

Chemical properties are represented by their laminar flame speed, auto ignition delay time, and adiabatic flame temperature, which are evaluated using appropriate chemical kinetic models and compared for methane and evaporated ethanol.

The impact of physical properties on fuel placement and mixing are evaluated using a CFD model of a simplified representation of the fuel injection and fuel-air mixing zone of the SGT5-2000E burner.

Adiabatic flame temperature of ethanol is similar to that from methane and the CFD study indicates that despite the different fuel jet velocities, overall fuel placement will be similar and mixing quality will be improved, suggesting that emissions performance will be similar or better.

The higher laminar flame speed of ethanol will lead to a higher turbulent flame speed and this will tend to reduce the flame length, and flame characteristic time, affecting the phase of the flame transfer function. This will have an impact on thermoacoustics, tending to favour higher frequency modes.

As the CFD study shows similar fuel placement and improved mixing, these factors will not adversely impact flashback or blow-off characteristics, but the higher flame speed will affect both. Blow-off risk will tend to reduce when firing evaporated ethanol, thus increasing turndown capability. However, the risk of flashback will increase, but the extent of the increase in risk may be to some extent offset by the improved mixing reducing areas of high fuel concentration. The flashback margin of this burner has not currently been evaluated and it may be necessary to de-rate the gas turbine if further investigations indicate insufficient flashback margin.

These initial findings suggest that the concept is viable and further investigations are under way. These will include the impact of ethanol quality (water content and contaminants such as alkali metals) and the impact of denaturants that may be required for alcohol tax purposes.

 

Presenting Author: David Abbott Uniper Technologies Ltd

Presenting Author Biography: Dr David Abbott is a gas turbine combustion specialist. His doctorate is in the combustion acoustics of solid rocket propellants. His early career focused on acoustic problems in gas fired boilers and furnaces. He has worked for a gas turbine manufacturer on the research, development and design of combustion systems. Subsequently he joined a power generator providing combustion support for all aspects of their gas turbine fleet. Since 2014 he has been a technical consult for Uniper Technologies Ltd and a visiting Fellow at Cranfield University with a particular interest in thermoacoustics, impacts of fuel composition, and low carbon fuels.

Authors:

David Abbott Uniper Technologies Ltd
Tijn Van Eil TU Delft, Process and Energy Depertment
Sikke Klein TU Delft,
Catherine Goy Uniper Technologies Limited