Novel Application of Pretreatment and Diagnostic Method Using Dynamic Pressure Fluctuations to Resolve and Detect Issues Related to Biogenic Residue Ash in Chemical Looping Gasification

Abstract

Biogenic residues are a promising feedstock to produce liquid biofuels via chemical looping gasification (CLG), but they form ashes with a high inorganic matter content, thus causing agglomeration and deposition in CLG-fluidized beds made of oxygen carriers (OC). The aim of this work is to develop pretreatments for residual biomasses to prevent this issue. Raw forest pine (as a reference material) and wheat straw residues were considered. The latter were pretreated by torrefaction at 250, 260, or 270 °C and through the washing of torrefied biomasses. Torrefaction encouraged a de-chlorinating effect, while washing allowed the removal of 30–40% of S, 60–70% of K, and 40–50% of P. The analysis of pressure fluctuation signals (standard deviations and dominant frequencies) was utilized to verify the improvement of the performance of treated biomass in fluidized beds: three OCs were, respectively, coupled with ashes from all biomasses, then fluidized from 700 to 1000 °C at two and three times the minimum fluidization velocity. The diagnostic method used to analyze pressure fluctuations was shown to be effective for detecting the incipient fading of bubbling fluidization. This phenomenon was related to the agglomeration or the severe fragmentation of OC particles mixed with ashes, thanks to scanning electron microscopy and particle-size measurements. These characterizations and pressure fluctuations analyses confirmed the general improvement of wheat straw performances after pretreatments.