Effects of fuel processing methods on industrial scale biogas-fuelled solid oxide fuel cell system for operating in wastewater treatment plants

The performance of three solid oxide fuel cell (SOFC) systems, fuelled by biogas produced through anaerobic digestion (AD) process, for heat and electricity generation in wastewater treatment plants (WWTPs) is studied. Each system has a different fuel processing method to prevent carbon deposition over the anode catalyst under biogas fuelling. Anode gas recirculation (AGR), steam reforming (SR), and partial oxidation (POX) are the methods employed in systems I-III, respectively. A planar SOFC stack used in these systems is based on the anode-supported cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode, operated at 800 °C. A computer code has been developed for the simulation of the planar SOFC in cell, stack and system levels and applied for the performance prediction of the SOFC systems. The key operational parameters affecting the performance of the SOFC systems are identified. The effect of these parameters on the electrical and CHP efficiencies, the generated electricity and heat, the total exergy destruction, and the number of cells in SOFC stack of the systems are studied. The results show that among the SOFC systems investigated in this study, the AGR and SR fuel processor-based systems with electrical efficiency of 45.1% and 43%, respectively, are suitable to be applied in WWTPs. If the entire biogas produced in a WWTP is used in the AGR or SR fuel processor-based SOFC system, the electricity and heat required to operate the WWTP can be completely self-supplied and the extra electricity generated can be sold to the electrical grid.     

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.     

Biohythane as an energy feedstock for solid oxide fuel cells

Biogas (60%-CH₄, 40%- CO₂) is a potential source of renewable energy when used as energy feedstock for solid oxide fuel cells (SOFC), but releases biogenic CO₂ emissions. Hybrid SOFC performance can be affected by fuel composition and reformer performance. Biohythane (58%-CH₄, 35%-CO₂ and 7% H₂) can be a better alternative providing balance between energy and biogenic emissions. Biohythane performance is studied for a 120 kW SOFC stack using ASPEN process model and compared with other feed stocks. This work is the first to study and report on the application of biohythane in SOFC systems. Biohythane was found to produce less biogenic CO₂ emissions and 6% less CO at the reformer than biogas. Comparisons show that biohythane provides better efficiencies in hybrid SOFC systems. Sensitivity studies recommends operation of stack with biohythane at Steam to Carbon Ratio (STCR) = 2.0, i = 200 mA cm⁻² and U_F = 0.85 respectively.     

Operation of solid oxide fuel cells with alternative hydrogen carriers

Future electricity production will use fossil-free sources with zero CO₂ emission or closed carbon cycle technologies based on renewable sources. While hydrogen is considered a key energy source, its production at present time relies heavily on fossil fuels. Furthermore, distribution and storage are not well established and require substantial investments. This is a strong motivation to identify alternative, safe, high power density hydrogen carriers, where existing logistics and infrastructure can be utilized. In this contribution, ammonia and biogas are considered for high-efficient electricity production in solid oxide fuel cells (SOFCs). It is demonstrated that the properties and operating conditions of SOFC allow for direct use of these fuels, with fuel pretreatment inside the SOFC anode. The high efficient electricity production using pure ammonia or real biogas was successfully proven on state-of-the-art SOFCs. Even without optimization of operating parameters, electrical efficiencies of 40–50% and high and stable power output were demonstrated.     

Carbon dioxide separation from high temperature fuel cell power plants

High temperature fuel cell technologies, solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs), are considered for their potential application to carbon dioxide emission control. Both technologies feature electrochemical oxidisation of natural gas reformed fuels, avoiding the mixture of air and fuel flows and dilution with nitrogen and oxygen of the oxidised products; a preliminary analysis shows how the different mechanism of ion transport attributes each technology a specific advantage for the application to CO₂ separation. The paper then compares in the first part the most promising cycle configurations based on high efficiency integrated SOFC/gas turbine “hybrid” cycles, where CO₂ is separated with absorption systems or with the eventual adoption of a second SOFC module acting as an “afterburner”. The second part of the paper discusses how a MCFC plant could be “retrofitted” to a conventional fossil-fuel power station, giving the possibility of draining the majority of CO₂ from the stack exhaust while keeping the overall cycle electrical efficiency approximately unchanged.     

Conversion of fossil and biomass fuels to electric power and transportation fuels by high efficiency integrated plasma fuel cell (IPFC) energy cycle

The IPFC is a high efficiency energy cycle, which converts fossil and biomass fuel to electricity and co-product hydrogen and liquid transportation fuels (gasoline and diesel). The cycle consists of two basic units, a hydrogen plasma black reactor (HPBR) which converts the carbonaceous fuel feedstock to elemental carbon and hydrogen and CO gas. The carbon is used as fuel in a direct carbon fuel cell (DCFC), which generates electricity, a small part of which is used to power the plasma reactor. The gases are cleaned and water gas shifted for either hydrogen or syngas formation. The hydrogen is separated for production or the syngas is catalytically converted in a Fischer–Tropsch (F–T) reactor to gasoline and/or diesel fuel. Based on the demonstrated efficiencies of each of the component reactors, the overall IPFC thermal efficiency for electricity and hydrogen or transportation fuel is estimated to vary from 70 to 90% depending on the feedstock and the co-product gas or liquid fuel produced. The CO₂ emissions are proportionately reduced and are in concentrated streams directly ready for sequestration. Preliminary cost estimates indicate that IPFC is highly competitive with respect to conventional integrated combined cycle plants (NGCC and IGCC) for production of electricity and hydrogen and transportation fuels.     

Determining marginal electricity for near-term plug-in and fuel cell vehicle demands in California: Impacts on vehicle greenhouse gas emissions

California has taken steps to reduce greenhouse gas emissions from the transportation sector. One example is the recent adoption of the Low Carbon Fuel Standard, which aims to reduce the carbon intensity of transportation fuels. To effectively implement this and similar policies, it is necessary to understand well-to-wheels emissions associated with distinct vehicle and fuel platforms, including those using electricity. This analysis uses an hourly electricity dispatch model to simulate and investigate operation of the current California grid and its response to added vehicle and fuel-related electricity demands in the near term. The model identifies the “marginal electricity mix” - the mix of power plants that is used to supply the incremental electricity demand from vehicles and fuels - and calculates greenhouse gas emissions from those plants. It also quantifies the contribution from electricity to well-to-wheels greenhouse gas emissions from battery-electric, plug-in hybrid, and fuel cell vehicles and explores sensitivities of electricity supply and emissions to hydro-power availability, timing of electricity demand (including vehicle recharging), and demand location within the state. The results suggest that the near-term marginal electricity mix for vehicles and fuels in California will come from natural gas-fired power plants, including a significant fraction (likely as much as 40%) from relatively inefficient steam- and combustion-turbine plants. The marginal electricity emissions rate will be higher than the average rate from all generation - likely to exceed 600 gCO₂ equiv. kWh⁻¹ during most hours of the day and months of the year - and will likely be more than 60% higher than the value estimated in the Low Carbon Fuel Standard. But despite the relatively high fuel carbon intensity of marginal electricity in California, alternative vehicle and fuel platforms still reduce emissions compared to conventional gasoline vehicles and hybrids, through improved vehicle efficiency.     

Combustion chemistry aspects of alternative fuels reforming for high-temperature fuel cell applications

Solid-oxide fuel cells (SOFC) constitute a particularly attractive technology for sustainable, combined heat and power generation, both at domestic and district levels. The elevated operating temperature of SOFC systems, allows the utilization of a wide spectrum of conventional and alternative fuels, through suitable reforming processes. The high temperatures and fuel rich conditions prevailing in SOFC reformers, enhance syngas yield and reforming efficiency but may give rise to unwanted effects, such as ignition, soot and coke formation and deposition. The above phenomena cannot be described via thermodynamic considerations and can only be effectively tackled through a detailed chemical kinetic approach. The present study provides a comparative assessment of SOFC reformer operation on conventional and alternative hydrocarbon fuels in terms of syngas yield, thermal efficiency and pollutants formation. In particular, the reforming of methane, a typical biogas (comprising of 60% CH₄ and 40% CO₂), methanol and ethanol is numerically assessed by utilizing a recently developed and validated comprehensive detailed kinetic mechanism for C₁–C₆ hydrocarbons, augmented with a PAH model. Chemical aspects of the fuel reforming process are investigated through rate-of-production path and sensitivity analyses. The study supports design guidelines aiming towards identification of optimum operating conditions, for specific applications and fuels. The analysis reveals that the extent of coupling between syngas formation and molecular growth processes is strongly dependent on fuel and operating conditions choice and identifies windows of efficient operation, for each case.     

Integration of solid oxide fuel cell and palladium membrane reactor: Technical and economic analysis

This paper presents a technical and economic analysis of a solid oxide fuel cell system equipped with a palladium membrane reactor (PMR–SOFC) with the aim of determining the benefits of such an integrated unit over the conventional reformer module (CON-SOFC). The performance of both SOFC systems under the conditions for energetically self-sustaining operation (Q_NET = 0) was achieved by varying the fuel utilization for each operating voltage. Two types of fuels, i.e., methane and desulphurized biogas, are considered. The simulation results show that the maximum power density of the CON-SOFC fuelled by methane (0.423 W/cm²) is higher than that of the CON-SOFC fuelled by biogas (0.399 W/cm²) due to the presence of CO₂ in biogas. For the PMR–SOFC, it is found that the operation at a higher permeation pressure offers higher power density because lower fuel utilization is required when operating the SOFC at the energy self-sustained condition. When the membrane reactor is operated at the permeation pressure of 1 bar, the methane-fuelled and biogas-fuelled PMR–SOFCs can achieve the maximum power density of 0.4398 and 0.4213 W/cm², respectively. Although the PMR–SOFC can offer higher power density, compared with the CON-SOFC, the capital costs of supporting units, i.e., palladium membrane reactor, high-pressure compressor, and vacuum pump, for PMR–SOFC need to be taken into account. The economic analysis shows that the PMR–SOFC is not a good choice from an economic viewpoint because of the requirement of a large high-pressure compressor for feeding gas to the membrane reactor.     

Fuel flexibility study of an integrated 25 kW SOFC reformer system

The operation of solid oxide fuel cells on various fuels, such as natural gas, biogas and gases derived from biomass or coal gasification and distillate fuel reforming has been an active area of SOFC research in recent years. In this study, we develop a theoretical understanding and thermodynamic simulation capability for investigation of an integrated SOFC reformer system operating on various fuels. The theoretical understanding and simulation results suggest that significant thermal management challenges may result from the use of different types of fuels in the same integrated fuel cell reformer system. Syngas derived from coal is simulated according to specifications from high-temperature entrained bed coal gasifiers. Diesel syngas is approximated from data obtained in a previous NFCRC study of JP-8 and diesel operation of the integrated 25 kW SOFC reformer system. The syngas streams consist of mixtures of hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. Although the SOFC can tolerate a wide variety in fuel composition, the current analyses suggest that performance of integrated SOFC reformer systems may require significant operating condition changes and/or system design changes in order to operate well on this variety of fuels.     

Design of highly efficient coal-based integrated gasification fuel cell power plants

Integrated gasification fuel cell (IGFC) technology combining coal gasification and solid oxide fuel cell (SOFC) is believed to be the only viable solution to achieving U.S. Department of Energy (DOE)’s performance goal for next generation coal-based power plants, producing electricity at 60% efficiency (coal HHV-AC) while capturing more than 90% of the evolved CO₂. Achieving this goal is challenging even with high performance SOFCs; design concepts published to date have not demonstrated this performance goal. In this work an IGFC system concept consisting of catalytic hydro-gasification, proven low-temperature gas cleaning and hybrid fuel cell-gas turbine power block (with SOFC operating at about 10 bar) is introduced. The system is demonstrating an electricity efficiency greater than 60% (coal HHV basis), with more than 90% of the carbon present in the syngas separated as CO₂ amenable to sequestration. A unique characteristic of the system is recycling de-carbonized, humidified anode exhaust back to the catalytic hydro-gasifier for improved energy integration. Alternative designs where: (1) anode exhaust is recycled directly back to SOFC stacks, (2) SOFC stack operating pressure is reduced to near atmospheric and (3) methanation reactor in the reactor/expander topping cycle is removed, have also been investigated and the system design and performance differences are discussed.     

State of the art of multi-fuel reformers for fuel cell vehicles: problem identification and research needs

This review is focused on discussions about multi-fuel reformer technology for fuel cell vehicles where techniques for onboard hydrogen generation and gas clean-up processes, as well as fuel considerations and emissions are included. Our conclusion is that the potential for developing a highly efficient, durable and reliable reformer system for automotive applications is considerably higher if dedicated fuel reformers are used instead of applications where all types of fuels ranging from natural gas to heavy diesel fuels can be used. The authors propose that petroleum-derived fuels should be designed for potential use in mobile fuel cell applications. The present literature review together with site visit discussions has led to the conclusion that there are relatively low emissions from fuel cell engines compared to internal combustion engines. However, the major research work on reformers/fuel cells have been performed during steady-state operation. Emissions during start-up, shutdown and transient operation are basically unknown and must be investigated in more detail.     

System modeling of an air-independent solid oxide fuel cell system for unmanned undersea vehicles

To examine the feasibility of a solid oxide fuel cell (SOFC)-powered unmanned undersea vehicle (UUV), a system level analysis is presented that projects a possible integration of the SOFC stack, fuel steam reformer, fuel/oxidant storage and balance of plant components into a 21-in. diameter UUV platform. Heavy hydrocarbon fuel (dodecane) and liquid oxygen (LOX) are chosen as the preferred reactants. A maximum efficiency of 45% based on the lower heating value of dodecane was calculated for a system that provides 2.5 kW for 40 h. Heat sources and sinks have been coupled to show viable means of thermal management. The critical design issues involve proper recycling of exhaust steam from the fuel cell back into the reformer and effective use of the SOFC stack radiant heat for steam reformation of the hydrocarbon fuel.     

Multi-objective optimization of biogas systems producing hydrogen and electricity with solid oxide fuel cells

The design of solid oxide fuel cells (SOFC) using biogas for distributed power generation is a promising alternative to reduce greenhouse gas emissions in the energy and waste management sectors. Furthermore, the high efficiency of SOFCs in conjunction with the possibility to produce hydrogen may be a financially attractive option for biogas plants. However, the influence of design variables in the optimization of revenues and efficiency has seldom been studied for these novel cogeneration systems. Thus, in order to fulfill this knowledge gap, a multi-objective optimization problem using the NSGA-II algorithm is proposed to evaluate optimal solutions for systems producing hydrogen and electricity from biogas. Moreover, a mixed-integer linear optimization routine is used to ensure an efficient heat recovery system with minimal number of heat exchanger units. The results indicate that hydrogen production with a fuel cell downstream is able to achieve high exergy efficiencies (65–66%) and a drastic improvement in net present value (1346%) compared with sole power generation. Despite the additional equipment, the investment costs are estimated to be quite similar (12% increase) to conventional steam reforming systems and the levelized cost of hydrogen is very competitive (2.27 USD/kgH₂).     

Numerical evaluation of a parallel fuel feeding SOFC–PEFC system using seal-less planar SOFC stack

We propose a system that combines a seal-less planar solid oxide fuel cell (SOFC) stack and polymer electrolyte fuel cell (PEFC) stack. In the proposed system, fuel for the SOFC (SOFC fuel) and fuel for the PEFC (PEFC fuel) are fed to each stack in parallel. The steam reformer for the PEFC fuel surrounds the seal-less planar SOFC stack. Combustion exhaust heat from the SOFC stack is used for reforming the PEFC fuel. We show that the electrical efficiency in the SOFC–PEFC system is 5% higher than that in a simple SOFC system using only a seal-less planar SOFC stack when the SOFC operation temperature is higher than 973 K.     

Coming clean with fuel cells

Fuel cells, mostly phosphoric acid, have been shown to operate well on renewable biogas fuels, such as anaerobic digester gas (ADC) produced at wastewater treatment plants as well as landfill gas (LFG) and gas produced in beer breweries. This paper discusses an innovative emission-offset project that utilizes anaerobic digester gas-powered fuel cells to produce electricity in New York. The use of fuel cells at wastewater treatment plants is also discussed.     

Fuel cell system modeling for solid oxide fuel cell/gas turbine hybrid power plants, Part I: Modeling and simulation framework

A sustainable future power supply requires high fuel-to-electricity conversion efficiencies even in small-scale power plants. A promising technology to reach this goal is a hybrid power plant in which a gas turbine (GT) is coupled with a solid oxide fuel cell (SOFC). This paper presents a dynamic model of a pressurized SOFC system consisting of the fuel cell stack with combustion zone and balance-of-plant components such as desulphurization, humidification, reformer, ejector and heat exchangers. The model includes thermal coupling between the different components. A number of control loops for fuel and air flows as well as power management are integrated in order to keep the system within the desired operation window. Models and controls are implemented in a MATLAB/SIMULINK environment. Different hybrid cycles proposed earlier are discussed and a preferred cycle is developed. Simulation results show the prospects of the developed modeling and control system.     

A comprehensive review of direct carbon fuel cell technology

Fuel cells are under development for a range of applications for transport, stationary and portable power appliances. Fuel cell technology has advanced to the stage where commercial field trials for both transport and stationary applications are in progress. The electric efficiency typically varies between 40 and 60% for gaseous or liquid fuels. About 30–40% of the energy of the fuel is available as heat, the quality of which varies based on the operating temperature of the fuel cell. The utilisation of this heat component to further boost system efficiency is dictated by the application and end-use requirements. Fuel cells utilise either a gaseous or liquid fuel with most using hydrogen or synthetic gas produced by a variety of different means (reforming of natural gas or liquefied petroleum gas, reforming of liquid fuels such as diesel and kerosene, coal or biomass gasification, or hydrogen produced via water splitting/electrolysis). Direct Carbon Fuel Cells (DCFC) utilise solid carbon as the fuel and have historically attracted less investment than other types of gas or liquid fed fuel cells. However, volatility in gas and oil commodity prices and the increasing concern about the environmental impact of burning heavy fossil fuels for power generation has led to DCFCs gaining more attention within the global research community. A DCFC converts the chemical energy in solid carbon directly into electricity through its direct electrochemical oxidation. The fuel utilisation can be almost 100% as the fuel feed and product gases are distinct phases and thus can be easily separated. This is not the case with other fuel cell types for which the fuel utilisation within the cell is typically limited to below 85%. The theoretical efficiency is also high, around 100%. The combination of these two factors, lead to the projected electric efficiency of DCFC approaching 80% - approximately twice the efficiency of current generation coal fired power plants, thus leading to a 50% reduction in greenhouse gas emissions. The amount of CO₂ for storage/sequestration is also halved. Moreover, the exit gas is an almost pure CO₂ stream, requiring little or no gas separation before compression for sequestration. Therefore, the energy and cost penalties to capture the CO₂ will also be significantly less than for other technologies. Furthermore, a variety of abundant fuels such as coal, coke, tar, biomass and organic waste can be used. Despite these advantages, the technology is at an early stage of development requiring solutions to many complex challenges related to materials degradation, fuel delivery, reaction kinetics, stack fabrication and system design, before it can be considered for commercialisation. This paper, following a brief introduction to other fuel cells, reviews in detail the current status of the direct carbon fuel cell technology, recent progress, technical challenges and discusses the future of the technology.     

Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Lignite as a fuel for direct carbon fuel cell system

Lignite, also known as brown coal, and char derived from lignite by pyrolysis were investigated as fuels for direct carbon solid oxide fuel cells (DC-SOFC). Experiments were carried out with 16 cm² active area, electrolyte supported solid oxide fuel cell (SOFC), using pulverized solid fuel directly fed to DC-SOFC anode compartment in a batch mode, fixed bed configuration. The maximum power density of 143 mW/cm² was observed with a char derived from lignite, much higher than 93 mW/cm² when operating on a lignite fuel. The cell was operating under electric load until fuel supply was almost completely exhausted. Reloading fixed lignite bed during a thermal cycle resulted in a similar initial cell performance, pointing to feasibility of fuel cell operation in a continuous fuel supply mode. The additional series of experiments were carried out in SOFC cell, in the absence of solid fuels, with (a) simulated CO/CO₂ gas mixtures in a wide range of compositions and (b) humidified hydrogen as a reference fuel composition for all cases considered. The solid oxide fuel cell, operated with 92%CO + 8%CO₂ gas mixture, generated the maximum power density of 342 mW/cm². The fuel cell performance has increased in the following order: lignite (DC-SOFC) < char derived from lignite (DC-SOFC) < CO + CO₂ gas mixture (SOFC) < humidified hydrogen (SOFC).