Fuel cells convert the chemical energy in fuel directly into electricity and heat, without combustion, leading to high efficiencies with low or even zero emissions. The SOFC is becoming a mature technology and can make the commercial breakthrough if cost targets can be met by achieving cost reductions through volume manufacturing, improved lifespan/performance, and lower cost materials [1
]. Research and development in the last twenty years have led to significant advances in all areas of the technology including cell, seal, interconnect, and stack design, as well as peripheral components and the entire balance of plant (BoP) [4
]. Manufacturing achievements have led to defect identification and minimization, quality control, and scale-up of stack components and the entire stack assembly manufacture.
The SOFC system is appropriate to operate on a pipeline fuel such as reticulated natural gas with its well-established supply infrastructure throughout the world. For such a fuel, minimal fuel processing is required, which includes desulphurization of the fuel to remove sulphur compounds that are naturally present in the hydrocarbon fuel source and those that are added as odorants to meet legislative requirements such as for natural gas, propane, and LPG for domestic applications. The preprocessing also includes a level of conversion of the hydrocarbon fuel, conventionally known as prereforming, which functions to convert the hydrocarbon feed such as natural gas to a hydrogen-rich mixture or a methane-rich mixture depending on the type of anode in the SOFC stack, i.e., noninternal reforming or internal reforming type. For an internal reforming type SOFC, where methane can be converted by steam reforming on the anode, it suffices to prereform the fuel to a level where all higher hydrocarbons (C1+
) are converted leading to a mixture of methane, hydrogen, and carbon oxides with little or no conversion of methane [6
]. For a noninternal reforming type SOFC, all hydrocarbon components including methane need to be fully converted to a mixture of hydrogen and carbon oxides. Owing to its high electrical efficiency, the SOFC technology results in reduced emissions of CO2
and is practically noise free. Furthermore, it is free of NOx
emissions due to its relatively low operating temperatures. The SOFC system is particularly attractive as a combined heat and power generation (CHP) system, since the waste heat generated can be used to supply heat to a hot-water system which can be interfaced to the SOFC system [8
A system-level flowsheet model of the SOFC system including the complete BoP is a useful platform for simulating the performance of the plant and for sizing of individual components of the BoP. Commercially available process simulation software such as Aspen Plus or Aspen HYSYS, PRO/II, etc., contains extensive thermodynamic and physical properties database and includes in-built modules for a number of components which are commonly used in a process plant, such as heat-exchangers of various types, reactors of various types, compressors, pumps, valves, separating columns, tanks, mixers, etc. It allows for energy optimization via heat and work integration of system components. However, it does not include a module for fuel cell reactions, i.e., it cannot directly account for the electrochemical reactions involving ions and electrons. There are two approaches for modeling SOFC-based systems with commercial process simulators. In one approach, the SOFC model is developed in a separate platform such as FORTRAN, VB, C++, MATLAB, etc. and then linked to the process simulator [9
]. In another approach, the SOFC reactions are modeled using the equilibrium reactor module GIBBS [16
]. Anderson et al. [17
] modeled the SOFC as a combination of an isothermal plug flow reactor (PFR) module, to account for methane reforming kinetics on the anode, and a GIBBS reactor for the fuel cell reactions of hydrogen and CO oxidation. However, this was not a system-level model and focused on reactions and mass transport processes at the cell level. Using established theoretical and/or empirical correlations from literature, they tested the validity of their model by comparing their simulation results with those of others reported in the literature. Two main drawbacks of this work are the assumptions of isothermal conditions for internal reforming and use of a GIBBS reactor for the fuel cell reactions. In a real system the SOFC stack does not operate in an isothermal mode. There are two opposing contributions to stack temperature profiles in the case of an internal reforming anode. The endothermic steam reforming reaction absorbs heat from the gas stream which results in cooling of the stack and the fuel cell reaction(s) release heat which results in heating of the stack; the net effect is determined by the extents of these reactions.
A conversion reactor is more appropriate for representing the fuel oxidation reactions by setting the percent conversion equal to fuel utilization. An equilibrium approach using the GIBBS reactor does not allow setting of the reaction conversion to match fuel utilization. In this work, the internal reforming of methane via steam reforming and the accompanying water-gas shift (WGS) reaction is modeled via the PFR module with the kinetic expressions from literature [18
] and the fuel cell reactions are modeled using the conversion reactor where the conversion is linked to the fuel utilization value calculated in a spreadsheet block. Another feature of the current work is that unlike the work of Anderson et al. [17
], where the PFR is modeled as an isothermal reactor, in this work the energy stream of the PFR is linked to the cell in the spreadsheet block which calculates the heat generated by the fuel cell reaction and is available as reaction heat in a direct internal reforming SOFC. The axial temperature profile created in the PFR is therefore, representative of the temperature profile on an SOFC anode with direct internal reforming, as the coupling of the endothermic methane steam reforming (MSR) reaction and accompanying mildly exothermic WGS reaction with heat available from the fuel cell reaction is appropriately captured with this approach. The corresponding composition profile under current load cannot be generated within the PFR module as this module only works with kinetic schemes and it is not possible to add the fuel cell reactions to the PFR as conversion reactions based on fuel utilization. An option to include the reaction kinetics of the electrochemical oxidation of hydrogen is also not available in the software. Nevertheless, the reaction extents and accompanying heat exchanges can be calculated in the spreadsheet module and linked to the PFR module. Firstly, this allows generation of open-circuit composition profiles of the internally reformed gas which sets the boundary for the Nernst voltage profile under load, after accounting for the extents of the fuel cell reactions. Secondly, the current density and composition profiles can be calculated within the spreadsheet block using appropriate correlations.
Previous work [9
] largely focused on the issues that can predict and improve the fuel cell operation in terms of current generation and voltage losses. For instance, the effect of air flow rate, steam to carbon ratio (S/C), current density, fuel utilization (Uf), inlet temperatures, or operating pressure have been extensively investigated. By contrast, in this work we have mainly focused on an analysis of processes that significantly affect anode performance and lifetime and consequently impact on the SOFC system as a whole. We analyzed anode performance for various levels of reforming activity. Three cases are considered: (i) Full reforming activity, (ii) 1/3rd reforming activity, and (iii) 1/6th reforming activity. Reduced reforming activity may be the result of engineered design [20
] or may result from progressive degradation of the anode from poisoning or sintering due to nickel coarsening over the useful life of the stack [4
], which extends the reaction zone and requires more of the anode segment from the leading edge to fully convert methane. Reforming kinetics reported by Ahmed and Föger [18
] and WGS reaction kinetics reported by Tingey [19
] were employed as the reaction rate details for PFR, leading to 1D pseudo-homogeneous results. The three different levels of activity were assigned by reducing the Arrhenius factor in the rate expression by the reduction factors 0.33 (~1/3rd) and 0.67 (~1/6th). In physical terms this signifies loss of reforming activity by poisoning or sintering. For these levels of activity, we assess the anode oxidation risk and carbon formation potential on the anode, both of which have severe life-limiting consequences on the anode [7
This paper contributes to the SOFC research considering its two novel contents including; (i) the novel simulation methodology: The simulation approach proposed for complicated internal reforming SOFC process offers simplicity and calculation speed without compromising the internal operations details. This is of particular interest for SOFC system modeling and design where several operational concepts including heat/mass transfer and electrochemical and fuel reformation reactions interactively occur at wide time and length scales. (ii) The understanding of distributed reformation potentials in controlling SOFC performance profiles. The incremental reformation process is demonstrated to be a promising strategy to moderate the undesirable gradients of SOFC internal profiles. This is promising to achieve higher homogeneity in temperature and concertation profiles inside the SOFC stack that subsequently offers enhanced current and voltage profiles. This is crucially important for SOFC efficiency and durability. In this paper, we demonstrate internal fuel reformation as an opportunity not only for heat integration and external reformer cost reduction but also for thermal management goals that eventually results in fuel cell longevity.
2. Simulation Methodology
The SOFC stack is simulated in the following way: Internal reforming of methane on the SOFC anode is modeled in a PFR module, using the reforming kinetics of Ahmed and Föger [18
] and the WGS reaction kinetics of Tingey [19
]. The electrochemical conversion of hydrogen and carbon monoxide are modeled as chemical conversions in a conversion reactor module. The associated electrical aspects including cell voltage, air utilization, and fuel utilization at a given operating current are calculated in a spreadsheet tool of Aspen HYSYS. The spreadsheet block in Aspen HYSYS is essentially an Excel-based spreadsheet with features of exporting data to and importing data from other modules of the flowsheet. This provides facility for post-processing of the calculated or entered values of the process variables. Computations of cell voltage entailing calculations of electrical losses, both ohmic and overpotential using empirical correlations [21
] and calculation of Nernst voltage from the concentrations of the reacted gas are carried out in the spreadsheet, with compositions imported from the PFR module. Similarly, the sum of electrical losses and entropy change, after allowing for losses to the surroundings from the stack, is calculated and this value is exported to the energy stream linked to the PFR, as heat available for direct internal reforming.
The average stack operating temperature is obtained by a trial-and-error method. An average stack temperature is assumed for calculating Nernst voltage, operating voltage, and electrical losses. The assumed temperature is then compared to the value returned by the PFR, which simulates internal reforming and utilizes heat generated from the fuel cell reaction to compute the temperature profile within the fuel cell and the composition profile of the internally reformed fuel. The amount of heat generated, i.e., the electrical and entropy losses are calculated at the assumed average temperature. These steps are iterated until agreement is reached between the assumed average cell temperature and the calculated average cell temperature.
The composition of utilized gas is obtained by matching the degree of conversion in the conversion reactor, with the fuel utilization level calculated in the spreadsheet, based on fuel flow rate and operating current. Since the electrochemical conversion of H2 and CO are modeled as combustion reactions, there is a temperature rise in the reactor due to the exothermicity of the combustion reactions. Since the exothermicity of the fuel cell reactions have already been accounted for by calculating the electrical losses and entropy change of the fuel cell reaction and entering this value as heat input to the internal reforming PFR, temperature rise in the conversion reactor is suppressed by use of the HYSYS object SET, to set the temperature of the stream leaving the conversion reactor to be the same as the temperature of the stream leaving the PFR.
and Table 1
show the integration of the modules representing the anode operation and the set of equations used in the calculation of all distributed variables within the stack—temperature, current density, and composition of all chemical species on the anode side.
The reactions taking place in the reaction modules, shown in Figure 1
, are as follows. Two parallel reactions take place in the internal reforming module (PFR) including MSR and WGS, presented by Equations (1) and (2), respectively:
Two parallel electrochemical reactions take place in the fuel cell reactor module (conversion reactor). The main reaction is hydrogen oxidation (Equation (3)) that may occur with the CO oxidation (Equation (4)), simultaneously.