- Title
- Ignition behaviour of individual pulverized coal particles in air and oxy-fuel environments
- Creator
- Abdul Gani, Zeenathul Farida
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2011
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In recent years, increasing concern over global warming and climate change and its connection to CO2 emissions have received huge attention worldwide. The alarming rise in CO2 levels and escalating demand for cleaner energy has lead to the development of novel technologies that may play a major role in addressing the issues related to CO2 emissions. Some of these technologies aim at reducing CO2 emissions by capturing and sequestrating it. Development of carbon abatement technologies, usually referred to as carbon capture and storage (CCS) technologies enables substantial reduction in CO2 emissions but still using fossil fuels. In conventional coal power plants, recovery of CO2 is energy intensive because of the low concentration of CO2 in the flue gas, which is due to the dilution by N2 in air. The concentration of CO2 can be increased and thereby the cost of CO2 recovery can be reduced by removing the nitrogen in the air before entering the furnace. Therefore, in oxy-fuel technology, combustion of coal takes place in the presence of pure oxygen and flue gas that is recycled from the furnace exit to moderate the temperature of the combustion process and for effective heat transfer. Techno-economic feasibility studies that have compared different CCS technologies have declared that oxy-fuel technology emerges as a favourable option among the CCS technologies that are close to commercialization. Apart from the capability to produce a highly concentrated CO2 stream, this technology has also been credited for its flexibility, as it can be adapted in new designs or used to retrofit existing coal fired (p.f.) power plants. Review of the literature shows that the change from air to oxy-fuel mode modifies the gas composition inside the furnace that leads to an altered temperature, species and heat transfer profiles causing undesirable effects inside an oxy-fired furnace. The difference in the combustion behaviour in an oxy-fuel furnace compared to conventional systems is summarized in the literature as due to the difference in the thermo-physical properties of the bulk gas (either N2 or CO2) and due to the change in the near burner aerodynamics caused due to different throughputs in air and oxy-fuel combustors. Extensive research work has been carried out in this novel mode of combustion, establishing the issues and addressing solutions. More research is needed especially on issues like ignition and devolatilization, as stable combustion in pulverized coal combustion system is ensured by the rapid volatile release, ignition and heat released during devolatilization. Therefore issues related to ignition and devolatilization is considered most important and requires continuous intensive research to take this technology to the next phase of commercialization. It has also been proposed by many researchers that undesirable behaviour during oxy-fuel combustion can be controlled by increasing the oxygen concentration in the oxidizer stream above 21% in CO2. The current research aims to carry out a fundamental investigation on the ignition behaviour of individual coal particles in oxy-fuel environments, comparing the results with the behaviour in air combustion. The objective is to compare the combustion parameters such as burnout times and particle burning temperatures in N2 and CO2 environments to understand the coal burning behaviour in oxy-fuel combustion and to quantify the impact of CO2 when substituted for N2. This was accomplished by carrying out a single particle experimental and theoretical investigation on the ignition behaviour of individual pulverized coal particles in air and oxy-fuel conditions utilising an entrained flow reactor supported by a flat flame burner. Optical pyrometry was used to collect thermal radiation emission-time history of the burning particles and burnout times and temperatures were obtained from the radiation traces. The temperatures of the burning particles were estimated by two colour technique while assuming the burning particle behaved as a grey body. Two coals of different rank (sub-bituminous and lignite), two particle size fractions (180-212 μm and 106-125 μm) at two furnace temperatures (1550 K and 1800 K), with oxygen concentration in the ambient gas varying from 10% to 50% by volume were used for the experiments. A single particle combustion model was also developed to simulate the combustion behaviour of coal particles in air and oxy-fuel conditions. Direct observations of the burning particles using a high speed camera were completed to validate the flame sheet concept used in the volatile combustion modelling. Cine and still images showed that the volatiles released from the particle form a volatile cloud surrounding the particle and are burnt in a spherical diffusion flame sheet enveloping the particle which shrinks in size as the volatiles are consumed. The size of the volatile cloud decreases with increasing oxygen concentration in the ambient gas which is consistent with the flame sheet radius predicted using the flame sheet model. Combustion data obtained in different gas mediums (O2/N2 and O2/CO2) show that, particles burn faster and hotter in the presence of N2 compared to CO2. Particles burning slower and colder in CO2 environments is attributed to the higher molar heat capacity of CO2 and lower mass diffusivity in CO2. The higher molar heat capacity acts as a heat sink, lowering the temperature of volatile combustion and particle temperature (cooling effect), while the lower mass diffusivity slows down the burning rate of both volatile and char. Irrespective of the bulk gas composition, burnout times and particle temperatures were strongly dependent on oxygen concentration and particle size. Smaller particles were found to burn rapidly and more intensely than larger particles due to the close proximity of the flame sheet to the particle surface. Since the energy imparted to the particle is higher, smaller particles burn hotter for shorter times, with little impact of the ambient gas temperature. Once the flame sheet, which is at a higher temperature than the ambient has formed, it becomes the dominant source of energy isolating the particle from the ambient. Oxygen concentration has a significant effect on the extent of devolatilization, which can be explained by the higher energy feed back to the particle due to higher flame sheet temperatures and closer flame sheet locations. The increased rate of char burning at higher oxygen concentrations can be explained by the higher accessibility of oxygen and higher heating rates from the flame sheet, ensuring the char particle reaches a higher temperature, thus increasing the char burning rate and lower burnout times. The observed trends in combustion durations and particle temperatures in air and oxy-fuel conditions are in accordance with previous investigations on single coal particles in air and oxy-fuel conditions and also with the predicted data using single particle modelling. Though the combustion duration trends predicted by the model are consistent with the measured times, the model over predicted the times irrespective of coal type and other experimental conditions. The relatively poorer behaviour of coal burning in an oxy-fuel environment compared to air can be overcome by increasing the oxygen concentration in the ambient gas above 21% by volume.
- Subject
- oxyfuel; climate change; global warming; clean energy
- Identifier
- http://hdl.handle.net/1959.13/1036739
- Identifier
- uon:13355
- Rights
- Copyright 2011 Zeenathul Farida Abdul Gani
- Language
- eng
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