The United Nations Framework Convention on Climate Change (UNFCCC) in 1992 first illustrated the social, economic and politic focus being placed on combating climate change caused by anthropogenic greenhouse gases. From there onwards research and development efforts have particularly centred on the reduction of CO2 emissions in the production of electrical power through the use of carbonaceous fossil fuels. The long-term goal is a conversion to sustainable and CO2 free means of producing power, utilizing in the main part renewable forms of energy such as solar, wind and hydro power. Currently, such renewable ways of creating electricity only represent a small percentage of global energy production. The technological and economic hurdles that are associated with a substantial increase of renewable energy production have greatly slowed their increased implementation. However, the goal of keeping the atmospheric CO2 concentration below 450 ppm requires a significantly faster reduction in the amount of greenhouse gas emissions. Therefore, considerations are being given to bridge technologies which would be able to capture and store the CO2 emissions from fossil fired power plants. These technologies are referred to as CCS (carbon capture and storage). Oxy-fuel combustion, combustion with pure oxygen instead of air, is one of those technologies and forms the focus of investigation of this work.
The Institute of Combustion and Power Plant Technology in Stuttgart, Germany, have researched this matter, carrying out combustion experiments in its 150 kWth circulating fluidized bed pilot facility. The experiments were aimed at investigating the influence of excess oxygen, combustion temperature and inlet oxygen concentration on the combustion process and comparing air to oxy-fuel combustion. These results were compared to the results of fundamental investigations and combustion experiments carried out by other research groups. The relationship between the operating parameters and the combustion characteristics were presented and the underlying mechanisms were identified.
During the tests CO, NOx, SO2, CO2 and O2 concentrations in the flue gas were measured, as well as the total organic carbon content of both the facility’s inventory and the fly ash from the combustion. The tests showed a similar behaviour regarding excess oxygen and combustion temperature as one would expect from air combustion.
The increased CO2 partial pressure slows down the homogenous CO oxidation reaction. Alongside that, higher CO concentrations in the flue gas can be observed to occur. Moreover, combustion losses cannot be estimated by the sole measurement of CO concentration as they show a deviating behaviour. Analysis of the solid combustion losses shows unchanged values when increasing the inlet oxygen concentration.
The flue gas recirculation would seem to have the greatest influence on the different varixv ables during oxy-fuel combustion. It further accelerates the capture of SO2 by the fuel ash. The reduction of the recirculation rate that results from higher inlet oxygen concentrations weakens the associated effects, for example this leads to a decrease of the reduction of NOx. The experiments carried out can be seen to be in very good agreement with the current findings of both fundamental and combustion research that has been conducted and published by other research groups. Current understanding has been increased through the effects associated with a change of the inlet oxygen concentration and the recirculation rate. A final evaluation of the results considering the transferability to large scale plants completes the investigations. As a result of this work it is possible to confirm the feasibility of oxy-fuel combustion in a circulating fluidized as an excellent technology for the capture of CO2 from coal fired plants.