Thermochemical conversion of multiple alkali metals in food waste pellet with a core-shell structure

The transformation and release of alkali metals during pyrolysis of heterogeneous food waste with high Na and K contents are experimentally characterized with unresolved mechanisms. A food waste pellet with a core-shell structure was designed and the release of K/Na from the core and the retention of K/Na by the char shell were quantified for different pyrolysis conditions. The retention effect of the char shell on the gaseous alkali released from the pellet core is strong at 800 °C while becomes negligible at lower temperatures (<600 °C). Based on the X-ray diffraction and chemical fractionation results, we attribute the retention phenomenon to the formation of non-soluble and water-soluble char-Na/K. It is also found that water-soluble alkali chlorides tend to form solid solutions, Na·1002K·8998Cl and Na·9003K·0997Cl, during pyrolysis, which promotes the release of gaseous NaCl. Released NaCl then reacts with the K-aluminosilicate to generate gaseous KCl. In consequence, the overall release of KCl is significantly enhanced by the addition of NaCl when compared to the samples without NaCl addition. At last, new thermochemical conversion pathways of Na/K during pyrolysis are proposed, in which the role of solid solutions and the retention effect of the heterogeneous fuel structure are simultaneously considered.