Layer formation on quartz bed particles during fast pyrolysis of grass

Abstract

Commercial fast pyrolysis technologies use bed materials, normally natural sand mainly consisting of quartz, acting as circulating heat carrier materials. Nowadays, the commercial conversion of biomass into fast pyrolysis bio-oil (FPBO) is still using ash-lean woody residues as a feedstock since the application of more abundant and possibly cheaper ash-rich agricultural biomass is currently at a significantly lower technology readiness level (TRL). To promote FPBO production from ash-rich biomass, the ash-related issues during the operation process need to be further studied. In the present investigation, the characteristics and formation process of layers formed on quartz bed particles, collected from a bench-scale fast pyrolysis unit based on the rotating cone technology, were studied. Two grass residues, representative of typical Si-K-rich agricultural biomass fuels, were used as feedstocks. Quartz bed particles at different sampling times from startup with fresh bed particles were collected. Scanning Electron Microscopy/Energy-Dispersive Spectroscopy (SEM/EDS) was employed to characterize the layer properties. Bed particle layers exhibited an uneven and discontinuous distribution on the quartz surface. This distribution over bed particles, as well as layer thickness, increased with the operational time. The dominating elements contained in layers were Si, K, Ca, and Cl (excluding O), which resembled that of individual bed ash particles found in the bed samples. In addition, the interpretation of the results was supported by thermodynamic equilibrium calculations. The findings suggest that the process of layer formation was governed by the direct adhesion of non-melted bed ash particles during the fast pyrolysis of grass.