Comparing the thermal conversion behavior of bio-wastes in three molten nitrates

Abstract

Molten salt could act as both heat transfer fluid and catalyst in bio-waste pyrolysis. The temperature and composition of anions and cations of molten salts have significantly different effect on the pyrolysis behavior of bio-waste. In this study, three kinds of molten nitrates have been applied to conduct thermal conversion experiments of two kinds of bio-waste (beech and waste paper). The results showed that NO2− has a higher reactivity towards the bio-waste than NO3−. In detail, a faster reaction rate of biowaste pyrolysis and higher yield of gas products were found in NaNO3–KNO3–NaNO2 and NaNO3–NaNO2 than NaNO3–KNO3. From the GC-MS analysis, it can be concluded that K+ inhibited the dehydroxylation process of phenols at 300 °C, while the phenols were completely converted to the aromatic hydrocarbonas the temperature went up. In addition, nitrogenous heterocycle was produced due to the reaction between furans, aldehydes, ketones and NO2−/NO3−, and the nitration process was found to be enhanced as the temperature rose. Inorganic components contained in bio-waste remaining in molten salt may affect its catalytic properties. Therefore, the effect of KCl–K2SO4–CaCO3 on the pyrolysis behavior of bio-waste in molten nitrate was also investigated. The inorganic components increased the weight loss temperature, while the thermal conversion of bio-wastes was promoted after reaching the reaction temperature. The study in this paper provides guidance for the selection of appropriate molten nitrate system and temperature for bio-waste treatment.