Effect of use of zero-carbon and low-carbon fuels on the performance of compact combined cycles for power generation

Abstract

Offshore electrical power is normally generated with several gas turbines running on natural gas produced on-site. These account for about 85% of the CO2 -emissions from the oil and gas sector in Norway. About 24% reduction of these emissions can be achieved by installing a steam bottoming cycle. Switching to zero-carbon fuels can be a future option to fully remove these emissions. Only a few offshore steam bottoming cycles are installed today, and gas turbines running on non-carbon fuels are technology under development. This study aims to quantify and analyze the effects on the performance of an offshore combined cycle after switching the gas turbine fuel from natural gas to alternatives with lower environmental impact. The fuels and fuel blends considered include hydrogen, ammonia, bio-methanol, and mixtures thereof — all of which have the potential to reduce overall CO2 intensity of power production. Hydrogen and ammonia, in particular, offer direct reductions in CO2 emissions. In our analysis we want to answer the following: - (1) will an installed steam bottoming cycle produce the same amount of power and operate under similar conditions when switching fuels? (2) For the different fuels, how much fuel would a combined cycle save with the same total power output? and, (3) What is the estimated total emission reduction potential for different fuel alternatives? The study is based on a common offshore configuration composed of four 40 MW gas turbines operating in simple cycle. In our analysis we replace one of them with a steam bottoming cycle and the results show that the net power output from the steam cycle across the fuels varied from 42.7 to 46.4 MW using equal steam cycle assumptions, with the ammonia-dominated mixtures showing the best performance. A once-through steam generator (OTSG) was designed for natural gas exhaust and applied in an improved cycle optimization aimed at maximizing steam bottoming cycle net power output while maintaining a fixed total power production. Results indicate significant fuel-saving potential ranging from 157 t/day for natural gas to 462 t/day for ammonia as fuel. These fuel-saving potentials were further used to estimate the potential for greenhouse gas emissions. The results showed that the reductions would be in the range of 85%–91% for the zero-carbon fuels. For bio-methanol, the emission reduction could vary from 4% to 70% depending on the production pathway.