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Designing a supercritical steam cycle to integrate the energy

requirements of CO2 amine scrubbing

Luis M. Romeo*, Sergio Espatolero, Irene Bolea

Centro de Investigación de Recursos y Consumos Energéticos (CIRCE).

Universidad de Zaragoza. Centro Politécnico Superior. María de Luna, 3, 50018 Zaragoza.

Abstract

Absorption by chemical solvents combined with CO2 long-term storage appears to offer

interesting and commercial applicable CO2 capture technology. However one of the main

disadvantages is related to the large quantities of heat required to regenerate the amine solvent that

means an important power plant efficiency penalty. Different studies have analyzed alternatives to

reduce the heat duty on the reboiler and the thermal integration requirements on existing power

cycles. In these studies integration principles have been well set up, but there is a lack of information

about how to achieve an integrated design and the thermal balances of the modified cycle flowsheet.

This paper proposes and provides details about a set of modifications of a supercritical steam cycle to

overcome the energy requirements through energetic integration with the aim of reducing the

efficiency and power output penalty associated with CO2 capture process. Modifications include a new

designed low-pressure heater flowsheet to take advantage of the CO2 compression cooling for

postcombustion systems and integration of amine reboiler into a steam cycle. It has been carried out

several simulations in order to obtain power plant performance depending on sorbent regeneration

requirements.

Keywords: amines, CO2 capture, power plant, energy integration, steam cycle

1. INTRODUCTION

Since Kyoto Conference and the knowledge of global warming caused by greenhouse gases,

intensive research is being carried out to remove and capture CO2 from gas mixtures, in particular

* Corresponding author (e-mail: luismi@unizar.es). Phone +34 976 762570 Fax +34 976 732078

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from combustion gases. As a result of this work, several pilot plants are being built and planned to

prove these technologies and verify its performance (European Commission, 2004; Feron, 2006).

Despite oxyfuel combustion has lately deserved a lot of efforts, special attention has been

historically focused on postcombustion schemes, in particular chemical absorption. Its advantages

include being a proven and mature technology, requiring little modifications to be integrated into a

existing power plant, and coming up as one of the less expensive technologies to be applied into

power plant (IPCC, 2005; Desideri and Paolucci, 1999; Singh et al., 2003). Nevertheless one of the

main obstacles is related to the large quantities of heat required to regenerate the amine solvent that

means important power plant efficiency penalties. A typical range is between 0.72 and 1.74MWt per

MWe generated in a coal-fired power plant (Ali, 2006). Consequently special efforts have been carried

out to analyze the retrofit of existing fleet of coal power plants as a necessity to explore the best near-

term strategy for CO2 capture. Outstanding studies have analyzed different alternatives to reduce the

heat duty on the reboiler (Abu-Zahra et al., 2006; Aroonwilas and Veawab, 2007) and the thermal

integration requirements on existing power cycles (Desideri and Paolucci, 1999; Singh et al., 2003; Ali,

2006; Mimura et al., 1995; Mimura et al., 1997; Bozutto et al., 2001; Dillon et al, 2005; Gibbins et al,

2005, Chalmers and Gibbins, 2007), but most of them treat integration as black box scheme. It is

usual to integrate the amine reboiler into a steam cycle but not details and thermal balances are

provided about modifications on low-pressure heaters, integration of CO2 compression cooling for

postcombustion systems,

A parametric study of the technical performance of absorption process with

monoethanolamine has been carried out to find out the optimal process specifications (Abu-Zahra et

al., 2006). Baseline case was Fluor Econamine FGTM and the thermal energy requirements were

reduced between 16 and 23% (depending on MEA concentration). Cooling water required was also

decreased by 3-10%. Moreover influence of blended-amines has been analysed. Important reductions

in thermal energy requirement and power plant efficiency penalty have been reported with MEA-

MDEA processes (Aroonwilas and Veawab, 2007). However no technical details were reported about

the integration of capture process into a power plant.

Other studies have been focused on the location of turbine steam extractions and the re-

injection of condensate from stripper to steam cycle. Power reduction around 17% has been reported,

for a 90MW coal-fired power plant, (Mimura et al., 1997), where 611t/h of CO2 are captured and

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compressed, using 737t/h of steam, which is the 54% of the steam leaving the boiler. Some

researchers have calculated power reduction up to 26%, with a decrease in power plant efficiency of

11.6 points for a 320MW coal-fired power plant (Desideri and Paolucci, 1999). In this case 335.2t/h of

steam were extracted at 5bar, in low pressure turbine stage, 33% of the steam leaving the boiler, to

capture 213.1t/h of CO2 and the condensate was re-injected into the deaerator. A novel strategy to

reduce the efficiency losses is based on an extraction from an IP/LP crossover pipe and an expansion

through a new auxiliary turbine (Ali, 2004), to get the adequate conditions for the steam to the reboiler.

In this case, 79% of the steam is drawn-off from a 450MW power plant.

It seems evident that the optimal option is to extract saturated steam midway through the low

pressure section of the turbine (Desideri and Paolucci, 1999; Mimura et al., 1997; Bozzuto et al., 2001;

Ali, 2004) with a pressure between 1.8 and 2.8bar using the lowest quality steam available to fit with

the reboiler requirements (Ali, 2004). Most of the steam turbines do not have an extraction at this

pressure range, as a consequence, perfect integration is only possible when steam cycle is designed

taking into account a future amine scrubbing installation. The present work is conceived as a

contribution to this question.

The aim of this paper is to design the modifications for a supercritical steam cycle (SCSC)

power plant in order to fully integrate a CO2 capture system based on chemical absorption and reduce

the penalties in efficiency and power output and the capture cost associated with capture process.

Detailed information about the power plant thermal balance is included. Integration includes: Heat

from SCSC to desorption reboiler and the modifications in SCSC to reduce efficiency penalty, design

modifications of low-pressure heaters in order to integrate heat of condensation and heat from CO2

intercooling compression.

2. CASE STUDY

The simulated power plant arranges a supercritical pulverized coal-fired boiler which produces

at base load 350 kg/s of live steam at 300bar and 590ºC (supercritical conditions). The reheat stage

arises temperature to 610ºC at 61bar, after two stages expansion. Coal composition and main

operating conditions are showed in table 1. Boiler efficiency has been assumed in 94% (LHV) and

produces 1015MWth at base load and yields 1578.96t/h of flue gas being 336.6tCO2/h (21.31%w). The

steam is expanded through nine turbine stages to condenser at 0.05bar, eight turbine bleedings are

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used for regenerative preheating (four high-pressure, deaerator, four low-pressure) to increase

feedwater temperature and steam cycle efficiency. Figure 1 illustrates the SCSC flow diagram for the

reference case. SCSC simulations have been carried out using Aspen Plus software (Aspen, 2003).

Results for flue gas and steam conditions throughout power cycle are showed in table 2. Coal

consumption, net plant efficiency, electricity output and emission rate are also included tin table 2.

3. INTEGRATION OF POWER PLANT AND MEA SCRUBBING

Initial condition of the simulation has been to capture around 90% of CO2 produced. It is used

a pure 30%w MEA aqueous solution. Five separate absorption/regeneration column trains have been

assumed. Flue gas, with a mass flow of 438.6t/s is drawn-off from desulphurization unit at 55ºC and

1atm. It is assumed no pollutants in flue gas such NOx and SOx. A purge of 5% of degraded MEA will

be also included within the model. Aspen RadFrac (Aspen, 2003) has been used to simulate the

multistage vapor-liquid absorption reactions.

The aim of this paper is to fully integrate a CO2 capture system based on chemical absorption

into a supercritical steam cycle in order to minimize the global energetic efficiency penalty. Amine

scrubbing is characterized by an important thermal heat requirement for amine regeneration at a given

steam pressure. Consequently, integration must include:

- heat from SCSC to desorption reboiler

- incorporation of condensate from reboiler to the SCSC

- modification of steam cycle low-pressure heaters and turbine bleedings

- integration of heat from CO2 intercooling

- integration of heat of condensation (if possible)

- incorporation of new heaters

- modification of original steam/water conditions

Although there are different solvent possibilities (MEA, MDEA, blended amines, KS-1), high

energy requirements are common in all cases, mainly heat for stripper reboiler. Fluor Econamine FGTM

process requires between 4.2GJ/tCO2 (Chapel et al., 1999) and 3.95GJ/tCO2 (Desideri, and Paolucci,

1999), KS-1 process requires 2.76GJ/tCO2 (Mimura et al., 1995; 1997), Fluor Econamine FG PlusTM

3.24GJ/tCO2 (IEA, 2004). These values depend on MEA concentration, with a process optimization

values around 3.89 could be reduced to 3.29GJ/tCO2 or 3.01 with MEA 30 or 40 wt.% (Abu-Zahra et

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al., 2006). Using blended-amines (MEA-MDEA) values of 4.8 or 3.1 could be reduced to 2.4 and

1.2GJ/tCO2 (Aroonwilas and Veawab, 2007). In this study a value of 3.1GJ/tCO2 has been chosen as

base case. Three additional cases will be analysed (4.8, 2.4 and 1.2GJ/tCO2) to study the result and

steam cycle modifications depending on energy requirements.

Absorption thermal heat demand depends on stripper pressure as well. There is a reduction in

thermal energy requirement by increasing stripper pressure, i.e. from 120kPa to 210kPa led to a 16%

reduction (Abu-Zahra et al., 2006). The consensus is that the reboiler temperature must not overcome

122ºC, value above which MEA degradation rates and corrosion problems will occur. Assuming 10ºC

as hot side temperature approach in the reboiler, the steam temperature limitation is 132ºC (Ali, 2004).

Saturation pressure at this temperature is 2.8bar. In the case of study, 257.4 MWth are required, for

providing desorption trains heat. It means that 118.7kg/s steam at 2.8bar must be extracted from a

turbine stage in the power plant. This mass flow increases up to 183.8kg/s (398.6 MWth) for the case

of 4.8GJ/tCO2, and reduces down to 91.9 (199.3 MWth) and 46.0kg/s (99.6 MWth) for the case of

lower specific regeneration energies (2.4 and 1.2GJ/tCO2).

Consequently, low-pressure turbine and low-pressure heaters must be modified in order to

include this pressure as a turbine bleeding and reduce the impact on power and efficiency reduction.

At this pressure, steam bleeding is superheated (T=206ºC, Tsat=132ºC for the case studied) so it has

to be cooled down in a new low-pressure heater to reduce steam temperature near the saturation.

This heater is located before the deaerator. Saturated liquid coming from the stripper reboiler, at a

temperature of 132ºC, will be added to the condensate coming from condenser and CO2 compression

cooling heaters, figure 2.

Integration of compression cooling

Total compression energy required to CO2 conditioning for transport, 100bar and 50ºC, is

around 26.6MWe which represents about 5.6% of the original power plant power output. Only few

studies (Desideri and Paolucci, 1999; Singh et al., 2003) have joined the capture process and its

compression necessities. Recently some works (Aspelund and Jordal, 2006; 2007; Heggum et al.,

2005; Oryshchyn et al., 2006; White et al., 2006; Aspelund et al., 2006; Romeo et al., 2007) have

carried out exhaustive analysis of CO2 compression. There is no thorough research including the

integration of the heat from the compression stages into the steam cycle. Compression process

requires intercooling stages to reduce power requirements and to avoid excessive CO2 temperature.

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A valuable heat stream is produced cooling down the CO2. If it were possible to transfer this energy to

the steam cycle it would not worth to reduce CO2 temperature down to the ambient temperature to

minimize compression power (Romeo et al., 2007). Optimum configuration is obtained when CO2

intercooling heat is transferred after the condensate pump and before any low-pressure heater.

Original low-pressure heaters (and turbine bleedings) should be eliminated and substituted by

different heat streams from compression cooling. An important steam mass flow is necessary for

amine regeneration so last steam turbine stages have lower steam mass flow. Thermal heat required

in the stripper defines the amount of heat transferred from intercooling to the low-pressure steam

cycle. Reducing stripper energy requirement increases the water mass flow in the LP heaters and it is

possible to take advantage of more energy from CO2 compression, reducing energy penalty. In order

to maximize the heat transferred to the steam cycle, intercooling heat has been divided into two

streams taking into account temperature levels. Figure 2 shows final arrangement for the base case.

Compression stages have been reduced from four (conventional configuration) to three to increase

CO2 temperature level in spite of slightly increasing compression power. Supposing a CO2 pressure at

compression inlet around 2 bar, stage pressures of 7, 30 and 100 bar and an intercooling temperature

of 50ºC (minimum temperature difference with condenser outlet of 17ºC), the compressor outlet

temperatures are respectively 160.8, 180.8 and 167.7ºC. LP heaters water mass flow is low compared

with a conventional steam cycle (same power output) due to steam extraction to the reboiler. That

makes it is difficult to seize all the intercooling heat. In particular a great amount of heat from CO2

drying (condensation) should be rejected, increasing cooling water demand.

Proposed configuration, figure 2, distributes the energy from second and third intercooling

stages (higher temperature levels) in two batteries of heat exchangers in order to avoid low

temperature difference between CO2 and condensate. The condensate from stripper increases the

water mass flow, reduces the water temperature difference and heaters work with higher temperature

differences. Table 3 presents the conditions of the steam cycle flowsheet.

Parallel heat exchangers arrangement in CO2 compression is between 160.8 and 50ºC. Water

increases temperature from 33ºC to t1. This temperature depends on stripper energy requirement,

table 4, and consequently on condensate mass flow. Higher stripper heat means a reduction on

condensate mass flow and higher condensate temperatures. Energy in these heat exchangers is

designated as Qinter1, table 5. The stripper heat for regeneration also influences heaters arrangement.

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When this value is low (high condensate flow) it is possible to take advantage of some condensation

heat from flue gases. CO2 outlet temperature from stripper is over 100ºC and it is needed to cool it

down for water condensation. This heat has a low temperature level that makes it difficult to integrate

into the steam cycle. Proposed configuration includes a heat exchanger in parallel with previous ones

to take advantage of some energy (Qcond) to increase condensate temperature up to 80ºC. The excess

of energy should be rejected into the cooling system (Qcooling) Table 5 shows that lower energy

requirements lead to a reduction in cooling necessities and better integration of condenser heat.

Saturated liquid coming from the stripper reboiler (132ºC) is added after these heaters. It increases

water temperature up to t2 (table 4) and mass flow up to mdea. Second parallel arrangement heat

exchangers take advantage of energy (Qint2, table 5) at higher temperature level between 180.0 and

160.8ºC. It increases condensate temperature to t3.

Final low-pressure heater with the superheated turbine bleeding before stripper commented

above (Qlp) increases temperature up to t4. With high regeneration energy the stripper mass flow

increases the value of available energy in this heater and t3 is elevated but it is not possible to

integrate all this energy into the steam cycle. Qcool should be also rejected to cooling equipment.

The supercritical steam cycle power plant was simulated with constant boiler input 1015 MWth

(constant plant size). In literature it is usual to use constant net output that implies to change the boiler

but it seems easier to adapt balance of plant (BOP) for a boiler design. An original 455.5MW net

power output was de-rated by the energy requirements for CO2 capture activities. Four values of

amine regeneration heat and low-pressure steam cycle modifications were considered as described

above.

Results in table 6 show the reduction both in net power and efficiency. Conventional MEA with

4.8 GJ/tCO2 reduces net efficiency in 10.01 points and net output 22.32%, 101.7MW. These values

could increase up to 10.88 points and 110.5MW if intercooling heat is not integrated into the steam

cycle.

Results for advanced MEA (Aroonwilas and Veawab, 2007) or optimized absorption process

(Abu-Zahra et al, 2006), with energy requirements of 3.1GJ/tCO2, show an important reduction in

energy penalty. Net output is reduced in 16.33% (74.4MW) and 7.32 points in net efficiency. These

values could increase up to 8.33 points and 84.6MW (18.6%) if intercooling heat is not integrated into

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the steam cycle. 50.2MWth are recovered from intercooling meaning a reduction in power penalty of

10.2 MW.

Simulation results with blended amines MEA-MDEA are similar to those reported elsewhere

(Aroonwilas and Veawab, 2007). Net output reduction between 13.65% (62.2MW) and 9.4% (43.1MW)

and net power output between 6.13 and 4.24 points drop, depending on energy requirements for

amine regeneration. Without integration these values increase up to 16.25-12.23% the net output and

7.28-5.49 points.

The effect of increasing deaerator pressure from 12 to 18bar is the augmentation of the net

output and efficiency values. Net output increases between 0.5-1% and 0.22-0.48 efficiency points.

Figure 3 shows these results in a net power output- net efficiency graph. It is important to note that the

slope of the line, that is constant, is related with boiler input. Reducing energy requirements from 4200

to 1200 GJ/tCO2 due to solvent improvements clearly reduces the penalties of capture systems.

Additionally the intercooling integration and low pressure SCSC modifications increase the plant

efficiency comparing with simple “black box” integration. Higher reductions are achieved with lower

regeneration energy requirements. For this reason the research and improvements in solvents with

lower energy requirements involve double positive effect in penalty reductions. That points out the

importance in solvent research and development. Figure 4 plots the same results in a lost output vs.

sorbent regeneration energy requirements .Intercooling integration for postcombustion capture

systems are essential to reduce lost output, for sorbent with high regeneration energy reductions are

around 8% from 328 to 320 kWh/tCO2. However reducing regeneration requirements the intercooling

integration could be reduced up to 23% from 166 to 128 kWh/tCO2. The same trend is observed

increasing deaerator pressure although the influence is lower that cooling integration. This fact points

out the importance of integration when improved sorbents are used.

6. CONCLUSIONS

Amine scrubbing is a well-known method for CO2 capture. Chemical reaction mechanisms and

solvent development have been studied in the last decade in order to reduce energy regeneration

requirements. However, the optimum integration of capture process into the power plant has not been

solved yet. The power output and efficiency penalties make that the efficiency optimization and the

economical optimization do not agree. This paper has proposed different possibilities to overcome the

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energy requirements by means of amine scrubbing integration into a commercial power plant, and has

presented different designs and an analysis of the performance of these approaches.

The proper integration of amine scrubbing for CO2 capture reduces the power output and

efficiency penalties for different regeneration energy requirements. Integration has included:

- the proposal to integrate the well-known modification of original steam bleeding

conditions to extract steam at 2.8bar,

- the incorporation of a new heater to cool down this bleeding down to the stripper

temperature requirement,

- the integration of CO2 intercooling, with the novelty to cool CO2 down to 50ºC (not to

25ºC) avoiding cooling requirements

- the integration of condensation heat in a two-stage parallel arrangement,

- the incorporation of condensate from reboiler to the SCSC after the first-stage

integration and

- the elimination of low-pressure turbine bleedings.

This integration scheme has been proposed and detailed conditions of the thermal balance

have been provided. Reductions in power output between 22.3 and 9.4% have been calculated

depending on regeneration requirements. These values increase up to 24.3 and 12.2% without

integration. Simulation results for efficiency penalty show a reduction from 10.9-5.5 to 10.0-4.2

efficiency points when integration is considered. Finally, reducing solvent energy requirement has a

double positive effect on power plant performance, suggesting the importance of researching and

developing advanced solvents with lower regeneration energy demand.

Acknowledgements

The authors are grateful for the financial support from the Spanish Government, without which,

this work could not have been undertaken. The work described in this paper was supported by the

R+D Spanish National Program from the Spanish Ministry of Science and Education under project

ENE2004-06053, Cuasi-zero CO2 emissions power plant technologies research. The Spanish case.

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