The subcritical and supercritical water gasification of residue of some agricultural biomass samples (cotton and tobacco stalks) was investigated in a batch reactor at temperature range of 300–600°C and experiments were performed by addition of 10wt.% of different natural mineral catalysts (Trona, Dolomite and Borax). The yield and composition of gaseous, aqueous products and residue were identified by different analytical techniques. Gaseous products consisted mainly of permanent gases such as hydrogen, carbon dioxide, methane, carbon monoxide and C2–C4 compounds. Aqueous products consisted mainly of carboxylic acids (hydroxyacetic acid, formic acid and acetic acid), furfurals, phenols (phenol and cresols), aldehydes (formaldehyde and acetaldehyde). The effectiveness of three different natural mineral catalysts in SCWG can be classified as being Trona [Na3(CO3)(HCO3)·2H2O]>Borax [Na2B4O7·10H2O]>Dolomite [CaMg(CO3)2]. The results showed that the addition of Trona as catalysts at reaction temperature of 600°C, hydrogen yields of tobacco and cotton stalks were increased by 42.9% and 39.9%, respectively.

In this study, production possibilities of hydrogen or methane via SCWG from various agricultural materials which are being a large potential and harvested to a large extent, such as fibrous formed agricultural residues i.e stalk of tobacco, cotton, sunflowers, corn, and lignocellulosic structured biomass materials such as cabbage, leek, cauliflower, and from lignocellulosic hard shell structured i.e. hazelnut, walnut, almond and valonia oak, will be investigated. Selected biomass samples were gasified in a batch autoclave at different temperatures (350-400- 450-500°C) and absence/presence of catalyst (K2CO3 or KOH). Organic fraction of biomass samples reacts with water at these conditions to produce gaseous products (mainly CO2, H2 and CH4, small amount of CO ve C2-C4 hydrocarbons), aqueous liquid products (phenols, carboxylated acids, alcohols, aldehydes, ketones and furfurals ) and trace amount of tar and coke.