Influence of lignin content on pyrolysis characteristics of biomass based on part of lignin removal

Abstract: Biomass pyrolysis is an important way for converting biomass to high-value products. Studies on pyrolysis behavior and products distribution of biomass are of great importance to the research on the interactions between 3 components of biomass. The interaction between lignin and holocellulose under the original cross-linking structure was firstly investigated, as well as the impact of the interaction on products' distribution. It offered theoretical basis to better understand pyrolysis of full components. This paper used sodium chlorite - acetic acid method to remove part of lignin of the peanut shell and walnut shell without removing holocellulose components. After different processing time, peanut shell and walnut shell with different lignin contents were obtained eventually. The pyrolysis of experimental sample was conducted in a fixed bed reactor with pyrolysis temperature of 300, 500, and 700 ℃, respectively. Materials and products characteristics were investigated with various testing approaches, such as elements analysis (Vario Micro cube, Germany), ultimate analysis, gas chromatography - mass spectrography (GC-MS) (7890A/5975C, Agilent, USA) and gas chromatography (micro GC 3000, Agilent, USA). The results showed that the solid yield increased with the increase of lignin content in sample at 300 ℃, while the liquid yield obviously decreased and gas yield decreased, too. With the temperature rising to 500 or 700 ℃, the solid yield visibly decreased compared with that at 300 ℃ and the solid yield almost didn't change at all with the increase of lignin content. Besides, there wasn't any decrease but slight increase of liquid yield at 500 and 700 ℃. With the lignin content of sample increasing, the proportion of CO2 decreased significantly and CH4 clearly increased while CO increased slightly and H2 content almost didn't change at all with low content at 500 ℃. The reason to the decrease of CO2 may be that lignin inhibited decarboxylation reaction during pyrolysis. When the temperature rose to 700 ℃, the content of H2 suddenly increased compared with that at 500 ℃ and decreased with the increase of lignin content. Besides, the CO content increased significantly with the increase of lignin content and the variation tendency of CH4 was consistent with that at 500 ℃. GC-MS analysis of pyrolysis oil at 500 ℃ showed that the relative amount of levoglucosan, which is the product of cellulose pyrolysis, gradually increased with the increase of lignin content while cellulose content gradually decreased. It indicated that lignin promoted the reaction of cellulose cracking into levoglucosan. Besides, the typical pyrolysis products of hemicellulose, such as hydroxyacetone and acetic acid, gradually decreased with lignin content increasing. Guaiacyl substituted derivative, the important lignin pyrolysis products, gradually increased with lignin increasing, while the trend of the yield of guaiacol, which is the primary pyrolysis product of lignin, turned out to be the opposite. It indicated that holocellulose slowed down the demethoxy reaction of guaiacyl and promoted removing reaction of lignin fatty hydrocarbon substituent. In conclusion, the study has the positive significance to understand the characteristics of products' formation and the results provide a theoretical basis for further usage of pyrolysis oil.