| Fast pyrolysis is an efficient method to convert biomass to fuels and chemicals.The biomass is initially converted to pyrolysis vapor,which immediately condense to liquid bio-oil.Due to its complex compounds and high oxygen content,crude bio-oil has many disadvantages,such as high acid value,high viscosity and poor thermal stability.Thus,it is essential to upgrade the bio-oil.Compared with treating the crude bio-oil,energy consumption caused by re-evaporation and possible secondary reaction can be avoid by immediately upgrading the pyrolysis vapor,meanwhile the oxygen content decreases and the hydrocarbon content increase.In the present study,lignin was selected as feedstock.Lignin pyrolysis and the effects of temperature and catalysts on the product distribution were investigated.The TG results indicated that lignin pyrolysis mainly occurred at 150-700℃,and the mass loss rate increased with the increasing heating rate.In the initial stage of pyrolysis,H2O and CO2 were first produced.With the temperature increased,the cleavages of bonds in biomass feedstock occurred,which leaded to the formation of aromatic oxygenates with hydroxyl,carboxyl and methoxy groups and released CH4 and CO.The structure and acid-base properties of catalysts had obvious effects on the distribution of pyrolysis products,but the effect of catalytic fast pyrolysis on improving the yield of aromatic hydrocarbons was not obvious.Due to the hydrogen deficient property of fast pyrolysis vapor,guaiacol was selected as the model compound of lignin pyrolysis product,and methanol was co-fed with guaicol during the catalytic cracking process.Co-feeding methanol could promote the conversion of guaiacol and inhibit coke formation.Among HZSM-5,Al-MCM-41 and γ-Al2O3,the γ-Al2O3 catalyst leaded to the highest guaiacol conversion and aromatic hydrocarbon yield.When the molar ratio of methanol to guaiacol was 25:1,reaction temperature was 400℃,W/F was 166 g·h·mol-1 and time on steam was 60 min,the conversion of guaiacol was 100%and the yield of aromatic hydrocarbons was 81.2%.In the catalytic co-cracking process,methanol acted as hydrogen donor,and phenolics were converted to aromatic hydrocarbons through hydrogen transfer and methylation.Besides,water formed from methanol dehydration could compete with coke precursors in the adsorption on acid sites to inhibit the coke deposition.The results of catalytic co-cracking other aromatics with methanol indicated that co-feeding methanol could also promote the conversion of phenol,anisole and the mixture of phenolic compound.2-Methylfuran(MF)was selected as model compound of cellulose/hemicellulose pyrolysis product,and catalytic co-cracking MF and methanol was catalyzed over HZSM-5 and γ-Al2O3,respectively.When the catalyst was HZSM-5,molar ratio of MF to methanol was 1:5,reaction temperature was 500℃,WHSV was 2 h-1 and time on steam was 60 min,the conversion of MF and methanol was 100.0%and 94.6%,and the yield of aromatic hydrocarbons was 42.0%.When the catalyst was γ-Al2O3,molar ratio of MF to methanol was 1:5,reaction temperature was 400℃,WHSV was 2.5 h-1 and time on steam was 60 min,the conversion of MF and methanol was 95.5%and 98.0%,and the yield of aromatic hydrocarbons and C5+cycloolefins was 40.6%and 56.8%.To make co-cracking with methanol suitable for upgrading biomass pyrolysis vapor,MF and cyclopentanone(CP)was added during the co-cracking reaction of guaiacol and methanol.Due to the reactions between MF(or CP)and methnol,the conversion of guaiacol decreased to 90.3%and 89.5%,while the yield of aromatic hydrocarbons increased and C5+ cycloolefins were detected in the product with the addition of MF(or CP).In the catalytic co-cracking of MF/guaiacol/methanol and CP/guaiacol/methanol,methanol played an important role.During the catalytic cracking of biomass pyrolysis vapor,co-feeding methanol could improve the yield of aromatic hydrocarbons and inhibit the coke formation during lignin and corn straw pyrolysis. |
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