Mesostructured metal oxide-based nanocomposites asudsorbents for H2S removal from syngas coal gasification

摘要

Desulphurization of gas phase sulphur compounds has been receiving dramatic attention sinceudhazardous, corrosive, and toxic gases that cause environmental damages (especially acid rain) andudindustrial challenges (i.e., corrosion of equipment and deactivation of catalysts).udThis dissertation presents results of R&D efforts to develop efficient MeOx/SBA-15-based sorbentsudfor H2S removal in view of possible applications in hydrogen purification, air pollution control, and deepuddesulphurization of fossil fuels. It is precisely in the latter topic that the research project was born. Theudproduction of power, fuels and chemicals in most countries is predominantly based on oil and, to a minorudextent, on natural gas. It is well-known that the reserves of both of these fuels are limited to a range ofud40, 60 years. On the contrary, coal is a widely available fossil fuel, and it is expected to last for aboutud230 years. The imminent oil production limitations and the longer availability of coal, the wish to improveudthe security of the energy supply, and the possibility to reduce greenhouse gas emissions by means ofudcarbon capture and sequestration (CCS) are sufficient motivations to increase the use of this resource.udIntegrated Gasification Combined Cycle (IGCC) process is a high efficiency power generation technologyudwhich gasifies coal to generate the fuel (syngas) for a high efficiency gas turbine. A key challenge forudproducing clean power or hydrogen via gasification is cost effective purification of the sour syngas.udThere are many commercial treatment techniques that are used to remove H2S, but their disadvantage isudthat hot coal gas must be cooled down near to ambient temperature for desulphurization. The coolingudequipment required, and the need to reheat the clean syngas before its use in a gas turbine result inudeconomic and thermodynamic penalties that decrease the efficiency of a gasification plant. It is for thisudreason that hot gas desulphurization technique has attracted more and more attention due to the factudthat it can reduce H2S down to 100 ppm level and avoid heat loss. Mid-temperature desulphurization isudachieved by the use of solid sorbents such as oxides of those metals that form stable sulphides, based onudthe non-catalytic reaction between a metal oxide and hydrogen sulphide. The optimum desulphurizationudtemperature has been recommended in the range of 300 to 450 °C, also in according to the moreudfavourable thermodynamic equilibrium of sulphur compounds removal. To accomplish this task, Zincudoxide- and Iron oxide-based materials have been successfully employed for decades in different domainsudof the chemical industry. The pure metal oxides used as sorbents, however, suffer from evaporation, lossudin the surface area and porosity due to sintering and mechanical disintegration that affect theirudperformance and life time adversely. With the purpose of overcoming this problem and to improve theirudperformance, metal oxides can be confined into a support, where under such conditions the materialsudare stable. The main properties required for support materials are inertness, high surface area, largeudpores and good mechanical strength.udThe thesis reports some simple and versatile routes which can be proposed to prepare a greatudvariety of MeOx/SBA-15 composites where the mesostructured SBA-15 silica, a high-surface area (up toud1000 m2/g) material, with 6–7 nm-wide regular channels and thick (3–4 nm) pore walls has been used asudefficient and stable support. MeOx active phase, formed inside the mesochannels, can reach the maximumudsize of 6-7 nm physically imposed by the pore diameter. Such a structure provides an ideal reactorudwhere the mesopores act as channels for the transport of reactant. As a consequence, enhancement ofudthe active phase reactivity might be expected. The proposed “Two-solvents” incipient impregnationudmethod is easily reproducible and easy to scale up. Furthermore, this method should provide, at least inudprinciple, ideal systems to be compared, and therefore to understand how the active phase natureudinfluence their performance.udFor the first time, a careful comparative study on the effect of the different nature of theudnanostructured MeOx (Me = Zn, Fe) dispersed into a mesostructured silica matrix (SBA-15) on the H2Sudremoval performance is carried out.udThe behaviour of the MeOx/SBA-15 composites in the removal of H2S is investigated in a fixed-bedudreactor and compared with that of an unsupported ZnO commercial sorbent. The morphological,udstructural, and textural features of fresh, sulphided, and regenerated sorbents have been assessed by audmulti-technique approach, including the study of the possible interactions between the guest oxide andudthe host silica support. Furthermore, the sorption-desorption behaviour, which is commonly justified onlyudon the basis of the different nature of the active phase and of the textural features (surface area andudpore volume), is discussed also considering the morphology and the crystallinity of the active phase.udIn the literature, to our best knowledge, no one have reported similar correlations. For this reasonudthis work can give an important contribution to improve the basic knowledge in the field of sorbents forudgas-removal.