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

Desulphurization of gas phase sulphur compounds has been receiving dramatic attention since hazardous, corrosive, and toxic gases that cause environmental damages (especially acid rain) and industrial challenges (i.e., corrosion of equipment and deactivation of catalysts). This dissertation presents results of R&D efforts to develop efficient MeOx/SBA-15-based sorbents for H2S removal in view of possible applications in hydrogen purification, air pollution control, and deep desulphurization of fossil fuels. It is precisely in the latter topic that the research project was born. The production of power, fuels and chemicals in most countries is predominantly based on oil and, to a minor extent, on natural gas. It is well-known that the reserves of both of these fuels are limited to a range of 40, 60 years. On the contrary, coal is a widely available fossil fuel, and it is expected to last for about 230 years. The imminent oil production limitations and the longer availability of coal, the wish to improve the security of the energy supply, and the possibility to reduce greenhouse gas emissions by means of carbon capture and sequestration (CCS) are sufficient motivations to increase the use of this resource. Integrated Gasification Combined Cycle (IGCC) process is a high efficiency power generation technology which gasifies coal to generate the fuel (syngas) for a high efficiency gas turbine. A key challenge for producing clean power or hydrogen via gasification is cost effective purification of the sour syngas. There are many commercial treatment techniques that are used to remove H2S, but their disadvantage is that hot coal gas must be cooled down near to ambient temperature for desulphurization. The cooling equipment required, and the need to reheat the clean syngas before its use in a gas turbine result in economic and thermodynamic penalties that decrease the efficiency of a gasification plant. It is for this reason that hot gas desulphurization technique has attracted more and more attention due to the fact that it can reduce H2S down to 100 ppm level and avoid heat loss. Mid-temperature desulphurization is achieved by the use of solid sorbents such as oxides of those metals that form stable sulphides, based on the non-catalytic reaction between a metal oxide and hydrogen sulphide. The optimum desulphurization temperature has been recommended in the range of 300 to 450 °C, also in according to the more favourable thermodynamic equilibrium of sulphur compounds removal. To accomplish this task, Zinc oxide- and Iron oxide-based materials have been successfully employed for decades in different domains of the chemical industry. The pure metal oxides used as sorbents, however, suffer from evaporation, loss in the surface area and porosity due to sintering and mechanical disintegration that affect their performance and life time adversely. With the purpose of overcoming this problem and to improve their performance, metal oxides can be confined into a support, where under such conditions the materials are stable. The main properties required for support materials are inertness, high surface area, large pores and good mechanical strength. The thesis reports some simple and versatile routes which can be proposed to prepare a great variety of MeOx/SBA-15 composites where the mesostructured SBA-15 silica, a high-surface area (up to 1000 m2/g) material, with 6–7 nm-wide regular channels and thick (3–4 nm) pore walls has been used as efficient and stable support. MeOx active phase, formed inside the mesochannels, can reach the maximum size of 6-7 nm physically imposed by the pore diameter. Such a structure provides an ideal reactor where the mesopores act as channels for the transport of reactant. As a consequence, enhancement of the active phase reactivity might be expected. The proposed “Two-solvents” incipient impregnation method is easily reproducible and easy to scale up. Furthermore, this method should provide, at least in principle, ideal systems to be compared, and therefore to understand how the active phase nature influence their performance. For the first time, a careful comparative study on the effect of the different nature of the nanostructured MeOx (Me = Zn, Fe) dispersed into a mesostructured silica matrix (SBA-15) on the H2S removal performance is carried out. The behaviour of the MeOx/SBA-15 composites in the removal of H2S is investigated in a fixed-bed reactor and compared with that of an unsupported ZnO commercial sorbent. The morphological, structural, and textural features of fresh, sulphided, and regenerated sorbents have been assessed by a multi-technique approach, including the study of the possible interactions between the guest oxide and the host silica support. Furthermore, the sorption-desorption behaviour, which is commonly justified only on the basis of the different nature of the active phase and of the textural features (surface area and pore volume), is discussed also considering the morphology and the crystallinity of the active phase. In the literature, to our best knowledge, no one have reported similar correlations. For this reason this work can give an important contribution to improve the basic knowledge in the field of sorbents for gas-removal.