- Title
- Pulverised coal combustibility in simulated oxyfuel (O₂/CO₂) and air (O₂/N₂) conditions
- Creator
- Rathnam, Renu Kumar
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2012
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The need for reducing emissions on a large scale to mitigate climate change has led to the development of new carbon capture and storage (CCS) technologies such as oxyfuel combustion. Oxyfuel combustion, as the name suggests, is a technology in which fuel is burnt in oxygen rather than air to reduce the nitrogen content of the flue gas and thereby producing a concentrated stream of carbon dioxide almost ready for sequestration. In oxyfuel combustion a part of the flue gas, concentrated in CO₂, is recirculated back into the furnace mainly to reduce the high furnace temperatures. Hence, the combustion atmosphere is mainly O₂/CO₂ under oxyfuel conditions whereas in conventional air combustion, the atmosphere is mainly O₂/N₂. This major difference in the combustion conditions (the high CO₂ concentrations) affects major processes such as fuel combustion, heat transfer, and emissions inside the furnace. Hence, further development of this technology greatly relies on understanding the combustion and other processes under O₂/CO₂ conditions, with high CO₂ levels, as most current data and models available from air combustion may not suit pulverised coal combustion under oxyfuel conditions. This study deals in studying the combustion of pulverised coal in both O₂/CO₂ and O₂/N₂conditions concurrently. Data from the current study will help understand pulverised coal combustion under oxyfuel conditions and also enable direct comparison with the results obtained under air conditions. Coals of different ranks obtained from various regions of the world were selected to widen the scope of the study. In total, eight coals were used for the experiments, out of which three of them were mainly studied. Coals ranged from lignite to semi-anthracite and were obtained from were obtained from Australia, South Africa, Poland, and Germany. The combustion rates and reactivities were measured using a Drop Tube Furnace (DTF) and a Thermogravimetric analyser (TGA). The coal samples were analysed using proximate, ultimate, and petrographic analyses. Particle size measurements and scanning electron microscope studies were carried out on coal and char samples. Internal surface area measurements were performed on char samples. Chars were formed in the DTF at 1400 °C in N₂ and CO₂ atmospheres for the isothermal TGA tests. Combustion measurements were mainly carried out in the DTF and TGA under various O₂ levels, ranging from 0 to 21% v/v basis, in O₂/N₂ and O₂/CO₂ atmospheres to simulate air and oxyfuel conditions respectively. Furnace temperatures were also varied in the DTF and the TGA tests in the range of 1000 to 1400 °C and 800 to 1000 °C respectively. Coal burnouts were measured in the DTF tests and char combustion rates were estimated from the coal burnout data. The char mass loss rates were measured in the TGA tests for reactivity assessment. Results from the current studies show that the char-CO₂ gasification reaction plays an important role under O₂/CO₂ conditions. Higher or similar volatile yields and coal/char burnouts were measured under CO₂ and O₂/CO₂ atmospheres respectively, in DTF experiments, in comparison to N₂ and O₂/N2₂conditions respectively. The results from the DTF experiments were supported by higher mass loss rates and endothermic reactions indicated in O₂/CO₂ atmospheres during TGA experiments. The effect of the char-CO₂ gasification reaction depended on the coal type, furnace (particle) temperatures, and O₂ levels in the gas. The lower rank lignite tested in the study exhibited significantly higher mass loss/burnout under O₂/CO₂ conditions when compared to the other higher rank coals used in the study. The increase in mass loss rates/burnouts due to the char-CO₂ gasification was enhanced by lower O₂vlevels and higher furnace temperatures. While the diffusion of O₂ in CO₂ is slower than that in N₂, similar burnouts and combustion rates were observed in O₂/N₂ and O₂/CO₂ conditions. Under diffusion limited conditions, at very high particle temperatures, the char combustion rates are slower under O₂/CO₂ conditions, however, the char-CO₂ gasification reaction may compensate for the slower diffusivity. However, this depends on the coal type/rank as well, as the char-O₂ oxidation is comparatively faster and under diffusion limited conditions, the char-CO₂ gasification may not have sufficient residence time to have a significant effect on the char combustion rate. This study has shown that the char-CO₂ gasification reaction may contribute to the overall char combustion rate under O₂/CO₂ conditions depending on the O₂ level, the particle temperature, and the coal type. Under practical combustion conditions, as simulated in the current study using the DTF experiments, there may not be major differences in the coal burnout in air and oxyfuel conditions due to the reasons mentioned above. Even though the coal burnouts may appear similar apparently, the mechanisms of coal combustion including the volatiles release and combustion, char formation, and char combustion, have to be modelled specifically for O₂/CO₂ conditions in order to understand and predict the coal burnout and emissions. The char-CO₂ gasification reaction should be included in the models. Experimental data from the current study may be used for future modelling studies under O₂/CO₂ conditions.
- Subject
- combustion; gasification; pyrolysis; oxyfuel; pulverised coal; TGA; drop tube furnace; char; reactivity; burnout
- Identifier
- http://hdl.handle.net/1959.13/936144
- Identifier
- uon:12224
- Rights
- Copyright 2012 Renu Kumar Rathnam
- Language
- eng
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