Study On The Properties Of Coke In Catalytic Steam Reforming Of Bio-oil For Hydrogen Production

Hydrogen is an ideal energy carrier,but most industrial hydrogen is produced from fossil fuels.Fossil fuel is a non-renewable energy,which causes serious environmental pollution in its use.Pyrolysis of biomass into bio-oil with high volume energy and density and steam reforming of bio-oil are considered to be feasible approaches for hydrogen production at present.Bio-oil is composed of condensing volatiles derived from the cracking of cellulose,hemicellulose and lignin in lignocellulosic biomass,which usually contains a series of oxygencontaining substances,such as sugar monomer/oligomer,sugar derivatives(such as carboxylic acids,aldehydes and ketones)and lignin-derived phenolic monomer/oligomer.These organic compounds with different molecular structures usually have different degrees of coking during reforming reaction,resulting in deactivation of catalyst and blockage of reactor bed.Therefore,in order to solve this problem,it is necessary to fundamentally understand the formation mechanism of coke and the nature of coke.From this point of view,this paper selected organic compounds with representative structures and functional groups in bio-oil as model compounds for the study of steam reforming reaction,and the main conclusions are as follows:1.Steam reforming of glucose,xylose,acetic acid and furfuryl alcohol(FA)was carried out,aiming to correlate coking behaviors with molecular structure of these organics.The rsults showed that acetic acid,as a small aliphatic molecule,could be effectively reformed and produced the lowest amount of coke.The carbonyl functionality,the multiple hydroxyl groups in the sugars and the furan ring in FA made polymerisation/cracking to form coke as the dominant reaction route in their steam reforming,diminished hydrogen production while led to rapid catalyst deactivation.The coke formed from acetic acid and FA was more aromatic,containing more C=C species,while that from glucose and xylose was more aliphatic,containing more carbonyl functionalities,which projected structural characteristics of the feedstock.2.Steam reforming of glucose,fructose and sorbitol were conducted to clarify the roles of these functionalities in the formation of coke.The results showed that reforming of sorbitol produced more hydrogen and less coke,unlike those of glucose and fructose which produced rather little hydrogen and too much coke.The carbonyl functionality was the main reason for the serious coking behavior but not the multiple hydroxyl groups.The coke from the reaction of sorbitol was mainly catalytic coke with a higher C/H and more defective large fused aromatic rings,which was more thermally stable,more resistant towards oxidation and having a higher crystallinity.Unlike those produced by the reforming of glucose and fructose.Functionalities of the sugars determined the properties of the coke generated.3.Steam reforming of xylose,xylitol and furfural,which are representatives of sugars,alcohols of multiple hydroxyl groups,and furans,were performed to correlate molecular structure and property of coke.The results showed that xylose polymerized rapidly during the steam reforming,forming initially π-conjugated polymeric organics and substantial amount of coke that was amorphous,aliphatic,thermally unstable with abundant olefin C=C and-OH.In comparison,steam reforming of xylitol generated little coke,as xylitol contains no C=O that was the primary reason for the serious coking in xylose reforming.Steam reforming of furfural also generated large amount of coke that was mainly aromatic with high thermal stability and high resistance towards oxidation,due to the reserve of the furan ring-like structure in the coke.Functionalities of the reactants dictated the thermal properties of the coke and also their morphologies.4.The small molecular organics in bio-oil could not be effectively transformed into the hydrocarbons with long aliphatic chains or aromatic rings via hydrotreatment,but they could be reformed with steam to generate hydrogen for the hydrotreatment.In this study,the pyrolysis of cellulose coupled with steam reforming of volatiles of small molecular size were performed at 400 to 600°C over Ni/Al2O3 catalyst,which is termed as a pyroreforming process for simultaneously production of hydrogen and heavy organics for further production of biofuels.The results indicated that the effective reforming of the small volatiles became dominant at 600℃.The coke formed at the low temperatures was mainly polymeric coke of aliphatic nature with low thermal stability,low carbon crystallinity,high reactivity towards oxidation and high hydrophilicity.Increase of temperature from 400 to 600°C reduced the formation of coke to a mild extent(from maximum of 15.0% to 12.8%)and suppressed the formation of polymeric coke by promoting gasification of precursors of coke.At the same time,high temperature promoted the formation of defect ring structure in coke.