MCFC-based CO2 capture system for small scale CHP plants

Carbon dioxide emissions into the atmosphere are considered among the main reasons of the greenhouse effect. The largest share of CO2 is emitted by power plants using fossil fuels. Nowadays there are several technologies to capture CO2 from power plants' exhaust gas but each of them consumes a significant part of the electric power generated by the plant. The Molten Carbonate Fuel Cell (MCFC) can be used as concentrator of CO2, due to the chemical reactions that occurs in the cell stack: carbon dioxide entering into the cathode side is transported to the anode side via CO3= ions and is finally concentrated in the anodic exhaust. MCFC systems can be integrated in existing power plants (retro fitting) to separate CO2 in the exhaust gas and, at the same time, produce additional energy. The aim of this study is to find a feasible system design for medium scale cogeneration plants which are not considered economically and technically interesting for existing technologies for carbon capture, but are increasing in numbers with respect to large size power plants. This trend, if confirmed, will increase number of medium cogeneration plants with consequent benefit for both MCFC market for this application and effect on global CO2 emissions. System concept has been developed in a numerical model, using AspenTech engineering software. The model simulates a plant, which separates CO2 from a cogeneration plant exhaust gases and produces electric power. Data showing the effect of CO2 on cell voltage and cogenerator exhaust gas composition were taken from experimental activities in the fuel cell laboratory of the University of Perugia, FCLab, and from existing CHP plants. The innovative aspect of this model is the introduction of recirculation to optimize the performance of the MCFC. Cathode recirculation allows to decrease the carbon dioxide utilization factor of the cell keeping at the same time system CO2 removal efficiency at high level. At anode side, recirculation is used to reduce the fuel consumption (due to the unreacted hydrogen) and to increase the CO2 purity in the stored gas. The system design was completely introduced in the model and several analyses were performed. CO2 removal efficiency of 63% was reached with correspondent total efficiency of about 35%. System outlet is also thermal power, due to the high temperature of cathode exhaust off gases, and it is possible to consider integration of this outlet with the cogeneration system. This system, compared to other post-combustion CO2 removal technologies, does not consume energy, but produces additional electrical and thermal power with a global efficiency of about 70%.