Synergistic effect of washing pretreatment and co-pyrolysis on physicochemical property evolution of biochar derived from bio-oil distillation residue and walnut shell

In this study, walnut shell was washed by deionized water, acetic acid and hydrochloric acid, followed by preparation of advanced biochar via co-pyrolysis with bio-oil distillation residue. The influence of washing pretreatment on feedstock was determined by thermogravimetric analyzer, Fourier transform infrared spectroscopy and X-ray diffraction, meanwhile, the physicochemical property evolution of biochar was characterized by Elemental analyzer and Raman spectroscopy. The results indicated washing pretreatment would change the distribution of components, decrease ash content and further strengthen the ordered arrangement of microcrystals. Moreover, washing pretreatment had a negative effect on biochar yield, yet co-pyrolysis of bio-oil distillation residue and washed walnut shell would enhance the relative increase rate of biochar yield and optimize the elemental distribution (i.e. increased carbon content). In addition, biochar yielded from washed walnut shell exhibited a relatively ordered carbon structure, whereas co-pyrolysis would significantly disrupt the arrangement of microcrystals. More importantly, hydrochloric acid washing and co-pyrolysis both had a positive effect on enhanced thermal stability and increased carbon purity of biochar.