ww.sciencedirect.com

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 3

Available online at w

journal homepage: www.elsevier .com/locate/he

MCFC-based CO2 capture system for small scale CHP plants

Umberto Desideri 1, Stefania Proietti 1, Paolo Sdringola 2, Giovanni Cinti 3, Filippo Curbis*

Universita di Perugia, Department of Industrial Engineering, 67 Via Duranti, 06100 Perugia, Italy

a r t i c l e i n f o

Article history:

Received 1 July 2011

Received in revised form

8 May 2012

Accepted 12 May 2012

Available online 29 July 2012

Keywords:

CO2

Cogeneration

MCFC

CCS

Aspen

* Corresponding author. Tel.: þ39 3491319634E-mail addresses: umberto.desideri@unip

giovanni.cinti@unipg.it (G. Cinti), filippocurb1 Tel.: þ39 075 5853743.2 Tel.: þ39 075 5853930.3 Tel.: þ39 075 5853991.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.05.048

a b s t r a c t

Carbon dioxide emissions into the atmosphere are considered among the main reasons of

the greenhouse effect. The largest share of CO2 is emitted by power plants using fossil fuels.

Nowadays there are several technologies to capture CO2 from power plants’ exhaust gas but

each of them consumes a significant part of the electric power generated by the plant. The

Molten Carbonate Fuel Cell (MCFC) can be used as concentrator of CO2, due to the chemical

reactions that occurs in the cell stack: carbon dioxide entering into the cathode side is

transported to the anode side via CO3¼ ions and is finally concentrated in the anodic exhaust.

MCFC systems can be integrated in existing power plants (retro fitting) to separate CO2 in the

exhaust gas and, at the same time, produce additional energy. The aimof this study is to find

a feasible system design for medium scale cogeneration plants which are not considered

economically and technically interesting for existing technologies for carbon capture, but

are increasing in numbers with respect to large size power plants. This trend, if confirmed,

will increase number ofmediumcogeneration plantswith consequent benefit for bothMCFC

market for this application and effect on global CO2 emissions. System concept has been

developed in a numerical model, using AspenTech engineering software. The model

simulates a plant, which separates CO2 from a cogeneration plant exhaust gases and

produces electric power. Data showing the effect of CO2 on cell voltage and cogenerator

exhaust gas composition were taken from experimental activities in the fuel cell laboratory

of the University of Perugia, FCLab, and from existing CHP plants. The innovative aspect of

this model is the introduction of recirculation to optimize the performance of the MCFC.

Cathode recirculation allows to decrease the carbon dioxide utilization factor of the cell

keeping at the same time system CO2 removal efficiency at high level. At anode side,

recirculation is used to reduce the fuel consumption (due to the unreacted hydrogen) and to

increase the CO2 purity in the stored gas. The system design was completely introduced in

themodel and several analyses were performed. CO2 removal efficiency of 63%was reached

with correspondent total efficiency of about 35%. Systemoutlet is also thermal power, due to

the high temperature of cathode exhaust off gases, and it is possible to consider integration

of this outlet with the cogeneration system. This system, compared to other post-

combustion CO2 removal technologies, does not consume energy, but produces additional

electrical and thermal power with a global efficiency of about 70%.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

.g.it (U. Desideri), stefania@unipg.it (S. Proietti), psdringola@mach.ing.unipg.it (P. Sdringola),is@gmail.com (F. Curbis).

2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 319296

1. Introduction

This paper presents a study of a novel technology for CO2

separation from the other components present in a cogene-

ration plant exhaust gases. The solution proposed is the use

of a particular fuel cell type, which may be used as a filter for

the entering CO2; Carbon dioxide is one of the outlet gases

mixed with hydrogen and water, from which it is better

separable than from other components of thermal engine

exhaust gases such as nitrogen and oxygen. CO2 is the most

important responsible for the increase of the Earth green-

house effect; and its emissions, produced especially by fossil

fuels fired power plants, should be also limited to avoid the

economic sanctions planned by the Kyoto Protocol for the

Nations that do not respect the limits [1]. The largest share of

these emissions could be avoided increasing the existing

energy system efficiency, switching to nuclear and renewable

energy sources; the remaining part could be avoided by

carbon capture and storage technologies. The future trend of

the CO2 emission is shown in Fig. 1 (left) in which three

trends are represented with different carbon mitigation

scenarios: with a moderate, medium and huge limitation on

CO2 emissions. Cogeneration, the combined electric and heat

production from the same fuel, is a great instrument to

increase the energy system efficiency which entails a reduc-

tion of consumptions and emissions: Fig. 1 (right) shows how

to obtain the same amount of heat and electricity from

cogeneration, that permits an important saving of primary

energy.

In Italy, several small-scale cogeneration plants are in

operation [2] and they could be an interesting test-bedmarket

for new technologies for carbon capture. In Fig. 2 (right) the

distribution of CHP plants in Italy is shown, where the yellow

points are the natural gas fuelled plants. Fig. 2 depicts, on the

left, the CO2 capture process that involves three phases after

the mining of fossil fuel: the capture, that takes place close to

the power plant; the transport, performed by pipelines; the

storage, that could be realized in various sites such as saline

aquifers, depleted oil and gas fields and oceans depth.

Carbon capture can be realized in different points of the

plant: carbon dioxide capture before combustion, after oxy-

fuel combustion and post-combustion [3,4]. These three

solutions are depicted in Fig. 3 (left). Only the third tech-

nology is really available at the commercial status: the

Fig. 1 e Carbon dioxide emissions trend (left) and Cogenera

solutions realized operate with high separation efficiencies

but with high energy consumption, that bear upon complete

plant balance. In this specific technological application, fuel

cells, electrochemical device producing electricity and heat

from hydrogen and air with high efficiencies, could play an

innovative role the drawback is their complexity that slows

down their development [5]. Molten carbonate fuel cells

could operate as a filter for the CO2 [6] which can be

concentrated in one of the two outlets, mixed with hydrogen

and water vapour. The idea is to feed the cathode inlet of an

MCFC with exhaust gases from a fossil fired power plant to

separate the CO2 content [7,8]. Fig. 3 (right) explains this

concept. Anode and cathode reactions are reported below

with the global reaction where it is possible to note how

a mole of CO2 moves from cathode to anode for each

hydrogen mole reacting in the cell.

Anode side H2 þ CO¼3/H2Oþ CO2 þ 2e� (1)

Cathode side 1=2O2 þ CO2 þ 2e�/CO¼3 (2)

Global Reaction H2 þ 1=2O2 þ CO2ðcatÞ/H2Oþ CO2ðanÞ (3)

2. Plant model

The target of this study is to build a numerical model of an

MCFC-based CO2 separation plant, using the commercial

software Aspentech, to verify the technical feasibility of

matching the power plant and the separation systems. The

objective is to separate at least 60% of the CO2 produced, as

shown in Fig. 4.

The starting points are the emission data of a cogeneration

and district heating plant operating in Umbria (Italy). This is

a natural gas fired plant, and the exhaust gases contain about

8%vol of CO2, mixed with nitrogen and oxygen, as shown in

Table 1 [10,11].

The First step was to create the cell model. In Aspentech

software there is no library for fuel cell components. Two

possible alternatives were considered: creating a Fortran

library to simulate the cell block or using a particular block

that passes part of the calculation to a calculation sheet. This

second option was selected and a calculation sheet was

implemented based on mass balance equilibrium for an

MCFC while voltage was calculated using Nernst’s equation

tion vs. Conventional generation comparison (right) [1].

Fig. 2 e CCS process (left) [3] and small cogeneration plants diffusion in Italy (right) [2].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 3 19297

to calculate OCV and ASR from experimental. The model

requires as input the fluid characteristics (composition,

pressure, temperature, total flow rate for both anode and

cathode side) and fuel cell operating parameters (tempera-

ture, pressure, current density, cell area); outputs are: utili-

zation factors (fuel, oxidant and CO2), stack voltage, electric

power produced and outlet flows composition. Base model

developed is shown in Fig. 5. The mentioned factors are

dimensionless parameters expressing the fuel, oxidant and

CO2 fraction consumed in the cell (eqs. (4)e(6)). The cell

usually operates in excess for these three elements to guar-

antee high performance and durability. Electric power is

produced in AC, the electric efficiency and the CO2 removal

efficiency are defined (eqs. (7)e(9)). In the cell model CO2

efficiency coincides with the CO2 utilization factor.

UF ¼ H2;in �H2;out

H2;in¼ H2;consumed

H2;in(4)

UOX ¼ O2;in �O2;out

O2;in¼ O2;consumed

O2;in(5)

UCO2¼ CO2;in � CO2;out

CO2;in¼ CO2;consumed

CO2;in(6)

Fig. 3 e The three ways of CCS and the principle of ope

WAC ¼ Vstack$AFC$J$hDC=AC (7)

hel ¼WAC

_mCH4$LHVCH4

(8)

hrem ¼ CO2;consumed

CO2;in(9)

Fig. 5 shows the cell’s polarization and the electric power

curve: theyareobtainedgiving themodel theCHPplant exhaust

gas composition at the cathode and SMR (S/C ratio 3) composi-

tion at the anode side. The operating point is not chosen near

the point of maximum power, but around 1000 A/m2 current

density, so that cell voltage is always higher than 0.7 V, that is

the lowest voltage limit for a stable operation of the cell [12].

The selection of the cell size was adjusted as trade off

between the highest possible CO2 utilization coinciding with

removal efficiency, and the lowest outlet gas temperature, to

safeguard the cell materials [13]. If the cell size increases,

UCO2 becomes higher, due to increase of chemical reactions;

similarly, the heat produced increases because the heat

produced is directly proportional to the number of reaction,

and, consequently the temperature of the exhaust gases

increases. Fig. 6 shows the curves used to determine cell area;

ration of molten carbonate fuel cells (on the right).

Fig. 4 e Target for CO2 capture using MCFC [9].

Table 1 e Upper cogeneration plant performances and exhaust gases data.

Power Fuel Exhaust gas flow Exhaust gas composition Temperature Pressure

1MWe þ 1.5 MWth Natural gas 1.71 kg/s N2 85.65 %vol 95.6 �C 1 bar

O2 6.03 %vol

CO2 8.32 %vol

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 319298

carbon dioxide utilization and outlet gas temperature versus

cell area, for different current density values: on this figure

the selection criterion explained above was applied. It was

assumed that 98% of the heat produced is available in the

exhaust gases, and the remaining 2% is released to the

environment.

Afterwards, innovative components of the systems have

been introduced: anode and cathode recirculation (Fig. 7, left);

they consist in the reinjection into the cell of a fraction

Fig. 5 e Base model and its polarization a

(recirculation degree R) of the outlet flow: each one (anode and

cathode side) entails advantages and drawbacks, however

they allow the cell model optimization; the recirculation

degree is a trade-off between some system parameters to

optimize the cell performance without damaging the oper-

ating stability.

The cathode recirculation allows the cell to work with the

CO2 removal efficiency previously chosen and to keep the CO2

and oxidant utilization factor lower (Fig. 7), with significant

nd electric power curve (on the right).

Fig. 6 e CO2 utilization factor and cathode side outlet temperature of the model varying active area.

Fig. 7 e Model with anode and cathode recirculation and effect of cathode recirculation into CO2 separation.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 3 19299

benefits for the fuel cell life and performance. The mathe-

matical relation describing the hrem dependence only from

CO2 utilization factor and from the recirculation degree, has

been determined, see eq (10). Cathode recirculation allows

fixing the correct operating temperature of the cell, and, in

particular, the typical gas temperature variation trend in the

fuel cell, as described in Fig. 8 (right).

hrem ¼ UCO2

R$UCO2þ 1� R

(10)

The system model was improved adding several auxiliary

components: reforming section, CO2 treatment section, and

heat exchangers. The reformer model simulates a reactor

where steam methane reforming of natural gas occurs so that

the cell is fedwith a stream reach of hydrogen. Steam to carbon

ratio is set to 3. The reformer block simulates an ideal reformer

where both SMR and shift reaction occurs so that carbon

monoxide does not enter into the cell, and no additional

reforming reaction occurs in the cell. This simplification

slightly modifies system parameters due to the fact that steam

reforming is always completed in the cell while, even consid-

ering CO shift as not completed, there is little effect in CO2

purity. In the CO2 treatment section there is a condenser that

separates the CO2 from vapour and a multi-step compressor

that increases the CO2 pressure up to 140 bars. The heat

produced by the cell during operation is used, in the system

design, to maintain a constant temperature in the reformer.

Table 2 contains reformer operating conditions and the oper-

ating conditions of the recirculation and CO2 treatment section

compressors. The heat exchangers are along the cathode

exhaust gases, from which heat is recovered, and provide all

the energy to bring inlet flows (air,water and natural gas) to the

Fig. 8 e CO2 utilization factor and inlet-outlet DT at cell’s cathode side varying cathode recirculation degree.

Table 2 e Reformer block and compressors blocksoperating conditions.

Reformer

Input Output

Parameter Fixed value Heat demand [kW]

Pressure [bar] 1.015

Temperature [�C] 600

Physical state Vapor

Compressors

Input Output

Parameter Fixed value Power [kW]

Isentropic efficiency 0.9 Pressure [bar]

Mechanical efficiency 0.95 Temperature [�C]

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 319300

aimed thermodynamic conditions. The last exchanger unit

recovers heat to produce hot water for district heating and

increases the heat produced by the main CHP plant. The

resulting system, Fig. 9, is an energy self-sufficient system, that

can be integrated in existing cogeneration systems fed with

natural gas. The anodic recirculation allows increasing the CO2

Fig. 9 e Full system

concentration in the flow designed for storage; as a conse-

quence the compression costs are reduced (the non-reacted

hydrogen in the previous transit through the cell is consumed

instead of being sent to compression). Therefore, the fuel flow

entering the system can be reduced, Fig. 10, but this does not

affect the efficiency, because the fuel cell is fed with a more

diluted hydrogen flow, keeping down the voltage and the

electric power produced (Table 3).

The main recirculation drawback is the energy consump-

tion related to compressors that raise the inlet pressure, after

the pressure drop in the cell [14]. Eventually, it can be said that

the cathode recirculation controls the separated CO2 quantity,

while the anodic recirculation refines the separated CO2

quality, as shown in Fig. 11 (right).

With the recirculation a new operation point is obtained so

that the new operating cell temperature (calculated as the

average temperature between the outlet gas temperature and

the average of the inlet gases temperature) is about 45 �Chigher than the previous temperature set, with consequences

for the system’s electric performance. Table 4 reports

temperature, pressure, mass flow and composition of main

system gas streams.

model scheme.

Fig. 10 e Fuel consumption and electric efficiency varying anode recirculation degree.

Table 3 e Advantages and drawbacks of anode andcathode recirculation.

Advantages Drawbacks

Anode � Increasing CO2 purity

� Reducing compression

costs

� Energy consumption

� Fuel mix

impoverishment

Cathode � Increasing CO2

separation

� Simplifying

temperature

control

� Energy consumption

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 3 19301

3. Performance

System performance is presented in Table 5. The previously

set target of 60% CO2 removal efficiency is achieved. There is

production of electric and thermal energy, so the total effi-

ciency is higher than 85%. The system has a suitable size,

about 400 kW, to be used as retrofitting in a 1MWe/1,5MWth

cogeneration plant.

Fig. 11 e CO2 compression power required and CO

A CO2 purity of 82.2% is a great result considering that this

is an active system, in comparison with amine-based sepa-

ration systems that can achieve 95% purity, but spending

a large amount of energy. In addition, separation of hydrogen

from CO2 can be easily realized with membranes or directly

from compressed flow due to the fact that at the temperature

at which CO2 liquefies, H2 remains in gaseous state and can be

easily separated by distillation. This process was not intro-

duced in the model but gives very high carbon dioxide storage

purity and permits to recover pure hydrogen that can be easily

valorised in the system directly at the anode or burned else-

where to produce additional heat. Finally a simple economic

comparisonwas performedwith a commercial system for CO2

separation, like an amine absorption system with MEA: this

kind of system requires energy to operate, and the capture and

storage of 1 ton of CO2 costs about 60V [15]. The separation

plant modelled in this paper, based on fuel cells, produces, on

the contrary, electrical and thermal energy, lowering the cost

of stored CO2 to 38 V/ton. The comparison is favourable to the

solution presented in this paper, that has on the contrary

considerable costs and plant design complexity. It is clear that

the current cost of anMCFC plant, about 5000 V/kWe installed

[16], has a consistent impact on the economics, and pays back

the investment costs after 20 operating years. If in the future

2 purity varying anode recirculation degree.

Table 5 e Model Parameters.

Inputs Outputs

Steam/Carbon ratio [e] 3 Fuel utilization [e] 0.704

Oxidant utilization [e] 0.234

Anode recirculation [e] 0.32 CO2 utilization [e] 0.400

Cathode recirculation [e] 0.59

Voltage [V] 0.706

DC/AC efficiency [e] 0.95 MCFC electric power [kWe] 386

FC temperature [�C] 695 MCFC electric efficiency 50.3%

FC pressure [bar] 1

Flow pressure loss [bar] 0.015 System electric power [kWe] 342

System electric efficiency 44.6%

Current density [A/m2] 1000

Cell area [m2] 575 CO2 removal efficiency 61.9%

Cell resistance [Ohm$cm2] 1.86 CO2 stored flux purity 82.2%

System thermal power [kWth] 320

Cogeneration efficiency 86.4%

Table 4 e Advantages and drawbacks of anode and cathode recirculation.

GASNAT H2O ICEOUT OXIDOUT FUELOUT CO2STOCK

Temperature [�C] 25.0 25.0 95.6 686.6 686.6 25.0

Pressure [bar] 1015 1015 1015 1000 1000 140.000

Mass flow [kg/hr] 55.184 186.246 6156.000 5323.618 1073.812 763.972

H2 0.000 0.000 0.000 0.000 0.092 0.178

CO 0.000 0.000 0.000 0.000 0.000 0.000

CO2 0.000 0.000 0.083 0.014 0.454 0.822

N2 0.000 0.000 0.856 0.887 0.000 0.000

AIR 0.000 0.000 0.000 0.000 0.000 0.000

H2O 0.000 1000 0.000 0.000 0.454 0.000

CH4 1000 0.000 0.000 0.000 0.000 0.000

O2 0.000 0.000 0.060 0.098 0.000 0.000

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 319302

the MCFC plant cost will decrease, this kind of system will be

economically attractive.

4. Conclusions

The CO2 separation system presented in this paper is char-

acterized by: high performance (removal efficiency, electric

and thermal production) and interesting economic return,

which is, better than commercial passive CO2 separation

systems. The model that was developed to perform calcula-

tions, starts from a real cogeneration power plant but meth-

odology and approach is scalable for any type of plant if the

MCFC size is compatible with the present development state

of this technology. The system model could be refined in all

the sections, and the next step is to perform experimental

tests of MCFC to validate the model and to evaluate the

pollutant effects on the cell performance and lifetime.

r e f e r e n c e s

[1] Desideri U. MCFC as potential concentrators in distribuitedgeneration systems: technical problems and challenges.Workshop in Fuel Cells in the Carbon Cycle, Napoli; 2010.

[2] Desideri U, Proietti S, Arcioni L. Implementation of ananalysis and advantages evaluation for distribuitedgeneration in Italy. proceedings of Turboexpo ASMEBarcellona; 2006.

[3] Girardi G. Carbone: Obbiettivo Zero Emission, le tecnologie dicarbon capture & storage. ENEA; 2007.

[4] Hendriks C, deVisser E, Jansen D, Carbo M, Ruijg GJ,Davison J. Capture of CO2 from medium scale emissionsources. Energy Procedia 2009;1:1497e504.

[5] Ronchetti M. Celle a combustibile, stato di sviluppo eprospettive della tecnologia. 2nd ed. Roma: Enea; 2008.

[6] Sugiura K, Takei K, Tanimoto K, Miyazaki Y. The carbondioxide concentrator by using MCFC. Journal of PowerSources 2003;118:218e27.

[7] Andreassi L, Chiappini D, Jannelli E, Ubertini S. Ultra Lowcarbon dioxide emission MCFC based power plant. Journal ofFuel Cell Science and Technology 2011;8(3).

[8] Desideri U, Proietti S, Cint G, Sdringola P, Rossi C.Analysis of pollutant emissions from cogeneration anddistrict heating systems aimed to a feasibility study ofMCFC technology for carbon dioxide separation asretrofitting of existing plants. International Journal ofGreenhouse Gas Control 2011;5(6):1663e73.

[9] Moreno A. Hydrogen and fuel cells in CCS power plant. Erice:ENEA, International School of Geophysics, 30� Course of CO2

Capture & Storage; 2007.[10] Desideri U, Proietti S, Sdringola P, Curbis F. Analysis of

emission into atmosphere of the cogeneration and

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 9 2 9 5e1 9 3 0 3 19303

district heating plant in Norcia (Italy). proceedings ofECOS; 2009.

[11] Energia Verde SpA. Relazione Tecnica di Riconoscimentofinalizzata alla qualificazione degli impianti dicogenerazione abbinati al teleriscaldamento, per il rilasciodei certificati verdi -. Impianto di cogenerazione di energiaelettrica e termica al servizio del teleriscaldamento dellacitta di Norcia (PG); 2008.

[12] EG&GTechnical Services Inc. Fuel cell handbook. 7thed.; 2004.[13] Campanari S, Chiesa P, Manzolini G. CO2 capture from

combined cycles integrated with Molten Carbonate Fuel

Cells. International Journal of Greenhouse Gas Control 2010;4(3):441e51 [Elsevier Ltd].

[14] Grillo O, Magistri L, Massardo AF. Hybrid systems fordistributed power generation based on pressurisation andheat recovering of an existing 100 kW molten carbonate fuelcell. Journal of Power Sources 2003;115(2):252e67.

[15] Romeo LM, Bolea I, Escosa JM. Integration of power plant andammine scrubbing to reduce CO2 capture cost. AppliedThermal Engineering 2008;28:1039e46.

[16] Massi E. Digestione anaerobica, Studi preliminari per unaricerca integrate di base. ENEA; 2010.