AbstractSolid oxide fuel cells (SOFCs) are highly efficient electrochemical energy conversion devices that can potentially reduce greenhouse gas emissions and revolutionize our energy infrastructures. This research investigates and demonstrates novel and highly efficient methods of utilising renewable biomass-derived gases using SOFC technology with commercially available anode (ASC) and electrolyte supported (ESC) button cells. The utilisation of biohydrogen and biohythane mixtures have been investigated in fuel cell, electrolysis and co-electrolysis modes. Cell electrical performance was characterised using potentiostatic techniques and fuel processing was characterised using online quadrupole mass spectroscopy.
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 SOFC 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 in both ASC and ESC supported cells. The effects of fuel variability on SOC overpotentials and outputs have been established and it is shown that cell performance for ESC and ASC is not significantly affected provided the fuel composition stays within 40-60 vol% H2. The effects of fuel variability are affiliated to the presence of the revere water-gas shift (RWGS) reaction, which takes place simultaneously alongside electrochemical processes. The ASC demonstrated better performance in fuel cell mode with more power being produced compared to an ESC, although ASC was more sensitive to fuel variability. QMS measurements indicated H2O and CO production took place in-situ via the RWGS reaction. Electrical power production in fuel cell mode was predominantly through H2 oxidation, whilst CO was converted in the WGS reaction to regenerate CO2 but did not contribute to electrical power production. In electrolysis mode, CO was produced simultaneously through electrochemical CO2 reduction and the RWGS reaction, H2O is electrochemically reduced to regenerate H2. Different operating conditions such as temperature have been studied and shown to have an effect on the performance and outputs of the cell.
Co-production of energy and useful chemicals using was demonstrated through investigations into the utilisation of biohythane (CH4/CO2/H2 - 60/30/10 vol%) produced from an optimised two-stage anaerobic digestion (AD) process. The gain in energy yield from two-stage AD was supplemented with additional gains in SOFC efficiency due to the presence of H2 in biohythane, giving up to 77% increased electrical energy yields from biomass overall compared with utilisation of biogas from single-stage AD in SOFCs. The results revealed that biohythane production rather than biogas is a highly advantageous route to energy production from biomass. Furthermore, the effects of fuel variability on the electrical performance and fuel processing of the cell operating on biohythane mixtures at different operating temperatures were studied. When H2/CO2 is blended with CH4 to make biohythane, the SOFC efficiency is significantly increased, high SOFC durability is achieved, and there are considerable savings in CH4 consumption. Enhanced electrical performance was due to the additional presence of H2 and promotion of CH4 dry reforming, the reverse Boudouard and reverse water-gas shift reactions. These processes alleviated carbon deposition and promoted electrochemical oxidation of H2 as the primary power production pathway. Substituting 50 vol% CH4 with 25/75 vol% H2/CO2 was shown to increase cell power output by 81.6% at 0.8 V compared with pure CH4. This corresponded to a 3.4-fold increase in the overall energy conversion efficiency and a 72% decrease in CH4 consumption. A 260 h durability test demonstrated very high cell durability when operating on a typical 60/30/10 vol% CH4/CO2/H2 biohythane mixture under high fuel utilisation due to inhibition of carbon deposition. A significant outcome of this work suggests that decarbonising gas grids by substituting natural gas with renewably produced H2/CO2 mixtures and utilising in SOFC technology gives considerable gains in energy conversion efficiency and carbon emissions savings.
Co-electrolysis of biohythane using ASCs was investigated and compared with biohydrogen and biogas (CH4/CO2 60/40 vol%) mixtures, operated with three different co-oxidants (steam, steam/CO2 and CO2). The overall performance and the composition of the output gases have been shown to be very sensitive to the co-oxidant used and by adding H2 to biogas feedstocks improves the overall performance of the cell compared with H2-free biogas.
|Date of Award||2021|
|Supervisor||Christian Laycock (Supervisor), Alan Guwy (Supervisor) & James Reed (Supervisor)|