Running solid oxide fuel cells on biogas

The feasibility of operating a solid oxide fuel cell on biogas has been studied over a wide compositional range of biogas, using a small tubular solid oxide fuel cell system operating at 850 °C. In addition the response of the SOFC towards waste ammonia has been studied. It is possible to run the SOFC on biogas, even at remarkably low levels of methane, at which conventional heat engines would not work, thus offering a valuable and environmentally friendly use for poor-quality biogas that is currently wasted by detrimental venting to the atmosphere. The power output varies with methane content of the biogas, with maximum power production occurring at 45% methane, corresponding to maximal production of H₂ and CO through internal dry reforming. Direct electrocatalytic oxidation of methane does not contribute to the power output of the cell. For biogas with higher methane contents methane decomposition becomes significant, leading to increased H₂ production, and hence transiently higher power production, and deleterious carbon deposition and thus eventual cell deactivation. SOFCs are tolerant to ammonia, actually utilising the ammonia present in biogas to produce electrical power, at the same time acting as an environmental clean-up device breaking down the ammonia pollutant to N₂ and water, with no formation of any undesirable nitrogen oxides. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Ni1−x−yMgxAlyO–ScSZ anodes for solid oxide fuel cells

Three types of cermets based on NiO–ScSZ (A), Ni_0.9Mg_0.1O–ScSZ (AMg) and Ni_0.9Mg_0.095Al_0.005O–ScSZ (AMgAl) were applied as SOFC anodes. Humidified H₂ and simulated biogas (CH₄:CO₂ = 6:4) were directly supplied to the anode side of SOFC single cell. Catalytic activities for the reforming and the electrochemical reactions were tested in a typical electrochemical measurement setup. When hydrogen (3% H₂O) was supplied as a fuel, the three anodes showed almost the same voltage losses (anodic overvoltages) of ca. 40 mV at 400 mA cm^− 2 at 1000 °C. However, supplying the simulated biogas, AMg and AMgAl showed smaller losses of 25 and 29 mV, respectively, than those in supplying hydrogen, whereas A showed the loss of more than 40 mV. Through this study, it was revealed that when the biogas is selected as a fuel, the electrochemical efficiency of the internal reforming SOFC is enhanced by using AMg or AMgAl as anode materials instead of A. Although the higher performances of AMg and AMgAl mainly result from the stability of small Ni particles against sintering, in addition to this effect, basic (Ni,Mg)O solid solution or MgO existing in the electrocatalysts contributes to further activity enhancement.     

氧化还原可逆Nb2TiO7复合阴极电解池高温水蒸气电解研究

系统地研究Nb₂TiO₇与Nb_1.33Ti_0.67O₄材料相互转变的氧化还原循环可逆性能,同时研究Nb2TiO7和Nb_1.33Ti_0.67O₄样品随温度和氧分压变化的电导率,并与复合电极对称电池和电解池的电化学性能相关联. 在830 oC下,对Nb_1.33Ti_0.67O₄复合电极电解池进行水蒸气的电解研究测试. 电流电压曲线和电解池短期性能测试表明在低电压下主要为电极的还原和活化过程;而在高电压下主要为水蒸气的电解. 当3%H₂O/Ar/4%H_{2气体通入阴极时电解池水蒸气电解的法拉第效率为98.9%;而当通入气体转换为3%H2O/Ar时效率为89%.}     

Enhancement of biogas production from individually or co-digested green algae Cheatomorpha linum using ultrasound and ozonation treated biochar

This paper proposes the use of modified biochar, derived from Sawdust (SD) biomass using sonication (SSDB) and Ozonation (OSDB) processes, as an additive for biogas production from green algae Cheatomorpha linum (C. linum) either individually or co-digested with natural diet for rotifer culture (S. parkel). Brunauer-Emmett-Teller (BET), Fourier-Transform Infrared (FTIR), thermal-gravimetric (TGA), and X-ray diffraction (XRD) analyses were used to characterize the generated biochar. Ultrasound (US) specific energy, dose, intensity and dissolved ozone (O₃) concentration were also calculated. FTIR analyses proved the capability of US and ozonation treatment of biochar to enhance the biogas production process. The kinetic model proposed fits successfully with the data of the experimental work and the modified Gompertz models that had the maximum R² value of 0.993 for 150 mg/L of OSDB. The results of this work confirmed the significant impact of US and ozonation processes on the use of biochar as an additive in biogas production. The highest biogas outputs 1059 mL/g VS and 1054 mL/g VS) were achieved when 50 mg of SSDB and 150 mg of OSDB were added to C. linum co-digested with S. parkle.     

Theoretical and experimental study of methane steam reforming reactions over nickel catalyst

In this work we perform DFT theoretical calculations of methane and steam interactions on Ni(1 1 1) surface. The calculations allow us to improve our understanding of the competition between these reactants by catalytic sites in methane steam reforming (MSR) process. For this purpose we compare theoretical results with kinetic measurements of MSR on a Ni(II)-Al(III) catalyst prepared from lamellar double hydroxides as precursor. This comparison shows that, for low H₂O/CH₄ ratios methane and water intermediate species adsorb on different catalytic sites. While CHO species adsorbs on top of Ni atom, OH one occupies preferentially a tri-coordinate surface site. On the other hand, for high H₂O/CH₄ ratios a competency between these species by Ni sites would establish, diminishing methane conversion. In addition competition between methane and steam for Ni sites would lead to a decrease in CO production. Nevertheless, intermediate species adsorbed on different active sites would produce CO₂, whatever the steam/methane ratio. Thus, it would be optimum steam concentration in hydrocarbon feed and active sites distribution on catalyst surface, which could maximize H₂ production and minimize CO selectivity. The theoretical findings agree with kinetic measurements, which show that maximum methane conversion depends on steam partial pressure in the feed; whereas always, selectivity to CO₂ increases and to CO diminishes.     

Demonstration of medium temperature,one atmosphere reversible H2/O2 fuel and steam electrolysis cell operation using polycrystalline H3O+β/β″ -Al2O3

The development of a reversible fuel and steam electrolysis cell based on an H₃O⁺β/β″ -Al₂O₃ solid electrolyte is described. The unit has been operated between 100 and 300°C at one atmosphere steam pressure. The major limitations are discussed with reference to the solid electrolyte and the method of electrode preparation.     

A novel DME steam-reforming catalyst designed with fact database on-demand

Novel catalysts for dimethyl ether (DME) steam reforming (SR) were designed based on catalysis database on-demand. A catalyst library consisting of precious metals loaded on various metal oxides was tested for DME SR and its elemental reactions of DME hydrolysis and MeOH SR. Platinum loaded on alumina, Pt/Al₂O₃, shows high activity for DME SR as reported previously. The drawback of the catalyst was also confirmed; the formation of methane leading to the reduction of hydrogen formation. From the fact database for DME hydrolysis and MeOH SR built up with high-throughput experimentation tools, the high activity of Pt/Al₂O₃ for DME SR is owing to its high activity on DME hydrolysis because its activity on MeOH steam reforming is not remarkable. Based on these facts, novel catalysts were designed and achieved by physical mixing of Pt/Al₂O₃ which reveals high activity on DME hydrolysis with an active catalyst on MeOH steam reforming. By mixing of Pt/Al₂O₃ with Pd/Al₂O₃, methane formation was suppressed without loss of hydrogen production activity.     

Application of Mössbauer spectroscopy in phase analysis of corrosion products from nuclear power plants

Magnetic phases of corrosion products from nuclear power plant WWER-440 (Jaslovské Bohunice) steam generators and secondary circuit pipes were investigated using transmission Mössbauer spectroscopy technique. The main components of secondary circuit’s corrosion products are magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) in different ratios. The results can be used for optimal control and maintenance of WWER-440 nuclear reactors as well as for optimisation of water chemistry regime.     

Two-stage thermochemical conversion of solid fuel in a setup with steam ejection

The paper presents a set of tests with a setup using steam supply into ejector instead of compressed air. Experi-ments measured the gas analysis data — volumetric concentrations O₂, CO, CO₂, C _n H _m , NO _x , H₂ at different propor-tions of air and steam. The data are compared with calculations for thermodynamic equilibrium compositions for the reacting mixture С+Н₂О+air performed by “Terra” computer code including the case of air excess (α ≤ 1). The cal-culations were also compared with available data on gasification output at a high content of ballasting gas. It was demonstrated that in these operation modes, the steam was an inert dilution agent, which did not exclude the outcome of coal gas production with high Н₂/СО and СО/СО₂ ratios corresponding to different modes of gasification.     

Absorption heat-pump regeneration in a rankine steam cycle

This paper presents a system of regenerative heating incorporating an absorption heat pump in a Rankine steam cycle which can improve cycle efficiency. A simulation has been performed to estimate the Rankine cycle efficiency in the proposed Absorption Heat Pump Regeneration (AHPRG) heating system using the working pair R21³-DMETEG. The results show that the cycle efficiency can be improved considerably without reducing the work output by the incorporation of AHPRG for low-temperature heating of steam condensate. Further, the temperature of the heat-pump evaporator which absorbs the heat rejected at the steam condenser plays a predominant role in the cycle efficiency.     

Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure

The electrocatalytic synthesis of ammonia from steam and nitrogen was studied in oxygen ion (O^2?) and proton (H⁺) conducting solid electrolyte cells at 450–700 °C and at atmospheric total pressure. A Ru-based industrial catalyst was used as the working electrode. In the H⁺ cell, steam was electrolyzed at the anode to produce protons and oxygen. Protons, transported to the cathode, reacted with nitrogen to produce ammonia. In the O^2? cell, H₂O and N₂ were fed in together at the cathode. Steam was electrolyzed and the produced hydrogen reacted with nitrogen. Ammonia formation was observed at temperatures between 500 and 700 °C. The conversions with respect to nitrogen or steam were low, primarily because of the poor conductivity of the working electrode. Both cells, however, exhibit promising features that make this alternative approach of ΝΗ₃ synthesis worthy of further investigation.     

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Methane steam reforming in an annular microchannel with Rh/Al2O3 catalyst

Regularities of methane conversion in the presence of water steam were obtained experimentally while activating chemical conversions on the inner convex wall of an annular microchannel. The steam methane reforming was done on the Rh/Al₂O₃ nanocatalyst with the heat applied through the microchannel gap from the outer wall. Concentrations of the products of chemical reactions in the outlet gas mixture are measured at different temperatures of the outer microchannel wall. The range of channel wall temperatures at which the ratio of hydrogen and carbon oxide in the outlet mixture grows substantially is determined. Data on the composition of methane conversion products for the ratio H₂O/CH₄ = 1.77 and the activation energy of methane steam reforming at reactor outer wall temperatures of up to 880°C are obtained. The effect of the radiation heat exchange and the external diffuse limitation on the rate of chemical conversions in methane steam reforming in an annular microchannel with external heat supply is determined.     

The chemical stability and conductivity improvement of protonic conductor BaCe0.8 − x Zr x Y0.2O3 − δ

A series of BaCe_0.8???x Zr _x Y_0.2O_3???δ (BCZYx) (x?=?0, 0.2, 0.4, 0.6, 0.8) powders were prepared by EDTA–citrate complexing sol–gel process in this paper. The electrical conducting behavior, as well as chemical stability, was investigated. X-ray diffraction (XRD) results reveal that all samples are homogenous perovskite phases. Observed from XRD patterns and thermogravimetric curves, the samples with x?≥?0.4 survive in the pure CO₂, while samples with various Zr contents all present structurally stable against steam at 800 °C. The Zr-free sample of BaCe_0.8Y_0.2O_3???δ possesses the maximum bulk conductivity, 4.25?×?10^?2 S/cm, but decomposes into Ba(OH)₂ and Ce_0.8Y_0.2O_3???δ in steam. A negative influence of increasing Zr content on the conductivity of BCZYx can be observed by impedance tests. Considering the effect of temperature on the bulk conductivity, BCZY0.4 is preferred to be applied in SOFC as a protonic conductor, ranging from 1.52?×?10^?4 to 1.51?×?10^?3 S/cm (500–850 °C) with E _a?=?0.859 eV, which is proved to be a good protonic conductor with t _H+?≥?0.9.     

Improvement of Cu–CeO2 anodes for SOFCs running on ethanol fuels

Ethanol is considered to be an attractive green fuel for solid oxide fuel cells (SOFCs) due to several advantages. In this paper, we presented recent progress of our group in Cu–CeO₂ anodes for SOFCs with ethanol steam as a fuel. Cu–CeO₂–ScSZ (scandia stabilized zirconia)anodes with different ratios of copper versus ceria were fabricated and the impedance spectra of symmetric cells were measured to optimize the anode composition. Area specific resistance (ASR) of these anodes was examined to prove the thermal stability of them, and possible reasons for degradation were analyzed. Furthermore, a Ni–ScSZ interlayer was added between Cu–CeO₂–YSZ (yttria stabilized zirconia) anode and ScSZ electrolyte to improve the anode performance, and the three-layer structure was fabricated by acid leaching of nickel and wet impregnation method. The maximum power density of the single cell reached 604 mW cm^? 2 and 408 mW cm^? 2 at 800 °C in hydrogen and ethanol steam respectively, and the cell obtained stable output in ethanol steam over an operation period of 50 h.     

XPS of a steam reforming NiAl2O4 catalyst

An XPS study of the behaviour of a Ni-αAl₂O₃ catalyst for a methane—steam reforming reaction was undertaken. The combined information from binding energy shifts and intensities is discussed in terms of the oxidation state of the Ni atoms in the fresh, used and reduced catalyst. Accumulations of Si and Pb were detected on the surface of the used catalyst, Si originating from the packing in the preheating zone in the reactor and Pb from the corrosion of a lead-containing valve by distilled water used to produce steam.     

Estimating the density of carbon atoms on a Ni catalyst surface in equilibrium with a carbonaceous gas

The amount of carbon adsorbed on the surface of Ni in contact with carbonaceous gas mixtures such as CH₄/H₂ and CO/CO₂, is estimated from equilibrium segregation data. The results are displayed on “gas composition versus temperature” plots for the above two gas mixtures. These plots provide basic thermodynamic information relevant to reactions such as steam reforming of hydrocarbons on supported Ni catalysts. For example, the plot for CO/CO₂ gas mixtures represents the Boudouard equilibrium on a single crystal Ni catalyst, whilst the plot for CH₄/H₂ gas mixtures provides information relevant to the equilibrium hydrogenation of adsorbed C to CH₄.     

Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell

The present work aims to explore the activity of Cu/CeO₂ composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, P_i-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO₂ with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO₂ were investigated by voltage-current density-power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H₂ production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and P_i-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu-CeO₂ electrodes remained essentially unaffected.     

The combined effect of H2O and SO2 on CO2 uptake and sorbent attrition during fluidised bed calcium looping

The effect of steam and sulphur dioxide on CO₂ capture by limestone during calcium looping was studied in a novel lab-scale twin fluidised bed device (Twin Beds – TB). The apparatus consists of two interconnected batch fluidised bed reactors which are connected to each other by a duct permitting a rapid and complete pneumatic transport of the sorbent (limestone) between the reactors. Tests were carried out under typical calcium looping operating conditions with or without the presence of H₂O and/or SO₂ during the carbonation stage. Carbonation was carried out at 650°C in presence of 15% CO₂, 10% steam (when present) and by investigating two SO₂ levels, representative of either raw (1500?ppm) or pre-desulphurised (75?ppm) typical flue gas derived from coal combustion. The sorbent used was a reactive German limestone. Its performance was evaluated in terms of CO₂ capture capacity, sulphur uptake, attrition and fragmentation. Results demonstrated the beneficial effect of H₂O and the detrimental effect of SO₂ on the CO₂ capture capacity. When both species were simultaneously present in the gas, steam was still able to enhance the CO₂ capture capacity even outweighing the negative effect of SO₂ at low SO₂ concentrations. A clear relationship between degrees of Ca carbonation and sulphation was observed. As regards the mechanical properties of the sorbent, both H₂O and SO₂ hardened the particle surface inducing a decrease of the measured attrition rate, that was indeed always very low. Conversely, the fragmentation tendency increased in presence of H₂O and SO₂ most likely due to the augmented internal stresses within the particles. Clear bimodal particle size distributions for in-bed sorbent fragments were observed. Microstructural scanning electron microscope and porosimetric characterisations aided in explaining the observed trends.     

Cooperative activation effect on H2O adsorption in MnO–Co catalyzed steam methane reforming

Steam methane reforming is a very important chemical process in hydrogen production and solid oxide fuel cells (SOFCs). Cobalt (Co) is an important catalyst for dry and steam methane reforming. However, previous studies have confirmed that metal Co surfaces only have weak adsorption activity for H₂O, which is evidently unfavorable for steam reforming. In this work we used first-principles simulations to study the activity of MnO–Co catalysts for the adsorption of H₂O. Compared with the Co (111) surface and pristine Co clusters, the MnO–Co catalytic layer has a stronger adsorption capability for H₂O because of the introduction of the MnO substrate, which is crucial for improving the steam reforming reaction and inhibiting carbon disposition in SOFCs. The cooperation mechanism between MnO and Co is discussed based on the analysis of electronic structures. The conclusions from this work are universal for other metal-oxide composite catalyst layers.