Anode Recycle Research Articles

Graphite Recycling from Spent Lithium-Ion Batteries.

The present work reports on challenges in utilization of spent lithium-ion batteries (LIBs)-an increasingly important aspect associated with a significantly rising demand for electric vehicles (EVs). In this context, the feasibility of anode recycling in combination with three different electrolyte extraction concepts is investigated. The first method is based on a thermal treatment of graphite without electrolyte recovery. The second method additionally utilizes a subcritical carbon-dioxide (subcritical CO2 )-assisted electrolyte extraction prior to thermal treatment. And the final investigated approach uses supercritical carbon dioxide (scCO2 ) as extractant, subsequently followed by the thermal treatment. It is demonstrated that the best performance of recycled graphite anodes can be achieved when electrolyte extraction is performed using subcritical CO2 . Comparative studies reveal that, in the best case, the electrochemical performance of recycled graphite exceeds the benchmark consisting of a newly synthesized graphite anode. As essential efforts towards electrolyte extraction and cathode recycling have been made in the past, the electrochemical behavior of recycled graphite, demonstrating the best performance, is investigated in combination with a recycled LiNi1/3 Co1/3 Mn1/3 O2 cathode.

触媒燃焼によるSOFCアノード再循環ガス中のメタン転化特性に関する研究

触媒燃焼によるSOFCアノード再循環ガス中のメタン転化特性に関する研究

Simulation of Start-up of Anode Gas Recycle SOFC System Using Catalytic Combustion Method

Solid oxide fuel cells (SOFCs) applied to stationary power generation systems have advantages such as the highest efficiency among all fuel cells and lower cost by the use of non-platinum catalysts. Furthermore, hybrid systems based on SOFCs are being actively researched to obtain even higher efficiencies by utilizing the high temperature exhaust gas from the SOFC. For an anode gas recycle (AGR) SOFCs, steam produced by the electrochemical reaction can be used in the steam reforming process to avoid carbon formation; therefore, there is no need for an external steam supply under a large recirculation ratio of anodic off-gas. In addition, the overall efficiency of an SOFC hybrid system, including bottoming engines, can be improved with such an approach. In our previous report, the experiments using a 1 kW tubular-type SOFC with an AGR system driven by a variable flow ejector were conducted, it was clarified that the AGR-SOFC with recirculation ratio of 0.62 was stable and generated about 2% more electricity compared to one-path SOFC systems under rated-load conditions. In this study, the cycle analyses were carried out for the AGR system using the catalytic combustion method, to remove methane efficiently in the anode gas at the start-up operations. In this systems, oxidation air is injected with main methane fuel, then methane can be converted by the catalytic combustion reaction in the combustion plenum located upstream the pre-reformer. Furthermore, steam produced by the reaction can be used in the steam reforming reaction at the pre-reformer and the fuel cell. To clarify the effects of operating conditions on the AGR system with catalytic combustion method, the chemical equilibrium calculations were conducted by using the experimental data on stack temperature and recirculation ratio determined using an actual SOFC stack. (1) Heat-up mode Cathode and anode gases were heated from ambient to high temperature by an electric heater to heat up the AGR-SOFC system. Methane fuel and oxidation air is injected into the anodic loop, thus methane is converted at the combustion plenum. It is assumed that catalytic combustion reaction fully processed in the combustion plenum heated by electric heater. Three operating thresholds were defined as a stable AGR-SOFC system operation as follows: (i) steam-to-carbon ratio S/C defined as the mole ratio of the H2O to CH4 and CO feed rate is larger than 2.0 to prevent carbon deposition; (ii) anode gas at the combustion plenum exit is not extremely high temperature; and (iii) oxygen is fully removed by the combustion process to maintain the reductive gas atmosphere. For recirculation ratio is larger than 0.7, the results shows enough steam product by the catalytic combustion reaction at air-to-methane ratio is about 7-9. In addition, excessive temperature rise of the anode gas can be prevented at high recirculation ratio greater than 0.7 for recycled gas temperature is 873 K, the required recirculation ratio strongly depends on the recycled gas temperature. An ejector drive system can easily function at the extremely high temperature, such as that of the anodic off-gas, because there are no rotating parts. Recirculation ratio 0.7 is the proven value in actual SOFC experiments under only rated-load condition, thus it is necessary to develop a high-efficiency binary fluid twin-fluid nozzle type ejector for the start-up operations. (2) Load-up mode CH4 fuel was increased gradually with an increase in current for the load. Finally, only CH4 is fed for a specified full load period. Fuel utilization is defined based on methane flow rate derived from feed rate by loss of consumption by catalytic combustion. The results shows that S/C decreases with increasing fuel utilization due to decrease in combustion reaction rate. In addition, the reduction of recirculation ratio cause a decrease of S/C and an increase of gas temperature at the exit of the combustion plenum. Consequently, it is important to keep up a sufficiently high recirculation ratio to maintain high S/C and low gas temperature at the exit of the combustion plenum. (3) Rated-load power-generating mode Only CH4 is fed in anode recycle system. AGR-SOFC showed stable output power with no external steam supply, because recycle enough steam via the electrical reaction can be used in steam reforming of methane fuel. It is important to maintain a sufficiently high recirculation ratio to avoid carbon deposition. The threshold of the recirculation ratio is dependent on fuel utilization; therefore, the AGR system requires a recirculation ratio greater than ca. 0.6.

Process simulation of biomass gasification integrated with a solid oxide fuel cell stack

Biomass gasification-solid oxide fuel cell (BG-SOFC) combined heat and power (CHP) systems are of major interest in the context of climate change mitigation, energy security and increasing energy efficiency. Aspen Plus is employed to simulate various BG-SOFC CHP systems. The aim of the research work is to investigate the technical feasibility of these systems and to study the influence of important operating parameters and examine integration options. Systems based on dual fluidised bed steam gasification and tubular SOFC technologies are modelled. The cathode recycle and electric heater integration options are not attractive in comparison to the base case anode recycle system. Thermal integration, i.e. using SOFC flue gas as gasifier oxidant, is desirable. Lowering the syngas preheat temperature (prior to SOFC anodes) is highly recommended and is more practical than lowering the cathode air preheat temperature. Results of the parametric study indicate that: steam to carbon ratio and biomass moisture content should be as low as possible; fuel utilisation factor can change the mode of operation of the plant (focus on electricity or heat); high temperature syngas cleaning is very attractive; gasification air preheating is more attractive than gasification steam superheating. High efficiencies are predicted, proving the technical feasibility of BG-SOFC CHP systems.

Analysis of solid oxide fuel cell system concepts with anode recycling

The main drivers for anode recirculation are the increased fuel efficiency and the independence of the external water supply for the fuel pre-reforming process. Different system flow-schemes have been defined and a set of parameters have been elaborated as basis for various calculations. Taking into account the combinations of layout, cell type, fuel utilization, fuel, and recycling ratio the total number of cases modeled was about 220. All calculated SOFC systems are on a high level of electrical net efficiency in the range of 50–66%. The electrical and thermal efficiencies are mainly influenced by the fuel utilization. The layout itself, the choice of fuel gas or the type of cell have minor effects on the system efficiency.

Modeling of a 5 kWe tubular solid oxide fuel cell based system operating on desulfurized JP-8 fuel for auxiliary and mobile power applications

An onboard autothermal reformer (ATR) integrated with a SOFC stack offers potential for high energy efficiency and utilization, low emission and quiet operation avoiding cost associated with hydrogen storage and infrastructure. Such a system can be a viable and attractive option especially for military's need for quiet and less pollutant Auxiliary Power Unit (APU) and Mobile Electric Power (MEP) units in temporary and permanent base camps [1,2]. A 5 kWe Solid Oxide Fuel Cell (SOFC) system operating on desulfurized JP-8 fuel was modeled using Aspen Plus process simulation software to examine the effects of oxygen to carbon ratio (O2/C) and steam to carbon ratio (H2O/C) at different ATR operating temperatures (700–850 °C), while keeping the SOFC stack temperature constant at 910 °C. Anode recycle steam and heat have been used to reform the desulfurized JP-8 fuel which would make the system lighter and compact for mobile application. The system modeling revealed a maximum net AC efficiency of 39.5% at 700 °C and a minimum of 32.6% at 850 °C ATR operating temperatures, respectively. Sensitivity analysis with respect to fuel utilization factor (Uf) and current density (j) were also conducted to identify the optimum operating window.

Measurement and analysis of carbon formation during diesel reforming for solid oxide fuel cells

Experiments and equilibrium analysis were conducted to study carbon formation during diesel reforming for a solid oxide fuel cell-based auxiliary power unit (APU) application. A photo-acoustic instrument provided direct measurements of solid carbon concentration in the reformer effluent stream, which could be correlated to reformate gas composition (as determined via mass spectrometer) and reformer temperature. These measurements were complimented by equilibrium calculations based upon minimization of total Gibbs free energy. It was determined that oxygen-to-carbon ratio (O/C), fuel utilization fraction and anode recycle fraction all influence the degree of carbon formation, and that once significant carbon concentration is measured, the reformer performance begins to show marked degradation. At a fixed operating point, lowering the reformer temperature produced by far the largest change in effluent carbon concentration. Systematic variation in O/C, fuel utilization and anode recycle revealed the interdependence among reformer temperature, effluent gas composition and carbon concentration, with a strong correlation between carbon and ethylene concentrations observed for [C2H4] > 0.8%. After each experiment, baseline reformer performance could be recovered by operation under methane partial oxidation (POx) conditions, indicating that reformer degradation results at least in part from carbon deposition on the reformer catalyst.

Highly Efficient IGFC Hybrid Power Systems Employing Bottoming Organic Rankine Cycles With Optional Carbon Capture

This study examines the performance of a solid oxide fuel cell- (SOFC-) based integrated gasification power plant concept at the utility scale (>100 MW). The primary system concept evaluated was a pressurized ∼150 MW SOFC hybrid power system integrated with an entrained-flow, dry-fed, oxygen-blown, slagging coal gasifier and a combined cycle in the form of a gas turbine and an organic Rankine cycle (ORC) power generator. The analyzed concepts include carbon capture via oxy-combustion followed by water knockout and gas compression to pipeline-ready CO2 sequestration conditions. The results of the study indicate that hybrid SOFC systems could achieve electric efficiencies approaching 66% [lower heating value (LHV)] when operating fueled by coal-derived clean syngas and without carbon dioxide capture. The system concept integrates SOFCs with the low-pressure turbine spool of a 50 MW Pratt & Whitney FT8-3 TwinPak gas turbine set and a scaled-up, water-cooled 20 MW version of the Pratt & Whitney (P&W) PureCycle ORC product line (approximately 260 kW). It was also found that a system efficiency performance of about 48% (LHV) is obtained when the system includes entrained-flow gasifier and carbon capture using oxygen combustion. In order to integrate the P&W FT8 into the SOFC system, the high-pressure turbine spool is removed which substantially lowers the FT8 capital cost and increases the expected life of the gas turbine engine. The impact of integrating an ORC bottoming cycle was found to be significant and can add as much as 8 percentage points of efficiency to the system. For sake of comparison, the performance of a higher temperature P&W ORC power system was also investigated. Use of a steam power cycle, in lieu of an ORC, could increase net plant efficiency by another 4%, however, operating costs are potentially much lower with ORCs than steam power cycles. Additionally, the use of cathode gas recycle is strongly relevant to efficiency performance when integrating with bottoming cycles. A parameter sensitivity analysis of the system revealed that SOFC power density is strongly influenced by design cell voltage, fuel utilization, and amount of anode recycle. To maximize the power output of the modified FT8, SOFC fuel utilization should be lower than 70%. Cathode side design parameters, such as pressure drop and temperature rise were observed to only mildly affect efficiency and power density.

Demonstration of Principle of Electrical Power Generation at High Fuel Utilization Efficiency by an SOFC System with an Anode Recycle Line

We show electric power generation of 380 W with fuel utilization of 90% using an anode recycle system, methane as fuel, and air as oxidant at a voltage of 7.5 V, which was approximately the same as that obtained with the anode recycling with fuel utilization of 80%.

Modeling and Performance Analysis of the Rolls-Royce Fuel Cell Systems Limited: 1 MW Plant

This paper is focused on the performance of the 1 MW plant designed and developed by Rolls-Royce Fuel Cell Systems Limited. The system consists of a two stage turbogenerator coupled with pressure vessels containing the fuel cell stack, internal reformer, cathode ejector, anode ejector, and off-gas burner. While the overall scheme is relatively simple, due to the limited number of components, the interaction between the components is complex and the system behavior is determined by many parameters. In particular, two important subsystems such as the cathode and the anode recycle loops must be carefully analyzed also considering their interaction with and influence on the turbogenerator performance. The system performance model represents the whole, and each physical component is modeled in detail as a subsystem. The component models have been validated or are under verification. The model provides all the operating parameters in each characteristic point of the plant and a complete distribution of thermodynamics and chemical parameters inside the solid oxide fuel cell (SOFC) stack and reformer. In order to characterize the system behavior, its operating envelope has been calculated taking into account the effect of ambient temperature and pressure, as described in the paper. Given the complexity of the system, various constraints have to be considered in order to obtain a safe operating condition not only for the system as a whole but also for each of its parts. In particular each point calculated has to comply with several constraints such as stack temperature distribution, maximum and minimum temperatures, and high and low pressure spool maximum rotational speeds. The model developed and the results presented in the paper provide important information for the definition of an appropriate control strategy and a first step in the development of a robust and optimized control system.

A techno-economic comparison of fuel processors utilizing diesel for solid oxide fuel cell auxiliary power units

Ultra-low sulphur diesel (ULSD) is the preferred fuel for mobile auxiliary power units (APU). The commercial available technologies in the kW-range are combustion engine based gensets, achieving system efficiencies about 20%. Solid oxide fuel cells (SOFC) promise improvements with respect to efficiency and emission, particularly for the low power range. Fuel processing methods i.e., catalytic partial oxidation, autothermal reforming and steam reforming have been demonstrated to operate on diesel with various sulphur contents. The choice of fuel processing method strongly affects the SOFC's system efficiency and power density. This paper investigates the impact of fuel processing methods on the economical potential in SOFC APUs, taking variable and capital cost into account. Autonomous concepts without any external water supply are compared with anode recycle configurations. The cost of electricity is very sensitive on the choice of the O/C ratio and the temperature conditions of the fuel processor. A sensitivity analysis is applied to identify the most cost effective concept for different economic boundary conditions. The favourite concepts are discussed with respect to technical challenges and requirements operating in the presence of sulphur.

Thermo-economic modeling of a solid oxide fuel cell/gas turbine power plant with semi-direct coupling and anode recycling

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency in order to improve system efficiencies and economics. The SOFC system is semi-directly coupled to the gas turbine power plant, with careful attention paid to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 21.6 MW at 49.2% efficiency. The model also predicts a breakeven per-unit energy cost of USD 4.70 ¢/kWh for the hybrid system based on futuristic mass generation SOFC costs. Results show that SOFCs can be semi-directly integrated into existing GT power systems to improve their thermodynamic and economic performance.

High Resolution Neutron Radiography Imaging of Operating PEM Fuel Cells: Effect of Flow Configuration and Gravity on Water Distribution

High resolution neutron radiography was used to image polymer electrolyte membrane fuel cells (PEMFCs) during operation under a variety of conditions. The effect of specific operating conditions, including flow direction (co-flow or counter-flow), orientation of the PEMFC with respect to gravity, and simulated anode recycle loop, were examined. Counter-flow operation was found to result in higher water content than co-flow operation, with more uniformly distributed water. Orientation with respect to gravity was also significant, with net water transport observed from the higher side of the cell to the lower side. The higher anode flow rate resulting from the simulated recycle loop facilitated convective removal of liquid water from the anode flow field, leading to lower overall levels of water in the PEMFC.

Effects of fuel cell anode recycle on catalytic fuel reforming

The presence of steam in the reactant gas of a catalytic fuel reformer decreases the formation of carbon, minimizing catalyst deactivation. However, the operation of the reformer without supplemental water reduces the size, weight, cost, and overall complexity of the system. The work presented here examines experimentally two options for adding steam to the reformer inlet: (I) recycle of a simulated fuel cell anode exit gas (comprised of mainly CO 2, H 2O, and N 2 and some H 2 and CO) and (II) recycle of the reformate from the reformer exit back to the reformer inlet (mainly comprised of H 2, CO, and N 2 and some H 2O and CO 2). As expected, anode gas recycle reduced the carbon formation and increased the hydrogen concentration in the reformate. However, reformer recycle was not as effective due principally to the lower water content in the reformate compared to the anode gas. In fact, reformate recycle showed slightly increased carbon formation compared to no recycle. In an attempt to understand the effects of individual gases in these recycle streams (H 2, CO, CO 2, N 2, and H 2O), individual gas species were independently introduced to the reformer feed.

Analysis of Integrated Coal Gasifier/Molten Carbonate Fuel Cell System. Reflection of Practical Generating Performance on Overall Power Plant.

Materials and heat balance in Integrated Coal Gasification/Molten Carbonate Fuel Cell (IG/MCFC) combined system have been calculated. In the calculation, an anode recycling system and an anode heat exchange system have been studied. The condition that non-deposition of carbon in anode inlet gas and that over 1% hydrogen composition in anode outlet gas have been supposed. In the case that the value of (H2·CO2)/(CO·H2O) ratio decreased to be 0.3 times than equilibrium value, the maximum fuel utilisation of the anode recycling system evaluated to be around 80%, while the maximum fuel utilisation of the anode heat exchange system estimated to be around 90%. From the results, fuel utilisation of anode heat exchange system could be set higher than that of anode gas recycling system. The analysis code has been applied to study a relation between partial pressure of CO2 in cathode gas and thermal efficiency of whole system. By changing the cooling value of cathode gas recycling line, the partial pressure of CO2 in cathode gas could be controlled. For the purpose to keep the CO2 partial pressure of cathode gas lower than 0.1 MPa, the cooling ratio of cathode recycling line should be decreased. By decrease of cooling ratio of cathode recycling line, the difference of thermal efficiency between two discussed systems has been decreased to be 1-2%.

Stack networking for system optimisation: an engineering approach

Direct internal reforming molten carbonate fuel cell (MCFC) systems are generally more efficient and more simple than equivalent external reforming systems. It is demonstrated that even better performance and system simplicity can be achieved by connecting the stacks in networks with the cathode flows and/or anode flows in series. The advantages and disadvantages of such networks are presented. Within the EC's Joule III programme, stack networking has been explored for direct internal reforming molten carbonate fuel cell (DIR-MCFC) systems. This study resulted in the selection of a system which employed anode recycle with the anodes connected in parallel and the cathodes connected in series. It achieved high performance, yet required few system components. In the project `Advanced DIR-MCFC development', led by BCN (P. Kortbeek and R. Ottervanger, J. Power Sources, 71 (1–2) (1998) 223), this system is being assessed with regard to cost, operating window, and controllability. The approach is to design the system to conform to the requirements of ECN's stacks. At the same time the MCFC stack development at ECN (G. Kraaij et al., J. Power Sources, 71 (1–2) (1998)) is geared towards fulfilment of the requirements in such a system with respect to performance, pressure management and lifetime, so the development of the system and the stack have an iterative character. A novel, patented, stack design with two anode outlet streams has been proposed by BG. It can be used in the chosen system and gives increased power output.

A novel configuration for direct internal reforming stacks

This paper presents a stack concept that can be applied to both molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) internal reforming stacks. It employs anode recycle and allows the design of very simple system configurations, while giving enhanced efficiencies and high specific power densities. The recycle of anode exit gas to the anode inlet has previously been proposed as a means of preventing carbon deposition in direct internal reforming (DIR) stacks. When applied to a normal stack this reduces the Nernst voltages because the recycle stream is relatively depleted in hydrogen. In the concept proposed here, known as the `Smarter' stack, there are two anode exit streams, one of which is depleted, while the other is relatively undepleted. The depleted stream passes directly to the burner, and the undepleted stream is recycled to the stack inlet. By this means high Nernst voltages are achieved in the stack. The concept has been simulated and assessed for parallel-flow and cross-flow MCFC and SOFC stacks and graphs are presented showing temperature distributions. The `Smarter' stacks employ a high recycle rate resulting in a reduced natural gas concentration at the stack inlet, and this reduces or eliminates the unfavourable temperature dip. Catalyst grading can further improve the temperature distribution. The concept allows simple system configurations in which the need for fuel pre-heat is eliminated. Efficiencies are up to 10 percentage points higher than for conventional stacks with the same cell area and maximum stack temperature. The concept presented here was devised in a project part-funded by the EU, and has been adopted by the European Advanced DIR–MCFC development programme led by BCN.

The effect of fuel cell operating conditions on DIR-MCFC system selection

The use of natural gas and ‘direct internal reforming’ allows very simple molten carbonate fuel cell systems to be devised. The compositions and flow rates of the anode and cathode stack inlet and outlet gas flows can be described by five dimensionless system variables. These variables describe the total air flow, the cathode recycle, the overall fuel utilisation, the anode recycle, and the amount of steam addition. The equilibrium stability limits of carbon are presented and related to the system variables. A simplified stack model has been developed and a parametric analysis has been performed to determine the effect of the system variables on the stack performance. Two atmospheric pressure systems are presented which are suitable for combined heat and power applications of less than 1 MWe.