Hydrogen Production from Supercritical Water Gasification of Lignin, Cellulose, and Other Biomass Residues
Kang, Kang 1986-
Hydrogen production from biomass is an attractive alternative for clean energy generation. This thesis presents the results from four research phases which were carried out to maximize the hydrogen yield from various biomass via supercritical water gasification (SCWG). In the first step, non-catalytic SCWG of lignin was optimized to achieve high hydrogen yield using Central Composite Design (CCD) method. The parameters that were optimized include temperature (400−650 °C), water to biomass mass ratio (3-8), and pressure (23−29 MPa). To achieve higher hydrogen production, higher temperature was desirable. The change of pressure does not show a significant effect on hydrogen yield. According to the response surface model, the highest hydrogen yield was predicted as 1.60 mmol/g. The optimum reaction conditions for highest hydrogen yield were predicted as temperature = 651 °C, water to biomass ratio = 3.9, and pressure = 25 MPa. In the second phase, Ni based catalysts were screened and modified for hydrogen production from lignin SCWG. The activity of Ni based catalysts using different supports follows the order of MgO < activated carbon (AC) < ZrO2 < TiO2 < Al2O3. The activity of the Ni based catalysts with different metal promoters follows the order of Cu < Co < Ce. The 20Ni-0.36Ce/Al2O3 catalyst showed highest hydrogen yield of 2.15 mmol/g at 650 ºC, 26 MPa, and water to biomass ratio of 5. In the third phase, two types of novel catalysts were prepared and tested, including NiCo/Mg-Al bimetallic catalysts and TiO2 supported Ni catalysts. For Ni-Co/Mg-Al bimetallic catalyst system, the hydrogen yield was well correlated to the quantity of strong acidic sites measured by NH3-TPD within the temperature range of 400-600 °C. Catalysts prepared by precipitation showed higher hydrogen yield than those prepared by impregnation. For Ni/TiO2 catalysts, 5 wt% Ni loading in the range of 0–20 wt% was the best for hydrogen production. However, improvement of hydrogen yield was not observed when Co, Ru, Ce or Mg was added to the 5Ni/TiO2 catalyst as promoters. A detailed mechanism for catalytic SCWG of lignin was proposed to address the role of different components of the catalyst in this process. The best catalysts found in this phase are iii Cop.2.6Ni-5.2Co/2.6Mg and 5Ni/TiO2, which showed hydrogen yield of 2.36 and 1.82 mmol/g, respectively. The last phase focuses on real biomass. K2CO3 and 20Ni-0.36Ce/Al2O3 catalyst were identified as the promising homogeneous and heterogeneous catalysts. The results from Taguchi optimization study indicate that the order of relative importance of the parameters was: biomass type < catalyst type < catalyst loading < temperature. Using different real biomass as hydrogen precursor, the order of hydrogen yield from various biomass was timothy grass < wheat straw < canola meal. The highest hydrogen yield of 3.31mmol/g was observed at 650 ºC, 26MPa, using K2CO3 as the catalyst at a loading of 100% and using canola meal as feedstock. During this study, the best composition of heterogeneous catalysts and better preparation methods were determined targeting higher hydrogen yield for the SCWG of biomass. The relative importance of different operating parameters was assessed using statistical methods. Also, the detailed mechanism of catalytic lignin SCWG was proposed to facilitate further understanding of the role of different catalyst building blocks in the process. The results presented in this work can lead to better understanding ofthe non-catalytic/catalytic SCWG processes, and provide a valuable reference for future study in the similar area.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeMeda, Venkatesh; Niu, Catherine; Sammynaiken, Ramaswami; Tyler, Bob
Copyright DateOctober 2016
Supercritical water gasification