Study On The Mechanism Of Low Temperature Pyrolysis Of Lignocellulosic Biomass In Molten Nitrate Salt

Molten salt,used as a carrier for heat transfer and biomass pyrolysis,is able to improve the pyrolysis efficiency and product quality.In view of the various lignocellulose components of biomass with high volatile content,complex devolatilization process,rich alkali metal and alkaline earth metal,this study focused on the low-temperature molten nitrate system applied to biomass pyrolysis technology.The temperature of gaseous product formation was reduced and the pyrolysis characteristics and mechanism of biomass in molten salt was revealed in this study.The specific contents of study included optimization of the nitrate system and working conditions,investigation of the reaction path and reaction kinetic characteristics of lignocellulosic components;exploration of the influence of ash components on the thermal and catalytic properties of molten salt.The main conclusions drawn from this research are as follows:NaNO₃-KNO₃-NaNO₂ system was screened out as the pyrolysis medium by comparing the pyrolysis characteristics of beech wood,a typical biomass,in NaNO₃-KNO₃-NaNO₂,NaNO₃-KNO₃ and NaNO₃-NaNO₂ systems.The NO₂^-in this system strongly promoted biomass pyrolysis.Compared with the traditional pyrolysis process under the same condition,the pyrolysis temperature point of beech wood can be reduced by 70 ^oC.At 300 ^oC,the yield of liquid and gaseous products increased by 26.2%and 45.1%respectively.By analyzing the pyrolysis behavior of biomass municipal solid waste with different components,the total yield of gas and liquid product from waste paper was 30.4%lower than the average yield from bamboo chopsticks and bark,indicating that cellulose was less reactive in this molten salt system.In order to reveal the pyrolysis mechanism of biomass lignocellulosic components in molten salt system,the pyrolysis behavior of three typical compounds and biomass,after removal of specific components in molten nitrate were deeply analyzed.The formation of CH₄ and H₂ came from methoxyl cleavage,secondary pyrolysis and reforming of feedstock,respectively.Molten salt promoted the decomposition of propyl chains and aromatic rings in lignin structure,and the cleavage of ether bonds was conducive to the generation of CO.A small amount of phenol（from pyrolysis of lignin）reacted with NO₃^-/NO₂^-to form nitrophenol.The lignin inhibited the reaction of part of hemicellulose in molten salt.Hemicellulose and cellulose reacted with NO₃^-/NO₂^-to produce amines and nitrogen-containing heterocycles.The online release process of gas products and the kinetic mechanism of biomass pyrolysis reaction in molten salt were investigated,based on the combination of molten salt pyrolysis reactor and process mass spectrometer.A kinetic model of gas release from beech pyrolysis in molten salt was constructed by elementary pyrolysis reaction of three typical components.The release of CO and H₂ was mainly from the pyrolysis of lignin,CO₂ was mainly from the pyrolysis of cellulose,and CH₄ was from the pyrolysis of both hemicellulose and cellulose.The fitting correlation coefficients of H₂ and CH₄ were higher than those of oxygen-containing gases including CO and CO₂.Combined with the chemical thermodynamic calculation results,it was shown that O element in the anion group of molten salts was involved in the generation and release of some oxygen-containing gases.Molten salt thermal treatment of simulated biomass ash was carried out to explore the impacts of the interaction between typical ash components in biomass and molten salts on the thermophysical properties and catalytic effects were explored.KCl and K₂SO₄ were mutually soluble with molten salt and decreased the melting point of molten salt.Meanwhile they also promoted the pyrolysis of beech wood,which volatile yield increased by more than20.6%.CaCO₃ was slightly soluble in molten salt and easy to precipitate at the bottom.Amorphous Si O₂ reacted with NaNO₂ in molten salt to form sodium silicate,which was immiscible with molten salt as well.In addition,the two precipitations have little effect on the chemical reaction characteristics of molten nitrate.