Document Sample
          2009, 30, 341–350



             Institute of Heat Engineering, Faculty of Power and Aeronautical Engineering,
            Warsaw University of Technology, 21/25 Nowowiejska, 00-665 Warsaw, Poland

     A molten carbonate fuel cell (MCFC) is shown to reduce CO2 emissions from a coal fired power
plant. The MCFC is placed in the flue gas stream of the coal fired boiler. The main advantages of this
solution are: higher total electric power generated by a hybrid system, reduced CO2 emissions and higher
system efficiency. The results obtained show that use of an MCFC could reduce CO2 emissions by 61%,
which gives a relative CO2 emission rate of 253 kg CO2/MWh.

     Przedstawiono koncepcję zastosowania węglanowego ogniwa paliwowego do zmniejszania emisji
CO2 z elektrowni opalanej pyłem węglowym. Ogniwo zostało umieszczone w strumieniu wylotowym
spalin. Głównymi zaletami takiego rozwiązania poza zmniejszoną emisją CO2 jest wzrost mocy genero-
wanej przez układ przy zwiększonej sprawności generowania energii elektrycznej. Otrzymane wyniki
wskazują, iż takie nietypowe zastosowanie ogniwa paliwowego umożliwia zredukowanie emisyjności
bloku parowego zasilanego pyłem węglowym o 61% do wartości 253 kg CO2/MWh.

                                       1. INTRODUCTION

     The European Union has placed limits on CO2 emissions by Member States as
a part of its emission trading scheme. This impacts fossil fuel power plants to a signif-
icant degree as their emissions are governed by the number of emission allowances
they receive from the Member State allocation. Excess CO2 emissions have to be cov-
ered by purchasing extra allowances, which is in effect a penalty (€100/Mg). In con-
trast, undershooting emission limits enables the emitter to sell CO2 allowances. The
selling price of a traded allowance is estimated at €20–30/Mg CO2. Fuel cells generate
electricity through electrochemical processes. There are many types of fuel cells, two
of which – molten carbonate fuel cell (MCFC) [1] and solid oxide fuel cell (SOFC) [2]
are high-temperature fuel cells. There is a variety of methods available to remove CO2
     Corresponding author, e-mail:
342                                     J. MILEWSKI et al.

from a fossil fuel power plant system [3]. The idea of adopting a molten carbonate fuel
cell to reduce CO2 emissions was developed by Campanari [4] who demonstrated that
an estimated reduction of 77% in CO2 emissions can be achieved in a steam turbine
power plant.

             Fig. 1. Working principles of MCFC: 1) fuel input, 2) mixture of CO2, H2
                       and H2O, 3) oxidant input, 4) exhaust, 5) CO3 − ions

     Fuel cells generate electricity through electrochemical processes. There are many
types of fuel cells; two of them – the molten carbonate fuel cell (MCFC) and the solid
oxide fuel cell (SOFC) – are high temperature fuel cells. They work at temperatures
ranging from 600 °C to 1000 °C. Amorelli et al. [5] described an experimental investi-
gation into the use of MCFC to capture CO2 from gas turbine exhaust gases. They
obtained the reduction of emission by 50%. Those experiments were performed using
a singular cell. Lusardi et al. [6] investigated the application of a fuel cell system for
CO2 separating from thermal plant exhaust. They found that, even without CO2 separa-
tion, the relative emission of carbon dioxide could be reduced to below the Kyoto
Protocol limit. If a separator is used, emissions could be reduced by 68%. The use of
an MCFC as a carbon dioxide concentrator was investigated by Sugiura et al. [7]. In
this work, the experimental results of CO2 sequestration by use of an MCFC are given.
One key conclusion from this work is that the CO2 removal rate can be obtained by
making calculations using the electrochemical theory. Novel methods whereby carbo-
nates were used as an electrochemical pump in carbon dioxide separation from gases
were described by Evan et al. [8]. Reducing CO2 emissions from a gas turbine power
plant by using a molten carbonate fuel cell was estimated by Milewski et al. [1].
                  Reducing CO2 emissions by using a molten carbonate fuel cell      343

    Based on the above review, a reduction of at least 50% in CO2 emissions could be
expected. Hydrogen, natural gas, methanol or biogas may be used as fuels for MCFCs.
On the cathode side, a mixture of oxygen and carbon dioxide is required. An MCFC
can work as a carbon dioxide separator/concentrator because CO2 is transported from
the cathode side to the anode side through a molten electrolyte. The combination of
CFPP with MCFC gives a hybrid system (HS) with increased efficiency and decreased
carbon dioxide emission. Negative ions are transferred through the molten electrolyte.
Each ion is composed of one molecule of carbon dioxide, one atom of oxygen and two
electrons. This means that an adequate ratio of carbon dioxide to oxygen is 2.75 (mass
based) or 2.0 (mole based).
    A typical composition of flue gas of CFPP is given in Table 1. The ratio of CO2 to
atomic oxygen is hence 2.5 (mole based) and 3.43 (mass based). This means that flue
gas contains an insufficient quantity of oxygen to trap all of the CO2.

                         Table 1. Coal-fired boiler flue gas composition

                                        Molar fraction Mass fraction
                                            [%]            [%]
                           CO2              15.00           21.43
                           N2               78.59           71.98
                           O2               5.98            6.25
                           H2O              0.50            0.29
                           CO2/O2           2.50            3.43

                                        2. THEORY

    Mathematical modelling is a basic method for analyzing systems incorporating
fuel cells. A zero-dimensional approach is used for the modelling of system elements.
The presented results are based on calculations made using an appropriate mathemati-
cal model. The governing equations of the MCFC and heat exchangers models have
been presented previously [1]. This section contains only the model of coal fired boiler
steam turbine cycle.
    The analyzed steam cycle consists of a coal fired boiler, a steam turbine, a con-
denser and a main pump. The flowsheet of the steam cycle is given in Fig. 2. The tem-
perature and pressure of the steam prior to entry into the turbine were assumed at
550 °C and 13 MPa, respectively. The steam turbine outlet pressure was assumed at
5 kPa. Based on these assumptions, the steam turbine power plant conditions were
calculated. The power generation efficiency and relative CO2 emission of the system
were 30% and 1137 kg/MWh, respectively. All analyzed cases were optimized with
the objective function being total power generation efficiency. Nevertheless, there is
room for discussion as to the choice of this as the objective function of the optimizing
344                                  J. MILEWSKI et al.

process. While the main task of an MCFC is to capture CO2 from flue gas, it also in-
creases total power generation efficiency due to its higher efficiency compared with
that of the steam cycle (44% vs. 30%).

                                    Fig. 2. GTPP system

                                2.1. OPTIMIZING PROCESS

     The BOX method was used for optimizing all systems [1]. This method is a se-
quential search technique which solves problems with non-linear objective functions,
subject to non-linear inequality constraints. No derivatives are required. It handles
inequality but not equality constraints. This method is not very efficient in terms of the
required number of function evaluations. It generally requires a large number of itera-
tions to converge on the solution.
     The size of the MCFC installed at the flue gas can be varied in wide range. On the
other hand, the same fuel utilization ratio can be realized with various size fuel cells.
There are three main parameters determining the MCFC size: the fuel utilization fac-
tor, maximum current density and inlet fuel flow. Two of these three parameters set
the size of the MCFC. The fuel utilization factor was chosen at the constant level of
90%. MCFC area is matched to obtain the assumed fuel utilization factor by changing
the imax value. The maximum current density and fuel mass flow were taken as primary
variables of the optimization process.
     Parameters to be optimized are:
     • MCFC fuel mass flow,
     • the value of imax in the range from 0.06 A/cm2 to 0.6 A/cm2
     • heat exchanger efficiency in the range from 0 to 85%,
     The optimizing process was carried out with the following constraint functions:
                 Reducing CO2 emissions by using a molten carbonate fuel cell      345

    • maximum temperature inside the stack not exceeding 650 °C,
    • cell voltage at the last cell higher than 0 V.
    Methane is mixed with steam to avoid carbon deposition. It was assumed that the
steam-to-carbon ratio of 1.4 was adequate to prevent carbon deposition [1].
    The steam-to-carbon ratio (s/c ratio) defines steam molar flow with respect to me-
thane and carbon monoxide molar flows delivered to the reformer:

                                     s    nH2 O
                                       =                                            (1)
                                     c nCH4 + nCO

where: nCH4 – methane molar flow, nH2O – water molar flow; nCO – carbon oxide molar
    Additional air is added to the flue gas prior to entry into the MCFC because the
quantity of oxygen is too low for total CO2 sequestration [1]. Additional air flow was
assumed at the value which gives CO2/O2 ratio of 2.0 (mass based).

                                       3. RESULTS

    The CO2 emission reduction factor is defined as follows:
                                                   mCO2 ,out
                                   ηCO = 1 −                                        (2)
                                                   mCO2 ,in
where: m – mass flow, kg/s, out – MCFC outlet cathode stream, in – MCFC inlet
cathode stream.
    Three cases were investigated for the application of an MCFC in a coal fired
steam cycle. The MCFC is fuelled by a mixture of methane and steam. In all analyzed
cases, an internal reforming of methane is assumed.

                                            3.1. CASE 1

    Case 1 concerns a situation where there is no intervention in either the coal fired
boiler or the steam cycle. The flue gas temperature is too low for the MCFC to be fed
directly. Two heat exchangers are added to heat up the flue gas prior to entry into the
cathode (Fig. 3). The MCFC is fed by a mixture of methane and steam on the anode
side and boiler outlet flue gas on the cathode side.
    Non-oxidized anode outlet gases are burned in a catalytic burner. The catalytic
burner is fed by pure oxygen to utilize the remaining methane, hydrogen and carbon
oxide. It must be borne in mind that oxygen extraction from air needs energy. Energy
required to produce 1 kg of oxygen at atmospheric pressure depends on the product
346                                  J. MILEWSKI et al.

purities and is a result of an optimisation between power cost and capital costs. In the
present work, it is assumed that oxygen is generated at 250 kJ/ kg O2 [9], which de-
creases the system efficiency depending on the amount of oxygen delivered. It should
be noted that energy demanded by the air separation unit (ASU) could be higher than
the assumed value.

                               Fig. 3. Flow diagram of case 1

                        Table 2. System parameters of analyzed cases

                             Parameter               Case 1 Case 2 Case 3
                  CFPP power/total power, %            58        61     63
                  MCFC power/total power, %            42        39     37
                  CFPP–MCFC efficiency (LHV), %       35         37     40
                  CO2 emission reduction factor, %    60        61     61
                  Relative CO2 emission, kg/MWh       304       268    253
                  MCFC efficiency (LHV), %            43        43     43
                  CFPP efficiency (LHV), %            30        35     38
                  Average cell voltage, mV            652       641    651
                  CO2/O2 mass flow based              3.3       3.3    3.3
                  HX-1 efficiency, %                   0         0      0
                  HX-2 efficiency, %                  27        12     12
                  HX-3 efficiency, %                  29        38     38
                  HX-4 efficiency, %                  61        25     25

    The system parameters for case 1 are given in Fig. 3 and Table 2. Optimization of
the main system parameters shows that heat exchanger HX-1 should be eliminated
from the system as it failed to deliver any additional efficiency.
                 Reducing CO2 emissions by using a molten carbonate fuel cell     347

                                        3.2. CASE 2

    The CO2 separator must be cooled for the purpose of water condensation. The heat
recuperation of the steam cycle can be realized by inserting a CO2 separator into the
feed boiler water stream. In case 2, a CO2 separator is placed in the condenser outlet
stream. The system parameters for case 2 are given in Fig. 4 and Table 2.

                                Fig. 4. Flow diagram of case 2

                                        3.3. CASE 3

    The temperature of the outlet cathode gas remains high even after a few heat ex-
changers (ca. 380 °C) which opens up the possibility of using this heat in the steam
cycle to preheat the boiler fed water.

                                Fig. 5. Flow diagram of case 3
348                                  J. MILEWSKI et al.

    In case 3, an additional heat exchanger is used as a second heat recuperator. It is
placed just after the CO2 separator. Hotter fed water delivered to the boiler enables the
steam turbine to produce more power, which decreases the relative CO2 emission
while leaving the CO2 emission reduction factor almost unchanged.The system pa-
rameters for case 3 are given in Fig. 5 and Table 2.

                                    4. DISCUSSION

    The size of a power plant, in which this solution can be adopted, is strongly de-
pended on state-of-the-art of MCFC technology. It was assumed that there is no inter-
vention to coal fired boiler architecture. It seems that more practical solution is to put
the MCFC inside the boiler which avoids using heat exchangers: HX-1 and HX-2.
    Mixing of flue gas with fresh air seems to be questionable but it should be re-
membered that in contrast to convectional CO2 separators (e.g., MEA), it is needed to
transport carbonate ions through the electrolyte. These ions consist of carbon dioxide
as well as oxygen.
    The CO2 emission reduction factor and CO2 relative emission were used to com-
pare the systems. These values for all analyzed cases are given in Table 2.
    Application of the MCFC in a coal fired power plant gives a relatively high reduc-
tion in CO2 emissions. The relative CO2 emission of the CFPP is estimated at 1137
kg CO2/MWh while in contrast the MFCF–CFPP hybrid system has the emission rate
of 300 kg CO2/MWh. The quantity of CO2 emitted by the MCFC–CFPP is 60% lower
than is the case with the CFPP.

                                   5. CONCLUSIONS

    MCFCs could be profitably used in existing power plants which have been given
CO2 limits. MCFCs could potentially decrease CO2 emissions, leaving the power gen-
eration capacity of the system at least the same, if not greater.
    As mentioned earlier, all cases were optimized to achieve maximum power gen-
eration efficiency. However, this may be open to challenge if it is accepted that the
main task of the MCFC is to limit CO2 emissions, which would result in the CO2
emission reduction factor being used as the objective function of the optimizing proc-
ess. If this factor is optimized, the cell voltage at the last cell can fall below zero and
the MCFC will work as a CO2 concentrator. At the very least, the MCFC would gen-
erate no power, and might even consume some. However, the main task of a power
plant is power generation; hence hybrid system efficiency was chosen as the objective
function for optimization.
                    Reducing CO2 emissions by using a molten carbonate fuel cell               349

    Important technical issues such as sulphur or dust resistances of the MCFC fell
outside the remit of this paper, although they can evidently limit the application of
MCFCs in coal fired power plants.

                              Fig. 6. Market prices for CO2 allowances
                                at European Energy Exchange, EEX

     It should be borne in mind that prices of tradeable CO2 allowances were relatively
low in the past (see Fig. 6) which afforded little opportunity to realize profits from
carbon trading. The market situation starts to be changed at present.
     • There is no unequivocal choice of objective function of the optimizing process
for this type of hybrid system
     • MCFCs could reduce CO2 emissions by the factor of 60%
     • A relative emission of 300 kg/MWh appears achievable for the hybrid system
     • Current price levels on the CO2 emissions trading market in the EU are too low
to allow the realization of carbon trading profits from the use of MCFCs.

    This work was sponsored by the Polish Ministry of Science and Higher Education in the period of

 [1] MILEWSKI J., LEWANDOWSKI J., MILLER A., Chem. Proc. Eng., 2008, 29, 939.
 [2] MILEWSKI J., MILLER A., SALACINSKI J., BADYDA K., Chem. Proc. Eng., 2006, 27, 237.
 [3] GÖTTLICHER G., PRUSCHEK R., Energ. Conv. Mgmt., 1997, 38.
 [4] CAMPANARI S., J. Power Sources 2002, 112, 273.
      Energy, 2004, 29, 1279.
 [6] LUSARDI M., BOSIO B., ARATO E., J. Power Sour., 2004, 131, 351.
350                                          J. MILEWSKI et al.

 [7] SUGIURA K., TAKEI K., TANIMOTO K., MIYAZAKI Y., J. Power Sour., 2003, 118, 218.
 [8] EVAN J., GRANITE T., O’BRIEN T., Fuel Proc. Techn., 2005, 86, 1423.
 [9] BOLLAND O., MATHIEU P., Energ. Conv. Mgmt., 39, 1998, 1653.

n       –   molar flow, kmol/s
η       –   efficiency, utilization factor
m       –   mass flow, kg/s
s/c     –   steam-to-carbon ratio
MCFC    –   molten carbonate fuel cell
MEA     –   monoethanolamine


     Członkostwo we Wspólnocie Europejskiej jest związane z ograniczeniami emisji pewnych związ-
ków do atmosfery, w tym CO2. Większości podmiotów emitujących znaczne ilości CO2 przyznaje się
wielkość emisji, której przekroczenie wiąże się z bardzo wysokimi karami (€100/Mg). Z drugiej jednak
strony, niewykorzystanie pełnego limitu pozwala na sprzedaż praw do emisji innemu podmiotowi. Obec-
nie stosuje się wiele metod umożliwiających ograniczenie emisyjności elektrowni zawodowych. Więk-
szość z nich, poza wzrostem kosztów wytwarzania energii elektrycznej, wiąże się ze zmniejszeniem
sprawności oraz mocy całego bloku. Ogniwa paliwowe wytwarzają energię elektryczną w wyniku reakcji
elektrochemicznych zachodzących na powierzchniach elektrod. Spośród wielu rodzajów ogniw paliwo-
wych tylko węglanowe ogniwa paliwowe (ang. molten carbonate fuel cell – MCFC) nadają się do sepa-
rowania CO2 ze strumienia spalin. Do prawidłowej pracy ogniwa wymagane jest bowiem dostarczenia
tlenu wraz z dwutlenkiem węgla w celu utworzenia ujemnie naładowanych jonów CO3 − , które są prze-

wodzone przez elektrolit. Ogniwo paliwowe MCFC, poza wodorem, może być zasilane paliwami węglo-
wodorowymi (w tym m. in. gazem ziemnym). Połączenie ogniwa paliwowego z układem siłowni parowej
tworzy układ hybrydowy (ang. hybrid system – HS) o potencjalnie większej sprawności od samego ukła-
du turbiny parowej. Spaliny zza kotła opalanego pyłem węglowym składają się głównie z azotu, tlenu,
pary wodnej oraz dwutlenku węgla. Mieszanina taka po oczyszczeniu z pyłu i związków siarki może
zostać wykorzystana jako utleniacz po stronie katody w ogniwie paliwowym MCFC. Spaliny pochodze-
nia kotłowego charakteryzują się zbyt niską temperaturą w stosunku do temperatury pracy ogniwa
MCFC, wobec czego wymagane jest zastosowanie dodatkowych wymienników ciepła bądź umieszczenie
ogniwa bezpośrednio w kotle. Z drugiej jednak strony obecność obiegu parowo-wodnego daje duże moż-
liwości w odzyskaniu ciepła odpadkowego z ogniwa przez odpowiednie zastosowanie wymienników
regeneracyjnych. Wszystkie te aspekty zostały w pracy uwzględnione. Stworzono odpowiednie modele
matematyczne ogniwa MCFC i układu turbiny parowej. Wykorzystując je, zbadano układ hybrydowy turbina
parowa–węglanowe ogniwo paliwowe w kilku wariantach oraz porównano je z układem bez ogniwa. W wyni-
ku przeprowadzonych obliczeń określono potencjał, jaki daje takie nietypowe zastosowanie ogniwa. Integracja
ogniwa paliwowego MCFC z układem turbiny parowej umożliwia zredukowanie emisyjności CO2 bloku paro-
wego o 61% do wartości 253 kg CO2/MWh wyprodukowanej energii elektrycznej.

                                                                                Received 9 February 2009