基于木质素部分脱除及其含量对生物质热解特性的影响

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

该研究采用固定床热解炉,研究不同木质素含量花生壳、核桃壳样品的裂解行为,利用元素分析、工业分析、气相色谱-质谱联用以及气相色谱法对原料和热解产物进行分析,探究木质素与综纤维素在原始交联结构下的交互作用及其对热解产物分布特性影响。研究结果发现,300℃热解条件下,随着木质素含量的增加,样品中固体产率增加,液体产率和气体产率下降。500、700℃热解条件下,固体产率相比300℃有大幅度的下降,且随样品中木质素含量的增加,固体产率无明显变化,液体产率稍微增加,气体产率下降。500℃时,H2产率很低,随样品中木质素含量的增加,CO2含量减少,CH4含量增加,CO含量有稍微的上升。而700℃时,综纤维素的脱氢、缩合、成环会生成大量的H2。同时,木质素能够促进综纤维素分解生成大量左旋葡聚糖,并抑制其分解;而综纤维素抑制木质素单体愈创木基的脱甲氧基反应,促进苯丙烷基的脱烷基反应,形成更多的酚类化合物。该研究对于生物质组分间交互和产物形成特性研究具有积极意义。

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 between3components 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 of300,500,and700℃,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 GC3000,Agilent,USA).The results showed that the solid yield increased with the increase of lignin content in sample at300℃,while the liquid yield obviously decreased and gas yield decreased,too.With the temperature rising to500or700℃,the solid yield visibly decreased compared with that at300℃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 at500and700℃.With the lignin content of sample increasing,the proportion of CO2decreased significantly and CH4clearly increased while CO increased slightly and H2content almost didn't change at all with low content at500℃.The reason to the decrease of CO2may be that lignin inhibited decarboxylation reaction during pyrolysis.When the temperature rose to700℃,the content of H2suddenly increased compared with that at500℃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 CH4was consistent with that at500℃.GC-MS analysis of pyrolysis oil at500℃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.