Oxy-fuel combustion of Victorian brown coal
thesisposted on 02.03.2017, 23:17 by Prationo, Wirhan
The consistent use of coal as a fuel source for power generation results in the significant emission of greenhouse gases into the environment. Using a low-rank coal, such as Victorian brown coal, results in a much higher CO₂ emission rate. Oxy-fuel combustion is identified as the promising CO₂ abatement technology for cleaner coal combustion. With the replacement of air with the mixture of high-purity oxygen and flue gas, the overall process is nitrogen-lean and can generate a flue gas ready that is rich in CO₂ and ready for carbon capture and storage (CCS). Due to the altered ignition and combustion feature of coal under this environment, the oxy-fuel burner has to be re-designed. With the construction and testing in various pilot-scale and demonstration-scale power plant, oxy-fuel combustion has been progressively advancing in recent years. However, most of these studies were centred towards the use of black coal as the main fuel source. Therefore, Victorian brown coal (VBC) is highlighted in this thesis due to the limited knowledge for its oxy-fuel combustion. VBC also possesses distinct properties and also burns distinctively from other coal. Although coal ignition has been widely studied, the impact of alkali and alkaline earth metal (AAEM) species and moisture, which are abundant in brown coal, are scarcely reported. The AAEM species are responsible for notorious slagging and fouling in the boiler which can be subsided via the injection of fuel additives. Additionally, the recirculation of flue gas is also likely to increase the overall steam concentration in the furnace. Understand of all these impacts on VBC ignition are essential for the deployment of oxy-fuel combustion in Latrobe Valley, Victoria. The scope of this research involves both experimental and modeling studies. For the experimental aspects of this research, a bench-scale entrained flow reactor with flat flame burner is commissioned for ignition study. Using an advance in-situ non-intrusive diagnostics facility, coal ignition behavior is captured in a series of photograph to elucidate the transient phenomena occurring during the particle heat up, devolatilisation and ignition. For the modeling approach, series of mathematical equations for coal combustion are written in MATLAB, including a single-film model to quantitatively describe the contribution of the two gasification reactions on char burnout and a transient ignition model for the ignition of dense particle stream. The calculated results are validated by experimental measurements. The first part of this research involves the investigation of the effect of AAEM species on coal ignition. The Chinese lignite from Xinjiang, which is rich in AAEM species, is used for this experiment. Further to that, ignition of coal upon the addition of kaolinite and the removal of AAEM are also investigated. The injection of kaolinite has a negligible effect on coal ignition but enhances the volatile decomposition rate. With the removal of AAEM species, the demineralised coal ignites considerably slower. This delay, however, can be eliminated by increasing the oxygen concentration to 30% in oxy-firing mode. Following that, the second distinct property in brown coal, abundant inherent moisture, is studied. Victorian brown coal with differing moisture content from 12% to 30% is prepared. It was later found that the coal ignition can occur although moisture in brown coal is not completely evaporated. This remaining moisture, referred to as inherent moisture, exerts influence on the subsequent devolatilisation and char combustion rate. Nonetheless, wet brown coal still ignites slightly later compared to its air-dried counterpart. An oxygen concentration of 30% in oxy-firing was also found sufficient to compensate for the detrimental effect of the inherent moisture on coal ignition. Next, the impact of the remaining moisture is investigated during char combustion process. From a modeling approach, it has been clarified that the un-evaporated moisture in wet coal is further released with the volatiles simultaneously in the air-firing mode. On the other hand, a portion of the inherent moisture still remains even after coal devolatilisation in the oxy-firing case. This residing moisture on particle surface later triggers a char-steam gasification reaction and its contribution is quantified through the modeling study in MATLAB. Finally, this research concludes with the ignition study of dense particle stream as means to extend these results to a real pulverized coal-fired burner. Here, it is more interesting to evaluate the effect of elevated steam concentration in the flue gas rather than evaluating the effect of the inherent moisture in the raw coal. Surprisingly, a faster ignition under the steam-rich condition was revealed from this research. This is later confirmed from modeling approach that the homogenous water-gas shift reaction is influential in accelerating the ignition of the volatiles released from Victorian brown coal. In summary, this thesis has identified a number of distinct features associated with the ignition and burnout of Victorian brown coal in the oxy-fuel combustion mode. Some of these findings can be further extended to commercial software for industrial applications. This research ultimately provides a clearer picture on ignition behavior of Victorian brown coal that is essential to develop an oxy-fuel burner.