Influence of alkali catalysts on the production of hydrogen-rich gas from the hydrothermal gasification of food processing waste

Abstract

Sub-critical water gasification by partial oxidation of glucose was carried out in the presence of various alkali catalysts; NaOH, KOH, Ca(OH)2, Na2CO3, K2CO3 and NaHCO3. Experiments were carried out in a closed batch reactor under sub-critical water conditions of 330°C temperature and 13.5MPa pressure. Hydrogen gas yield in relation to the alkali catalyst was in the following order; NaOH>KOH>Ca(OH)2>K2CO3>Na2CO3>NaHCO3. The metal hydroxides produced higher H2 gas yield than the carbonates or bicarbonate. Carbon dioxide was the predominant gas product in the presence of metal carbonate and bicarbonates. Catalysts of the hydroxide ion group, NaOH, KOH and Ca(OH)2, were selected to react with representative food processing wastes in the form of molasses and rice bran under hydrothermal condition. The results indicated that NaOH, KOH and Ca(OH)2 could promote biomass decomposition and improve the amount of H2 product via the water-gas shift reaction by intermediate formation of formate salts. In addition, NaOH, KOH and Ca(OH)2 inhibited and suppressed tar and char formation. NaOH was also effective in capturing the CO2 formed, thereby shifting the water-gas shift reaction equilibrium in the forward direction. In addition, results showed that hydrothermal gasification at low temperature depended not only on the metal ion (Na+, K+ and Ca2+) but also on their species and concentrations. It is suggested that the basicity strengths of the aqueous solutions of alkali additives may have enhanced the initial decomposition of biomass into gasifiable intermediates, especially during the heat-up period.