Hydrothermal gasification of different agricultural wastes in supercritical water media for hydrogen production: a comparative study

Conversion of different agricultural residues including almond shell, walnut shell, barley straw, canola stalk, rice straw and wheat straw to hydrogen-rich gas was performed via gasification in supercritical water media in a determined condition. Elemental characterization was performed using CHNSO analyzer. Besides, cellulose and lignin contents in biomass structure were determined according to TAPPI test methods T264cm-97 and T222om-02, respectively. The correlations between the yields of the product gas components with C/H/O ratio in the initial forms of used feedstocks were investigated. In addition, the relation between the components of biomass structure with the yields of main gaseous products was also studied for each feedstock. The maximum hydrogen yield of 8.38 mmol/g was observed for barley straw which has the highest H percentage of 6.5 wt%. Canola stalk with the highest C/H ratio showed the highest total gas yield of 25.3 mmol/g. Canola stalk with highest amount of cellulose and hemicellulose of 67.42 % had the highest CGE of 45 % and barley straw had the highest HGE of 20 %. Higher C/H value and lower oxygen percentage in the initial form of feedstocks resulted in higher total gas and CO2 yields and lower hydrogen yields. Lignin content in the initial form of the feedstocks was inversely proportional with total gas yields whereas cellulose content showed a straight relation with the total gas yield.