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Production of hydrogen rich gas by biomass gasification: application to small scale, fuel cell electricity generation in rural areas

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The research project is addressed to a preliminary technical and economic evaluation of a small/medium capacity biomass-to-energy system, based on gasification processes able to convert (under proper conditions) the energy of biomass directly to hydrogen, which is utilised for electricity and heat generation in fuel cell modular units. The suitability of the integrated system in rural areas is also investigated. Two different gasification technologies are considered: the entrained flow gasifier, originally developed by NOELL Energie und Entsorgungstechnik GmbH for the conversion of residual and waste materials; the catalytic dual interconnected fluidised bed system, developed by University College London (UCL) on the base of experimental tests of fluidised bed biomass gasification previously performed at the University of LAquila (UNIVAQ). All gas cleaning operations have been identified and quantified in details. The results of the work carried out show that the integrated plant envisaged in the original proposal is technically feasible by means of both gasification systems, with considerable advantages linked to the high level of integration that can be obtained between the gasification and the electricity production sub-systems. Net electric efficiencies higher than 30 % are readily obtainable, with additional thermal power available for district heating or different utilities. The catalyst characterisation and reactivity tests carried out at University Louis Pasteur of Strasbourg (ECPMS) have shown that trimetallic perovskite structures are able to assure high yield and selectivity for the conversion of tar and methane, with low levels of carbon build-up on the active surfaces and negligible sintering phenomena. A sample of such catalyst has been tested in a secondary fixed bed reactor downstream of a bench-scale fluidised bed gasifier, with very positive results: more than 2 Nm3 of dry gas have been obtained per kg of biomass d.a.f., with a hydrogen content of about 60 % by volume and less than 0.3 g/Nm3 of tar. Hydrogen fed 850 kW phosphoric acid fuel cell (PAFC) modules are considered. They are studied by CLC s.r.l., based on the 200 kW natural gas units presently commercialised in Europe. The data produced by ABIOTEC, the Emilia Romagna Agency for promotion of new agro-industrial systems, show that biomass sources can be identified, specifically from among the perennial crops (arundo donax, miscanthus), which are compatible with either the technical requirements of the integrated system (among these: moisture level, sulphur content, feedstock size range), and the considered local rural context; they allow a round the year utilisation of the electricity generation plant, with logistically and economically reasonable solutions for biomass storage and transportation. Cost estimates have been performed, both for a small scale application based on a single PAFC module, and for a medium size plant based on 10 PAFC modules, which gives sensitivity to the system scaling-up. The analysis shows in general high investment and operation costs, which were expected as a result of joining new technologies (gasification and fuel cells), both at their first market entry. With a single 850 kW module system, revenues do not even balance overall running costs (investment, operation, maintenance, taxes and insurance). The larger plant with the dual fluidised bed gasifier hits the target of power production costs slightly lower than the selling price of the electric kWh obtained from renewable energy sources. These costs are obviously expected to reach more acceptable levels as new technologies come to the market and get beneficial effects from operation experience and increasing production volumes.