Study On The Mechanisms Of CO₂ Capture And Tar Decomposition Of Herb Residues Pyrolysis With Ni/CaO Composite Catalyst For Gas Production

In the process of biomass thermal conversion,two key issues of tar decomposition and CO₂ capture in gas products need to be solved in order to obtain the high-quality gas production.In this paper,the simulation and experimental methods were used to investigate the mechanisms of CO₂ capture and tar decomposition in herb residue catalytic conversion for gas production.The mechanisms of the adsorption and catalytic reactions of different molecules on the Ni/CaO catalyst?s?were studied based on Density Functional Theory simulations.Based on the Gibbs minimum free energy method,the thermodynamic equilibrium analysis of herb residues catalytic conversion was carried out and the experimental suitable operating parameters were obtained.The catalyst was characterized by XRD,SEM,BET and thermogravimetric experiments,etc.The mechanisms of CO₂ capture and tar decomposition in herb residue catalytic conversion for gas production was explored by using thermogravimetry-infrared experiments,pyrolysis-combined gas chromatography/mass spectrometry and moving bed experiments.The results are as follows:The molecular adsorption mechanism on Ni/CaO catalyst?s?surface.The order of adsorption was H₂O>CO₂>CO>CH₄>H₂ according to the absolute value of molecular adsorption energy.O_surf was the active center of decarbonization.The adsorption energy of carbon dioxide on the catalyst surface increased with nickel added to CaO.Toluene,as the model compound of biomass pyrolysis tar,the Mulliken bond population of carbon-carbon bonds in its benzene ring was reduced with the catalyst?s?.The carbon-carbon bond was more susceptible to cleavage.The catalysts promoted the decomposition of toluene into small molecules.Based on the Ni/CaO surface,CH₃+H?CH₄ was the rate determining step of the methanation reaction.CO+O?CO₂ was the rate determining step of water gas shift reaction.CO₂?CO+O was the rate determining step of the carbon reduction reaction.The catalysts promoted the carbon reduction reaction,which enhanced the reduction of CO₂ to generate a large amount of CO.The CO₂ concentration was reduced by the in-situ decarburization of Ni/CaO catalyst?s?.Furthermore,the addition of catalyst promoted water gas reaction and the content of H₂ increased.The thermodynamic equilibrium analysis showed that the suitable operating parameters for the herb residue catalytic pyrolysis were 1 atm,Ca/HR=0.65 and700°C.The molecular sieve was used as the carrier-supported Ni/CaO to improve uniform dispersibility.The CO₂ absorption rate of calcium oxide could reach more than 85%.After 8 thermogravimetric cycles,the CO₂ absorption rate still reached70%,while the absorption rate decreased with the increase of cycle numbers.Increasing the heating rate promoted gas production.And the in-situ CO₂ capture of Ni/CaO promoted the methanation temperature range moving to a higher temperature region,which relieved thermodynamic constraints.The methane selectivity was maintained at the higher temperature required for tar catalytic decomposition and biomass thermal conversion.The product composition of the entire reaction system was adjusted and the quality of pyrolysis gas was improved on the basis of promoting the water gas shift reaction,the reaction of char and carbon dioxide,etc.Moreover,it improved H₂ production,which was consistent with the results of the simulation.The molecular sieve-supported Ni/CaO catalyst had an excellent catalytic cracking effect on aromatics.Ni/CaO exhibited the catalytic effect of promoting toluene decomposition.Meanwhile,Ni/CaO was beneficial to the reduction of acid substances such as acetic acid.With the case of Ni/CaO addition,the hydrogen content increased to 48.13vol.%,carbon monoxide and carbon dioxide decreased to 21.39 vol.%and 5.74vol.%,respectively.The methane content was 17.5 vol.%.The absorption rate of carbon dioxide reached 81.5%.The gaseous low heating value reached 14.1MJ/Nm³.