Off-Gas Recycle Research Articles

Tuning selectivity in low-temperature Fischer-Tropsch synthesis by applying gas recycle mode

While hydrocarbons production by Fischer-Tropsch synthesis (FTS) is widely recognized, commercial interest in FTS products is mainly constrained by oil prices. The FTS-derived fuels, although higher quality and greener, cannot currently compete with cheaper fossil fuels, motivating to shift the FTS products distribution to higher value-added chemicals, such as olefins-C5+. The high olefin selectivity in FTS over Co-based catalysts is usually ascribed to Co2C on the catalyst surface with its inherent deficiencies. This study demonstrates a novel approach to alter low-temperature FTS selectivity by applying off-gas recycling. At slow gas recycling rates, C19+ selectivity increases due to re-adsorption and re-incorporation into the growing chain of gaseous olefins from the off-gas. High-intensity gas recycling facilitates C5+ olefin vapors removal from the reactor by enhanced gas flow with their subsequent cut-off in the cold trap, thus reducing residence time of condensed olefins and probability of their secondary reactions.

System model and performance analysis of a solid oxide fuel cell system self-humidified with anode off-gas recycling

Recycling the anode off-gas of the solid oxide fuel cells can effectively improve the system fuel utilization (Uf,sys) under a low stack fuel utilization (Uf,1), prolonging the service life and reducing the power generation cost. A compact self-humidified anode off-gas recycle (SAOR) system fueled by natural gas is proposed and analyzed. Clear mathematical relationships for key physical quantities in all system components are derived. Analytical expressions concerning the controllable parameters, Uf,sys and the recycling ratio (λ), and the controlling targets, Uf,1 and the oxygen to carbon ratio, are presented to facilitate the accurate system control for high efficiency and moderate Uf,1 and free of carbon deposition. The core system model is found to agree very well with multiple sets of available experimental data. The SAOR system performance for representative fuel flow rates and Uf,1 and for all λ are presented. The results show the SAOR system can achieve a DC efficiency similar to, but with Uf,1 10% lower than the best known non-SAOR system, demonstrating the presumed superiority of SAOR system. The SAOR system is a promising low cost design capable of achieving high efficiency and durability and should be pursuit for commercial development.

Conception and thermo-economic performance investigation of a novel solid oxide fuel cell/ gas turbine/Kalina cycle cascade system using ammonia-water as fuel

In order to develop a zero-carbon, high-efficiency and low-cost energy system, the conception of an innovative ammonia-water fueled solid oxide fuel cell (SOFC) - gas turbine (GT) - Kalina cycle (KC) cascade power system is proposed. The performance superiority of novel cascade system is verified by comparing with SOFC system using anode off-gas recycle (AOR). The influence of system key operation parameters including fuel ammonia mass fraction on novel system total energy efficiency and levelized cost of electricity (LCOE) is revealed. The rapid prediction and multi-objective optimization of novel system thermo-economic performance are conducted based on artificial neural network - multi-objective Harris hawks optimizer (ANN-MHHO). The results show that the total energy efficiency and LCOE of novel ammonia-water fed SOFC-GT-KC system are 12.92% higher and 8.91% lower than the SOFC system using AOR under preliminary condition, respectively. Parametric analysis reveals that fuel ammonia mass fraction presents a more significant effect on novel system thermo-economic performance than cell stack fuel utilization ratio. A high cell stack fuel utilization ratio of more than 0.64 is not conducive to improving novel system economic performance. The novel system LCOE reaches the minimum of 0.13807 $/kWh at SOFC operation pressure of 9.5 bar. The multi-objective optimization results demonstrate that the novel system total energy efficiency is increased by 1.14% and LCOE is reduced by 1.23% under optimal condition compared with preliminary condition. The novel system LCOE is more sensitive to ammonia price than interest rate.

Comparison and analysis of heat exchange and off-gas recycle strategies in tri-reforming-SOFC system

Integrating tri-reforming into SOFC system can convert fuels flexibly and reduce carbon emissions. The tri-reformer-SOFC system requires heat management to reduce energy consumption and to average temperature profiles. In this paper, four system configurations were built with two gas recirculation strategies (anode off gas recycling and burner off-gas recycling) and two reformer types (with heat exchange and oxygen injections). 1-D spatial distributions in the reformer/SOFC and the effect of recirculation ratio (RR) are explored. It is found that the two recirculation strategies can enhance the system electric efficiency due to the recycled fuel and heat, and the two reformers can both average the temperature profiles. Among the four configurations, configuration A exhibits the best performance, and higher RR increases the system electric efficiency, decreases the max temperature gradient in the SOFC and decreases the system global efficiency slightly. This work proposes novel strategies for SOFC system and provides important data for practice.

Performance analysis, techno-economic and life cycle assessment of Jatropha curcas L. (Euphorbiaceae) seedcake gasification and Fischer-Tropsch integrated process for bio-methanol production

The overall economic performance of biodiesel production can be improved by reducing the cost of methanol required for the transesterification process. Gasification integrated with Fischer-Tropsch synthesis provides an alternative route to biomass conversion. In this study, a thermodynamic model of steam gasification of Jatropha curcas L. (Euphorbiaceae) seedcake and Fischer-Tropsch synthesis of the syngas is developed to forecast the production of bio-methanol using Aspen Plus. The combined effects of the gasification temperature, steam flow rate, and off-gas recycling percentage were investigated using a response surface methodology to pinpoint the optimal operating conditions. The results showed that a gasification temperature of 820 °C, steam flow rate of 740 kg/h and off-gas recycle fraction of 0.6 were the optimum operating conditions for producing the highest amount of bio-methanol (53.13 wt.%). An initial capital investment of 7.4 million dollars and a minimum production price of bio-methanol of $0.91/L was determined. Integrating the bio-methanol production process in biodiesel plants reduces the cost of producing biodiesel by 26.36%. The environmental impact analysis showed that the process had an overall effect of −10 potential environmental impacts/kg of bio-methanol generated. Utilizing optimized process parameters may improve the process’ competitiveness on a commercial scale and improve sustainability in the biorefinery process.

Development of a Micro-Bubble System for Ozone Off-Gas Recycling in the Ozone Treatment Process

Development of a Micro-Bubble System for Ozone Off-Gas Recycling in the Ozone Treatment Process

Vacuum pressure swing adsorption for efficient off-gas recycling: Techno-economic and CO2 abatement study

We suggest a vacuum pressure swing adsorption that uses CuCl/Boehmite adsorbent as a novel material to efficiently recycle a blast furnace gas into the blast furnace and discuss the techno-economic and CO2 abatement impact. To identify the crucial factors of the separation, five cases are simulated: three adsorption pressures at moderate CO purity, and three purity levels at moderate adsorption pressure. Also, the coke-replacement effects are estimated through the blast furnace simulation to consider the economic benefits. The result shows that the energy efficiency of the separation process varied 72–82%, and injection of highly purified off-gas (99% purity) was the most economically profitable, giving 86.7 US-MM$ of net present value. Cost sensitivity showed that the coke price is the most influential, but the adsorbent cost and carbon taxes have relatively little effect. In the best case, the coke-replacement ratio is 0.26kgCoke mgas−3. The suggested process reduces net emission by 0.19tCO2-eq tHM−1, and this corresponds to the 10% of net reduction which is the competitive strategy compared with the renewable hydrogen blast furnace. This study broadened the understanding of the separation process for off-gas recycling, and optimization of the process should be studied further.

Optimization of Anode Off-Gas Recycle Ratio for a Natural Gas-Fueled 1 kW SOFC CHP System

Anode off-gas recycle is an effective method to improve fuel utilization (Uf) and efficiency of solid oxide fuel cell (SOFC) systems. One of the key issues is to determine the optimum recycle ratio of anode off-gas, which mainly contains H2O, CO2 and unused H2, CO. In this study, a process model of a 1 kW SOFC combined heat and power (CHP) system using natural gas as fuel was established, and the effect of anode off-gas recycle ratio on the operation of the system was studied. The recycle ratio of anode off-gas used ranged from 0 to 0.5. Based on the results, the recycle ratio around 0.3 is recommended. In this case, the net electrical efficiency of the system is 54.4%, the thermal efficiency is 28.8%, the total efficiency is 83.2% (LHV), and the actual Uf is 77.1%.

Economic assessment of membrane-based power-to-gas processes for the European biogas market

Four different power-to-gas process chains implementing catalytic methanation and membrane upgrading are presented. Their economics for synthetic natural gas production and hydrogen storage are assessed for the first time in the context of multiple different European gas grid standards by simulation. Influence of two different fermentation setups for biogas production as well as pressure levels from 6 to 15 bar and catalyst specific flow rates from 100 to 150 Nl/(h gCAT) were considered. Based on this simulation an economic analysis was done. Capital expenditures and operational expenditures for electrolysis, methanation, membrane gas upgrading, and compressors of each chain are presented.It is shown that the integration of a simple recycle of the off-gas stream can reduce the costs from up to 2.93 €/Nm³ produced synthetic natural gas to at least 1.63 €/Nm³. Hydrogen storage cost can be decreased from up to 1.81 €/Nm³ stored hydrogen to at least 0.95 €/Nm³. Variation of the fermentation setup only leads to savings in the range of 0.03–0.08 €/Nm³ synthetic natural gas if off-gas recycling is included. Catalyst specific gas flow rate is shown to be of neglectable influence, contributing less than 0.01 €/Nm³ produced synthetic natural gas or stored hydrogen. Pressure influence is more pronounced at up to 0.10 €/Nm³ produced synthetic natural gas and up to 0.05 €/Nm³ stored hydrogen. Optimal operation pressure for simple off-gas recycling is shown to be below 10 bar.

Control System Based on Anode Offgas Recycle for Solid Oxide Fuel Cell System

The conflicting operation objectives between rapid load following and the fuel depletion avoidance as well as the strong interactions between the thermal and electrical parameters make the SOFC system difficult to control. This study focuses on the design of the decoupling control for the thermal and electrical characteristics of the SOFC system through anode offgas recycling (AOR). The decoupling control system can independently manipulate the thermal and electrical parameters, which interact with one another in most cases, such as stack temperatures, burner temperature, system current, and system power. Under the decoupling control scheme, the AOR is taken as a manipulation variable. The burner controller maintains the burner temperature without being affected by abrupt power change. The stack temperature controller properly coordinates with the burner temperature controller to independently modulate the stack thermal parameters. For the electrical problems, the decoupling control scheme shows its superiority over the conventional controller in alleviating rapid load following and fuel depletion avoidance. System-level simulation under a power-changing case is performed to validate the control freedom between the thermal and electrical characteristics as well as the stability, efficiency, and robustness of the novel system control scheme.

Industrial Production of Formaldehyde Using Polycrystalline Silver Catalyst

We report the first comprehensive review of the industrial process for the catalytic partial oxidation of methanol to formaldehyde using silver catalyst. Formaldehyde remains a key, large volume industrial chemical which is ubiquitous to household, commercial, aviation, pharmaceutical and automotive products due to its high reactivity and versatility. Critical analysis of the various technological approaches used to manufacture formaldehyde, characteristics of silver catalysts employed, process conditions, and performance values has been presented. The three common plant types are almost complete conversion (BASF type); incomplete conversion followed by distillation (ICI type); and the off-gas recycle process which dominates world production due to its extensive use in China. Polycrystalline silver remains the most popular catalyst, with only minimal use of alternate supported catalysts or gauze materials. Catalyst physical properties such as bulk density, purity, mechanical strength, size fraction and sh...

Analysis of Transient Response of Solid Oxide Fuel Cells with Anode Off-Gas Recycle

An anode off-gas recycle (AGR) technology is currently focused as a way for further improvement of efficiency of solid oxide fuel cells (SOFC) which have higher performance compared to other types of fuel cells. AGR is a technology that recycles a part of the anode off-gas containing electrochemically unreacted fuel that comes out from the SOFC anode and reutilizes it for power generation. Because the unreacted fuel component in the recycled gas is added to the input fuel to the system, the fuel flow in the cell becomes more than that for a conventional SOFC without AGR, even though the fuel supply to the system is equivalent. Therefore, fuel utilization can be easily increased and improvement of electrical efficiency of the system is expected. Another advantage is that the system, in principle, does not require external water supply due to reutilization of steam in the recycled gas. However, in the SOFC system in which AGR is introduced (AGR-SOFC), the changes in the current, the flow rate and the composition of the gasses supplied to the cell lead to the changes in the flow rate and the composition of the recycled gas. The effect of changes in the recycled gas is then feedbacked to the system inlet. Because a delay in response of a blower which recycles the anode off-gas (recycle blower) is added in this situation, the transient response performance of the system may become more complex and worse than conventional systems. Thus, in this study, we constructed an AGR-SOFC model for transient response analysis under power increase situations and analyzed the transient response of the system and dependency of the response on a degree of power increase by numerical simulation. The AGR-SOFC model is composed of a reformer, an SOFC, and a recycle blower assuming the plug flow. Transient behavior of the system was analyzed where the increase of output power in accordance with the step wise increases in required current and fuel supply was assumed. Because the transport of gasses in the SOFC systems is relatively slow, increase in the current and fuel supply without any limit leads to fuel starvation due to shortage of the fuel in the cell and carbon deposition due to shortage of the steam for fuel reforming. Due to these effects, the system may be damaged. Hence the operating conditions for avoiding these causes of damage were investigated. In this model the current and the fuel supply are chosen in accordance with the investigation. As a result of the analysis, the transient response of the AGR-SOFC is classified into three types according to the way of increase in the current and the fuel supply and characteristics of each type of responses are clarified. In addition, it is revealed that AGR-SOFC can afford more fuel and steam to the reformer and cell and can increase both the current and the fuel supply instantaneously compared to conventional non AGR SOFC systems under identical fuel utilization conditions. Therefore, superior transient response over wide range of power increase is clarified to be expected in AGR-SOFC, even though there had been a concern that transient response performance might become worse due to the complex recycling system.

Modeling of Methanol-Fueled SOFC with Anode Offgas Recycling for Simplified System Design

Objective :Methanol is a fuel that can be synthesized from either fossil fuel or biomass. Use of methanol as a fuel in SOFCs offers several advantages. A major advantage lies in its ease of storage, handling, distribution, and input, being found in liquid form under ambient conditions. Compared to most other fuels, methanol is rather easily produced, and is known to have a lesser tendency for carbon deposition when vaporized and heated at high temperature.In this work, we conducted modeling and simulations of a methanol-fed SOFC system with anode gas recycling. Anode gas recycling provides water vapor in the fuel inlet, thus decreasing the tendency of the feed gas to form carbon deposits in the fuel cell stacks. This strategy also helps reduce thermal stresses in the stacks that would arise if the reforming was attempted in-situ with 100% water vapor originating from outside the SOFC system. The recycling also helps increase the fuel utilization. A thermodynamic analysis was conducted to calculate the energy generated, absorbed, or exchanged in the various blocks of the system, the expected system energy output, as well as conditions under which carbon deposition is prevented. Results:The model is based on an integrated energy process with anode gas recycling, a rated output electrical power of 100 kW, and an electrical efficiency of 50% to initiate simulations. Using the CHEMCAD software, equilibrium gaseous compositions were calculated. The oxygen flow rate and the cell temperature were set at five times the stoichiometric value and 800°C, respectively. Thermodynamic calculations show that the gas mixture at the anode inlet will not lead to carbon deposition above 575°C. The energy generated in the cell by electrooxidation is calculated at 144.6 kW. 44.6 kW of thermal energy is thus generated at the stack level. This heat is removed by the anode and cathode outlet flows, as well as minor heat losses through the system walls. A mass recycle ratio of 1.47 was obtained. Energy involved in each of 9 system blocks was calculated. Overall system and electrical efficiencies of 75% and 57% were obtained from simulations based on lower heating value. A voltage ranging from 0.91 to 0.94 V across the cell stacks was obtained at a fuel utilization factor of 80% and a temperature of 800°C. Finally, simulation iterations lead to an output fuel cell power of 98.8 kW, which is close to the power value used as input at the start of simulations.Conclusion:The recycle ratio of 1.47 contributes to system efficiency. By contrast, it was reported that for propane, a fuel utilized in Northern Canada, a recycle ratio greater than 5 is required for reliable functioning of SOFC systems. Modeling and simulation results clearly showed that methanol has very little tendency to form carbon deposits during cracking in situ when used in a simple anode gas recycling system. A very good electrical efficiency is thus obtained.

Analysis of Transient Response of Solid Oxide Fuel Cells with Anode Off-Gas Recycle

In this work, a solid oxide fuel cell system with anode off-gas recycle (AGR-SOFC) model which is composed of a reformer, an SOFC, and a recycle blower was constructed, assuming plug flow. Transient behavior of the system was analyzed where the increase of output power in accordance with the stepwise increases in required current and fuel supply was assumed. As a result of the analysis, the transient response of the AGR-SOFC was classified into three types according to the way of increase in the current and the fuel supply and characteristics of each type of responses are clarified. It was also found that the AGR-SOFC can increase the current and fuel supply more instantaneously and to a greater extent compared to conventional non AGR systems under identical fuel utilization conditions and superior output response is expected.

Modeling of Methanol-Fueled SOFC with Anode Offgas Recycling for Simplified System Design

Use of methanol as a fuel in SOFCs offers several advantages, such as having a lesser tendency for carbon deposition when vaporized and heated at high temperature. In this work, we conducted modeling and simulations of a methanol-fed SOFC system with anode offgas recycling. Anode offgas recycling provides water vapor in the fuel inlet, thus decreasing the tendency of the feed gas to form carbon deposits in the fuel cell stacks. The recycling also helps increase the fuel utilization. A thermodynamic analysis was conducted to calculate the energy generated, absorbed, or exchanged in the various blocks of the system, and the expected system energy output. Modeling and simulation results clearly showed that methanol has very little tendency to form carbon deposits during cracking in situ when used in a simple anode gas recycling system. A very good electrical efficiency is thus obtained.

A model-based diagnostic technique to enhance faults isolability in Solid Oxide Fuel Cell systems

This work illustrates an innovative diagnostic technique able to improve fault isolability in Solid Oxide Fuel Cell (SOFC) energy conversion systems. On-board sensor reduction may induce fault clustering and, thus, hinder univocal fault isolation. According to the proposed technique, isolated system component sub-models, fed with faulty inputs, can be used to solve this issue. These models provide a set of redundant residuals, which react only if the related component is under faulty state. The technique is characterized and tested in simulated environment on an SOFC Anode Off-Gas Recycling (AOGR) system. Hydrogen external leakage, fuel and air heat exchangers efficiency reduction and recirculation unit malfunction are addressed and implemented in the complete system model. This latter is used to simulate system variables in both nominal and faulty conditions and compute residuals for fault detection and isolation. The sub-models are then used to introduce further residuals, and their behaviour is investigated at different fault magnitudes. The analysis is firstly performed in an ideal case scenario, considering the fault isolability that can be theoretically achieved. Then, practical application of the diagnostic algorithm is discussed, considering quantitative residuals deviation and properly analysing the effects of feeding the sub-models with inputs provided by both faulty and nominal models. The achieved results confirm the capability of the proposed approach to univocally isolate the considered faults in all the investigated conditions. Moreover, the analysis of the real case scenario proved the proposed algorithm suitable also for real applications.

Improved Fault Isolability for Solid Oxide Fuel Cell Diagnosis Through Sub-system Analysis

Abstract This work illustrates an advanced technique able to improve fault isolability. Univocal fault isolation may be hindered by on-board sensor reduction, since fault clustering can occur. According to the proposed technique, isolated system component models, fed with faulty inputs, are used to compute a set of redundant residuals reacting only if the related component is under faulty state. The technique is characterized and tested via simulation analysis, on a Solid Oxide Fuel Cell Anode Off-Gas Recycling system. Stack internal leakage and fuel heat exchanger crossover models are implemented in the complete system model and used to simulate both nominal and faulty conditions. The achieved results confirm the capability of the proposed approach to univocally isolate the considered faults.

Methane/steam global reforming kinetics over the Ni/YSZ of planar pre-reformers for SOFC systems

A solid oxide fuel cell (SOFC) is a promising all-solid-state energy conversion device that produces electricity by electrochemically combining a fuel with an oxidant across an oxide electrolyte. To optimize SOFC performance, it is necessary to investigate the effects of the design and operating parameters on system components such as pre-reformers and afterburners. In pursuit of this, a multi-component SOFC design based on a planar stack was developed. Two specially designed planar-type steam pre-reformers (the first consisting of five layers with air heating and the other of one with electric heating) utilizing nickel yttria-stabilized zirconia (Ni/YSZ) as catalyst were built to investigate the kinetics of steam reforming with methane. Whereas experimental results on the global reaction kinetics of a Ni-based catalyst in the 5-layer planar pre-reformers within a temperature range of between 350°C and 610°C have been published, this work reports the results of reforming kinetics using a 1-layer reformer as a flow reactor in a higher temperature range (500°C–740°C), as well as on reforming kinetics in combination with anode off-gas recycling. The results confirm the proposed kinetic expression of the Arrhenius type (second order with respect to the mole fraction of methane and first order with respect to the mole fraction of water) for different steam-to-carbon ratios and also for anode off-gas recycling. The activation energy for methane steam reforming corresponds to the desorption energy of CO or H2O from the catalyst surface depending on the inlet composition of the mixture.

Improvement of Single Solid Oxide Fuel Cell Performance by Using Anode Off-Gas Recycle

The purpose of this research is to elucidate quantitatively the degree of performance improvement in solid oxide fuel cells (SOFCs) which use methane as a fuel by introducing anode off-gas recycle (AGR) through experiments and simulation. In the experiments, power generation characteristics of a commercially available disc-type SOFC under AGR conditions were measured using an apparatus that can simulate an anode feed gas for AGR conditions with high accuracy. Because the experiments are conducted using the cells with seal-less structure, the possible diffusion of surrounding air into anode negatively affects the cell performance. This effect of air penetration which may appear in experimental data was evaluated by numerical simulation and taken into consideration. As a result of the experiment and the simulation, an improvement of the cell voltage by conducting AGR was evaluated and factors of the performance improvement were quantitatively discussed.

Techno-economic analysis of the biomass gasification and Fischer–Tropsch integrated process with off-gas recirculation

The techno-economic analysis of a BG-FT (biomass gasification and Fischer–Tropsch) integrated process with different configurations, once-through and with recirculation concepts, for green fuel production is presented. Rice straw is considered a feedstock for the gasification process operated under the thermal self-sufficiency condition. Modeling of such an integrated process is performed by using Aspen Custom Modeler program. Regarding one technical aspect, the influence of changing an off-gas recycle fraction at different values of the FT reactor volume on the performance of the syngas processor, the FT synthesis and the energy efficiency is discussed. The production rate of syngas, diesel product and FT off-gas, as well as electricity from the BG-FT process, can be maximized via suitable adjustment of the recycle fraction and selection of the FT reactor volume. The economic analysis using an incremental NPV (net present value) as an economic indicator implies that the use of the recycle concept in the BG-FT process without the installation of any secondary equipment is less feasible than the once-through. Although the maximum diesel production rate is found when the FT reactor volume of 190 m3 and the FT off-gas recycle fraction of 0.4 are chosen, the incremental NPV shows a negative value.