An investigation into the interaction of biomass ash and K-feldspar as bed material in fluidized-bed combustion and gasification

Abstract

Increasing concentrations of greenhouse gases such as CO2 in the atmosphere are driving a development towards technologies using renewable sources to provide for the global energy demand. Biomass plays a major role in the substitution of fossil fuels with renewable energy carriers. It constitutes a suitable feedstock for biomass gasification technologies in which biomass is converted into a secondary energy carrier, called product gas. Product gas can be used to generate electricity and chemical products such as hydrogen, Fischer-Tropsch diesel, or alcohols. Dual fluidized-bed gasification systems circulate the bed material olivine between two separate combustion and gasification units. Interaction of biomass ash and bed material causes the formation of calcium-rich layers on bed materials which provide catalytic activity towards tar reduction. A shift from more expensive bed materials such as olivine to low-cost bed materials such as K-feldspar could further improve the gasification process. The empirical part of this study comprised three combustion experiments in a 5 kW bench-scale fluidized-bed reactor. Three pelletized fuel mixtures were combusted at 780-800°C: Straw (100 % straw), Chicken Litter Low (CLH, 90 % straw, 10 % chicken litter), and Chicken Litter High (CLH, 70 % straw, 30 % chicken litter). 540 g of fresh potassium (K)-feldspar par-ticles with a grain size between 200-250 µm were used as bed material. Bed samples were taken during combustion and after defluidization. The samples were analyzed with scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS). Individ-ual and layered element mappings were evaluated to determine qualitative layer formation during initial hours of operation. Additional quantitative line scans were used to establish the thickness and elemental distribution of the qualitative layers. The total elemental composition of the used K-feldspar particles was obtained with X-ray fluorescence (XRF) spectrometry. The experiments showed that the combustion of Straw and CLL resulted in very little inter-action between the produced biomass ash and the K-feldspar particles. The relatively low calcium (Ca) content of both fuel mixtures lead to significantly diminished layer formation over time. No layers were found on the bed particles. Conversely, the high potassium (K) and silicon (Si) content in the ashes made for the formation of low-melting K-silicate ash par-ticles that adhered to the surface of the bed materials and caused faster bed agglomeration. The results from the combustion of CLH showed initial layer formation on the bed particles. The higher Ca content in CLH facilitated the formation of continuous Ca-layers as found with the qualitative element mappings. Phosphorus (P) from the biomass ash was also found in the Ca-layers. Additionally, the total elemental composition of the used K-feldspar bed material included the elements Ca, Al, P, S, and Mg which were part of the biomass ash composition of CLH. This led to the conclusion that the layer formation process on K-feldspar particles could be ash-related. Future experiments should aim for advanced layer formation on K-feldspar particles to gain further insights into the layer formation mechanism on K-feldspar.