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Materials development against hot corrosion in biomass gasification facilities

: Wiltner, A.; Gaitzsch, U.; Walther, G.; Weißgärber, T.; Drechsler, E.; Wolf, G; Masset, P.; Hülscher, M.; Hill, H.; Ossenberg-Engels, A.

Karlsruhe Institute of Technology -KIT-:
Energy, Science and Technology 2015. Book of Abstracts : The energy conference for scientists and researchers; International Conference & Exhibition, 20-22 May 2015, Karlsruhe, Germany
Karlsruhe: KIT, 2015
Energy, Science and Technology Conference & Exhibition (EST) <2015, Karlsruhe>
Fraunhofer IFAM, Institutsteil Pulvermetallurgie und Verbundwerkstoffe Dresden ()
Fraunhofer UMSICHT SuRo ()

Biomass gasification is a promising technique within the energy mix. Nowadays, wood pellets are used as fuel. Smaller gasification units in decentralised operation could use waste materials, such as wood chips, straw or chicken dung as alternative feedstocks. In this case, the ashes, slugs and gaseous compositions carry higher sulphur and chloride contents. This results in aggravated hot corrosion at distinct components, such as gas inlet nozzles. Moreover, these parts are stressed by high temperatures as well. In order to avoid premature damage of such parts, materials are developed within the research project KorrMat which provide chemically resistant oxide layers for passivation. Since temperatures above 800°C and 1000°C are reached under operating conditions together with low oxygen contents (λ < 1), alumina or silica former based materials are favoured. Consequently, the investigated materials are tested in terms of oxide layer formation and hot corrosion resistance. For the oxidation behaviour tests thermal treatments in dry and wet air are used as screening. The hot corrosion conditions are adjusted similar to those in the gasifier from Qalovis by using original ashes from wood chips and controlled sulphur and chloride additions inside the test gas atmospheres. In this paper, the materials choice, first results from thermal treatments in hot corrosion experiments and a detailed characterization of the different gaseous and solid species within the hot zone of the gasification device are thoroughly discussed.
Within the project KorrMat materials are developed which provide high Al or Si contents resulting in passivating alumina or silica scales. In addition, the high Al or Si concentrations act as a reservoir at the surface for a possible self-healing behaviour under operating conditions. These new compositions are compared to well established materials exhibiting high Cr contents together with Ni based alloys which are already in use under extreme conditions; such as alloy 625 in waste incinerators. The materials under investigation are based on alloy powders which are consolidated to bulk materials by using hot pressing or the fast sintering technique SPS. Moreover, this powders were deposited by atmospheric plasma spraying (APS) or high velocity oxygen fuel spraying (HVOF) on high temperature materials (Nicrofer 3220H). Bulk materials as well as sintered powders for application experiments were tested in terms of oxide layer formation in a first step. After the oxidation experiments at 850°C and 1000°C in dry and wet air a first sample selection has been done. For the thermal treatments under hot corrosion conditions densified powders as well as coated specimens have been investigated. Since the hot corrosion conditions are the major factors determining the life time of the gas inlet nozzles, the gasification process and especially the ashes and atmospheres have been investigated in detail. From these studies the composition of the ashes and gas components carried by the fuels were derived and used in laboratory hot corrosion experiments. For these experiments test pieces were partially embedded in the original ashes of the biomass gasifier and tested at temperatures of 850 °C and 1000 °C with a constant flow of the named gas composition. This offers the advantage of the investigating three different contact zones at one test piece simultaneously: Zone 1: ash / test piece; zone 2: zone ash / gas / test piece; zone 3: zone gas / test piece. Using alternative feedstocks considerably higher concentrations of elements which initiate corrosive attacks have been determined. Moreover, the change of feedstocks will influence the sticking coefficient of the resulting ashes which, therefore, have been evaluated as an additional possible reason for accelerated corrosion.
Conclusions and/or Outlook
First results on materials development, which can be used in highly corrosive and hot atmospheres, are presented. Parallel to the material development detailed studies on ash composition and gas phases have been done leading to proposals for further materials development. The materials showing the best performances at lab scale will be used for near net shape consolidation of tools as well as a long time application within the gasifier.
Acknowledgement: The project KorrMat is funded by the Federal Ministry of Education and research under grant agreement no. [03X3585 KorrMat].