Doctor of Philosophy
Chemical and Biochemical Engineering
George Nakhla and Paul Charpentier
Supercritical water gasification (SCWG) is an innovative, modern, and effective destruction process for the treatment of organic compounds. Hydrogen production using SCWG of biomass or waste feedstocks is a promising approach towards cleaner fuel production while simultaneously providing novel solution for hard-to-treat organic wastes. The main premise of this work was to experimentally examine real waste biomass sources i.e. hog manure and waste biomass model compounds using SCW gasification while examining various commercial catalysts. The improvement of the SCWG process requires an understanding of the waste/biomass reaction chemistry, and thus the knowledge of the reaction mechanisms is critically important for proper catalyst selection and design. The main possible reactions that occurred through the formation and disappearance of intermediate compounds as well as the final gaseous and liquid final products are reported. In this work, gasification and partial oxidation of glucose 0.25 Molarity (M) was conducted using different metallic Ni loadings (7.5, 11, and 18 wt %) on different catalyst supports (θ-Al2O3 and γ-Al2O3) in supercritical water at 400-500°C, and compared with a commercial catalyst (65 wt % Ni on Silica-Alumina). Results showed that the presence of metallic Nickel increases the yield of gases and the total gas yield increased with increasing nickel in the 7.5-18 wt % Ni/Al2O3 catalyst. This study showed that the same hydrogen yield can be obtained from the synthesized low nickel alumina loading (18 wt %) catalyst as the high (65 wt %) nickel on silica-alumina commercial catalyst. In this work, oleic acid was examined as a model compound for lipids. Results showed that an increase of temperature coupled with the use of catalyst enhanced the gas yield dramatically. The H2 yield was 15 mol/ mol oleic acid converted using both the pelletilized Ru/Al2O3 and powder Ni/Silica-alumina catalysts which yielded 4 times higher than the calculated equilibrium yield of 3.5 mol/mol oleic acid fed. The composition of residual liquid products was studied and a generalized reaction pathway of oleic acid decomposition in SCW reported. The catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated. Employing TiO2 as a catalyst alone had no significant effect on the H2 yield but when combined with CaO increased the hydrogen yield by 35%, while promoting higher total carbon (TOC) reduction efficiencies. The process liquid effluent characterization showed that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme was provided. Cysteine gasification in supercritical water in the presence of Ru/Al2O3, Ru/AC and activated carbon (AC) catalysts was also investigated. The main sulfur-containing compound in the gaseous effluents in all experiments was H2S. It was found that the formation of H2S was neither dependent on temperature nor on the catalyst. The composition of residual liquid products revealed the presence of residual organic sulfur components that include diethyl sulfide, diethyl tri sulfide, and ethanone. A generalized reaction pathway for organo-sulfur compounds was reported. Catalytic hydrogen production with various catalysts from hog manure using supercritical water partial oxidation was investigated. The order of H2 production was the following: Pd/AC > Ru/Al2O3 > Ru/AC > AC > NaOH. A 35% reduction in the H2 and CH4 yields was observed in the sequential gasification partial oxidation (oxidant at an 80% of theoretical requirement) experiments compared to the gasification experiments (catalyst only). Moreover, this reduction in gas yields coincided with a 45% reduction in the liquid effluent chemical oxygen demand (COD), 60% reduction of the ammonia concentration in the liquid effluent, and 20% reduction in the H2S concentration in the effluent gas. The scientific contribution of this study culminated in the development of a qualitative mechanistic understanding of the reaction chemistry of organic matters presented in waste streams such as waste biomass, sewage sludge, and hog manure. This understanding of the SCW reaction chemistry is required for the potential applications of SCW for energy recovery from waste streams through hydrogen production.
Youssef, Emhemmed A.E.A, "Supercritical Water Gasification of Biomass & Biomass Model Compounds" (2011). Electronic Thesis and Dissertation Repository. 295.