SOFC
Influence of main gas components of biomass-gasification product gas on SOFC performance
One of the main goals of this project is the optimization of SOFC operating conditions in order to increase the overall system efficiency of the proposed BIO-CCHP system. A first step towards achieving this goal was the investigation of the impact of varying main product gas compounds (H2, H2O, CO, CO2 and CH4) on the performance of a SOFC. This investigation was conducted on a single-cell SOFC test bench at Graz University of Technology. Based on a literature research, it was decided to use an electrolyte supported cell with Ni/ceria-based anode at 850°C for a comprehensive parameter study. For each operating point, a detailed electrochemical analysis by the means of electrochemical impedance spectroscopy analysis, a current-voltage characterization and post-mortem analysis was conducted.
Based on the results of these analyses, a high H2/CO ratio besides a H2O/CH4 ratio larger than 1 as well as a low CO2 content could be determined to have a performance-increasing effect on the cell when operated at 850°C. Product gas from steam gasification meets these requirements to a larger extent than product gas from air gasification. Therefore, a subsequent 500 h stability experiment was conducted using a representative synthetic steam gasifier product gas in order to clarify if re-oxidation of anodic nickel occurs, a possible risk of the higher steam content (36 vol%) in steam gasifier product gas according to literature. This risk could not be confirmed. Instead, 500 h of stable fuel cell operation was demonstrated without performance degradation as demonstrated in Fig. 1. Moreover, no signs of re-oxidized nickel were found on the cell during post mortem analyses.
Concluding, valuable results for the understanding of beneficial gas component ratios could be generated. Moreover, the operation of a promising cell design (Ni/ceria-based electrolyte supported cell) with steam gasifier product gas could be demonstrated at 850°C. These outcomes provided the basis for further experimental and numerical works conducted within this project at Graz University of Technology. Results were presented at the International Conference for Polygeneration Strategies (ICPS) in November 2019 in Vienna and summarized in the following journal paper:
G. Pongratz et al., “Investigation of solid oxide fuel cell operation with synthetic biomass gasification product gases as a basis for enhancing its performance,” Biomass Conv. Bioref., vol. 11, no. 1, pp. 121–139, Feb. 2021, doi: 10.1007/s13399-020-00726-w.
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a) current voltage correlations |
b) EIS spectra at 300 mA/cm2 |
Fig. 1 Current-voltage-curves a) and impedance spectra b) of cell before and after 500 h stability experiment with synthetic product gas typical of fluidized bed gasification with steam, containing 36 vol% H2O.
H2S-induced short-term cell degradation and regeneration
H2S is a known catalyst poison that deactivates active sites of nickel-based fuel cell anodes, which consequently results in the inhibition of reactions and a degradation of the fuel cell performance. In a gasification-based biomass-to-power system, like investigated in this project, the removal of H2S from the product gas before entering the fuel cells is therefore crucial to ensure their stable operation. However, a reduced efficiency or malfunction of desulfurization units might result in a short-term exposure of SOFCs to H2S, the most common sulfur compound released during biomass gasification. The severity of this exposure on the cell performance and its course in time were investigated in an experimental campaign using two industrial-relevant cell types with different sulfur tolerance (electrolyte supported cell with Ni/ceria-based anode; anode supported cell with Ni/zirconia-based anode) at temperatures relevant for gasifier-SOFC coupling (750 and 800°C).
The cells were exposed to a synthetic gas mixture representing the product gas of a wood steam gasifier containing up to 10 ppmv H2S until cell performance degradation stabilized. Then, H2S was removed from the gas mixture and the regeneration behavior was observed as exemplarily shown in Fig. 1. Full performance regeneration could be observed at 750 and 800°C for both cells after poisoning with to 10 ppmv H2S, even for the cell type with less sulfur tolerant anode (Ni/zirconia-based). Moreover, a maximum performance degradation of 7.6% could be observed (see Fig. 2b), whereby the anode supported cell showed a much lower degradation rate than the electrolyte supported cell (see Fig. 2a and c). The high H2O content of 36 vol% in the synthetic gas mixture representing wood steam gasification possibly contributed to a lower degradation and better regeneration of the cells. Steam gasifiers might therefore be more suitable for the coupling with SOFCs than air gasifiers under the aspect of sulfur tolerance.
Using the results observed it is possible to identify if degradation based on sulfur poisoning occurs during SOFC operation, which can be used for the development of methods to detect sulfur poisoning at an early stage in order to avoid major efficiency losses. Thus, it is possible to take appropriate counteractions and prolong the lifetime of the system under operation. Results were presented at the 14th European Fuel Cell Forum (EFCF) in October 2020 in Lucerne and published in the following journal paper:
G. Pongratz et al., “Analysis of H2S-related short-term degradation and regeneration of anode- and electrolyte supported solid oxide fuel cells fueled with biomass steam gasifier product gas,” Energy, vol. 218, p. 119556, Mar. 2021, doi: 10.1016/j.energy.2020.119556.
Considering the higher expectable cell performance when using steam gasifier product gas in comparison to air gasifier product gas due to its beneficial gas component ratios (discussed in article “Influence of main gas components of biomass-gasification product gas on SOFC performance”) besides a possibly higher sulfur tolerance of cells when operated with steam gasifier product gas, it was decided to foster further experimental works on the coupling of steam gasifiers with SOFCs, aiming at real couplings as presented in the article “Operating limits for real gasifier-SOFC coupling”.
Fig. 1 Testing procedure with analysis parameter. Example: Ni/zirconia-based anode supported cell at 750°C, 5 ppmv H2S.
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Fig. 2 Decrease of cell voltage for H2S concentrations of 1, 5 and 10 ppmv after 1 h of degradation (a) and at steady state (b) after Dtpois (c).
Operating limits for real gasifier-SOFC coupling
The demonstration of real gasifier-SOFC coupling is crucial to bring BIO-CCHP systems using SOFCs as power generators closer to market maturity. In the past, gasifier-SOFC coupling could already be demonstrated for more than 100 h without performance degradation of the used cells or the occurrence of carbon deposition within the cell anode or the periphery. However, these experiments were predominantly relying either on the almost complete avoidance of tars or on a large steam content and consequently steam-to-carbon ratio (SCR) in the product gas (SCR>5). The complete removal of tars from a biomass gasification product gas is an error-prone and cost-intensive task, whereby high steam contents significantly reduce the achievable power output. For this reason, the demonstration of SOFC operation with real product gas showing a lower SCR and the presence of tars are of high value for the scientific community due to a large potential in increasing the cell performance besides reducing gas cleaning requirements. This would significantly contribute to reduce operational- and capital expenditure.
For this reason, a gasifier-SOFC coupling test bench was developed at Graz University of Technology in order to investigate the operation of SOFCs with product gas from steam gasification with a more industrial-relevant SCR (approx. 2) and varying tar contents (2.8-4.8 g·Nm-3 gravimetric tars), a novelty for the scientific community. An extensive experimental campaign was conducted in which changes in cell performance, anode- and contact mesh material morphology and fuel cell tar reforming capability were investigated for industrial-relevant cell types at 850°C and 800°C. Moreover, a focus was laid on the understanding of the quantity and type of solid carbon that was formed during the experiments as well as on the amount and composition of tars entering- and leaving the cell. Fig. 1 shows the test bench developed for gasifier-SOFC coupling experiments, whereby a scheme of the test bench is depicted in Fig. 2. Heavy tars were partially removed in some tests in a condensation unit which resulted in a gravimetric tar content of 2.8-3.7 g·Nm-3 in the product gas, whereby raw gas experiments were conducted with a gravimetric tar content ranging from 4.6 to 4.8 g·Nm-3.
Four coupling experiments were conducted for up to 60 h using the product gas from an in-house steam gasifier, whereby the product gas showed a more industrial-relevant SCR of 2 and a typical tar content for fluidized bed gasification. This could not be achieved in the past, so the following conclusions can be drawn for further consideration of optimal operating conditions for gasification-SOFC coupling: Successful coupling without performance degradation could be demonstrated at 850°C for an electrolyte supported cell with Ni/ceria-based anode (higher tar tolerance) and an anode supported cell with Ni/zirconia-based anode for 30 h when heavy tars were partially reduced from the product gas. The reduction of heavy tars, although modest in the percentage of reduction of the total tar content, proved to be extremely relevant to avoid changes in the anode morphology and consequently to avoid performance degradation. At 850°C, an SCR of 2 might be sufficient to avoid carbon deposits on cell anodes and to ensure a graphitic structure of deposits on the cell support thus reducing the risk of fast blockages in the flow channels. Moreover, structural degradation of zirconia-based anodes might be avoided despite their known lower tolerance to tars in comparison to ceria-based anodes. At 800°C, successful operation of an anode supported cell with Ni/zirconia-based anode could also be demonstrated for 30 h. Although heavy tars were also reduced, a significantly larger quantity of carbon deposition could be observed besides a more pyrolytic structure of the deposited carbon. Thus, the addition of steam or a more intensive reduction of the tar content might be required measures to ensure stable long-term fuel cell operation at 800°C or below, as favorable for a facilitated heat recovery in biomass-to-power systems. Operating boundaries at these temperatures have to be determined in future works. Fuel cell operation with raw product is not recommended as heavy tars significantly increase the risk for nickel dusting in contact meshes and the cell anode. Results were summarized in a journal paper which is currently under review.
This experimental study did not consider the utilization of product gas from air gasification as it was discussed to be less beneficial for the utilization in SOFCs (less expectable performance output and a possibly lower sulfur tolerance of the cells) in the previous works of the project (see articles “Influence of main gas components of biomass-gasification product gas on SOFC performance” and “H2S-induced short-term cell degradation and regeneration”). However, air gasifiers show a large potential for the future usage in smaller-scale BIO-CCHP systems due to their lower complexity and therefore investment costs. Therefore, the quantification of performance differences between the utilization of steam- and air gasifier product gas in SOFCs were discussed in the last period of the project based on the result of a numerical study, which is presented in the article “Performance- and carbon deposition risk evaluation for coupling air- and steam gasifiers with SOFC via CFD simulations”.
Fig. 1 Test bench for gasifier-SOFC coupling experiments
Fig. 2 Scheme of test bench for gasifier-SOFC coupling
Performance- and carbon deposition risk evaluation for coupling air- and steam gasifiers with SOFC via CFD simulations
As discussed in the previous works conducted in this project (see articles “Influence of main gas components of biomass-gasification product gas on SOFC performance”, “H2S-induced short-term cell degradation and regeneration” and “Operating limits for real gasifier-SOFC coupling”), product gas from steam gasification is very promising for the usage in SOFCs. In comparison to product gases from air gasification, higher fuel cell performance can be expected due to the larger H2/CO ratio and lower CO2 content. Besides, the higher steam-to-carbon ratio (SCR) of approximately 2 reduces the thermodynamic risk for carbon deposition within the cell anode. However, fixed bed air gasification is an established technology in smaller-scale biomass-to-power systems due their lower complexity in comparison to fluidized bed gasifiers, which are often used for medium scale systems. Therefore, using air gasifier product gas in SOFCs is also very promising despite a lower expectable cell performance and higher carbon deposition risk.
This work aimed for a quantification of differences in cell performance regarding the achievable cell power output and electrical efficiency when using product gas from steam and air gasification. A sub-system of a biomass-to-power plant with a focus on the SOFC module was considered in order to calculate the net SOFC performance and additional heat demands (see Fig. 1). 2D-CFD models of two industrial-relevant cell designs (an anode supported cell (ASC) with Ni/zirconia-based anode and an electrolyte supported cell (ESC) with Ni/ceria-based anode) were developed with an implemented detailed mechanism for methane steam reforming. The CFD modelling approach allowed to resolve local species concentrations within the fuel cell electrodes and therefore a more accurate determination of the fuel cell performance. Moreover, carbon adsorbed on the reactive anode surface (C(s)) could be modeled with the detailed kinetic mechanism which allowed a more reliable prediction of the carbon deposition risk than approaches based on thermodynamic equilibrium calculations, as C(s) is claimed to be a precursor for the formation of solid carbon in literature. Required steam dilution rates for an air gasifier product gas in order to minimize the carbon deposition risk could be estimated. Simulations were conducted for a variety of fuel cell operating points relevant for gasifier-SOFC coupling for air- and steam gasifier product gas. Based on the obtained results, promising gasifier-SOFC configurations and fuel cell operating conditions could be defined.
ASCs with Ni/zirconia-based anode can be recommended for the usage with air gasifier product gas in small scales. A generally lower carbon surface coverage could be demonstrated even at a SCR lower than for the investigated steam gasifier product gas. This could be attributed to the lower CH4 content of the gas mixture from air gasifiers, which allows the use of Ni/zirconia-based anodes which are less tolerant to carbon deposition. The lower risk for carbon deposition from the main gas components besides the generally lower content of tars released during air gasification allows the operation of the SOFC unit at 750°C, which is beneficial for the thermal integration of the fuel cell unit and its lifetime. Besides, the reduction of the cell efficiency for ASCs is limited with lower temperatures. At this temperature of 750°C, steam dilution is required to increase the SCR of the air gasifier product gas only to around 1.5 in order to minimize the risk for carbon deposition.
As mentioned before, the usage of product gas from steam gasification offers a generally higher electrical efficiency and power output in SOFCs due to the higher LHV and H2/CO ratio. Therefore, the use of steam gasification product gases is recommended at medium scales. However, a higher risk for carbon deposition was determined in comparison to the usage of air gasifier product gas which is why a cell temperature of 850°C is recommended for this product gas, eventually with the usage of more resistant ESCs with Ni/ceria-based anodes. Nevertheless, both recommended gasifier-SOFC configurations (small scales: air gasifier at 750°C with ASC and Ni/zirconia-based anode; medium scales: steam gasifier at 850°C, eventually with ESC and Ni/ceria-based anode), showed a comparable power output and total heat demand for air/fuel pre-heating and steam generation. The slightly lower electrical net efficiency achievable with air gasification might be compensated by a facilitated heat recovery due to the lower possible cell temperature. Concluding, both configurations are promising for the implementation in gasification-based biomass-to-power systems, which should be confirmed via long-term experimental investigations in the future and detailed techno-economic analysis of whole biomass-to-power systems. Results were summarized in a journal paper which is currently under review. Besides, the use of a steam gasifier product gas with a SCR of around 2 and a moderate tar content could be demonstrated in real couplings lasting up to 60 hours, where degradation could be hindered with cell temperatures of 850°C and the reduction of the heavy tar content, as described in the article “Operating limits for real gasifier-SOFC coupling”.
Fig. 1 Scheme of the biomass-to-power sub system considered for the performance- and carbon deposition risk analysis of the SOFC unit.
Summary of the work at IEn and MTF including long-term measurements of the SOFC stack using fuelled with purified syngas from gasifier at the Institute of Power Engineering (IEn)
Within the BIOCCHP project, polish consortium (IEn and MTF) realized several tasks focused on improving the integration between biomass gasifier and solid oxide fuel cell stack. In the first step, different mediums for gasification process were analysed. Air, which is commonly used in similar setups, was replaced with air enriched with oxygen (40% mol.) and water steam. Conducted experiments showed the improvement in gasifier performance and production of gases with properties causing lower risk of potential carbon deposition in the fuel cells. Proper control strategy for the gasifier in such conditions was determined using CFD model of the reactor. Next, realized tasks included experimental studies of a mobile purification system in conjunction with various biomass gasification installations, taking into account the types of substances poisoning the catalysts, i.e. solid particles, sulphur, alkali metals, tar compounds or chlorine compounds, as well as the effect of water and hydrogen content in the gas, which can significantly differ between different gasification technologies. For the solid oxide fuel cells, IEn objectives were to determine the optimal operating conditions of the stack coupled with the gasification reactor in order to obtain the conversion efficiency above 40%. Within the project, multiple 5-cell and shorter SOFC stacks were constructed and investigated experimentally in various conditions. Realized work was focused on minimization of degradation of SOFC cells fuelled with various gases generated in gasification reactors, with particular emphasis on less known pollutants such as ethene, ethane and ammonia traces. In chosen experiments, it was decided to verify the conditions of soot deposition in the anodic part of the stack monitoring its influence at stack level.
Fig. 1 Long-term measurements of the SOFC stack using fuelled with purified syngas from gasifier. Experiments were performed at the Institute of Power Engineering
For the techno-economic analysis, main tasks were focused on carrying out optimization of costs and efficiency of CCHP installations for various boundary conditions, including local technological and financial conditions for the reference countries. The analysis took into account the impact of delivered fuels, power of the installation, integration with the heating system and usage of ORC system with an absorption chiller. To summarize, all works done by the polish part of the project consortium were realized in accordance with the BIOCCHP project plans and assumptions resulting in a big step forward towards developing of biomass-fuelled, SOFC-based multi-cogeneration systems.
