AbstractThis research investigates utilisation of coke oven gas (COG) and its impurities (H2S and phenol) in the means of co-electrolysis using a commercially available anode supported solid oxide cell (SOC). SOCs are highly efficient electrochemical energy conversion devices that can potentially reduce greenhouse gas emissions and utilise complex gas mixtures in means of energy production (operation in fuel cell mode) or useful gases production when operating in electrolysis mode. COG was simulated using a mixture of methane and hydrogen (CH4/H2 30/70 vol%). The electrochemical performance of an anode supported button cell was characterised using open circuit potential measurements, current-voltage curves, and electrochemical impedance spectroscopy. The product gas composition was analysed using quadrupole mass spectrometry (QMS). The anode surface and its composition were analysed in ex-situ analysis performed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).
Co-electrolysis of simulated COG with 50 vol% steam achieved a H2 amplification of 119 % and a purity of 91.7 vol%, balanced mainly in carbon dioxide (CO2) and carbon monoxide (CO). Theoretically, this corresponds to a worldwide H2 production from COG of 87.6 Mt, which is in excess of the current global demand for H2 (70 Mt). This was achieved via combination of catalytic steam reforming of CH4 and the water-gas shift reactions and electrochemical processes with catalytic processes accounting for the most of H2 production. The work demonstrates the considerable potential to upgrade COG using SOFC technology, which could enable greater downstream recovery and purification of H2 from an under-utilised industrial waste resource.
Co-electrolysis of simulated coke oven gas with CO2 using an anode-supported solid oxide electrolysis cell was investigated. Use of CO2 as the co-oxidant promoted the catalytic dry reforming of CH4 and reverse water-gas shift (RWGS) reactions to yield synthesis gas mixtures balanced in 11-45 vol% CO2 and with H2/CO ratios in the range 1.1-2.4. Coelectrolysis of COG is a possible route towards synthesis gas production and CO2 utilization that will require advances in SOEC design and the performance and durability of their anode materials.
The effects of tars on utilisation of COG with steam was investigation via the study of addition of phenol in concentrations of 0-100 g m-3. The study has revealed that high levels of steam allowed a catalytic conversion of phenol into hydrogen and carbon dioxide via steam reforming of phenol and water gas shift reaction. Short-term tolerance towards phenol at was achieved as electrochemical processes were not affected at concentrations up to 30 g m-3. Additionally, the hydrogen production and purity were improved by increasing the operating voltage of the cell due to increased presence of O2-ions. Carbon deposition which most likely occurred when operating under phenol concentrations above 75 g m-3 was observed in ex-situ analysis.
Finally, the influence of the presence of 10 ppm H2S on the operation of an SOC fuelled by COG was investigated. H2S was converted to H2 and SO2 via reaction with H2O decreasing its influence on the SOC operation, but also lower current outputs in electrolysis mode due to consumption of H2O. In fuel cell mode the exposure to H2S caused inhibition of SMR and WGS catalytic reactions. In electrolysis mode the highest contents of H2 were detected in the product gases, reaching 86.8 vol% at 1.5 V. Exposure to H2S did not affect the composition of the product gases at 1.2 V as increased oxygen ions flux allowed the elimination of the degradation caused by the presence of H2S. Ex-situ analysis revealed that carbon deposition was likely alleviated via sulphur passivated reforming (SPARG) process, that occurred at tested conditions due to the presence of H2S and H2O. Additionally, no sulfur was observed at all view fields studied, suggesting full removal of sulfur via oxidation.
|Date of Award
|Christian Laycock (Supervisor), Jon Maddy (Supervisor) & Stephen Carr (Supervisor)