- Title
- Utilisation of ventilation air methane in chemical looping systems
- Creator
- Zhang, Yongxing
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- A major source of greenhouse gas emissions from the mining sector is the release of fugitive methane emissions (CH₄ with a global warming potential 25 times greater than that of CO₂) from the ventilation air systems in coalmines. Abatement of ventilation air methane (VAM) has been a priority for the coal industry in recent years and has become an even higher priority following the introduction of a carbon price in many parts of the world including Australia. Capture and use of VAM (i.e. utilisation) is considered the most effective means of abating fugitive methane emissions from coal mining. The key challenge though is to develop abatement platforms which can effectively deal with (i) high gas volumes; as high as 600 m³/s; (ii) low methane concentrations; typically 0.1-1 %V/V; and (iii) the highly variable nature of VAM both in terms of the flow rate and concentration. This PhD thesis has been motivated by the desire to overcome the above challenge and has resulted in the development of three chemical looping based alternative pathways to utilise VAM. These are: 1. Conversion of VAM to hydrogen as a value added product using a dual loop chemical looping process. 2. Thermal oxidation of hydrogen enriched VAM using H2 produced by a novel Integrated Gasification Chemical Looping Combustion (IGCLC) process. 3. Ancillary use of VAM as an oxidizing agent in chemical looping combustion of synthesis gas. In the first option a Cu-based chemical loop for air separation process is integrated with a Fe-based chemical loop (i.e. dual loop) where VAM is used as a feedstock to produce pure hydrogen. It was proved that oxygen in VAM can be removed with Cu₂O at temperatures between 573K and 673K without the occurrence of methane oxidation. With a moderate iron oxide circulation rate, pure hydrogen can be produced with methane concentration as low as 0.4 vol%, and a hydrogen efficiency of 60% was obtained with methane concentration of 1 vol%. The reduction of Fe₂O₃/Al₂O₃ by ultra-low concentration methane was investigated experimentally in TGA and fixed bed setup, showing that 45 wt% Fe₂O₃/Al₂O₃ was the most suitable oxygen carrier for chemical looping hydrogen production with methane concentration as low as 0.1 vol%. Reduction of Fe₂O₃ with ultra-low concentration methane was found to be a two-step process. The first step of Fe₂O₃ to Fe₃O₄ was controlled by a phase change mechanism followed by the step of Fe₃O₄ to FeO by a diffusion controlled mechanism. In the second option the thermal oxidation of VAM takes place in a VAM combustor (VC) along with hydrogen as a supplementary fuel. The hydrogen in this process is produced from the integrated gasification chemical looping combustion (IGCLC) of coal. With a moderate VAM flow rate (~25 kg/kg coal), the temperature in VC was found to be higher than 1188 K even with methane concentration as low as 0.1 vol%. In most cases, the overall efficiency for the whole system was higher than 45% and CO₂ capture rate higher than 80%. However, the variation in VAM flow rate and methane concentration led to a significant change in the VC temperature. In particular, the temperature was increasing linearly with the increase in methane concentration, which increased by 200K from the methane concentration of 0.1 vol% to 1 vol%. Similarly in Option 3, VAM was utilised as an oxidation agent in chemical looping combustion of synthesis gas where methane was oxidised in an air reactor in the presence of oxygen carriers. Due to the circulation of oxygen carriers between the fuel and air reactors, the temperature deference was decreased to 34 K when methane concentration varied from 0.1 vol% to 1 vol%. Moreover, the process delivered a higher CO₂ capture rate of over 90% and similar overall system efficiency compared with the abovementioned process. The methane conversion with reaction temperature was reported for the oxidation of VAM in the presence of iron oxide as an oxygen carrier. Complete methane conversion can be achieved at temperatures around 873 K even with the methane concentration as low as 0.23 vol% and iron oxide content as low as 1 wt%. Overall, we found that the above chemical looping processes are quite suitable for applications involving utilisation of methane. However, selection of one technology over the others for a given applications hinges upon a careful analysis of the pros and cons of the three technology options discussed earlier.
- Subject
- ventilation air methane; chemical looping
- Identifier
- http://hdl.handle.net/1959.13/1052975
- Identifier
- uon:15502
- Rights
- Copyright 2014 Yongxing Zhang
- Language
- eng
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