The interaction of biomass gasification syngas components with tar in a solid oxide fuel cell and operational conditions to mitigate carbon deposition on nickel-gadolinium doped ceria anodes

The combination of biomass gasification with solid oxide fuel cells (SOFCs) is gaining increasing interest as an efficient and environmentally benign method of producing electricity and heat. However, tars in the gas stream arising from the gasification of biomass material can deposit carbon on the SOFC anode, having detrimental effects to the life cycle and operational characteristics of the fuel cell. This work examines the impact of biomass gasification syngas components combined with benzene as a model tar, on carbon formation on Ni/CGO (gadolinium-doped ceria) SOFC anodes. Thermodynamic calculations suggest that SOFCs operating at temperatures > 750 °C are not susceptible to carbon deposition from a typical biomass gasification syngas containing 15 g m⁻³ benzene.However, intermediate temperature SOFCs operating at temperatures < 650 °C require threshold current densities well above what is technologically achievable to inhibit the effects of carbon deposition. SOFC anodes have been shown to withstand tar levels of 2-15 g m⁻³ benzene at 765 °C for 3 h at a current density of 300 mA cm⁻², with negligible impact on the electrochemical performance of the anode. Furthermore, no carbon could be detected on the anode at this current density when benzene levels were <5 g m⁻³.     

Catalytic steam reforming of methane,methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC

This study investigated the possible use of methane, methanol, and ethanol with steam as a direct feed to Ni/YSZ anode of a direct internal reforming Solid Oxide Fuel Cell (DIR-SOFC). It was found that methane with appropriate steam content can be directly fed to Ni/YSZ anode without the problem of carbon formation, while methanol can also be introduced at a temperature as high as 1000 °C. In contrast, ethanol cannot be used as the direct fuel for DIR-SOFC operation even at high steam content and high operating temperature due to the easy degradation of Ni/YSZ by carbon deposition. From the steam reforming of ethanol over Ni/YSZ, significant amounts of ethane and ethylene were present in the product gas due to the incomplete reforming of ethanol. These formations are the major reason for the high rate of carbon formation as these components act as very strong promoters for carbon formation.     

Carbon deposition on Ni/YSZ anodes exposed to CO/H2 feeds

Solid oxide fuel cells (SOFC) can be operated with a variety of fuels. In anode-supported SOFC, these fuels may decompose or react catalytically in the anode compartment resulting in mixtures that, in most cases, include high concentrations of H₂ and CO. In this study, the formation of carbon from CO and H₂ mixtures on Ni/YSZ anodes at 1073 K has been investigated using electrochemical and carbon characterization techniques. More carbon is deposited when Ni/YSZ anodes are exposed to CO/H₂ mixtures than to pure CO. Polarization of the anodes reduced the amount of carbon deposited but the extent of the reduction depended on the gas composition.     

The study of the performance of Ni supported on gadolinium doped ceria SOFC anode on the steam reforming of ethanol

The catalytic performance of Ni/CeGd SOFC anodes prepared by co-precipitation for steam reforming of ethanol at different reaction temperatures was evaluated. The Ni/CeGd SOFC anode calcined at 1073 K exhibited the highest activity and the lowest by-products formation rates during SR at 773 K. The TG and SEM analyses of the used catalysts showed that the deactivation observed for SR at 773 K was associated with the formation of carbon filaments. It was also observed that the increase of reaction temperature from 773 to 1073 K decreased coke formation, which was no longer detected at 1073 K. This result was attributed to the reverse of the Boudouard reaction and the promoting effect of the support on carbon gasification.     

Direct utilization of methanol on impregnated Ni/YSZ and Ni-Zr0.35Ce0.65O2/YSZ anodes for solid oxide fuel cells

Some of the limits on fuel cell development include the issues of hydrogen availability and storage. Methanol has many advantages as an alternative fuel for fuel cells but depending on the anode composition, the formation of carbon may be a problem. In this paper, the direct utilization of methanol in solid oxide fuel cells with impregnated Ni/YSZ and Ni-Zr_0.35Ce_0.65O_2−δ (ZDC)/YSZ anodes was investigated at 1073 K. Performance and stability of these anodes, as measured by steady-state polarization and electrochemical impedance spectroscopy, were improved by the presence of ZDC; although, the deposition of carbon, as detected by scanning electron microscopy and temperature-programmed oxidation analysis, was not entirely avoided. The impact of the carbon, however, was different depending on the anode. That is, carbon formation caused the delamination of impregnated Ni/YSZ anodes, while the structural integrity of Ni-ZDC/YSZ anodes was maintained and the cell performance was not negatively impacted. Increasing the fuel utilization decreased coking, as predicted by equilibrium calculations.     

Behavior and mechanisms of Ni/ScSZ cermet anode deterioration by trace tar in wood gas in a solid oxide fuel cell

In order to determine both the criterion for diagnosing the deterioration of Ni/ScSZ cermet anodes in solid oxide fuel cells (SOFCs) by tar contaminants in wood gas and the tolerance limit of tar in wood gas for such anodes, the influence of tar concentration in wood gas on anode deterioration behavior was examined by means of scanning electron microscopy. We found that the anode degradation mechanism consisted of three phenomena: the disappearance of Ni particles, the destruction of sintered ScSZ, and carbon deposition. Furthermore, the Ni particle disappearance occurred at lower tar concentrations than did sintered ScSZ destruction and apparent carbon deposition. Therefore, we propose that the disappearance of Ni particles be set as the criterion for confirming deterioration of Ni/ScSZ cermet anodes in SOFCs by tar. On the basis of this criterion, the tolerance limit of toluene in fuel gas was determined to be 3 g/Nm³ when the operating temperature, steam to carbon molar ratio, and current density were 1073 K, 1, and 0.5 A/cm², respectively. The tolerance limit for tar for the fuel cell constructed herein was one to two orders of magnitude higher than that for internal combustion engines.     

Operation of solid oxide fuel cell on biomass product gas with tar levels >10 g Nm−3

This work assesses experimentally the feasibility of feeding a high tar load product gas from biomass gasification to a planar solid oxide fuel cell (SOFC) for renewable electricity generation. The SOFC had a nickel gadolinium-doped ceria anode (Ni-GDC) and the gasifier was a pilot scale circulating fluidized bed, employing hot gas-cleaning to remove particulates, HCl and H₂S. The SOFC operated for several hours on either pre-reformed gas (reduced tar levels < 0.5 g Nm^?3) as well as on high tar-laden wood gas (tar levels > 10 g Nm^?3) i.e. with no pre-reforming of tars. The tests were carried out at low fuel utilization U_f of around 20% at a current density j = 130 mA cm^?2. In all cases stable continuous SOFC performance was established. Post experimental examination of the SOFC showed that the anode was not affected by carbon deposition or other impurity accumulation.     

Effect of Sm0.2Ce0.8O1.9 on the carbon coking in Ni-based anodes for solid oxide fuel cells running on methane fuel

To directly use hydrocarbon fuel without a reforming process, a new microstructure for Ni/Sm_0.2Ce_0.8O_2−δ (Ni/SDC) anodes, in which the Ni surface of the anode is covered with a porous Sm_0.2Ce_0.8O_2−δ thin film, was investigated as an alternative to conventional Ni/YSZ anodes. The porous SDC thin layer was coated on the pores of the anode using the sol–gel coating method. The cell performance was improved by 20%–25% with the Ni/SDC anode relative to the cell performance with the Ni/YSZ anode due to the high ionic conductivity of the Ni/SDC anode and the increase of electrochemical reaction sites. For the SDC-coated Ni/SDC anode operating with methane fuel, no significant degradation of the cell performance was observed after 180 h due to the surface modification with the SDC film on the Ni surface, which opposes the severe degradation of the cell performance that was observed for the Ni/YSZ anode, which results from carbon deposition by methane cracking. Carbon was hardly detected in the SDC-coated Ni/SDC anode due to the catalytic oxidation of the deposited carbon on the SDC film as well as the electrochemical oxidation of methane in the triple-phase-boundary.     

Investigation of a multi-generation system using a hybrid steam biomass gasification for hydrogen, power and heat

In this paper, an integrated process of steam biomass gasification and a solid oxide fuel cell (SOFC) is investigated energetically to evaluate both electrical and energy efficiencies. This system is conceptualized as a combined system, based on steam biomass gasification and with a high temperature, pressurized SOFC. The SOFC system uses hydrogen obtained from steam sawdust gasification. Due to the utilization of the hydrogen content of steam in the reforming and shift reaction stages, the system efficiencies reach appreciable levels. This study essentially investigates the utilization of steam biomass gasification derived hydrogen that was produced from an earlier work in a system combines gasifier and SOFC to perform multi-duties (power and heat). A thermodynamic model is developed to explore a combination of steam biomass gasification, which produces 70–75 g of hydrogen/kg of biomass to fuel a planar SOFC, and generate both heat and power. Furthermore, processes are emerged in the system to increase the hydrogen yield by further processing the rest of gasification products: carbon monoxide, methane, char and tar. The conceptualized scheme combines SOFC operates at 1000 K and 1.2 bar and gasifier scheme based on steam biomass gasification which operates close to the atmospheric pressure, a temperature range of 1023–1423 K and a steam-biomass ratio of 0.8 kmol/kmol. A parametric study is also performed to evaluate the effect of various parameters such as hydrogen yield, air flow rate etc. on the system performance. The results show that SOFC with an efficiency of 50.3% operates in a good fit with the steam biomass gasification module with an efficiency, based on hydrogen yield, of 55.3%, and the overall system then works efficiently with an electric efficiency of ∼82%.     

Construction and reaction characteristics of pyrochlore-supported Ni catalysts for tar steam reforming

Tar is a common by-product during the gasification of biomass and its presence largely limits the subsequent application of syngas generated. Although biomass tar could be converted into hydrogen-rich syngas by catalytic steam reforming, the frequently adopted high-activity and low-cost Ni catalysts suffer from the problem of easy deactivation as a result of carbon deposition, and more efficient and stable catalyst needs to be developed for tar removal in biomass gasification. In the work, various Ni/pyrochlore catalysts characterized with redox properties were constructed and further modified through partial replacement of A-site in support, and their reaction characteristics in toluene steam reforming were comprehensively investigated. Results show that catalysts of Ni/La₂Ce₂ and Ni/Y₂Ce₂ have good catalytic performance due to the strong interaction between Ni and pyrochlore. Although a small amount doping of Sr in A-site is observed to decrease Ni/pyrochlore interaction, the great promotion in surface oxygen mobility make Ni/La_1.8Sr_0.2Ce₂ possess the best reactivity among all catalysts studied, and the optimum operating conditions is determined to be 650 °C and S/C = 2. Moreover, Ni/La_1.8Sr_0.2Ce₂ is found to be very stable during toluene steam reforming, which is proved to be a result of the superior capability in resisting coke formation.     

Performance of SOFC coupled with n-C4H10 autothermal reformer: Carbon deposition and development of anode structure

The performance deterioration of solid oxide fuel cells (SOFCs, Nickel-Yttria stabilized zirconia (Ni-YSZ)/YSZ/lanthanum doped strontium manganite-YSZ (LSM–YSZ)) coupled with n–C₄H₁₀ steam reformers (SR), autothermal reformers (ATR), or catalytic partial oxidation reformers (CPOX) was examined using an integrated system of a micro-reactor reformer and SOFC unit. The terminal voltage rapidly degraded in CPOX-driven SOFC (oxygen to carbon ratio (OCR) = 0.5) while it was fairly stable for SR-driven SOFC (steam to carbon ratio (SCR) = 2) over 250 h. For ATR-driven SOFC at near the thermoneutral point (OCR = 0.5 and steam to carbon ration (SCR) = 1.3), significant deterioration of the terminal voltage was observed in 100 h of operation. The main precursors of carbon deposition on the SOFC were identified by reformate gas analysis during the tests. In this study, we reveal that the carbon deposition on the SOFC anode can be affected by not only lower-order hydrocarbons (C₁∼C₄), but also by the CO/H₂ gas mixture. The change in electrical conductivity of the Ni-YSZ cermet used for the SOFC anode was investigated under different gas mixtures. To investigate the propensity for carbon deposition by each carbon-containing gas mixture, we defined the ratios of steam to specific carbon (C₁∼C₄ lower-order hydrocarbons and CO) in the reformate gas (SSCR, steam to specific carbon ratio). To inhibit carbon deposition on SOFC anode, the SSCR must be sufficiently high. However, the reformer operates near its maximum efficiency at low SSCR value and the higher the SSCR value, the lower the open circuit voltage and operating power density due to Nernst potential. In this study, a metal-foam supported SOFC single cell (Ni-YSZ/YSZ/Gd-doped ceria (CGO) buffer layer/lanthanum strontium cobalt ferrite-samarium doped ceria (LSCF-SDC)), impregnated with catalyst was designed; this novel SOFC was then examined for operation at a low SSCR value of the autothermal reformer conditions (near maximum efficiency of n-C₄H₁₀ reformer and thermal neutral point, SSCR = 0.5, OCR = 0.5 and SCR = 1.3). The voltage for the metal-foam supported SOFC impregnated with 0.5 wt% Rh/CGO remained at a nearly constant value, around 0.8 V, for 200 h under a constant temperature of 750 °C and current load of 250 mA cm⁻².     

Study on steam reforming of CH4 and C2 hydrocarbons and carbon deposition on Ni-YSZ cermets

Equilibrium partial pressure of oxygen and the boundary of carbon deposition region were calculated in the CHO phase diagram at temperatures ranging from 400 to 1000 °C. The open circuit voltage for the solid oxide fuel cell (SOFC) was directly connected to the calculated partial pressure of oxygen at higher temperatures. These calculations suggested that the development of the anode catalyst without carbon deposition was one of the most promising ways to achieve high efficiency in SOFC because the amount of added water could be reduced. The characteristics of steam reforming of methane and carbon deposition on Ni-Y₂O₃-stabilized zirconia (Ni-YSZ) cermets anodes were examined. The effect of MgO, CaO, SrO and CeO₂ addition to Ni-YSZ cermets on their catalytic activity and carbon deposition was investigated. Although, the CaO addition slightly deteriorated the electrochemical activity as anode, the CaO addition was effective in suppressing carbon deposition and promoted steam reforming of CH₄.     

Durability of solid oxide fuel cells using sulfur containing fuels

The usability of hydrogen and also carbon containing fuels is one of the important advantages of solid oxide fuel cells (SOFCs), which opens the possibility to use fuels derived from conventional sources such as natural gas and from renewable sources such as biogas. Impurities like sulfur compounds are critical in this respect. State-of-the-art Ni/YSZ SOFC anodes suffer from being rather sensitive towards sulfur impurities. In the current study, anode supported SOFCs with Ni/YSZ or Ni/ScYSZ anodes were exposed to H₂S in the ppm range both for short periods of 24 h and for a few hundred hours. In a fuel containing significant shares of methane, the reforming activities of the Ni/YSZ and Ni/ScYSZ anodes were severely poisoned already at low H₂S concentrations of ∼2 ppm H₂S. The poisoning effect on the cell voltage was reversible only to a certain degree after exposure of 500 h in the state-of-the-art cell, due to a loss of percolation of Ni particles in the Ni/YSZ anode layers closest to the electrolyte. Using SOFCs with Ni/ScYSZ anodes improved the H₂S tolerance considerably, even at larger H₂S concentrations of 10 and 20 ppm over a few hundred hours.     

Nickel/gadolinium-doped ceria anode for direct ethanol solid oxide fuel cell

This report investigates the properties of nickel/gadolinium-doped ceria (Ni/GDC) as anode material for bio-ethanol fueled SOFC. The Ni/GDC cermets with 18 and 44 wt.% Ni were prepared by a hydrothermal method. Ethanol decomposition, steam reforming, and partial oxidation of ethanol were studied using a fixed-bed reactor at 1123 K. Carbon was formed only under dry ethanol for both catalysts. The addition of water or oxygen to the feed inhibited the formation of carbon. Ni/GDC was used as the anode current collector layer and as a catalytic layer in single cells tests. No deposits of carbon were detected in single cells with Ni/GDC catalytic layer after 50 h of continuous operation under direct (dry) ethanol. This result was attributed to the catalytic properties of the Ni/GDC layer and the operation mechanism of gradual internal reforming, in which the oxidation of hydrogen provides the steam for ethanol reforming, thus avoiding carbon deposition.     

Preparation of Cu-Ni/YSZ solid oxide fuel cell anodes using microwave irradiation

A microwave irradiation process is used to deposit Cu nanoparticles on the Ni/YSZ anode of an electrolyte-supported solid oxide fuel cell (SOFC). The reaction time in the microwave is only 15 s for the deposition of 6 wt% Cu (with respect to Ni) from a solution of Cu(NO₃)₂·3H₂O and ethylene glycol (HOCH₂CH₂OH). The morphology of the deposited Cu particles is spherical and the average size of the particles is less than 100 nm. The electrochemical performance of the microwave Cu-coated Ni/YSZ anodes is tested in dry H₂ and dry CH₄ at 1073 K, and the anodes are characterized with scanning electron microscopy, electrochemical impedance spectroscopy, and temperature-programmed oxidation. The results indicate that preparation of the anodes by the microwave technique produces similar performance trend as those reported for Cu-Ni/YSZ/CeO₂ anodes prepared by impregnation. Specifically, less carbon is formed on the Cu-Ni/YSZ than on conventional Ni/YSZ anodes when exposed to dry methane and the carbon that does form is more reactive.     

The role of the ceria dopant on Ni / doped-ceria anodic layer cermets for direct ethanol solid oxide fuel cell

The effect of ceria dopant aiming at stability in Ni/doped-ceria anodic layers for direct ethanol solid oxide fuel cells (SOFC) was studied. Solid solutions of ceria doped with Y, Gd, Zr, or Nb (10 mol%) impregnated with NiO were tested in a fixed bed reactor for ethanol conversion reactions and for direct (dry) ethanol SOFC. The ceria dopant showed a marked effect on both the catalytic and the electrical transport properties of the ceramic support. Catalytic activity data revealed that the studied materials deactivate in ethanol decomposition reaction but are stable for ethanol steam reforming. Thus, feeding dry ethanol to the SOFC with a Ni/doped-ceria anodic catalytic layer evidenced that water produced from the electrochemical hydrogen oxidation provides steam for the internal reforming resulting in great stability of the fuel cells tested during ~100 h. The combined catalysis and SOFC results demonstrate Ni/doped-ceria is as candidate anode layer for stable SOFC running on bioethanol.     

In situ Van der Pauw measurements of the Ni/YSZ anode during exposure to syngas with phosphine contaminant

Solid oxide fuel cells (SOFCs) represent an option to provide a bridging technology for energy conversion (coal syngas) as well as a long-term technology (hydrogen from biomass). Whether the fuel is coal syngas or hydrogen from biomass, the effect of impurities on the performance of the anode is a vital question. The anode resistivity during SOFC operation with phosphine-contaminated syngas was studied using the in situ Van der Pauw method. Commercial anode-supported solid oxide fuel cells (Ni/YSZ composite anodes, YSZ electrolytes) were exposed to a synthetic coal syngas mixture (H₂, H₂O, CO, and CO₂) at a constant current and their performance evaluated periodically with electrochemical methods (cyclic voltammetry, impedance spectroscopy, and polarization curves). In one test, after 170 h of phosphine exposure, a significant degradation of cell performance (loss of cell voltage, increase of series resistance and increase of polarization resistance) was evident. The rate of voltage loss was 1.4 mV h⁻¹. The resistivity measurements on Ni/YSZ anode by the in situ Van der Pauw method showed that there were no significant changes in anode resistivity both under clean syngas and syngas with 10 ppm PH₃. XRD analysis suggested that Ni₅P₂ and P₂O₅ are two compounds accumulated on the anode. XPS studies provided support for the presence of two phosphorus phases with different oxidation states on the external anode surface. Phosphorus, in a positive oxidation state, was observed in the anode active layer. Based on these observations, the effect of 10 ppm phosphine impurity (or its reaction products with coal syngas) is assigned to the loss of performance of the Ni/YSZ active layer next to the electrolyte, and not to any changes in the thick Ni/YSZ support layer.     

Main issues of the impact of tar,H2S,HCl and alkali metal from biomass-gasification derived syngas on the SOFC anode and the related gas cleaning technologies for feeding a SOFC system: A review

Biomass gasification has acquired considerable interest as a sustainable and environmentally friendly way to produce heat, hydrogen or electricity from agro-industrial wastes or other kinds of biomass. A very effective solution for the achievement of high electrical efficiency (up to 55%) is the integration of biomass gasification with solid oxide fuel cell (SOFC) technology, including the necessary gas cleanup to avoid degradation of the SOFC. For this reason, this paper first shows in detail the risk and the impact of carbon deposition, exposure to tar, hydrogen sulphide, hydrogen chloride and alkali metals on SOFC anode and then, considering the tolerance limit for inorganic and organic contaminants (<1 ppmv for H₂S, HCl and alkali and <10 ppmv for tar), offers an overview of the most relevant and effective technologies to remove these contaminants and to feed safely a SOFC system.     

Resistance of Ni/perovskite catalysts to H2S in toluene steam reforming for H2 production

H₂S is a kind of common impurity produced during the gasification of biomass, and it will poison the catalysts used in biomass tar steam reforming, leading to a rapid degradation on the catalytic performance. The purpose of this work is to investigate the characteristics of biomass tar steam reforming using Ni/perovskite catalysts with the presence of H₂S. Results show that H₂S could significantly reduce catalytic activity due to the adsorption of sulfur on Ni surface, and Ni/perovskite catalysts are less susceptible to this poisoning in comparison to the Ni-catalyst loaded on γ-Al₂O₃. To understand the mechanism, fresh and spent catalysts were characterized using various techniques of XRD, SEM-EDS, XPS and TPO. It is proved that the lattice oxygen in perovskite could transform into surface species, inhibit the adsorption of sulfur and thus benefit to the reactivity of catalysts during biomass tar steam reforming.     

Investigating the impact and reaction pathway of toluene on a SOFC running on syngas

The integration of solid oxide fuel cells (SOFCs) with gasification systems have theoretically been shown to have a great potential to provide highly efficient distributed generation energy systems that can be fuelled by biomass including municipal solid waste. The syngas produced from the gasification of carbonaceous material is rich in hydrogen, carbon monoxide and methane that can fuel SOFCs. However, other constituents such as tar can cause catalyst deactivation, and blockage of the diffusion pathways. This work examines the impact of increasing concentrations of toluene as a model tar in a typical syngas composition fed to a NiO-GDC/TZ3Y/8YSZ/LSM–LSM SOFC membrane electrode assembly operating at 850°C and atmospheric pressure. Results suggest that up to 20 g/Nm³ of toluene and a low fuel utilisation factor (c.a. 17%) does not negatively impact cell performance and rather acts to increase the available hydrogen by undergoing reformation. At these conditions carbon deposition does occur, detected through EDS analysis, but serves to decrease the ASR rather than degrade the cell.