Study On Catalyzed Biomass Activated Carbon-CO₂ Gasification By Molten Salt Modified Calcium Oxide

In 2021,the global demand for fossil energy has increased by 5.8%,while the cumulative emissions of CO₂ from global energy and industrial processes has reach36.3 billion tons.Under the pressure of energy shortages and mitigation of CO₂accumulation,China and the world urgently need renewable,clean energy to alleviate the shortage of fossil raw materials and ecological environment problems.Biomass energy was a renewable and low-pollution energy source with great development prospects,and it was the fourth largest energy system in the world.The current main technologies for biomass energy conversion and utilization include:hydrothermal liquefaction,fermentation,alcoholization,pyrolysis,etc.They could convert biomass into energy,but they all had certain shortcomings.In recent years,biomass gasification technology has been regarded as the best high value added conversion and utilization method of biomass because it could convert biomass into high quality fuel and syngas.In order to achieve the goal of overall carbon emission reduction and industrial technology upgrading,China had formulated strict“Double-Carbon”goals,and emission reduction and resource utilization of CO₂ waste gas were imminent.The carbon capture and storage technologies based on chemical absorption,physical adsorption,membrane separation,cryogenic distillation,etc.just temporarily seal up the CO₂ in the flue gas and have not completely solved the problem of CO₂accumulation.Biomass-CO₂ gasification was an important measure to achieve carbon emission reduction and biomass resource utilization,and it provided a reliable solution to alleviate the accumulation of CO₂ in the atmosphere.However,the carbon activity of biomass decreased after the volatile matter precipitated,and the subsequent semi-coke gasification（Boudouard）process always limited the application of this technology.Current research mainly focused on adding or impregnating catalysts to promote the Boudouard reaction.Single alkali metal salt catalysis was widely used,but there was also easy volatilization,deactivation at high temperatures,and reaction deactivation with aluminosilicate.Therefore,in this paper,after selecting the best single metal salt catalyst Ca O,it was modified by molten salt（M/Ca O）with excellent heat transfer performance and catalytic performance,so as to improve the catalytic stability of Ca O,increase the conversion rate of CO₂,and improve the quality of synthetic gas,lays the foundation for the realization of China’s“Double-Carbon”goal.The main research conclusions were as follows:（1）Common alkaline earth metal salts and alkali metal salts could promote the gasification reaction of biomass activated carbon-CO₂,and the order of catalytic effect was:Ca O>K₂CO₃>Na₂CO₃.Compared with no catalyst,the average gasification rate increased by 1.87 times,1.65 times and 1.33 times,respectively.Among them,the CO yield first increased and then decreased with the increase of Ca O loading,and the optimal loading was 15 wt.%,at which point the average gasification rate increased by 3.14 times.The reaction temperature had the greatest impact on the gasification effect.As the temperature increased,the gasification rate increased rapidly.800°C-850°C was an important watershed in the Ca O-catalyzed Boudouard reaction.（2）The ternary carbonate Li₂CO₃-Na₂CO₃-K₂CO₃ had the lowest melting point,the highest thermal stability,the best catalytic effect,and the best molten salt system.With the increased of CO₂ flow rate,the CO production first increased and then decreased,reaching its maximum at 160 m L/min.The small particle size had a slight advantage in the gasification process.Compared with pure Ca O,the optimal loading of molten salt was 10 mol%,and the average gasification rate was increased by 1.29times;the optimal loading of M/Ca O is 20 wt.%,and the average gasification rate was increased by 1.57 times,and the molten salt could increase the optimal loading of Ca O.The gasification rate increased rapidly with the increase of reaction temperature.The CO yields of Ca O-850°C and M/Ca O-800°C were almost the same,and the molten salt could decrease the reaction temperature.（3）Through the mechanism analysis,it was concluded that the molten salt could promote the Boudouard reaction in the following ways:1)Improved the thermal weight loss characteristics of biomass activated carbon.After loading M/Ca O,the pyrolysis process of the sample was promoted,so that the weight loss at 800°C reached 20.01 wt.%,increased 4.77 wt.%.2)The molten layer formed at high temperature could improve the dispersion state of Ca atoms,avoid agglomeration,and better load Ca on the active sites of the sample to improve the reaction efficiency.The molten salt could enter Ca through the pores on the surface of the sample to participate in the catalytic gasification reaction and promote the formation of narrow fracture pores.3)Molten salt could hinder the formation of large aromatic ring structures and promote the formation of small aromatic ring structures during the gasification process of biomass activated carbon.The existence of molten salt could alleviate the reaction deactivation phenomenon of Ca O and improve its catalytic stability.