A process integration approach to the synthesis of hybrid post-combustion carbon dioxide capture processes

It is widely acknowledged that man-made CO₂ emissions to the atmosphere must be significantly reduced to mitigate the damaging effects of global climate change. The energy sector contributes to a large portion of the carbon emissions and a wide range of technologies need to be implemented to make the progression towards the low carbon dioxide emission. CO₂ capture and storage (CCS) is seen as a technology that can reduce the carbon emissions in the coal fired power stations. This will help reduce the rate of climate change by removing greenhouse gases that would otherwise be emitted to the atmosphere. Carbon capture and storage involves the capture of carbon dioxide gas from within the CO₂ generation process, compressing it into a supercritical fluid and finally sequestrating it. Implementing CCS technology for electricity production has an impact on the net power output of the power plant and it also has a high capital and operating cost. Among the different capture technologies, solvent absorption is considered to be the benchmark amongst the post-combustion carbon capture technologies. However, there are other technologies that have potential to be energy and cost competitive such as: adsorption, membranes and low-temperature separation. The aim of the PhD is to improve the integration of the capture processes with the power plant stations to reduce the energy penalty associated with the addition of CCS by combining current carbon capture technologies to form hybrid post-combustion carbon capture processes and evaluate their performances. A hybrid post-combustion carbon capture process consists of two steps: an initial recovery step would increase the concentration of CO₂ in the flue gas whilst trying to limit the amount of CO₂ lost in the waste gas. The second step would then be a purification step, where the CO₂ gas is purified and pressurised to the sequestration requirement. The research will develop a methodology to assess carbon capture technologies using exergy analysis, in combination with pinch analysis and optimisation methods, such as multi-objective optimisation (MOO). The hybrid carbon capture technologies is modelled using Aspen HYSYS®. These models provide the information to perform the analysis and optimisation of the power plants to determine the energy and cost targets. The two hybrid processes that were developed are VSA/low-temperature separation hybrid carbon capture processes and membrane/low-temperature separation hybrid carbon capture processes. The processes were then evaluated using MOO and the energetic and economic performance were compared to the MEA solvent absorption carbon capture processes. Hybrid carbon capture processes using technologies such as VSA, membranes and low-temperature carbon separation have shown potential to be energetically and economically competitive with the established MEA solvent absorption. The VSA/low-temperature hybrid process requires a specific shaft work of 1.46 GJe/t (CO₂ captured) when 90.0 % of the CO₂ is being recovered. The membrane/low-temperature hybrid carbon capture process required a specific shaft work of 1.38 GJe/t (CO₂ captured) at 90% CO₂ recovery rate when mixed refrigerant is used to achieve the low temperatures. On an economic perspective, the VSA/low-temperature separation hybrid process had a higher cost of avoidance of $78/t (CO₂ avoided) compared to the $67//t (CO₂ avoided) of the membrane/low-temperature separation hybrid process. Finally, beyond analysing the overall performance of the hybrid processes, the thesis allowed each hybrid process to be analysed in depth through heat integration and MOO; the energy trade-off between the CO₂ recovery stage and CO₂ purification stage was studied. The difference in using mixed ethane/propane refrigerant versus propane only refrigeration was also investigated.