Effect of operating parameters on performance of an integrated biomass gasifier,solid oxide fuel cells and micro gas turbine system

An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and micro gas turbine has been investigated by thermodynamic model. A zero-dimensional electrochemical model of SOFC and one-dimensional chemical kinetics model of downdraft biomass gasifier have been developed to analyze overall performance of the power system. Effects of various parameters such as moisture content in biomass, equivalence ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis.It is found that char in the biomass tends to be converted with decreasing of moisture content and increasing of equivalence ratio due to higher temperature in reduction zone of gasifier. Electric and combined heat and power efficiencies of the power system increase with decreasing of moisture content and increasing of equivalence ratio, the electrical efficiency of this system could reach a level of approximately 56%.Regarding entire conversion of char in gasifier and acceptable electrical efficiency above 45%, operating condition in this study is suggested to be in the range of moisture content less than 0.2, equivalence ratio more than 0.46 and mass flow rate of biomass less than 20  kg h⁻¹.     

High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas

This paper presents the results from a 150 h test of a commercial high temperature single planar solid oxide fuel cell (SOFC) operating on wood gas from the Viking two-stage fixed-bed downdraft gasifier, which produces an almost tar-free gas, that was further cleaned for particulates, sulphur and tar traces. The chosen SOFC was electrolyte supported with a nickel/gadolinium-doped cerium oxide (Ni-GDC) anode, known for its carbon deposition resistance. Through humidification the steam to carbon ratio (S/C) was adjusted to 0.5, which results in a thermodynamically carbon free condition at the SOFC operating temperature T = 850 °C. The cell operated with a fuel utilisation factor (U_f) around 30% and a current density of 260 mA cm⁻² resulting in an average power density of 207 mW cm⁻². Throughout the duration of the test, only a minor cell overpotential increase of 10 mV was observed. Nevertheless, the V–j (voltage–current density) curves on H₂/N₂ before and after the wood gas test proved identical. Extensive SEM/EDS examination of the cell's anode showed that there was neither carbon deposition nor significant shifts in the anode microstructure or contamination when compared to an identical cell tested on H₂/N₂ only.     

Process simulation of a SOFC and double bubbling fluidized bed gasifier power plant

The development of reliable fuel cells power plant based on renewable fuels stands out as one of the promising energy systems solutions for the future. Indeed fuel cells can increase the efficiency and the cleaning of the electrical energy production from renewable fuels. Process simulations of advanced power plants fed by low cost renewable fuels like biomass waste are a key step to develop renewable resources based on high temperature fuel cells applications. The aim of this work is to predict the component behaviour of a specific power plant mainly composed of a small indirectly heated gasifier and a Solid Oxide Fuel Cell (SOFC) and fed by chestnut coppice, waste available in great quantity in Central Italy, as well as in several other European regions. The plant's thermodynamic behaviour is analysed by means of the process simulator CHEMCAD© in which particular models for the SOFC and the gasifier have been developed in FORTRAN by the authors and then interfaced to commercial software. The results of the predictive model are presented and discussed, showing the possibility of an extremely interesting “carbon neutral” small plant configuration with high electrical and global efficiency exclusively based on the use of low cost renewable resources.     

Process simulation of a hybrid SOFC/mGT and enriched air/steam fluidized bed gasifier power plant

The aim of this work is to experimentally and numerically analyze the performance of a integrated power plant composed by a steam oxygen fluidized bed biomass gasifier fed by woods, a Solid Oxide Fuel Cell (SOFC) and a micro Gas Turbine (mGT). The numerical analysis is carried out by using ChemCAD software. In particular, SOFC and gasifier were modeled using proper developed Fortran subroutines interfaced to the basic software. The adopted SOFC model was already validated by the authors in previous works, while the gasifier model was here developed and validated by means of experimental activities carried out by using a bench scale gasifier. Different compounds (Benzene, Toluene, Naphthalene, Phenols) were chosen to analyze the tar evolution in the gaseous stream during the gasification process. Hot gas cleaning (based on catalytic ceramic filter candles inserted in the freeboard of the gasifier – UNIQUE concept) was adopted to remove tar and particulates from the fuel hot gas stream. Different moisture contents in the range between 10 and 30% (i.e. in a deviation of 10% around the usual wood moisture content of 20%) were numerically simulated as well as the degree of purity of the oxygen utilized in the power plant (between 25% and 95%, the rest being N₂). The power requirement for pure oxygen production leads to a reduction of the electrical efficiency of the whole power plant. For this reason, a sensitivity analysis was conducted to find the optimal operation conditions in order to maximise the syngas (H₂, CO) content in the produced gas, while maintaining a high overall electrical efficiency.     

Mechanical reliability and durability of SOFC stacks. Part I : Modelling of the effect of operating conditions and design alternatives on the reliability

Electrochemical and mechanical aspects in solid oxide fuel cell stack must be understood to meet the reliability targets for market implementation. This study presents a stack modelling framework that combines thermo-electrochemical models, including degradation and a contact finite-element thermo-mechanical model. It considers rate-independent plasticity and creep of the component materials and proposes periodic boundary conditions to model the stacking of repeating units. This Part I focuses on the effects of the operating conditions and design alternatives.     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.     

Study of a renewable biomass fueled SOFC: The effect of catalysts

This study demonstrated the feasibility of a tubular solid oxide fuel cell (SOFC) operating on fuels gasified from a carbon-bed containing a renewable biomass. Coconut shell carbon was used as the renewable biomass for the gasification to power the SOFC at 800 °C. The study was particularly focused on the effect of catalysts on the efficiency of the gasification process, thus the performance of tubular SOFC powered by these gasified fuels. SOFCs with catalysts such as Fe₂O₃ and K₂CO₃ added coconut shell carbon showed higher power density compared to that without catalyst. The improved cell performance can be ascribed to a promoted carbon conversion rate by the catalysts, and consequently higher concentration of gaseous fuels.     

Evaluation of cost reduction potential for 1 kW class SOFC stack production: Implications for SOFC technology scenario

Recently, a commercial version of a residential solid oxide fuel cell (SOFC) system with a flat tubular cell has been developed. However, the system cost still remains very high, which is a barrier to its widespread use. In this study, the potential for cost reductions in SOFC stack production was investigated in order to contribute to the viability of the widespread use of such residential SOFC systems in future. A cost analysis of 700 W SOFC stack production based on a process integration modeling was conducted. The present bottom–up approach enabled us to perform a sensitivity analysis with a variety of parameters in terms of cell design, the production process and cell performance. This allowed us to investigate the effects of these factors on the production cost, thereby revealing the quantitative impact of each technological improvement on the cost reduction potential. The present analysis also revealed innovation pathways which could result in technology scenarios where residential SOFC systems could reach a break-even point in comparison with the baseload electricity cost. The analysis of the cost reduction potential presented here provides a useful viewpoint for developing a research strategy for state-of-the-art SOFC technology.     

Thermodynamic analysis of a proton conducting SOFC integrated system fuelled by different renewable fuels

This work proposes a power generation system consisting of steam reformer and SOFC–H⁺ fuelled by different types of fuel, i.e., ethanol, glycerol and biogas. The performance analysis of integrated system is performed based on thermodynamic calculation through Aspen Plus simulator. The total of the Gibbs free energy minimization is used to determine product composition at equilibrium. The electrochemical model not only considers all voltage losses but also includes the effect of current leakage as a result from the electrolyte used. Considering the operating condition of steam reformer, it is found that the gas product contains the highest amount of hydrogen without the carbon formation when reformer is operated at 973 K with steam to carbon ratio of 1. In addition, the simulation results show that the SOFC–H⁺ operated at 973 K and 1 A/cm² can provide a suitable compromise between system performances and exhaust gas composition. The use of glycerol reformate has the highest cell and system efficiencies and fuel utilization compared to the others. In addition, the integrated system fuelled by glycerol can release low CO amount whereas there is more heat provided to the surrounding. Therefore, it can be concluded that glycerol is suitable renewable fuel for SOFC–H⁺ integrated system.     

Weight analysis on geometric parameters of ejector under high back pressure condition of SOFC recirculation system

With advantages of no parasitic energy consumption, small size, and low noise, ejector is a promising choice for the solid oxide fuel cell (SOFC) anode gas recirculation system. However, it is difficult to design an ejector with good performance under the high back pressure condition of the SOFC system. In this paper, weight analysis on key geometric parameters of ejector is carried out based on the result of an experimentally validated ejector simulation model. Four main geometric parameters that have the most significant effect on ejector performance, namely the ejector diameter ratio (D_r), mixing chamber length (L_m), diffuser length (L_d), and nozzle outlet position (NXP), are analyzed in detail. The D_r has a decisive influence on the momentum exchange in the mixing phenomenon between the primary flow and secondary flow in the mixing chamber where the normal shock position changes accordingly. The L_m mainly affects the intensive mixing flow which leads the normal shock to appear prematurely. The L_d should be long enough for boosting back pressure and reducing the effect on the mixing process. The NXP has no effect on the normal shock position. The results show that the critical back pressure increases with the rise of normal shock position and the impact weight of L_m, L_d and NXP can be treated as 21∶10∶1 approximately and the D_r is thought to be a decisive factor. This weight analysis method will be helpful for designing ejectors used in the high back pressure condition of the SOFC anode recirculation system.     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

Effect of operating conditions and gas flow patterns on the system performances of IIR-SOFC fueled by methanol

Mathematical models of an indirect internal reforming solid oxide fuel cells (IIR-SOFC) fueled by methanol were developed to analyze the thermal coupling of the internal endothermic steam reforming with exothermic electrochemical reactions and predict the system performance. The simulations indicated that IIR-SOFC fueled by methanol can be well performed as autothermal operation, although slight temperature gradient occurred at the entrance of the reformer chamber. Sensitivity analysis of five important parameters (i.e. operating voltage, reforming catalyst reactivity, inlet steam to carbon ratio, operating pressure and flow direction) was then performed. The increase of operating voltage lowered the average temperature along the reformer chamber and improved the electrical efficiency, but it oppositely reduced the average current density. Greater temperature profile along the system can be obtained by applying the catalyst with lower reforming reactivity; nevertheless, the current density and electrical efficiency slightly decreased. By using high inlet steam to carbon ratio, the cooling spot at the entrance of the reformer can be reduced but both current density and electrical efficiency were decreased. Lastly, with increasing operating pressure, the system efficiency increased and the temperature dropping at the reformer chamber was minimized.     

Enhanced performance of counter flow SOFC with partial internal reformation

We study a counter-flow solid oxide fuel cell system and consider the challenges faced in minimizing thermal variations from the nominal operating conditions for a reasonable range of power tracking. Blower dynamics, reformer transport delays, spatial distribution of the heat generated and the resulting thermal response are among the issues considered. A novel approach, relying on partial internal reformation of the feedstock is proposed as a remedy to maintain a strong level of power tracking with minimal thermal stress to the fuel cell.     

High performance solid oxide fuel cell operating on dry gasified coal

A fluidized coal bed-solid oxide fuel cell (FB-SOFC) arrangement is employed for efficient conversion of dry gasified coal into electricity at 850 °C. It consists of an anode-supported tubular solid oxide fuel cell of 24 cm² active area coupled to a Boudouard gasifier. A minimally fluidized bed of low sulfur (0.15 wt%) Alaska coal is gasified at 930 °C by flowing CO₂ to generate CO. The resulting CO fuel is oxidized at the Ni/YSZ cermet anode. The highest cell power density achieved is 0.45 W cm⁻² at 0.64 V with 35.7% electrical conversion efficiency based on CO utilization. This power density is the highest reported in the literature for such systems and corresponds to a total power generation of 10.8 W by this cell. Similarly, 48.4% is the highest conversion efficiency measured at a power density of 0.30 W cm⁻² and 0.7 V. The open circuit voltages are in good agreement with values expected based on thermodynamic data.     

Energetic analysis of SOFC co-generation system integrated with EV charging station installed in multifamily apartment

The authors propose a solid oxide fuel cell (SOFC) co-generation system integrated with an electric vehicle (EV) charging station. To examine the feasibility of the proposed system, the authors numerically simulated the operation pattern of the proposed system, using the electric and thermal demands measured at a multifamily apartment from May 2003 to April 2004. Based on the simulation results, the authors calculated the overall efficiency of the system as well as the primary energy saving rate. The available electric energy for EV charging was also evaluated for the case of the proposed system installed in the multifamily apartment. As a result, it is made clear that the annually averaged overall efficiency reaches about 77% (LHV) and the expected primary energy saving rate exceeds 30% throughout the year. In addition, it is also found that sufficient amount of electric energy for EV charging can be obtained from the proposed system. Therefore, it can be concluded that the proposed SOFC system can successfully combine electrical/thermal energy co-generation and electric vehicle charging while maintaining high energy efficiency and good matching to energy consumption patterns.     

Numerical investigation of different loads effect on the performance of planar electrode supported SOFC with syngas as fuel

Reducing greenhouse gas emission such as carbon dioxide is the goal of each country. As a developing country with coal as the dominant energy source, China confronts the pressure of saving energy and reducing emission by using coal efficiently and cleanly. Integrated gasification fuel cell (IGFC) hybrid power generation system is one of optimal system for clean coal utilization. In this work, the three-dimensional mathematical models for planar solid oxide fuel cell (SOFC) with syngas as fuel was constructed, and its performance was numerically investigated at different loads. Under all calculating conditions, the optimal power density is obtained at current density of 4700 A m⁻², where the output voltage is 0.57 V and the power density is 2671.01 W m⁻². With the increasing of current density, the temperature increase as well. And also the difference of max- and min-temperature in SOFC enhances. But the ohmic over-potential changes unobvious. Furthermore, the change rate of species is nearly linear with the increment of current density.     

The importance of fuel variability on the performance of solid oxide cells operating on H2/CO2 mixtures from biohydrogen processes

Biologically produced mixtures of H₂ and CO₂ (biohydrogen) from processes such as dark fermentation or photo-fermentation are versatile feedstocks which can potentially be utilised in solid oxide cell (SOC) devices. In this work, solid oxide electrolysis of biohydrogen has been investigated for the first time and is compared directly with fuel cell mode utilisation. The performance and fuel processing of SOCs utilising biohydrogen have been characterised in greater detail than has been achieved previously through the use of experiments which combine electrochemical techniques with quadrupole mass spectrometry (QMS). The effects of fuel variability on SOC overpotentials and outputs have been established and it is shown that cell performance is not significantly affected provided the fuel composition stays within 40–60 vol% H₂. QMS measurements indicate H₂O and CO production takes place in-situ via the reverse water-gas shift (RWGS) reaction. Electrical power production in fuel cell mode is predominantly through H₂ oxidation, whilst CO is converted in the WGS reaction to regenerate CO₂ but does not contribute to electrical power production. In electrolysis mode, CO is produced simultaneously through electrochemical CO₂ reduction and the RWGS reaction; H₂O is electrochemically reduced to regenerate H₂.     

The effect of IGFC warm gas cleanup system conditions on the gas–solid partitioning and form of trace species in coal syngas and their interactions with SOFC anodes

The U.S. Department of Energy is currently working on coupling coal gasification and high temperature fuel cell to produce electrical power in a highly efficient manner while being emissions free. Many investigations have already investigated the effects of major coal syngas species such as CO and H₂S. However coal contains many trace species and the effect of these species on solid oxide fuel cell anode is not presently known. Warm gas cleanup systems are planned to be used with these advanced power generation systems for the removal of major constituents such as H₂S and HCl but the operational parameters of such systems is not well defined at this point in time. This paper focuses on the effect of anticipated warm gas cleanup conditions has on trace specie partitioning between the vapor and condensed phase and the effects the trace vapor species have on the SOFC anode. Results show that Be, Cr, K, Na, V, and Z trace species will form condensed phases and should not effect SOFC anode performance since it is anticipated that the warm gas cleanup systems will have a high removal efficiency of particulate matter. Also the results show that Sb, As, Cd, Hg, Pb, P, and Se trace species form vapor phases and the Sb, As, and P vapor phase species show the ability to form secondary Ni phases in the SOFC anode.     

Effects of PEMFC operating parameters on the performance of an integrated ethanol processor

In this paper the performance of a complete fuel cell system processing ethanol fuel has been analyzed as a function of the main fuel cell operating parameters. The fuel processor is based on the steam reforming process, followed by high- and low-temperature shift reactors, and carbon monoxide preferential oxidation reactor, which are coupled to a polymeric fuel cell (PEMFC). The goal was to analyze and improve the fuel cell system performance by simulation techniques. PEMFC operation has been analyzed using an available parametric model, which was implemented within HYSYS environment software. Pinch Analysis concepts were used to investigate the process energy integration and determine the maximum efficiency minimizing ethanol consumption. The system performance was analyzed for the SR-12 Modular PEM Generator, the Ballard Mark V fuel cell and the BCS 500 W stack. The net system efficiency is dependent on the required power demand. Efficiency values higher than 50% at low loads and less than 30% at high power demands are computed. In addition, the effect of fuel cell temperature, pressure and hydrogen utilization was analyzed. The trade-off between the reformer yield and the fuel cell performance defines the optimal operation pressure. The cell temperature determines operating zones where the water, involved in the reforming reactions, can be produced or demanded.