Part I : Modelling and feasibility study

Biomass gasification derived fuel gas is a renewable fuel that can be used by high temperature fuel cells. In this two-part work an attempt is made to investigate the integration of a near atmospheric pressure 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 nominal output range. Heat for steam gasification is supplied from SOFC depleted fuel into a fluidised bed combustor via high temperature sodium heat pipes. The integrated system model was built in Aspen PlusTM simulation software and is described in detail. Part I investigates the feasibility and critical aspects of the system based on modelling results.

A low gasification steam to biomass ratio (STBR = 0.6) is used to avoid excess heat demands and to allow effective H2S high temperature removal. Water vapour is added prior to the anode to avoid carbon deposition. The SOFC off gases adequately provide gasification heat when fuel utilisation factors are <0.75; otherwise extra biomass must be combusted with overall efficiency penalty. For SOFC operation with Uf = 0.7 and current density 2500A m-2 the electrical efficiency is estimated at 36% while thermal efficiency at 14%. An exergy analysis is presented in Part II.

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 PlusTM 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 2500Am-2, the system uses 90 kg h.-1 biomass, operates with electrical exergetic efficiency 32% producing 140kWe, while the combined electrical and thermal exergetic efficiency is 35%.
 

Keywords : Modelling, SOFC, Biomass, Gasification, Aspen PlusTM