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Experimental investigation of syngas composition evolution along the gasifier height during the updraft gasification of lignocellulosic feedstock

Fixed bed gasification is one of the most suitable process to exploit lignocellulosics because it can be carried out at small scale (<1MWth) with a relatively simple and robust technology. From fixed bed gasifier syngas can be draft from the bottom (downdraft type) of from the top (updraft type). The latter is characterized by higher energy efficiency conversion and enhanced flexibility of feedstock, as regard moisture content, size and composition. One of the most important parameter for the syngas use is the ratio H2/CO. In the reported work the focus is placed onto tuning syngas composition as a desirable feature of biomass gasification. Such process flexibility can be achieved by introducing steam in the system, by using oxygen or enriched air by a special reactor design (indirect gasification) ; by adding a catalytic step of water gas shift (WGS) or a CO2 adsorption step. Novelty of this work can be seen in the measuring evolution of gasification at different heights of the reactive biomass bed. This was achieved by probing syngas streams directly from the core and analysing it onsite to measure the content of H2, CO2, CO, CH4, CnHm, N2, O2. The chemical composition was correlated to the thermal profile provided by in bed measurements of 7 thermocouples, plus 4 in freeboard. The plant is also equipped with section based on Water Gas Shift to increase the H2 yield. The use of steam increased the hydrogen content and positively affected the molar ratio H2/CO that reached the steady value of 1 by using air and steam as gasifying medium. In the tests, the biomass was completely converted in gaseous and liquid energy carriers with cold gas efficiency (CGE) ranging from 61% to 75%. With WGS the level of H2 in gas reached 37 vol % (53 vol % on nitrogen free basis) corresponding to a new increased H2/CO ratio of 9.

updraft gasification ; syngas composition ; almond shells ; lignocellulosic feedstock ; water gas shift

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