Effects of fuel cell anode recycle on catalytic fuel reforming

The presence of steam in the reactant gas of a catalytic fuel reformer decreases the formation of carbon, minimizing catalyst deactivation. However, the operation of the reformer without supplemental water reduces the size, weight, cost, and overall complexity of the system. The work presented here examines experimentally two options for adding steam to the reformer inlet: (I) recycle of a simulated fuel cell anode exit gas (comprised of mainly CO₂, H₂O, and N₂ and some H₂ and CO) and (II) recycle of the reformate from the reformer exit back to the reformer inlet (mainly comprised of H₂, CO, and N₂ and some H₂O and CO₂). As expected, anode gas recycle reduced the carbon formation and increased the hydrogen concentration in the reformate. However, reformer recycle was not as effective due principally to the lower water content in the reformate compared to the anode gas. In fact, reformate recycle showed slightly increased carbon formation compared to no recycle. In an attempt to understand the effects of individual gases in these recycle streams (H₂, CO, CO₂, N₂, and H₂O), individual gas species were independently introduced to the reformer feed.     

Bio-fuel reforming for solid oxide fuel cell applications. Part 2: Biodiesel

Biodiesel is considered as a renewable hydrogen source for solid oxide fuel cells (SOFCs). This study contributes to a fundamental understanding of biodiesel autothermal reforming (ATR), which has not yet been widely explored in the open literature. Ultra-low sulfur diesel (ULSD) ATR is established as a baseline for this analysis. This work applies a micro-soot meter based on a photo-acoustic method to quantify the condensed carbon from a single-tube reactor, and uses a mass spectrometer to measure the effluent gas composition under different operating conditions (reformer temperature, steam/carbon ratio, oxygen/carbon ratio, and gas hourly space velocity). The key objective is to identify the optimum operating environment for biodiesel ATR with carbon-free deposition and peak hydrogen yield. Thermodynamic analysis based on the method of total Gibbs free energy minimization is used to evaluate the equilibrium composition of effluent from the reformer. The experimental investigations complimented with this theoretical analysis of biodiesel ATR enable effectively optimizing the onboard reforming conditions. This study is one component of a three-part investigation of bio-fuel reforming, also including fuel vaporization and reactant mixing (Part 1) and biodiesel–diesel blends (Part 3).     

Bio-fuel reformation for solid oxide fuel cell applications. Part 1: Fuel vaporization and reactant mixing

A new configuration of a mixing chamber integrated with a customized porous nozzle has been developed to completely vaporize heavy hydrocarbon fuels (e.g., diesel, biodiesel) and achieve homogenous mixing of fuel/air/steam. This proposed configuration suppresses hydrocarbon thermal pyrolysis and solid carbon formation in the fuel vaporization step. The porous nozzle promotes the micro-explosion of emulsified fuel and accelerates secondary atomization to reduce the droplet size. The mixing chamber with customized nozzle was integrated in a single-tube reformer system in order to analyze its effect on diesel and biodiesel auto-thermal reforming (ATR). It has been demonstrated that the customized nozzle not only improved the hydrogen production rate and the reforming efficiency, but it also stabilized the chemical reactions within the reformer and prevented the reactor inlet from high temperature sintering. For diesel ATR, this mixing chamber–reformer combination enabled operation at relatively low reformer temperature without forming solid carbon. This study is one component of a three-part investigation of bio-fuel reforming, also including biodiesel (Part 2) and biodiesel–diesel blends (Part 3).     

Heat-up and start-up modeling of direct internal reforming solid oxide fuel cells

In this paper, a transient heat transfer model to simulate the heat-up and start-up periods of co- and counter-flow direct internal reforming solid oxide fuel cells is developed and presented. In this comprehensive model, all the heat transfer mechanisms, i.e. conduction, convection, and radiation, and all the polarization nodes, i.e. ohmic, activation, and concentration, are considered. The heat transfer model is validated using the results of a benchmark test and two numerical studies obtained from the literature. After validating the model, the heat-up, start-up, and steady-state behaviors of the cell are investigated. In addition, the first principal thermal stresses are calculated to find the probability of failure of the cell during its operation. The results of the present model are in good agreement with the literature data. It is also shown for the given input data that counter-flow case yields a higher average current density and power density, but a lower electrical efficiency of the cell. For the temperature controlled heat-up and start-up strategy, the maximum probability of failure during the operation of the cell is found to be 0.068% and 0.078% for co- and counter-flow configurations, respectively.     

Modelling of solid oxide fuel cells with internal glycerol steam reforming

The direct application of glycerol in solid oxide fuel cell (SOFC) for power generation has been demonstrated experimentally but the detailed mechanisms are not well understood due to the lack of comprehensive modeling study. In this paper, a numerical model is developed to study the glycerol-fueled SOFC. After model validation, the simulated SOFC demonstrates a performance of 7827 A m^?2 at 0.6 V, with a glycerol conversion rate of 49% at 1073 K. Then, parametric analyses are conducted to understand the effects of operation conditions on cell performance. It is found that the SOFC performance increases with decreasing operating voltage or increasing inlet temperature. However, increasing either the fuel flow rate or steam to glycerol ratio could decrease the cell performance. It is also interesting to find out that the contribution of H₂ and CO to the total current density is significantly different under various operating conditions, even sometimes CO dominates while H₂ plays a negative role. This is different from our conventional understanding that usually H₂ contributes more significantly to current generation. In addition, cooling measures are needed to ensure the long-term stability of the cell when operating at a high current density.     

Quantitative analysis of sub-ppm traces of hydrocarbons in the product gas from diesel reforming

The autothermal reforming of middle distillates is a favored reforming technique for fuel cell-based auxiliary power units (APUs) in the transport sector. The efforts made in recent years to produce a high quality reformate have been reflected in a variety of improvements to the reactors and mode of operation of the fuel processing systems. By these means, the concentrations of contaminants, especially traces of hydrocarbons in the gas phase, frequently drops below the previous detection limit. In this paper, a new GC/MS method is developed to reduce the detection limit of the analytics into the sub-ppm range. This work is intended to serve as a valuable step to showing which traces of hydrocarbons can still be qualitatively and quantitatively expected in a high quality reformate. This contribution may help determine the multitude of hydrocarbonous substances that can have a detrimental long-term effect on catalysts and adsorbents in a fuel cell-based APU system operated with middle distillates.     

A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

A direct carbon fuel cell based on a conventional anode-supported tubular solid oxide fuel cell, which consisted of a NiO-YSZ anode support tube, a NiO-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode, has been successfully achieved. It used the carbon black as fuel and oxygen as the oxidant, and a preliminary examination of the DCFC has been carried out. The cell generated an acceptable performance with the maximum power densities of 104, 75, and 47 mW cm⁻² at 850, 800, and 750 °C, respectively. These results demonstrate the feasibility for carbon directly converting to electricity in tubular solid oxide fuel cells.     

An autothermal reforming system for diesel and jet fuel with quick start-up capability

A quick, low energy consuming and reliable start-up is essential for fuel cell systems utilizing diesel and jet fuel. A compact fuel processor for coupling with a high-temperature polymer electrolyte fuel cell is developed with electrically-heated reactors in the 28 kW_th power class. Based on this set-up, start-up strategies are developed and validated. With the basic strategy, 14 min are required in the best case to commence reforming and achieve self-sustaining operation with desired CO concentration at full load using NExBTL diesel and, respectively, 16 min using Jet A-1. However, using premium diesel, the basic strategy leads to a strong increase in the concentrations of ethane and benzene. An advanced strategy enables 16 min start time with premium diesel suppressing these undesired side products. This result is within the 30 min start-up time target for auxiliary power units for 2020 and offers a reliable option for real world applications.     

Durability of direct internal reforming of methanol as fuel for solid oxide fuel cell with double-sided cathodes

Direct internal reforming of methanol is applied as fuel for a Ni-YSZ anode-supported solid oxide fuel cell with a flat tube based on double-sided cathodes. It achieves a power density (PD) of 0.25 W/cm² at 0.8 V, reaching about 90% of that is fueled by H₂. And the cell has been operated for more than 120 h by the direct internal reforming of methanol. The durability and apparent advantage for using humidified methanol may lead to widespread applications by direct internal reforming method for this new designed SOFC in the future.     

Thermodynamic modeling of direct internal reforming solid oxide fuel cells operating with syngas

In this paper a direct internal reforming solid oxide fuel cell (DIR-SOFC) is modeled thermodynamically from the energy point of view. Syngas produced from a gasification process is selected as a fuel for the SOFC. The modeling consists of several steps. First, equilibrium gas composition at the fuel channel exit is derived in terms mass flow rate of fuel inlet, fuel utilization ratio, recirculation ratio and extents of steam reforming and water–gas shift reaction. Second, air utilization ratio is determined according to the cooling necessity of the cell. Finally, terminal voltage, power output and electrical efficiency of the cell are calculated. Then, the model is validated with experimental data taken from the literature. The methodology proposed is applied to an intermediate temperature, anode-supported planar SOFC operating with a typical gas produced from a pyrolysis process. For parametric analysis, the effects of recirculation ratio and fuel utilization ratio are investigated. The results show that recirculation ratio does not have a significant effect for low current density conditions. At higher current densities, increasing the recirculation ratio decreases the power output and electrical efficiency of the cell. The results also show that the selection of the fuel utilization ratio is very critical. High fuel utilization ratio conditions result in low power output and air utilization ratio but higher electrical efficiency of the cell.     

Numerical analysis of electrical power generation and internal reforming characteristics in seal-less disk-type solid oxide fuel cells

For seal-less type solid oxide fuel cells, its power generation characteristics and distribution of the gas composition depend on not only the electrochemical reaction, but also complex kinetics and transport phenomena, because the internal reforming reaction and the water-gas shift reaction take place together with reverse diffusion of the ambient gas from the surroundings of the cell. The purpose of this paper is to theoretically explain the experimental results of the anodic concentration profile of gaseous species previously reported in a practical seal-less disk-type cell which used pre-reforming methane with steam as a fuel. A numerical model that takes into account the transport phenomena of the gaseous species and the internal reforming reaction with the water-gas shift reaction together with the assumption of the cell outlet boundary condition was constructed to numerically analyse the gas composition distribution and power generation characteristics. Numerical analyses by the model were conducted for the several cases reported as the experiment. The calculated results in the anode gas concentration profile and in the voltage–current characteristics show good agreement with the experimental data in every case, and then the validity of the simulation model was verified. Therefore, the model is useful for a seal-less disk-type cell which is operated by a fuel including non-reformed methane.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Combined methane reforming by carbon dioxide and steam in proton conducting solid oxide fuel cells for syngas/power co-generation

Methane and carbon dioxide mixture can be used as the fuel in a proton conducting solid oxide fuel cell (SOFC) for power/syngas co-generation and greenhouse gas reduction. However, carbon deposition and low conversion ratio are potential problems for this technology. Apart from using functional catalytic layer in the SOFC to enhance CH₄ dry reforming, adding H₂O into the fuel stream could facilitate the CH₄ conversion and enhance the co-generation performance of the SOFC. In this work, the effects of adding H₂O to the CO₂CH₄ fuel on the performance of a tubular proton conducting SOFC are studied numerically. Results show that the CH₄ conversion is improved from 0.830 to 0.898 after adding 20% H₂O to the anode. Meanwhile, the current density is increased from 2832 A m⁻² to 3064 A m⁻² at 0.7 V. Sensitivity studies indicate that the H₂:CO ratio can be effectively controlled by the amount of H₂O addition and the H₂ starvation can be alleviated, especially at high current density conditions.     

Performance degradation of solid oxide fuel cells due to sulfur poisoning of the electrochemical reaction and internal reforming reaction

Hydrogen sulfide is known to degrade the solid oxide fuel cell (SOFC) performance by adsorbing on the nickel anode catalyst. In this research, the mechanism underlying such SOFC degradation was evaluated based on a theoretical mathematical modeling approach and the sulfur coverage was calculated from a Temkin-like isotherm which is related to both temperature and hydrogen sulfide (H₂S) concentration. The influences of the cell temperature, H₂S concentration and electrochemical performance on both the sulfur coverage and cell polarization are studied in detail. Two specific models were considered to identify whether sulfur poisoning has a larger impact on cell performance through its effect on the electrochemical reaction or on the internal reforming reaction. It was found that sulfur poisoning has different effects on the hydrogen oxidation reaction and internal reforming reaction, leading to competing changes in cell performance with temperature and H₂S concentration.     

Highly efficient utilization of industrial barium slag for carbon gasification in direct carbon solid oxide fuel cells

Direct carbon solid oxide fuel cells (DC-SOFCs) are recognized as an efficient energy conversion device. With regard to their operation mechanism, the reverse Boudouard reaction rate is the crucial factor influencing cell performance. In this work, a new-type catalyst derived from industrial barium slag (BS) was first developed to enhance the reverse Boudouard reaction and DC-SOFC performance. The chemical composition and micro-morphologies of BS and barium slag-derived catalyst (BSC) were characterized in detail. The superiorities of BS and BSC were reflected in the enhanced DC-SOFC performance and high fuel utilization. The single cell fueled by BSC-loaded carbon yielded the best output of 249 mW cm⁻² at 850 °C. This result was comparable to the 266 mW cm⁻² output of a hydrogen-fueled SOFC due to the superior catalytic activity of metallic catalysts toward carbon gasification. The advantage of the BSC was also observed in the durable operation of the corresponding DC-SOFCs, which lasted for 36.2 h at 50 mA with the fuel utilization of 29.0%. This work provides a new channel for green and efficient utilization of BS and other industrial residues, and a novel option to the development of energy conversion technology.     

Electrochemical properties and thermal neutral state of solid oxide fuel cells with direct internal reforming of methane

Solid oxide fuel cells (SOFCs) with direct internal reforming (DIR) provide a promising method to realize clean and efficient utilization of hydrocarbon fuels. Thse endothermic reforming reactions occur simultaneously with exothermic electrochemical reactions at the anode, making thermal neutral state achievable inside a fuel cell, providing reference to the thermal management. In this study, a calculation model combining experimental data and thermodynamic results was established, validating the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics in direct internal steam and dry reforming were elaborately compared, contributing to a more scientific understanding of anode reaction mechanism. Detailed experimental investigation was carried out to determine the influence of H₂O/CO₂ on the electrochemical properties of DIR-SOFCs, based on which the optimum steam-carbon ratio (S/C) and CO₂ to CH₄ ratios were obtained. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH₄H₂O and CH₄CO₂ atmospheres were proposed to distinguish different physical and chemical processes within anodes. The results of this study can be conducive to a more precise understanding of reaction mechanism on SOFC anodes and meaningful for practical application of DIR-SOFCs.     

Investigation of La0.6Sr0.4Co1-xNixO3-δ (x=0, 0.2, 0.4, 0.6, 0.8) catalysts on solid oxide fuel cells anode for biogas dry reforming

The introduction of catalyst on anode of solid oxide fuel cell (SOFC) has been an effective way to alleviate the carbon deposition when utilizing biogas as the fuel. A series of La_0.6Sr_0.4Co_1-xNi_xO_3-δ (x = 0, 0.2, 0.4, 0.6, 0.8) oxides are synthesized by sol-gel method and used as catalysts precursors for biogas dry reforming. The phase structure of La_0.6Sr_0.4Co_1-xNi_xO_3-δ oxides before and after reduction are characterized by X-ray diffraction (XRD). The texture properties, carbon deposition, CH₄ and CO₂ conversion rate of La_0.6Sr_0.4Co_1-xNi_xO_3-δ catalysts are evaluated and compared. The peak power density of 739 mW cm^?2 is obtained by a commercial SOFC with La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst at 850 °C when using a mixture of CH₄: CO₂ = 2:1 as fuel. This shows a great improvement from the cell without catalyst for internal dry reforming, which is attributed to the formation of NiCo alloy active species after reduction in H₂ atmosphere. The results indicate the benefits of inhibiting the carbon deposition on Ni-based anode through introducing the La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst precursor. Additionally, the dry reforming technology will also help to convert part of the exhaust heat into chemical energy and improve the efficiency of SOFC system with biogas fuel.     

A diesel fuel processor for stable operation of solid oxide fuel cells system: II. Integrated diesel fuel processor for the operation of solid oxide fuel cells

Post-reforming experimental results for the complete removal of light hydrocarbons from diesel reformate are introduced in part I. In part II of the paper, an integrated diesel fuel processor is investigated for the stable operation of SOFCs. Several post-reforming processors have been operated to suppress both sulfur poisoning and carbon deposition on the anode catalyst. The integrated diesel fuel processor is composed of an autothermal reformer, a desulfurizer, and a post-reformer. The autothermal reforming section in the integrated diesel fuel processor effectively decomposes aromatics, and converts fuel into H₂-rich syngas. The subsequent desulfurizer removes sulfur-containing compounds present in the diesel reformate. Finally, the post-reformer completely removes the light hydrocarbons, which are carbon precursors, in the diesel reformate. We successfully operate the diesel reformer, desulfurizer, and post-reformer as microreactors for about 2500 h in an integrated mode. The degradation rate of the overall reforming performance is negligible for the 2000 h, and light hydrocarbons and sulfur-containing compounds are completely removed from the diesel reformate.