Energy Systems and Technologies for the Coming Century ...

power production from utilizing the high-temperature SOFC off gases containingresidual fuel. Furthermore, the MGT pressurizes the SOFC stack, thus enhancing theSOFC performance.Heat for district heating purposes is mainly extracted from cooling the plant exhaust gaswith minor contributions from cooling the product gas in the gas conditioning step. Someof the produced heat should be applied to the drying and pyrolysis step, though.Based on the plant concept described above, a system design without integrationbetween the gasification and the CHP producing plant parts were initially studied in aprevious publication [11]. A flow sheet of this initial plant design can be found in Figure1 and it is used as reference plant 1 in this study. As depicted in Figure 1, wet wood isfed to the steam dryer, producing dry wood and steam, which both are led to the gasifier.For reasons of simplification, the pyrolysis takes place inside the gasification reactor inthis modelling study. Heat for the wood drying is provided by the hot product gas fromthe gasifier (and transferred via superheated (SH) steam), thus ensuring possibleindependent operation of the gasifier plant part. A bag filter removing particulates is theonly gas cleaning device in the considered system. The product gas is cooled to 90°Cbefore it is led through the filter. It is assumed that no alkali compounds leave thegasifier plant entrained in the product gas flow, since all alkalis should be condensed atsuch low temperatures, thus removed along with particulates in the bag filter. Thesulphur content is expected to be very low (cf., [12]-[13]), so no sulphur clean-up step isincluded. If it was found necessary, a ZnO bed could be located after the gasifier airpreheater depending on the preferred operating temperature of such a sulphur removalunit. The condensing gas cooler lowers the water content in the product gas to 12.7 vol-% resulting in a steam to carbon ratio (S/C) of 0.41, which is somewhat low [17] butjustified by the very low tar content in the product gas. Hofmann et al. [8] showed thatcarbon formation did not occur when fuelling an SOFC with product gas from a twostagegasifier at low fuel utilization and a S/C ratio of 0.5. Higher fuel utilization shouldenable operation at lower S/C ratios [17].The conditioned product gas is then converted to electricity and heat in the CHPproducing plant part consisting of a pressurized SOFC stack and a recuperated MGT. Inaddition, the product gas compressor works as a suction blower for the gasifier system. Agenerator (not illustrated) is situated on the shaft of the gas turbine and it produces thenet electric MGT power. A DC/AC inverter (not illustrated) converts the DC power fromthe SOFC stack to AC power.The size of the plant is defined by the thermal input of biomass fed to the dryer, and inthis study it is approximately 0.5 MW th (LHV).Based on an exergy analysis, some modifications of the heat management can beimplemented to enhance the plant performance. For details, the reader is referred to [12]and [13]. The resulting plant layout is depicted in Figure 2 and it is used as referenceplant 2 in this study. In reference plant 2, the exhaust gas downstream the recuperator isdelivering heat to the biomass drying process by heating dryer steam in the steam heater.The hot product gas leaving the gasifier air preheater, that previously heated the dryingprocess, preheats the pressurized product gas in a first step preheater, while the anode offgas preheats the pressurized product gas in a second step preheater. The added first steppreheater ensures a higher temperature of the anode off gas leaving the second steppreheater and entering the burner. Since the first step product gas preheater works as arecuperator, a heat exchanger effectiveness of the same level as the MGT recuperator isapplied.Risø International Energy Conference 2011 Proceedings Page 300