Upgrading Of Bio-oil In Supercritical Fluids

Bio-oil,a liquid product from renewable biomass via liquefaction or pyrolysis,by the virtue of its environmentally friendly potential such as CO₂ neutrality,is regarded as a promising renewable energy source and received more and more attention.The crude bio-oil is a complicated oxygenated mixture which generally contains a wide variety of acids,phenols. esters,ketones,aldehydes and sugars,and as a result,crude bio-oil exhibits some undesired properties such as acidity,high viscosity,and thermal instability.On the other hand,high water and oxygen content in crude bio-oil leads to its low heating value compared with that of fossil fuels.Therefore bio-oil needs to be upgraded to improve its quality for its practical application. At present,the main aim of bio-oil upgrading processes carried out by most investigators is to obtain hydrogecarbons by removing the O-compounds in bio-oil in which catalytic hydrodeoxygenation process is mostly used.The main disadvantage of catalytic hydrodeoxygenation process is the high consumption of hydrogen and energy due to the high temperature and hydrogen pressure required in this method.The aim of this work is to produce oxygenated fuel through bio-oil upgrading using an environmentally friendly method with low energy consumption.One-pot reaction in supercritical fluids,an innovative route for bio-oil upgrading was developed in this work.The instable compounds in bio-oil are converted into stable O-compounds or other compounds through one-pot reaction in which many reactions are coupled and merged.The process was carried out under mild conditions and in which,the crude bio-oil was converted into oxygenated fuel.This method can enhance the use efficiency of elements in bio-oil.The basic properties and compositions of bio-oil from fast pyrolysis of rice husk were analyzed using GC-MS.TG.vacuum distillation and other conventional methods.Crude bio-oil has high water content(28.0%),high oxygen content(51.2%) and low heating value(16.5 MJ/kg).The organic components of crude bio-oils include acids(33.17%).phenols(16.06%). esters(6.85%),ketones(17.82%).aldehydes(4.45%) and sugars(15.38%).TG analvsis showed that completed combustion of bio-oil in oxygen occurred at 430℃. Ethanol and methanol were selected as supercritical reaction media in the supercritical upgrading processes due to their mild critical conditions and advantageous effect on the upgrading reactions.On the other hand,ethanol is mainly produced from biomass while methanol is mainly produced from fossil fuels and these two alcohols all can be used as vehicle fuels.Therefore the investigation of upgrading of bio-oil in supercritical ethanol and methanol is an innovative work for combined utility of alcohol fuels and bio-oil.The crude bio-oil was upgraded in supercritical ethanol using Al₂(SiO₃)₃ as catalyst at 260℃with 10:1 ethanol/oil ratio.The results showed that supercritical upgrading process performed effectively and the components of upgraded bio-oil were optimized greatly.Acidic aluminum silicate facilitates esterification in supercritical ethanol to convert most acids contained in crude bio-oil into various kinds of esters.The amount of esters in upgraded bio-oil increased from 6.85%to 69.57%and the amount of acids decreased from 33.17%to 1.76%.The amount of phenols decreased from 16.06%to 3.05%.Aldehydes such as furfural and vanillin which are typically present in crude bio-oil were removed during supercritical upgrading process and not detected in upgraded bio-oil.Sugars,which are largely(15.38%) contained in crude bio-oil were converted completedly in supercritical upgrading process.The pH value and heating value of upgraded bio-oil increased compared with that of crude bio-oil.The viscosity of upgraded bio-oil decreased.The changes of these properties showed that the quality of upgraded bio-oil was improved.It proved that supercritical upgrading process is so effective and promising that can be developed into an industrial method.The crude bio-oil was upgraded in supercritical ethanol using HZSM-5(Si/Al=22,100,300) as catalyst.The amount of distillation residue of upgraded bio-oil decreased compared to that of crude bio-oil.With the increasement of acidity of HZSM-5(decrease of Si/Al ratio),the amount of distillation residue of upgraded bio-oil decreased and at the same time,the amounts of ethylene and CO₂ in gas phase products increased.These results showed that stronger acidic HZSM-5(low Si/Al ratio) can facilitate cracking of heavy,components of bio-oil more effectively in supercritical upgrading process.Methanol is another effective reactive medium for bio-oil upgrading.It is interesting to found that bio-oil upgrading process works effectively in supercritical methanol even without catalyst. Hydrogenation of light and heavy bio-oil(defined as distillate and residue of crude bio-oil through vacuum distillation,respectively) was investigated.It was found that separation of heavy bio-oil from crude bio-oil can facilitate hydrogenation process.Furfural and vanillin ware used as molecular probes to examine the mechanism of hydrogenation process and it showed that cracking,reforming and hydrogenolysis were involved in hydrogenation process.