Experimental Research On Catalytic Hydrogenation Reduction Of CO₂ From Coal-fired Flue Gas

The emission reduction of carbon dioxide (CO2), as a major international issue for influencing the economics and environment of the world in the future, is an important part of implementing sustainable development strategy in our country. As an effective technology for the emission reduction of CO2, the catalytic transfer hydrogenation (CTH) has been widely studied by scholars from home and abroad. But it has the issues of high energy consumption, complex operation conditions and low economy, which may limit its industrial application. On the basis of CTH theory, a new technology about the conversion and utilization of CO2 from coal-fired flue gas was developed under atmospheric pressure and moderate temperature in this paper, which would provide the theoretical prop and the technical base for the resource utilization of CO2.In the self-made reactor, the hydrogen donor of M was selected as the high effective reagent for CO2 conversion under atmospheric pressure and moderate temperature. The experiments of influencing factors such as volume content of solvents, M concentration, reaction temperature and solution pH were carried out, from which, the optimal conditions were obtained with 80% of volume content of ethanol,0.439mol·L-1 of M concentration,45℃ of reaction temperature and 9.0 of pH. The competition or inhibition mechanism of coexisting gases such as sulfur dioxide, nitric oxide and oxygen on CO2 conversion was investigated, and the results showed that the coexisting gases had no obvious effect on CO2 conversion. According to the results of penetration experiments and the relevant theories, the average amount of CO2 absorbed by M solution was calculated at about 2.65×107mg under the optimal conditions.For improving the conversion efficiency of CO2, the nickel/phosphorus (Ni/P) material was prepared as the most suitable catalyst for CO2 conversion. Through the optimization tests for the preparation of the catalyst, the catalyst carrier was determined as aluminum oxide (Al2O3), the molar ratio of nickel to phosphorus was 1:1, the impregnation time was 12h and the calcination temperature was 550℃. The catalyst (Ni-P/Al2O3) prepared at the above conditions were applied in experiments to test its catalytic property on CO2 conversion, from which, the optimum reaction conditions were achieved with 1.0% of mass ratio of Ni-P/Al2O3 to M,0.175mol·L-1 of M concentration,8.0 of solution pH,90% of ethanol content,55℃ of reaction temperature, 0.5L·min-1 of gas flow and 0.15L of absorption solution. Under these optimum conditions, the average conversion efficiency for CO2 was increased by 36.72%; meanwhile, the usage amount of M was deceased by 60.14%, which showed that Ni-P/Al2O3 catalyst was effective for the reduction of CO2 by M.In order to determine the products of CO2 conversion, the product samples were analyzed by Ultraviolet Adsorption Spectrometry (UAS), Ion Chromatography (IC), Fourier Transform Infrared Spectrometry (FTIR) and High Performance Liquid Chromatography-Mass Spectrometry (HPLC/MS), respectively, from which, the main reduction product was confirmed as formate. Combined with the previous results, the mechanism of formate produced from CO2 reduction by M was proposed, based on the theory of electrochemical theories and the reaction characteristics of CO2 and M. The active borohydride intermediates of BHi(OH)4-i-1 (i=3,2,1) and HCO2BHj(OH)3-j-1 (j=3,2,1,0) were considered as the important mediums for the formation of formate.According to the thermodynamic theories of state functions of chemical reactions and Helmholtz functions of constant temperature procedures, the thermodynamic parameters of the standard conditions and the temperatures of 5℃～65℃ were calculated respectively. The thermodynamics feasibility of the reduction reaction between CO2 and M was verified. The models of thermodynamic parameter and temperature were obtained, and the influence of temperature on the reaction of CO2 and M was investigated.The kinetics experiments were carried out under non-catalytic conditions and catalytic conditions, respectively. The results showed that under non-catalytic conditions, the reaction partial order of CO2 conversion was 1.08, and the apparent activation energy was about 20.25 kJ·mol-1. Under catalytic conditions, the partial order of CO2 reaction was 1.44, and the apparent activation energy was about 12.64 kJ·mol-1, which indicated that the energy barrier of the reduction reaction of M and CO2 could be reduced effectively, and the reduction conversion of CO2 could be promoted by Ni-P/Al2O3 catalyst.