4. Carbon Capture in Natural Gas Combined Cycle plants (cont.)Laura Herraiz (l.herraiz@ed.ac.uk)PGRAInstitute for Energy Systems, The University of Edinburgh, UK

CO2 Capture Workshop, Mexico, January 2018

4.2. Alternatives to optimise PCC system integration in Natural Gas Combined Cycle plants

4.2.1 Exhaust Gas Recirculation

4.2.2 Selective Exhaust Gas Recirculation

4.2.3 Supplementary Fired Combined Cycle

4.2.4. Sequential Supplementary Fired Combined Cycle

4.2.5. Gas Turbine humidification

Post-combustion Carbon Capture in NGCC

• Flue gases from NGCC plants raise challenges for CO2 separation with Post-combustion Carbon capture technologies compared to flue gases from coal-fired power plants, due to:

Low CO2 concentration (3-4 vol% CO2),

Large volumetric flow rates,

High O2 concentration (12-13 vol% O2),

This is a consequence of the large amount of excess air required to limit temperatures in the combustor.

3

Replace part of the excess of ambient air with recirculated flue gases and/or steam injection

Burn additional fuel using the excess of oxygen in exhaust flue gases

How to increase CO2 concentration?

4.2. Alternatives to optimise PCC integration in NGCCs

Exhaust Gas Recirculation (EGR)

Selective Exhaust Gas Recirculation (S-EGR)

Supplementary Fired Steam Cycle (SFSC)

Sequential Supplementary Fired Steam Cycle (SSFSC)

Gas turbine humidification: Steam injection (STIG) or Evaporative gas turbines (EvGT)

Operating strategies are investigated to optimise the integration of PCC technologies in a NGCC…

… With the objective of:

o Reducing the energy consumption and electricity output penalty.

o Reducing the PCC plant size

o Minimising capital and operation costs.

By increasing CO2 concentration in flue gases and reducing flow rates

4

4.2.1 Exhaust Gas Recirculation • EGR consists of recycling a portion of the exhaust flue gas leaving the HRSG back to the

compressor inlet.

• The recycled flue gas replaces part of the excess of air which results in a higher CO2

concentration and a smaller volume of flue gases treated in the PCC system.

Schematic diagram of a NGCC plant with EGR (Herraiz, 2016)

O2 vol% in combustor

CO2 vol% in exhaust gas

Herraiz (2016)

5

4.2.1. Exhaust Gas Recirculation

• The EGR ratio and the CO2 concentration is limited by the minimum O2 level in the combustor that ensures complete combustion with low levels of CO and UHCs emissions.

(Herraiz, 2016)

• A limiting oxygen conc. of 16-17 vol% (*)

is found at 35% EGR ratio. The CO2 conc. in the exhaust gases is 6-6.5 vol%.

(*) Limiting O2 level experimentally determined for a Dry Low-NOx combustion system employed in GE F-class GT technology (ElKady et al. 2009; Evulet et al. 2009)

6

4.2.2. Selective Exhaust Gas Recirculation

• SEGR consist of selectively transferring CO2 from a flue gas stream into an air stream which enters the GT compressor. Other components in the flue gases (e.g. N2, H2O) are ideally not recirculated.

Schematic diagram of a NGCC with S-EGR in parallel and in series (Herraiz, 2016)

SEGR in Parallel SEGR in Series

7

S-EGR in Parallel • CO2 conc. of 14 vol% in exhaust gases is possible at 70% recirculation ratio, 96% post-

combustion capture (PCC) efficiency and 97% selective CO2 transfer (SCT) efficiency.

(Herraiz, 2016)

8

For Overall CO2 capture level 90 %

S-EGR in Parallel

• Higher CO2 concentrations are possible for lower overall CO2 capture levels.

9

(Diego et al., 2017b)

Selective CO2 transfer eff. 95%

S-EGR in Series• CO2 conc. of 13 vol% concentration is possible, operating at 95% SCT efficiency and 31 %

PCC capture efficiency.

(Herraiz, 2016)

10

For Overall CO2 capture level 90 %

4.2.2. Selective Exhaust Gas Recirculation • A high CO2 concentration in the exhaust gases (> 13 %vol) is possible while maintaining

oxygen levels in the GT combustor above 19 vol%.

• The CO2 concentration is limited by the highest efficiencies that can be achieved in practice with the technologies used for CO2 capture and Selective CO2 transfer

SEGR in Parallel SEGR in Series

• Higher CO2 concentration.

• Lower volume of flue gases treated in the PCC system.

• Higher CO2 concentration.

• Lower CO2 capture efficiency in the PCC system.

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• Compared to a conventional configuration of NGCC with PCC …..

Effects of SEGR on CCGT performance STEAM CYCLE

Changes:

• High GT exhaust flue gas temperature

Effects:

Larger amount of heat available heat in the bottoming cycle

Consequence:

Steam Turbines power output increase

GAS TURBINE

Changes:

• Additional P drop in gas side through the SCT system

• Increase in T at GT compressor inlet • High CO2 vol% in working fluid

Effects:

• Despite the high CO2 density, the inlet mass flow rate is smaller due to the high temperature

• Small deviation on compressor and turbines performance with SEGR from design conditions

• Decrease in compression outlet T and increase expansion outlet T.

Consequence:

GT power output decreases12

POST-COMBUSTION CAPTURE

Changes:

• High CO2 partial pressure • Smaller flow rate or

Smaller CO2 capture rate

Effects:

Lower Energy consumption (reboiler duty) and

Smaller plant size (packing volume)

CCGT Net power output increase

Net thermal efficiency increase

• Compared to a conventional configuration of NGCC with PCC …..

Effects of SEGR on CCGT performance

13

Effects of SEGR on the GT combustion system

14

• The technical challenges of S-EGR are associated to the combustion system, since CO2 acts as a combustion inhibitor and high concentrations would lead to flame instability, blow-off and eventual extinction of the flame.

• Effect of S-EGR on the Combustion system has been experimentally investigated at the Gas Turbine Research Center at Cardiff, UK (Marsh, 2016) under the scope of the SELECT project.

http://www.cu-gtrc.co.uk/

Effects of SEGR on GT combustion system

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• For the range of CO2 concentrations expected with SEGR in parallel or SEGR in series, the objectives of the test campaigns are:

o To determine the range of equivalence ratios (Ø) for which the flame is stable, and thus to identify blowout and technical flashback limits, at different combustion pressures.

o To measure CO, NOx and UBHC emissions in the exhaust flue gases

𝜙 =𝐹𝐴𝑅 𝑎𝑐𝑡𝑢𝑎𝑙

𝐹𝐴𝑅 𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐

=𝑚 𝑎𝑖𝑟 𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐

𝑚 𝑎𝑖𝑟 𝑎𝑐𝑡𝑢𝑎𝑙

(Marsh, 2016)

Effects of SEGR on the PCC system

(Herraiz, 2016)

SEGR in parallel

• At 70% recirculation ratio, 96% PCC efficiency and 97% STC efficiency leads to: > 40% reduction in the absorber packing volume and 5% reduction in reboiler duty.

SEGR in series

• At 95% SCT efficiency and 31 % PCC capture efficiency results in: > 60% reduction in the absorber packing volume and 6% reduction in reboiler duty

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Technologies for Selective CO2 transfer • Rotary Adsorption

Physical adsorption with structured materials in a rotary wheel.

Low pressure drop.

(Herraiz, 2016) (Diego et al. 2017a)

• Selective CO2 membrane system

Low energy input, operating with a moderate ratio of feed pressure to permeate pressure.

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Technologies for Selective CO2 transfer

Key aspects for selective CO2 transfer technologies:

• High selectivity for CO2 transfer over other components in the flue gas (e.g. N2)

• Rotary adsorption: trade-off between CO2 affinity and possibility to regenerate the solid with ambient air.

• Membranes: trade-off between permeability and selectivity.

• Low Oxygen transfer from ambient air to flue gases

• Low heat transfer rate from flue gases into the air entering the compressor

• Low pressure drop (typically overcome by an air fan)

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Minimise Gas Turbine derating

4.2.3. Supplementary Fired Combined Cycle• SFCC increases CO2 concentration in the exhaust gases by burning additional fuel in a

secondary combustor (in-duct burner) using the excess of oxygen still remaining in the gases as oxydizer.

• The O2 content in the exhaust gases decreases, leading to reduced solvent degradation.

• The high flue gases temperature helps to stabilize the flame, despite the relatively low oxygen concentration of 11-12 vol%, compared to 21 vol% in ambient air.

19(Diego et al., 2017a)

4.2.3. Supplementary Fired Combined Cycle• The CO2 conc. in the flue gases can be increased up to 6-7 vol% CO2.

• It is restricted by the maximum allowable gas temperature in the HRSG (~ 820 ºC using conventional materials, ~900 – 1300 if insulated casing or water-cooled furnace are used).

• Using biomass/biogas instead of fossil fuels in the supplementary firing stages would contribute to a further reduction of CO2 emissions.

• Additional power is generated in the bottoming cycle. Yet the overall efficiency of the combined cycle plant decreases to ~43-48% (Gonzalez et al. 2016), compared to 57-63% for unabated NGCC plant (w/o SF).

Higher CO2 concentration possible with:

Sequential Supplementary Firing

Smaller efficiency penalty possible with:

Optimisation of the steam cycle

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4.2.4. Sequential Supplementary Fired Combined Cycle • SSFCC consists of burning additional fuel sequentially in several stages in between the heat

transfer banks along the HRSG, keeping gas temperatures at 800 -900 ºC.

• The CO2 concentration in the exhaust gases is constrained by the minimum excess of O2 in the combustion gas to ensure complete combustion, typically of the order of 1-2 vol% (Kitto &Stult 1992). The CO2 concentration can be increased up to 9.4 vol%.

Schematic process flow diagram of a SSFCC with a subcritical steam cycle(González et al. 2016) 21

4.2.4. Sequential Supplementary Fired Combined Cycle • SSFCC configurations: subcritical and supercritical steam conditions.

Temperature/heat diagram of a SSFCC with a subcritical steam cycle(González et al. 2016)

Temperature/heat diagram of a SSFCC with a supercritical steam cycle(González et al. 2016)

Power output 781 MWThermal efficiency 43.1 %LHV

Power output 824 MWThermal efficiency 45.6 %LHV

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4.2.4. Sequential Supplementary Fired Combined Cycle • SSFCC configurations: subcritical and supercritical steam conditions.

Parameters involved in the optimization to maximise overall thermal efficiency:

• No. of additional firing stages,

• Amount of NG burnt

• Pinch temperature points,

• No. of pressure levels and steam pressure.

Temperature/heat diagram of a SSFCC with a subcritical steam cycle(González et al. 2016)

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4.2.4. Sequential Supplementary Fired Combined Cycle

Technical performance, compared to a conventional CCGT with PCC:

• SSFCC presents a lower overall thermal efficiency (e.g. 43.1 %LHV vs. 51.3 %LHV)

• Yet, the higher steam turbines power generation leads to a reduction of the number of GT-HRSG trains (from 2 to 1) with a 50% reduction in flue gases flow rate, for the same power output.

• The capture plant size is significantly smaller.

Economic analysis: CCGT with SSFCC could be an attractive alternative for markets with access to competitive natural gas prices, with an emphasis on capital cost reduction, and were supply of carbon dioxide for Enhance Oil Recovery (EOR) is important.

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4.2.4. Sequential Supplementary Fired Combined Cycle • Key parameters to evaluate the economic feasibility of SSFCC:

• Natural Gas price

• CO2 price for EOR

Total Revenue Requirement:TRR = LCOE − EOR revenue

(González et al. 2016)

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4.2.5. Gas turbine humidification

Water or steam replace part of the excess of air used for cooling. The water vapour can easily be separated from the flue gas by condensation, and a high CO2 concentrations in the flue gas are possible, compared to conventional GTs.

• Inlet air cooling systems – limited by ambient air relative humidity and T

• Combustion chamber injection systems

o Direct steam injection (STIG)

o Evaporative Gas Turbines (EvGT), where water injection in an humidification tower and evaporation.

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The highest possible CO2 concentration depends on the water to air ratio (WAR) and on the cooled temperature of flue gas prior to entering the PCC plant.

4.2.5. Gas turbine humidification

Humid air turbines or Evaporative Gas turbines (EvGT)

27(Diego et al., 2017a)

• A humidification tower inject water to the compressed air stream leaving the compressor. Heat from flue gases is used to saturate the air and obtain a single-phase mixture.

4.2.5. Gas turbine humidification

Humid air turbines or Evaporative Gas turbines (EvGT)

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• EvGTs results in a higher power output due to a large mass flow rate through the expansion stages.

• EvGTs present a higher thermal efficiency compared to open-cycle GT but a lower compared to CCGTs (unless low-T heat is used for water evaporation).

• Yet suitable turbomachinery for HAT-systems still needs to be developed (to cope with flow mismatch between compressor and turbine)

• Limitations on the water to air ratio ( ~ 0.12 – 0.14, higher ratios may result in incomplete combustion and high CO and UHC emissions). Thus limited CO2 increase (up to 5 vol%) is possible.

4.2.5. Gas turbine humidification

Steam Injection Gas Turbine (STIG) systems:

29(Diego et al., 2017a)

• Direct steam injection in the combustion chamber.

HRSG

4.2.5. Gas turbine humidification

30

• STIG results in a higher GT power output due to a large mass flow rate through the expansion stages, yet the thermal efficiency decreases compared to conventional GTCCs.

• STIG technology is available at commercial scale, yet Low-NOx Dry burners are currently being used in the combustion system of currently employed GT engines.

• The CO2 achievable in the flue gases is limited by the highest possible steam to air ratio that ensure stable and complete combustion.

Steam Injection Gas Turbine (STIG) systems:

4.2.5. Gas turbine humidification

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• STIG results in a higher GT power output due to a large mass flow rate through the expansion stages, yet the thermal efficiency decreases compared to conventional GTCCs.

• STIG technology is available at commercial scale, yet Low-NOx Dry burners are currently being used in the combustion system of currently employed GT engines.

• The CO2 achievable in the flue gases is limited by the highest possible steam to air ratio that ensure stable and complete combustion.

Steam Injection Gas Turbine (STIG) systems:

4.3. Summary

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Strategies / Key Challenges: Exhaust gas

recirculation

Evaporative Gas

Turbines

Supplementary

fuel combustion

Sequential

supplementary

fuel combustion

Selective Exhaust

Gas Recirculation

in Parallel

Selective Exhaust

Gas Recirculation

in Series

Impact on

combustion

Comburent

composition

compared to

air-

combustion

Low O2 conc. »

16-17 vol% and

moderately high

CO2 conc. » 2-3

vol%

Low O2 conc.

and high H2O

conc.

Low O2 conc. »12

vol% in secondary

combustor

Low O2 conc. <

12 vol% in

secondary

combustors

Low O2 conc. »

18 %vol and

considerably high

CO2 conc. » 10

vol%

Low O2 conc. »

18 %vol and

considerably high

CO2 conc. » 10

vol%

Potential

problems

Flame stability,Flame stability,

CO and UHC

emissions, soot

formation

Combustion in in-

duct burners

benefits from

high flue gas

temperature

Combustion in in-

duct burners

benefits from

high flue gas

temperature

Flame stability,

CO and UHC

emissions, soot

formation

Flame stability,

CO and UHC

emissions, soot

formation

CO and UHC

emissions, soot

formation

NOx reduction yes yes yes yes yes yes

Impact on

compressor

and turbine

Deviations

from design

operation

point due to:

Compressor inlet

conditions and

working fluid

composition

Increase in

turbine mass

flow affects

pressure ratio

and working

fluid

composition

None None

Compressor inlet

conditions and

working fluid

composition

Compressor inlet

conditions and

working fluid

composition

4.3. Summary

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Strategies / Key Challenges: Exhaust gas

recirculation

Evaporative Gas

Turbines

Supplementary

fuel combustion

Sequential

supplementary

fuel combustion

Selective Exhaust

Gas Recirculation

in Parallel

Selective Exhaust

Gas Recirculation

in Series

Impact on

CCGT

efficiency

Compared to

conventional

CCGT with PCC

Small increase

Moderate

reduction in

combined cycle

Significant

reduction of

several

percentage

points

Significant

reduction of

several

percentage

points

Not investigated/

not available

Not investigated/

not available

Main effect

Increase in

compressor inlet

temperature

Lower heat

available in HRSG

or steam

extraction for

injection

Reheated flue gas

only generates

work in the

steam cycle

Reheated flue

gas only

generates work

in the steam

cycle

Impact on

the CO2

capture

process

Max. CO2 conc.

achievableUp to 6 vol% Up to 6 vol% Up to 6 vol% Up to 9 vol%. Up to 18-20 vol% Up to 18-20 vol%

Flue gas flow

rate reductionUp to 35% Moderate Moderate Up to 50% Up to 70%

None

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Strategies /

Key

Challenges:

Exhaust gas

recirculation

Evaporative Gas

Turbines

Supplementary fuel

combustion

Sequential

supplementary fuel

combustion

Selective Exhaust

Gas Recirculation in

Parallel

Selective Exhaust

Gas Recirculation in

Series

Operating

variable to

increase CO2

Recirculation ratio Water to Air ratio Additional natural

gas fired

Additional natural

gas fired

Recirculation ratio

and selective CO2

transfer efficiency

Selective CO2

transfer efficiency

Limiting

factor

Min. O2 level in the

combustor

Min. O2 level in the

combustor and

water/steam

injection system

Max. temperature

limited by

metallurgical

constraints in the

HRSG

Min. O2 excess in

flue gas for complete

combustion of the

fuel in in-duct

burners

Max. practical

efficiencies in PCC

and selective CO2

transfer process.

Max. CO2 level in the

combustor for

operating O2 conc.

Max. practical

efficiencies in PCC

and selective CO2

transfer process.

Max. CO2 level in the

combustor for

operating O2 conc.

Technology

readiness

Experimental work

reach industrial

scale burners

Process modelling.

Further

development of the

burners required

for combustion

efficiency.

Commercially

availableProcess modelling Process modelling Process modelling

4.3. Summary

References• Diego, M.E., Akram, M., Bellas, J.M., Finney, K. N. and Pourkashanian, M. (2017). Making gas-CCS a commercial reality: The

challenges of scaling up.

• Diego, M.E., Bellas, J.M. and Pourkashanian, M. (2017) Process Analysis of Selective Exhaust Gas Recirculation for CO2 capture in Natural Gas Combined Cycle Power Plants Using Amines, J. of Engineering for Gas Turbines and Power, 139, 121701-1.

• ElKady, A. M., Evulet, A., Brand, A., Ursin, T. P. and Lynghjem, A. (2009) ‘Application of Exhaust Gas Recirculation in a DLN F-Class Combustion System for Postcombustion Carbon Capture’, J.l of Engineering for Gas Turbines and Power, 131(May 2009), p. 34505. doi: 10.1115/1.2982158.

• Evulet, A. T., ELKady, A. M., Branda, A. R. and Chinn, D. (2009) ‘On the Performance and Operability of GE’s Dry Low NOx Combustors utilizing Exhaust Gas Recirculation for PostCombustion Carbon Capture’, Energy Procedia, 1, 3809–3816.

• González Díaz, A., Sánchez Fernández, E., Gibbins, J. and Lucquiaud, M. (2016). Sequential supplementary firing in natural gas combined cycle with carbon capture: A technology option for Mexico for low-carbon electricity generation and CO2 enhanced oil recovery. Int. Journal of Greenhouse Gas Control, 51, 330-345.

• Herraiz, L. (2016) Selective Exhaust Gas Recirculation in Combined Cycle Gas Turbine power plants with Post-combustion Carbon Capture. The University of Edinburgh.

• Herraiz, L., Palfi, E., Sanchez Fernandez, E. and Lucquiaud, M. (2018) Selective Exhaust Gas Recirculation in Combined Cycle Gas Turbine power plants with Post-combustion Carbon Capture. Int. Journal of Greenhouse Gases, accepted.

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References• Li, H., Ditaranto, M., Yan, J. (2012). Carbon capture with low energy penalty: supplementary fired natural gas combined

cycles. Appl. Energy, 97, 164-169.

• Marsh, R., Giles, A., Runyon, J., Pugh, D., Bowen, P., Morris, S., Valera-Medina, A., Best, T., Finney, K. and Pourkashanian, M. (2016) ‘Selective Exhaust Gas Recycling for Carbon Capture Applications: Combustion and Operability Measurement’, The Future of gas Turbine Technology 8th International Gas Turbine Conference, 12-13 October, Brussels, Belgium.

• Merkel, T. C., Wei, X., He, Z., White, L. S., Wijmans, J. G. and Baker, R. W. (2013) ‘Selective Exhaust Gas Recycle with Membranes for CO2 Capture from Natural Gas Combined Cycle Power Plants’, Industrial & Engineering Chemistry Research, 52, 1150–1159

• SELECT (2014) Selective Exhaust Gas Recirculation for Carbon Capture with Gas Turbines: Integration, Intensification, Scale-up and Optimisation. Available at: http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/M001482/1.

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Carbon Capture in Natural Gas Combined Cycle plants (NGCC)Laura Herraiz (l.herraiz@ed.ac.uk)PGRAInstitute for Energy Systems, The University of Edinburgh, UK

CO2 Capture Workshop, Mexico, January 2018