Journal papers
This paper presents an overview of biogas compositions originating from agriculture and the organic fraction of municipal solid waste. An intensive data compilation was performed from literature, plant data from an EU project (Waste2Watts) and from sampling campaigns at 5 different anaerobic digesters in Switzerland. Besides reporting the major components of biogas i.e. methane and carbon dioxide, the concentration of minor components such as nitrogen and oxygen, as well as trace amounts of sulfur compounds (H2S, mercaptans, sulfides, etc.), silicon compounds (siloxanes, silanes), ammonia, halogenated compounds, and other volatile organic compounds (VOCs) are reported. These trace compounds can present a significant challenge to the energetic use of biogas, specifically in the use of novel, high-efficient processes such as high temperature fuel cells or catalytic fuel upgrading units. H2S and other sulfur compounds are the major concern, as they are abundantly found in agriculture biogas; unlike silicon compounds, which are generally exist in low or undetectable levels.
Solid Oxide Fuel Cells (SOFC) are efficient, modular and fuel-flexible high temperature electrochemical devices. SOFC systems can be coupled to biogas from anaerobic digestion plants to obtain efficient and decentralized CHP systems, maximizing the valorization of biogas in virtuous waste-to-energy schemes. The main challenges for biogas-SOFC plants are related to performance stability and degradation at process level. In this work the performance and stability of an electrolyte supported SOFC single cell (100 cm2) fed with biogas mixtures derived from different integrated biogas-SOFC CHP plant configurations (hot/cold recirculation; UFf 65e85% – obtained from previous simulation work) has been analyzed with an integrated experimental and simulative approach. To support the experimental results, a chemical equilibrium model of the gas conversion processes coupled with the electrochemical conversion route is developed in MATLAB in order to simulate the gas composition at the anode outlet, which is compared and validated with experimental data obtained by Gas Chromatography (GC). Results show that suitable and stable cell performances are obtained while feeding the SOFC samples by biogas (720e800 mV; 0.16e0.2 W/cm2 at 0.25 A/cm2) where the main performance losses are related to steam content – as well as other gas species, deriving from the pre-processing of the biogas. The gas composition and UFf simulation results show good correspondence with the GC data (error range <5% for the matrix gases and <10% for water) highlighting that the SOFC processes under clean biogas can be successfully represented – to a certain extent – by a chemical equilibrium model.
The biogas needs to be reformed before electro-chemical conversion in the solid-oxide fuel cell, which can be promoted efficiently with wise thermal management and reforming conditions. To ensure the system safety and catalysts durability, additional mineral-bearing water and carbon deposition should be avoided. This paper conducted a detailed biogas-SOFC CHP system analysis considering four layouts, featuring hot and cold recirculation of the anode off-gas, partial oxidation and complete internal reforming. The process optimization and sensitivity analysis are performed with the design variables including the recirculation ratio, and external reformer temperature. The anode supported SOFC operates at 800 °C and 0.4 A/cm2 current density. The results show that pre-reforming with hot recirculation and cold recirculation schemes achieve the highest system efficiency between 56% and 63%. The pre-reforming with hot recirculation scheme has a broader self-sufficient water range eliminating the carbon deposition risk at the recirculation ratio of 42–78% and reforming temperature of 400–650 °C. The no pre-reforming with hot recirculation scheme achieves maximum system efficiency of 58% due to the fuel dilution. Moreover, the partial oxidation with hot recirculation scheme maximum efficiency is limited to 58.9%, given that the partial oxidation reaction is less efficient than steam and dry reforming reactions. The proposed system layout could demonstrate the feasibility of biogas-SOFC with different reforming options especially on small scale with high efficiency and optimal thermal integration opportunities.
The increasing penetration of variable renewable energies poses new challenges for grid management. The economic feasibility of grid-balancing plants may be limited by low annual operating hours if they work either only for power generation or only for power storage. This issue might be addressed by a dual-function power plant with power-to-x capability, which can produce electricity or store excess renewable electricity into chemicals at different periods. Such a plant can be uniquely enabled by a solid-oxide cell stack, which can switch between fuel cell and electrolysis with the same stack. This paper investigates the optimal conceptual design of this type of plant, represented by power-to-x-to-power process chains with x being hydrogen, syngas, methane, methanol and ammonia, concerning the efficiency (on a lower heating value) and power densities. The results show that an increase in current density leads to an increased oxygen flow rate and a decreased reactant utilization at the stack level for its thermal management, and an increased power density and a decreased efficiency at the system level. The power-generation efficiency is ranked as methane (65.9%), methanol (60.2%), ammonia (58.2%), hydrogen (58.3%), syngas (53.3%) at 0.4 A/cm2, due to the benefit of heat-to-chemical-energy conversion by chemical reformulating and the deterioration of electrochemical performance by the dilution of hydrogen. The power-storage efficiency is ranked as syngas (80%), hydrogen (74%), methane (72%), methanol (68%), ammonia (66%) at 0.7 A/cm2, mainly due to the benefit of co-electrolysis and the chemical energy loss occurring in the chemical synthesis reactions. The lost chemical energy improves plant-wise heat integration and compensates for its adverse effect on power-storage efficiency. Combining these efficiency numbers of the two modes results in a rank of round-trip efficiency: methane (47.5%) > syngas (43.3%) ≈ hydrogen (42.6%) > methanol (40.7%) > ammonia (38.6%). The pool of plant designs obtained lays the basis for the optimal deployment of this balancing technology for specific applications.
Biomass-to-electricity or -chemical via power-to-x can be potential flexibility means for future electrical grid with high penetration of variable renewable power. However, biomass-to-electricity will not be dispatched frequently and becomes less economically- beneficial due to low annual operating hours. This issue can be addressed by integrating biomass-to-electricity and -chemical via ‘‘reversible’’ solid-oxide cell stacks to form a triple-mode grid-balancing plant, which could flexibly switch among power generation, power storage and power neutral (with chemical production) modes. This paper investigates the optimal designs of such a plant concept with a multi-time heat and mass integration platform considering different technology combinations and multiple objective functions to obtain a variety of design alternatives. The results show that increasing plant efficiencies will increase the total cell area needed for a given biomass feed. The efficiency difference among different technology combinations with the same gasifier type is less than 5% points. The efficiency reaches up to 50%–60% for power generation mode, 72%–76% for power storage mode and 47%–55% for power neutral mode. When penalizing the syngas not converted in the stacks, the optimal plant designs interact with the electrical and gas grids in a limited range. Steam turbine network can recover 0.21–0.24 kW electricity per kW dry biomass energy (lower heating value), corresponding to an efficiency enhancement of up to 20% points. The difference in the amounts of heat transferred in different modes challenges the design of a common heat exchange network.
Fuel cells powered by biogas for decentralised cogeneration of heat and power are an attractive alternative to combustion tech- nologies. However, biogas contains sulfur-based compounds (H2S, COS, DMS, siloxanes), which are harmful to fuel cells. This work was carried out in the framework of the European project Waste2Watts, involving the laboratories of Politecnico di Torino, ENEA, and PSI. The aim is to design and test a flexible and cost-effective cleaning unit to remove impurities for the use of biogas in high-efficiency fuel cell systems. The focus is on small- to medium-sized farms for which deep cleaning of biogas by adsorption materials is a suitable techno-economic solution to avoid intensive gas processing treatments. The ability of commercial adsorption materials (activated carbons, metal oxides, and metal hydroxides) to remove hydrogen sulphide and carbonyl sulphide was tested under different biogas compositions (oxygen and humidity). After evaluating the results, three plant configurations were proposed to optimally utilise the potential of the sorbents. Indeed, the RGM3 sorbent has proven to be an effective solution for removing H2S and COS under humid conditions (50% RH), whilst R7H and R8C sorbents are better suited for removing H2S and COS, respectively, in dry biogas conditions.
Conference papers and abstracts
Solid oxide fuel cell (SOFC) systems show immense potential for efficient biogas utilization, which can significantly reduce carbon emissions by integrating with biogas cleaning units and carbon capture technologies. Despite the significant benefits, the high cost of the integrated system presents a challenge. Therefore, a techno- economic assessment is necessary to evaluate the feasibility of the integrated system and provide guidelines for cost reduction. The ideal capacity for an integrated biogas-fed SOFC power system ranges between 20 kW to 200 kW. Firstly, a comparison of different biogas types and stack technologies is conducted to evaluate the system performance. Secondly, a techno-economic investigation of the feasibility of different system designs is performed by comparing different carbon capture technologies with various biogas cleaning units. The study results can facilitate the optimization of the integrated system’s design and operation and pave the way for large-scale implementation of SOFC technology in the energy industry.
Solid oxide fuel cell (SOFC) system is of great potential for high-efficient biogas utilization. Integrated with carbon capture storage (CCS) and utilization (CCU) technologies, a green power system with low carbon emission will be achieved, which however is of high system cost. In this case, it is necessary to carry out techno-economic assessment to evaluate the feasibility of the integrated system and draw the guidelines for further cost reduction. The preferred capacity for a biogas-fed SOFC power system integrated with CCS and CCU ranges from 20 kW to 200 kW. The comparison of different biogas types and stack technologies is performed first for system performance. Regardless of CCS and CCU parts, the breakdown of levelized cost of electricity (LCOE) is performed to assess the contribution of component costs.
When biogases are directly used as fuels in SOFCs, catalytic reforming of methane into hydrogen and carbon monoxide occurs at the nickel anode. This may expose the anode to localized cooling and high risk of carbon deposition. These issues can be tackled by preprocessing the biogas. Diluting it with steam or CO2 recovered from the SOFC anode off-gas may suppress the risk of carbon deposition whereas reforming it externally can protect the cell from thermal instabilities while offering additional protection against contaminants. Ruthenium has an outstanding catalytic activity with demonstrated coking resistance. Its cost however limits its use. In this work, dry reforming of diluted biogases was performed on a low-ruthenium-exsolution catalyst. An approach by total material balance allowed to determine near 100 % methane conversion at GHSV up to 2000 h-1 while the reaction was close to the computed thermodynamic equilibrium.
The direct conversion of biogas to electricity using solid oxide fuel cells (SOFC) could enable a more efficient exploitation of biomass resources. However, raw biogas needs a number of pretreatment steps that include cleaning, diluting, and pre-reforming in order to protect the SOFC anode material against impurities, coking, and local cooling, respectively. Following theoretical investigations on ideal biogas dilution and reforming, SOFC single cells were operated with several gas mixtures of H2, CO, CH4, H2O, and CO2 simulating dry-reformed and steam-reformed biogas. Results showed that the SOFC performances slowly degraded during the first hours of operation before they stabilized. In a second step, hydrogen sulfide and dimethyl sulfide, two contaminants often found in biogas that may leak through the cleaning and reforming units, were added to the fuel at concentrations up to 5 ppm. Although a fast but limited drop in the operating voltage was observed, only a small fraction of these losses was irreversible and the SOFC rapidly recovered under clean conditions. Short-term and long-term expositions were studied. Online monitoring of the output voltage may serve as a good indicator of the cleaning unit state.
When deploying an SOFC system on a biogas plant, one of the main concerns is the management of sulfur impurities. Sorbent- or reactive beds are usually employed to reduce sulfur concentrations well below 1 ppm. However, during long term operations, there still exists a risk that hydrogen sulfide, dimethyl sulfide (DMS), or other organosulfur compounds leak through the cleaning unit and pre-reformer. It is shown through impedance spectroscopy that even low levels (< 1 ppm) of sulfides degrade the anode charge transfer resistance and deactivates the water gas shift reaction; this in itself acting as indicator for sulfur breakthrough. Deactivation mechanisms take place through adsorption steps which are therefore highly dependent upon the temperature. We show how temperature control can mitigate sulfur-induced degradation and/or help in the regeneration of deactivated anode material. H2S and DMS were used as sulfur impurities which exhibit different interactions with the anode surface. Their specific deactivation mechanisms are discussed based on electroanalytical data and gas chromatography.
The Solid-oxide fuel cell is a highly efficient prime mover for biogas conversion, but a part of biogas needs to be reformulated externally to facilitate the electrochemical conversion, easy control of reforming conditions, and thermal management of the stack. Carbon deposition and external mineralcarrying water should be avoided to ensure the durability of the fuel processor and stack catalysts. This paper investigates four plant layouts with different anode off-gas recirculation schemes and biogas reforming methods: (1) pre-reforming with hot recirculation (HR), (2) pre-reforming with cold recirculation (CR), (3) no pre-reforming and hot recirculation (NR), (4) partial oxidation with hot recirculation (PO). All the schemes feature an electrolyte supported SOFC working at 860°C and 0.23 A/cm2 current density. A sensitivity analysis of the plant efficiency as a function of the Recirculation Ratio (RR) and the Reformer Temperature (RT) is performed. The results show that HR and CR schemes achieve the highest efficiency (58-63%). The HR scheme benefits from the recirculated water and does not require external water for RR > 50% and RT > 600°C; the CR scheme achieves the same result for RR > 80% and RT > 700°C. The optimal RR is within 50 – 80% for the highest system efficiency, as a trade-off between the overall fuel utilization and electrochemistry performance. The RT should be between 600 and 700°C. The HR scheme is the overall best performing if the re-circulator and stack designs do not limit the flow rates at a high RR.