Preparation And Fire Extinguishing Mechanism Of Micro-nano Ammonium Dihydrogen Phosphate Composite Powder Fire Extinguishing Agent

As the halon fire extinguishing agent is phased out,the development and research of a new generation of clean,efficient and low-toxic fire extinguishing media is one of the focuses and hotspots in the field of fire protection today.Ultrafine powder fire extinguishing agent has become one of the most ideal halon substitute fire extinguishing agents because of its high fire extinguishing efficiency,environmental protection and no pollution,good insulation,convenient use and high safety.The preparation,fire-extinguishing characteristics and fire-extinguishing mechanism of powder fire-extinguishing agent are studied,and the possibility of using it as a fire-extinguishing agent to replace halon is explored.Aiming at the problems of large particle size and uneven particle size distribution of the current ammonium dihydrogen phosphate ultrafine powder,which affects the fire extinguishing efficiency,this paper adopts the anti-solvent ultrasonic method to carry out the ultra-fine ammonium dihydrogen phosphate,and explores the best process.The conditions are that the initial concentration of ammonium dihydrogen phosphate is 0.5mol/L,the solvent-antisolvent ratio is 1:6,the reaction temperature is 15°C,the ultrasonic time is 6 min,and the ultrasonic power is 120 W,and the crystal form control agent（sucrose）is added as 2 wt%,under this condition,ammonium dihydrogen phosphate powder with D₉₀of 6.56μm is prepared,and the prepared samples are characterized by X-ray diffraction analysis,laser particle size analysis,scanning electron microscope and other means,indicating that the prepared phosphoric acid is ammonium dihydrogen powder has uniform particle size and complete shape.Secondly,the fire extinguishing concentration of propane/air flame of ammonium dihydrogen phosphate fire extinguishing agent with D₉₀of 8μm,12μm,16μm and 20μm is measured by the combustion cup experiment,and the superfine ammonium dihydrogen phosphate powder fire extinguishing agent with D₉₀of 8μm has the best fire extinguishing effect,and the fire extinguishing concentration is 27.46 g·m^-3,and with the decrease of powder particle size,the fire extinguishing concentration decreases.The research also carried out the fire extinguishing concentration measurement on the ultrafine ammonium dihydrogen phosphate powder with a D₉₀particle size of 8μm compounded with 10%,20%,30%,40%magnesium hydroxide powder,and obtained the compound Mg（OH）₂content of20%is the best,and the fire extinguishing concentration is 25.74 g·m^-3.At the same time,it is found by comparing with single ammonium dihydrogen phosphate fire extinguishing agent that under the conditions of the same particle size of D₉₀,magnesium hydroxide composite powder fire extinguishing effect better than single ammonium dihydrogen phosphate powder.Finally,the fire-extinguishing mechanism of NH₄H₂PO₄and Mg（OH）₂is studied by means of density functional theory,and it is concluded that the reaction of H₃PO₄,the pyrolysis product of NH₄H₂PO₄,with H?,?OH,CH₃?radicals is the main pathway at high temperature;In the process of Mg（OH）₂unimolecular reaction,Mg（OH）₂and its intermediates（Mg OH,Mg）mainly react with free radicals,and can also react with intermediates（H₂PO₃,H₂PO₄）generated in the reaction process of H₃PO₄to form fire extinguishing substances H₂Mg PO₃and H₂Mg PO₄,to achieve the effect of circulating fire extinguishing;in the calculation of Mg（OH）₂clusters,Mg（OH）₂clusters can react with H·radicals,and the generated intermediates continue to react with H?,?OH and?O radicals to form cyclic reaction.In conclusion,the micro-nano-scale NH₄H₂PO₄and Mg（OH）₂composite powder can interrupt the flame chain reaction and have excellent fire extinguishing performance,which can be used as an alternative product for"halon".