Comprehensive Description of the Interactions between Silicate Bed Materials and Biomass Ash during DFB gasification

Licentiate thesis, 2020

The utilization of biomass for energy generation is a CO2 neutral alternative to fossil fuel. A suitable way of thermal conversion of biomass is dual fluidized bed (DFB) gasification where biomass is converted to a raw gas which can subsequently be used for chemical synthesis. DFB gasification relies on bed material which functions as a heat carrier to transport thermal energy from the combustor to the gasifier. The bed material’s interaction with the inorganic ash fraction released from the biomass can results in detrimental agglomeration which necessitates replacement of the bed material, or beneficial activation which reduces problems associated with unwanted side-products (tar) of the process. Whether agglomeration or activation occurs is dependent on ash composition as well as on the utilized bed material. Different materials have been tested as bed materials for gasification of which three (quartz, olivine, and alkali-feldspar) are the focus of this work. The materials resemble each other in being naturally occurring minerals which are based on silicate networks.

The bed material samples were analyzed with multiple analysis methods after their use in the Chalmers DFB gasifier (such as SEM-EDS, XRD and TEM). The interaction of biomass ash and bed material led to the accumulation of ash-derived elements on the surface of the bed particles. For all three materials, the development of a three layered structure was observed. Common for all materials was that a Mg-rich layer was found on the surface and a Ca-rich layer underneath. The layers between the particles’ core and the Ca-rich layer were different for the three materials and is therefore dependent on the utilized bed material. The accumulation of ash-derived elements led to an increase in activity towards tar reduction while at the same time depolymerization of the silicate network of the minerals could be observed.