High temperature solid oxide fuel cell integrated with novel allothermal biomass gasification: Part II: Exergy analysis

Biomass gasification derived gas is a renewable fuel, which can be used for SOFC applications. This work investigates the integration of a near atmospheric solid oxide fuel cell (SOFC) with a novel allothermal biomass steam gasification process into a combined heat and power (CHP) system of less than MWe range. Heat for steam gasification is supplied from SOFC depleted fuel in a fluidised bed (FB) combustor via high temperature sodium heat pipes. In the first paper, the integrated system was modelled in Aspen Plus™ and critical aspects for its feasibility were identified. The aim of this second part is the evaluation of the integrated system in exergy terms. Satisfying allothermal gasification heat demand is illustrated by examining each sub-process involved separately as well as combined. For a relatively low STBR = 0.6, the SOFC fuel utilisation for which the system operates under optimum conditions is Uf = 0.7. Above that value additional biomass has to be used in the FB combustor to provide gasification heat with considerable exergy losses. For SOFC operation at current density 2500 A m−2, the system uses 90 kg h−1 biomass, operates with electrical exergetic efficiency 32% producing 140 kWe, while the combined electrical and thermal exergetic efficiency is 35%.