Valorization Of Furfural Residue By Hydrothermal Carbonization:Process Optimization,Biochar And Process Water Characterization

The decrease of fossil fuels and the exacerbate of environmental pollution prompt people to look for clean and cheap renewable energy.As an abundant,clean,and renewable energy source,it is significant to investigate the biomass conversion to fuel and high-value chemicals.Hydrothermal carbonization（HTC）is a thermochemical treatment process that allows for the conversion of relatively dilute biomass with simple operating process under mild reaction condition.It is considered an efficient disposal method of waste biomass to provide functional carbon materials with properties such as friable,hydrophobic,and increased in mass and energy densification compared to the raw biomass.Furfural residue（FR）is a low-cost by-product generated in the furfural production from corncobs,and there is no efficient technique to utilize FR.The FR is mainly composed of cellulose and lignin.As an acidic residue,the FR contains a high moisture content after the process from corncob to furfural.HTC could be a feasible approach for utilization of this industrial waste.In this paper,the valorization of furfural residue by hydrothermal carbonization（HTC）is conducted at various reaction conditions,and the process optimization,solid structure and liquid phase composition are studied exhaustively.The main contents and results are as follow:Firstly,the HTC of deashed FR was conducted at various pH,reaction temperatures（200,220 and 240°C）and reaction times（1-24 h）.The chemical and structural properties of obtained biochars were investigated by elemental analysis,high-resolution solid state ¹³C NMR,and X-ray photoelectron spectroscopy（XPS）.The morphology of biochar was characterization by scanning electron microscope.The analysis of solid product indicated that the biochar yields declined with increasing of reaction temperature and time.According to results from SS-NMR and XPS,240°C is an essential temperature for biochar formation.At this temperature,changes in the surface properties initiated within 1 h,and the major transformation occurred within 8 h.The dominating change was cellulose fraction reaction forming furanic and aromatic structures in the biochar,which is faster at higher temperatures.The composition of lignin is changed few in this process,while it is separated from cellulose and becomes a loose structure.In addition,these biochars showed high thermal stability below 300°C according to thermogravimetric analysis.Secondly,the main components in process water gathered from different HTC reaction conditions are investigated comprehensive.Various qualitative and quantitative NMR techniques(¹H and ¹³C NMR,¹H-¹H COSY and ¹H-¹³C HSQC etc.)especially 1D selective gradient total correlation spectroscopy（TOCSY NMR）were strategically applied in the analysis of HTC stock process water.The results from the NMR study illustrated that the main compounds detected in process water are 5-hydroxymethylfurfural（5-HMF）,formic acid,methanol,acetic acid,levulinic acid（LA）,glycerol,hydroxyacetone and acetaldehyde.Furthermore,the relationship between the concentration of major products and the reaction conditions（180-240°C at 8 h,and 1-24 h at 240°C）was established.Finally,reasonable reaction pathways for hydrothermal conversion of FR were proposed based on this result and our previously obtained characteristics of biochars.