Energy Systems and Technologies for the Coming Century ...

The moister content for these three scenarios were less than the limit value of 16%. TheHRSG effectiveness were less than 70%, indicating that the size of the HRGS werewithin acceptable cost. It could also be concluded form the values of HRSGeffectiveness that the rather high plant efficiency with the 780C scenario was not onlyproducing steam at higher temperature but also using the HRSG more effectively. Thenet power produced from the SOFC cycle was directly related to the operatingtemperature, meaning that the higher SOFC operating temperature was the higher SOFCnet power became.4 ConclusionsA novel IGSS (Integrated Gasification SOFC – Steam turbine) was presented, designedand thermodynamically analyzed for the first time in an open literature. A novelrecuperated system configuration versus non-recuperated design was compared in termsof plant efficiency, net power output and other important parameters. Woodchips wassupplied to a two-stage gasifier with cold efficiency of 93% to produce wood gas. Theproduced wood gas was adequately clean to be used in SOFC stacks without any prereformingprocess. However, for the sake of safety a desulfurization reactor was used toremove the small amount of sulfur from the wood gas. The wood gas was then suppliedto an SOFC cycle for electricity production. The energy of the waste gases from theSOFC cycle was then recovered in an HRSG to generate steam. The generated steamwas then used to produce power in a Rankine cycle which was accounted as a bottomingcycle for the topping SOFC cycle. In addition, the energy of the off-gases after theHRSG was recovered in a hybrid recuperator for further usage. The later recoveredenergy was sent back to the topping cycle again. Such recovering treatment was shownto increase the plant efficiency significantly. Plant efficiencies of 56% were reportedunder normal operation which was considerably higher than the tradition IGCC plants.Under certain operating condition plant efficiency could be as high as 63%, with currentSOFC technology.It was thermodynamically shown that the corresponding non-hybrid recuperated planthad an optimal live steam pressure at 8 bar with a plant efficiency of about 48% whichwas comparable with traditional IGCC plants. Applying a hybrid recuperator andrecovering more energy increased the plant efficiency by about 17% (or 8 pointpercentage from 48% to 56%).Finally, it was also discussed that if it was possible to increase the operating temperatureof the SOFC to 894C, then the plant efficiency of 65% would be achieved under certainoperating condition such as SOFC utilization factor of 90% and SOFC current density of100 mA/cm 2 .5 References[1] Fontell, E., Kivisaari, T., Christiansen, N., Hansen, J.-B. and Pålsson, J. 2004,”Conceptional study of a 250 kW planar SOFC System for CHP Application”, J.Power Sources, Vol. 131, pp. 49 – 56.[2] Rokni, M., 2003, “Introduction of a Fuel Cell into Combined Cycle: ACompetitive Choice for Future Cogeneration”, ASME Cogen – Turbo, IGTI vol. 8,pp. 255 – 261.[3] Riensche, E., Achenbach, E., Froning, D., Haines, M. R., Heidug, W. K., Lokurlu,A., and von Andrian, S., 2000, “Clean Combined-cycle SOFC Power Plant – CellModeling and Process Analysis”, J. Power Sources, Vol. 86, pp. 404 – 410.[4] Pålsson, J., Selimovic, A. and Sjunnesson, L., 2000 “Combined Solid Oxide FuelCell and Gas Turbine System for Efficient Power and Heat Generation”, J. PowerSources, Vol. 86, pp. 442 – 448.Risø International Energy Conference 2011 Proceedings Page 296