MCFC-based CO2 capture system for small scale CHP plants

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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

article info

abstract

Article history:

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

Received 1 July 2011

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

Received in revised form

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

8 May 2012

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

Accepted 12 May 2012

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

Available online 29 July 2012

reactions that occurs in the cell stack: carbon dioxide entering into the cathode side is transported to the anode side via CO¼ 3 ions and is finally concentrated in the anodic exhaust.

Keywords:

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

CO2

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

Cogeneration

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

MCFC

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

CCS

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

Aspen

will increase number of medium cogeneration plants with consequent benefit for both MCFC 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 the model and several analyses were performed. CO2 removal efficiency of 63% was reached with correspondent total efficiency of about 35%. System outlet 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 postcombustion 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.

* Corresponding author. Tel.: þ39 3491319634. E-mail addresses: [email protected] (U. Desideri), [email protected] (S. Proietti), [email protected] (P. Sdringola), [email protected] (G. Cinti), [email protected] (F. Curbis). 1 Tel.: þ39 075 5853743. 2 Tel.: þ39 075 5853930. 3 Tel.: þ39 075 5853991. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.048

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1.

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Introduction

This paper presents a study of a novel technology for CO2 separation from the other components present in a cogeneration 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 greenhouse 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 reduction 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-bed market 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 oxyfuel combustion and post-combustion [3,4]. These three solutions are depicted in Fig. 3 (left). Only the third technology is really available at the commercial status: the

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 CO¼ 3 /H2 O þ CO2 þ 2e

(1)

Cathode side 1=2O2 þ CO2 þ 2e /CO¼ 3

(2)

Global Reaction H2 þ 1=2O2 þ CO2ðcatÞ /H2 O þ 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

Fig. 1 e Carbon dioxide emissions trend (left) and Cogeneration vs. Conventional generation comparison (right) [1].

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

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 (temperature, pressure, current density, cell area); outputs are: utilization 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 guarantee 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;consumed ¼ H2;in H2;in

UOX ¼

O2;in  O2;out O2;consumed ¼ O2;in O2;in

UCO2 ¼

CO2;in  CO2;out CO2;consumed ¼ CO2;in CO2;in

(4)

(5)

(6)

WAC ¼ Vstack $AFC $J$hDC=AC hel ¼

(7)

WAC _ CH4 $LHVCH4 m

(8)

CO2;consumed CO2;in

(9)

hrem ¼

Fig. 5 shows the cell’s polarization and the electric power curve: they are obtained giving the model the CHP plant exhaust gas composition at the cathode and SMR (S/C ratio 3) composition 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;

Fig. 3 e The three ways of CCS and the principle of operation of molten carbonate fuel cells (on the right).

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

Table 1 e Upper cogeneration plant performances and exhaust gases data. Power 1MWe þ 1.5 MWth

Fuel

Exhaust gas flow

Exhaust gas composition

Temperature

Pressure

Natural gas

1.71 kg/s

N2 85.65 %vol O2 6.03 %vol CO2 8.32 %vol

95.6  C

1 bar

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

(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 operating 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

Fig. 5 e Base model and its polarization and electric power curve (on the right).

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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.

benefits for the fuel cell life and performance. The mathematical 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$UCO2 þ 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 fed with 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 considering 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 operating 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

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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 blocks operating conditions. Reformer Input Parameter Pressure [bar] Temperature [ C] Physical state Compressors Input Parameter Isentropic efficiency Mechanical efficiency

Fixed value 1.015 600 Vapor

Fixed value 0.9 0.95

Output Heat demand [kW]

Output Power [kW] Pressure [bar] Temperature [ C]

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

concentration in the flow designed for storage; as a consequence 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 consumption 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  C higher 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.

Fig. 9 e Full system model scheme.

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

Table 3 e Advantages and drawbacks of anode and cathode recirculation.

Anode

Cathode

3.

Advantages

Drawbacks

 Increasing CO2 purity  Reducing compression costs  Increasing CO2 separation  Simplifying temperature control

 Energy consumption  Fuel mix impoverishment  Energy consumption

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 efficiency 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.

A CO2 purity of 82.2% is a great result considering that this is an active system, in comparison with amine-based separation 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 introduced 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 elsewhere to produce additional heat. Finally a simple economic comparison was performed with 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 an MCFC 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

Fig. 11 e CO2 compression power required and CO2 purity varying anode recirculation degree.

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



Temperature [ C] Pressure [bar] Mass flow [kg/hr] H2 CO CO2 N2 AIR H2O CH4 O2

GASNAT

H2O

ICEOUT

OXIDOUT

FUELOUT

CO2STOCK

25.0 1015 55.184 0.000 0.000 0.000 0.000 0.000 0.000 1000 0.000

25.0 1015 186.246 0.000 0.000 0.000 0.000 0.000 1000 0.000 0.000

95.6 1015 6156.000 0.000 0.000 0.083 0.856 0.000 0.000 0.000 0.060

686.6 1000 5323.618 0.000 0.000 0.014 0.887 0.000 0.000 0.000 0.098

686.6 1000 1073.812 0.092 0.000 0.454 0.000 0.000 0.454 0.000 0.000

25.0 140.000 763.972 0.178 0.000 0.822 0.000 0.000 0.000 0.000 0.000

Table 5 e Model Parameters. Inputs

Outputs

Steam/Carbon ratio [e]

3

Anode recirculation [e] Cathode recirculation [e]

0.32 0.59

DC/AC efficiency [e] FC temperature [ C] FC pressure [bar] Flow pressure loss [bar] Current density [A/m2] Cell area [m2] Cell resistance [Ohm$cm2]

0.95 695 1 0.015 1000 575 1.86

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 characterized 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 calculations, starts from a real cogeneration power plant but methodology 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.

references

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

Fuel utilization [e] Oxidant utilization [e] CO2 utilization [e]

0.704 0.234 0.400

Voltage [V] MCFC electric power [kWe] MCFC electric efficiency

0.706 386 50.3%

System electric power [kWe] System electric efficiency

342 44.6%

CO2 removal efficiency CO2 stored flux purity System thermal power [kWth] Cogeneration efficiency

61.9% 82.2% 320 86.4%

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