Developing Solid Oxide F uel Cell Based Power Plant For Water Treatment Plants: Experimental and System Modelling Studies

Fossil fuels are currently the primary source for electrical power generation, which subsequently increases the rate of greenhouse gas (CO2, CH4) emission. It has been agreed at the Climate Change Conference 2015 in Paris (COP21) to reduce greenhouse gas emissions in order to limit the global temperature increase to less than 2°C compared to pre-industrial era temperature. The GHG (Greenhouse Gas) effect is mostly attributed to methane and carbon dioxide emissions into the atmosphere. In order to reduce the use of fossil fuels and their negative impact on the environment, renewable energy resources have been receiving much attention in recent years. Sanitation systems, centralized Wastewater Treatment Plants (WWTPs) and organic waste digesters give an ample opportunity for resource recovery to produce biogas that contains mainly methane and carbon dioxide. The low conversion efficiency of conventional energy conversion devices like internal combustion engines and turbines prevents biogas from reaching its full potential as over 50% of chemical energy is dissipated. Wastewater treatment is a developed technology from human health and environmental-friendliness points of view. However, from energy aspects, it is still an energy-intensive process step. Wastewaters might contain significant amounts of organic matter and nutrient (nitrogen and phosphorus) compounds. The chemical energy in domestic wastewater is approximately 3.8 kWh.m-3 based on theoretical Chemical Oxygen Demand (COD) of 1 kg m-3. At wastewater treatment plants (WWTPs), collecting and treating wastewater streams need a considerable amount of electricity (0.5 kWh m-3) to reach an acceptable quality of discharge requirements. In a conventional WWTP, nitrogen is removed through nitrification, and biodegradable organic matter is converted to methane in anaerobic digestion. The energy demand at WWTPs could be partially offset by an efficient recovery of nutrient and organic matter from the wastewater stream. Biogas production is an important technology widely applied in Europe. Biogas can be converted to energy through thermal conversion with combined heat and power (CHP) plants. However, the electrical efficiency of the system is limited to 25-30%. In parallel, nitrogen can be removed from wastewater and converted and stored in the form of an ammonia-water mixture from ammonium-rich streams after anaerobic digestion. Solid oxide fuel cell (SOFC) is an energy conversion device that directly converts chemical energy into electrical energy based on electrochemical reactions. SOFC can operate with different types of fuels, especially unconventional or renewable fuels. The efficiency of SOFC is higher compared to conventional combustionbased processes. Therefore, the sustainability of WWTPs can be improved first by a recovery of nutrient and organic material from the wastewater stream and then, replacing the inefficient combustion process with an efficient high-temperature electrochemical reaction in SOFC. Due to the modularity of SOFC, this can be used for a wide range of biogas production capacities at WWTPs. However, the development of SOFC is still facing many challenges, and a better understanding of the constraints is needed. This dissertation aims to provide design concepts and thermodynamic system analysis for the biogas-ammonia fuelled SOFC system at wastewater treatment plants with a focus on achieving a safe operating condition and high electrical efficiencies. Thereupon, extended experimental studies have been conducted in this work on biogas dry and combined reforming. Moreover, the influence of mixing ammonia-water to biogas in SOFC has been experimentally investigated. After indicating the safe operating condition of biogas-ammonia fuelled SOFC, system modelling studies have been carried out in order to design an efficient conceptual biogas-ammonia fuelled SOFC system at wastewater treatment plants. Additionally, a complete biogas SOFC pilot system consists of a gas cleaning unit and an external gas processing system has been designed.Energy Technolog

Direct internal methane reforming in biogas fuelled solid oxide fuel cell; the influence of operating parameters

Internal dry reforming (IDR) of methane for biogas-fed solid oxide fuel cell (SOFC) applications has been experimentally investigated on planar Ni-GDC (cermet anode) electrolyte-supported cells. This study focuses on the effect of CO2 concentration, current density, operating temperature, and residence time on internal methane dry reforming. A single cell is fed with different CH4/CO2 mixture ratios between 0.6 and 1.5. Extra CO2 recovered from carbon capture plants can be utilized here as a reforming agent. The I-V characterization curves are recorded at different operating conditions in order to determine the best electrochemical performance while the power production is maximized, and carbon deposition is suppressed. The outlet gas from the anode is analyzed by a micro gas chromatograph to investigate methane conversion inside the anode fuel channel and to understand its influence on the cell performance. Relatively long-term experiments have been performed for all gas mixtures at 850°C under a current density of 2000 A m−2. The results indicate that when the cell is fed with biogas with an equimolar amount of CH4 and CO2, carbon deposition is prevented, and maximum power density is obtained.Energy Technolog

Thermodynamic analysis of solid oxide fuel cell integrated system fuelled by ammonia from struvite precipitation process

Energy and exergy performance of ammonia fuelled solid oxide fuel cell (SOFC) integrated system in wastewater treatment plants (WWTPs) is evaluated in this study. Ammonia can be recovered through a struvite precipitation process in the form of an ammonia-water mixture (with 14 mol.% ammonia) and used as a carbon-free fuel. A series of experiments has been conducted for SOFC single cell to evaluate the performance with different ammonia-water mixture ratios. An ammonia-SOFC system was modeled in Cycle Tempo for detailed thermodynamic analysis. The heat from the electrochemical reaction in the SOFC and catalytic combustion in an afterburner is used in the struvite decomposition process. However, the generated heat is not sufficient to meet the heat demand of the struvite decomposition reactor. To improve the sustainability of the system in terms of heat demand, the system can be integrated into a heat pump assisted distillation tower, meanwhile, the ammonia concentration of the fuel stream increases. Increasing the ammonia concentration to 90 mol.% increases the energy and exergy efficiencies of the SOFC system. The net energy efficiency of the integrated system with a heat pump assisted distillation tower is 39%, based on the LHV of the ammonia-water mixture.Energy Technology3mE Algemee

A solid oxide fuel cell fuelled by methane recovered from groundwater

This study investigates the feasibility of electricity production in a solid oxide fuel cell using methane recovered from groundwater as the fuel. Methane must be removed from groundwater for the production of drinking water to, amongst others, avoid bacterial regrowth. Instead of releasing methane to the atmosphere or converting it to carbon dioxide by flaring, methane can also be recovered by vacuum stripping and served as a fuel. However, the electrical efficiency of currently used combustion-based technologies fuelled with methane-rich gas is limited to 35% due to the low heating value of the recovered gas (70 mol. % methane) and power derating due to the presence of carbon dioxide (25 mol.%). We propose to use a solid oxide fuel cell to use the methane-rich gas as fuel. Solid Oxide Fuel Cells are fuel-flexible and potentially attain higher electrical efficiencies up to 60%. To this end, specific gas processing, including cleaning and methane reforming, is required to allow for durable operation in a solid oxide fuel cell. We assessed whether electricity could be generated by a solid oxide fuel cell using methane recovered from a full-scale drinking water treatment plant as a fuel. The groundwater had a methane concentration of 45 mg∙L-1, and the recovered gas by vacuum towers contained 70 mol% methane. We used a gas cleaning reactor with impregnated activated carbon to remove hydrogen sulfide traces from the methane-rich gas. Thermodynamic calculations showed that additional steam is required to achieve a high methane reforming. The added steam and the carbon dioxide content in the recovered gas simultaneously contribute to the methane reforming to prevent carbon deposition. The measured open circuit potential corresponded with the theoretical Nernst voltage, implying high methane reforming in the solid oxide fuel cell. The achieved power density of the cell fuelled with the methane-rich gas (mixed with steam) was 27% less than the hydrogen-fuelled cell. Ultimately, 51.2% of the power demand of the plant can be covered by replacing the gas engine in a drinking water treatment with a 915 kW solid oxide fuel cell system fuelled by the methane recovered from the groundwater, while the greenhouse gas emission can be reduced by 17.6%.Energy TechnologySanitary Engineerin

Solid oxide fuel cells (SOFCs) fed with biogas containing hydrogen chloride traces: Impact on direct internal reforming and electrochemical performance

This study is particularly aimed at investigating the influence of hydrogen chloride traces in biogas on direct internal reforming in solid oxide fuel cells (SOFCs). The experiments are performed with simulated biogas containing methane to carbon dioxide ratio of 3:2, the usual average proportion in biogas. To the best of our knowledge, there are no reported studies that investigated the effect of hydrogen chloride on direct internal reforming by clearly establishing the effect of reforming with outlet gas composition measurements. The experiments at SOFC operating temperature of 850 °C reveals no negative effect on reforming or cell performance, with 4, 8, and 12 ppm(v) of hydrogen chloride in biogas. At 800 °C, there is no visible performance degradation, but a negligible amount of methane (∼ 1%) is detected in the anode off gas. Both the reforming and electrochemical performance are marginally affected at 750 °C. Further, post-test analyses (FESEM-EDS, XRD) of the used SOFC reveals no damage to the cell at microstructure level or chlorine poisoning. All the experiments are performed in the context of utilizing the biogas generated from sewage treatment plants in an SOFC system. The reported level of chlorine traces in biogas generated from sewage sludge is < 10 ppm(v) and hence the limit set for experiments is at par with this value.Energy TechnologySanitary Engineerin

The effect of H₂S on internal dry reforming in biogas fuelled solid oxide fuel cells

Internal dry reforming of methane is envisaged as a possibility to reduce on capital and operation costs of biogas fuelled solid oxide fuel cells (SOFCs) system by using the CO2 present in the biogas. Due to envisaged internal dry reforming, the requirement for biogas upgrading becomes obsolete, thereby simplifying the system complexity and increasing its technology readiness level. However, impurities prevailing in biogas such as H2S have been reported in literature as one of the parameters which affect the internal reforming process in SOFCs. This research has been carried out to investigate the effects of H2S on internal dry reforming of methane on nickel-scandia-stabilised zirconia (Ni-ScSZ) electrolyte supported SOFCs. Results showed that at 800°C and a CH4:CO2 ratio of 2:3, H2S at concentrations as low as 0.125 ppm affects both the catalytic and electric performance of a SOFC. At 0.125 ppm H2S concentration, the CH4 reforming process is affected and it is reduced from over 95% to below 10% in 10 h. Therefore, future biogas SOFC cost reduction seems to become a trade-off between biogas upgrading for CO2 removal and biogas cleaning of impurities to facilitate efficient internal dry reforming.Sanitary EngineeringEnergy Technolog

Design, construction, and testing of a gasifier-specific solid oxide fuel cell system

This paper describes the steps involved in the design, construction, and testing of a gasifier-specific solid oxide fuel cell (SOFC) system. The design choices are based on reported thermodynamic simulation results for the entire gasifier- gas cleanup-SOFC system. The constructed SOFC system is tested and the measured parameters are compared with those given by a system simulation. Furthermore, a detailed exergy analysis is performed to determine the components responsible for poor efficiency. It is concluded that the SOFC system demonstrates reasonable agreement with the simulated results. Furthermore, based on the exergy results, the components causing major irreversible performance losses are identified.Energy Technology3mE Algemee