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Chemical looping combustion of Victorian brown coal using Fe-based oxygen carriers
thesisposted on 2017-03-02, 00:16 authored by Rajendran, Sharmen
The vast reserves of Victorian brown coal (VBC), over 400 years at the current rate of consumption, is predominantly used for power generation with over 80% of Victoria’s electricity generated from the combustion of this fuel. This results in the emission of vast amounts of greenhouse gases such as CO2. Hence, it is important to investigate carbon dioxide capture and storage technologies for use in power stations employing fossil fuels. Chemical Looping Combustion (CLC) is an emerging CO2 capture technology which is capable of inherently capturing CO2. In CLC, the Oxygen Carrier (OC) provides the oxygen for the combustion of the fuel hence eliminating dilution with N2 from air. Once the oxygen in the OC is depleted, it is regenerated through oxidation in air and is then sent back to react with another batch of fuel. The vast majority of research in the field of CLC has been focussed on gaseous fuels such as natural gas and syngas due to the simplicity of such a process. In recent times, there has been a shift towards the use of solid fuels due to their abundance, widespread availability and lower cost. As such, there are a number of gaps in the field of CLC employing solid fuels; additionally, the only information relating to CLC of VBC is limited to experiments using small scale laboratory equipment. This thesis serves to fill some of the gaps in the field of VBC-fuelled CLC. The first study investigated the effect of inherent coal minerals on the performance of a CLC system; a high ash Canadian lignite was also used as part of this comparative study. The results highlighted that the low ash VBC was more suitable for use as a fuel in CLC as it was highly reactive and its low ash content led to a smaller amount of ash deposition on the OC. The second study involved using synchrotron radiation to perform in-situ X-ray Diffraction studies of a VBC-fuelled CLC process to understand both the changes that the OC undergoes as part of the redox reaction as well as carbon deposition on the OC. The results showed that the reduction of Fe₂O₃ beyond Fe₃O₄ was not favourable over long periods of time when using CO₂ as the gasification agent as it led to carbon deposition on the OC. The third study is a first-of-its-kind investigation, where the reduction kinetics of an Fe-based OC was determined in the presence of a char derived from VBC. The Shrinking Core Model (SCM) and the Modified Volume Reaction Model (MVRM) were used to model the reduction of the OC. The results showed that both models were capable of predicting the reduction of Fe₂O₃ in the presence of a solid fuel. The calculations also verified that the rate limiting step in CLC was that of char gasification. The fourth study investigated the effect of the reactor configuration on the performance of the CLC system as such a comparison has never been attempted. A fluidized bed reactor, an atmospheric fixed bed reactor and a pressurized fixed bed reactor operated at 5 bar were used. The amount of the fuel and the OC together with the flow rates of the gases were kept constant so that the results from the different setups could be compared accurately. It was found that using the fluidized bed reactor allowed for the fastest gasification of the fuel due to better contact between the gasification agent and fuel. Although the CO₂ yield and carbon conversion in the fluidized bed reactor was lower compared to the other two fixed bed reactors, it is expected that the use of a circulating fluidized bed (CFB) reactor with cyclones, a carbon stripper and a taller expanded freeboard would improve these two parameters. The fifth study involved fabricating and trialling 18 synthetic OCs in which NiO, CuO and Mn₂O₃ were supported on Fe₂O₃. This was done as most studies in literature utilize an inert support that is not able to take part in the redox reaction; as such a greater quantity of the OC is needed to provide the necessary oxygen. The results highlight that impregnated OCs were more reactive relative to their physically mixed counterparts. The use of high levels of CuO should be avoided as it led to the defluidization of the bed. Although NiO performed well, it may not be suitable for use due to its toxicity. Taking numerous considerations into account, the use of Mn₂O₃ was recommended as it led to a synergistic effect with Fe₂O₃. The sixth and final study of this thesis utilized a 10 kWth alternating fluidized bed reactor to trial the performance of VBC in a large scale reactor. A number of studies on the effects of temperature, fuel type, OC particle size range and long term operation on the performance of the CLC system were done. The NOx emissions were quantified and a carbon balance was also performed. The NOx emissions were found to average around 25 ppm over the course of the reduction reaction. Based on the carbon balance, 6.8% of the introduced carbon was unaccounted for due to the low capture efficiency of the cyclones. The optimum parameters were found to be 900°C for the temperature, 150-350 µm for the OC particle size range and VBC for the fuel. The average carbon conversion and CO2 yield over 35 reduction reactions was found to be 86% and 81% respectively for the conditions optimized for this reactor setup. These studies show that the use of Fe-based OCs is highly promising with VBC. The main recommendation from this thesis is the use of VBC in a CFB reactor as this is expected to significantly improve the carbon conversion and CO₂ yield.