The Effects Of Additives On The Combustion Characteristics Of Aluminum Powder In Heated Steam

In this paper,to solve the ignition and combustion problems of aluminum（Al）-based hydro-reactive metal fuels,and develop the Al reaction degree,magnesium（Mg）,sodium fluoride（NaF）,and sodium borohydride（NaBH₄）were selected as three suitable additives and applied to nano-aluminum（nAl）and micro-aluminum（μAl）powders.We conducted a combustion experiment to systemtically investigate the effects of theses additives on the ignition and combustion characteristics of Al powder in heated steam,including ignition temperature,maximum combustion temperature,ignition delay time,combustion flame,volume concentration of hydrogen methods,etc.At the same time,the morphology,components,and Al combustion efficiencies of these different combustion products were analyzed by X-ray diffraction（XRD）,scanning electron microscopy-energy dispersive spectrometer（SEM-EDS）,and thermogravimetric analysis（TGA）.Based on the experimental results,the reaction mechanisms of Al powder with the addition of the three kinds of additives in heated steam were further revealed.The main conclusions in present work are as follows:（1）The combustion process of Al powder with steam can be divided into three stages: a slowly heating zone,a rapidly reaction zone,and a cooling zone.Increasing steam temperature could enhance the heat flux in the furnace,which helps to decrease the ignition temperature and the ignition delay time of Al powder.Moreover,the ignition temperature,maximum combustion temperature,and ignition delay time of Al powder are all significantly increased as the Al particle size increases from nanometer to micrometer.Based on analysis of the combustion products,nAl particle with the ruptured oxide shell can directly react with steam and then appears aggregation.At this point,the oxide shell fistly breaks and then melts and forms an oxide cap covering a portion of the surface for μAl particle.Thus,μAl particle continues to absorb heat and then burns in gas phase.（2）For the nAl-Mg sample,nAl-steam reactions may synchronize with Mg combustion,so Mg addition cannot lead to an abvious reduction in the ignition temperature and the ignition delay time of Al powder in steam.Although increasing the Mg content contributes to enhance the maximum combustion temperature and combustion intensity of nAl powder,the combustion efficiency of nAl powder gradually decreases.The results demonstrate that Mg addition helps to develop the exothermic capacity of the Al-steam reaction,but the generated MgAl2O4 will cover on the product surface and hinder the complete combustion of nAl powder.（3）For the μAl-Mg sample,Mg preferentially ignites before μAl powder,so it can be seen that the temperature-time profile obviously performs a double-peak feature.Increasing the Mg content leads to an increase in the ignition temperature and the maximum combustion temperature,and a remarkably decrease in the ignition delay times of Mg and μAl powder.However,the addition of Mg to μAl powder significantly decreases the combustion efficiency.（4）The higher steam temperature,the more reduction in the ignition temperature and the ignition delay time of nAl powder with the addition of NaF.In addition,as the NaF content increases,the combustion efficiencies of nAl powder significantly increase.The combustion products of the nAl-NaF sample become more loose and porous as compared to that of Al powder without addition of additive.The results demonstrate that increasing the steam temperature contributes to accelerate the hydrolysis and ionization processes of NaF,and the producted OH-can deatroy the oxide shell on the surface of nAl powder.Additionally,the released melting F-can penetrate the dense oxidation product and crack the product surface,which helps to improve combustion performance of nAl.（5）For the μAl-NaF sample,the ignition temperature,the maximum combustion temperature,and the ignition delay time of μAl powder in steam significantly decrease as the NaF content increases steadily from 0 % to 7 %.However,the decreases in the flame brightness,combustion region,and combustion time are caused by the loss of a portion of heat released from the μAl-steam reactions to NaF hydrolysis and ionization processes.The combustion efficiency of μAl powder shows tendency to ascend with increasing NaF content.（6）For the nAl-NaBH₄ sample,the ignition temperature,the maximum combustion temperature,and the ignition delay time of nAl powder in steam decrease with the increase of NaBH₄ content.At the same time,the addition of NaBH₄ is conducive to reduce the starting time of hydrogen generation.The results show that NaBH₄ presents high reactivity in steam compared with nAl powder,and the hydrolysis products NaOH can greatly improve the ignition and combustion characteristics of nAl powder.At 450 °C and 650 °C,the volume concentration of hydrogen dramatically increases with the increase of NaBH₄ content.However,the addition of NaBH₄ has no influence on improving the combustion efficiency of nAl powder.（7）Increasing NaBH₄ content or steam temperature is helpful to significantly decrease the ignition temperature and the ignition delay time of μAl powder in steam.Moreover,NaBH₄ addition has positive influence on improving the combustion intensity and shortening the combustion time of μAl powder.It can be found that many Al particles on the combustion product surface can rupture and form the hollow shell structures as the stem temperature is raised.In addition,the combustion efficiency of μAl powder is dramatically decreased with the addition of NaBH₄.The reason is that the producted OH-by hydrolysis reaction of NaBH₄ acts on the oxide shell and generates very stable meta aluminate.The meta aluminate prevents steam from contacting the fresh Al,which leads to the μAl-steam reaction cannot go on.