Session: 03-05 Efficiency Improvement and Carbon Footprint reduction in turbomachinery Systems

Paper Number: 151881

Decarbonisation of Industrial Power Generation Gas Turbines With Bio-Alcohols 

Despite the worldwide ambition for a low-carbon economy, the intermittency of renewable power sources means that dispatchable power sources are required to meet global energy demands. Gas turbines offer flexible power generation but traditionally burn carbon-based fossil fuels. Whilst zero-carbon fuels such as hydrogen and ammonia show promise as future green fuels, carbon-based biodiesels, bio-alcohols and e-alcohols can also offer significant carbon dioxide reduction benefits. Uniper has already demonstrated the suitability of hydrotreated vegetable oil to replace diesel in liquid-fired gas turbines.

Bio-ethanol and bio/e-methanol are promising low-carbon alternative fuels for gas turbines and could be combusted in the gaseous (evaporated) or liquid phase. Despite gaseous ethanol having a lower Wobbe index than natural gas, it is within the range of experience of current natural gas-fired lean-premixed burners. Therefore, in principle, evaporated ethanol could be burnt in existing lean-premixed natural gas burners without modification. However, the Wobbe index of methanol is too low to be acceptable in the evaporated state in existing premix combustors without significant modification.

Previous studies demonstrated the suitability of evaporated pure ethanol to replace natural gas in a Siemens SGT5-2000E gas turbine lean-premixed burner without modification. However, in practice, gas turbine operators are unlikely to choose pure ethanol due to the cost of water removal and the need to add denaturants to avoid alcohol tax. This study investigates the impact of water and methanol content on evaporated ethanol combustion in the SGT5-2000E gas turbine and extends the previous simplified mixing studies to a representative burner geometry.

The previous study found that, when firing evaporated ethanol, NO_x emissions would be similar to when firing natural gas. However, this more detailed study found that NO_x emissions are likely to be higher for ethanol due to kinetic effects not evaluated in the initial assessment, and the poorer mixing relative to natural gas revealed when the detailed geometry was assessed.

The assessment of the impacts of water and methanol addition to ethanol found no significant effect on combustion performance compared to pure ethanol, but the decrease in heating value would have an adverse effect on fuel flow/pressure drop. The use of evaporated ethanol including moderate water content with added methanol may still be attractive, possibly with de-rating to minimise flashback risk, reduce the fuel system pressure drop, and reduce NO_x. However, some efficiency losses relative to natural gas firing due to de-rating would be offset by recovering waste exhaust heat for evaporation.

Curtailed renewable power may be used to produce e-methanol and this may favour methanol over ethanol as a future low-carbon fuel. However, it is unlikely that the  SGT5-2000E could burn evaporated methanol in unmodified burners. This study also considers the potential to burn methanol (evaporated or liquid) in modified dual fuel (natural gas/distillate) burners.

In principle using both premix and diffusion liquid fuel passages simultaneously would accommodate the increased fuel flow needed for liquid methanol combustion. Increasing the size or number of gaseous fuel nozzles would allow for the required flow of evaporated methanol. Using waste exhaust heat for evaporation would result in approximately 5-6% reduction in fuel requirement. Thus, a concept is proposed using a modified dual-fuel burner. The gas turbine would be started using liquid methanol, avoiding the need for supplementary heating for initial evaporation. Switching to evaporated methanol when waste heat becomes available will allow efficiency benefits to be realised.

Chemical kinetic studies and CFD mixing studies for evaporated methanol using a representative burner geometry suggest that combustion risks are acceptable and similar to those determined for ethanol, but NO_x emissions for methanol are likely to be significantly lower.

In principle, either ethanol or methanol could be used as a low-carbon fuel for power generation, but each application would require an assessment to identify the optimum choice of fuel and burner configuration.

Presenting Author: James Harman-Thomas Uniper Technologies

Presenting Author Biography: Dr James Harman-Thomas is a Gas Turbine Combustion Engineer at Uniper Technologies, based in the United Kingdom. He graduated with an Engineering Doctorate (EngD) from the University of Sheffield in 2023 studying the chemical kinetics of high-pressure combustion. Since joining Uniper Technologies in June 2023, James has worked extensively on the conversion of existing fossil-fired assets to green fuels.

Authors:

James Harman-Thomas Uniper Technologies
Carlo Antonio Caputo Cranfield University
Krzysztof Danielak Cranfield University
David Abbott Uniper Technologies
Xiaoxiao Sun Cranfield University