Study On Thermochemical Conversion Mechanism And Product Regulation Of Bio-oil Distillation Sludge

Biomass refinery system as an emerging technology for waste management and carbon neutral is composed of biomass liquefaction and bio-oil purification.Bio-oil,the main product from biomass liquefaction,is of potential to be further processed to replace petroleum or extract commodity chemicals.However,bio-oil from conventional pyrolysis is composed by hundreds of compounds,which are enriched by water and acids,endowing drawbacks such as high moisture,low calorific values,high acidity,and easy aging.As such,bio-oil could be further purified by distillation and rectification.However,such chemical processing is usually accompanied by a heating process,which results in the polymerization reactions among bio-oil components,and thus the accelerated aging reaction for bio-oil.After the heating process,a residue is generated at the bottom of environmental equipment and such solid waste is difficult to utilize directly.The presence of bio-oil distillation sludge significantly suppressed the progress of bio-oil purification,and harm equipment as well as production safety.In these contexts,exploring the valorization process of bio-oil distillation sludge is the last link for completing biomass refinery system.1.Pyrolysis mechanism and product regulation of bio-oil distillation sludgeBio-oil distillation sludge was formed by polymerization of organics in bio-oil.Since the scissions of chemical bonds were usually disordered in conventional pyrolysis platform,it was difficult to achieve the directional regulation of the pyrolytic product from bio-oil distillation sludge.Torrefaction and alkaline impregnation could optimize the structural characteristics of bio-oil distillation sludge,from which torrefaction promoted the release of oxygen and synergistically retained the carbon and hydrogen in feedstock,leading to the further polymerization of organics and thus the reduction of branch chain structures.Alkaline impregnation destroyed the carbon skeleton and exposed more chain structures in feedstock,thus inducing the increase of pyrolysis reaction rate.The coupling effects of torrefaction and alkaline impregnation on sample pyrolysis rate were different.Low torrefaction temperatures facilitated the strengthened pyrolysis reaction rate and increase of pyrolysate concentration,but its synergistic effect with alkaline impregnation weakened the competitions among active molecules and inhibited the increase of reaction rate.High-temperature torrefaction enhanced the thermostability of sample,and the pyrolysis rate is correspondingly suppressed.However,its coupled impacts with alkaline impregnation reduced the activation energies of 10.48～14.38%,further reducing the energy barrier required for pyrolysis.The pyrolytic product distribution of bio-oil distillation sludge mainly contained hydrocarbons,phenols,aromatics,and alcohols,among which phenols accounted for the highest proportion in the products,reaching 47.08%.The specific phenols were guaiacol and 4-ethyl phenol.Hydrocarbons accounted for 12.62～17.30%,which were mainly aromatic hydrocarbons of phenanthrene and biphenyl.2.Co-pyrolytic interaction and product purification of bio-oil distillation sludgeThe product distribution of pyrolysis vapors was improved by the co-pyrolysis mode of bio-oil distillation sludge and walnut shell,and further directionally and selectively valorized by fractional condensation technology.The fractional bio-oil yield from co-pyrolysis was mainly affected by the temperatures in first condenser and feedstock blend ratios,while the condensation efficiency and total bio-oil yield would be increased by enhancing condensation temperatures.The total bio-oil could be recovered and enriched to the maximum extent by using a lower condensation temperature in the tandem condensers.The co-pyrolysis system would significantly promote the increased carbon content,reduced oxygen content of 24.04～26.58%,weakened moisture content and thus improved fuel properties of bio-oil.The addition of biomass introduced more active free radicals into the co-pyrolysis system,endowing the changes in the parallel reactions between pyrolysis vapors,which resulted in the dehydration and hydrogenation reactions and promoted the secondary cracks of large aromatic compounds.Additionally,the lyophilization technology was first applied to the separation of bio-oil as a pathway for purifying heat-sensitive organics.The miosture content of bio-oil was the key to affect the lyophilization efficiency.The high miosture content resulted in the mixed solution to tend to the eutectic point of water,making more small-molecule organics to be synergistically released during the lyophilization.According to the bulk properties of co-pyrolytic bio-oil,the impacts of different induction atmospheres on the pyrolysis characteristics and synergies were further analyzed.The co-pyrolysis system could inhibit the heterogeneous reactions of biochar under high-temperature pyrolysis,enhanced the biochar yield and further decreased reaction rate.Meanwhile,CO2 partial pressure more than 75%would significantly facilitate the reaction rate of lignin pyrolysis and promote the depolymerization of lignin.Co-pyrolysis system also reduced the energy barriers of reaction process and accelerated the pyrolysis reaction.However,CO2 participated in the secondary reaction of pyrolysis vapors due to its own acidity,resulting in more complex reaction system and thus increasing the reaction activation energies.The impacts of co-pyrolysis blend ratios and CO2 partial pressures on the kinetic mechanism model were significantly different according to the conversion.The kinetic mechanism model in the early stage of reaction was mainly affected by co-pyrolysis blend ratios,while the mechanism model with conversion higher than 50%was controlled by CO2 partial pressures.Also,the contents of hydrocarbons and phenols in the pyrolysis products were significantly increased by co-pyrolysis,while the CO2 atmosphere changed the pyrolysis path of tar,and co-induced the production of more phenols and aromatic monomers,reducing the formation of aromatic hydrocarbons.In addition,the co-pyrolysis interaction promoted the improvement of biochar yield.The phase change of bio-oil distillation sludge during co-pyrolysis would inhibit the development of porous structures in the biochar,and made volatile organic compounds deposit in the particles.The deposited carbon increased the carbon content and calorific value of solids,inhibited the enrichment of oxygen elements and thus improved the carbon purity and thermostability of biochar.3.Study on gasification reactivity optimization and structural regulation of bio-oil distillation sludgeBy comparing the gasification reactivity of bio-oil distillation sludge and walnut shell,the possibility of clean conversion of bio-oil distillation sludge into synthetic gas was explored.The gasification reactivity of bio-oil distillation sludge was far lower than that of walnut shell,and it was a kind of feedstock with inert physicochemical properties similar to municipal sludge.The gasification reactivity of feedstock can be significantly improved by increasing the gasification temperatures and heating rates.Based on the three gasification kinetic models,the gasification characteristic curves of the two feedstocks can be well fitted.The random pore model was the best fitting model for the gasification characteristics of walnut shell due to considering the effect of pore development and inorganic catalysis on biochar gasification process.However,bio-oil distillation sludge will not form pore structures in the gasification process due to its inertness.The reaction active sites on the surface promoted its gasification reaction process,so the volume model and particle model were the best kinetic models for gasifying bio-oil distillation sludge.The back-propagation neural network topology,for the first time,was used to predict the gasification behaviors of the two feedstocks.The adaptability of machine learning itself fitted the gasification characteristic curves very well,and the fitting mean square deviation was less than 0.035%,with the correlation coefficient reaching more than 0.9983.Through further matching the gasification reactivity indice with their structural characteristic parameters,it was found that the AG/Aall value derived from Raman spectrum had a strong linear relationship with the isothermal gasification parameters,so it was judged that amorphous carbons were the main factors to promote the gasification reactivity.The ash composition and evolution were comprehensively evaluated.According to the experimental and fitting results,it was determined that the bio-oil distillation sludge ash had anti-slagging characteristics,while the biomass ash had potential slagging and fouling risk.Introducing rapeseed cake into the bio-oil distillation sludge for co-gasification significantly increased the amount of amorphous carbon and edge defects in the biochar,thus improving the gasification reactivity of bio-oil distillation sludge and reducing the activation energies of co-gasification.4.Co-combustion behaviors and gas-solid phase product evolution of bio-oil distillation sludgeThe co-combustion characteristics of bio-oil distillation sludge and walnut shell were mainly affected by the blend ratios and combustion atmospheres.A large number of volatiles escaped from the walnut shell during devolatilization stage,resulting in a significant reduction in the proportion of biochar combustion stage.The combustion process of bio-oil distillation sludge was mainly the heterogeneous reaction of biochar.Co-combustion would weaken the release of volatile organic compounds,thus increasing the proportion of biochar combustion stage and improving combustion efficiency.The co-combustion kinetics based on the multiple distributed activation energy model showed that the co-combustion process could be fitted by four pseudocomponents,in which the activation energy of E1,represented by devolatilization stage,decreaseed with the enhancement of the walnut shell proportion,while the biochar combustion stage represented by E4 presented an opposite trend.The activation energy sensitivity of the four pseudo-components changeed significantly when the proportion of walnut shell was 90%.Meanwhile,the control variable of the overall reaction gradually transited from the devolatilization stage to the biochar combustion stage,resulting in the biochar combustion stage becoming a strong response area for the whole combustion process.The oxy-fuel combustion would promote the secondary reaction of flue gas with CO2,and synergistically reduced the CO2 concentration in the cocombustion system.Additionally,the oxy-fuel combustion also inhibited the formation of aromatics and further facilitated the cracking of macrocyclic aromatics to phenols rather than aromatic hydrocarbons.However,the co-combusted ash still showed slagging tendency due to the large walnut shell proportion,but the relevant antislagging coefficient had been alleviated owing to the introduction of bio-oil distillation sludge.The oxy-fuel combustion retained the conversion of potassium salt in ash chemistry,which also made the molten ash have the slagging and fouling risk.