Hydrogen Production from Supercritical Water Gasification of Lignin, Cellulose, and Other Biomass Residues
Date
2016-11-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
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.
Description
Keywords
Hydrogen, Supercritical water gasification, Biomass, Catalysis.
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Environmental Engineering
Program
Environmental Engineering