Release and Transformation of Sodium in Kitchen Waste during Torrefaction

The release and transformation of typical chemical forms of sodium (i.e., H2O-soluble salts and carboxylates) in kitchen waste during torrefaction at 200-300 degrees C was investigated in this study. It was found that sodium release is negligible (<2%) at 200 degrees C after torrefaction for 15 min, but the release increases to similar to 10% when the temperature is increased to 300 degrees C. Because of the high concentration of water-soluble sodium in raw noodles, the release of sodium during torrefaction is mainly in the form of NaHCO3 or Na2CO3, as NaCl is relatively stable at torrefaction temperatures. The transformation of Na from a water-soluble form to a CH3COONH4-soluble form during torrefaction was found to be an important factor in the release of sodium from raw noodles, since sodium release occurs much more readily from a Na-exchanged sample during torrefaction. CH3COONH4-soluble sodium is also found to transform back into a water-soluble form during torrefaction. For example, similar to 18% of the sodium in a Na-exchanged sample transforms to a water-soluble form (i.e., Na2CO3) at 300 degrees C, indicating the existence of an interconversion mechanism between the water-soluble and CH3COONH4-soluble forms. A small amount of water-soluble or CH3COONH4-soluble sodium could also transform to acid-soluble and stable forms during torrefaction at high temperatures (i.e., 300 degrees C).