Study On The Thermal Behavior And Synergistic Effect During Co-pyrolysis Of Biomass And Coal

Biomass is the only renewable carbon resource,and coal would continue to be the main part of the energy consumption structure in China in the forseeable future.Biomass and coal are partly compatible in thermochemical utilization,which means it is possible to develop co-conversion technologies to take full advantage of their superiorities.It would be significant in both energy strategy and fundamental research.Co-pyrolysis could be applied for direct production of three-phase products,and is the first step of other thermal co-conversion including gasification and combustion.Thus,co-pyrolysis is the key step in the relevant technology development,which has became a hotspot gradually in the past decade.Most of the early studies have focused on the trend of product yields during co-pyrolysis,however,because of the complexity of pyrolysis process and the diversity of materials,the knowledge about co-pyrolysis synergy is still superficial,which makes it difficult in guilding the technology development.Based on the current situation,experimental study on co-pyrolysis behavior and synergy between biomass and coal was conducted.Multiple approaches were combined to seek a breakthrough for deeper insights into synergistic effect during co-pyrolysis.In this work,thermogravimetry coupled with Fourier transform infrared spectroscopy(TG-FTIR)was applied for co-pyrolysis,by which the overall thermal behavior and volatile evolution were figured out.The results evidenced some slight synergies during co-pyrolysis of cornstalk-lignite and cornstalk-bituminous coal.The mass of final residue in high-temperature stage increased,and the shoulder peak in the derivative weight curve of cornstalk weakened.The infrared spectroscopy realized whole-process monitoring and tracking for co-pyrolysis synergy,which demonstrated that the weakened shoulder peak might be from C=O and C-O groups(small oxy-containing organics)during devolatilization.Furthermore,co-pyrolysis char was prepared by simulating the condition of thermogravimetry experiments.The influence of slow co-pyrolysis on surface morphology and porosity of char was revealed,which approved the synergy observed by TG-FTIR.Based on the self-designed micro-scale free-fall reactor,the product distribution during co-pyrolysis of biomass and coal was studied.Furthermore,non-linear surface fitting and contour map were employed to describe and visualize the synergistic effect based on the yield data.The results showed that the synergistic effect was still weak,even though the conditions for synergy enhancement were satisfied,which were stable contact and high heating rate.The char yields after co-pyrolysis of cornstalk-lignite and bamboo fiber power-lignite slightly increased,however,the changes of gas and tar yields were different depending on the biomass varieties.The synergistic effect observed in the combustible fractions in gas product(CO?H₂?CH₄)was consistent with the overall gas yield.The influence of coal on biomass pyrolysis kinetics was studied by advanced differential isoconversional method.By using anthracite,the interference on the process of biomass pyrolysis greatly decreased,which contributed to acquiring more representative kinetic parameters.After blending anthracite with 1:1 mass ratio to biomass,the activation energy of cornstalk pyrolysis decreased,however,that of bamboo fiber powder pyrolysis increased.The opposite change was probably due to the influenced thermal diffusivity of biomass due to anthracite additive.Fraser-Suzuki function was applied for deconvolution of the overall pyrolysis weight curves,and isoconversional kinetic analysis was conducted to all pseudo components.The results were basically consistent with the overall kinetic parameters.The reliability of the kinetic calculation was validated by simulating the rate curves based on the obtained kinetic parameters.Co-pyrolysis char was prepared by a tube resistance furnace simulating the condition of fast pyrolysis.Multiple approaches were combined for analysis on the physicochemical properties and combustion behavior of co-pyrolysis char.The results showed that the surface area and porosity of mesopore and large micropore were remarkably influenced;co-pyrolysis favored retaining carbon and alkali,alkaline-earth metals(AAEMs)in the char;combustion behavior of co-pyrolysis char resembled a single fuel rather than a mixture,which means the combustion temperatures of bio-char and coal-char shifted and merged;the total heat releases during combustion of co-pyrolysis char and char blends were both higher than the calculated values.The findings revealed the role of synergistic effect in char evolution,which might include the secondary reaction of volatile,secondary char formation,interfering free radical reaction,and AAEMs migration.Study on two-stage pyrolysis characteristics of cornstalk with lignite or bituminous coal was conducted by using free-fall reactor.By separating biomass and coal instead of co-pyrolysis,the role of volatile-char interaction between them was investigated.Compared with normal single-stage pyrolysis,two-stage pyrolysis promoted the secondary reaction of volatile,which increased the gas yields and promoted the formation of secondary char.Raman spectrum evidenced the remarkable change of the surface carbon structure of the second-stage char.Combustion experiments revealed that combustion reactivities of char after high-temperature volatile-char interaction were changed similarly as those in co-pyrolysis,however,the degree was much slighter.It might be attributed to the greatly decreased contact between volatile and char,as well as the much lower activity of the volatile compared with the case of co-pyrolysis.