Experimental Study On Catalytic Cracking And Supercritical Ethanol Hydrocracking Of Wood Tar

Biomass can be converted into bio-oil,bio-char,and combustible gas with high added value through thermal conversion technologies,which will help solve the problems of China's energy shortage,large dependence on fossil fuels from foreign countries,unreasonable energy structure,and environmental pollution.However,Wood tar is inevitably produced during the thermal conversion of biomass.Wood tar has complex compositions,including phenols,ketones,heterocyclics and other substances,especially being rich in a variety of macromolecular aromatic hydrocarbons.These aromatic compounds have not been used effectively for the time being.In addition,it also has strong acidity,high moisture content,low calorific value and other properties.These characteristics of wood tar seriously hinder its effective use and application of biomass energy technology,and cause energy waste and environmental pollution.Therefore,it's of great significance to explore suitable methods for the upgrading and utilization of wood tar.Firstly,various analytical methods were carried out to analyze the physicochemical properties of wood tar,such as elemental analysis,water content analysis,pH testing,and gas chromatography-mass spectrometry?GC-MS?.Through these analysis,it was found that compared with gasified tar and bio-oil,wood tar has higher C content and lower H content.In addition,the wood tar has a relatively strong acidity?PH=3.32?.The calorific value is low and the high heat value is 24.87 MJ/Kg.The moisture content is high?10.3wt%?.The composition of wood tar is extremely complicated.According to GC-MS analysis,there are phenols,ketones,ethers,furans,and polycyclic aromatic hydrocarbons in wood tar,among which phenols account for a large proportion.These phenols are mainly derived from the cracking of lignin in biomass.Then,the catalytic pyrolysis experiment of wood tar was carried out on a vertical two-stage furnace.A catalyst-free control group was carried out to study the effect of ZSM-5 catalyst.And the catalytic cracking characteristics of wood tar at different catalytic temperatures?600,700,800°C?were investigated.The results showed that the gas yield increased,the liquid yield decreased and the solids yield remained basically unchanged with the increase of catalytic temperature.Under the catalysis of ZSM-5 zeolite,the change trend was more obvious.The main components of the gas product are CO and H₂ gases.The total output of these two gases can reach 11.1mmol/g,which can achieve further industrial utilization and achieve effective utilization of wood tar.Aromatics is the main component in the catalytic cracking oil.And the aromatics'content of non-catalytic and catalytic experimental group was 80.54%and 89.13%respectively at 700?.After adding ZSM-5 zeolite catalyst,the content of amino acids,aromatic hydrocarbons and ethers increased and the content of phenols,furans,carbonyl compounds and ketones decreased.The reduction or disappearance of these small molecules of oxygenated substances indicated the wood tars underwent deoxygenation reactions during the reaction.This paper proposed that lightweight bio-oil upgrading method-supercritical ethanol hydrocracking was used to upgrade wood tar.Because of the properties of the superior solubility and hydrogen supply capacity of the supercritical ethanol and the cracking capacity of hydrocracking,the wood tar can be converted into lower molecular weight and more stable substances,which is beneficial to the further effective utilization of wood tar.The supercritical ethanol hydrocracking reaction was carried out on a high-temperature and high-pressure reactor.The wood tar was upgraded in supercritical ethanol using Pd/C catalyst under different temperature or different reaction time.The three-state reaction products were analyzed.Especially the characteristics of the upgraded oils were analyzed by various methods such as GC-MS analysis,purging light component in fixed-bed,gel permeation chromatography?GPC?and ultraviolet fluorescence analysis.The results showed that the molecular weight of the liquid product was the smallest at the atmosphere of H₂ and with the catalysts.The removal of oxygen in the wood tar was mainly achieved by decarbonylation and the decarboxylation reaction at low temperature.With the increase of temperature,the removal of oxygen was mainly achieved by esterification reaction and generating H₂O and the cracking reaction of wood tar became stronger and stronger.But when the temperature was too high,it caused polymerization of the substance in the oil which affected the quality of upgrading bio-oil.In this paper,the best temperature for upgrading was 320°C and the best reaction time is 3 hours.It's the highest content of light components and monocyclic aromatic compounds in the upgraded oil at 320°C and in 3 hours,and the content of phenolic compounds is increased from44.34wt.%of the original wood tar into 51.83wt.%.These small molecular phenols have good industrial application value and are conducive to realize further use of wood tar.Finally,the effect of different kinds of catalysts on the hydrocracking of biomass tar was explored at 320°C in 3 hours.The results showed that zeolite catalysts were beneficial to the cracking reaction of heavy components in wood tar,which can effectively promote the formation of gas and have a higher selectivity for olefin gases,while the selectivity of alkanes is better when the carrier of catalyst was activated carbon.The catalysts Pd/C and Ru/C have the most obvious effect of upgrading among all kinds of catalysts because of much more content of single-ring aromatics and small-molecule phenols in the upgrading oil of these two catalysts,while the effect of Ru/H-Beta is most evident among all zeolite catalysts.