| dc.description.abstract | The present research addresses the complexities involved in the use of syngas as a fuel in a Solid Oxide Fuel Cell (SOFC). Solid oxide fuel cell is an efficient, clean direct energy conversion device. Being a fuel flexible device, it can potentially be fuelled with syngas generated from a biomass gasification system. Typically, syngas generated from thermochemical conversion of biomass has a mixture of gases and some contaminants. The composition and the level of contaminants vary with the reactor configuration. Biomass gasification generates a combustible gaseous fuel mixture consisting of CO, H2, CH4 and CO2, trace compounds like H2S, HCl, HCN, NH3 and condensable hydrocarbons like “tar”. The current research focuses on a twofold approach; establishing protocols for the characterization of syngas obtained from biomass gasification, and the performance evaluation of SOFC single cell for a range of gas compositions. Even though the challenges involved in using a carbonaceous gaseous species as fuel in SOFC have been addressed, the issue of carbon deposition on SOFC anodes is a research focus, globally.
To estimate the condensable hydrocarbons like tar, a reliable, generic method for analyzing tar has been developed, using Cold Solvent Trapping (CST) method for collecting the samples, and a Gas Chromatography-Mass Spectroscopy/Flame Ionization Detector (GC-MS/FID) system to analyze the collected samples. The composition of tar was determined, and the amount of the tar was evaluated after classifying the identified compounds as oxygenated and non-oxygenated compounds. The author proposes the use of the method adopted, by using GC-MS for identifying the compounds, GC-FID for the quantification. The use of internal standard compound with reference compounds, naphthalene and phenol helps to quantify the tar. The estimated tar level in the clean gas was ~37.61 mg. Nm-3.
Further gas purification and separation of hydrogen or a mixture of hydrogen and CO from the fuel gas mixture is possible by using Vacuum Swing Adsorption (VSA) system. The major focus of the gas characterisation study was on exit gas from the oxy-steam gasification system. Limited studies were conducted on VSA along with the oxy-steam gasification system, during the present study. Gas composition after VSA: 70% hydrogen, 5% methane, 25% CO.
A commercially available state-of-the-art electrolyte supported SOFC button cell was tested using an SOFC test rig. The cell is multi-layered and combines the advantages of Ni, Co, YSZ, SDC. Parametric studies were carried out with hydrogen to establish the baseline scenario. Simulated syngas, consisting of varying composition of CO and H2, was used to study the effect of CO-H2 mixture on the cell performance. Use of CO-H2 mixture as fuel resulted in performance derating, even though the Nernst potentials for both CO and H2 are similar. Investigations suggest that carbon deposition (coking) is predominantly an issue, which could result in degradation of the cell, and causing a performance drop. To address this issue, humidification of syngas has been considered.
Thermodynamic equilibrium calculations suggest that the Water Gas Shift Reaction helps in the utilization of CO at the cell level in the presence of steam. Excess steam can have an adverse effect on the anode due to Ni oxidation, while lower steam levels promote carbon deposition. The work presents the case of using a suitable CO-H2-H2O ratio to reduce or eliminate carbon deposition and also to improve the performance of the cell. Steam required for the syngas, after cleaning and separation, if obtained with a CO:H2 molar ratio of 1:3, would be similar to the simulated syngas experimental conditions of this study, needing ~5% by volume steam, in the gas. From the study, the carbon deposition regime and carbon-free regime (coking and non-coking regimes) for a range of gas compositions was established, to determine the safe operating conditions for SOFC.
From the study, it can also be concluded that the use of advanced anodes and the design of SOFCs to facilitate WGSR on the anodes, by using the steam produced during H2 oxidation, could pave the way for clean, economic, decentralized power generation using carbonaceous fuels like syngas, resulting in commercial deployment of the technology in future. | en_US |
