Carbon dioxide emissions will continue to be a major environmental concern due to the fact that coal will remain a major fossil-fuel energy resource during the next few decades. Various CO2 capture and storage strategies have been considered for coal-fired power-generation plants to mitigate CO2 emissions. These technologies include pre-combustion or fuel decarbonisation, post-combustion, and oxy-fuel technology. In oxy-fuel combustion, fuel is combusted in oxygen rather than air and the oxygen stream is usually diluted by recycled flue gas to control the flame temperature and ensure a proper heat transfer without major changes in the layout of the boiler. In this thesis, oxy-fuel combustion technology is investigated to evaluate the effect of the O2/CO2 environment on coal combustion.
Coal combustion can be regarded as taking place in two steps: coal devolatilization followed by solid char combustion. In this thesis, the effect of oxy-fuel conditions on coal combustion is evaluated by investigating coal devolatilization and char combustion separately. Coal pyrolysis in 100% N2 and 100% CO2 environments was performed in an electrically entrained-flow reactor. Pyrolysis experiments were performed at different temperatures ranging from 700 °C to 1150 °C with high heating rates of K/s and residence times between 1-1.5 s. Chars were collected at 1150 °C and residence time of 1 s for combustion experiments. Char combustion was performed in a once-through 20 kW test facility with a residence time of around 4.2 s. Two coals were handled in this thesis: Klein Kopje coal from South Africa, a medium-rank coal and Lausitz coal from Germany, a low-rank coal. A commercial Rhenish char was also used for char combustion.
The results of the investigations are of high value because the link between coal pyrolysis and char combustion, the parameter study, and the scale of the test set up are an extremely good approach. Conclusions are drawn with respect to a CO2-rich environment, temperature, residence time, and coal rank for coal pyrolysis. On the other hand, char-combustion conclusions with respect to a CO2-rich environment, O2 partial pressure, temperature, and char quality are drawn.
It was found that the CO2 environment has an important effect on coal pyrolysis and char combustion. During pyrolysis, the CO2 environment enhances the mass release as the temperature and residence time of pyrolysis increase through Boudouard reaction. It was found that the CO2 environment enhances the mass release during coal pyrolysis above 850 °C by approximately 10% for Klein Kopje coal and 11-14% for Lausitz coal. In addition, the CO2 environment has a significant effect on the pyrolysis gas speciation. Carbon monoxide concentration is higher during pyrolysis in the CO2 environment than in the N2
environment for both coals especially above 850 °C due to Boudouard reaction and homogeneous water gas shift reaction. This finding could explain why higher CO concentrations during coal combustion under oxy-fuel conditions are measured in the burner zone. On the other hand, for char combustion, it was found that the O2/CO2 atmosphere has a retardation effect on char burnout and O2 consumption compared to O2/N2 combustion, especially at 15% O2. Moreover, the CO peak is shifted in 15% O2/CO2 combustion compared to 15% O2/N2 combustion. However, the O2/CO2 atmosphere has a significant effect on the CO concentration near the burner zone during char combustion; the maximum CO concentration in O2/CO2 is significantly higher than in O2/N2 combustion.
Coal rank has a significant effect on coal pyrolysis and char combustion. The mass release for Lausitz coal is approximately 40-50% higher than for Klein Kopje coal in both environments. Klein Kopje coal and Lausitz coal behave differently during pyrolysis: Klein Kopje coal exhibits swelling in both environments, while Lausitz coal exhibits fragmentation. In addition, Klein Kopje coal produces solid or low porous char structures, whereas Lausitz chars are highly porous in both N2 and CO2 environments. On the other hand, Lausitz chars are the most reactive during combustion from the investigated chars, 95% burnout is achieved in shorter residence time than Klein Kopje chars and Rhenish char. Klein Kopje chars are more effective in the reduction of recycled NO than Lausitz chars.