Hydrolysis of eutectic compositions in the ZnCl2:KCl:NaCl ternary system and effect of adding ZnO

Sepideh Niazi,Espen Olsen,Heidi S Nygård

doi:10.1016/j.molliq.2020.114069

Sepideh Niazi, Espen Olsen + Show 1 more

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https://doi.org/10.1016/j.molliq.2020.114069

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Abstract

Molten salt systems have been considered as proper liquefiers, solvents and transfer media due to their transport and thermodynamic properties. Employing molten salts to liquify biomass could be performed to make it pumpable and transfer it more easily through thermochemical conversion processes to e.g. bio-oil. The first challenge for this application is to find a salt with relatively low melting point. It needs to be low enough to avoid producing ash or char and at the same time high enough to liquify biomass. The selected molten salt requires high thermal stability to avoid salt decomposition at high temperatures and make salt recycling possible. Another challenge is minimising the hydrolysis rate of the molten salt in contact with water molecules originated from the biomass, because this can lead to undesired formation of highly corrosive acids. ZnCl2:KCl:NaCl is a promising molten salt with relatively low melting point, high thermal stability and good properties in contact with biomass. The objective of this work is to investigate the properties of the eutectic mixtures of ZnCl2:KCl:NaCl that are of importance for thermochemical conversion of biomass. Four compositions of ZnCl2:KCl:NaCl are investigated, including Salt #1: 60: 20: 20, Salt #2: 59.5: 21.9: 18.6, Salt #3: 52.9: 33.7: 13.4 and Salt #4: 44.3: 41.9:13.8 in mole fraction. Salt #4 is found to exhibit the best properties, with low melting point, the highest thermal stability and the lowest hydrolysis rate. Salt #1 had the highest hydrolysis rate. However, addition of ZnO showed a marked, limiting effect on the hydrolysis, especially at temperatures below 400 °C.

Highlights

Molten salts have been used in various applications since the 19th century and their technologies are still under development [1,2]
One of the molten salt usages which has attracted a lot of attention in recent years is their role in the emerging renewable energy applications such as thermochemical conversion of biomass
The results showed that using molten salts could improve the yield production of for example phenolic compounds [3,4,5]

Summary

Introduction

Molten salts have been used in various applications since the 19th century and their technologies are still under development [1,2]. In the late 20th century, molten salts were employed to develop pyrolysis of biomass in a few studies. The results showed that using molten salts could improve the yield production of for example phenolic compounds [3,4,5]. This is indicating that the molten salt could have catalytic effects. Another possible advantage is that the heating rate of biomass particles in molten salts is found to be higher than in inert atmosphere [6] or in a fluidized sand bed [7]

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