Direct carbon fuel cell: Fundamentals and recent developments

The direct carbon fuel cell is a special type of high temperature fuel cell that directly uses solid carbon as anode and fuel. As an electrical power generator for power plants, it has a higher achievable efficiency (80%) than the molten carbonate and solid oxide fuel cells, and has less emissions than conventional coal-burning power plants. More importantly, its solid carbon-rich fuels (e.g. coal, biomass, organic garbage) are readily available and abundant. In this review, some fundamental study results of electrochemical oxidation of carbon in molten salts are summarized. Recent developments in direct carbon fuel cell configurations and performance are also discussed.     

Alkaline falling-film fuel cell A breakthrough in technology and cost

The work described in this paper was oriented towards fuel cells for practical applications, but mainly presents data obtained using half-cells. The economic significance of these data is discussed, together with the technical concept of fuel cell power stations and for transportation applications. The proposed fuel cell with generate power at much lower costs than conventional power plants, and a zero-emission vehicle with fuel cells will operate at lower fuel cost than a car with an internal combustion engine. The simple falling-film process leads to high power densities (6 kW/l) and low cost. The details given are valid for the use of hydrogen produced from fossil energy sources. Concentrated CO₂, a byproduct of this technology can be stored in discussed oil and gas fields at a very low cost to avoid global warming. Thus, this ‘down-to-earth’ hydrogen technology is a free from CO₂ emissions as solar-hydrogen technology.     

PEM fuel cell performance with solar air preheating

Proton Exchange Membrane Fuel Cells (PEMFC) have proven to be a promising energy conversion technology in various power applications and since it was developed, it has been a potential alternative over fossil fuel-based engines and power plants, all of which produce harmful by-products. The inlet air coolant and reactants have an important effect on the performance degradation of the PEMFC and certain power outputs. In this work, a theoretical model of a PEM fuel cell with solar air heating system for the preheating hydrogen of PEM fuel cell to mitigate the performance degradation when the fuel cell operates in cold environment, is proposed and evaluated by using energy analysis. Considering these heating and energy losses of heat generation by hydrogen fuel cells, the idea of using transpired solar collectors (TSC) for air preheating to increase the inlet air temperature of the low-temperature fuel cell could be a potential development. The aim of the current article is applying solar air preheating for the hydrogen fuel cells system by applying TSC and analyzing system performance. Results aim to attention fellow scholars as well as industrial engineers in the deployment of solar air heating together with hydrogen fuel cell systems that could be useful for coping with fossil fuel-based power supply systems.     

Power generation using a mesoscale fuel cell integrated with a microscale fuel processor

An integrated fuel reformer and fuel cell system for microscale (10–500 mW) power generation is being developed and demonstrated as an alternative to conventional batteries. In this system, thermal energy is transformed to electricity by stripping the hydrogen from the hydrocarbon fuel (reforming) and converting the hydrogen to electricity in a proton exchange membrane (PEM) fuel cell. The fabrication and operation of a mesoscale fuel cell based on phosphoric acid doped polybenzimidazole (PBI) technology is discussed, along with tests integrating the methanol processor with the fuel cell. The PBI membrane had high ionic conductivity at high temperatures (>150 °C), and sustained the high conductivity at low relative humidity at these temperatures. This high-temperature stability and high ionic conductivity enabled the membrane to tolerate extremely high levels of carbon monoxide up to 10% without significant degradation in performance. The combined fuel cell/reformer system was successfully operated to enable the production of 23 mW of electrical power.     

The utilization of hydrogen in hydrogen/diesel dual fuel engine

A hydrogen fueled internal combustion engine has great advantages on exhaust emissions including carbon dioxide (CO₂) emission in comparison with a conventional engine fueling fossil fuel. In addition, if it is compared with a hydrogen fuel cell, the hydrogen engine has some advantages on price, power density, and required purity of hydrogen. Therefore, they expect that hydrogen will be utilized for several applications, especially for a combined heat and power (CHP) system which currently uses diesel or natural gas as a fuel.A final goal of this study is to develop combustion technologies of hydrogen in an internal combustion engine with high efficiency and clean emission. This study especially focuses on a diesel dual fuel (DDF) combustion technology. The DDF combustion technology uses two different fuels. One of them is diesel fuel, and the other one is hydrogen in this study. Because the DDF engine is not customized for hydrogen which has significant flammability, it is concerned that serious problems occur in the hydrogen DDF engine such as abnormal combustion, worse emission and thermal efficiency.In this study, a single cylinder diesel engine is used with gas injectors at an intake port to evaluate performance swung the hydrogen DDF engine with changing conditions of amount of hydrogen injected, engine speed, and engine loads. The engine experiments show that the hydrogen DDF operation could achieve higher thermal efficiency than a conventional diesel operation at relatively high engine load conditions. However, it is also shown that pre-ignition with relatively high input energy fraction of hydrogen occurred before diesel fuel injection and its ignition. Therefore, such abnormal combustion limited amount of hydrogen injected. Fire-deck temperature was measured to investigate causal relationship between fire-deck temperature and occurrence of pre-ignition with changing operative conditions of the hydrogen DDF engine.     

中国中长期碳减排战略目标初探(Ⅱ)——中国煤炭消费过程的碳排放及减排措施

中国能源领域排放的二氧化碳主要来自煤炭,因此煤炭消费过程中的碳减排措施尤为重要。煤炭的主要用户是发电部门,基于应对气候变化的需要,煤电行业的低碳途径不得不考虑采用CCS技术。不论是新建燃煤电厂,还是今后在传统电厂改建过程中增设CCS设施已是大势所趋,预计多数仍将采用MEA法脱除烟气中二氧化碳这一成熟技术。由于MEA法技术经济指标不够先进,估计10~20年内必将出现更先进的脱二氧化碳工艺技术。传统的燃煤锅炉增加CCS的经济效益已经逊于IGCC-CCS,预计2020年后IGCC电厂将成为新建煤电厂的首选方案。20年后采用临氢气化炉与燃料电池FC发电相结合、把高温的热能和甲烷的化学能直接转化为电力的IGFC高效燃煤电厂或将成功应用,IGFC综合能量转化效率比IGCC相对高出1/2~3/4,发展前景不可低估。钢铁、水泥和化工等高耗煤工业部门可通过节能和采用CCS技术降低碳排放,其余用煤的工业部门和分散用户则应考虑节能或用天然气等低碳燃料替代,间接起到减排效果。预计2050年燃煤发电和高耗煤工业总计将排放二氧化碳4.6Gt,如果二氧化碳捕集量是2.9Gt,则净排放量为1.7Gt。加上其他难以捕集二氧化碳的工业、部门及民用煤排放二氧化碳1.0Gt,合计二氧化碳净排放量为2.7Gt(情景A)。如果采用更先进的技术和严格的节能减排措施,可减少煤炭消耗0.31Gt标煤,减少二氧化碳排放0.5Gt,使煤源二氧化碳净排放量减少到2.2Gt(情景B)。无论哪种情景,实施CCS的任务都十分艰巨。     

Electrochemical high temperature technology for hydrogen production or direct electricity generation

Solid oxide electrolyte cells have been developed for hydrogen production from water vapour with high overall efficiency. On the basis of experimental data plant concepts have been designed for autothermal electrolysis operation with low temperature (∼150°C) steam input at a mean cell voltage of ∼1.3 V. The production of modules of series connected cells is under way in order to demonstrate the vapour electrolysis in a small, but complete plant for ∼3.5 kW hydrogen output. The actual state of the art of this technology will be described. It has been demonstrated that the solid oxide cells can be operated also in the reverse mode acting as fuel cells for hydrogen as well as for carbon monoxide. Calculations of overall performance of high temperature fuel cell plants show that, e.g. for electricity generation from natural gas, efficiencies of about 60% can be achieved with the additional advantage of extremely low NO_x emission.     

Performance assessment of concentrated solar power plants based on carbon and hydrogen fuel cells

In spite of the recent success on the implementation of Concentrating Solar Power (CSP), still this technology needs a substantial enhancement to achieve competitiveness. This paper provides thorough insight after previous analyses on an alternative concept for higher efficiency CSP systems based on the replacement of the power block by an electrochemical conversion system. Concentrating solar energy is herewith used to decompose methane into hydrogen and carbon, which are used in hydrogen and carbon fuel cells for electricity generation. This approach envisages modular, efficient and flexible generation plants. Dispatchability can be achieved by storing the solid carbon. Solar-to-electricity efficiency was calculated assuming thermodynamic equilibrium composition and experimental data available from literature, and compared with those of conventional power generation systems and commercial CSP plants. It is concluded that this new-generation CSP concept is potentially able to produce power more efficiently than the current state-of-the art solar thermal power plants.     

Sensitivity of nickel cermet anodes to reduction conditions

The direct use of methane as fuel for solid oxide fuel cell (SOFC) without pre-reforming would reduce running costs and enable higher efficiencies. But methane generally causes carbon deposition on the nickel anode and subsequent power degradation. This paper shows that carbon deposition from methane is very sensitive to anode reduction conditions. The effect of direct methane on microtubular SOFC reduced at two different conditions was studied at temperatures above 800 °C. Reducing the cells at high temperature gave good performance on hydrogen but the current degraded quickly on methane, suggesting that carbon was blocking the nickel surfaces. This was not recoverable by bringing in hydrogen to replace the methane. Cells reduced under low temperature conditions gave higher current on methane than on hydrogen, showing that carbon deposited from the methane improved nickel anode conductivity in this case. These cells also did not degrade on methane under certain conditions but lasted for a long period. Extracting the carbon by feeding the cell with hydrogen interrupted this newly formed linkage between the nickel particles, reducing the electrical conductivity, which could be recovered by reintroducing methane. The conclusion was that nickel cermet anodes are very sensitive to reduction conditions, with low temperature reduction being preferred if methane is to be used as the chosen fuel.     

Cathode electrocatalyst selection and deposition for a direct borohydride/hydrogen peroxide fuel cell

Catalyst selection, deposition method and substrate material selection are essential aspects for the design of efficient electrodes for fuel cells. Research is described to identify a potential catalyst for hydrogen peroxide reduction, an effective catalyst deposition method, and supporting material for a direct borohydride/hydrogen peroxide fuel cell. Several conclusions are reached. Using Pourbaix diagrams to guide experimental testing, gold is identified as an effective catalyst which minimizes gas evolution of hydrogen peroxide while providing high power density. Activated carbon cloth which features high surface area and high microporosity is found to be well suited for the supporting material for catalyst deposition. Electrodeposition and plasma sputtering deposition methods are compared to conventional techniques for depositing gold on diffusion layers. Both methods provide much higher power densities than the conventional method. The sputtering method however allows a much lower catalyst loading and well-dispersed deposits of nanoscale particles. Using these techniques, a peak power density of 680 mW cm⁻² is achieved at 60 °C with a direct borohydride/hydrogen peroxide fuel cell which employs palladium as the anode catalyst and gold as the cathode catalyst.     

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.     

Biomass to power conversion in a direct carbon fuel cell

Direct carbon fuel cells (DCFC) offer clear advantages over conventional power generation systems including higher conversion efficiency, low emissions and production of a near pure CO₂ exit stream which can be easily captured for storage. When operated on biomass-derived fuels and combined with carbon capture and storage they have the potential to be a carbon negative technology. Currently most studies relating to DCFC's focus on the use of synthetic high purity fuels. Although of significant academic interest, the high energy requirements for the production of such fuels and high cost would negate the advantages offered by DCFCs over conventional combustion technologies that can produce power from lower-grade fuels. A number of industrial processes (such as pyrolysis or gasification) can produce high carbon containing and low cost chars from biomass sources. This paper describes the operation of a novel solid state direct carbon fuel cell operated on two such commercially available bio-mass derived chars, an agricultural waste derived bio-char used for soil enrichment and coconut char used for the processing of ceramics. Chemical analysis (ICP, XRF), X-ray diffraction and thermo-gravimetric analysis have been used to characterise the fuels. Testing on small button cells showed that it is possible to operate fuel cells directly on low grade unprocessed chars. Although initial power densities were low, significant improvements to cell materials and designs can lead to practical devices. Overall the stability of the fuel cell materials in contact with bio-chars appeared to be good with no phase decomposition of any material observed.     

Using harmonised life-cycle indicators to explore the role of hydrogen in the environmental performance of fuel cell electric vehicles

This work uses harmonised life-cycle indicators of hydrogen to explore its role in the environmental performance of proton exchange membrane fuel cell (PEMFC) passenger vehicles. To that end, three hydrogen fuel options were considered: (i) conventional, fossil-based hydrogen from steam methane reforming; (ii) renewable hydrogen from biomass gasification; and (iii) renewable hydrogen from wind power electrolysis. In order to increase the robustness of the life-cycle study, the environmental profile of each hydrogen option was characterised by three harmonised indicators: carbon footprint, non-renewable energy footprint, and acidification footprint. When enlarging the scope of the assessment according to a well-to-wheels perspective, the results show that the choice of hydrogen fuel significantly affects the life-cycle performance of PEMFC vehicles. In this regard, the use of renewable hydrogen –instead of conventional hydrogen from steam methane reforming– is essential when pursuing low carbon and energy footprints. Nevertheless, the identification of the most favourable renewable hydrogen option was found to be conditioned by the prioritised life-cycle indicators.     

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.     

Numerical modeling of an automotive derivative polymer electrolyte membrane fuel cell cogeneration system with selective membranes

Cogeneration power plants based on fuel cells are a promising technology to produce electric and thermal energy with reduced costs and environmental impact. The most mature fuel cell technology for this kind of applications are polymer electrolyte membrane fuel cells, which require high-purity hydrogen.The most common and least expensive way to produce hydrogen within today's energy infrastructure is steam reforming of natural gas. Such a process produces a syngas rich in hydrogen that has to be purified to be properly used in low temperature fuel cells. However, the hydrogen production and purification processes strongly affect the performance, the cost, and the complexity of the energy system.Purification is usually performed through pressure swing adsorption, which is a semi-batch process that increases the plant complexity and incorporates a substantial efficiency penalty. A promising alternative option for hydrogen purification is the use of selective metal membranes that can be integrated in the reactors of the fuel processing plant. Such a membrane separation may improve the thermo-chemical performance of the energy system, while reducing the power plant complexity, and potentially its cost. Herein, we perform a technical analysis, through thermo-chemical models, to evaluate the integration of Pd-based H₂-selective membranes in different sections of the fuel processing plant: (i) steam reforming reactor, (ii) water gas shift reactor, (iii) at the outlet of the fuel processor as a separator device. The results show that a drastic fuel processing plant simplification is achievable by integrating the Pd-membranes in the water gas shift and reforming reactors. Moreover, the natural gas reforming membrane reactor yields significant efficiency improvements.     

Potential of molten carbonate fuel cells to enhance the performance of CHP plants in sewage treatment facilities

In spite of its slow commercial deployment, fuel cells are amongst the most efficient and environmentally friendly electric power generators. The case of Molten Carbonate Fuel Cells is even more interesting since, in addition to these features that are common to all fuel cells, these systems can be used as active carbon capture devices due to their capability to migrate carbon dioxide from one electrode (cathode) to another (anode). In this context, this work presents the operation of a fuel cell of this type coupled to a combined heat and power plant based on gas reciprocating engines as typically used in wastewater treatment plants. The biogas produced in the water sludge digestion process is burnt in the reciprocating engines, whose exhaust gases are mixed with air and blown into the fuel cell cathode. The carbon dioxide contained in this stream is conveyed in the form of carbonate ions (CO₃⁼) through the electrolyte to the anode where it reacts with the hydrogen fuel, being released as carbon dioxide. The exhaust gases from the anode comprise carbon dioxide, water steam and a small fraction of unspent hydrogen fuel. The combustion of the latter species with pure oxygen followed by a cooling process permits separating a gaseous stream of pure carbon dioxide from a liquid stream of water.     

Recent challenges of hydrogen storage technologies for fuel cell vehicles

Fuel cell vehicles have a high potential to reduce both energy consumption and carbon dioxide emissions. However, due to the low density, hydrogen gas limits the amount of hydrogen stored on board. This restriction also prevents wide penetration of fuel cells. Hydrogen storage is the key technology towards the hydrogen society. Currently high-pressure tanks and liquid hydrogen tanks are used for road tests, but both technologies do not meet all the requirements of future fuel cell vehicles. This paper briefly explains the current status of conventional technologies (simple containment) such as high-pressure tank systems and cryogenic storage. Another method, hydrogen-absorbing alloy has been long investigated but it has several difficulties for the vehicle applications such as low temperature discharge characteristics and quick charge capability due to its reaction heat. We tested a new idea of combining metal hydride and high pressure. It will solve some difficulties and improve performance such as gravimetric density. This paper describes the latest material and system development.     

The extraction,conversion and use of energy resources: future challenge to materials technology

Abstract

The growing shortage of all types of fossil fuels, and concerns about global warming are beginning to have a major impact on the power generation and transport sectors. Renewable energy, in conjunction with energy conservation measures such as cogeneration, and the use of biofuels and hydrogen in transport will help extend fossil fuel resources, but there is real need to find and exploit new coal, oil, and gas reserves. Materials technology must play its part in these new developments. Future fossil fuel generating plants will have to capture the carbon dioxide produced, and to compensate for the parasitic power losses this implies, this will require conventional plants to operate at very high temperatures. Likely targets for the strength of advanced austenitics are postulated, and the need to carry out R&D into type IV cracking, dissimilar metal weld issues, and steam side oxidation is emphasised.     

Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods

Fuel cell and hydrogen technologies are re-gaining momentum in a number of sectors including industrial, tertiary and residential ones. Integrated biogas fuel cell plants in wastewater treatment plants and other bioenergy recovery plants are nowadays on the verge of becoming a clear opportunity for the market entry of high-temperature fuel cells in distributed generation (power production from a few kW to the MW scale).High-temperature fuel cell technologies like molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) are especially fit to operate with carbon fuels due to their (direct or indirect) internal reforming capability. Especially, systems based on SOFC technology show the highest conversion efficiency of gaseous carbon fuels (e.g., natural gas, digester gas, and biomass-derived syngas) into electricity when compared to engines or gas turbines. Also, lower CO₂ emissions and ultra-low emissions of atmospheric contaminants (SO_X, CO, VOC, especially NO_X) are generated per unit of electricity output. Nonetheless, stringent requirements apply regarding fuel purity. The presence of contaminants within the anode fuel stream, even at trace levels (sometimes ppb levels) can reduce the lifetime of key components like the fuel cell stack and reformer. In this work, we review the complex matrix (typology and amount) of different contaminants that is found in different biogas types (anaerobic digestion gas and landfill gas). We analyze the impact of contaminants on the fuel reformer and the SOFC stack to identify the threshold limits of the fuel cell system towards specific contaminants. Finally, technological solutions and related adsorbent materials to remove contaminants in a dedicated clean-up unit upstream of the fuel cell plant are also reviewed.     

The design of long-life,high-efficiency PEM fuel cell power supplies for low power sensor networks

Field sensor networks have important applications in environmental monitoring, wildlife preservation, in disaster monitoring and in border security. The reduced cost of electronics, sensors and actuators make it possible to deploy hundreds if not thousands of these sensor modules. However, power technology has not kept pace. Current power supply technologies such as batteries limit many applications due to their low specific energy. Photovoltaics typically requires large bulky panels and is dependent on varying solar insolation and therefore requires backup power sources. Polymer Electrolyte Membrane (PEM) fuel cells are a promising alternative, because they are clean, quiet and operate at high efficiencies. However, challenges remain in achieving long lives due to factors such as degradation and hydrogen storage. In this work, we devise a framework for designing fuel cells power supplies for field sensor networks. This design framework utilize lithium hydride hydrogen storage technology that offers high energy density of up to 5000 Wh/kg. Using this design framework, we identify operating conditions to maximize the life of the power supply, meet the required power output and minimize fuel consumption. We devise a series of controllers to achieve this capability and demonstrate it using a bench-top experiment that operated for 5000 h. The laboratory experiments point towards a pathway to demonstrate these fuel cell power supplies in the field. Our studies show that the proposed PEM fuel cell hybrid system fueled using lithium hydride offers at least a 3 fold reduction in mass compared to state-of-the-art batteries and 3-5 fold reduction in mass compared to current fuel cell technologies.