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International Journal of Greenhouse Gas Control 39 (2015) 116

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International Journal of Greenhouse Gas Control

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Evaluating Pressure Swing Adsorption as a CO2 separation techniquein coal-red power plants

Luca Riboldi , Olav BollandEnergy and Process Engineering Department, The Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

a r t i c l e i n f o

Article history:Received 2 December 2014Received in revised form 28 January 2015Accepted 4 February 2015Available online 16 May 2015

Keywords:CO2 captureProcess simulationsCoal-red power plantsPressure Swing AdsorptionEfciency penalty

a b s t r a c t

The paper provides with a rst assessment on the suitability of Pressure Swing Adsorption (PSA) as avalid option for Carbon Capture and Storage (CCS) in coal-red power plants. A full-plant analysis ofan Advanced SuperCritical (ASC) pulverized coal plant and of an Integrated Gasication Combined Cycle(IGCC) plant, operating with a PSA unit, is presented. The systems selected aim to represent the most diff-used options for coal-based power generation, respectively in a post- and pre-combustion application ofCO2 separation. The denition of the PSA process is tailored for the two different scenarios considered,starting from the adsorbent selected (zeolite 5A and activated carbon, respectively for post- and pre-combustion). The objective is to investigate the competitiveness of PSA with respect to the benchmarktechnology for CCS, namely absorption. In order to consider the different aspects measuring the effec-tiveness of a CO2 separation technique, the performance of the power plants is evaluated in terms of CO2separation performance, energy efciency and footprint of the technology. The post-combustion scenarioanalysis shows that PSA can be competitive with regard to the separation and the energy performance.PSA is able to match the CO2 separation requirements, and the relative energy penalty is slightly lowerthan that resulting from amine-absorption. Despite that, the footprint of the PSA unit demonstrates to beway larger than that related to absorption and unlikely acceptable. PSA in the pre-combustion scenarioreturns more encouraging results, approaching the outcomes achieved with absorption both in terms ofCO2 separation performance and plant energy efciency. The footprint, even though signicantly largerthan with absorption, appears to be reasonable for actual implementation.

2015 Elsevier Ltd. All rights reserved.

1. Introduction

The atmospheric concentration of carbon dioxide (CO2) hasincreased by 40% since pre-industrial times, and recently passedthe 400ppm milestone. CO2 is regarded as the main responsi-ble for the atmospheric greenhouse effect, which is producing thewarming of the climate system. It is extremely likely that humaninuence has been the dominant cause of the observed warm-ing (IPCC, 2013). One possible mitigation action for stabilizing theatmospheric CO2 concentration, while continuing exploiting fossilfuel resources, is Carbon dioxide Capture and Storage (CCS). CCSconsists in separating CO2 from large anthropogenic point sources,such as thermal power plants, compressing it for transportationand permanently storing it in underground geological formation.There are different types of CO2 capture systems: post-combustion,

Corresponding author at: Kolbjrn Hejes vei 1a, NO-7491 Trondheim, Norway.Tel.: +47 735 93559.

E-mail address: [email protected] (L. Riboldi).

pre-combustion and oxyfuel combustion (IPCC, 2005). Many dif-ferent techniques have been proposed for capturing CO2. Theseincludes: chemical or physical absorption, adsorption, reactivesolids, membranes, cryogenic processes (Ebner and Ritter, 2009).To date, all commercial CO2 capture plants are based on absorptionfor separating CO2 (Herzog et al., 2009), as it is themostmature andwell understood technology.However, its large scale deployment ishindered by the large power consumption,whichnegatively affectsthe energy efciency of the plant. That, summed to other concernsrelated to the solvent toxicity and to the potentially high corro-sion rate,makes advisable to investigate alternatives. In the currentwork, Pressure Swing Adsorption (PSA) process is analyzed as anoption for post- and pre-combustion CO2 capture. PSA is a cyclicprocess. During the adsorption step, the CO2 present in the feedgas stream is xed on the surface of the selected adsorbent. Fol-lowing, the regeneration of the bed is carried out by a pressureswing operation. The potential advantage connected to this processis the absence of any thermal energy duty during the regenera-tion step. Adsorption processes have been successfully employedfor CO2 removal from synthesis gas for hydrogen production

http://dx.doi.org/10.1016/j.ijggc.2015.02.0011750-5836/ 2015 Elsevier Ltd. All rights reserved.

2 L. Riboldi, O. Bolland / International Journal of Greenhouse Gas Control 39 (2015) 116

Nomenclature

ai number of neighboring sites occupied by adsorbatemolecule for species i

Ci gas concentration of species i, mol/m3

Cp specic heat at constant pressure, MJ/(kgK)Cp,ads adsorbed phase specic heat at constant pressure,

J/(kgK)Cp,g gas specic heat at constant pressure, J/(molK)Cp,s particle specic heat at constant pressure, J/(kgK)Ctot total gas concentration, mol/m3

Dax,i axial dispersion coefcient of species i, m2/sDc,i micropore diffusivity of species i, m2/sD0c,i limiting micropore diffusivity at innite tempera-

ture of species i, m2/sDmg,i multicomponent diffusion coefcient of species i,

m2/sDg,ii binary diffusion coefcient of the ij system, m2/sdp particle diameter, mEa,i activation energy of species i, J/molHr,i heat of adsorption of species i, J/molkf gas conductivity, J/(smK)ki equilibrium constant of species i, Pa1

k,i adsorption constant at innite temperature ofspecies i, Pa1

kLDF,i linear driving force coefcient, s1

m mass ow rate, kg/sn mole ow rate, mol/sP pressure, PaPCO2 CO2 purityPr Prandtl numberqi

equilibrium adsorbed concentration of species i,mol/kg

qi averaged adsorbed concentration of species i,mol/kg

qm,i specic saturation adsorption capacity of species i,mol/kg

R universal gas constant, Pam3/(molK)RCO2 CO2 recoveryRH2 H2 recoveryRe Reynolds numberrc crystal radius, mT temperature, Kus supercial velocity, m/syi mole fraction of species iz axial direction, m

Greek letters specic heat ratio bed porosityp particle porosityCO2 CO2 capture efciencyis isentropic efciencynet net electric efciencyax axial thermal dispersion coefcient, J/(smK) dynamic viscosity, Pa si diffusion parameter for species ig gas volumetric mass density, kg/m3

p volumetric mass density of the particle, kg/m3

LDF linear driving force geometrical factor (15 for zeolite5A, 3 for activated carbon)

AcronymsASC advanced supercriticalC compressorCCS carbon capture and storageDHU dehydration unitFS ash separatorIC inter-coolerIGCC integrated gasication combined cycleLHV lower heating valueLDF linear driving forceMSHE multi stream heat exchangerPSA pressure swing adsorptionTV throttling valveWGS water gas shift

Subscriptsi species

SuperscriptsNC number of components

(Cen and Yang, 1986; Ribeiro et al., 2008, 2009; Yang and Lee,1998; Yang et al., 1997). With regard to CCS applications, PSA pro-cess suitability has to be proven yet. A large number of studieshave been done in order to assess PSA processes operating in thecondition typical of post- (Agarwal et al., 2010; Choi et al., 2003;Chou and Chen, 2004; Ishibashi et al., 1996; Kikkinides et al., 1993;Ko et al., 2005; Liu et al., 2011a; Mehrotra et al., 2010; Na et al.,2001, 2002; Nikolic et al., 2008; Plaza et al., 2010; Reynolds et al.,2006; Takamura et al., 2001; Tlili et al., 2009) and pre-combustion(Casas et al., 2013; Schell et al., 2013) applications. A signicantlack was found in the analysis of more comprehensive systems(The Future of Coal, 2007), where the PSA process is integratedwith the rest of the plant. Few works deal with the understandingof such complex arrangements. In post-combustion applications,only preliminary studies have been carried out, whose results canbe considered partial (Panowski et al., 2010) and/or focusing ona particular side of the topic (e.g., economic considerations) (Hoet al., 2008). In pre-combustion applications, more thorough anal-yses have been performed. Liu and Green (2014) evaluated theapplicability of PSA as CO2 removal technology in an IntegratedGasication Combined Cycle (IGCC). They simulated a warm PSAprocess based on a tailored adsorbent, able to perform at elevatedtemperature. The results achieved are in linewith those of a Selexolabsorption process. Other studies investigated the performance ofSorption Enhanced Water Gas Shift (SEWGS), an innovative CO2capture process for pre-combustion applications, applied to bothIGCC (Gazzani et al., 2013) andNatural Gas Combined Cycle (NGCC)(Manzolini et al., 2011). In either case the outcome appears to beextremely promising. The objective of this paper is to provide a full-plant analysis of coal-red plants implementing CO2 capture by acold PSA process, meaning that the process takes place at tempera-ture levels suitable for many of the most common adsorbents. Coalwas selected as fuel because of its higher emission index (higherCO2 emission per unit of energy released). Further, coal utiliza-tion is predicted to increase in the future, under any foreseeablescenario (The Future of Coal, 2007). Thus, CCS will become a crit-ical tool in order to enable a sustainable exploitation of coal. Twoplant congurations were considered, respectively to account fora post- and a pre-combustion scenario. Post-combustion CO2 cap-ture is implemented by integrating a PSA process into an AdvancedSuperCritical (ASC) pulverized coal plant. Pre-combustion CO2 cap-ture is implemented by integrating a PSAprocess into an Integrated

L. Riboldi, O. Bolland / International Journal of Greenhouse Gas Control 39 (2015) 116 3

Gasication Combined Cycle (IGCC) plant. First, the layout of thethermal power plant, to be coupled with the CO2 capture unit, isdened and modeled. Following, the modeling of the PSA processis presented resulting in a dynamic computational model. The pro-cedure for the choice of the optimal PSA process conguration isoutlined. A full-plant analysis is then provided for both the sce-narios. Simulations were also implemented for the reference casewithout CO2 capture and for the case with CO2 capture based on anabsorption process. A plant-level comparison is carried out, retur-ning the competitiveness of PSA process with regard to anothertechnique of decarbonization (i.e., state-of-the-art absorption pro-cesses). The performance of the system is evaluated on three levels,namely CO2 separation performance, energy efciency and foot-print of the technology.

2. Modeling of the power plant

The model of the power plant was developed by ThermoowInc. products: STEAM PRO, GT PRO and THERMOFLEX. The focus ison coal-red power plants, since combustion of coal produces highspecic emission of CO2 per unit of electricity generated. Accord-ingly, two thermal power plant layouts were selected to representthe most common systems for coal-based power generation. Thesesystems are an Advanced SuperCritical (ASC) pulverized coal plantandan IntegratedGasicationCombinedCycle (IGCC)plant, respec-tively constituting the basis for the post- and pre-combustion CO2capture scenario.

First, a baseline casewithoutCO2 capturewasmodeled. Thepur-pose was to benchmark the coal-red power plant, in order to havea reference case for comparisons.However, theobject of the study isto assess the plant performancewhen CO2 capture is implemented.Therefore, thementioned plantswere equippedwith a CO2 captureunit. A mature technology for separating CO2 from the gas streamalready exists (i.e., absorption). For the sake of fair comparisonsbetween different CO2 capture technologies, models were devel-oped for the plants with state-of-the-art absorption CO2 captureprocesses. For the post-combustion scenario, a MEA-based chem-ical absorption process was considered. For the pre-combustionscenario, a Selexol-based two-stage absorption process was con-sidered. Finally, the same power plants coupled to a PSA processfor CO2 capture were modeled, as this constitutes the core of thecurrent work. Six cases were, hence, simulated:

1. ASC plant without CO2 capture2. ASC plant with CO2 capture by absorption3. ASC plant with CO2 capture by PSA4. IGCC plant without CO2 capture5. IGCC plant with CO2 capture by absorption6. IGCC plant with CO2 capture by PSA

All the cases discussed were based on the European Bench-marking Task Force (EBTF) recommendations (DECARBit, 2011).The purpose was to dene a common set of assumptions andparameters for the different simulations, in order to guaranteethe consistency of the comparisons. A description of the referencecoal-red power plants and of the same plants implementing CO2capture by absorption can be found in the EBTF report (DECARBit,2011). In the present work, only the denition of the additionalunits in the plant layout integrating a PSA process is reported, asthis constitutes the novelty of the analysis.

2.1. ASC plant with CO2 capture by PSA

The integration of a PSA unit in the ASC plant is notaffecting much the general layout. The additional units are

all downstream the ue gas treatment units, and consist ina water removal section, a PSA process and a compressionstage for CO2 transport. The plant upstream remains basicallyunchanged. The resultant plant layout is represented in Fig. 1.The characteristics of the most relevant streams are given inTable 1.

The water removal unit is added because water is known tohinder the CO2 adsorption process. An equilibrium separation iscarried out. The ue gas stream is cooled down to approximately20 C and fed to a ash separator. This simple process can onlylower the water content down to about 2%. It would be advis-able to reach water contents much lower than that, but it wouldrequire a different dehydration strategy. This has not been includedin the simulation. For a deeper insight regarding thewater presenceissue, refer to the dedicated Section 3.3. The partially dehydratedue gas stream is entering the PSA unit, where CO2 is separatedfrom the other components in a two stages PSA process. The neces-sity of two PSA stages will be illustrated later. The pressure of theCO2-rich gas stream leaving the PSA unit needs to be raised toan appropriate level for transportation and storage. A target pres-sure of 110bar was assumed. The CO2-rich stream undergoes acompression process in a ve-stage intercooled compressor. TheCO2-lean stream resulting from the PSA process is vented to theatmosphere.

2.2. IGCC plant with CO2 capture by PSA

The addition of a PSA unit to the IGCC plant requires a higherdegree of integration compared to the post-combustion scenario.A major difference is that the CO2-lean gas stream leaving thePSA process (i.e., the H2-rich gas stream) is further processed inthe plant, constituting the fuel for the gas turbine. The additionalunits, with respect to the reference IGCC plant (DECARBit, 2011),consist in a water-gas shift section, a PSA process and a compres-sion stage for CO2 transport. The plant layout is represented inFig. 2. The characteristics of the most relevant streams are givenin Table 2.

The Water-Gas Shift (WGS) converts CO and H2O into CO2 andH2, providing a benecial effect on the following CO2 separationdue to the increase in the CO2 partial pressure. COS hydrolysis isalso carriedout in theWGSprocess. The syngas is thencooleddown.During thecoolingprocess, condensingwater is removed. Thanks tothe relatively highpressure,water presence is drastically decreased(0.6%). The syngas stream at an appropriate temperature is fed tothe H2S removal unit and successively to the PSA unit. The outputsof the PSA process are a CO2-rich stream and a H2-rich stream. Thelatter is the fuel for the gas turbine cycle and is preheated by thesyngas leaving theWGSprocess. Since theCO2-rich gas streamdoesnot achieve the requirements for being processed and transported,a furtherpurication step is implemented. It consists in the removalof impurities bymeans of twoash separators integrated in theCO2compression section (see Fig. 3). This approach has already beensuggested for removing a selection of non-CO2 gases from oxy-combustion power plants (Pipitone and Bolland, 2009; Posch andHaider, 2012). After a rst partial compression (up to 30bar) anda dehydration process, the CO2-rich gas stream enters a system oftwomulti-streamheat exchangers, each followedby aash separa-tor. The appropriately set temperature levels (30 C and 54.5 C(Posch and Haider, 2012)) allow to separate two different streams:a CO2-rich stream, matching the requested purity specications,which completes the compression process; a CO2-lean stream, richin H2, which can be added to the syngas injected as fuel in the gasturbine. The CO2-rich stream is further compressed to 110bar in anintercooled-compressor. An air expander is also present, providingan additional power output. It partially expands the air extracted


HP steamAir

Coal

IP steam

Cold R/H

Feedwater

Limestone

Fly ash

Bottom ashCO2 to transport

and storage

Gypsum Effluent

Waterremoval

Selective CatalyticReduction

Coal and ashhandling ASC boiler island

Water

ElectroStaticPrecipitator

Flue Gas Desulphurization

CO2compression

PSACO2 capture

ASC steam turbine cycle

Flue gas to atmosphere1

2

3

5

6

7

8

9

4

10

Fig. 1. ASC plant with integrated a PSA unit for CO2 capture and a CO2 compression unit.

Table 1Stream table of the ASC plant integrated with a PSA unit for CO2 capture and a CO2 compression unit.

Stream m T P MW Composition (%mol.)

(kg/s) (C) (bar) (g/mol) CO2 N2 O2 Ar SO2 H2O

1 66.2 66.2 1.0 2 744.2 744.2 1.0 28.9 0.03 77.3 20.7 0.9 1.03 735.7 735.7 1.0 29.9 14.9 74.1 2.9 0.9 0.04 7.24 800.8 800.8 1.0 29.8 13.6 74.4 4.4 0.9 0.04 6.75 800.8 800.8 1.0 29.8 13.6 74.4 4.4 0.9 0.04 6.76 823.3 823.3 1.0 29.3 13.1 71.3 4.2 0.9 0.002 10.57 781.1 781.1 1.0 30.4 14.3 77.8 4.6 0.94 0.002 2.38 150.4 150.4 1.0 43.2 95.1 4.6 0.3 0.02 9 619.7 619.7 1.0 28.6 1.7 91.8 5.4 1.1

10 150.4 150.4 110.0 43.2 95.1 4.6 0.3 0.02

Nitrogen to the gas turbine

Air from the gas turbineAir

O2N2

Coal Slag

Syngas

Air SeparationUnit

Gasifier

H2-richsyngas

CO2 to transportand storage

Convective coolerand scrubber

PSA CO2 capture

H2S absorber

Gas turbineHRSG

CO2 compressionand flash separation

H2-richsyngas

Steam turbinecycle

Acid gas

Syngas reheaterand cooling

section

H2-richsyngas

Flue gas to atmosphere

1

2

3 4

56

1311

12 10

9

17

Water GasShift7 8

14

16

Air expander 15

Fig. 2. IGCC plant with integrated a PSA unit for CO2 capture and a CO2 compression unit.


Table 2Stream table of the IGCC plant integrated with a PSA unit for CO2 capture and a CO2 compression unit.

Stream m T P MW Composition (%mol.)

(kg/s) (C) (bar) (g/mol) H2 CO2 CO CH4 N2 O2 Ar H2S H2O

1 38.5 25.0 1.0 7.7 2 64.6 15.0 1.0 28.9 0.03 77.3 20.7 0.9 1.03 8.5 82.5 1.0 8.5 0 100 4 31.2 123.9 44.9 32.0 0 3.5 95.0 1.5 5 64.6 351.8 10.6 28.9 0.03 77.3 20.7 0.9 1.06 87.5 116.2 24.1 28.0 0 100 7 76.3 497.1 43.1 21.3 26.2 3.1 55.7 10.0 0.4 0.2 4.38 108.7 47.2 39.4 20.2 53.1 37.7 1.5 0.06 6.7 0.3 0.1 0.69 107.6 64.0 38.8 20.2 53.5 37.9 1.5 0.06 6.7 0.3 0.0001 0.03

10 19.1 62.5 38.8 6.5 84.7 2.6 2.0 0.1 10.1 0.5 11 88.6 38.6 1.0 37.2 14.8 81.6 0.9 0.03 2.5 0.0612 8.2 17.6 27.7 15.1 63.5 22.8 3.5 0.1 10.0 13 80.4 28.0 110.0 43.7 0.6 98.9 0.1 0.01 0.4 14 27.2 230.0 24.1 7.8 81.5 5.7 2.2 0.08 10.1 0.4 15 64.6 432.3 17.6 28.9 0.03 77.3 20.7 0.9 1.016 656.1 579.5 1.0 27.4 1.2 75.1 10.1 0.8 12.717 656.1 113.8 1.0 27.4 1.2 75.1 10.1 0.8 12.718 88.5 28.0 30.0 37.2 14.8 81.7 0.9 0.03 2.6 19 88.5 30.0 30.0 37.2 14.8 81.7 0.9 0.03 2.6 20 24.5 54.5 28.8 26.9 37.7 54.0 2.2 0.07 6.1 21 16.4 17.7 7.2 43.7 0.6 98.8 0.2 0.01 0.5 22 64.0 17.7 17.4 43.7 0.6 99.0 0.1 0.01 0.3

from the gas turbine compressor and fed to the ASU, in order torecover part of the compression work.

3. Modeling of the PSA unit

3.1. Adsorption bed model

The mathematical model for the dynamic simulation of anadsorption bed relies onmaterial, energy andmomentumbalances.The adsorbents are considered to have a bi-disperse structure (i.e.,a population of macro and micropores). Three material balanceswould be theoretically necessary, one for the bulk gas phase, onefor the macropores and one for the micropores. In order to reducethe computational time requested to solve the set of equations,a simplication was introduced. This simplication is based onthe evaluation of the mass transfer resistances, and it assumes

the limiting case where one mass transfer mechanism is control-ling, namely the diffusion in the micropores. Accordingly, the othermass transfer resistances have been neglected (i.e., macropore andlmdiffusion). This simplicationhavebeen supportedbypreviousstudies (Lopeset al., 2009a;Ruthvenet al., 1980;Yucel andRuthven,1980) and have been already successfully applied by other workssimulating the behavior of PSA units (Ribeiro et al., 2008; Casaset al., 2013). Thekineticof themass transferprocess is accounted forthe LinearDriving Force (LDF) approximation (Yang, 1997;Azevedoand Rodrigues, 1999; Rodrigues and Dias, 1998; Sircar and Hufton,2000). Its application is in line with the material balance simpli-cations above-mentioned. Similarly the energy balances have beensimplied assuming thermal equilibriumbetween the gas and solidphases, reducing to one the equation needed (Ribeiro et al., 2008).An energy balance with the wall and the environment should beconsidered. It is commonpractice to describe the heat transferwith

MSHE MSHE

FS FSTV

TV

C CIC IC

PSA product gas stream

CO2-lean stream

CO2-rich stream

CO2-rich stream

CO2-rich stream to

transport and storage

C

13

12

18 19 20

2122

DHU

11

IC

Fig. 3. CO2 compression unit integrated with a double ash separation process.


the wall and the environment by average heat transfer coefcients.However, the inuence of these terms is decreasing with the size ofthe unit. Given that the novelty of this work is to evaluate the PSAunit performance in actual operating arrangements (large diameterreactors), the reactors have been considered to be adiabatic. Thisapproach seems to provide satisfactory predicting capabilities andit simplies the model. The additional assumptions adopted in themodel are listed below:

The gas in the bulk phase is considered to follow the ideal gaslaw.

Thebed is assumeduniform throughout all its length (10m). Con-stant bulk density (735kg/m3 for the zeolite 5Aand522kg/m3 forthe activated carbon) andbedporosity (0.32 for the zeolite 5Aand0.38 for the activated carbon).

The ow pattern is described by the axially dispersed plug ow. The radial diffusion effects are ignored. The momentum balance is described by the use of the well-

known Ergun equation (Froment et al., 2010). The heat of adsorption is independent of temperature and

adsorbed phase loading.

Based on these assumptions, the governing equations utilizedare the following.

Component mass balance:

Cit

[ + p(1 )] = (usCi)z

+ z

(Dax,iCtot

yiz

) p(1 )qi

t(1)

LDF model:

qit

= kLDF,i(qi qi) with kLDF,i = LDFDc,i

r2c(2)

Overall mass balance:

Ctott

[ + p(1 )] = (usCtot)z

p(1 )NCi

qit

(3)

Energy balance:[Cp,GCtot + p(1 )Cp,GCtot + (1 )Cp,Sp

+(1 )pNCi

Cp,ads,iqi

]T

t= usCp,GCtot

T

z+

z

(ax

T

z

)

+p(1 )NCi

(Hr,i)qit

(4)

Momentum balance:

P

z=

[150

d2p

(1 )23

us + 1.75dp

(1 )3

Gus|us|]

(5)

The transportparameters areevaluated through frequentlyusedcorrelations (see Table A.1 in Appendix A). Averaged values weresuccessively used for the simulations. Physical properties of thegas were evaluated in all the points of the bed through an exter-nal physical property package (i.e., MultiashInfochem ComputerServices Ltd.) interfaced with the main simulation tool.

The adsorbent selected for the post-combustion scenario is azeolite 5A (Liu et al., 2011b). Zeolites are well studied CO2 adsor-bents, which proved to perform well in the conditions typical ofpost-combustion applications (i.e., very low CO2 partial pressure)

(Siriwardane et al., 2001, 2005; Harlick and Tezel, 2004). Eventhough zeolites 13X are normally regarded as the most effectivezeolites for CO2 adsorption processes, a zeolite 5A was considered.This choice was driven by the availability of data and comparativeresults (Liu et al., 2011a). Bearing in mind that the simulation out-puts would possibly be slightly superior with a zeolite 13X, it isopinion of the authors that the key outcomes presented afterwardsare still valid. The same considerations can be applied discussingthe possibility of utilizing two different adsorbents in the two PSAstages. Tailored adsorbents can suit better the specic operatingconditions providing a performance enhancement but hardly sig-nicant.

Theuptakecapacityof theadsorbent isdescribedbyanextendedmulti-site Langmuir model:

qi

qm,i= aikiPi

[1

NCi

(qi

qm,i

)]ai, with ki = k,i exp

(Hr,i

RT

)(6)

Datawere available just for CO2 andN2, themain constituents oftheue gas to process. The fractionofO2 has been includedwithN2.This approximation has been suggested by the similar selectivity ofCO2 with regard to N2 and O2 (Choi et al., 2003; Siriwardane et al.,2001) and it is therefore thought not to meaningfully affect theresults.

The adsorbent selected for the pre-combustion scenario isan activated carbon (Lopes et al., 2009a). Activated carbonsdemonstrated to outperform zeolites when overpassing a certainthreshold (7bar) of CO2 partial pressure (Siriwardane et al., 2001).Thus, in the typical pre-combustion operating conditions (e.g.,PCO2 = 14.7bar) activated carbon has been considered to be themost suitable option. The adsorption isotherm was again describedby an extended multi-site Langmuir model, represented by Eq. (6).Even though equilibrium data were available also for CH4, the syn-gas components given as an input in the PSA model were just CO2,H2, CO andN2. The smallmole fraction ofmethanewould not reallyinuence the performance of the whole unit. Nevertheless, addinganother component resulted in less stability of themodel and addi-tional computational efforts. Thus, the fraction of CH4 has beenincluded with CO.

The physical properties, the kinetic and the equilibrium data ofthe adsorbents are reported in Table 3.

3.2. PSA process

PSA is a gas separation process in which the adsorbent is regen-erated by rapidly reducing the partial pressure of the adsorbedcomponent, either by lowering the total pressure or by using apurge gas. The process is inherently discontinuous, since duringthe regeneration step the gas feed to a column has necessarilyto be interrupted. Thus, different columns working in tandem arerequested in order to enable the processing of a continuous feed.A coordinated group of columns is dened as PSA train. If differenttrains are present, the process gas stream is equally split betweenthem. The columns of a train cyclically undergo a series of stepsin an asynchronous manner. Some of these steps are closely inter-connected, implying restrictions to the scheduling of the cycle. Thesteps that have been considered for the PSA process are:

Feed (F): the feed gas is co-currently injected at the bottom of thecolumn. The components of the gas stream starts to be selectivelyadsorbed on the surface of the adsorbent.

Rinse (R): before starting the regeneration of the bed, part of theproduct gas is fed to the column. This gas, rich in CO2, displacesthe inert bulk gas remained in the column after the feed step.


Table 3Bed characteristics, physical properties, kinetic data equilibrium data of the adsorbents.

Physical properties

dp (mm) p p (kg/m3) Cp,s (J/(kgK))

Zeolite 5A (Liu et al., 2011b) 2.70 0.30 1083 920Activated carbon (Lopes et al., 2009a) 2.34 0.57 842 709

Equilibrium and kinetic parameters

a k (Pa1) qm (mol/kg) Hr (kJ/mol) D0c/rc2 (s1) Ea (kJ/mol)

Zeolite 5A (Liu et al., 2011b)CO2 2.1 1.47E11 3.92 37.9 14.8 26.3N2 2.5 3.79E11 3.28 19.4 0.1 6.3

Activated carbon (Lopes et al., 2009a)CO2 3.0 2.13E11 7.86 29.1 17.5 15.8N2 4.0 2.34E10 5.89 16.3 1.0 7.0H2 1.0 7.69E11 23.57 12.8 14.8 10.4CO 2.6 2.68E11 9.06 22.6 59.2 17.5

Depressurization (D): the pressure is reduced by putting in con-tact the column with another at a lower pressure level.

Blowdown (BD): the pressure is reduced to the lowest level inorder to regenerate the bed. A stream of CO2-rich gas is leavingthe column during this step.

Purge (Pu): the regeneration is completed by injecting a purg-ing gas into the column, normally counter-currently. This stepis again carried out at the lowest pressure of the system andproduces a CO2-rich gas stream.

Pressurization (P): the pressure is increased by putting in contactthe column with another at a higher pressure level.

Null (N): the column is left idle. Feed Pressurization (FP): part of the feed gas is used to pressur-

ize the column to the highest pressure level necessary for theadsorption process.

The different operating conditions in which the PSA processis supposed to perform in post- and pre-combustions scenarios,necessarily led to different congurations, in terms of number ofbeds and type of steps. The guiding criterion, for the selectionof the process layout, was the necessity of approaching values ofCO2 recovery and purity sufcient for a CCS application (i.e., CO2recovery 90% and purity 95%). A multitude of different processcongurations exists and may be employed. Given the large num-ber of variables to consider (i.e., type and number of steps, duration

of the cycle, adsorbent material, etc.) there is not a well-denedframework to pinpoint the most suitable alternative. In the presentwork, it was decided to refer to cycle congurations successfullyemployed in the literature (Liu et al., 2011a; Casas et al., 2013).Minor changeshavebeendonewith respect to those cycles, in orderto deal with the slightly different operating conditions considered.However, other congurations are possible and may lead to similargood performance. For the post-combustion scenario, a rst PSAstage consists in a three-bed ve-step cycle, while a second stageconsists ina two-bedve-stepcycle. The sequenceofdifferent stepsundergone by a column is shown in Fig. 4. For the pre-combustionscenario, the PSA conguration adopted in the present work is aseven-bed twelve-step cycle, where the sequence of different stepsundergone by a column is shown in Fig. 5.

Different boundary conditions have to be established for eachstep of the PSA cycle. The Danckwerts boundary conditions areapplied. Theyassumenodispersionor radial variation in concentra-tion or temperature either upstreamor downstreamof the reactionsection. Table B.1 in Appendix B reports those boundary conditions.

The energy consumptiondirectly related to the PSAprocess con-sists in the power necessary to a fan to overcome the pressuredrops and the power necessary to the vacuum pump to create anunder-atmospheric pressure (when requested from the regenera-tion strategy). If a rinse step is implemented, a fan is necessary forfeeding the rinse ow rate into the column and overcoming the

Bed 1

Bed 2

Bed 3

F BD Pu FP

Pu BDFFP R

R

BD Pu FP F

F R BD Pu FP

FFP D BD

P

F D BD P FP

Product of other

PSA trains

Feed

CO2 to compression and storage

P

DBD FFPBD

R

Bed 1

Bed 2

WastePurge

Product

Waste

Feed

Feed Feed

First

PSA

stage

Second

PSA

stage

Fig. 4. PSA processes for the post-combustion scenario. Representation of the sequence of steps undergone by a single column in the rst and second PSA stage.


D2

FP F D1 D2 D3 D4 BD Pu P4 P3 P2 P1N N N N

P1 N FP F D1 D2 D3 D4 BD Pu P4 N P3 N P2P2 N

FP F D1 D2 D3 D4 BD Pu P4P2 P1N N N N

P4 P3 P2 P1N N N N FP

P3

F D1 D2 D3 D4 BDPu Pu

P4 P3 P2 P1N N N N FP F D1 D2 D3 D4BD Pu

P4 P3 P2 P1N N N N FP F D1BD Pu D2D3 D4

D1 D2 D3 D4 BD Pu P4 P3 P2 P1N N N FP FNF

F D X 4 BD Pu P X 4 N X 4 FP

Bed 1

Bed 2

Bed 3

Bed 4

Bed 5

Bed 6

Bed 7

OCdeeF -rich

CO -rich

Feed

H -richPurge

Fig. 5. PSA process for the pre-combustion scenario. Representation of the sequence of steps undergone by a single column.

pressure drops. Furthermore, a gas compression may be applied,with the relative compression power duty. These energy consump-tions were evaluated within the PSA model as following:

Fan power = 1is

fanfan 1

RTin

[(PinPout

)(fan1)/fan 1]nin (7)

Compressor power

= 1is

comprcompr 1RTin

[(PinPout

)(compr1)/compr 1]nin (8)

Vacuum power

= 1is

vacuumvacuum 1RTin

[(Patm

Pvacuum

)(vacuum1)/vacuum 1]nin (9)

3.3. Water and adsorption

Presence of water is often troublesome in PSA processes. Watercompetitively adsorb on the solid sorbents and tend to accumulatesince classical pressure swing operation may be not sufcient todesorb it. Both zeolites and activated carbons have demonstratedto experience this negative effect (zeolites appears to be more sen-sible to water presence). Few studies really dealt with this issuein detail when analyzing the suitability of CO2 capture throughPSA processes. Some experimental studies have been conductedbothwith zeolites (Brandani andRuthven, 2004; Gallei and Stumpf,1976; Li et al., 2011) andwith activated carbons (Lopes et al., 2009a;Adams et al., 1988; Wang et al., 2008). However, not much hasbeen done regarding modeling. This can be considered as a big gap,especially when considering post-combustion application wheresignicant amount of water is present in the ue gas. The commonapproach suggested in the literature is to remove water prior theCO2 capture unit by means of a separate PSA unit or a pre-layer ofselective adsorbents like activated alumina or silica gel desiccants(Liu et al., 2011a; Chue et al., 1995). These methods have to proveto perform satisfactorily integrated in the complex arrangement ofa power plant with CO2 capture systems. Further, they will resultin additional power consumption.

In the post-combustion simulation proposed, water is removedto as large extent as possible by condensation, and the remaining

water is neglected in the PSA process due to lack of modeling data.The effect of this approximation could not be evaluated and wouldneed to be investigated. For pre-combustion applications the con-tent of water in the syngas entering the PSA unit is down to tracelevel (0.03%). As long as a more efcient regeneration procedure(e.g., heating of the bed) is planned after a certain number of cycles,in order to avoid water accumulation, the performance should notbe signicantly affected (Ribeiro et al., 2009). Thus, the water con-tent was neglected in the present work without further concerns.

3.4. Solution of the PSA model

The described modeling framework for the PSA process resultsin a set of partial differential and algebraic equations (PDAEs). Thesolution was obtained implementing the modeling equations ingPROMS environment (Process System Enterprise, London, UK).The set of PDAEs requires a considerable computational effort inorder to be solved. One way to simplify the model, thus to reducethe computational time, was to adopt a one-column approach. Thismodeling strategy consists in simulating just one of the columns ofthe whole train (Ribeiro et al., 2009; Casas et al., 2013; Jiang et al.,2004; Park et al., 2000). The interactionsbetweendifferent columnsare accounted for by virtual gas streams which are dened throughthe information stored in the previous cycles. The rinse, purge andpressure equalization-pressurization steps rely on this modelingtechnique. Adopting this simplication, it is essential to assure thatthe mass balances are always closed. This is rather straightforwardfor the rinse and purge steps, while the pressure equalization stepsrequires an additional effort. In fact, an appropriate value of theequalization pressure needs to be set, in order to avoid inconsis-tency in the mass balances. The procedure outlined by Casas et al.(2013) was applied to determine this pressure level.

The discretization algorithm applied for the numerical solutionof the model is the Centered Finite Difference Method (CFDM). Thespatial domain was discretized in 150 intervals. A higher numberof discretization points was not used, because it would have signif-icantly increased the computational time, without increasing in asimilar manner the accuracy of the simulation.

The columns are considered to be initially lled with nitrogenand hydrogen, respectively in the post and pre-combustion sce-nario. The simulation is stopped when the Cycle Steady State (CSS)


arises. At CSS the process repeats itself invariably, meaning thatthe conditions at the end of each cycle are the same as those atthe beginning. Whilst the operation of a single column remainsbatchwise, the process reaches a steady condition. All the resultspresented refer to the cycles at CSS.

4. Results and discussion

4.1. Denition of the performance parameters

The CO2 separation performance is primarily evaluated in termsof CO2 recovery (RCO2 ) and purity (PCO2 ). In the pre-combustionscenario it is also useful to dene the H2 recovery (RH2), giving thatH2 is fuelling the downstream gas turbine cycle. The CO2 recov-ery may be misleading when large energy penalties result from theCO2 separation process. For this reason, an additional parameterwas introduced, namely the CO2 capture efciency (CO2 ). The CO2capture efciency is the real measure to what extent the CO2 iscaptured from a power plant, relatively to a reference plant with-out CO2 capture. The aforementioned parameters are dened asfollowing:

RCO2 =mof CO2 in the product stream

mof CO2 formed(10)

PCO2 = CO2 volumetric concentration in the product stream (11)

RH2 =mof H2 entering the gas turbine as fuelmof H2 entering the CO2 separation unit

(12)

CO2

= 1 net for the reference plant without CO2 capturenet for the plant implementing CO2 capture

(1 RCO2

)(13)

The energy efciency of the plant is evaluated through the netelectric efciency (net), referred to the LHV:

net = Net electric outputNet fuel input (14)

The footprint of the CO2 separation technology is evaluated interms of square meters occupied by the relative unit. The prelim-inary analysis carried out considers the size and the number ofcolumns necessary for the CO2 separation process. A more thor-oughanalysis, includingall theequipment relative to the separationprocess, would be needed in order to obtain more reliable outputs.However, it has been considered beyond the sake of the presentwork, which aims to give a rst assessment on the possible dimen-sions of the units and on the difference between the separationtechniques.

4.2. Post-combustion PSA process

Liu et al. (2011a) demonstrated that, in order to achieve therequestedperformance in termsofCO2 recoveryandpurity, theuegas resulting from the combustion of coal needs to undergo a two-stage PSA process. The rst stage considered in the current workconsists in a three-bed and ve-step cycle (Fig. 4). Since no ue gascompression is implemented upstream the PSA unit, the ue gasenters at about atmospheric pressure. The aim of the rst stage isto achieve the highest possible CO2 recovery. As a tradeoff, it is notpossible to achieve very high CO2 purity. The regeneration processis carried out by decreasing the pressure to 0.1 bar. This pressurevalue has been suggested in many studies (Kikkinides et al., 1993;Liu et al., 2011a; Na et al., 2001, 2002; Takamura et al., 2001). Theregeneration pressure to be applied is dependent on the shape

of the adsorbent isotherm and on the degree of vacuum to reachin order to guarantee proper bed regeneration. 0.1 bar seemed tobalance the different requirements. Other values may have beenconsidered but the advantages in terms of energy savings obtainedwith a higher regeneration pressure would be counterbalanced bylower separation performance. As an example, some simulationswere implemented with the vacuum level set to 0.2 bar. Whilstthe energy penalty could be effectively reduced of about 0.5%, theoverall CO2 recovery dropped under the target value (86.8 %). TheCO2 enriched-gas leaving from the blowdown and purge steps arethen collected and sent to the second PSA stage, a two-bed ve-step cycle (two-bed six-step if purge is implemented), where it isfurther puried. In order to enhance the second PSAprocess perfor-mance, a compression of the gas stream is implemented betweenthe PSA stages. The gas is brought up to 2bar before undergoingthe second adsorption process. Fig. 6 shows the overall levels ofCO2 recovery and CO2 purity obtained in the PSA process (after thetwo PSA stages) by varying the Purge-to-Feed mole ow rate ratio(P/F) of the secondPSA stage. It is clear from thegure that there is atradeoff between CO2 recovery and purity. The highlighted point inFig. 6 (PCO2 = 95.1% andRCO2 = 90.2%) represents the PSAoperatingconditions selected for the process to be matched with the powerplant. It refers to a PSA process in which the purge step has notbeen implemented, hence with a P/F ratio equal to zero. This con-guration was chosen because it is able to contemporary fulll thespecication of CO2 recovery and purity. Additionally, the absenceof a purge step simplies the process conguration. The resultantcharacteristics of the two PSA stages, which were selected to beintegrated in the ASC plant, are reported in Table 4.

The PSA columns were initially sized in order to be able to pro-cess the entire ow rate. Since an excessively large diameter wouldhave been required, a maximum size of 8m was stated. A limita-tion to the supercial velocity was also introduced (0.15m/s), inorder to maintain the pressure drop in the column within a cer-tain threshold (0.1bar). The supercial velocity adopted was alsoveried to be lower than the minimum uidization velocity. Thesedesign considerations implied the need for splitting the total owrate in a number of trains, respectively 73 and 23 for the rst andsecond PSA stage. Fewer trains are needed in the second PSA stagebecause large part of the undesired components has already beenseparated in the rst PSA stage.

4.3. Post-combustion scenario analysis

Table 5 summarizes the outcome of the full-plant analysis car-ried out on the three cases considered for the post-combustion

90.0 %

90.5 %

91.0 %

91.5 %

92.0 %

92.5 %

93.0 %

93.5 %

91.0 % 92.0 % 93.0 % 94.0 % 95.0 % 96.0 %

CO

2 rec

over

y

CO2 purity

P/F = 0,056

P/F = 0,042

P/F = 0,028

P/F = 0,014

P/F = 0,007

P/F = 0

Fig. 6. CO2 separation performance of the PSA process in the post-combustion sce-nario. Results reported refer to different Purge-to-Feed ratio (P/F) of the mole owrates in the second PSA stage.


Table 4Scheduling, characteristics and performance of the PSA process in the post-combustion scenario.

Step time (s) Mole ow rate (mol/s) CO2 (%)

Stage F R D BD Pu P FP Feed Purge Rinse Purity Recovery

1 702 234 0 702 234 234 304.3 91.3 91.3 49.7 93.82 650 50 830 50 180 360.0 95.1 96.1

scenario. TheplantwithoutCO2 capture facilities and theplantwitha state-of-the-art absorption unit were dened in compliance withthe framework determined in the EBTF project (DECARBit, 2011).They are meant to be the basis for comparison with the ASC plantintegrated with PSA, dened in this work. All the simulations werecarried out with the same net fuel input.

4.3.1. Separation performanceThe CO2 separation performance of the PSA process succeeds to

meet the required specications (RCO2 = 90.2%andPCO2 = 95.1%). Ifnecessary, those values can be further increased at the expense of ahigher energy consumption. As an example, a ue gas compressioncan be implemented before the PSA unit. The resulting increase intheuegas totalpressurewould implyan increaseof theCO2 partialpressure, positively affecting the adsorption process. A simulationwas run to evaluate this option, considering a ue gas compressionfrom 1bar to 1.5 bar. The outputs fully met the CO2-rich streamspecications (RCO2 = 90.85% and PCO2 = 95.42%) even applying alower pressure at the entrance of the second PSA unit (i.e., 1.5 barinstead of 2bar). However, the compression of the ue gas wouldbe an energy demanding process and the impact on the energy bal-ance of the system is evaluated later. The general outcome is thatthe CO2 separation performance of the PSA unit, dened includingtwo following PSA stages, is able to reach the target levels of CO2recovery and purity, and to return a CO2 efciency even slightlyhigher than absorption. Moreover, by playing with the PSA processconguration, it is possible to further raise or lower down the sepa-rationperformancewith a consistent impact on the energypenalty:the highest the desired separation performance, the highest is theexpected energy penalty.

4.3.2. Energy performancePSAdemonstrates tobecompetitivewithabsorptionwhen look-

ing at the energy analysis. The attained net electric efciency is

Table 5Main outputs of the full-plant analysis in the post-combustion scenario.

Plant summary No capture Absorption PSA

Power inputsCoal ow rate (kg/s) 66.2 66.2 66.2Coal LHV (MJ/kg) 25.2 25.2 25.2Net fuel input (MWth) 1665.5 1665.5 1665.6

Power outputsSteam turbine output (MW) 828.1 714.6 827.3Gross electric output (MW) 828.1 714.6 827.3CO2 separation powerconsumption (MW)

10.4 102.8

Flue gas compression powerconsumption (MW)

0.0 15.4

CO2 compression powerconsumption (MW)

47.5 52.8

Miscellaneous auxiliaries(MW)

77.4 87.0 77.5

Total auxiliary powerconsumption (MW)

77.4 144.8 248.4

Net electric output (MW) 750.7 569.7 578.9Plant performance

Net electric efciency (%) 45.1 34.2 34.8CO2 purity (%) 100.0 95.1CO2 recovery (%) 90.0 90.2CO2 capture efciency (%) 86.8 87.3

slightly higher to that associated with the absorption-based plant.The reference ASC plant without CO2 capture displayed a net of45.1%. It drops to 34.2% and 34.8%, respectively with CO2 captureby absorption and PSA. Before it was mentioned the possibility ofcarrying out a ue gas compression (up to 1.5 bar) upstream thePSA process, attaining enhanced CO2 separation performance. Theenergy spent for the compression would have a signicant impacton the energy balance of the plant, lowering the nal net downto 33.6%. A reason that may justify such a procedure is the ben-et that would be obtained in terms of sizes and footprint of theseparation unit. Thus, the possibility will be still mentioned in thefootprint section, but, otherwise, this option does not appear to beworth of further analyses. The most signicant power consump-tions, contributing to reduce the net of the plant in the presence ofCO2 capture processes, are shown in Fig. 7. It is worthwhile tomen-tion that, in order to be able to compare the difference sources ofpower losses, the power consumption connected to steam extrac-tions needs to be dened (while all the others are direct electricpower consumptions). In fact, the reduction in power output isless than the heat content of the steam. It was evaluated consid-ering the missing expansion of the steam between the extractionpoint and the downstream condenser, the steam condition at theextraction point and the steam turbine efciency. Eq. (15) showsthe methodology adopted:

Power consumption due to steam extraction

= is,stmsteamcpT1[1

(P2P1

)(1)/](15)

The total power consumption is slightly lower for the PSA case,as was easily predictable given the higher net. When applying anabsorption process for capturing CO2, the largest share of powerconsumption is connected to the reboiler heating duty for theregeneration of the solvent. In order to comply with this energydemand, steam is extracted fromthe turbine. Thisprocedure resultsin a decrease of the gross power output of the plant of about113.6MW. The other signicant power consumption is related tothe compression of the CO2-rich stream. A ve-stage intercooledcompressor is used to raise the pressure from 1.7bar to 110bar fortransport (47.5MW). In the PSA case the process is not demand-ing for any steam extraction. However, other sources of powerconsumptions are present. They are related to the pressure mod-ications undergone by the ue gas, necessary to carry out theadsorptiondesorption process. The term dened as CO2 sepa-ration power consumption includes in the PSA case: the powerrequested by the vacuum pumps to establish the vacuum for theregeneration of the bed (95.5MW); the power supplied to the fan toovercome the pressure drops during the feed, feed pressurizationand rinse step (7.3MW). The CO2 separation power consumptionresults to be the largest source of power loss (102.8MW), while inthe absorption case it has a limited impact (10.4MW mainly due tothe consumption of the pumps for the solvent circulation). The uegas compression occurring between the two PSA stages has a non-negligible impact on the energy balance, accounting for 15.4MW.In the PSA case the CO2-rich stream compression displays a powerconsumption of 52.8MW. The compression power duty is largerthan in the absorption case mainly because of the higher pressureratio to provide. The CO2-rich stream leaves the PSA process at a


0

20

40

60

80

100

120

140

160

180

200

ASPnoitprosbA

Pow

er c

onsu

mpt

ion

[MW

] Steam extraction for reboiler [MW] -STEAM

CO separation [MW] - SEP

Flue gas compression [MW] - FG COMPR

CO compression [MW] - COMPR

Total power consumption [MW] - TOTST

EAM

STEA

M

SEP

SEP

FG C

OM

PR

FG C

OM

PR

CO

MPR

CO

MPR

TOT

TOT

Fig. 7. Power consumptions related to the CO2 capture and compression process for the post-combustion scenario.

lower pressure level (1 bar) compared to that resulting from theabsorption process (1.7 bar). In conclusion, capturing CO2 in a PSAprocess displays the big advantage of not requiring any steam, leav-ing untouched the steam turbine cycle. The implementation of aPSA process introduces new sources of power consumptions con-nected to the pressure swing processes necessary to comply withthe requested CO2 separation performance. However, the overallbalance seems positive under an energy point of view. It is worthto mention that the pumps and compressors simulated have beenconsidered to operate at steady state. This is a strong simplica-tion given the inherent dynamic behavior of a PSA process. It is notknown to what extent a discontinuous feed to those devices cannegatively affect their performance.

4.3.3. FootprintThe mole ow rate entering a single PSA train cannot be further

increased, compared to the level reported in Table 4, for limita-tions related to the pressure drop and the minimum uidizationvelocity. Treating the total ue gas volume, the plant needs a largenumber of PSA trains (i.e., about 73 and 23 trains for the rstand second PSA stage). Each PSA train is constituted by 3 and 2columns, respectively in the rst and second PSA stage, and thediameter of a column was set to 8m. Table 6 shows an estima-tion of the footprints of the two separation techniques considered.The absorption column diameter was calculated by dening a rea-sonable supercial velocity of the ue gas entering the column(i.e., 2m/s). It becomes clear that the total footprint of the PSA-based CO2 capture unit is excessive to be considered feasible. Away to partially reduce the footprint could be to introduce a uegas compression before the PSA unit. Compressing the ue gasup to 1.5 bar demonstrated to lead to a reduction in the numberof necessary PSA trains of about 9units. It was already veriedthat this operation would also be benecial for the CO2 sepa-ration process. However, the nal footprint would still be muchlarger than that of the absorption-counterpart. Not to mention theadditional power consumption introduced which would severelyaffect the process competitiveness under an energy efciency pointof view.

Table 6Footprint analysis for the post-combustion scenario.

Absorption PSA

Column diameter (m) 20.7 8.0Number of columns 2 264Footprint (m2) 674 13285

4.4. Pre-combustion PSA process

The PSA process is supposed to be able to process the syngas andreturn two streams: a CO2-rich stream to be sent to compressionand transportation; and a CO2-lean stream, rich in H2, to be fed tothe gas turbine as fuel. Both streams request some purity charac-teristics to be fullled, namely CO2 and/or H2 purity and recovery.Previous studies (Casas et al., 2013) suggested that a single PSAstage would have been able to fulll these requirements in con-ditions typical for a pre-combustion application. However, Casaset al. (2013) simulated a gas stream which contains only H2 andCO2. When applying a realistic syngas composition, the results ofthe simulations becamedifferent from those expected. The PSA lay-out adopted in the present work is a seven-bed and twelve-stepcycle and the regeneration pressure was set to 1bar. Some demon-strative simulations were run to assess the effectiveness of theselected regeneration pressure. Higher regeneration pressure lev-els can bring an improvement on an energy point of view, althoughthe reduced purity could partially even out the expected reductionin compression power consumption. Conversely, the separationperformance decreases according to the less effective regenera-tion process. 1 bar appeared to be the regeneration pressure whichwas closer to meet both separation and energy specications. Fig. 8shows the levels of CO2 recovery and CO2 purity obtained in theassessed PSA process by varying the Purge-to-Feed ratio (P/F). Thevalues reported in the gure refer only to the PSA unit. The overallplant CO2 purity and recovery will be different since an additionalash separationprocess is implementedafter thePSAprocess. Fig. 8

75%

80%

85%

90%

95%

100%

76% 78% 80% 82% 84% 86%

CO

rec

over

y

CO purity

P/F = 0,193 P/F = 0,167P/F = 0,140

P/F = 0,113

P/F = 0,087

P/F = 0,060

P/F = 0,040

P/F = 0,013

Fig. 8. CO2 separation performance of the PSA process in the pre-combustion sce-nario. Results reported refer to different Purge-to-Feed ratio (P/F) of the mole owrates.


Table 7Scheduling, characteristics and performance of the PSA process in the pre-combustion scenario.

Step time (s) Moleowrate (mol/s) CO2 (%)

F D4 BD Pu P4 N4 FP Feed Purge Purity RecoveryPSA 90 41 80 59 41 8 41 3771.6 525.0 81.6 96.2PSA+ash 98.9 89.8

makes clear that the PSAprocess is not quite able tomatch the spec-ications. Whilst the CO2 recovery can be pushed easily over thetarget value of 90%, the CO2 purity hardly reaches values around85%. A further increase of the CO2 purity appears difcult to achieveand would come at the expense of the CO2 recovery, which woulddrasticallydecrease. Realizing the impossibility to reach thedesiredoutput streams characteristicswithin thePSAunit, the strategywasmodied. A solution could have been to introduce an additionalPSA stage (likewise post-combustion scenario) or better to apply adual PSA process (Grande and Blom, 2012). Considerations mainlyregarding the possible footprint related to a second PSA train leadus to choose a different option. Nevertheless, the dual PSA processcould result competitive and should be matter of further investi-gations. To comply with the selected alternative, the CO2 recoverytarget was set to the highest possible level, while a relatively lowervalue of purity was accepted. It was then introduced a further CO2purication process downstream of the PSA unit. It consists of adouble ash separation integrated in the CO2 compression process(Fig. 3). Referring to Posch and Haider (2012), the temperaturesselected at the outlet of each heat exchanger were set respec-tively to 30 C and 54.5 C. The gas stream is compressed up to30bar before entering the ash separation unit. Implementing thisadditional separation step, the nal result in terms of CO2 purity(PCO2 = 98.9%) and recovery (RCO2 = 89.8%) basically fullled therequirements. The H2 recovery (RH2 = 99.6%) was satisfactory aswell. The operating conditions selected for the full-plant analy-sis are those represented by the highlighted point in Fig. 8 (i.e.,P/F=0.140). This conguration was chosen because it provides agoodbalancebetweenseparationandenergyperformances. Table7displays the relative PSA characteristics, together with the sepa-ration performance obtained. The overall separation performance,resulting from the integration of the ash separation unit, is alsoreported.

The criteria adopted for the design of the pre-combustion PSAunit are similar to those discussed in the post-combustion scenario.

A less stringent limitation was imposed to the maximum pressuredrop (0.15bar) and a lower supercial velocity had to be utilized(0.08m/s) in order to make up for the higher operating pressure(as can be inferred from the Ergun equation, the higher the operat-ing pressure, the larger the pressure drop). However, a single PSAtrain was evaluated as able to process the entire syngas ow rate.Accordingly, the columns diameter was set to 6.6m.

4.5. Pre-combustion scenario analysis

Table 8 summarizes the outcome of the full-plant analysis car-ried out on the three cases considered for the pre-combustionscenario. TheplantwithoutCO2 capture facilities and theplantwitha state-of-the-art absorption unit were dened in compliance withthe framework determined in the EBTF project (DECARBit, 2011).They are meant to be the basis for comparison with the IGCC plantintegratedwith PSA, dened in thiswork. The simulationswere runsuch as to obtain similar exhaust gas ow rates at the gas turbineoutlet. This assumption meant to support following comparisonsof the results by allowing same size gas turbines to be used for thesimulations. The typology of gas turbine considered is large-scaleF class 50Hz.

4.5.1. Separation performanceWhenevaluating theCO2 separationperformance, PSA anddou-

ble ash process seems to match the requirements. The PCO2 isabove 95% and RCO2 is slightly lower than the target, with a value of89.8% (at least when considering the CO2 recovery only for the sep-aration technology). It is important to achieve a high value of PCO2(98.9%), because this is strictly related to the H2 recovery, whichis, in fact, very high as well (RH2 = 99.6%). Recovering large part ofH2 is essential in order to guarantee good energy performance ofthe system. However, the syngas fuelling the gas turbine containstraces of CO and CH4, products of the gasication process. Theircombustion results in the formation of additional CO2 which has to

Table 8Main outputs of the full-plant analysis in the pre-combustion scenario.

Plant summary No capture Absorption PSA+ash

Power inputsCoal ow rate (kg/s) 33.3 38.5 38.5Coal LHV (MJ/kg) 25.2 25.2 25.2Net fuel input (MWth) 837.3 968.1 968.2

Power outputsGas turbine output (MW) 253.1 287.9 287.1Steam turbine output (MW) 192.6 167.6 167.4Air expander output (MW) 4.5 5.7 5.4Gross electric output (MW) 450.2 461.1 459.9CO2 separation power consumption (MW) 16.5 0.0CO2 compression power consumption (MW) 18.7 41.3ASU power consumption (MW) 38.9 51.5 51.6Miscellaneous auxiliaries (MW) 15.5 16.3 16.7Total auxiliary power consumption (MW) 54.3 103.0 109.6Net electric output (MW) 395.8 358.1 350.2

Plant performanceNet electric efciency (%) 47.3 37.1 36.2CO2 purity (%) 100.0 98.9CO2 recoveryseparation technology (%) 94.6 89.8CO2 recoveryoverall plant (%) 90.6 86.1H2 recovery (%) 100.0 99.6CO2 capture efciency (%) 88.1 81.8


0

50

100

hsalf+ASPnoitprosbA

Pow

er c

onsu

mpt

ion

[MW

]WGS LHV reduction [MW] - LHV1

LHV lost in CO separation [MW] - LHV2

Steam extraction for WGS [MW] - STEAM

CO separation [MW] - SEP

CO compression [MW] - COMPR

Total power consumption [MW] - TOT

LHV

1

LHV

2

LHV

1

LHV

2

STEA

M

STEA

M

SEP

SEP

CO

MPR

CO

MPR

TOT TO

T

Fig. 9. Power consumptions related to the CO2 capture and compression process for the pre-combustion scenario.

be taken into account in the CO2 balance of the overall plant. Forthis reason, there is an additional CO2 recovery parameter, which isconsidering the total CO2 formed. The RCO2 for the PSA case dropsthen to 86.1% which is not fully fullling the requirement. Con-versely, absorption as decarbonization technique succeeds to reachthe suggested target values, attaining PCO2 , overall RCO2 and RH2 of100%, 90.5% and 100% respectively. The CO2 capture efciency wellsummarizes the discussed picture. CO2 for the PSA-based plant is81.8%, a value that can be considered acceptable, although lowerthan that achieved with absorption (88.1%).

4.5.2. Energy performanceThe energy analysis of the pre-combustion scenario reveals that

absorption is not clearly outperforming PSA. The reference IGCCplant without CO2 capture attains a net of 47.3%. Introducing anabsorption unit or a PSAunit for CO2 capture drops thenet down to37.1% and 36.2% respectively. The difference between the two casesis rather small (0.9%). A breakdownanalysis of the power consump-tion, related to the integration of a CO2 capture unit, highlightssome differences (see Fig. 9). Since some power consumptions arecharacteristic of a pre-combustion application, they are describedhereafter (the calculation of the equivalent power consumption isalso explained, if the term reported is not a direct electric powerconsumption):

WGSLHV reduction: theWGSprocess produces a reductionof thesyngas LHV (partially balanced by a higher mass ow rate). Thereduction in the fuel energy is converted intopower consumptionby considering the net efciency of the plant.

LHV lost in CO2 separation: since traces of hydrogen and car-bonmonoxide are leavingwith theCO2-rich stream, their heatingvalue iswasted. The reduction in the fuel energy is converted intopower consumption considering the net efciency of the plant.

Steam extraction for WGS: some steam need to be extracted bythe steam turbine in order to be fed to the WGS process. Themissing expansion of that steam causes a reduction in the steamturbine power output. The power consumption is calculated asdescribed in the post-combustion scenario for steam extractions.

The PSA unit does not directly require much energy. The CO2separation power consumption is very small (0.05MW) andmainly due to the fans for overcoming the pressure drop in thebed. Since the regeneration pressure is atmospheric, no vacuumpumps need to be installed. The avoidance of a rinse step in thePSA process conguration is also contributing to limit the power

consumption. In the absorption case the CO2 separation powerconsumption is larger. The required 16.5MW are mostly suppliedto the pumps for the solvent circulation. However, the particularconguration of the absorption/regeneration process is favorablewhen considering the power for the compression of the CO2-richstream. The regeneration process for the absorption case is occur-ring at three different pressure levels (12.7, 7.5 and 1.1bar). In thePSA process the CO2-rich stream leaves the unit at 1bar, meaningthat the pressure ratio that the compressor has to provide is, onaverage, larger. Moreover, in the double ash separation process,the CO2-rich streams leaving the ashes are partially expanded inadiabatic throttles, since by entering counter-currently the heatexchangers they assure the necessary cooling potential. Hence, theCO2 compression duty is further increased. The CO2 compressionpower consumption results to be 41.3MW for the PSA case, whilefor the absorption case is 18.7MW. It can be argued that the powersaved in the separation process, adopting PSA, is more than bal-anced by the additional power demand for CO2 compression. Theother power consumptions evaluated are very similar in both cases,so they do not modify the picture outlined. Summing up, the CO2capture through a PSA unit shifts the power consumption fromthe capture process to the CO2 compression, while all the otherpower loss contributions remain almost unchanged. However, theincrease in the compression power results to prevail. Accordinglythe energy efciency penalty relative to the PSA case is slightlyhigher than that relative to the absorption case.

4.5.3. FootprintGiven the high pressure at which the syngas enters the PSA

unit (38.8 bar), resulting in a relatively low volumetric ow rate,it was possible to design the PSA unit in a way that all the syn-gas is processed by a single PSA train. The supercial velocityadopted is able to maintain the pressure drop within acceptablelimits (0.15bar). The value was also veried not to overpass theminimum uidization velocity at the operating conditions consid-ered. Established the velocity and knowing the volumetric owrate, the cross sectional area was evaluated and, hence, the diame-ter of the column. It resulted to be 6.6m. Even though a PSA train isformed by 7 columns working in parallel, the footprint of the PSAunit appears to be acceptable. However, the footprint of an absorp-tion unit would be much smaller. Table 9 compares the estimationsof the two footprints, highlighting the remarks of the analysis. Theabsorption column diameter was calculated by dening a reason-able supercial velocity of the ue gas entering the column (i.e.,1m/s).


Table 9Footprint analysis for the pre-combustion scenario.

Absorption PSA

Column diameter (m) 2.2 6.6Number of columns 2 7Footprint (m2) 8 239

5. Conclusions

In the current work, the suitability of PSA process for CO2 cap-ture in coal-red power plants has been assessed. The effectivenessof PSA is evaluated on three different levels: CO2 separation perfor-mance, energy efciency and footprint of the technology. A post-and a pre-combustion scenario have been considered.

In the post-combustion scenario a PSA process is integratedwith an Advanced SuperCritical (ASC) pulverized coal plant. Theoutputs of the full-plant analysis were compared to those of a sim-ilar plant implementing a state-of-the-art absorption process forcapturing CO2. A two stage PSA process is necessary in order toachieve satisfactory characteristics of the CO2-rich stream to betransported and stored. The rst PSA stage is a three-bed ve-stepcycle, the second is a two-bed ve-step cycle. The resulting CO2purity (PCO2 = 95.1%) and recovery (RCO2 = 90.2%) fulll the targetlevels established (i.e., PCO2 95%andRCO2 90%). The utilizationof a PSA process shifts the power consumption related to CO2 cap-ture from a thermal duty for regenerating the solvent (i.e., amineabsorption) to direct electrical power for vacuum pumps and com-pressors. The resultant energy penalty is competitive with that ofthe benchmark absorption-based plant, as it was possible to obtaina net electric efciency slightly higher. The main obstacle for thesuitability of PSA in post-combustion application is related to itsfootprint. The ue gas ow rate has to be split in a large num-ber of PSA trains (about 73 and 23 for rst and second PSA stage)to be processed. Given the diameter (8m) of each of the columnsconstituting a train, the footprint of the PSA unit is much largercompared to the reference absorption unit. Modications in theprocess conguration may bring an improvement in this sense, atthe expense of other performance indicators. However, the gap isso large that is difcult to imagine lling it within the consideredprocess framework.

The application of a PSA process in a pre-combustion scenarioreturns more promising results. The PSA process is integrated inan Integrated Gasication Combined Cycle (IGCC) plant. The out-puts of the full-plant analysis were compared to those of a similarplant implementing a state-of-the-art absorption process for cap-turing CO2. The PSA process considered was a seven-bed andtwelve-step cycle. In order to comply with the separation perfor-mance specications, an additional double ash separation processwas integrated in the CO2 compression unit. The obtained purity(PCO2 = 98.9%) of the CO2-rich stream fullls the requirement. Theoverall CO2 recovery (RCO2 = 86.1%) is slightly lower compared tothe level aimed (i.e., 90%). However, a rearrangement of the processcould be able to trade off part of the purity for a higher recovery,

so that the process meets both the requirements. The absorptionprocess fully complies with the target values. The energy analysisof the simulated PSA-based plant yields a net of 36.2%. The netof the reference IGCC plant without CO2 capture is 47.3%, whileintegrating an absorption process for CO2 capture drops it to 37.1%.The difference of energy efciency between the two cases studiedis lower than 1%. The footprint of the PSA unit is not problematic,since a single PSA train (7 columns of 6.6mdiameter) is able to pro-cess the volumetric ow rate of syngas. In conclusion, PSA processhas the chance to become competitive in a pre-combustion sce-nario for CO2 capture. The general performance obtained is slightlylower compared to that relative to a plant implementing an absorp-tion process. On the other hand, PSA is a less mature technologyfor CO2 capture applications. Therefore, substantial improvementsare likely achievable. For instance, the layout of the whole processmay be further optimized. Advancements in material technologymay also introduce adsorbents with increased uptake capacity andselectivity, and possibly with higher thermal resistance. Such anaccomplishment would make possible better separation perfor-mance and a higher degree of process integration. Hence, thereis reason to believe that PSA can become a suitable alternative toabsorption for pre-combustion CO2 capture.

Acknowledgement

The authors gratefully acknowledge the nancial support pro-vided through the EnPe NORADs Programme within the energyand petroleum sector.

Appendix A. Calculation of the transport parameters

Table A.1Transport parameters equations.

Axial dispersion (Ruthven, 1984)

(16) Dax,i = (0.45 + 0.55)Dmg,i + 0.35dp2 |us|

Wilke model for a single-phase mixture of gases (Fuller et al., 1966; Polinget al., 2000)

(17) Dmg,i

= 1yii /= i

(yi /Dg,ii )

(18) Dg,ii =103T1.75g [(1/Mi )+(1/Mi )]

1/2

(pg/101325)

(

i

i

)1/3+

(i

i

)1/32

Micropore diffusivity (Ruthven, 1984)

(19)Dc,i

r2c=

D0c,i

r2cexp(

Ea,iRT)

Axial thermal dispersion coefcient (Lopes et al., 2009b)

(20) axkf

= 7 + 0.5PrRe


Appendix B. Boundary conditions for the PSA processes

Table B.1Boundary conditions adopted for the PSA processes. The co-current blowdown and co-current pressure equalization boundary conditions are the same as the counter-currentcounterpart applied inverted at the extremities of the column.

Feed: z=0 z= L

Dax,iCiz

= us(CF,i Ci)n = nF

axT

z= usCpCtot(TF T)

Ciz

= 0P = PFT

z= 0

Rinse: z=0 z= L

Dax,iCiz

= us(CR,i Ci)n = nR

axT

z= usCpCtot(TR T)

Ciz

= 0P = PFT

z= 0

Pressure equalizationdepressurization: z=0 z= L

Ciz

= 0n = 0T

z= 0

Ciz

= 0P = PeqT

z= 0

Counter-current blowdown: z=0 z= L

Ciz

= 0P = PBDT

z= 0

Ciz

= 0n = 0T

z= 0

Purge: z=0 z= L

Ciz

= 0P = PBDT

z= 0

Dax,iCiz

= us(CPu,i Ci)n = nPu

axT

z= usCpCtot(TPu T)

Counter-current pressure equalizationpressurization: z=0 z= L

Ciz

= 0n = 0T

z= 0

Dax,iCiz

= us(Ceq,i Ci)n = neq

axT

z= usCpCtot(Teq T)

Feed pressurization: z=0 z= L

Ciz

= 0n = 0T

z= 0

Dax,iCiz

= us(CFP,i Ci)P = PFP

axT

z= usCpCtot(TFP T)

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Evaluating Pressure Swing Adsorption as a CO2 separation technique in coal-fired power plants1 Introduction2 Modeling of the power plant2.1 ASC plant with CO2 capture by PSA2.2 IGCC plant with CO2 capture by PSA

3 Modeling of the PSA unit3.1 Adsorption bed model3.2 PSA process3.3 Water and adsorption3.4 Solution of the PSA model

4 Results and discussion4.1 Definition of the performance parameters4.2 Post-combustion PSA process4.3 Post-combustion scenario analysis4.3.1 Separation performance4.3.2 Energy performance4.3.3 Footprint

4.4 Pre-combustion PSA process4.5 Pre-combustion scenario analysis4.5.1 Separation performance4.5.2 Energy performance4.5.3 Footprint

5 ConclusionsAcknowledgementAppendix A Calculation of the transport parametersAppendix B Boundary conditions for the PSA processesReferences

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