Mechanisms Of Lignocellulosic Biomass Pyrolysis At Low Temperature With Generalized Two-dimensional Correlation Infrared Spectroscopy

Biomass is an important resource for the development of human society.The thermochemical conversion of biomass into gas,liquid and solid fuel can effectively replace petrochemical products,thereby reduce fossil energy consumption and CO₂ emission.The characteristics of pyrolysis products are related to biomass component and ash ingredient.Meanwhile,the pyrolysis condition is also critical to control products quality.This paper was intended to study the biomass pyrolysis mechanism below 600℃based on the pyrolysis behavior of cellulose,xylan,alkali lignin and nature biomass（bamboo and corncob）.Generalized two-dimensional infrared correlation spectroscopy（2D-PCIS）was introduced to study the molecular structure evolution of char.The correlation of gas,liquid and solid products formation were investigated based on 2D-PCIS results.The pyrolysis experimental of cellulose,xylan,alkali lignin was performed on a vertical reactor.The results showed that the C-OH at C₂/C₃/C₆ position of pyran ring was easily to dehydrate during the pyrolysis of cellulose at low temperature（≤350℃）.The dehydration of C₂-C₃ hydroxyl and subsequent keto-enol tautomerism reaction promoted the formation of levoglucosenone（LGO）precursor,and the dehydration of C₃-C₆ hydroxyl was easily to form 1,4:3,6-Dianhydro-α-D-glucopyranose（DAGP）precursor.After 350℃,the glycosidic bond was rapidly broken and the etherification reaction of C₁-C₆ position accelerated the levoglucosan（LG）generation.According to the 2D-PCIS results,the change of C=O groups were changed earlier than that of aliphatic C-O-C in cellulose char,which indicated that the dehydration and keto-enol tautomerism reaction of pyran-OH was earlier than ring scission in the pyrolysis process of cellulose.No anhydrosugar was detected in xylan bio-oil because of few C₆ structure was exit in xylan.Xylan was easily to decomposed,and the depolymerization and ring scission occurred below 200℃.According to the results of 2D-PCIS,the dehydration and keto-enol tautomerism reaction of C-OH maybe earlier than the ring scission of xylose.After 225℃,the main structure of xylan was completely fragmented,and a large amount of C=O,C=C,C-O-C intermediates are formed.The intermediates then reconstructed into small aliphatic molecules and furan,cyclopentanone,aromatic ring compounds,which was similar to the molecular reconstruction mechanism of cellulose.Lignin was more difficult to decompose compared with cellulose and xylan.The weight loss of alkali lignin before 250℃ was derived from the cleavage of unstable structure in the side-chain of benzene ring.After 300℃,the depolymerization of lignin was increase rapidly,meanwhile the fractures of C-OH,-CxHy,C=O,C=C structures produced small aliphatic compounds,such as CO₂,CO,acetic acid,sulfides,etc.When the pyrolysis temperature exceeded 350℃,the char structure of cellulose,xylan,alkali lignin was composed of large amounts of low-order fused ring（2⁵ ring）.The aromatic rings were substituted with each other to form an amorphous,three-dimensional network.Furtherly,the removal of aliphatic structure and the dehydrogenation reaction after 450℃ accelerated the production of H₂ and CH₄,the char structure was consisting of high-order fused ring（2×2⁴×4 ring）.In order to determine the detailed effects of ash composition on biomass pyrolysis,11alkali and alkaline earth metal salts were used to study the cracking products of cellulose and bamboo.The results indicated that all metal additives could promote ring scission of cellulose.Na₂CO₃ and K₂CO₃ were easily to attach to the surface of biomass which could strengthen the solid-solid contact reaction in the process of biomass pyrolysis.The solid-solid contact reaction promoted the cracking of cellulose and bamboo to form a large amount of gas products,and the char structure was more condensed.The relative content of cyclopentanone and small aliphatic compounds were high in cellulose bio-oil,and phenyl compound content was as high as 75%in bamboo bio-oil.MgCl₂ and CaCl₂ were prone to deliquescence in the process of mechanical mixing and pyrolysis heating up,which resulted to the weakening of H-bonding network and biomass components linkage.MgCl₂ and CaCl₂ were broken into HCl in the atmosphere of biomass pyrolysis over 500℃.The effect of deliquescence and acid atmosphere promoted cellulose dehydration and ring scission which leaded pyran products reduction in cellulose bio-oil.However,the relative contents of pyran product in bamboo bio-oil was increased,it may be related to the fiber structure damage of biomass.NaCl,KCl,Na₂SO₄,K₂SO₄,MgSO₄,CaCO₃ addition promoted the volatile cracking and char formation throughby particle contact,but the influence of gas,bio-oil composition was little.Bamboo and corn cob were used as natural biomass to investigate the influence of ash and fiber structure to biomass pyrolysis.During the pyrolysis of bamboo and deashed corncob,the cellulose cracking was inhibited.The pyrolysis char obtained at 300℃ showed strong diffraction peaks（101,002）of cellulose and large amounts of C-O structure appeared in FTIR.The pyrolysis products of bamboo and corncob indicated that the cleavage of hemicellulose-lignin linkage and hemicellulose-cellulose H-bonding maybe earlier than the main component of biomass,which resulted in a high production of 4-vinylphenol and low production of levoglucosan.Hemicellulose decomposed rapidly in the isothermal pyrolysis of bamboo at 350℃,the release peak of CO₂ and C=O structures is earlier than CO.In the bamboo pyrolysis of 550℃,the release peak of CO and C-O-C structures were earlier than CO₂ and C=O because of quickly degradation of cellulose.Ash removal and ash addition had a great influence on biomass pyrolysis products.The pyrolysis results of corncob and deashed corncob showed that K,Ca minerals in ash could accelerate the depolymerization of biomass macromolecules.Cellulose and hemicellulose were promoted to open ring and then reconstructed into small aliphatic molecules,lignin depolymerized into large amounts of monophenyls.Meanwhile,K,Ca minerals promoted the deoxygenation,dehydrogenation of char,and the condensation structure of char was significantly increased.In addition,ash addition promoted the secondary cracking of volatiles,the ring scission of pyran product formed a large amount of small aliphatic molecules,cyclopentanone and phenyl compounds.