Study on two-step pyrolysis of walnut shell coupled with acid washing pretreatment

Abstract

Two-step pyrolysis(TSP) of walnut shell(WS) coupled with acid washing pretreatment was carried out to investigate the coupling effects on improving the quality of bio-oil and selectivities of value-added chemicals by Pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS). The first step temperature(T1) was set from 350 °C to 550 °C, and the second step temperature was set as 650 °C. Thermogravimetric analyzer coupled with FTIR spectrophotometer(TG-FTIR) was also used to analyze the pyrolysis characteristics of the sample. The results indicated that metallic species in walnut shell was obviously reduced after acid washing (5%HCl), hence the pyrolysis process was postponed and the bio-char yield decreased. TSP coupled with acid washing could reduce the contents of acids, ketones, alcohols, aldehydes, phenols and increased the contents of N-containg compounds, sugars and furans in the first step. The hydrocarbons content decreased and the sugars content further increased in the second step. Besides, some value-added chemicals were also enriched, such as furfural(6.19%), levoglucosan(34.29%) and 2-methoxy-4-methylphenol(6.03%), etc. It could be preliminarily concluded that T1 from 400 °C to 450 °C was suitable for achieving the high quality bio-oil and high selectivities of value-added chemicals.