CO2 capture by Condensed Rotational Separation

R.J. van Benthum, H.P. van kemenade, J.J.H. Brouwers, M. GolombokEindhoven University of Technology, Netherlands

∗Corresponding author e-mail: r.j.v.benthum@tue.nlwww.tue.nl/ptc

Abstract

Condensed rotational separation is a technique in which flue gas is cleaned by condensation ofthe CO2 and mechanical centrifugal separation. It requires a purification of CO2 in the flue gas,prior to separation. This purification can be realized with existing techniques like oxygen enrichedcoal combustion or CO2 separating membranes. Combined with an enrichment technique, con-densed rotational separation provides an answer that can compete with promising conventionaltechniques for CO2 capture, like oxy–fuel combustion or amine absorption. These conventionaltechniques produce a waste stream with a high CO2 purity that can be compressed to supercriticalpressure for transport and storage. It is shown that energy consumption of CRS is only slightlymore than gas compression of a sequestration stream resulting from conventional separation tech-niques.

1 Introduction

Application of CO2 capture and storage (CCS) in conventional air blown coal combustion powerplants involves the separation of CO2 from the flue gas at low concentration and low partial vapourpressure. Currently the method of choice in industry is the absorption of CO2 with amine. Thisprocess requires energy however, mostly in the form of low pressure steam to regenerate the solvent.This steam cannot be used for electricity generation and the costs, including compression of the CO2to supercritical pressure, are estimated at 1150 MJ per ton of separated CO2 for 90% CO2 removal[1].

Another approach, currently investigated at pilot scale by Vattenfall [2], is the combustion ofcoal with pure oxygen and is referred to as oxy–fuel combustion. Since no nitrogen is present inthe combustion air, flue gas consists of almost pure CO2. Oxy–fuel enables the capture of CO2 bydirect compression of the flue gas but requires an air separation unit (ASU) to supply the oxygen.The energy penalty of oxy–fuel consists of the air separation and compression of the flue gas and isestimated at 990 MJ per ton of separated CO2, for 90% CO2 removal [1].

In this study we explore a breakthrough compact CO2 capture method that can be either retrofittedto existing power plants or incorporated in traditionally designed new ones: condensed rotationalseparation, abbreviated as CRS. The technology relies on separation by cooling, a technology pio-neered by von Linde in the 1900’s [3]. Gases with a positive Joule–Thomson coefficient decrease intemperature and condense when throttled. By controlling how much the temperature decreases byadiabatic cooling and expansion components can be separated as they have different boiling temper-atures. Several processes have been developed on this principle, for example the Total Sprex processto remove H2S from natural gas [4], Cryocell by CoolEnergy to remove CO2 from natural gas [5],Controlled Freeze Zone by Exxon for sour gas fields [6] and the Alstom anti-sublimation process forflue gases [7].

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Rapid cooling of binary– or multi–component mixtures of gases to temperatures where one, orsome of the components preferentially condense, leads to a mist of very small droplets with diam-eters of 1 to 10 micron. The phenomenon is known to occur by aerosol formation in flue gases ofbiomass combustion installations [8], condensate droplets resulting from cooling of wet natural gas[9] and has also been measured in experiments with CH4/CO2 mixtures and CO2/N2 mixtures [10].For a process which relies on fast phase change as a means of separation to be economical and prac-tical, it is necessary to have a device capable of capturing micron-sized droplets with high collectionefficiency, low pressure drop and small building space.

Cyclones are the standard for liquid/gas separation in hydrocarbon processing plants [11, 12],but can only handle droplet sizes above 15 µm [13–15] . A more feasible solution is provided by therotational particle separator, abbreviated by RPS. The RPS consists of a cylindrical pipe in which arotating element is placed. The rotating element is a simple rotating body consisting of a very largenumber of axial channels of a few millimeters in diameter. In the channels the micron-sized dropletsare centrifuged to form a liquid film at the walls which is ripped of at the exit of the channels inthe form of droplets typically 20 µm or larger. The RPS thus agglomerates droplets to a size, suchthat they can be separated according to the working principle of ordinary axial cyclones [16–18].The rotating element can receive its momentum for rotation by pre-rotation of the gas entering therotating element, and/or by external drive through an electrical motor which is indirectly connectedthrough a magnetic field.

In this paper we do not go into the RPS separation technology, which is standard, but assess thepossibility of CRS for CO2 capture in coal combustion. To that end, the thermodynamics of CRS arediscussed in §2 and translated into separation conditions, separation performance and the involvedenergy penalties. In section 3 a first attempt is made to get an estimation for the energy costs ofCRS. As reference point the 500 MWe supercritical pulverized–coal power plant described in theMIT study "The Future of Coal" [1] is used. After a discussion of the results and future work, themain conclusions are summarized in section 5.

2 Condensed Rotational Separation

The essence of CRS is the mechanical separation of a partially condensed gas mixture into a vaporand a liquid phase by means of cooling, condensation and centrifugal separation.

The concept of CRS was originally developed for cleaning contaminated natural gas (CH4, CO2,H2S) [16]. In natural gas (see figure 1A), contaminated gas from a high pressure gas well (typically100 bar) is cooled by expansion (EXP1) such that the gas mixture enters the vapor–liquid region.The condensed mixture contains at this point a CH4 rich vapor phase and a CO2 rich liquid phase.During the expansion the liquid phase is formed as a mist of micron–size droplets as a result ofheterogeneous condensation in the gas. After separation by the RPS, the cleaned natural gas (theproduct) is compressed to pipeline pressure (typically 100 bar). The liquid waste is pumped tosupercritical pressure (typically 100 bar) [10, 18–20].

2.1 single stage CRS

In contrast to natural gas (figure 1A), flue gas (N2, CO2) from power plants is usually at ambientpressure (1 bar) and moderate temperature. This means that cooling cannot be provided by expan-sion. A heat exchanger has to be used to cool down the gas mixture (figure 1B), and a chiller has tobe provided (HEX1). This is the first energy penalty.

In figure 2A the phase envelope is shown in the p–T diagram for a 21/79 %mole CO2/N2 com-position, corresponding to a typical dry flue gas. This flue gas composition is derived from thestoichiometric combustion of coal (represented by carbon) with normal air (O2,N2).

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COM1

HEX1

RPS

Feed(N2, CO2) Product (vapor)

(N2 rich)

EXP1

RPS

Feed(CH4, H2S, CO2)

Waste (liquid)(H2S, CO2 rich)

Product (vapor)(CH4 rich)

A B

Waste (liquid)(CO2 rich)

figure 1: Comparison of single stage CRS in natural gas and in flue gas. The dotted line below the heatexchanger presents a part of the waste stream that is separated by wall condensation and drainage in theheat exchanger.

For phase equilibrium calculations, an extended equation of state program is developed, basedon a cubic equation of state of the Peng–Robinson type with pure component parameters fitted tovapor pressures and liquid densities along with a composition dependent mixing rule. A separatefreeze out model to predict the boundary of solid CO2 formation is incorporated. The motivationfor developing an in–house code instead of using commercially available programs is that flexibilityin the optimization studies was needed.

Although the vapor–liquid–solid regime is depicted by the grey areas in figures 2 A and B, we donot acces this region with CRS because of the forming of solid particles. It is known that for temper-atures below the freeze–line, solid formation can occur on the walls in the heat exchanger, therebyclogging the heat exchanger. It can also occur in the bulk flow, which is comparable to heteroge-neous condensation. Separation of a mixture in the vapor–liquid–solid regime may result in a vaporphase containing small solid particles and a liquid phase in which solid particles are dissolved. Thisliquid–solid slurry can clog the channels of the RPS and other down–stream equipment. Althoughit might be possible, no research is done sofar on operating CRS in the vapor–liquid–solid area.

temperature [ oC]

−80 −60 −40 −20 0 200

50

100

150

200

250

temperature [ oC]

Pres

sure

[bar

]

CO2

Recovery

−80 −70 −60 −50 −40 −30 −20 −10 00

50

100

150

200

250

0

10

20

30

40

50

60

70

80

90

100

Pres

sure

[bar

]

A B

TRTR

figure 2: Phase diagrams of a A:21/79 CO2/N2 mixture and B:70/30 CO2/N2 mixture. The white solidline represents the boundary of the vapor–liquid region. The grey area and the white dashed line denotethe region in which solid formation occurs and the solids line of CO2. The color scale denotes the CO2recovery on a scale of 0 to 100%.

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With this restraint, the point with the lowest pressure at the interface between the vapor–liquidregime and the solids–line (white dotted lines in figures 2 A and B), is at a temperature of -58 oC and31 bar. This point is mentioned as the lower triple point and is shown in figures 2 A and B by ’TR’.The consequence is that the flue gas has to be compressed (COM1 in figure 1B) in order to applyCRS, thus adding an additional energy penalty to the previous mentioned energy for cooling.

The separation performance of CRS can be expressed in two parameters: The waste CO2 purityand the CO2 recovery. The waste CO2 purity is the concentration of CO2 in the waste stream. Therecovery is defined as the fraction of the total amount of CO2 in the feed that ends up in the waste.An expression for the CO2 recovery is given as:

RCO2 =amount o f CO2 in wasteamount o f CO2 in f eed

=Mliq xCO2

M f eed zCO2

=xCO2

(zCO2 − yCO2

)zCO2

(xCO2 − yCO2

) . (1)

In equation 1, M f eed and Mliq denote the feed and waste flow in [mole s−1], zCO2 , xCO2 and yCO2 themole fractions of CO2 in the feed, waste and product stream.

Strictly defined targets concerning the capture and sequestration of CO2 from flue gases are notwell documented. However, in agreement with the EU framework proposal for CCS a target forwaste CO2 purity (the CO2 concentration in the waste stream) can be set at ≥ 95% and we aim for aCO2 recovery range of 70–90% [21, 22]. This pipeline specification for transport and storage of CO2for sequestration is of great importance since it determines the separation conditions of CRS.

In figures 2A and 2B the color scale denotes the CO2 recovery. The colors in the figures show theCO2 recovery that can be achieved over the range of pressure and temperature in which the vapor–liquid regime is found. For typical flue gas (figure 2A) the maximum CO2 recovery is a meagre 37 %and not worth the effort of applying CRS.

From eq. (1) it can be derived that an increase in feed CO2 concentration is required to increase theCO2 recovery. The flue gas entering CRS must contain a higher CO2 concentration; higher than 21%.To support this conclusion the phase diagram of a 70/30 CO2/N2 mixture is depicted in figure 2B.For this example a maximum recovery 70–90 % can easily be achieved. Next to temperature andpressure a third parameter is now introduced that affects the CO2 recovery: the CO2 concentrationin the feed stream.

From figures 2A and 2B and the evaluation of phase diagrams in between, four important con-clusions can be drawn:

• For a binary mixture within the vapor–liquid regime, the mole fractions of CO2 and N2 in both the vaporand liquid are only dependent on pressure and temperature.

• Highest CO2 recovery is found in the vapor–liquid region at the interface between the solids line and thevapor–liquid region.

• The solids line (dashed white line) is positioned around -60 oC for CO2/N2 mixtures and is not muchinfluenced by the CO2 concentration in the feed.

• The increase of CO2 concentration in the feed results in extension of the phase diagram towards highertemperatures. As a result, the CO2 recovery increases overall.

The first conclusion can be proven with the Gibbs Phase rule. The conclusion implies that CO2and N2 fractions in the vapor and liquid phase do not change if mole fractions in the feed are changedwhile pressure and temperature are kept constant. The total amount of CO2 and N2 in the vapor andliquid phase however do change.

The second and third conclusion lead to the choice of the separation temperature as close aspossible to the solids line (white dashed line). Since the boundary line of the solids area is hardlyaffected by the CO2 concentration, the separation temperature can be set at -55oC for any binaryCO2/N2 mixture. As an implication of the first three conclusions, 95% waste CO2 purity is foundat -55oC and 36 bar, independent from the the feed concentrations. Two parameters that determineseparation performance remain: separation pressure and the CO2 concentration in the CRS feed

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stream. The separation pressure influences both CO2 recovery and the CO2 purity of the wastestream. The CO2 concentration in the feed only influences the CO2 recovery.

Evaluation of the separation performance for different feed concentrations and different separa-tion pressures is shown in figure 3. Figure 3 presents the separation performance as CO2 recoveryversus the waste CO2 purity for different feed concentrations. The pressure increases by clockwisedirection of the lines. It shows that a 40–70% CO2 concentration in the feed is required to achieve≥ 95% waste CO2 purity and a CO2 recovery in the range of 70 to 90% (green area in figure 3).

The results of figure 3 lead to the requirement of CO2 enrichment: The CO2 concentration inthe flue gas, prior to the CRS process needs to be increased. CO2 enrichment can be achieved forexample by combustion of the fuel with oxygen enriched air, or by CO2/N2 separating membranesafter combustion and prior to the CRS process. This enrichment step is the third additional energypenalty.

0 20 40 60 80 10050

55

60

65

70

75

80

85

90

95

100

CO2

Recovery (%)

x CO2

(%

mo

le)

20 %mole

CO2

30 %mole

CO2

50 %mole

CO2

70 %mole

CO2

80 %mole

CO2

figure 3: CO2 recovery against waste CO2 purity (xCO2 ) for a single stage CRS process. The legenda showsthe CO2 concentrations in the feed from 20 to 80%. The green rectangle in the upper right corner depictsthe targets for CCS: 70–90% CO2 recovery and a waste CO2 purity ≥ 95%. Note: Following the lines inclockwise direction corresponds to an increasing separation pressure.

In summary, there are three energy penalty’s involved with CO2 capture by CRS: cooling by achiller, compression, and the CO2 enrichment of the flue gas, prior to CRS. We identified that theseparation performance of CRS can be expressed as a CO2 recovery and a waste CO2 purity. TheCO2 recovery of CRS is determined by the separation pressure and the composition of the CRS feedstream. The waste CO2 purity is solely dependent on pressure, as long as the feed stream consists ofonly CO2 and N2.

2.2 two stage CRS

Single stage CRS operating at a separation pressure of 36 bar results in a waste CO2 purity of 95%as stated in section 2.1. At this pressure, the maximum CO2 recovery is however not reached, as canbe derived from figure 3. At separation pressures above 36 bar, the CO2 recovery is increased but alot of nitrogen is dissolved into the waste stream and the requirement of ≥ 95% is not met. At low

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separation pressures (< 36 bar), the amount of condensed matter is small. However the waste CO2purity is above 95%. It thus meets the requirement for waste CO2 purity, but only a small part of theCO2 is recovered from the feed in the waste stream.

Single stage CRS operating at a pressure above 36 bar can be improved with a second stage inthe waste stream that operates at 36 bar (figure 4A). This second stage produces a waste CO2 purityof 95%. Single stage CRS operating at a pressure below 36 bar can be improved by a second stageadded to the vapor stream (figure 4B), which operates at a pressure above 36 bar. Both options canbe realized as a sequential process (figures 4A and 4B) or a recycle process (figures 4C and 4D). Inthe latter, the product (vapor) or the waste stream (liquid) from the second stage is fed back to thefeed stream.

Waste(CO2 rich)

Product(N2 rich)

Feed(CO2 , N2)

Waste(CO2 rich)

Product(N2 rich)

Feed(CO2 , N2)

Product(N2 rich)

Waste(CO2 rich)

Feed(CO2 , N2)

2

1

12 1

2

Waste(CO2 rich)

Product(N2 rich)

Feed(CO2 , N2)

2

1

(T , P1) (T , P2 > P1)

(T , P2 > P1)

(T , P2 < P1)

(T , P2 < P1) (T , P1)

(T , P1)

(T , P1)

A) Liquid-Sequential B) Vapor-Sequential

C) Liquid-Recycle D) Vapor-Recycle

figure 4: The different combinations of staging that can be distinguished in two stage CRS.

The analysis of separation performance for all depicted configurations in figure 4 is similar. Asan example we discuss the vapor–sequential CRS layout: A single stage CRS process operating at apressure below 36 bar produces a waste CO2 purity ≥ 95% in the waste stream. However, a greatpart of the CO2 leaves the single stage CRS process trough the product stream, resulting in low CO2recovery. Adding a second stage to the product stream that operates at a higher pressure couldimprove the amount of captured CO2. Such a two stage vapor–sequential CRS layout, is shown inmore detail in figure 5.

The second stage consists of compression (COM2) and isobaric cooling (HE2), to reach the separa-tion condition of the second stage. The first stage (psep < 36 bar) results in a liquid CO2 concentration> 95%. The second stage (psep > 36 bar) results in a liquid CO2 concentration < 95%. The secondstage separation pressure is determined by the CO2 concentration of the combined waste stream(waste CO2 purity).

The procedure for finding the second stage pressure is as follows: a first stage pressure is fixedand the second stage pressure is varied over an interval of pressures, such that it operates in thevapor–liquid region of the second stage gas mixture. The interval is further limited by the ≥ 95%requirement of the CO2 concentration (waste CO2 purity) in the combined waste stream. The secondstage pressure is determined by finding the maximum CO2 recovery within this limited interval.

The operating limits of the vapor–sequential layout are determined by the boundaries of the

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vapor–liquid region (white solid line in figures 2A and 2B) and by the requirement for 95% wasteCO2 purity of the combined waste stream.

In comparison with single stage CRS, vapor–sequential CRS results in improved CO2 recovery,while the CO2 concentration in the waste stream is kept at 95% or more.

COM1

HE1

RPS1

Feed(CO2 , N2)

Waste (liquid)(CO2 rich)

Product (vapor)(N2 rich)

HE2

RPS2

COM2

PUMP

figure 5: Vapor–sequential CRS: The second CRS stage is added to the product stream (vapor) of the firststage. The pump illustrates that both liquid streams are at equal pressure before the two streams merge.

In figure 6 the investigated single and two stage processes are compared for the CO2 recovery asa function of the CO2 concentration in the feed. All layouts, depicted in figure 4 produce a waste CO2purity of ≥ 95%. The CO2 recovery is thus determined by a variation of the separation pressure(s),such that the requirement of the waste CO2 purity is satisfied. For the two stage layouts, only themaximum CO2 recovery at a given CO2 concentration in the feed is depicted.

20 30 40 50 60 70 8045

50

55

60

65

70

75

80

85

90

95

CO2

concentration in CRS feed (%mole

)

CO2 re

cove

ry

(%)

Single StageLiquid SequentialVapor SequentialLiquid RecycleVapor Recycle

figure 6: Maximum recovery that can be achieved with the different CRS layouts, while producing a wastestream with a CO2 purity of ≥ 95%.

All two stage processes have a clear advantage over the single stage process in terms of CO2recovery that can be reached at similar CO2 concentration in the feed. In the range of 70–90% CO2recovery, the vapor–sequential CRS process reaches highest CO2 recovery and is therefore the layoutof choice.

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3 Energy costs

In section 2.2 it is found that a vapor–sequential CRS layout has the advantage over the other lay-outs in terms of recovery. To achieve a CO2 recovery of 90% and a waste CO2 purity of 95%, theCO2 concentration in the CRS feed must be at least 64% (see figure 6). For this case an exampleof oxygen enriched coal combustion combined with vapor–sequential CRS is discussed below. Theassumptions for this example can be found in table 1.

tabel 1: Assumptions for the energy calculations of CRS

Power plantNett electrical power output 500 MWethermal–to–electric efficiency 40% (found on gross electric power)coal type Carbon (HV: 32800 kJ/kg)composition normal air 21/79%mole O2/N2

CRSηT/C(isentropice f f iciency) / ηpump 80% (per stage)max. compression ratio 4 (per stage)heat exchanger effectivity 80-95%inlet conditions feed: 1 bar , 40oCexit conditions product:1 bar, 40oC

waste: 100 bar, 40oC

The oxygen enriched air for combustion is assumed to be provided by mixing of normal airand a high purity oxygen stream from an air separation unit, often denoted as ASU. Although it isunlikely to be the best method to get partial enrichment, it is a proven technique and therefore usedas reference for pre–combustion enrichment.

The required high purity oxygen for combustion purposes can be achieved by an air separationunit (ASU) using cryogenic distillation. Typical costs of cryogenic distillation to produce low pres-sure high purity oxygen (∼90% O2) from ambient air can be estimated at a basic separation energyof 576 kJ/kg O2 [23]. This basic separation energy does not include the efficiency of the compres-sors, heat of regeneration of driers and the power consumption of the cooling system of the ASU.However it gives a basic magnitude of today’s feasible energy costs for oxygen enrichment. For thisexample a process overview for the CRS installation is constructed with use of Aspen Plus (figure 7).The CRS feed stream, which is the mass flow of dry flue gas from the power plant, is compressed inthree stages (COM1, COM2 and COM3) with equal pressure ratio to first separation pressure, such thatthe compression ratio of each compressor is <4. The compression ratio between the vapor streamfrom RPS1 and the second separation pressure is also <4. Single stage compressors as commonlyused in LNG and air separation industry can thus be applied.

Separation of the vapor and the formed CO2 rich liquid during cooling takes place in rotatingparticle separators RPS1 and RPS2 (see figure 7). In the first CRS stage, most liquid is formed andseparated, leaving only a small vapor stream for the second CRS stage.

After compressors COM1 to COM3 the flue gas is intercooled in heat exchangers HE1, HE2 and HE3.Cooling water is used to bring the flue gas down to 40oC.

HE4 in figure 7 is cooled against the high pressure waste stream (HE12) and the low temperatureexpanded product streams from the second and third expansion stage (HE9, HE10, HE11). The outlettemperature of HE4 and the outlet pressures of expanders EXP1, EXP2 and EXP3 are chosen such, thatthe inlet and outlet temperatures of HE9, HE10 and HE11 match with the inlet and outlet temperatureof HE4 and that thermal power of HE9 to HE12 match with the thermal power of HE4.

In HE6 the vapor stream from RPS1 is internally cooled against HE8 in the product stream (vapor)from RPS2. The outlet temperature of HE8 is set by the inlet temperature of HE6. The outlet tempera-ture of HE6 is set such that the thermal power of HE6 and HE8 match.

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The additional required cooling is provided by chillers. These are represented by HE5 and HE7

in figure 7. The chillers work as reversed Carnot cycles and their efficiency is assumed equal to theinverse Carnot efficiency. The liquid waste stream is pumped to a pressure of 100 bar by PUMP1 andPUMP2.

For 90% CO2 capture with a waste CO2 purity of 95% by combination of vapor–sequential CRSand oxygen enriched coal combustion, an energy penalty is paid of 729 MJ/t separated CO2. This isequal to 18.4% of the gross electric power output or 7.4% of the produced gross thermal power. Theenergy consumption per component is given in table 2:

tabel 2: Energy operating costs of two stage vapor–sequential CRS combined with oxygen enriched coalcombustion, for a 500 MWe power plant with CO2 capture. (nett electrical output: 500MWe)

[MJ/t CO2][%Pelec

]Oxygen enrichment 246 6.2Compressor 547 13.8Expander -111 -2.8Chiller 41 1Pump 7 0.2total 729 18.4

The results from the example in table 2 show that the total energy requirement is dominated bythe CO2 enrichment by oxygen enriched coal combustion (34% of energy consumption) and the com-pression of the CO2 within the CRS process (75% of energy consumption). Enrichment is thereforean important contributor to the overall energy penalty. The second conclusion that can be drawnfrom this example is that the total energy requirement CRS combined with CO2 enrichment is ofthe same order but somewhat better as other reported technologies like oxy–fuel combustion (990MJ/t, CO2) and amine absorption (1150 MJ/t, CO2) with CO2 compression [1], that produce 90%CO2 recovery and a CO2 purity of 95%.

To achieve a CO2 recovery of <90%, requires less CO2 enrichment, compared to 90% CO2 recov-ery, as can be seen in figure 6. The energy penalty for CO2 enrichment is thus expected to be less. Ifadditionally, the requirement for waste CO2 purity is increased, the separation pressure(s) decrease,along with a further decrease in CO2 recovery. This implies reduced compression costs. Using CRSin combination with an enrichment technique as a bulk separator (CO2 recovery <90% and/or awaste CO2 purity ≥ 95%), can thus substantially lower the costs.

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CO

M1

2.65

. 104 kW

CO

M2

2.64

. 104 kW

CO

M3

2.59

. 104 kW

HE1

-2.6

9. 104 kW

HE2

-2.7

5. 104 kW

HE3

-2.9

6. 104 kW

HE4

-5.7

1. 104 kW

HE5

-1.0

7. 104 kW

RPS

1R

PS2

CO

M4

5.51

. 103 kW

HE6

-6.8

6. 103 kW

HE7

-3.7

9. 103 kW

HE8

7.47

. 103 kW

EXP1

-4.3

2. 103 kW

EXP2

-6.2

4. 103 kW

EXP3

-6.5

6. 103 kW

HE9

7.25

. 103 kW

HE1

06.

73. 10

3 kW

HE1

16.

67. 10

3 kW

V

3.2

bar

160o C

40o C

10.4

bar

161o C

40o C

33.5

bar

162o C

40o C

-43o C

V+L

-55o C

V+L

LV

83.5

bar

22o C

-40o C

-55o C

V+L

V L

100

bar

100

bar

100

bar

-52o C

HE1

24.

19. 10

4 kW

S

21o C

25.1

bar

-52o C

5.1

bar

-52o C

40o C

1 ba

r-5

2o C

FEE

D S

TRE

AM

:m

ixtur

e:

64 %

mol

e, C

O2

36 %

mol

e, N

2m

ass

flow

: 23

2.5

kg/s

pres

sure

: 1

bar

tem

pera

ture

: 40

o C

22.4

%m

ole,

CO

2

77.6

%m

ole,

N2

82.7

kg/

s33

.5 b

ar-5

5 o C

95.5

%m

ole,

CO

2

4.5

%m

ole,

N2

149.

8 kg

/s33

.5 b

ar-5

5o C

WA

STE

STR

EA

M:

( to

sequ

estr

atio

n )95

%m

ole,

CO

25

%m

ole,

N2

159.

3 kg

/s10

0 ba

r40

o C

PR

OD

UC

T S

TRE

AM

:( t

o ch

imne

y )16

.2 %

mol

e, C

O2

83.8

%m

ole,

N2

73.2

kg/

s1

bar

40o C

16.2

%m

ole,

CO

283

.8 %

mol

e, N

2

73.2

kg/

s83

.5 b

ar-5

5o C

87.3

%m

ole,

CO

2

12.7

%m

ole,

N2

9.5

kg/s

83.5

bar

-55o C

40o C

PUM

P11.

09. 10

3 kW

PUM

P21.

86. 10

1 kW

V =

Vap

orL

= Li

quid

S =

Sup

ercr

itica

l

Hea

tEx

chan

ger

1

Hea

tE

xcha

nger

2

Hea

tEx

chan

ger

1

figure 7: Flow and energy diagram of the two stage vapor–sequential CRS process for oxygen enrichedcoal combustion.

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3.1 Energy costs versus recovery

The example calculation in the beginning of § 3 is only a point calculation in terms of CO2 feedconcentration (64%) and CO2 recovery (90%). It is however of interest to take a look at the behaviorof the energy penalty caused by CRS only, over a range of CO2 recoveries and for different CRSlayouts.

For derivation of the energy penalty, the feed mass flow of CRS is required. To derive the CO2enriched feed mass flow for CRS, CO2 enrichment is assumed to be a black box in which N2 isseparated from the flue gas stream to obtain the required CO2 concentration in the CRS feed. Thefeed mass flow is found by solving the componentwise mass balance of this black box.

In figure 8 the energy penalty of single stage CRS is evaluated for 60, 70 and 80% CO2 feedconcentration and a waste CO2 purity between 99 and 95%. For single stage CRS, operating at afixed CO2 feed concentration, a decrease in CO2 recovery comes together with an increase in CO2concentration in the waste stream (waste CO2 purity), as can be seen in figure 3.

0 10 20 30 40 50 60 70 80 90 1005

6

7

8

9

10

11

12

CO2

recovery [%]

%P

elec

tric

[%

]

60% 70%

80%

figure 8: Energy penalty of CRS versus CO2 recovery for 60, 70 and 80% CO2 concentration in the feed.The energy penalty is expressed as percentage of the gross electrical power output

From figure 8 it can be concluded that energy costs of CRS decrease for increasing CO2 feed con-centration and for decreasing CO2 recovery. Further investigation towards possibilities of enrich-ment (pre– and post combustion) is needed to complete the total energy requirement picture. Forevery CO2 recovery an optimum is expected in total energy requirement versus CO2 concentrationin the CRS feed.

In figure 6 it is seen that vapor–sequential CRS results in higher CO2 recovery than single stageCRS for the same CO2 concentration in the feed. In the end, it is however not the CO2 recoverythat determines the layout of choice, but the energy requirement of CRS for the same CO2 recoveryand same CO2 concentration in the feed. For 64% CO2 concentration in the CRS feed (equal to theexample in the beginning of § 3) an example is illustrated in figure 9, where the energy penalty ofsingle stage CRS is compared to vapor–sequential CRS. The line in figure 9 corresponding to vapor–sequential CRS is given over the range of minimum–maximum recovery that can be reached, whileproducing a 95% CO2 concentration in the waste.

Figure 9 shows that the CO2 recovery of single stage CRS, at which the waste CO2 purity is 95%(86% recovery), can also be approximated with vapor–sequential CRS but with a smaller energyrequirement (∼1%). Vapor–sequential CRS has therefore not only the advantage of higher CO2 re-covery, compared to single stage CRS, but also the advantage of a lower energy penalty at equal CO2recovery.

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50 55 60 65 70 75 80 85 90 957

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

CO2

recovery [%]

%P

elec

tric

[%

]

Single stage

Vapor-Sequential

figure 9: Comparison of the energy penalty between single stage and vapor–sequential CRS as a functionof recovery for a CO2 concentration of 64% in the feed stream. The line for vapor–sequential CRS iscalculated for the minimum and maximum recovery that can be achieved while producing a waste of≥95% CO2 purity.

3.2 Compression of the waste CO2 stream

As pointed out previously in this paper, CRS produces a CO2 waste stream as a liquid which is thenpumped to supercritical pressure. Oxy–fuel combustion and amine absorption for example alsoproduce a high purity gaseous CO2 waste stream, but at atmospheric pressure. For transport andunderground injection, the CO2 waste stream must still be compressed to supercritical pressure. Ingeneral all carbon capture processes that produce CO2 for transport and injection have to deal withthis compression of the CO2 waste stream to supercritical pressure. The penalty of this compressionis basically equal for every carbon capture technique. It is therefore interesting to investigate howmuch extra energy is required by CRS, compared to gas compression of the CO2 waste stream fromconventional techniques.

For transport and underground injection, the CO2 waste stream must satisfy the pipeline spec-ification of ≥ 95% CO2 concentration. Since this is almost pure CO2, the CO2 waste stream fromconventional CO2 capture processes can be assumed as a pure CO2 stream. Gas compression of thewaste CO2 stream can be assumed as isothermal compression of an ideal gas. More realistic is how-ever a four stage isentropic compression of a real gas with intercooling and an isentropic efficiency of80%. Ideal gas isothermal compression represents the absolute minimum amount of energy requiredto bring the sequestration stream from atmospheric to supercritical pressure.

Both the energy costs of isothermal compression and 4–stage compression are depicted in figure10, together with the energy costs of single stage CRS from figure 8. Figure 10 shows that if 90% ofthe CO2 in flue gas is captured, about 7–9% of the electric power output of the plant is required tocompress it from 1 bar to a sequestration pressure of 100 bar. To do the same with single stage CRSrequires 11% for the CO2 concentration in the feed for which 90% CO2 recovery can just be achieved.The extra energy required by CRS in this case, is only 2% of the electric power of the power plant.

When the CO2 concentration in the feed increases or the CO2 recovery decreases, in figure 10, theenergy consumption of CRS approaches the energy consumption of the 4 stage gas compression ofthe CO2 waste stream. This has two causes. To explain these two causes, it is convenient to describea single stage CRS process as two physically separated streams (illustrated in figure 11): a N2 richstream and a CO2 rich stream. Both streams are compressed and cooled to separation pressure andtemperature. After separation the N2 rich stream is heated and expanded to the same pressure

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0 10 20 30 40 50 60 70 80 90 1000

2

4

6

8

10

12

CO2

recovery [%]

%P

elec

tric

[%

]

Isothermal compression ideal gas 1−>100 bar4−stage compression 1−>100bar, η

isenth=0.8

60% 70% 80%

figure 10: Energy penalty of CRS for 60, 70 and 80% CO2 concentration in the feed stream, compared togas compression of the sequestration stream as a function of CO2 recovery. The gas compression from 1 to100 bar is calculated for a pure CO2 stream and for both isothermal compression of ideal gas and 4 stageisentropic compression with an isentropic efficiency of 0.8.

and temperature as the feed. For the CO2 rich stream there are two options: Case A: heating ofthe CO2 rich stream and gas compression to supercritical pressure. Case B: pumping the liquid tosupercritical pressure and then heat it up to the same output temperature as case A.

Suppose there are no irreversible losses. Then the thermal cooling energy of the N2 rich streambefore separation in HE1 can be fully exchanged with the thermal energy after separation in HE2.Similarly the compression work before separation in COM1 can be produced completely by expansionafter separation in EXP. Therefore the N2 rich stream is energy neutral.

N2 rich stream

CO2 rich stream

A

B

COM1

COM2

COM4

COM3

EXP

PUMPHEX5 CHIL

HEX3

HEX1 HEX2

HEX4

HEX6

s e

p a

r a t

i o n

figure 11: A schematic presentation of CRS in which the product and waste streams are depicted as phys-ically separated streams. For the CO2 rich stream, case A corresponds to gas compression of the CO2 richwaste stream. Case B represents the liquid pumping as occurs in CRS.

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For case A in the CO2 rich stream, heat before (HE3) and after separation HE4 can be fully ex-changed. Heat exchangers HE3 and HE4 can therefore be neglected from the picture and only a gascompression of the CO2 rich stream (COM2 and COM3) is left. In case B, the liquid is pumped to super-critical pressure, thereby heating up only a few degrees. The thermal energy in HE6 is therefore less,compared to HE4, when heated up to the same outlet temperature as case A. If heat before (HE5) andafter (HE6) is exchanged, there is a lack of cooling power before separation. This cooling power hasto be provided at the separation temperature by a chiller (CHIL). In this paper we suppose chillersthat work on mechanical power and water cooling. For the conditions used in this paper a COPR of2 is used, so the mechanical power required by the chiller is 0.5 times the thermal energy that mustbe cooled from the CO2 rich stream. The energy that must be extracted from the chiller by coolingwater is 1.5 times the thermal energy that must be cooled from the CO2 rich stream.

A lower CO2 recovery in CRS corresponds to a lower separation pressure, as can be derivedfrom the phase diagrams (figure 2). The temperature after compression will therefore be lower,which means less cooling power is needed to cool down the CO2 rich stream to separation tem-perature. As a consequence, the required cooling power by the chiller (CHIL) decreases in case B.The difference between the mechanical energy required in case B (chiller, compressor, pump) andcase A (compressor) becomes therefore smaller with decreasing separation pressure and thus withdecreasing CO2 recovery. One can interpret this as a result of the heat leakage by water cooling inthe chiller in case B, which is not present in case A. Water cooling has a zero energy penalty, sincewater pumping requires a negligible amount of energy compared to the total costs of CRS and it isusually available at power plant locations. The difference in heat leakage between CRS (case B) andgas compression (case A) is the primary effect causing the energy penalty of CRS to approximate 4stage gas compression for decreasing CO2 recovery.

The secondary cause is related to the efficiencies in the system, to describe irreversible losses.CRS operates with a compression efficiency of 80% and heat exchanger effectiveness of 0.8–0.95.The 4–stage gas compression operates at 80% isentropic efficiency. The total efficiency can thereforediffer between CRS (case B) and gas compression (case A). Irreversible losses in the N2 rich streamalso explain the shift of the CRS energy penalty towards 4 stage gas compression in figure 10. Ahigher CO2 concentration in the CRS feed implies that less nitrogen is present. The N2 rich massflow is therefore smaller. Nett irreversible losses in the N2 rich stream (figure 11) thus become lessand the energy consumption of CRS tends more towards 4 stage gas compression of the pure CO2stream (figure 10).

It can be concluded that CRS adds only a small extra energy penalty to the gas compression ofthe CO2 waste stream. This extra energy penalty decreases if the CO2 concentration in the CRSfeed increases or if the CO2 recovery is allowed to be lower then 90%. If one wants to apply aconventional technique like oxy–fuel for the capture of CO2, then one can think of replacing theordinary gas compression by CRS. A small energy penalty is then added by CRS, but the requiredCO2 concentration resulting from oxy–fuel combustion is allowed to be lower then the pipelinespecification. Energy can then be saved on the air separation unit to provide oxygen enriched airfor oxy-fuel combustion and/or costs can be saved on the quality of the fuel without loss of overallpower plant efficiency.

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4 Discussion

Condensed rotational separation can only capture a considerable amount of CO2 (CO2 recovery be-tween 70 and 90%) and bring it to a pipeline specification of ≥ 95%, if combined with a methodto enrich the flue gas in CO2 prior to CRS to a CO2 concentration of 40 to 70%. Furthermore theapplication of CRS is most effective when bulk separation of CO2 is required (CO2 recovery ∼ 70 to90%). In that case, the energy penalty for CRS is about 9 to 12% of the electric power output, as canbe seen in figure 10. Estimated from figure 10 and the example calculation in § 3.1, the energy costsof CRS combined with oxygen enriched coal combustion for 70 to 90% CO2 removal are between 16and 19% of the electric power output. This energy penalty range may vary somewhat for a differentconcentration of CO2 in the CRS feed and/or for a different CRS configuration.

All carbon capture processes require compression of the CO2 stream to approximately 100 bar fortransportation and/or subsurface injection. CRS itself consumes only marginally more energy thanthe gas compression of a sequestration stream from other separation techniques, as was shown in§ 3.2. However, by using CRS instead of conventional compression, the sequestration stream can bebrought to pipeline specifications (≥ 95%). As a consequence, conventional techniques like oxy–fuelcombustion, become a purification step prior to CRS, which have to produce a flue gas stream withonly a moderate CO2 purity. The implication is that the CO2 concentration in the flue gas after oxy–fuel combustion (the CRS feed stream) does not have to satisfy the pipeline specification anymore.A CO2 concentration in the CRS feed steam of 40 to 70% is satisfactory. This means that we canrelax the requirements of oxy–fuel combustion on the leakage of ambient air into the boiler, the fuelquality and the air separation. The operating and capital costs can therefore be potentially lower,then applying oxy–fuel with CO2 as a CO2 capture method.

From sections 2 and 3 it is seen that two stage configurations of CRS can improve the CO2 re-covery for the same concentration of CO2 in the feed stream and if the same waste CO2 purity isrequired. It can also produce a CO2 recovery equal to single stage CRS at lower energy costs, as canbe seen in figure 8. In the range of 70–90% CO2 recovery all configurations are within a variationof CO2 recovery of 10% (figure 6). Thus vapor–sequential CRS not only improves the CO2 recov-ery in the range of 70 to 90%, it also can decrease the energy penalty for the same CO2 recovery ascompared to single stage CRS.

For an estimation of the energy penalty of CRS, a simple model consisting of stoichiometric com-bustion of carbon with normal air (N2, O2) was used, to predict flue gas concentration and flue gasmass flow. Real coal does not exist solely of carbon but also contains hydrogen, nitrogen, sulfur,oxygen moisture and small amounts of other components and its composition and energetic valuevaries widely over the different mining locations. Furthermore, coal is normally combusted withexcess air. Therefore flue gas from coal–fired power plants also contains H2O and O2. Water is con-densed during cooling, or can be removed prior to the CRS process. The oxygen left over from excessair combustion might influence the separation performance of CRS, since O2 behaves like N2 in theflue gas. The influence of O2 on separation performance should be checked for flue gas mixtures ofN2/O2/CO2.

CRS in combination with another enrichment technology is most effective as a bulk separator ascan also be concluded from other CO2 removal applications, i.g. sour gas [24] and coal gasification[25]. Even when CRS by itself cannot reach the required CO2 recovery it can be very cost effective touse CRS in tandem with a conventional technology. The energy consumption for the classic absorp-tion process for example, increases rapidly with both the volume flow and CO2 concentration. CRSmight be used to lower this energy penalty.

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5 Conclusions

• Condensed rotational separation, abbreviated as CRS, requires a CO2 enrichment step prior tothe CRS process, in order to separate a considerable amount of CO2 at a sufficiently high CO2purity. This enrichment can be realized by oxygen enriched coal combustion. It could alsobe realized by CO2/N2 separation after combustion in the case of membranes. Applicationof CRS for CO2 capture can thus supplement other separation techniques to achieve possiblelower overall energy costs, while guaranteeing the pipeline specification for transport andsubsurface injection of CO2.

• All CO2 capture processes have to compress the sequestration stream to a supercritical trans-port and or injection pressure. In general this is a gas compression. CRS produces the CO2 ina liquid form which requires the liquid pumping to supercritical pressure. The energy penaltyof CRS is found to be only slightly higher than gas compression.

• By replacing gas compression with CRS in oxy–fuel combustion, the requirement for the CO2content of the flue gas after combustion can be lowered. This allows for reduced demands onair tightness of the boiler and coal feed system, the fuel quality and the air separation.

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