Evaluation of nitrates as igniters for microwave-induced combustion: understanding the mechanism of ignition

Abstract

Recent studies have reported the use of microwave-induced combustion (MIC) for digestion of several kinds of matrices. In spite of several applications of MIC, relatively few information is available regarding the mechanism of ignition. In this work, a systematic study related to the role of NH₄NO₃ solution and other nitrates for the ignition step in the MIC system was performed. In this sense, aqueous solutions of Ca(NO₃)₂, KNO₃, Mg(NO₃)₂, NaNO₃ and NH₄NO₃ were evaluated. It was observed that the ignition is dependent on nitrate concentration and microwave power and seems to be related to the oxidation of organic matter by NO₃⁻, which releases enough energy for starting a chain reaction leading to combustion. Additionally, a special action promoted by microwaves without using nitrate solutions was not observed. According to the results, all the evaluated nitrate solutions can be used as igniters with microwave power of 750 W or higher. It was also possible to use nitrate solutions as diluted as 1 mol L⁻¹ and relatively short time was required for ignition of filter paper (below 10 s). Furthermore, the use of higher microwave power allowed a more reproducible ignition.