299 / 2018-07-26 14:40:44

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

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Two-step pyrolysis is a method to release pyrolysis products stepwise for improving the quality of bio-oil and enriching value-added chemicals, and the acid washing pretreatment has been proved to be an effective approach to remove alkali and alkali earth metals and increase the yield and quality of bio-oil. Therefore, the two-step pyrolysis combined with acid washing pretreatment was carried out to investigate the coupling effects of the two methods on improving the quality of bio-oil and selectivities of value-added chemicals by Py-GC/MS. In the experiment, walnut shell was used as material and TG-FTIR was used to analyze the pyrolysis characteristics of the sample. The first step pyrolysis temperature (T1) was set from 350°C to 550°C, and the second step pyrolysis temperature was 650°C. The results indicated that the metallic species in walnut shell was obviously reduced after acid washing, hence the pyrolysis process was postponed and the bio-char yield decreased. Compared with two-step pyrolysis without washing pretreatment, the coupled method reduced the contents (peak area/%) of acids, ketones, alcohols, aldehydes, phenols and increased the contents of N-containing compounds, sugars and furans in the first step. The hydrocarbons content decreased and the sugars content was further increased in the second step. Besides, some single value-added chemicals were also enriched, such as furfural (6.19%), 2-Methoxy-4-methylphenol (6.03%), guaiacol (3.41%), levoglucosan (34.29%) and 4-vinylphenol (6.99%), etc. It could be preliminarily concluded that T1 from 400°C to 450°C was the optimum condition to achieve high selectivities toward value-added chemicals and high quality bio-oil in the first or second step of the two-step pyrolysis of lignocelluloses biomass coupled with acid washing pretreatment.