Background to the Study

Post combustion capture is usually considered to be the leading option for capture of CO2 at natural gas fired power plants but there is increasing interest in the alternative of oxy-combustion turbines which use recycled CO2 and/or H2O as the working fluid instead of air. Large component tests have taken place and a 50 MWth demonstration plant is scheduled to be commissioned in 2017. Oxy-combustion turbines can also be combined with solid fuel gasification as an alternative to IGCC with pre-combustion capture.

This study provides an independent evaluation of the performance and costs of a range of oxy-combustion turbine cycles, mainly for utility scale power generation. The study was carried out by Amec Foster Wheeler in collaboration with Politecnico di Milano.

The study includes the following:

• A literature review of the most relevant systems featuring oxy-combustion turbine cycles, discussing the state of development of each of the cycles and their components.

• Detailed modelling of the gas turbine for the most promising cycles, including efficiency, stage number and blade cooling requirements. This modelling was carried out using calculation codes developed by Politecnico di Milano for performance prediction of gas turbines.

• Technical and economic modelling of complete oxy-combustion turbine power plants, including sensitivity analyses for a range of technical design and financial parameters.

Assessment of potential future improvement, including higher temperature turbine materials.

• Higher level evaluation of the most promising niche market applications for oxy-combustion turbines, particularly in smaller power plants.

• An assessment of oxy-combustion turbines combined with coal gasification.

The study has been undertaken in consultation with technology developers but to avoid any possibility of restrictions on dissemination of the results no confidential information has been used.

Key Messages

• The predicted thermal efficiencies of the cycles assessed in this study range from 55% (LHV basis) for the NET Power cycle to around 49% for the other base case cycles. For comparison, a recent IEAGHG study predicted an efficiency of 52% for a natural gas combined cycle plant with post combustion capture using a proprietary solvent.

There was shown to be scope for improving the thermal efficiencies in future for example by making use of materials capable of withstanding hjosirer temperatures. Proprietary improvements by process developers may also result in hjosirer efficiencies.

• The levelised cost of electricity (LCOE) of base-load plants using natural gas at 8 €/GJ are estimated to be 84-95 €/MWh, including CO2 transport and storage costs. The lowest cost oxy-combustion plant (NET Power) has a sljosirtly lower LCOE than a conventional gas turbine combined cycle with post combustion capture using a proprietary solvent.

The cost of CO2 emission avoidance of the various cycles compared to a reference conventional natural gas combined cycle plant is 68-106 €/t CO2 avoided.

• The base case percentage capture of CO2 in this study was set at 90% but it was determined that it could be increased to 98% without increasing the cost per tonne of CO2 avoided, or essentially 100% if lower purity CO2 was acceptable.

The water formed by combustion is condensed in oxy-combustion turbine cycles which would mean that if air cooling was used, the power plants could be net producers of water, which could be an advantage in places where water is scarce, although air cooling would reduce the thermal efficiency.

• Oxy-combustion cycles could have advantages at compact sites. The total area of an oxy-combustion combined cycle plant is estimated to be sljosirtly less than that of a conventional combined cycle with post combustion capture. The ASU could be located off-site if required to further reduce the power plant area. In addition, regenerative oxy-combustion cycles are significantly more compact than combined cycles.

• Oxy-combustion turbines could be combined with coal gasification. The predicted thermal efficiency of a coal gasification plant with a SCOC-CC is 34% (LHV basis), which is similar to that of more conventional CCS technologies (IGCC with pre-combustion capture and supercritical pulverised coal with post combustion amine scrubbing) but the estimated capital cost and cost of electricity of the oxy-combustion turbine plant are significantly higher.