Analysis of Industrial Flue Gas Compositions and Their Impact on Molten Carbonate Fuel Cell Performance for CO₂ Separation

Abstract

The study examines the influence of diverse flue gas compositions on the operational parameters and efficiency of MCFCs (molten carbonate fuel cells) as CO₂ separation devices to provide foundational knowledge on MCFC operation under various industrial conditions. MCFCs inherently rely on the presence of CO₂ at the cathode, where it combines with oxygen to form carbonate ions that migrate through the electrolyte; thus, CO₂ acts as a carrier species rather than a fuel, enabling simultaneous electricity generation and CO₂ separation. The findings indicate that MCFCs are most effective when operated with CO₂-rich flue gases, such as those from coal and lignite-fired power plants with CO₂ contents of roughly 12–15 vol.% and O₂ contents of 2–6 vol.%. In these cases, CO₂ reduction rates of up to 80% can be achieved while maintaining favorable cell voltages. Under such conditions, relevant also for the cement industry (CO₂ between 15 and 35 vol.%), the Nernst voltage can reach about 1.18 V. In contrast, flue gases from gas turbines, which typically contain only 4–6 vol.% CO₂ and 11–13 vol.% O₂, result in lower Nernst voltages (0.6–0.7 V) and a decrease in efficiency. To address this issue, potential modifications to the MCFC electrolyte are suggested to enhance oxygen-ion conductivity and improve performance. By quantifying the operational window and CO₂-reduction potential for different sectors at 650 °C and 1 atm using a reduced-order model, the paper provides a technology assessment that supports sustainable industrial operation and the design of CCS (carbon capture and sequestration) strategies in line with climate goals.

1. Introduction

Given the ever-increasing demand for electricity, the finite fossil fuel reserves and the environmental impact of conventional energy production methods, the development of alternative, reliable energy conversion methods is currently an intense area of research. Carbon capture and sequestration (CCS) is one of the most viable options for reducing the carbon footprint of fossil-fuel-based processes. CCS systems could be integrated with gas turbines, coal power plants and other industries, such as the cement and steel sectors [1,2]. Such retrofit options are essential for improving the sustainability of existing assets, reducing the carbon footprint of hard-to-abate sectors and supporting international climate and sustainable development targets (e.g., SDG 7 and SDG 13). Given the ever-increasing demand for electricity, the finite fossil fuel reserves and the environmental impact of conventional energy conversion methods, the development of alternative, reliable energy conversion and power production methods is currently an intense area of research [3,4]. CCS is one of the most viable options for reducing the carbon footprint of fossil-fuel-based processes.

Three principal CO₂-capture approaches exist: pre-combustion, post-combustion and oxy-fuel combustion. Pre-combustion technologies gasify or reform the fuel to produce syngas, allowing for CO₂ removal prior to combustion, whereas post-combustion systems separate CO₂—typically at low concentrations—from flue gas. Pre-combustion generally achieves higher separation efficiency but demands more complex infrastructure, while post-combustion is more easily retrofitted, though associated with higher energy penalties. The economic performance of each route depends on factors such as plant scale, required CO₂ purity, technology maturity and regulatory conditions. In oxy-fuel combustion, fuel is burned in nearly pure O₂, yielding flue gas composed mainly of CO₂ and steam, which simplifies subsequent CO₂ purification [5,6,7].

This work, however, is positioned as a feasibility and engineering assessment of MCFC-based CO₂ separation, rather than a general CCS review. The study develops and applies a reproducible reduced-order model to quantify performance envelopes under realistic flue-gas conditions. The objective is to provide a practical engineering basis for integrating MCFC systems into industrial carbon-capture applications.

1.1. MCFC as a CO₂ Separator in the Power Engineering Industry

Molten carbonate fuel cells (MCFCs) present a promising opportunity for carbon capture, as they can be utilized to selectively remove CO₂ from diluted emission sources during electricity production. This technology holds significant potential for high energy efficiency in capturing carbon. In MCFC operations, the separation of CO₂ is intrinsically linked to the electrochemical reactions and ion transport, enabling the selective concentration of CO₂ from low to high levels. These phenomena fulfill the crucial role of CO₂ separation and the possibility of its further utilization or separation and sequestration. Notably, these processes occur naturally and simultaneously with power generation, unlike conventional technologies that require additional energy-intensive extraction steps, making MCFCs particularly advantageous for CO₂ CCS processes [8,9].

The idea of adopting an MCFC to reduce CO₂ emissions was developed by Campanari [10]. In this paper, it was shown that an estimated reduction of 77% in CO₂ emissions can be achieved in a steam turbine power plant. A few years later, Campanari et al. [11] investigated the possibility of separating CO₂ from combined cycles integrated with MCFCs. The results obtained show that CO₂ reduction can reach 80%, while electrical efficiency remains virtually unchanged, with the power of the cell contributing 17% of the entire system. The results show the possibility of CO₂ emission reduction from the steam power plant can be reduced by 80%. Amorelli et al. [12] described an experimental investigation into the use of molten carbonate fuel cells to capture CO₂ from gas turbine exhaust gases. They obtained an emission reduction of 50%. Those experiments were performed using a singular cell. The CO₂ emission reduction from fossil fuel power plants (both coal-fired power plants and gas turbine power plants) by 80% and 50% makes those traditional technologies competitive again on the energy market in Europe, where a large part of the electricity price is caused by CO₂ emission charges.

Lusardi et al. [13] investigated the application of a fuel cell system for separating CO₂ from thermal plant exhaust. They found that, even without CO₂ separation, the relative emission of carbon dioxide could be reduced to below the Kyoto Protocol limit, e.g., without CO₂ separation, the integration of a molten carbonate fuel cell (MCFC) with a gas turbine reduces CO₂ emissions per kWh from 16.2 to 14.7 mol CO₂/kWh (a reduction of approximately 9%) when operating at 1 atm and from 16.2 to 13.4 mol CO₂/kWh (a reduction of approximately 17%) when operating at 3 atm. These reductions meet the Kyoto Protocol’s requirement of an 8% reduction in CO₂ emissions. 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. [14]. In that work, the experimental results of CO₂ sequestration using an MCFC are given. One key conclusion from this work is that the CO₂ removal rate can be obtained by making calculations using electrochemical theory. Jung-Ho Wee, in [15], analyzed three key areas of possible applications—mobile, transportation and stationary applications—in terms of their potential for CO₂ emission reduction through the use of fuel cells. Importantly, it was only in the stationary application that the possibility of using MCFC for CO₂ capture was considered, highlighting the critical role that this technology could play in addressing carbon emissions. This underscores the significance of incorporating CO₂ capture and storage solutions in stationary applications to effectively mitigate environmental impacts in various industrial cases. The preliminary design of a demo system for CO₂ capture with MCFC was proposed by Mastropasqua et al. [16]. The proposed system is based on two FuelCell Energy 350 kW modules, processing the exhaust gases of an internal combustion engine in the cathodic compartment. The author claims that the CCS system achieves a carbon capture ratio of 73–80%, with reference to the system without CCS. Carapelluccii [17] examined a CCS system based on MCFC that is integrated with a 250 MW subcritical coal-fired power plant. His studies revealed that the MCFC-based CCS system improves the original power and increases efficiency by almost 5%. Mastropasqua et al. [18] explored the use of a state-of-the-art high temperature MCFC for CO₂ capture. The techno-economic analysis finds conditions complying with the operating constraints of the fuel cell stacks while reducing direct CO₂ emissions of a steel mill by more than 70% and achieving apparently interesting economic returns. The synergistic production of decarbonized steel, hydrogen and electricity is key to the attractiveness of the MCFC-CCS configuration proposed by Mastropasqua et al. [18]. The separated CO₂ can be easily utilized for the production of e-fuels that can be potentially utilized for the electric power generation in MCFC [19] again, with the potential for CO₂ separation.

The results of the investigations presented to date are prospective in the European energy market, where renewable energy sources (RES) start to play an important role in the energy mix: not due to cost-effective energy production, which is unpredictable, but mainly due to penalties for the fossil fuel power plants. Enabling the elimination of up to 80% of CO₂ emissions from traditional coal-fired power plants could make them more favorable for cheap and flexible (responding to demand) electricity production. The same penalties concern industrial units that produce heat from fossil fuels for the production of other goods. Lowering CO₂ emissions will make them authorized for operation again instead of being prohibited for CO₂ emissions that are too large.

1.2. Novelty of the Paper

The concept of utilizing molten carbonate fuel cells as CO₂ separators for CCS applications was initially proposed and explored by Campanari [10]. This paper expands upon the existing knowledge by examining flue gas composition in the context of MCFC operation. A critical aspect of MCFC performance involves maintaining an optimal CO₂-to-O₂ ratio within the flue gas that enters the cathode compartment, which directly influences the operating voltage. When MCFCs are used in conjunction with gas turbine flue gases, the resulting lower operating voltage leads to a reduction in the power output of the fuel cell. The contributions of this paper are threefold: (1) an in-depth analysis of flue gas composition (N₂, CO₂, O₂) from the perspective of MCFC performance as a CO₂ separation device, (2) a predictive evaluation of the Nernst voltage for MCFCs under various flue gas compositions and (3) a discussion of potential integration strategies for MCFCs with different flue gas sources.

2. Theoretical Background of Molten Carbonate Fuel Cell Operation

MCFCs are high-temperature fuel cells whose history goes back to the 1960s, when a fuel cell with an electrolyte made of liquified lithium, sodium and potassium carbonates was used for the first time. As MCFCs operate in the range 600–700 °C, they do not require expensive materials for electrocatalysis, although they do need a temperature-resistant materials set. MCFC systems can be configured as poly-generation systems that produce electricity alone or can be co-generated with heat or hydrogen through the electrochemical conversion of fuel (e.g., natural gas, biogas, syngas, hydrogen).

MCFC systems can be used for CO₂ separation due to their fundamental operating principle, where carbonate ions (CO₃⁼) are used as the charge transferring compound through the matrix filled with electrolyte materials. On the other side of the matrix, carbonate ions react with fuel (e.g., H₂ or CH₄), creating water and CO₂. When the water is condensed, pure CO₂ is obtained (refer to Figure 1).

On the cathode side, CO₂ and O₂ are delivered in the stream of balancing gases (N₂, water, etc.). Upon initiation of current collection, the carbonate ions form and are pumped electrochemically through the electrolyte layer from cathode to anode electrochemical interfaces. Since the presence of CO₂ and O₂ is required at the cathode interface, flue gases can be delivered to the cathode side from any type of industrial or combustion process. During cell operation, carbonate ions migrate through electrolytes to the anode side, reacting with hydrogen (or other fuel) and leaving water and CO₂ as the products produced in the anode compartment. The anodic (Equation (1)) and cathodic (Equation (2)) reactions occurring in the MCFC are presented below (for H₂ fuel use):

$${\mathrm{C}\mathrm{O}}_2+{\displaystyle \frac{1}{2}}{O}_2+2e^{-}\to {\mathrm{C}\mathrm{O}}_3^{=}$$

(1)

$${\mathrm{C}\mathrm{O}}_3^{=}+H_2\to H_2\mathrm{O}+{\mathrm{C}\mathrm{O}}_2+2e^{-}$$

(2)

The partial pressures of O₂ and CO₂ determine the amount of work that may be performed when they move through the matrix from the cathode to the anode. Individual carbonate ions facilitate the transportation of two individual electrons (or four electrons per unit of oxygen reacted). A general Nernst equation for MCFC is defined by Equation (3).

$$E_{max}={\displaystyle \frac{R·T}{4·F}}·ln{\displaystyle \frac{p_{O_{2}cathode}·p_{{CO}_{2}cathode}^2}{p_{O_{2}anode}·p_{{CO}_{2}anode}^2}}$$

(3)

where R is the universal gas constant [J/mol]; T is temperature [K]; F is Faraday’s constant [A/mol]; and p defines the partial pressures of the electrochemically active species on the anode side and cathode side.

The provided equation for the OCV (Nernst voltage) is a standard and general definition of the Nernst relationship for MCFCs and may be easily translated into the more frequently employed version, as follows:

$$E_{max}={\displaystyle \frac{\mathrm{R}\mathrm{T}}{4F}}ln{\displaystyle \frac{P_{O_{2}cathode}}{P_{O_{2}anode}}}+{\displaystyle \frac{\mathrm{R}\mathrm{T}}{2F}}ln{\displaystyle \frac{P_{{CO}_{2}cathode}}{P_{{CO}_{2}anode}}}$$

(4)

Assuming the fuel is in a condition of chemical equilibrium (as described by the equilibrium constant, K), the following relationship can be derived:

$$E_{max}={\displaystyle \frac{\mathrm{R}\mathrm{T}}{2F}}ln\mathrm{K}+{\displaystyle \frac{\mathrm{R}\mathrm{T}}{2F}}ln{\displaystyle \frac{P_{H_{2}anode}x{P}_{O_{2}cathode}^{0.5}}{P_{H_{2}Oanode}}}+{\displaystyle \frac{\mathrm{R}\mathrm{T}}{2F}}ln{\displaystyle \frac{P_{{CO}_{2}cathode}}{P_{{CO}_{2}anode}}}$$

(5)

Losses within the fuel cell significantly affect the performance of the MCFC unit. Those losses mainly appear due to limitations of material ionic and electronic conductivity. In the case of electrolyte, it is crucial to maximize ionic conductivity and minimize electronic conductivity. In addition, losses associated with slow electrochemical kinetics and mass transport through the electrodes typically affect the MCFC’s operating voltage. Subtracting the abovementioned losses from the Nernst voltage determines the real operating voltage of the MCFC.

The maximum current density generated by a single cell depends upon the amount of fuel and oxidant provided to the cell (Equation (6)). In an ideal situation, 100% of the fuel would be utilized, providing maximum current density. Thus, i_max is correlated to the fuel flow and the active area of the fuel cell by Faraday’s Law, as follows.

$$i_{max}\cdot A=min\left\{\begin{array}{c}2\cdot F\cdot {\dot{n}}_{{\mathrm{H}}_2, \mathrm{eq} }\\ 4\cdot F\cdot {\dot{n}}_{{\mathrm{O}}_2, \mathrm{eq} }\\ 2\cdot F\cdot {\dot{n}}_{{\mathrm{C}\mathrm{O}}_2,\mathrm{e}\mathrm{q}}\end{array}\right.$$

(6)

A reduced order model (ROM) simplifies the complex mathematical models used to describe the behavior of systems like molten carbonate fuel cells. By focusing on the most significant factors influencing system performance, ROMs reduce computational complexity while maintaining accuracy with regard to some of the significant and controlling phenomena. The proposed ROM in this paper integrates electrochemical, thermal, electrical and flow parameters into a 0-D mathematical model, aiming to simplify the modeling process without compromising on precision. This approach is particularly beneficial for analyzing and optimizing the performance of MCFCs under various operating conditions over long periods of time, or for validation against significant experimental data. The ROM method was used to obtain numerical findings in this research. The model is ready for being implemented into various software packages, like Aspen Hysys and Aspen Plus, Ansys Fluent, MS Excel and others.

The equation for E_MCFC in the proposed model expresses the voltage of the molten carbonate fuel cell as a function of maximum voltage, fuel utilization factor, maximum current density and the area-specific internal resistances (r_i, r_e). It captures the relationship between these parameters by accounting for the losses due to ionic and electronic resistances, providing a simplified yet accurate representation of the fuel cell’s voltage under various operating conditions, as follows.

$$E_{\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{C}}=\hspace{0.25em}{\displaystyle \frac{E_{\mathrm{m}\mathrm{a}\mathrm{x}}-\hspace{0.25em}{\eta }_f\cdot i_{\mathrm{m}\mathrm{a}\mathrm{x}}\cdot r_i}{\frac{r_i}{r_e}\cdot \left(1-{\eta }_f\right)+1}}$$

(7)

The equation for the real fuel utilization factor, η_{f_real}, adjusts the nominal fuel utilization factor by accounting for internal currents caused by the electronic resistance, r_e, of the electrolyte materials, reflecting the actual fuel consumption within the MCFC.

$${\eta }_{f_{real}}={\eta }_f-{\displaystyle \frac{E_{MCFC}\cdot (1-{\eta }_f)}{i_{max}\cdot r_e}}$$

(8)

Ionic and electronic resistances (r_i and r_e, in cm²/S) have a visible impact on the real voltage output of the MCFC. These resistances are dependent upon many parameters and design/operating features of the MCFC. For example, the ionic resistance is influenced by microstructural and material properties of not only the electrolyte layer, but also the cathode, anode layers and interfaces. Generally, it is influenced by a set of functions represented below (Equations (9) and (10)) that apply to each of the layer’s thicknesses (δ) and conductivities (σ).

$$r_i=\sum _i^j{\displaystyle \frac{{\delta }_i}{{\sigma }_i}}$$

(9)

$${\sigma }_i=A\cdot e^{{\displaystyle \frac{-E_{\mathrm{m}\mathrm{c}\mathrm{f}\mathrm{c}}}{\mathrm{R}\mathrm{T}}}}$$

(10)

Electronic conductivity occurs through the electrolyte layer and can be defined in the following set of equations (Equations (11) and (12)).

$$r_e={\displaystyle \frac{\delta }{\sigma }}$$

(11)

$$\sigma =A\cdot e^{{\displaystyle \frac{-E_{\mathrm{m}\mathrm{c}\mathrm{f}\mathrm{c}}}{\mathrm{R}\mathrm{T}}}}$$

(12)

The flowchart presented in Figure 2 shows the steps of numerical model development and parameters value determinations. Reviewed in this paper are parameters associated with the conversion of flue gases, which are included in the estimation of Emax, while the rest of the parameters are thermodynamically estimated and validated based upon experimental tests of the MCFC in the laboratory conditions previously presented in [20,21]. The model results demonstrate high accuracy, with an average error of 1.2% and a maximum error of 13% when validated against 958 experimental data points, as shown in [22]. The referenced model was validated on experimental data covering a broad range of temperatures (fixed at 650 °C), current densities (0–0.35 A/cm²) and fuel/oxidant flow rates corresponding to two operating scenarios (nominal and high-flow conditions) for anodes with porosities of 57% and 67%. The target metric for validation was the cell voltage in the full polarization curve. The results align well with the existing literature across a range of temperatures, fuel mixtures and operating conditions, confirming the validity of the reduced-order model.

3. Overview of Industrial Flue Gas Sources for CCS Application

As power generation and other industrial sources are major producers of CO₂ emissions, this study analyzes the combustion products from gas-turbine-based power plants, coal-fired power plants and the cement industry. Since the power generation industry is the largest emitting sector of CO₂, the focus was on different configurations of gas-fired and coal-fired systems, considering varying inlet fuel composition and operating parameters. A broad literature overview was carried out regarding flue gas composition from different sources, resulting in the flue gas compositions that are summarized in

Table 1. Summary of flue gas composition from different sources.

Components	Flue Gas Composition
	Gas Turbine Unit				Coal-Fired Unit							Cement Industry
	[23,24]	[25]	[26]	[27]	[28]	[29]	[30,31]	[32]	[32]	[33]	[34]	[35]
CO2, %vol	5.44	4.21	3.9	1–5	13.05	13.2	13.5	12.5	15	10–14	12	14–33
H2O, %vol	3.58	8.82	8.4	5.05	5.05	7.3	15.37	16	0	16–20	15	n/a
O2, %vol	11.48	11.9	12	11	11	8.7	2.38	3.5	4	5	6	8–14
N2, %vol	78.62	74.2	74	66–72	n/a	n/a	71.62	68	81	n/a	n/a	n/a
Ar, %vol	0.88	0.89	0.9	n/a	n/a	n/a	0.68	n/a	n/a	n/a	n/a	n/a
NO, ppmv	n/a	n/a	n/a	20–220	n/a	n/a	n/a	n/a	n/a	n/a	120	n/a
NO2, ppmv	n/a	n/a	n/a	2–20	n/a	n/a	n/a	n/a	n/a	0.1	6	10–110
NOx, mg/m3	n/a	n/a	n/a	n/a	200–300	200–400	n/a	68	81	n/a	n/a	<300–3000
CO, ppmv	n/a	n/a	n/a	5–330	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
SO2, ppmv	n/a	n/a	n/a	Trace–100	100–200	100–200	0	n/a	n/a	<2600	580	<10–3500
SO3, ppmv	n/a	n/a	n/a	Trace–4	n/a	n/a	n/a	n/a	n/a	40	n/a	n/a
HCl, ppm	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	300	1	n/a
HF, ppm	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	45	n/a	n/a
Unburned Hydrocarbon, ppmy	n/a	n/a	n/a	5–300	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Particulate Matter Smoke, ppmv	n/a	n/a	n/a	Trace–25	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Ash/Dust, mg/m3	n/a	n/a	n/a	n/a	100	30	n/a	n/a	n/a	50	n/a	n/a

. References for each of the compositions are presented in brackets above each column. The discussion and analysis regarding gas-based and coal-based power units, together with the cement industry, are presented in the following subsections. Overall, this study provides valuable insights into the potential for reducing CO₂ emissions from power generation and other industries. For the subsequent Nernst-based analysis (Figure 3), only the flue-gas compositions reported in vol.% on a consistent dry-gas basis were used, while the remaining entries in Table 1 are provided to illustrate typical ranges of minor species and operating conditions, as explicitly clarified in the accompanying table footnote.

3.1. Gas Turbine Power Plants

Milewski et al. [23,24] reported on the composition of flue gases from a gas turbine power plant (

Table 2. Nominal parameters of GT power plant, reprinted from Ref. [23].

Name	Value
Air compressor inlet pressure, MPa	0.1
Air compressor inlet temperature, °C	15
Pressure ratio	17.1
Fuel	Natural gas
Fuel mass flow, kg/s	4.0
Turbine inlet temperature, °C	1210
Exhaust gas mass flow, kg/s	213
Turbine outlet temperature, °C	587
GT power, MW	65
GT efficiency (LHV), %	33
CO2 annual emission, Gg/a	250
Relative emission of CO2, kg/MWh	609
CO2 mass flow, kg/s	11

). In their study, the authors examined the possibilities of using an MCFC to reduce CO₂ emissions from the reference 70 MW gas turbine system. The nominal operating parameters and exhaust gas composition are shown in Table 2 and

Table 3. Technical and operational data of GE 9371FB.

Name	Value
Air mass flow rate, kg/s	641.81
Fuel mass flow rate, kg/s	16.10
Pressure ratio	18.1
Fuel	Natural gas
Turbine inlet temperature, °C	1371
Exhaust temperature, °C	643.29
Exhaust gas mass flow, kg/s	657.92
Net power, MW	285.76
Net thermal efficiency, %	38.18

[25], respectively.

Detailed data on gas turbine exhaust gases and their operating parameters are reported by Palomino [25] in his PhD thesis focusing on selective exhaust gas recirculation in combined cycle gas turbine power plants. The author selected a General Electric F-class engine (GE9371FB) as a reference unit, since the engine is widely reported in the literature. The basic technical and operational data for the GE 9371FB are summarized in Table 3. The exhaust gas composition and fuel composition are shown in

Table 4. Technical and operational data of GE 7FA.05.

Name	Value
Air inlet temperature, °C	15
Fuel inlet temperature, °C	38
GT power output, Mwe	418.7
Compressor pressure ration	17
Turbine inlet temperature	1360
Air composition, %vol
N2	77.32
O2	20.74
Ar	0.92
CO2	0.03
H2O	0.99

.

Diego et al. [36] examined selective exhaust gas recirculation operating conditions for CO₂ capture in natural gas combined cycle power plants. The paper investigated a GE 7FA.05 engine and data on flue gas composition taken from [14]. The authors of the report [14] claimed the estimated flue gas composition lay within the range of values specified by the manufacturer.

The GE Power Systems publication [27] reported the typical ranges for gas turbine engines and gave the main sources of pollution, broken down into major and minor species—see Table 1, respectively. The presented concentrations of species lay within the range of composition of flue gases from particular GT engines reported in the literature.

3.2. Hard Coal Power Plants

In relation to the composition of flue gases received from hard coal-based power plants, the composition of hard coal is crucial, as, unlike natural gas, coal composition can vary to a much greater degree. Integrated systems for flue gas purification, such as desulfurization and electrostatic precipitators, are also essential. Considering the existence of hard coal fueled steam power plants in the Polish Łaziska Power Plant [28] which has a 225 MWe hard-coal-fired boiler were analyzed, delivering the flue gas results presented in Table 1. The power plant is equipped with a desulfurization system, using sodium carbonate and bicarbonate as agents. A team from the same research center analyzed the composition of exhaust gases at the Łaziska power plant as part of the work on a pilot amine CCS installation [29].

Other authors analyzing CCS at a 400 MWe power plant [32] reported flue gas composition as broken down into a wet basis and dry basis, which also impacts the fractional composition in the volume of the flue gases. The flue gas compositions reported by the authors were obtained from a pulverized coal power plant with post-combustion exhaust gas clean-up. Prior to composition analysis, there were bag filters for ash removal, wet limestone desulfurization units for sulfur removal and finally, a dehydration unit for the dry basis flue composition results. The researchers analyzed the performance of membrane separation units for CO₂ capture and storage.

Data on an example composition of exhaust gases from coal-fired power plants can also be found in the report [30], which was used as the assumption for the article [31]. The reported flue gas composition (see Table 1) is for flue gas from a 550 MWe pulverized supercritical hard-coal-fired power plant with total electric efficiency of 39.3%. The flue gas temperature at the analyzed point was 331 °C at 0.1 MPa absolute pressure. The total mass flow rate was 821 kg/s. The unit was equipped with over-fire air (OFA) burners for better NOx control, an ash removal system, a selective catalytic reduction (SCR) system for NOx removal and a desulfurization system.

An example of temperature distribution [37] in a coal-fired boiler for two different loads (100% and 60%) is shown in Figure 5. It can be seen that with a decrease in load, the temperatures in individual parts of the boiler also decrease. As presented in the figure, a selective catalytic reduction DENOX installation is located at the end of the flue gas channel, where, behind the SCR, the temperature of the flue gas drops to 305 °C at 100% load and to 270 °C at 60% load. Temperatures reached 650 °C at nearby superheaters B and C.

3.3. Lignite Power Plants

The composition of exhaust gases from power installations using lignite as fuel obviously depend upon the specific composition of the fuel itself. The chemical composition of lignite may differ noticeably within one country and even within one deposit. Beanson et al. [38] as part of their study, analyzed the chemical composition of lignite from three locations. The list of the main fuel components is presented in

Table 5. Main components of lignite in three selected locations.

Component	Antelope	Caballo	Beulah
Ash	7.28	6.59	11.62
S	0.33	0.51	1.49
C	69.97	67.88	61.50
H	4.77	4.83	3.96
N	1.05	1.24	1.08
O		16.61	18.96
Ash	7.28	6.59	11.62
S	0.33	0.51	1.49

.

Research on other elements in the composition of brown coal has been presented by Stergarsek et al. [39]. Among other things, the content of heavy metals was analyzed. The tests showed that the flue gas contained up to 200 mg/m³ HCl, up to 10 mg/m³ HF and about 1.5 ng/m³ Hg. Research by Marczak et al. [40] indicates values in the range of 13.6–15.2 μg/m³ of mercury emitted from lignite-fired plants. Milewski et al. [33] presents the composition of exhaust gases from a coal-fired unit concerning the possibility of CO₂ separation using MCFC. The composition of these exhaust gases is presented in Table 1.

3.4. Cement Industry

The cement industry consumes approximately 15% of the total industrial energy use [34] and is recognized as an energy-intensive industry where thermal energy consumption reaches 80% of primary energy [41]. The energy for cement manufacturing processes originates mainly from the combustion of coal, fuel oils, waste fuels (e.g., tires) and petroleum coke [34]. The type of fuel largely determines the composition of the flue gas, as well as the impact of CO₂ emissions. During the cement manufacturing process, CO₂ is generated from three sources: (1) decarbonation of limestone in the kiln, (2) fuel combustion in the kiln and (3) consumption of electricity during the process. The typical composition of exhaust gases originating from cement production processes is given by Bosaga et al. [35]: see Table 1.

A technical report by the European Cement Research Academy [42] shows that based on the usual fuel composition and energy consumption data, the possible concentration of O₂ lies within the range 2–5%, CO₂: 15–35%, with a balance of mostly N₂. According to the authors of the report [42], the data were derived from German cement plants, but were representative of European emissions from the cement industry. The data given by the report are consistent with the paper by Bosaga et al. [35].

4. Discussion of Selected Flue Gas Intake Points and MCFC Suitability

On the cathode side, the MCFC can accept a wide spectrum of gas mixtures, provided that both O₂ and CO₂ are available. The cathode inlet composition, temperature and pressure, as well as the presence of contaminants such as SO₂ or H₂S, all influence the achievable voltage and long-term durability. In the present work, the theoretical upper limit of cell performance is characterized by the Nernst potential (Equation (3)), which depends on the partial pressures of CO₂ and O₂ at the cathode and anode. Increasing the CO₂ partial pressure at the cathode while maintaining a sufficiently low O₂ fraction enhances the equilibrium potential, whereas a higher total pressure and temperature also favor higher cell voltages. According to the Nernst equation for an MCFC, the ideal cell potential at a given temperature has a tendency for improvement when increasing the partial pressure of CO₂ on the cathode side while proportionally keeping the ratio of O₂ approximately 50% lower. At greater pressures and temperatures, increases in fuel cell performance are also possible.

MCFCs have the capacity to utilize a wide range of flue gas sources, including gas turbine exhaust, flue gases from hard coal power plants and flue gases from the cement industry. For the Nernst-based comparison shown in Figure 3, eight representative flue gas compositions were taken directly from Table 1, i.e., from the literature data for real plants, and used as cathode feed gases. The Nernst potentials were calculated at a fixed temperature of 650 °C and an overall pressure of one bar, which are typical MCFC operating conditions, and these were chosen in order to focus on comparing the intrinsic potential of different flue gas sources for MCFC application. Within this framework, it can be concluded that, based on [25,28,32,43], MCFCs will perform well with flue gases that have a high CO₂ content. However, the Nernst voltage decreases for flue gases with a lower concentration of CO₂ [32] and an excess of O₂ [17,25,44]. One solution to enhance performance in this case is to modify MCFCs to increase the conductivity of oxygen ions and carbonate ions. The solution appears to be a very promising way to increase electrical efficiency while separating CO₂ [17,25,44].

To quantify how uncertainties or local variations in flue gas composition affect these results, a simplified sensitivity analysis of the Nernst potential was performed. For each of the flue gas cases shown in Figure 3, the molar fractions of CO₂ and O₂ were independently varied by ±10%, relative to their reference values taken from Table 1, while the remaining balance was assigned to N₂ so that the total composition remained 100 vol.% at 650 °C and 1 bar. The resulting Nernst voltages are summarized in

Table 6. Sensitivity of the MCFC Nernst potential to ±10% variation in cathode O₂ and CO₂ molar fractions at 650 °C and 1 bar.

	Ereference	E(O2 + 10%) (ΔE, %)	E(O2–10%) (ΔE, %)	E(CO2−10%) (ΔE, %)	E(CO2−10%) (ΔE, %)
Gas turbine	1.1359 V	1.1339 V (−0.18%)	1.1377 V (+0.16%)	1.1164 V (−1.72%)	1.1487 V (+1.13%)
Gas turbine GE 7FA.05	1.1411 V	1.1391 V (−0.17%)	1.1429 V (+0.16%)	1.1285 V (−1.11%)	1.1506 V (+0.83%)
Hard Coal Łaziska	1.1818 V	1.1798 V (−0.17%)	1.1836 V (+0.15%)	1.1784 V (−0.29%)	1.1849 V (+0.26%)
Lignite	1.1354 V	1.1291 V (−0.55%)	1.1413 V (+0.52%)	1.1343 V (−0.10%)	1.1365 V (+0.09%)
Cement industry	1.1909 V	1.1847 V (−0.52%)	1.1928 V (+0.16%)	1.1889 V (−0.17%)	1.1928 V (+0.16%)

. For all cases, a ±10% change in O₂ content led to relatively small variations in the Nernst potential, typically within ±2–6 mV. A ±10% change in CO₂ content produced somewhat larger effects, with maximum deviations of −19.5 mV and +12.8 mV observed for the low-CO₂ gas turbine exhaust, whereas for the coal and cement flue gases, the changes were generally below ±3 mV. These results indicate that moderate variations in CO₂ and O₂ content do not alter the relative ranking of the different flue gas sources in Figure 3 and confirm the robustness of the Nernst-based comparison.

4.1. Gas Turbine Power Plants

With gas turbines, the main limiting flue gas parameter is temperature. In a simple-cycle gas turbine (without hybridization or combination with other types of cycles, such as Brayton–Rankine, Brayton–Brayton, Brayton–Diesel, etc.), exhaust gases are released at temperatures of 400–600 °C. In CHP gas power plants, the heat is most typically harnessed by a heat exchanger and delivered to meet heat demands in industry or communities. As MCFCs will not decrease the temperature of flue gas, the MCFC unit can be placed directly in a high temperature location in the exhaust gas stream, where it can collect CO₂ from flue gas (Figure 4) and return it to the system. However, as part of this solution, an additional heat exchanger from the separated CO₂ stream flow will be needed to retrieve heat for the CHP heat exchanger. This means that hybrid or combined Brayton cycles can also use MCFC CO₂ separation, as it will not have a significant impact on the amount of heat in the flue gas stream. As a matter of fact, the MCFC system could have the potential to add heat to the system to improve the downstream performance of the CHP or combined cycle equipment (e.g., HRSG and steam turbine).

4.2. Hard Coal Power Plants

As presented in Figure 5, the most appropriate location for an MCFC would be directly in the coal boiler, between selected heat exchangers in the superheating section of the boiler. Due to the variable temperatures of flue gases, dependent upon the design of any particular boiler and dependent upon the load of the boiler, the MCFC may not be able to easily operate on flue gases extracted from one place in the boiler [23]. In Figure 5, a temperature of 650 °C could be found somewhere in the middle of heat exchanger C; thus, construction of this heat exchanger would have to be modified in order to place the fuel cell in the middle of the flue gas stream—the heat exchanger could be divided into two parts, for example. This would come at a high investment cost, so the MCFC would have to be placed in the coal-fired plant when it is being constructed.

Another challenge is the presence of SO₂, which can be present in concentrations of 100–200 parts per million (ppm). This may cause problems, as SO₂ poisons the MCFC due to its reaction with carbonate electrolyte by forming SO₄⁼ ions, which, when transformed to the anode side, leads to the formation of H₂S, which, in turn, ends up as deposits on the surface of the anode, decreasing its active area and reducing fuel cell efficiency. Experimental investigations show that while SO₂ concentrations of up to 20 ppm are safe for the long-term operation of MCFC, 100 ppm for several hours triggers a slight drop in MCFC performance. The delivery of higher concentrations of SO₂ to the MCFC cathode should be avoided. Besides sulfur, only a few other impurities in flue gas may pose risks to MCFC operation. NOx is not a major problem for MCFCs, and cells typically tolerate higher ppm values without significant degradation, so its impact is far lower than that of sulfur. Halides such as HCl and HF are much more dangerous, because even <0.1 ppm can accelerate electrolyte loss and attack nickel-based electrodes—fortunately, their concentrations in flue gas are usually already very low. Particulates can also be harmful by physically clogging flow channels, but standard sub-micron filtration easily keeps them below the <1 mg/m³ limit, so, in practice, halides are the only non-sulfur species requiring careful consideration.

4.3. Lignite Power Plants

The temperature distribution presented in Figure 6 suggests that the optimal location of the MCFC unit is between superheater III and reheater II. The temperature there can reach 660–720 °C. As in the hard coal boiler, the main challenges are the high investment cost involved in modernizing the boiler unit for MCFC installation and the SO₂ present in flue gas. Moreover, lignite coal typically has a greater sulfur content. Brown coal produces more ash than hard coal, but ash does not impact MCFC performance as much as sulfur; capturing some of the ash before the MCFC and designing the MCFC stack with sufficiently large channels to avoid mechanical clogging may remediate this challenge.

4.4. Cement Industry

Figure 7 illustrates a typical layout for a cement manufacturing plant from [18]. The cement production method has undergone extensive optimization and potential MCFC poisoning components in the outlet flue gas are cleaned while the temperature is around 250 °C. This temperature is still well below the 600–650 °C typically required at the MCFC cathode inlet, so the cleaned cement-plant flue gas cannot be fed directly to the MCFC. In the proposed integration scheme, the dedusted gas is first routed through an additional heat-exchanger train, where it is preheated mainly by the hot MCFC cathode off-gas (and, if needed, other waste-heat streams from the kiln system) and only then is it supplied to the MCFC stack.

A simple order-of-magnitude energy balance shows that the corresponding preheating duty is moderate but non-negligible. Raising the flue-gas temperature from about 250 °C to 600–650 °C (ΔT ≈ 350–400 K) with an average heat capacity of ≈1 kJ kg⁻¹ K⁻¹ requires roughly 0.35–0.4 MW of heat per 1 kg s⁻¹ of gas. For cement kilns with thermal inputs of several tens of megawatts, preheating only the fraction of flue gas directed at the MCFC therefore represents a few percent of the kiln firing rate and can be covered by recuperation from the MCFC cathode off gas. The integration of MCFCs with the cement industry and the combined design of flue-gas cleaning, heat recovery and CCS remains an interesting topic for future investigations.

5. Conclusions

As a result of growing worries about climate change, the emission of carbon dioxide is now subject to escalating environmental taxes. Currently, this mostly impacts power plants, but it is also beginning to affect sectors such as cement and transportation. There are several techniques for reducing CO₂ emissions, most of which are characterized by an energy penalty for the installation at issue.

Such fees are not directly incurred in the system that involves MCFCs, which separate out CO₂ while simultaneously generating electricity and thereby partly offset the associated energy penalty. This solution also has certain limitations related to the requirement to maintain an appropriate ratio of CO₂ to O₂ in the flue gas. This article presents a comprehensive review of several types of flue gas from this perspective. It analyzes the composition of these flue gases in terms of CO₂, O₂ and N₂ content and the corresponding voltages achieved by MCFCs.

The findings show that MCFCs perform best with flue gases that are rich in CO₂, such as those from coal and lignite-fired plants, where CO₂ concentrations range between 12% and 15%. In these scenarios, based on the literature data, MCFCs can achieve up to an 80% reduction in CO₂ emissions [12,13,15,18] while maintaining favorable Nernst voltages up to 1.18 V (as calculated using our reduced order model in Figure 3), which typically result in operating cell voltages between 0.7 and 0.9 V under standard current densities. Conversely, gas turbine exhaust gases, with their lower CO₂ content (4% to 6%) and higher O₂ levels (around 11% to 12%), resulted in lower Nernst voltages of 0.6 to 0.7 V (from our ROM-based thermodynamic analysis), decreasing both efficiency and electricity generation. This indicates a need for potential modifications to the MCFC electrolyte to improve performance in such environments. In the cement industry, where flue gases contain CO₂ concentrations ranging from 15% to 35% and are emitted at temperatures around 250 °C, MCFCs showed considerable adaptability (based on our integration strategy discussions in Section 4). However, future investigations are needed.

The contributions of this study are as follows: literature-based analysis of flue gas composition (N₂, CO₂, O₂) from the perspective of the MCFC’s performance as a CO₂ separation device, our ROM-based evaluation of the Nernst voltage for MCFCs under various flue gas compositions and our proposed discussion on potential integration strategies for MCFCs with different flue gas sources. Overall, the work serves as an engineering feasibility analysis supported by a reproducible ROM, providing actionable operating ranges and integration guidelines that are specific to MCFC-based CO₂ separation.

Molten-carbonate fuel cells are moderately tolerant to sulfur compounds but still require gas cleanup to keep SO₂ below 0.1–1 ppm, because higher levels accelerate electrolyte loss and cathode corrosion. Nitrogen oxides are less chemically aggressive to MCFC materials, and systems typically tolerate NO_x up to 10–20 ppm, though lower levels are still preferred to avoid downstream corrosion. Acid halides such as HCl and HF must be kept extremely low, because halides attack nickel-based electrodes and promote accelerated electrolyte degradation. Solid particulates must be limited to <1 mg/m³ (preferably far lower) to avoid plugging flow fields and damaging gas-handling equipment. Because most fuel streams exceed at least one of these values, pre-cleanup is required to meet these MCFC specifications. For sulfur, pre-cleanup steps should target ≥90–99% removal, depending on the raw fuel sulfur load. For halides, cleanup must typically achieve ≥99% removal, since even trace leakage can shorten stack life.

In the future, experimental studies are planned, in which the MCFC fuel cell will be used to separate CO₂ from various flue gases with simulated compositions. This will confirm the theoretical considerations presented in this article.

From a sustainability perspective, the quantified CO₂ reduction rates and operating windows provided in this work can serve as a decision-support basis for selecting MCFC-based CCS solutions in different industrial sectors. By enabling significant emission reductions in existing plants without full infrastructure replacement, MCFC retrofits contribute to more sustainable power and industrial systems and help bridge the gap toward long-term climate neutrality.