Carbon capture for CCGT | The energy


required for the CO2 capture is reduced


by increasing the CO2


concentration (%) at the


concentration (%), but limited by O2


combustor inlet. To push this to higher EGR rates, that is a further reduction in energy demand, hydrogen piloting (0% in this plot) is being considered (Source: SINTEF)


level and stabilising the flame are dominant, with an overall positive impact on system performance. The additional energy penalties of the overall system were also analysed, confirming that the CO2


capture energy to be processed by the CO2 capture section,


due to the recycle of a portion of the whole gas. Lower volumes translate into smaller sizes for the equipment involved in the capture.


As a result, the costs associated with the CO2 capture system decrease, again moving the needle towards more economic operation.


Current status of EGR for gas turbines


Although still not commercially deployed in the gas turbine industry, EGR is a technology add-on that has drawn a lot of attention recently at both research and industrial level. SINTEF has long investigated the concept for the sake of lowering the barrier to capture CO2


from


low concentration gases such as in natural gas power generation. Efficiency and cost are of course obvious targets, but in applications with severe space constraints like the offshore sector, decreasing the size and weight becomes an equally important consideration. In the early 2010s SINTEF [1]


showed that EGR has the


best capability for increasing carbon dioxide concentration with the highest electrical efficiency, compared with other methods for increasing CO2


concentration in the exhaust gas of gas turbines, such as humidification, supplementary firing, and external firing. Actually, with EGR there are further positive side effects, such as a decrease in NOx


formation due to lower local


flame temperature and a potential for part load efficiency improvement, as shown in the work of the TRANSITION partner TotalEnergies [2] to an inlet air heating effect.


, thanks


But a major technology bottleneck lies in the combustor of the gas turbine, where it has been established that the amount of exhaust gas that can be recycled is limited to 30-40% before combustion stability and efficiency issues arise, a point already confirmed in the course of GE’s high pressure test facility trials, as reported in 2009 [3]


.


Challenges and opportunities for EGR


A SINTEF simulation study, in collaboration


with TRANSITION project partners from the University of Florence, investigated the impact of exhaust gas recirculation on a natural gas fuelled combined cycle power plant, highlighting challenges and potential. The effect of increasing the EGR ratios (ie, the fraction of the exhaust gas that is recycled) on the downstream CO2


capture process was first


assessed. The CO2


capture technology selected is


chemical absorption using monoethanolamine (MEA) as solvent. This is a mature technology, using steam for regeneration of the solvent, widely regarded as benchmark technology. The results first reaffirm what was already observed in the literature, in that increasing the EGR ratio results in turn in reduced energy penalty and smaller equipment size. In the study, fairly high and impractical EGR ratios (well above the accepted 35-40% limit) were purposely tested. The goal was to investigate whether the positive impact of EGR is attenuating over a certain threshold, irrespective of its feasibility from a combustion perspective. Interestingly, the trends remained stable, seemingly suggesting that: the more EGR, the better. This result means that effort put into further pushing the combustion limit will positively affect the capture plant. And this is the ultimate objective of the TRANSITION project, to investigate hydrogen piloting of the combustor at high EGR ratio. The piloting consists of adding a relatively small amount of hydrogen into the combustor, just enough to maintain stable combustion and allow higher EGR levels. To minimise the amount of hydrogen needed, and its associated cost, clever injection strategies in the burner– combustor arrangement are being developed. The most promising configuration designed by Baker-Hughes and the University of Florence will finally be tested in relevant high-pressure conditions at the partner site DLR, Germany. Despite the H2


decrease of the CO2


piloting resulting in a small concentration in the


exhaust gases, the simulation results show that the advantages derived from increasing the EGR


26 | October 2024| www.modernpowersystems.com


requirement is predominant, and that the design of the system should aim at minimising it. As the energy consumption is a key driver of operating costs, and equipment sizes are a key driver of capital costs, these results represent a basis for further techno-economic evaluations. Based on these conclusions, SINTEF aims at focusing its efforts on developing a more efficient power plant with integrated carbon capture and EGR. To achieve this, future steps in the TRANSITION project will include conducting detailed simulations at selected EGR levels, supported by experimental investigation of combustion. This will allow us to further optimise the system and perform techno- economic analyses to explore the feasibility and benefits of optimising exhaust gas recirculation strategies for natural gas fuelled combined cycle power plants.


Although the power sector is going through a radical change where renewable electricity is gradually and inexorably increasing its share, natural gas power generation is expected to continue play an important role in balancing the power system and increasing its flexibility and reliability. Reducing the emissions from natural gas fuelled combined cycle plants is, therefore, a necessity. CO2


the lone option available on the table to pursue such reduction. In spite of that, the


economic penalties incurred by deploying CO2 capture are slowing down its take up. Exhaust gas recirculation shows substantial promise as a means of improving cost effectiveness but is not yet commercially available. SINTEF, in collaboration with its partners from the TRANSITION project, will strive to further advance EGR towards technological maturity continuing to address remaining limitations via focused research.


References


[1] H. Li, M. Ditaranto, and D. Berstad, ‘Technologies for increasing CO2 concentration in  with post-combustion, amine-based CO2 capture’, Energy, vol. 36, no. 2, pp. 1124–1133, Feb. 2011, doi: 10.1016/j.energy.2010.11.037.


[2] J. Hachem, T. Schuhler, D. Orhon, M. Cuif-Sjostrand, A. Zoughaib, and M. Molière, ‘Exhaust gas recirculation applied to single-shaft gas turbines: An energy and exergy approach’, Energy, vol. 238, p. 121656, Jan. 2022, doi: 10.1016/j. energy.2021.121656.


[3] A. M. ElKady, A. Evulet, A. Brand, T. P. Ursin, and A. Lynghjem, ‘Application of exhaust gas recirculation in a DLN F-Class combustion system for postcombustion carbon capture’, J. Eng. Gas Turbines Power, vol. 131, p. 034505, Feb. 2009, doi: 10.1115/1.2982158.


capture is currently


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