The modelling of a hybrid combined cycle with pressurised fluidised bed combustion and CO_2 capture

摘要

This study investigates the possibility of capturing CO_2 from flue gas under pressurised conditions, which could prove to be beneficial in comparison to working under atmospheric conditions. Simulations of two hybrid combined cycles with pressurisedfluidised bed combustion and CO_2 capture are presented. CO_2 is captured from pressurised flue gas by means of chemical absorption after the boiler but before expansion. The results show a CO_2 capture penalty of approximately 8 percentage points (including 90% CO_2 capture rate and compression to 110 bar), which makes the efficiency for the best performing cycle 43.9%. It is 5.2 percentage points higher than the most probable alternative, i.e. using a natural gas fired combined cycle and a pulverisedcoal fired condensing plant separately with the same fuel split ratio. The largest part of the penalty is associated with the lower mass flow of flue gas after CO_2 capture, which leads to a decrease in work output in the expander and potential for feedwater heating. The penalty caused by the regeneration of absorbent is quite low, since the high pressure permits the use of potassium carbonate, which requires less regeneration heat than for example the more commonly proposed monoethanolamine. Althoughthe efficiencies of the cycles look promising it will be important to perform a cost estimate to be able to make a fair comparison with other systems. Such a cost estimate has not been done in this study. A significant drawback of these hybrid cycles inthat respect is the complex nature of the systems that will have a negative effect on the economy.