Mesoporous and hierarchically porous CO2 sorbents and nickel-based catalysts for use in the generation of H2 and synthesis gas from biomass

Abstract

A hydrogen (H2) energy economy has the potential to lower fossil carbon dioxide (CO2) emissions, which have been shown to be the major cause of climate change. Biomass gasification as a source of H2 and synthesis gas has many environmental advantages and is the most favourable process in terms of commercialisation in the near future. In this study, steam methane reforming (SMR), water-gas shift (WGS) and dry methane reforming (DMR) were selected as model reactions for downstream processing of biomass gasification. The selective generation of hydrogen gas (H2) from the pyrolysis-gasification of renewable biomass feedstocks, using in-situ carbon dioxide (CO2) capture is a promising process that has been investigated extensively in the pursuit of practical and sustainable processes for hydrogen production. The high CO2 uptake capacity of bulk calcium oxide (CaO) as a CO2 sorbent is well known in literature, as are its limitations in terms of decay in activity, due to sintering effects, over multiple carbonation and regeneration cycles. A number of research groups have reported that dispersion of nanoparticulate calcium oxide on/through support materials can enhance the reactivity, thermal stability and longevity of the sorbent. The first key objective of this project was to synthesise mesoporous and hierarchically porous alumina and aluminate materials to use as supports for calcium oxide sorbent, as well as, nickel catalysts. The nanocasting technique using a soft-templating method and evaporationinduced self-assembly was employed to synthesise these materials. Pluronic P123 was used as structure-directing agent for the mesopores and polyurethane foam or polystyrene beads were used as co-templates for the hierarchically porous alumina, calcium aluminate, cerium aluminate and nickel aluminate in which both two-dimensional and three-dimensional porous structures were obtained. ii CaO/porous calcium aluminate sorbents were synthesised and tested for their CO2 uptake capacity and thermal stability by means of thermogravimetric analysis. They remained stable and active in CO2 sorption for at least thirty carbonation and regeneration cycles, using a typical steam methane reforming (SMR) reaction temperature of 700 °C as the carbonation temperature (also tested at 560 °C) and a relatively high regeneration temperature of 850 °C. The sorbent materials synthesised in this study displayed a high CO2 uptake and a resistance to sintering effects. Initial SMR and WGS experiments showed that mesoporous nickel aluminate (meso- NixAlyOz) catalyst generated a high hydrogen yield comparable to that of a commercial nonporous Ni/Al2O3 catalyst. The physical mixture of the CaO/mesoporous calcium aluminate sorbents and the meso-NixAlyOz led to improved hydrogen yields during CO2-sorption enhanced SMR and WGS, via the removal of the CO2 from the product gas stream. Hydrogen can also be produced from the reaction between methane, a biomass gasification product, and CO2; this process is known as dry methane reforming (DMR). DMR is even more endothermic than SMR, but if it is used in conjunction with sustainable processes, such as biomass gasification of renewable carbonaceous feedstocks, this energy cost could be justified. Both SMR and DMR afford a synthesis gas but for DMR a lower H2/CO ratio is obtained, which is more suitable for use in e.g. gas-to-liquids (GTL) technologies such as Fischer-Tropsch synthesis. Nickel-based catalysts are known to have a high DMR activity but are prone to coking. Aluminate support materials have been shown to improve coking-resistance in catalysts, therefore, mesoporous and macro-mesoporous nickel aluminates were synthesised, as well as, nickel and nickel-cobalt bimetallic catalysts on porous alumina and aluminate supports. Nickel/non-porous Al2O3 catalysts were synthesised, for comparison. All of these materials were tested for catalytic activity towards DMR of a model product gas from pyrolysis- gasification of biomass. The performances of these materials were screened along with a commercial nickel/alumina (Ni/Al2O3) catalyst at 700, 800 and 900 °C. Most of the catalysts tested displayed a high degree of catalytic activity and showed very little weight loss in post- DMR temperature-programmed oxidations (TPO), which suggests that coking of these catalysts was negligible. Hierarchically porous nickel aluminate (hier-NixAlyOz) showed a high H2 and CO yield compared with other materials and also a high degree of stability during long-term DMR catalytic experiments (20 and 665 hours) with the aforementioned model product gas at 800 °C. The catalysts that displayed a high degree of catalytic activity and stability during these screening experiments were performance-tested for dry methane reforming, at 800 °C, of a real product gas from pyrolysis-gasification of beechwood chips. The hier-NixAlyOz catalyst proved to have the highest catalytic activity and stability in DMR experiments compared to other materials tested, including a commercial Ni/Al2O3 catalyst. The use of porous alumina and aluminate materials as supports for CaO sorbent and nickel catalyst proved to be beneficial in terms of thermal and hydrothermal stability, as well as maintaining catalytic activity and CO2-uptake capacity through resistance to coking and sintering.