Energy Systems and Technologies for the Coming Century ...

exhaust gas temperature, which can be utilized for additional power generation in heatengines or used for other heating purposes, whether internal in or external to the system.The Danish SOFC developer Topsoe Fuel Cell A/S states that the electrical efficiency ofdistributed power generation can be increased from average 40% in traditional powerplants to 55% when using SOFC technology [2]. SOFCs can electrochemically convertH 2 and CO to electricity and heat as well as internally reform CH 4 into more H 2 and COdue to their high operating temperature and the presence of a nickel catalyst. Comparedto other fuel cell types, these conversion pathways make SOFCs very fuel flexible andideal for conversion of product gas from thermal gasification.Generally, a major issue of combining gasification and SOFC technology has proved tobe gas conditioning, as SOFCs have strict requirements for fuel cleanliness [3]. In typicalgasifiers, relatively high levels of contaminants are entrained in the product gas flow,thus advanced gas cleaning is needed. In this study, a novel two-stage gasificationconcept producing a very clean product gas at high cold gas efficiency is used, thus onlysimple gas conditioning is necessary. General documentation of the demonstrated twostagegasification concept has previously been published [4]-[5]. The concept is scalableup to the range of 3-10 MW th [6], and it has been demonstrated up to 0.6 MW th (based onlower heating value, LHV) at present [7]. Hoffman et al. [8] operated an SOFC oncleaned product gas from a demonstrated two-stage gasifier for 150 hours without anyprove of cell degradation.Previous studies by the authors have shown that combining SOFC and gas turbinetechnology significantly enhances the power yield from thermally gasified biomasscompared to using only one of the two technologies [9]-[11]. A small-scale gasifier-SOFC-MGT hybrid plant can reach an electrical efficiency around 50% (LHV).Furthermore, an exergy analysis study has revealed that optimization of the heatmanagement can increase the electrical efficiency even more [12]-[13].This work presents thermodynamic assessments of alternative plant layouts of apreviously studied novel hybrid CHP plant combining two-stage biomass gasification,simple gas conditioning, an SOFC stack and a MGT. These alternative plant layoutsinclude optimization efforts by introducing a methanation reactor to increase the CH 4content in the anode feed, thus ensuring more SOFC cooling by the endothermic internalreforming reactions. Hereby, the parasitic loss of the plant can be reduced due tolowering of the excess air flow to the SOFCs. The thermodynamic assessments areconducted by use of mathematical models describing the thermodynamic processes. Themodels rely on connecting zero-dimensional component models to generate the completeplant-level models. The simulation tool used in this modelling study is DNA (DynamicNetwork Analysis), which is made for simulations of mathematical models representingthermodynamic processes [14]-[16].2 Reference Plant DesignsThe plant is divided into a gasification plant part and a CHP producing plant part. Woodchips are converted to product gas in the gasification part, and in the CHP producingpart, the product gas is converted to electric power and heat by use of an SOFC stack anda MGT.The gasification plant part is based on the two-stage gasification concept, whichcomprises a technique where drying and pyrolysis take place prior to an autothermaldowndraft gasification reactor, and where partial oxidation creates a high-temperaturetar-cracking zone in between the pyrolysis and char gasification steps. By thisgasification concept, a very clean product gas can be produced, thus avoiding advancedgas conditioning and easier utilization in an SOFC. The simple gas conditioningperformed in this plant includes gas cooling, a bag filter and a condensing gas cooler.The CHP producing plant part comprises an SOFC stack and a MGT. The SOFC ensuresefficient power production from the product gas, while the MGT ensures additionalRisø International Energy Conference 2011 Proceedings Page 299