Reactions of different food classes during subcritical water gasification for hydrogen gas production

The reactions of different food classes during alkaline subcritical water gasification have been investigated with a view on hydrogen gas production. Experiments were conducted with sub-stoichiometric amounts of H2O2 for partial oxidation. NaOH was added to aid sample decomposition, reduce char/tar formation and to promote water–gas shift reaction. In general, hydrogen gas production depended on the class of food wastes including their chemical structure. Carbohydrate-rich food waste (glucose, molasses, tropical fruit mixture, whey powder) produced higher H2 gas yields than others (proteins and lipids). Lipid-rich samples were the most difficult to decompose into gasifiable intermediates and therefore produced the lowest H2 yield. Generally, the addition of NaOH led to higher H2 generation from all sample types. However, two separate side reactions namely, neutralization and saponification involving NaOH with protein- and lipid-rich samples, respectively were significant. Hydrogen production from carbohydrate-rich samples was most suited for the reaction conditions applied.

► Gas composition from the hydrothermal gasification of food wastes.
► H2 generation is dependent on the composition of the main food classes.
► A mechanism for the hydrothermal gasification of the three main classes of food waste.
► Addition of NaOH decreases the formation of tars/oils and char/solid and enhances H2.