PhD project title: Towards a circular sulphur economy (# of pos: 14)

Topic background
Since biogas and landfill gas streams are renewable energy sources, their global use has increased during the last decades and is expected to remain rising. Typically, toxic hydrogen sulphide (H2 S) needs to be removed from these gas streams to prevent harmful sulphur dioxide emissions. The biological gas desulphurisation process under haloalkaline conditions is a cost-effective and environmentally friendly alternative for the conventional physical-chemical gas desulphurisation processes. In addition to H2 S removal from these gas streams, biologically produced elemental sulfur can be harvested from the process, as well. Especially the relatively small particle size and hydrophilicity of the biologically produced elemental sulfur are benefits for application as fertilizer and/or fungicide. As the research in the Sulfur Theme of Wetsus focusses on the optimization of the biological desulfurization process, it facilitates the transition towards a circular sulphur economy.

Research challenges
In the biological gas desulphurisation process, gas, liquid, and solid phases co-exist. For instance, in the absorber, a gas phase (sour gas) is contacted with a liquid phase (haloalkaline solution), which contains solid phases (biosulphur particles and microorganisms). While the microbial community and its associated kinetics have been extensively studied, a number of phenomena in the biological gas desulphurisation process are not yet fully understood. The recently discovered electron shuttling capacity of sulfide oxidizing bacteria is one of these. Due to these phenomena, dissolved sulfide is removed from process solution, without consuming oxygen. Other not fully understood phenomena are, for instance, the enhancement of H2 S absorption by bacteria and the sulfur crystals formation. All of these processes are hypothesized to occur at the interfaces of the biological desulfurization process. Hence, research is required to investigate the interplays between kinetics of chemical and biological reactions and transfer rates at the interfaces.

Objectives and methodology
This project aims to understand the interplays between the kinetics of the biological reactions, the kinetics of the chemical reactions, and the transfer rates around the various gas-liquid and liquid-solid interfaces in the biological gas desulphurisation process. In the last decennia, a large number of projects have been executed, resulting in a vast amount of experimental data. However, a minimum amount of the full potential of the work has been utilized, since the majority has not been used for modelling to unravel the aforementioned phenomena. Hence, part of this work will be focussing on developing models describing the transfer and reaction kinetics at the interfaces. In addition, next to utilizing data obtained in previous work, experimental work will be performed in batch and/or continuous bench scale setups to gather data for model validation and calibration. Furthermore, when required, pilot plant facilities (owned by Paqell B.V.) may be used validate the developed models.