| Carbon capture and storage


High pressure throttle


Reheater Low pressure superheater (LPSH) Crossover


compression 19.3%


CO2


Stripper system 24.1%


Cold reheat/intermediate pressure superheater (IPSH)


Figure 3. Steam turbine arrangement for a combined cycle plant. Steam for the carbon capture plant is extracted from the steam turbine IP/LP crossover (Image: GE Vernova)


The bolt-on solution is a straightforward way of generating steam with least to no integration with the combined cycle power plant. The bolt-on approach involves installing an external auxiliary boiler to provide the necessary steam to the reboiler. To achieve the desired capture rate for the site, it


is necessary to take account of the CO2 emissions generated by the auxiliary boiler. Thus, the carbon dioxide capture plant will also receive the exhaust gases from the auxiliary boiler. To deal with these CO2


emissions


generated by the auxiliary boiler, both the carbon capture plant and the auxiliary boiler must be larger.


In contrast, the steam integration solution involves the extraction of steam from the combined cycle plant. The choice of steam extraction point in the combined cycle power plant is based on the steam temperature and quality needed in the reboiler. Generally, the steam quality at the reboiler interface is around 4-6 bar saturated. This implies that the intermediate pressure (IP) and low pressure (LP) crossover is the most favoured location for steam extraction. A throttle valve allows steam to be drawn from the IP/LP turbine crossover, taking account of the pressure loss in the steam pipelines and the mean temperature differential in the reboiler. An attemperator is required to manage the temperature of the steam draw by spraying condensate to keep the temperature just above saturation when it enters the reboiler. When compared to bolt-on solutions, the steam integration solution offers significant


carbon capture cost savings. There is no need for an auxiliary boiler because the steam to the carbon dioxide capture plant is extracted from the steam turbine IP/LP crossover, as already noted.


The bolt-on solution increases the size of the CO2


capture plant needed and the total amount of fuel consumed on site by installing an additional external auxiliary boiler to supply steam to the capture plant. The natural gas fuelled combined cycle power plant and the auxiliary boiler both produce carbon dioxide emissions, which the capture plant is designed to capture.


As noted, this will lead to an increase in the capture plant’s size, power consumption, and footprint. The bolt-on solution increases the overall amount of carbon dioxide emissions sent to the capture plant by up to 30%, assuming a 95% capture rate for the capture plant.


Implementing a bolt-on solution reduces the natural gas fuelled combined cycle system’s net efficiency by about 20% when compared with the NGCC base case without CO2


capture.


When using the steam integration option, however, the net NGCC efficiency decreases by approximately 8%. The bolt-on solution’s NGCC output drop is roughly 6%, whereas the steam integration solution’s is 12%.


It is also possible to employ an optimised combination of steam integration and auxiliary boiler to achieve the required balance of capex, opex and operability needed for a given plant.


Total plant cost


Carbon capture plant


Absorption system


20% Capex reduction with 30% EGR


Figure 5. Combined cycle capture plant capex reduction with 30% EGR (Image: GE Vernova)


15% 5% 8% 18% 25%


120% 100% 80% 60% 40% 20% 0%


Figure 4. Carbon capture equipment capex breakdown (Image: GE Vernova)


Why exhaust gas recirculation? With exhaust gas recirculation (EGR), a portion of the HRSG exhaust flue gas flow is reintroduced into the inlet of the gas turbine after appropriate cleaning and conditioning. EGR offers two key advantages. First, the flue gas directed to the carbon dioxide capture plant is reduced proportionally with the EGR rate. This reduction in mass flow to the capture plant decreases the size of major capture plant components, including the direct contact cooler and the absorber, leading to a decrease in both footprint and capex of the capture plant. Second, since EGR increases the concentration of CO2 resulting in a higher CO2


in the exhaust gases, partial pressure in


the flue gas, the driving force in the separation process is enhanced. In other words, EGR enables the capture plant to capture the same amount of CO2


from exhaust gases


while treating a smaller volume of flue gases. This leads to a reduction in the capture plant footprint, capex, and opex. Furthermore, EGR helps mitigate amine degradation linked to O2


in the exhaust gas, with a consequent decrease in amine make-up flow, thereby improving the overall opex of the CO2


plant. Combustion dynamics, emissions, and the


concentration at the gas turbine combustor input, limit the EGR ratio. For example, at a 30% EGR ratio, CO2


O2 concentrations in exhaust gases increase by nearly 40%. However,


increasing the EGR ratio to 30% lowers the O2 concentration in the combustor by nearly 20%.


100%


Figure 6. Combined cycle capture plant capex saving with EGR and steam integration (Image: GE Vernova)


71% 63% capture Pre-


scrubber system 12.2%


Absorption system 44.3%


Bolt-on solution


Steam integration


Steam integration +30% EGR


www.modernpowersystems.com | November/December 2024 | 15


CCP capex


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