Investigation Of Agricultural Straw Towards High-valued Matrials

Agricultural straws,mainly consisting of cellulose,lignin and hemicelluloses,have advantages over the fossil resources as their short growth period,abundant production,renewable and biodegradable features.There is a great potential demand in such market as green materials,e.g.textile fiber,food packaging,environmental protection materials,and environmentally friendly chemicals.Recently,agricultural straw also obtained extensive interests in the field of biomass nanomaterials,such as cellulose nanocrystals,cellulose nanofibers and biomass-based electronic materials.But actually,except a small part of the agricultural straws is used as feed,most of them are directly burned or discarded.The agricultural straw resources are serious under valorization in China.The main reason lies in the absence of efficient utilization technology.In the earlier stage,we have developed highly efficient and environmentally friendly process for separating the main components.The separated straw cellulose can meet the requirement of wet spinning.While,the isolated straw cellulose was dispersive in the degree of polymerization(DP),and with high ash content,which affects the spinnability obviously.Hence,on the premise of ensuring the DP of the straw cellulose,to improve the process efficiency through multi-physics-assisted steam explosion technology was conducted.We take the wheat straw as the starting material for exploring the process and conditions of increase the components separation efficiency,and the quality of straw cellulose in the same procedure by optimize the multi-physics assisted steam explosion pretreatment conditions combined with the following treatment.Furtherly,the process for removing impurities in the straw cellulose was optimized,and the treatments of the wastewater generated in each process stage were optimized.At last,the biomass derived porous carbon materials were prepared from the steam exploded wheat straw for supercapacitor and microwave absorbant,and the pore structure regulation mechanisms were studied.The main results of this work are as follows:(1)A multi-physical-field assisted steam explosion system was developed.An ultrasonic and microwave coupling assisted steam explosion device was designed and manufactured.A series of technical problems,such as connection,sealing and power matching between the ultrasonic(and/or microwave)and steam explosion chamber,were investigated.Based on the device,the technique conditions were systematically optimized through orthogonal designed experiments.The optimized conditions were summarized as:cooking pressure 1.8 MPa,cooking time 30 min,ultrasonic power 300 W,and microwave power 1500 W.Under the optimized conditions,78.4%of hemicellulose was extracted.Under the decreased pressure condition,the DP of the isolated straw cellulose was considerably improved from 320 to 423.In addition,the only ultrasonic(or microwave)assisted steam explosion were also compared,and the results indicated that multi-physical-field assisted steam explosion led to higher hemicellulose removal efficiency.(2)A new deashing process was developed for purification of the isolated straw cellulose.The steam-exploded wheat straw was delignified by N,N-dimethyl formamide(DMF),water and NaOH mixed solvent,and then bleached with NaOH/H₂O₂aqueous solution to get the straw cellulose.A new process for deashing from the as obtained straw cellulose was developed.It was found that KOH/H₂O₂ and HCOOH solutions could remove silicon impurities and metal ions in straw cellulose.Furtherly,the deashing process was also optimized by orthogonal experiment.The total ash content of the deashed straw cellulose decreased from 2.1 to 0.11 wt%,under the optimized composition of the deashing solution,consisting 8.0 wt%KOH,2.0 wt%H₂O₂ and 2.0 wt%HCOOH.(3)The process conditions for the wastewater treatment was optimized.The waste water generated during the separation procedures of straw components were named as wastewater?,wastewater?,and wastewater?,corresponding to the steam explosion,the delignification and the bleaching stages separately.All the waste water species were treated by anaerobic fermentation technology.It was proved that the potential for methane production of the three wastewaters(methane volume/waste volume)ranked as wastewater?>wastewater I>wastewater II.According to the analysis,the low potential of methane production in wastewater I is due to the fact that the wastewater mainly contains hemicellulose and hemicellulose hydrolysate dissolved in water during steam explosion,and the organic matter content is low.As to the wastewater II,it still contains a small amount of DMF after concentration,which is not conductive to microbial activity.For wastewater?,which contained a small amount of H₂O₂ to oxidize the organic matter,as well as the residual CH₃CH₂OH,showed better potential of methane production than the other two waste water species.It was also found that the biochemical reaction of bacteria was inhibited when the DMF content in wastewater II was>10%.However,when the content of DMF was lower than 10%,there was no obvious effect on the anaerobic fermentation process.The COD value of the wastewater can be significantly decreased through the anaerobic fermentation process with mixing the three kinds of wastewater with a ratio of 1:2:2(volume ratio)and pretreated by iron carbon material.(4)The new biomass-based porous carbon for supercapacitors was developed from the steam exploded wheat straw.Firstly,the biomass-based porous material is prepared by a dissolution-gelation process using a mixed solvent system of tetrabutylammonium hydroxide(TBAH),dimethyl sulfoxide(DMSO)and water from the steam exploded wheat straw.Secondly,biomass-derived hierarchical porous carbon(BHPC)were prepared by pre-carbonization at 400?,activation by KOH.The microstructure of the biomass-based porous carbon was regulated by tuning the concentrations of the straw in the solution from 1,3 to7wt%,and the subsequent activation temperature from 500,600 to 700?.The morphology of porous carbon could be regulated as honeycomb-like,graphene-like and layer-like shape by increasing of the straw concentration from 1,3 to 7 wt%,respectively.Generally,we proposed a method for regulating the morphology of biomass-based materials from mesoporous to microspores by controlling the activation process and conditions.The electrode material BHPC-3(prepared condition:the biomass concentration 3 wt%,and the activation temperature600?)exhibited the highest specific capacitance with a specific surface area of 772 m²·g^-1,a mesoporous volume of 0.41 cm³·g^-1(62.1%of the total pore volume)and a micropore volume of 0.25 cm³·g^-1(37.9%of the total pore volume).The specific capacitance of BHPC-3 at current densities of 0.5 and 10 A·g^-1 are 226.2 and 176.0 F·g^-1,respectively.Meanwhile,BHPC-3 displayed a high area specific capacitance(29.3 u F·cm^-2),which meant that BHPC-3 made full use of its specific surface area.This was mainly owing to the proper ratio between meso-and micropores,which effectively reduced the transport distance of electrolyte ions and promoted their rapid transport.(5)The new biomass-based porous carbon for absorbing electromagnetic was developed from the steam-exploded wheat straw.The structure of the biomass-based porous carbon material was controlled by adjusting the carbonization temperature(500,600 and700?)and the amount of FeCl₃·6H₂O(0,44,55 and 66 wt%).Meanwhile,the electromagnetic properties of the materials were studied.The dispersion and diameter of the iron nanoparticles could be controlled by tuning the carbonization temperature,to furtherly control the electromagnetic wave absorption properties of the materials.The magnetic carbon foam MLPC-600-1.0(the mass percentage of FeCl₃·6H₂O is 55 wt%,and the carbonization temperature is 600?)exhibited the highest electromagnetic wave loss intensity with an optimum reflection loss(RL)value of-43.6 d B(7.1 GHz)and an effective bandwidth of 11.5 GHz(RL<-10 d B).It was also found that the microwave absorption properties of the porous materials were mainly attributed to the multiple loss mechanisms such as interface loss,electro-magnetic coupling loss,multiple reflection loss and magnetic loss.