Energy Systems and Technologies for the Coming Century ...

where g 0 , R an T are the specific Gibbs free energy, universal gas constant and gastemperature respectively. Each atomic element in the inlet gas is balance with the outletgas composition, which yields the flow of each atom has to be conserved. For Nelements this is expressed ask w ni, in 1i1m1Aij n m,out Amjfor j ,N(13)The N elements correspond to H 2 , CO 2 , H 2 O and CH 4 in this pre-reforming process. A mjis the number of atoms of element j (H, C, O, N) in each molecule of entering compoundi (H 2 , CH 4 , CO, CO 2 , H 2 O, O 2 , N 2 and Ar), while A ij is the number of atoms of element jin each molecule of leaving compound m (H 2 , CH 4 , CO, CO 2 , H 2 O, N 2 and Ar). Theminimization of the Gibbs free energy can be formulated by introducing a Lagrangemultiplier, , for each of the N constraints obtained in Eq. (15). After adding theconstraints, the expression to be minimized is thenN k w G tot, out jnout ij m in mj j i , A n , A(14)1 i1 m1The partial derivation of this equation with respect ton i,outcan be writes as ni,outg G ni0i,outtot,out, out RT lnNj1 Ajij 0 for i 1, kNni, outpout jAij 0 for i 1, kj1(15)At the minimum each of these is then zero. The additional equation to make the systemconsistent is the summation of molar fractions of the outlet gas to beki1n 1(16)i, out3 Suggested Plants ConfigurationsThe suggested plants configurations are presented in Figs. 1. Wood chips are supplied tothe two-stage gasification plant for biogas production. The first reactor accounts fordrying and pyrolysing processes while the second reactor is the fixed bed gasifier. Asreported in Hoffman et al (2006), the product gas is pure enough to be fed to a SOFC cellwithout any problem. However, in this study a simple hot gas cleaner is used to removethe small amount of sulfur which exists after the gasifier. It is assumed that thedesulfurizer is working at about 240C. The cleaned fuel is preheated and in an anodepre-heater (AP) to 650C before entering to the anode side of the SOFC stacks. Theoperating temperature of the SOFC stacks as well as the outlet temperatures is assumedto be 780C. The burned fuel after the stacks is used to preheat the incoming fuel to AP.On the other side of the fuel cell, air is compressed and preheated in a cathode pre-heater(CP) to 600C before entering the cathode side of the SOFC stacks. Because some fuel isleft after the anode side of the SOFC stacks, the rest of the fuel together with the aircoming out of the cathode side are sent to a burner for further combustion. The off-gasesfrom the burner is used to generate steam in a heat recovery steam generator (HRSG)using an economizer (ECO), an evaporator (EVA) and a super-heater (SUP). Thegenerated steam can then be expanded in a steam turbine to produce power. In this study,part of the expanded steam is extracted for a deaerator (feed water tank). The expandedsteam after the turbine is then send to a condenser before pumping to deaerator.Risø International Energy Conference 2011 Proceedings Page 292