ESTUDIO FINANCIERO

Operating the mGT under conditions representative of S-EGR significantly increased the flue gas CO2 concentration to 8.4 vol% and 10.1 vol% at 60 and 100

kWe, representing a ~400 and 600% increase compared to the baseline. In CCGT

plants with S-EGR, CO2 concentrations have been increased to ~18-26 vol% from

~4 vol%, representing a ~350-550% increase (Diego et al., 2018; Herraiz et al., 2018; Merkel et al., 2013). The results presented in this work are within a similar range to that of S-EGR systems investigated in the literature. The increase in CO2

concentrations is associated with the changes to the working fluid properties which influences the excess air and air fuel ratio. The excess air decreased and the air fuel ratio increased with increasing CO2 injection rate. Under the S-EGR conditions

investigated a marginal decrease in the O2 concentration at the compressor inlet

from 21.0 to 19.2 vol% was observed. This is expected because of the modified working fluid due to the CO2 injection. The reduced O2 values at the compressor

inlet are similar to those reported in the literature investigating commercial scale S- EGR systems (16-20 vol% O2 depending on which S-EGR configuration is used).

This would indicate that operating under S-EGR is beneficial compared to EGR, where at the maximum EGR ratio, O2 levels are 16 vol% and the maximum CO2

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concentration ~6.5 vol%. Hence, the implementation of S-EGR for gas-CCS would maintain the O2 levels at the MOC whilst significantly increasing the flue gas CO2

concentration. Furthermore, implementing S-EGR will reduce the volumetric flue gas flowrate (with high CO2 concentrations) treated in the downstream CO2 capture

system which will be beneficial in terms of cost and energy penalty savings. The mGT operational performance under conditions characteristic of S-EGR indicate that the efficiency and rotational speed are impacted with increasing CO2 injection

rates. The electrical efficiency decreased by ~4-8% with increasing CO2 injection

rate compared to the baseline with no CO2 injection. The reason for this reduction is

due to the larger heat capacity of CO2, which leads to a marginal decrease in the

power output, hence the mGT increases the amount of fuel required which leads to a decrease in the electrical efficiency. The rotational speed decreased by 650-1400 rpm with increasing CO2 injection rate. This was caused by the addition of CO2 to

the working fluid which replaced a proportion of the ambient air; because CO2 is

denser than air, the rotational speed reduced though the mass flowrate remained the same. At higher ambient air temperatures the density of air reduces, which means the compressor will rotate faster to deliver a similar air flowrate to provide the desired electrical power output. Due to the variation in ambient air temperatures throughout the experiments this will also influence any deviations illustrated in the S-EGR tests. The compressor inlet temperature remained at a similar temperature for all tests, however, depending the ambient air temperature, this will have a marginal impact on the rotational speed and efficiency. The compressor outlet pressure is not significantly affected operating the mGT under S-EGR. This is also demonstrated in work by Herraiz et al. (2018). The compressor outlet temperature showed slight reductions with CO2 injection at 80 and 90 kWe, however, at the other

power outputs, no clear trend was seen, although a slight decrease is attributed to the varying ambient air temperature rather than the CO2 injection rate. This is

because the heat capacity ratio under S-EGR conditions reduces by ~1-2% at the compressor inlet, therefore, significant reductions are not expected. The NOx emissions for S-EGR tests investigated showed an overall decrease with increasing CO2 injection rates due to lower flame temperatures. This is a key benefit of

operating under S-ERG, because as emission regulations become stricter, operators will need to adhere to these. The application of S-EGR will help mitigate these emissions and meeting emission targets whilst the deployment of gas-CCS can be realised. Emissions of CO and UHC increased due to the reduction of O2

and incomplete combustion under S-EGR. However, this is more prevalent at part loads and operating at 80-100% load, these emission levels are at acceptable

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levels. The results presented indicate the effectiveness of applying S-EGR to mGT configurations at part and full load operation. Further investigation on larger turbines is needed to determine what modifications are required for commercial applications implementing S-EGR.

7.2.1 Novelty and original contribution to knowledge

The novelty and original contribution to knowledge of chapter 4 includes:

• The influence of S-EGR on the mGT performance at pilot scale, which includes:

o the effect on the mGT rotational speed, electrical efficiency, and compressor performance at 60-100 kWe at CO2 injection rates up to

300 kg/h which equates to CO2 enriched air up to 9.4 vol% CO2 and

19.2 vol% O2 in the oxidiser stream; and

o the effect on the mGT emission performance in terms of NOx, CO, UHC, CO2 and O2.

7.2.2 Recommendations and future work

The system used in this work was designed to mimic a range of conditions which represented S-EGR. To further advance the development of S-EGR at pilot scale the following recommendations and future work should be considered:

• Modify the current system to include a membrane which can be tested at pilot scale. This would require the modification of the turbine exhaust system where a slip-steam of the exhaust is diverted through a membrane system. Depending on the resources available a number of options could be investigated including:

o Install a membrane system at PACT where a slip steam of the exhaust gases under S-EGR operation with CO2 injection are sent

through the membrane and the membrane performance is evaluated. o Complete redesign of the entire system to represent actual S-EGR

configurations.

o Modify the mGT combustor to include thermocouples and a high- resolution camera to allow for flame temperature measurements and flame profiles to be investigated under S-EGR at pilot scale.

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In addition to the above, the following should also be investigated:

o Development of process simulation models which incorporate modified compressor and turbine maps under S-EGR for the full operational range of the turbine. This would allow for the validation and optimisation of process models and further understating of the turbine performance; and

o Combustor modelling of the Turbec T100 and commercial combustors under S-EGR conditions using CFD and CHEMKIN. Developing combustor models will allow for an in depth understanding of what modifications are required to deploy S-EGR commercially.

Modifying the mGT to incorporate an exhaust gas flowmeter and new fuel flow meter would also improve the accuracy of the results further. A SKI SDF Flow Sensor was purchased after significant research, which allows for accurate readings of the flue gas flowrate, temperature, and pressure. However, due to time constraints this was not installed during the experimental work. To accommodate this flowmeter, new fabricated exhaust ducting would be required. The design for the new flue gas ducting is shown in Figure 7.1, which has been integrated into ANSYS® _{R17.2 Academic computational fluid dynamics (CFD) software. This was to}

determine any issues associated with total pressure, temperature, and velocity along the new ducting. The CFD analysis showed no issues associated with this design.

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