Comparative study on the pyrolysis behaviors of rice straw under different washing pretreatments of water, acid solution, and aqueous phase bio-oil by using TG-FTIR and Py-GC/MS

Washing pretreatment is a promising method for upgrading biomass by removing alkali metal and alkaline earth metals (AAEMs). In this study, three types of solution with different pretreatment severities (i.e., water (H2O), dilute hydrochloric acid (HCl) solution, and aqueous phase bio-oil (APBO)) were used to study the effects of the washing pretreatment on the pyrolysis behaviors and kinetics of rice straw. The pyrolysis experiments of raw and pretreated rice straw (Raw-RS, H2O-RS, HCl-RS, and APBO-RS) were carried out using TG-FTIR and Py-GC/MS. Results show that among the three types of washing pretreatment methods, the APBO washing pretreatment has the highest removal efficiency of AAEMs: 99.7% of K, 91.7% of Na, 96.6% of Mg, and 95.2% of Ca. According to TG-FTIR results, only one sharp mass loss peak was present in the DTG curves of Raw-RS and H2O-RS, whereas two mass loss peaks (one sharp peak and one shoulder peak) were observed in HCl-RS and APBO-RS. The evolution pattern of the volatile compounds (H2O, CH4, CO, CO2, and C=O stretching) was thoroughly investigated, and the absorbance intensity of these components increased with the washing pretreatment of APBO and HCl. The APBO washing pretreatment had the highest activation energy in the conversion rate of 0.2-0.75, followed by HCl and H2O washing pretreatment. According to the Py-GC/MS results, the washing pretreatment reduced the relative contents of acids, ketones, furans, and phenols in bio-oil, whereas removing of AAEMs increased the relative contents of anhydrosugars (mainly levoglucosan). Among the three types of washing pretreatments, the APBO washing pretreatment had the most remarkable influence on the property of bio-oil, and this indicates that the APBO washing pretreatment is a promising method for upgrading biomass and its pyrolytic product.