| The utilization of fossil fuels results in serious environmental problems and national energy crisis.Biomass including agricultural residues and wood wastes is one of the most promising renewable sources.This renewable materials is abundant in nature,and thus has been given increasing attentions in recent decades.Fast pyrolysis technology is considered as one of the most important ways to utilize biomass,which converts complex solid biomass into liquid product known as bio-oil.The chemical composition of biomass is very complex,including hundreds of organic compounds and most of them in low contents,which greatly hinders the application of bio-oil,as well as separation of specific chemicals.To improve the selectivity of biomass pyrolysis to specific chemicals,some pretreatment methods have been developed.As a more environmentally-friendly and less energy consumption process,bio-pretreatment gained more and more attentions.Currently,white rot fungi and brown rot fungi have been verified to be capable of selectively degrading chemical components in biomass to increase the pyrolysis selectivity.In order to systematically study the effects of white rot fungi and brown rot fungi pretreatment on biomass pyrolytic characteristics,two representative fungi(white rot fungi Ganoderma lucidum and brown rot fungi Fomitopsis palustris)were selected as the main microorganisms to conduct bio-pretreatment process of birch wood for different times(0-16 weeks).The effects of bio-pretreatment on the chemical structure and pyrolysis characteristics of birch were investigated."The changes of chemical structures were characterized by chemical components,X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),andsolid-state cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance(CP/MAS 13C NMR).The results showed that bio-pretreatment significantly changed the chemical composition and structure of birch,but different effects were also observed between the two pretreatment methods.After 16 weeks pretreated by Ganoderma lucidum,the contents of lignin and hemicellulose decreased from 26.39 and 24.21 wt.%to 24.81 and 18.72 wt.%,respectively.Content of cellulose increased correspondingly.XRD results showed a decrease of the peak of amorphous cellulose at 16.4°,while the peak of crystalline cellulose at 22.5°was changed slightly with an increase of crystallinity from 53.53 to 56.94.FT-IR results showed that with the increase of pretreatment time,the intensity of cellulose?-glycosidic bond peak at 899 cm-1 increased,and the intensity of hemicellulose acetyl peak at 1740 cm-1 decreased.The intensity of the lignin aromatic ring skeleton peak at 1505 cm-1,1593 cm-1 and the methoxy group of syringyl and guaiacyl lignin peak at 1319 cm-1 decreased significantly,and a new lignin C=O peak was observed at 1651 cm-1,which indicated that the pretreatment facilitated the degradation,demethoxylation and oxidation of lignin.CP/MAS 13C NMR results showed a decrease of amorphous cellulose C-4 and C-6 at 83.51 ppm and 62.90 ppm,as well as hemicellulose acetyl carbon peak at 21.21 ppm and lignin methoxy carbon peak at 55.85 ppm.These results indicated that a large amount of crystalline cellulose structure was retained after white rot fungi pretreatment,which was advantageous for the selectivity of cellulose products.After brown rot fungi Fomitopsis palustris pretreatment,the content of lignin increased to 31.38 wt.%and the hemicellulose content decreased to 22.22 wt.%gradually,while the cellulose content changed slightly.The results of structural characterization indicated that Fomitopsis palustris selectively degraded the hemicellulose acetyl group and partially degraded amorphous cellulose in birch,with the cellulose ?-glycosidic bond increased and the crystallinity accordingly increased to 56.94.The intensity of lignin peaks such as aromatic ring,methoxy and C=O group increased after brown rot fungi pretreatment,which suggested a large amount of lignin structure retained.The effects of fungi pretreatment on pyrolysis characteristics were investigated by thermogravimetric analysis(TGA)and analytical pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS)experiments.The results showed the initial pyrolysis temperature of birch was increased by 40? and the activation energy increased from 92.8 kJ-mol-1 to 99.3 kJ-mol-1 after 16 weeks pretreated by Ganoderma lucidum.The maximum weight loss rate increased significantly and the carbon residue was gradually decreased,which indicated the pretreatment improved the thermal stability and raised the pyrolysis efficiency and reaction rate of birch.With the increase of pretreatment time,the yields of pyrolysis products such as furans,anhydrosugars,ketones and aldehydes increased from 10.96,7.41,5.43 and 3.72%to 14.60,8.68,6.45 and 5.46%,respectively.The phenolic yield significantly decreased from 40.65 to 25.61%.The yield of levoglucosan,as a typical cellulose product,increased from 6.69 to 7.82%.The yield of acetic acid as a hemicellulose product,decreased slightly from 3.04 to 2.52%,and the lignin products 2,6-dimethoxyphenol and 2-methoxyphenol were significantly reduced from 5,70 and 2.63%to 1.66%and 1.29%,respectively.The pyrolysis characteristics of enzymatic lignin isolated from pretreated and untreated birch were further investigated.The yield of lignin phenolic products decreased from 71.69 to 69.84%,and the yield of their side chain oxidized products was also increased from 16.76 to 19.13%,which indicated the lignin was oxidized after white rot fungi pretreatment.Different fungi had different effects on the pyrolysis characteristics of birch.After brown rot fungi pretreatment,the initial pyrolysis temperature of birch decreased,the activation energy gradually reduced to 84.8 kJ·mol-1,and the maximum weight loss rate increased firstly and then decreased.Moreover,the amount of residual carbon increased,which indicated that the pretreatment reduced the thermal stability of birch,and the change of residual carbon content was the result of lignin content increase.With the increase of pretreatment time,the yield of cellulose products anhydrosugars and ketones significantly increased from 7.38 and 3.70%to 10.14 and 4.96%after 12 weeks pretreatment,respectively.The yield of lignin phenolic product significantly increased from 40.49%to 49.18%.The yield of acetic acid as hemicellulose product decreased from 3.02 to 2.10%.In addition,the yields of short chain phenols,side chain oxidized,demethylated,and hydroxyphenyl phenol products were gradually increased from 17.61,8.23,3.23 and 2.19%to 21.76,10.00,5.61 and 4.97%respectively.The above results confirmed that white rot fungi and brown rot fungi pretreatment can significantly change the chemical structures and pyrolysis characteristics of wood.However,the effects heavily depended on the fungi species.This study provided a theoretical basis for improving the selective pyrolysis of biomass,as well as a new way for the high value utilization of decayed biomass. |
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