Experimental Study On Biomass Pyrolysis And Gasification With Molten Carbonates

Biomass,as the only renewable carbon resource,has attracted wide attention for its high efficiency and high quality utilization.Gasification can convert biomass directly into high-quality syngas and provide raw materials for liquid fuel and chemical synthesis,which is very important for efficient conversion and utilization of biomass.The application of molten salt thermal conversion technology in biomass gasification has unique advantages,which can effectively improve the quality and yield of syngas.However,at present,there is a lack of relevant research on the mechanism of the thermal conversion process of biomass with molten salts.In this paper,a series of experimental studies on the pyrolysis and gasification of biomass with molten salt are carried out.The applicability of the molten salt system,the catalytic pyrolysis mechanism of molten salt and the evolution of char structure during the process of molten salt gasification were studied.The results will provide theoretical guidance for the application of pyrolysis and gasification of biomass with molten salt.The main research work are as follows:Firstly,four kinds of binary molten salts and three kinds of ternary molten salts were prepared.The thermogravimetric and differential scanning calorimetry analyzer,fixed-bed reaction system and thermodynamic equilibrium simulation were combined to study the thermal melting characteristics of molten salt and the selectivity for biomass pyrolysis.The results indicated the melting point and stability of alkali metal carbonates are between chlorides and sulfates.The alkali metal carbonates promoted the increase of H₂ and CO content in the pyrolysis gas and the content of phenols and aromatics significantly,while reduced the content of carboxylic acid substances in the bio-oil greatly.At the same time,it also have better activation effect on char to form mesoporous porous carbon.Considering the melting point and stability of molten salt and the influence on pyrolysis products,ternary alkali metal carbonate(Li₂CO₃-Na₂CO₃-K₂CO₃)showed the best applicability to the pyrolysis and gasification of biomass and the improvement of products quality.Secondly,the effects of the addition amount of ternary molten carbonates and the reaction temperature on the pyrolysis characteristics of biomass were investigated with thermogravimetric analyzer and fixed bed.The mechanism of the effect of molten carbonate on the pyrolysis characteristics of biomass was studied.It was found that at450-650°C,the catalytic effect of molten salt on biomass pyrolysis increased the yield of H₂and CO₂ greatly.At the same time the content of carboxylic acids in liquid products decreased significantly while ketones increased but the yield decreased.The O/C ratio and the char yield increased;At 750-850°C,the molten carbonate could react with char and generate a large amount of CO,further increased the gas yield.The char yield decreases but the pore structure was improved.At 850°C,when the mass ratio of molten carbonates to biomass was 10,the gas yield was increased significantly from 41.35%(the original pyrolysis)to 82.57%.When the mass ratio of molten carbonates to biomass was 5,the char has a maximum specific surface area of 972.57 m²/g and a maximum total pore volume of0.6203 cm³/g.Then,the catalytic mechanism of molten carbonate to biomass pyrolysis combined with heat and mass transfer process was further analyzed,and the catalytic cracking effect of molten carbonate on biomass pyrolysis volatiles was emphatically investigated.At450-550°C,the carboxylation of carboxylic acids under the catalysis of molten carbonate was converted into linear ketones,while furan is converted into linear alcohol ketones or cyclopentanone,which resulted in the formation of a large number of linear alkanone hydrocarbons,cyclopentanone and phenols in the oil.At 650-850°C,the pyrolysis intermediate further deoxidized and aromatized under the catalysis of molten salt,forming a large number of polycyclic aromatic hydrocarbons,its content was more than 90%in the bio-oil at 850°C.At the same time,the yield of H₂ and CO increased significantly,and the yield of syngas reached 29.51 mmol/g.Next,the gasification process of biomass in molten salt was studied using on-line gas analyzer.The gasification process dynamics was studied,and the effects of different gasification parameters on gas release law,gas yield and gasification efficiency were investigated.The results indicated the biomass CO₂ gasification process in molten salt was close to the homogeneous reaction model at 750°C and 850°C,and closer to the phase interface reaction model at 950°C.The activation energy of biomass CO₂gasification in molten salt decreased significantly.The increased of gasification temperature,CO₂ flow rate and CO₂ concentration were beneficial to the improvement of gas yield and gasification efficiency.Considering the gas production,gasification efficiency and thermal stability of molten carbonate,it was found that it was found that the suitable conditions for biomass gasification in molten salt are:gasification temperature 850-900°C,CO₂ concentration greater than 50%and raw material particle size about 1mm.Finally,the gasification characteristics of biochar in molten salt were studied,the structural evolution of char in molten salt was discussed,and the mechanism of catalytic gasification of molten salt was revealed.The results show that with the introduction of molten salt,the reactivity of char was significantly enhanced,and the structure of char was also significantly improved.The mesoporous structure of char has increeased greatly.The Raman spectra showed that molten salts could promote the decomposition of large aromatic ring systems(no less than 6 rings)into small aromatic ring systems(with 3 to 5 rings),while inhibited the polymerization of small aromatic structures into larger aromatic ring structures,and also promoted the ring-opening of aromatic rings to form saturated branched chains.Moreover,the aromatic microcrystalline carbon in char was converted into amorphous carbon after ring opening,which reduced the aromaticity of char,resulting in the reduction of aromatic microcrystalline size and the increase of layer spacing,further improving gasification reactivity.