Study On Preparation Of Clean Fuel By Reaction Of Alkali Metal Hydride With Gaseous Small Molecule Compounds

Efficient hydrogen storage is one of the key technical challenge of turning hydrogen economy into reality. However, there are still many problems to be solved because of the bottleneck about hydrogen storage as a clean energy resources. Conventional high pressure storage and cryo-storage are not suitable ways for practical vehicular application due to their hazardous properties and their low volumetric energy density, which limit application of methane as a kind of clean energy by carbon dioxide methanation. The previous studies indicate that a kind of reversible hydrogen storage material with high capacity that can release hydrogen spontaneously under mild condition is promising to be prepared by combining the alkali metal hydrides with ammonia. The hydrogen desorption reaction can be described as follows. MH+NH3→MNH2+H2 (M=Li, Na, and K) (1)Among these systems, the LiH-NH3 system with the highest hydrogen capacity (8.1 wt%) can proceed at relatively lower temperature due to a hydrolysis-type exothermic reaction. However, the reaction kinetic of the system is slow for a realistic application. The core problem to developing an alkali metal hydrides-ammonia hydrogen storage material is how to make it have high capacity and possess excellent hydrogenation and dehydrogenation kinetics performance simultaneously.Firstly, In the present work, the hydrogen desorption properties for the reactions between lithium hydride with different particle sizes and NH3 at 50-200 ℃ were systematically investigated to design a system with better kinetic properties. Moreover, various kinds of potassium compounds were also examined as potential catalysts to improve the kinetic properties of the hydrogen desorption reaction in the LiH-NH3 system. It is found that three ways are effective to improve the desorption kinetics of the LiH-NH3 system. One way is to decrease the size of the LiH particles through ball milling, anther is to increase the temperature, and the third is to use the potassium compounds doping, among the adopted potassium compounds, KBr shows the best effect.Secondly, in this paper, the KH-added LiH-NH3, KH-added LiH-LiNH2, KH-added LiNH2, and KNH2-added LiNH2 systems were systematically investigated as they have the better dehydrogenation/hydrogenation properties. It was found that the ternary amide KLi3(NH2)4 was an important intermediate that was inclined to be formed in the dehydrogenation and hydrogenation processes of the potassium compounds-added Li-N-H system. Further investigations revealed that both the solid state reaction of LiNH2 with KNH2 and the solid state reaction of LiNH2 with KH under mechanical ball milling or heat treatment condition will lead to the formation of the KLi3(NH2)4 ternary amide. Moreover, the ternary amide KLi3(NH2)4 single phase was successfully synthesized by the mechanical ball milling and its ammonia desorption and hydrogenation properties were investigated. It was observed that the ammonia desorption rate of KLi3(NH2)4 was faster than that of LiNH2 and the hydrogen absorption kinetic of KLi3(NH2)4 was between those of KNH2 and LiNH2.Lastly, we use alkali metal hydride with strong reducibility and low density to react with carbon dioxide to realize its methanation with hydrogen producing. the mechanism for reactions of MH with CO2 was proposed as following reaction equations. It is worth noting that the enthalpy change and gibbs free energy values in the system of carbon dioxide and alkali metal hydride is calculated to be negative(-515—890 KJ/mol, MH=LiH), exhibiting an exothermic nature. These characteristics determine that combining the alkali metal hydride with carbon dioxide is a promising way to prepare a kind of methanation material with high energy capacity that can release methane spontaneously under moderate condition. 4LiH+CO2→2Li2O+C+H2→2Li2O+CH4 (2) 4NaH+3CO2→2Na2CO3+C+H2→2Na2CO3+CH4 (3)We demonstrate for the first time that carbon dioxide can be effectively captured and reduced by the reduction properties of alkali metal hydrides producing methane and hydrogen only through heat treatment at moderate temperature. The yield and mole fraction of methane in the gas products depend on the reaction temperature, pressure and time. The reactions around 450 ℃ show superior yield and mole fraction of methane. The yield and mole fraction of methane are increasing with the reaction time lengthening within 48 h. The analysis of mechanism indicates that the amorphous carbon as the intermediate plays important roles in the process of the conversion of carbon dioxide to methane.