Metal oxide-molten salt catalyzed pyrolysis: Improving the energy conversion efficiency of wheat straw

Abstract

This study systematically investigates the pyrolysis process of wheat straw in a ternary carbonate molten salt system (Li₂CO₃-Na₂CO₃-K₂CO₃) with the addition of different metal oxides (CaO, Al₂O₃, Fe₂O₃, TiO₂). The effects of the ratio of molten salt, type of metal oxide, and pyrolysis temperature on the pyrolysis products were examined. As the temperature increased, in the MS15 (wheat straw: molten salt = 1:15) system, the yield of gas-phase products rose from 21.21 wt% to 82.57 wt%, while the yields of solid products and bio-oil rapidly decreased to 23.76 wt% and 37.60 wt%, respectively. Additionally, the system exhibited the highest lower heating value (LHV) within the temperature range of 450–750 ℃. Therefore, the optimal ratio of wheat straw to carbonate molten salt is 1:15. Furthermore, when adding four metal oxides (Al₂O₃, CaO, Fe₂O₃, TiO₂) to the MS15 system, it was found that the addition of CaO (CaO-MS15) resulted in the highest LHV across all pyrolysis temperatures. Notably, at 550 ℃, the maximum LHV was 21.89 MJ/Nm³, an increase of 5 MJ/Nm³ compared to MS15. Analysis of the solid phase using XRD, XPS, and SEM indicated that the active sites on the surface of CaO facilitated the cleavage and deoxygenation of bio-oil on the biochar during the pyrolysis process, breaking it down into small molecular gases.