Off-Gas Recycle Research Articles

Development of a fuel feeder for a solid oxide fuel cell test station

Solid oxide fuel cells (SOFC) can utilize various fuels, such as natural gas, hydrogen and biogas, but often, it is sensible to use a pre-reformer that converts the fuel into a hydrogen-rich gas stream. Relevant testing conditions, including the fuel to be used in SOFC systems, are important because cell performance depends on test conditions, such as fuel composition. Still, a majority of the reported single-cell and short stack tests are performed with pure hydrogen or synthetic reformate mixed from gas bottles. In this article, the development of a fuel feeder used to pre-reform natural gas for a single cell SOFC test station is presented. To mimic SOFC system conditions, natural gas is taken from the grid, desulfurized with commercial sulfur sorbent and reformed with a commercial precious metal catalyst. The fuel feeder is designed to be a versatile and efficient research tool, capable to be used in a wide temperature and gas flow range and with different reforming techniques, such as steam reforming, catalytic partial oxidation and simulated anode off-gas recycling. The construction, operation and characterization of the fuel feeder as well as methods of avoiding carbon formation are discussed. The performance is evaluated by comparing measured outlet temperatures and compositions against equilibrium values. All measured gas compositions matched closely with the calculated equilibrium values, and the identified deviations were small and to no harm in practical use. The operator can control the product gas composition by setting the fuel feeder heater to the temperature corresponding to the targeted composition. Results show that the fuel feeder design can be used as such for single-cell testing or scaled to fit larger stack test stations. Copyright © 2015 John Wiley & Sons, Ltd.

Analysis of Transient Behavior of Fuel Reformer to Fuel Supply Variation in Solid Oxide Fuel Cell Systems with Anode Off-Gas Recycle

Solid oxide fuel cells (SOFCs) are known for having high power generating efficiency among various types of fuel cells and being able to utilize various fuels such as fossil fuels. In the case of applying SOFC to vehicles, improvement of vehicle efficiency can be expected, using fossil fuels. However, in the case of using fossil fuels for fuel cells, fuels must be reformed on board using H2O, CO2, and so on. Therefore, in order to utilize SOFCs to vehicles, methods of supplying these gasses on board need to be considered. To solve this problem, one of the promising methods is the anode off-gas recycle (AGR). The AGR is a technology in which SOFC exhaust gas, which contains residual fuels, is partially mixed with supplied fuels into the system prior to entering a fuel reformer, and recirculates these gasses in the system. By introducing AGR to SOFC system, i.e., by utilizing H2O and CO2included in the SOFC exhaust,, fuel utilization of the system can be improved and the steam reforming of fuel become possible without mounting water tanks on vehicles. In addition, in the case of utilizing SOFC to vehicles, complicated responses to precipitous load variations must be considered, unlike stationary systems operated under moderate load variations. For example, in the case of increasing fuel supply in order to increase output power from the system, it could be possible that the output power is temporarily lowered because of decrease of temperature of the SOFC and reformer due to increase of heat requirement for fuel reforming. This behavior is due to transient variation of heat balance in the reformer. Therefore, for the analysis of transient response of SOFC systems, the response of reformer is very important and needs to be paid attention. Especially, further complicated behavior of reformers is expected in SOFC systems with AGR (AGR SOFC systems) because of off-gas recycle. For instance, in the case of increasing fuel supply to the system in order to increase the output power from the SOFC, possible delay of fuel reforming in the reformer may cause the decrease in the ratio of increase of recycled gas to that of fuel supply. In such a case, the amount of H2O and CO2in the recycled gas decreases compared to the fuel supply, and the carbon deposition occurs, depending on the ratio of increase of fuel supply. Hence, in this research, the transient behavior of the fuel reformer in AGR SOFC systems under various fuel supply rate is discussed by numerical simulation. Specifically, the effect of the change ratio of fuel supply and the delay of fuel reforming in the fuel reformer on system response is analyzed, by modeling composition of input gas to the fuel reformer. As a result of numerical simulation, the operating conditions in which the carbon deposition occurs are clarified. In addition, because ratio of increase of fuel supply needs to be limited in order to prevent carbon deposition, mitigation methods for this limitation are also discussed.

A Lumped Dynamic Modelling Approach for Model-Based Control and Diagnosis of Solid Oxide Fuel Cell System with Anode Off-Gas Recycling

The interest towards anode off-gas recycling (AOGR) in Solid Oxide Fuel Cell (SOFC) systems has recently increased, mainly due to the potential electrical efficiency improvement, as well as to the possibility of avoiding feeding external water for fuel reforming, when adopting such a configuration. However, advanced system control is necessary to ensure such relevant benefits are achieved. Proper control actions and system performance optimization, when AOGR is introduced, entail developing detailed thermodynamic system analysis, in such a way as to remove or at least limit any hazard associated to solid carbon formation, while suitably meeting thermal and electric requirements. To this end, an adequate SOFC stack model is necessary. This work focuses on the identification and validation of an SOFC stack model that accounts for AOGR inclusion. A lumped dynamic modelling approach is adopted to simulate temperature and gas composition at the stack anode outlet. The model is able to simulate the dynamic response of the stack during transients, as shown in Figure 1. Experimental data from a real SOFC system is used in model validation and the modeling approach adopted here ensures achieving a satisfactory compromise between estimation accuracy and computational burden. These factors support using the model as state estimator in model-based control and diagnosis algorithms. Figure 1 – Comparison between measured and simulated fuel gas temperature at anode outlet Figure 1

Numerical Simulation of SOFC System Performance at 90% Fuel Utilization with or without Anode Off-Gas Recycle for Enhancing Efficiency

To investigate possibility to enhance electrical efficiency, performance of methane-fueled 10 kW SOFC systems was numerically simulated with calculated voltage-current-density curves at 750ºC of a single cell with a bipolar plate, which approximated a stack performance. Operation at 90% fuel utilization (U f) with anode off-gas recycle (AGR) and with steam reforming (no recycle) led to 75-67%-LHV and 72-66%-LHV DC electrical efficiency at 0.05-0.40 A/cm2, respectively. System simulations considering heat loss from a cell stack clarified that 750ºC stack temperature can be maintained at 0.12-0.40 A/cm2 and 64-58% and 63-59% net AC electrical efficiency will be attainable at U f = 90% with and without AGR, respectively. Furthermore, enhancement of net AC efficiencies was found to be 11-10 percentage points at those current densities against ones of a Ene-farm-like conventional system using steam reforming at U f = 73%.

A Lumped Dynamic Modelling Approach for Model-Based Control and Diagnosis of Solid Oxide Fuel Cell System with Anode Off-Gas Recycling

This work focuses on the identification and validation of a Solid Oxide Fuel Cell (SOFC) stack model that accounts for anode off-gas recycling (AOGR) inclusion. A lumped dynamic modelling approach is adopted to simulate temperature and gas composition at the stack anode outlet. The model is able to simulate the dynamic response of the stack during transients. Experimental data from a real SOFC system are used in model validation and the modeling approach adopted here ensures achieving a satisfactory compromise between estimation accuracy and computational burden. These factors support using the model as state estimator in model-based control and diagnosis algorithms.

Analysis of Transient Behavior of Fuel Reformer to Fuel Supply Variation in Solid Oxide Fuel Cell Systems with Anode Off-Gas Recycle

In the case of increasing fuel supply to solid oxide fuel cell (SOFC) systems, mass transport delay generally occurs in reformers and SOFCs. Therefore, heat used for fuel reforming becomes larger than heat supplied to reformers, and reformer heat tends to be absorbed. In this paper, transient behavior of reformers in SOFC systems with anode off-gas recycle, in the case of increasing fuel supply, is analyzed by numerical simulation. As a result, it is suggested that reformer temperature decreases from several to several tens of K, when heat is absorbed from the reformer, which is small scale, lightweight and has low heat capacity, for several seconds. In addition, methane flow rate outputted from the reformer increases, and carbon deposition occurs, depending on conditions. This paper will present details of these results and discuss methods to prevent carbon deposition and increase of CH4 output from the reformer.

Numerical Simulation of SOFC System Performance at 90% Fuel Utilization with or without Anode Off-Gas Recycle for Enhancing Efficiency

To investigate possibility to enhance electrical efficiency, performance of methane-fueled 10 kW SOFC systems was numerically simulated with calculated voltage-current-density curves at 750ºC of a single cell with a bipolar plate, which approximated a stack performance. Operation at 90 % fuel utilization (U f) with anode off-gas recycle (AGR) and with steam reforming (no recycle) led to 75-67 %-LHV and 72-66 %-LHV DC electrical efficiency at 0.05-0.40 A/cm2, respectively. System simulations considering heat loss from a stack clarified that 750ºC stack temperature can be maintained at 0.12-0.40 A/cm2 and 64-59 % and 63-59 % net AC electrical efficiency will be attainable at U f = 90% with and without AGR, respectively. Furthermore, enhancement of net AC efficiencies was found to be 10 percentage points at those current densities against ones of a conventional system using steam reforming at U f = 73 %.

Diesel auto-thermal reforming for solid oxide fuel cell systems: Anode off-gas recycle simulation

Diesel auto-thermal reformation (ATR) with solid oxide fuel cell (SOFC) stack anode off-gas recycle (AOGR) has a reliable steam recycling supply to the reformer and improves overall system efficiency. For the lab-scale experiments, it is crucial to develop a cost-effective technique to simulate the AOGR effects on hydrocarbon catalytic reformation due to safety and cost considerations of providing the full recycle composition in the absence of fuel cell stack hardware. The present work combined thermodynamic modeling and experiments to compare diesel ATR performance with AOGR and with direct water/air inputs as recycle simulation (RS). Variations of input water and air flow were employed to simulate the effects of recycle gas on syngas production and to analyze the contribution of recycled CO2 dry reforming. A single-tube reformer with Rh/CeO2–ZrO2 catalyst was used for diesel ATR experiments with a photo-acoustic micro-soot meter to monitor carbon formation in the reformate effluent. Experimental results suggest water and air input flows are two key variables to simulate performance of diesel ATR with AOGR, whereas gas hourly space velocity and reforming temperature do not significantly affect the recycle simulation process in syngas production. The optimum AOGR ratio for an SOFC stack with 65% fuel utilization was identified as 45% for diesel ATR to achieve maximum syngas production and reforming efficiency with a given input air flow.

Development of Anode Off-Gas Recycle Blowers for High Efficiency SOFC Systems

Prototype of efficient anode-off gas recycle blowers was developed with a 3D turbo impeller and a DC brush-less built-in motor for high efficiency SOFCs. Performance test of the blowers clarified 26% and 52% total blower efficiency for the type SSR-70 for 10 kW-class SOFC and SSR-140 for 250 kW, respectively. System performance simulation of an SOFC system with the SSR-70 blower elucidated that 59% net AC electrical efficiency can be expected. When the blower efficiency is improved to 50% as of SSR-140, electrical efficiency will become 60%. We will continue developing more efficient recycle blowers.

Effect of Anode off‐gas Recycling on Reforming of Natural Gas for Solid Oxide Fuel Cell Systems

AbstractThe effect of anode off‐gas recycling (AOGR) on the characteristic performance of a natural gas reformer equipped with a precious metal catalyst is investigated experimentally. The reformer is operated both with synthetic AOGR gas and in steam reforming (SR) conditions. The characteristic performance in SR and AOGR mode are compared with equilibrium, and it is found that equilibrium is more readily achieved in AOGR mode. The reformer is used for extended periods of time (100–1,000 h) in conditions where carbon formation is thermodynamically possible to measure any changes in characteristic performance. No significant change in the performance is observed due to carbon formation or catalyst deactivation. The reformer could be successfully implemented in a 10 kW SOFC system with an anode off‐gas recycling loop.

Comparison of efficiencies of low, mean and high temperature fuel cell Systems

Fuel cell systems are always said to show high electrical efficiency. The results achieved up to now, however, differ considerably, especially between the various fuel cell types all using natural gas as fuel. With the presented study the reasons for the different results and general potentials for fuel cell systems are highlighted. For that purpose several system lay-out concepts were elaborated for PEFC, PAFC and SOFC. The performed energy balance calculations for eight different plant concepts (three PEFC, two PAFC and three SOFC) for steady state operation with methane revealed that because of external reforming PEFC and PAFC systems are limited to about 67% and 70%, respectively, for the fuel utilisation. High temperature fuel cells can achieve at least 80% because of the possibility of internal reforming, or even over 90% in case of anode off-gas recycling. In combination with a cell voltage which is about 100 mV lower than that of MCFC and planar SOFC, PEFC can only achieve 38% of electrical net efficiency, PAFC 42% and tubular SOFC 54%. The latter is similar to MCFC, which is operated at higher cell voltage but lower fuel utilisation. The highest efficiency with up to 63% can be achieved with planar SOFC systems, because this concept allows high fuel utilisation together with high cell voltages.

Design and optimization of a combined fuel reforming and solid oxide fuel cell system with anode off-gas recycling

An energy conversion and management concept for a combined system of a solid oxide fuel cell coupled with a fuel reforming device is developed and analyzed by a thermodynamic and electrochemical model. The model is verified by an experiment and then used to evaluate the overall system performance and to further suggest an optimal design strategy. The unique feature of the system is the inclusion of the anode off-gas recycle that eliminates the need of external water consumption for practical applications. The system performance is evaluated as a function of the steam to carbon ratio, fuel cell temperature, anode off gas recycle ratio and CO 2 adsorption percentage. For most of the operating conditions investigated, the system efficiency starts at around 70% and then monotonically decreases to the average of 50% at the peak power density before dropping down to zero at the limiting current density point. From an engineering application point of view, the proposed combined fuel reforming and SOFC system with a range of efficiency between 50% and 70% is considered very attractive. It is suggested that the optimal system is the one where the SOFC operates around 900 °C with S/ C ratio higher than 3, maximum CO 2 capture, and minimum AOG recirculation.

Numerical Simulation of Anode-Supported Disc-Type Single Cell at Anode Off-Gas Recycle

Numerical simulations were implemented to predict a disc-type single cell performance with gas seal at anode off-gas recycle from the previous experimental results without gas seal. It was clarified that 70% DC electrical efficiency with seal could be expected at 700ºC, 387 mA cm-2 (90% fuel and 45% air utilizations) and recycle ratio r = 0.67 by 17 mV enhancement in the cell voltage with gas sealing. The efficiency would be increased by 1 percent points compared to one without seal. Distribution of current density, anode gas composition, and electromotive force in the cell with seal was also elucidated. Furthermore, it was revealed that change in current density, H2 concentration, and electromotive force between the cell inlet and outlet could become smaller as the recycle ratio increases. It was also found that differences in recycle ratio (r = 0.56, 0.67, 0.74) could influence cell voltage only by ± 1%.

Numerical Simulation of Anode-Supported Disc-Type Single Cell at Anode Off-Gas Recycle

not Available.

Economic evaluation of off-gas recycle pressure swing adsorption (PSA) in industrial scale poly(3-hydroxybutyrate) fermentation

The use of high purity oxygen as a feeding gas in microbial fermentation has recent gained huge attention in industrial scale process due to its advantageous effect to increase viable cell density as well as target metabolite productivity. However, usage of highly pure oxygen is currently too expensive for industrialization. To overcome this limitation, off-gas recycle pressure swing adsorption (PSA) was employed which assist in continuous production of high purity oxygen during fermentation process. The off-gas from fermentation process contained about 70 ∼ 80% oxygen which was re-utilized as the input gas in PSA, whereas conventional PSA process utilized air with 21% oxygen content. This difference in the oxygen content of the input gas decreased the size of PSA, resulting in reduction of equipment costs. Using the characteristics of off-gas recycle PSA, we analyzed and compared the economic feasibility of a highly pure oxygen supplying strategy for the production of poly(3-hydroxybutyrate) (PHB) by Ralstonia eutropha based on previously reported data, using both air and highly pure oxygen. To achieve annual production of 3,630 tonnes of PHB the aforementioned method was adopted, by doing so the production cost of PHB was significantly reduced to 4.2 $/kg. While, the conventional process (utilizing air as the feeding gas) resulted in the highest cost (5.9 $/kg). The low cost of off-gas recycle PSA was due to reduction of fixed capital cost, which was about 3 times lower than that of the conventional fermentation process. Therefore, the production cost of fermentation was considerably decreased by using a highly pure oxygen supplying strategy based on off-gas recycle PSA.

ChemInform Abstract: Offgas Recycle for Increased Heat Production from Aerobic Thermophilic Treatment of Swine Waste: Pilot Studies and Full‐Scale Design

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Effect of temperature, hydraulic residence time and elevated [formula omitted] on acid neutralization within a pulsed limestone bed reactor

Limestone has potential for reducing reagent costs and sludge volume associated with treatment of acid mine drainage, but its use is restricted by slow dissolution rates and the deposition of Fe, Al and Mn-based hydrolysis products on reactive surfaces. We evaluated a pulsed limestone bed (PLB) reactor (15 L/min capacity) that uses a CO 2 pretreatment step to accelerate dissolution and hydraulic shearing forces provided by intermittent fluidization to abrade and carry away surface scales. We established the effects of hydraulic residence time (HRT, 5.1–15.9 min), temperature ( T, 12–22 °C) and CO 2 tension ( P CO 2 , 34.5–206.8 kPa) on effluent quality when inlet acidity (Acy) was fixed at 440 mg/L (pH=2.48) with H 2SO 4. The PLB reactor neutralized all H + acidity ( N=80) while concurrently providing unusually high levels of effluent alkalinity (247–1028 mg/L as CaCO 3) that allow for side-stream treatment with blending. Alkalinity (Alk) yields rose with increases in P CO 2 , HRT and settled bed height (BH, cm) and decreased with T following the relationship ( R 2=0.926; p<0.001): (Alk) non-filtered=−548.726+33.571·( P CO 2 ) 0.5+33.671·(HRT)+7.734·(BH)−5.197·( T). Numerical modeling showed CO 2 feed requirements for a target Alk yield decrease with increases in HRT, T and the efficiency of off-gas (CO 2) recycling.

Reduction of Combustion Irreversibility in a Gas Turbine Power Plant Through Off-Gas Recycling

Combustion in conventional fossil-fueled power plants is highly irreversible, resulting in poor overall energy conversion efficiency values (less than 40 percent in many cases). The objective of this paper is to discuss means by which this combustion irreversibility might be reduced in gas turbine power cycles, and the conversion efficiency thus improved upon. One such means is thermochemical recuperation of exhaust heat. The proposed cycle recycles part of the exhaust gases, then mixes them with fuel prior to injection into a reformer. The heat required for the endothermic reforming reactions is provided by the hot turbine exhaust gases. Assuming state-of-the-art technology, and making a number of simplifying assumptions, an overall efficiency of 65.4 percent was attained for the cycle, based on the lower heating value (LHV) of the methane fuel. The proposed cycle is compared to a Humid Air Turbine (HAT) cycle with similar features that achieves an overall efficiency of 64.0 percent. The gain in cycle efficiency that can be attributed to the improved fuel oxidation process is 1.4 percentage points. Compared to current high-efficiency gas turbine cycles, the high efficiency of both cycles studied therefore results mainly from the use of staged compression and expansion with intermediate cooling and reheating, respectively.