Study On Reaction Kinetics And Characteristics Of Producing Hydrogen And Heat By Al-based Metallic Fuels And Water

Aluminum is a metal with high energy density.It has a great prospect in the field of underwater propulsion and aerospace.Aluminum-water reaction is also a solution to hydrogen production and storage.The main problem in the application of metallic aluminum is that the presence of the oxide film causes the reaction to be difficult to start.Therefore,suitable methods need to be adopted to improve the reactivity of the aluminum.In this paper,the thermodynamic theory of aluminum-water reaction was first studied by using FactSage software.The heat of the aluminum water reaction was reduced and the Gibbs free energy was reduced?the absolute value increased?,and the reaction was carried out more thoroughly.It could be known that the temperature was an important factor affecting the reaction of aluminum water.And the impact of pressure was relatively weak.The reactivity of aluminum in water vapor below 1000? was then studied using a Cahn Thermax 500 pressurized thermo gravimetric analyzer.In the case of less than 800?,the reaction of aluminum in water vapor was weak.The rate of weight gain started to accelerate after 900 ?.The start temperature of Al-H₂O reaction was too high and was not conducive to rapid production of hydrogen.Proper activator was sought by adding different low-melting point metals to the aluminum to react with water.Thermogravimetric analysis was adopted to study the reaction characteristics of samples with the addition of lithium,magnesium,zinc,bismuth and tin.The results indicated that lithium showed the best result,followed by magnesium.After adding 20%of lithium,the reaction efficiency of aluminum was more than 66.64%under the temperature of 1000 ?.The kinetic parameters of Al-220%Li Al-20%Mg,Al-20%Zn,and Al-20%Bi and Al-20%Sn in water vapor were calculated by single curve integral method.The reaction rate constant of Al/H2O was 2.8×109exp?-47994/T?,while for Al-20%Li in water vapor the reaction rate constant was 5.5×104exp?-3869/T?.The reaction product was analyzed using XRD.The results showed that Al Al2O3 and A12.667O4 were found in the product of aluminum-water reaction.For Al-20%Li,Al,Al2O3 and LiAlO2 were found in the product,and no lithium oxide or hydroxide was found.The same was withAl-20%Mg The products of Al-20%Zn,Al-20%Bi and Al-20%Sn in water vapor were characterized by XRD.The respective oxides and elements appeared in products.And zinc,tin and bismuth existed in the form of oxides rather than aluminates.The effect of Li contents was studied using the same method?thermogravimetric analysis?.The results showed that the increase of lithium content led to the increase of final weight gain.The ignition temperature decreases with the increase of lithium content.When the lithium content exceeded 5%,ignition tenperature did not change much and was about 380?.Based on the results of thermogravimetric experiments,magnesium and lithium were used as the additives and initiator for aluminum-water reaction.Using the self-built test bed,the corresponding experimental equipment recorded temperature and hydrogen production during the reaction process.Al-10%Li,Al-20%Li,Al-10%Mg and Al-20%Mg were used in the following experiments.When the elemental aluminum was heated to 700 ?,the yield of hydrogen production was only 142 ml.g-1.The hydrogen production rate was much lower than that mixed with magnesium and lithium.The hydrogen production yield of Al-20%Li reached 1038.85 ml·g-1,which was the largest in the five samples.The results showed that the hydrogen production rate and yield of Al-20%Mg were much lower than that of Al-20%Li The results of hydrogen production were consistent with the experimental results in thermo gravimetric experiments.Using the scanning electron microscope and transmission electron microscopy to test the samples after the reaction,it can be seen that the surface of product for pure aluminum was basically no damage.Many complete spheres could be found.For Al-20%Mg,surface morphology of the reaction product was relatively complex.Something like needles appeared.In the case of Al-20%Li,hexahedral crystal was formed in the product.According to the results of XRD,hexagonal crystal was speculated to be lithium aluminate?LiAlO2?.No solid oxide film was found in the product,and there was a gap between the hexagonal crystals,which favored continued contact with the water vapor and continued reactionThe effect of different lithium content on the hydrogen production of aluminum was also studied.When the lithium content was 5%,the hydrogen production yield and the average hydrogen production rate were 53%and 112 mL·min-1·g-1.The maximum hydrogen production rate for Al-20%Li was slightly smaller than that of Al-5%Li and Al-10%Li.But Al-20%Li has the highest hydrogen production yield.The main reason lay in the longer reaction time.Then,the opration conditions of the samples in the reactor were optimized,and the law of heat conduction and heat release during the reaction was studied.The effects of temperature and inflow on hydrogen production of aluminum-based metals were studied.The optimum reaction conditions were selected by comparing the results of hydrogen production and internal temperature changes.When the starting temperature was too high,the surface of the sample was covered with fluidizedsubstances to prevent the further progress of the reaction.But when the temperature was lower,it was difficult to promote the reaction of aluminum-water.The water inflow mainly affected the supply of reactants and changes in internal temperature,which showed opposite effects.In the study of the heat release,the temperature measured by infrared imager and the thermocouples could be used to compare the surface temperature and the internal temperature inside the sample.The evolution in surface temperature of the sample was studied using the infrared temperature map taken at different times.Finally,the dynamic reaction process of Al-based metals in the reactor was studied.In this paper,it was focused on Al-Li and Al-Mg-Li alloy.Using the arrangement of thermocouples,the temperatures of different sections and different heights in reactor during the reaction were recorded and analyzed.And the change of internal pressure was also monitored.For the reaction of Al-Li alloy in the reactor,the change of pressure and hydrogen generation rate had the same trend and could be divided into three stages.By analyzing the temperature in the horizontal and vertical directions,the following conclusions could be drawn:the reaction in the reactor began from the inlet,and then the reaction core transferred to the central position and finally to the farthest from the inlet.The rate of reaction propagation in the horizontal direction was approximately 0.568 to 1.105 mm/s.Al-Mg-Li had a similar trend.For the study of the dynamics of aluminum-based metals,heat release and hydrogen generation,it provided the exqperimental basis for aluminum-based energy utilization system.The energy system proposed in this paper could be applied in special occasions.It had unique advantages and great development potential in underwater propulsion field.