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- ItemRice husk derived porous silica as support for pd and CeO2 for low temperature catalytic methane combustion(Basel : MDPI, 2019) Liu, Dongjing; Seeburg, Dominik; Kreft, Stefanie; Bindig, René; Hartmann, Ingo; Schneider, Denise; Enke, Dirk; Wohlrab, SebastianThe separation of Pd and CeO2 on the inner surface of controlled porous glass (CPG, obtained from phase-separated borosilicate glass after extraction) yields long-term stable and highly active methane combustion catalysts. However, the limited availability of the CPG makes such catalysts highly expensive and limits their applicability. In this work, porous silica obtained from acid leached rice husks after calcination (RHS) was used as a sustainable, cheap and broadly available substitute for the above mentioned CPG. RHS-supported Pd-CeO2 with separated CeO2 clusters and Pd nanoparticles was fabricated via subsequent impregnation/calcination of molten cerium nitrate and different amounts of palladium nitrate solution. The Pd/CeO2/RHS catalysts were employed for the catalytic methane combustion in the temperature range of 150–500◦C under methane lean conditions (1000 ppm) in a simulated off-gas consisting of 9.0 vol% O2, and 5.5 vol% CO2 balanced with N2. Additionally, tests with 10.5 vol% H2O as co-feed were carried out. The results revealed that the RHS-supported catalysts reached the performance of the cost intensive benchmark catalyst based on CPG. The incorporation of Pd-CeO2 into RHS additionally improved water-resistance compared to solely Pd/CeO2 lowering the required temperature for methane combustion in presence of 10.5 vol% H2O to values significantly below 500◦C (T90 = 425◦C). © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemAsh transformation mechanism during combustion of rice husk and rice straw(New York, NY [u.a.] : Elsevier, 2022) Beidaghy Dizaji, Hossein; Zeng, Thomas; Hölzig, Hieronymus; Bauer, Jens; Klöß, Gert; Enke, DirkBiomass is an alternative energy resource to fossil fuels because of its potential to reduce greenhouse gas emissions. However, ash-related problems are serious obstacles for this development, especially for the use in combustion plants. Thus, design and operation of biomass boilers require detailed understanding of ash transformation reactions during thermochemical conversion. To evaluate ash transformation in silica-rich biomass fuels, rice husk and rice straw were selected because of their abundance, limited utilization conflicts with the food sector, as well as their potential in both energy and material applications. This paper reveals ash transformation mechanisms relevant for the ash melting behaviour of silica-rich biomass fuels considering chemical and phase composition of the ashes. In this regard, several advanced spectroscopic methods and diffractometry were employed to characterize the materials. The ash transformation reactions and the viscosity were simulated using thermodynamic equilibrium calculations and a slag viscosity modeling toolbox. The results illustrate the impact of impurities on the atomic structure of the silica resulting in an altered ash melting behaviour and viscosity of the silica-rich ashes. Chemical water washing, acid leaching, and blending of rice straw with rice husk strongly influenced the chemical composition of the ashes and improved ash melting behaviour. The analysis also revealed the correlation between the crystalline fraction and the porosity in silica-rich biomass ashes, as well as a crystallinity threshold. These findings are highly relevant for future investigations in boiler designs and production of biogenic silica for material applications.