The reduction of CO2 emissions is of highest concern in relation to limiting the anthropogenic impacts on the environment. Primary focus has gathered on the large point sources of CO2 emissions constituted by large heat and power stations and other heavy, energy-consuming industry. Solutions are sought which will enable a significant reduction of the anthropogenic CO2 emissions during the transformation period from the use of fossil fuels to renewable sources of energy. Carbon capture and storage (CCS) has the potential to significantly reduce CO2 emissions from power stations while allowing for the continuous utilisation of the existing energy producing system in the transformation period. Oxyfuel combustion is one of the possible CCS technologies which show promising perspectives for implementation in industrial scale within a relatively short period of time. Oxyfuel combustion deviates from conventional combustion in air by using a mixture of pure oxygen and recirculated flue gas as the combustion medium thereby creating a flue gas highly concentrated in CO2 making the capture process economically more feasible compared to technologies with capture from more dilute CO2 streams. This project has investigated a number of the fundamental and practical issues of the oxyfuel combustion process by experimental, theoretical, and modelling investigations in order to improve the knowledge of the technology. The subjects investigated cover: general combustion characteristics of coal and biomass (straw) and mixtures thereof, formation and emission of pollutants, ash characteristics, flue gas cleaning for SO2 by wet scrubbing with limestone and for NOx by selective catalytic reduction (SCR), corrosion of boiler heat transfer surfaces, operation and control of large suspension-fired boilers, and the perspectives for the implementation of oxyfuel combustion s a CO2 sequestration solution in the Danish power production system. Regarding the fundamental combustion characteristics (combustion, emissions, and ash), the project has not identified any disqualifying characteristics. On the contrary, oxyfuel has the potential to improve fuel burnout and significantly reduce NOx emissions compared to conventional combustion in air. However, the significantly increased levels of CO2, H2O, CO (and SO2) within the boiler will have a negative effect on the risk of corrosion through a number of mechanisms such as carburisation (CO2 and H2O), water wall corrosion due to reducing conditions (CO), and both high- and low-temperature sulphur-induced corrosion (SO2/SO3). Both the wet flue gas desulphurisation and the selective catalytic reduction process for NOx removal have shown satisfying performance in oxyfuel atmospheres. At the same time, process calculations have shown that it is possible to retrofit an existing boiler to oxyfuel combustion. Different configurations; cold and hot recirculation of flue gas; are possible each with differences in the associated uncertainty, necessary level of process re-design, and reductions in the plant efficiency. It was generally seen that the configuration with the highest level of re-design, i.e. hot recirculation of flue gas, provided the possibility of the highest electrical efficiency but also the largest number of technical challenges. Generally, it has been concluded that it would be beneficial to mainly apply the oxyfuel technology to new-build plants rather than as a retrofit solution. In that respect, it is unlikely that oxyfuel power plants are commissioned in Denmark before 2020. However, in order to meet the very strict demands for the reduction of CO2 emissions within EU by 2050 application of oxyfuel combustion capture at power stations burning CO2 neutral fuels (biomass) could be an advantageous solution due to the associated, negative CO2 emissions.
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Technical University of Denmark, Department of Chemical Engineering, 2011