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Metal Oxides as Sulphur Sorbents for use in Supercritical Algae Gasification - An in & ex situ XAS Study

English title Metal Oxides as Sulphur Sorbents for use in Supercritical Algae Gasification - An in & ex situ XAS Study
Applicant Ludwig Christian
Number 153314
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Material Sciences
Start/End 01.04.2015 - 31.03.2018
Approved amount 233'313.00
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All Disciplines (2)

Discipline
Material Sciences
Chemical Engineering

Keywords (7)

Supercritical water; Microalgae ; Sulphur-sorbents; Bioenergy; Waste; Residues; Algae

Lay Summary (German)

Lead
Algen haben ein hohes Potenzial in einer zukünftigen, nachhaltigen Energieversorgung, wobei die Algenbiomasse mit einem Ru-katalysierten Verfahren in überkritischem Wasser (SCW) zur Methanerzeugung genutzt wird. Doch enthält Biomasse auch relevante Mengen Schwefel, welches ein wirksames Katalysatorgift ist. Das Hauptziel dieses Projektes ist es Materialien zu untersuchen, die in der Lage sind, Schwefel aus anorganischen sowie organischen S-haltigen Verbindungen, unter SCW Bedingungen zu adsorbieren. Metalloxide, die im Handel erhältlich sind, sowie eigene synthetisierte Materialien sollen untersucht werden.Dieses Projekt zielt darauf ab, einen Beitrag auf dem Gebiet der katalytischen Umwandlung von Algenbiomasse in Methan bei SCW Bedingungen zu leisten. Allerdings werden die Projektergebnisse auch für andere nasse Biomassen, das in der Schweiz größte noch nicht erschlossene Bioenergiepotential, relevant sein.
Lay summary

Motivation. Erneuerbare Bioenergie ist eine der kostengünstigsten Möglichkeiten fossile Brennstoffe zu ersetzen, vorausgesetzt, dass ihre Produktion nicht mit der Nahrungsmittelproduktion konkurrenziert. Die hydrothermale Verarbeitung nasser Biomasse in überkritischem Wasser (SCW, Tc = 647 K, Pc = 22,1 MPa) hat ein großes Potenzial für die Herstellung von Biokraftstoffen und Biochemikalien aus verschiedenen nassen Biomassen. Insbesondere Algen haben ein hohes Potenzial, um in der Zukunft eine wichtige Rolle bei der nachhaltigen Energieversorgung zu spielen, wobei durch Umwandlung der Algen, mittels einem Ru-katalysierten Prozess, Methan erzeugt wird. Doch enthalten Algen relevante Mengen an Schwefel-Verbindungen, welches wirksame Katalysatorgifte sind.

Forschungsinhalt. Das Hauptziel dieses Projektes ist es Materialien zu untersuchen, welche in der Lage sind, Schwefel aus anorganischen als auch organischen S-haltigen Verbindungen unter SCW Bedingungen zu adsorbieren. Metalloxide, die im Handel erhältlich sind, sowie eigene synthetisierte Materialien sollen untersucht werden. Die Oxide werden unter Bedingungen hergestellt, um Teilchengröße, kristalline Phase und Morphologie zu kontrollieren. Ein neuer Reaktor wird aufgebaut, um damit die funktionelle Aktivität der Materialien unter SCW Bedingungen mit synthetischen Stoffen (z.B. mit S verunreinigte organische Substrate in wässrigen Lösungen), sowie mit realer Biomasse zu untersuchen. Erweiterte XAS-Studien sollen durchgeführt werden, um den Wechselwirkungsmechanismus von S-Verbindungen mit dem Metalloxid aufzuklären.

Relevanz. Die wichtigsten Ergebnisse des Projekts werden die wissenschaftlichen Erkenntnisse über Materialien für die S-Adsorption bei SCW Bedingungen aus der Sicht der Synthese, Charakterisierung, Funktionalität und Wirkungsmechanismus sein. Zudem sind die Erkenntnisse bedeutend für die Weiterentwicklung der katalytischen Umwandlung von nasser Biomasse (Algen, Biomassereste und Bioabfälle) zu Methan.

Direct link to Lay Summary Last update: 30.01.2015

Lay Summary (English)

Lead
Algae have a high potential as future sustainable energy supply by converting the algal biomass to methane with a Ru-catalyzed process at supercritical water (SCW) conditions. But, biomass also contains relevant amounts of sulphur, which is an effective catalyst poison. The main objective of the present project is devoted to study materials able to capture sulfur from inorganic, as well as, organic S-containing compounds at SCW conditions. Metal oxides which are commercially available as well as own synthesized materials will be investigated.This project aims to create impact in the field of catalytic conversion of algal biomass into methane at SCW condition. However, the project results will also be relevant for other wet biomass feedstocks, the largest unexploited bioenergy source in Switzerland.
Lay summary

Motivation. Renewable bio energy is one of the most cost-effective options of substituting fossil fuel, provided that its production does not compete with critical resources needed for the food production. Hydrothermal processing of wet biomass at the supercritical point of water (SCW, Tc=647 K, Pc=22.1 MPa) has a great potential for producing bio-fuels and bio-chemicals from various wet biomass sources. In particular, algae have a high potential to play a relevant role in the future sustainable energy supply by converting the algal biomass to methane with a Ru-catalyzed process at SCW conditions. But, biomass contains relevant amount of sulphur compounds, which are effective catalyst poisons.

Research content. The main objective of the present project is devoted to study materials able to capture sulfur from inorganic, as well as organic S-containing compounds at SCW conditions. Metal oxides which are commercially available as well as own synthesized materials will be investigated. The oxides will be prepared in batch hydrothermal or solvothermal conditions to control the particle size, crystalline phase, and morphology. A new tubular reactor will be constructed and put into operation to study the functional activity of the materials working at SCW conditions with synthetic streams (e.g. organic substrates in aqueous solutions contaminated by S compounds) as well as with biomass streams. Advanced XAS (X-ray Absorption Spectroscopy) studies will be carried out in order to elucidate the interaction mechanism of S-compounds with the metal oxide.

Relevance. The major outcomes of the project will be the scientific insight on materials for S-capturing at SCW conditions from the point of view of synthesis, characterization, functionality, and mechanism of action, as well as from the applicative impact in the field of catalytic conversion of wet biomass (algae, biomass residues, and bio-wastes) into methane under these conditions.

Direct link to Lay Summary Last update: 30.01.2015

Responsible applicant and co-applicants

Employees

Publications

Publication
The impact of sorbent geometry on the sulphur adsorption under supercritical water conditions: a numerical study
Maxim Florentina, Niceno Bojan, Testino Andrea, Ludwig Christian (2017), The impact of sorbent geometry on the sulphur adsorption under supercritical water conditions: a numerical study, in Biomass Conversion and Biorefinery, 1-7.

Collaboration

Group / person Country
Types of collaboration
Prof. Jefferson J. Tester United States of America (North America)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
Dr. Maarten Nachtegaal (SLS/PSI) Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Dr. Thomas Huthwelker (SLS/PSI) Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
Dr. Bojan Niceno (NES/PSI) Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
- Exchange of personnel

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
16th International Conference of Physical Chemistry ROMPHYSCHEM–16 Talk given at a conference Sulfur chemisorption on metal oxides 21.09.2017 Galati, Romania Ludwig Christian; Maxim Florentina;


Associated projects

Number Title Start Funding scheme
183663 Behavior of sulfur in biomass under hydrothermal conditions: removal strategies for catalytic processes 01.02.2020 Bilateral programmes

Abstract

In this postdoc study the interaction of sulphur with metal oxides as potential sulphur sorbents in supercritical water (Tc=647 K; Pc=22.1 MPa) will be investigated, because sulphur derogates the Ru-catalysed methanation of gases (CO, H2) from biomass gasified in the same medium (SCWG). Research of PSI and EPFL showed that a major challenge for a successful demonstration of PSI’s catalytic SCWG process for algae slurries on a technical scale is to deal with its native sulphur content regarding catalyst deactivation. The motivation for the project is our observation of high sulphate tolerance of Ru/ZrO2 in SCWG of model biomass (ethanol) compared with activated carbon as catalyst support (Ru/C). This bears the promise that ZrO2 (and possibly also other, cheaper, metal oxides) may serve as sulphur sorbent in PSI’s catalytic SCWG process at T~400°C. There are different options that possibly will be needed to overcome the technical challenges. A most promising and efficient possibility is the implementation of a sulphur capture reactor prior to performing the Ru-based SCW-Methanation step downstream in a separate reactor unit. With respect to ZrO2 as sulphur-catcher, the literature shows that surface-sulphated ZrO2 can also enfold substantial acidic activity (“solid superacid”), albeit mainly after calcination at much higher temperatures (T~650°C). The enhanced acidic activity is attributed to the binding of surface SO4 with Zr elements in a bridging bidentated state, allowing the double-bond nature of the surface complex to be much stronger than with simple metal sulphate or monodentate-bound surface sulphate. Also other metal oxides, such as TiO2, SnO2, Fe2O3, show enhanced activity after this preparation method. But, enhanced acidic activity is not desired with the PSI SCW-Algae process, because the acidity may also catalyse e.g. dehydrogenation, esterification, or polymerisation reactions with the SCW-gasified biomass (or biomass representative) compounds, such as alcohols, organic acids, alkanes, and alkenes, which might deactivate the Ru/C methanation catalyst as well. Also unknown with SCWG is whether sulphate is being reduced into H2S (by H2 from model or real biomass). If so, ZrO2-based gas clean-up studies suggest that H2S could then be adsorbed dissociatively at the ZrO2 surface. However, fact is that hardly anything is known of what happens with these, and other (organic, aromatic) sulphur forms under SCWG conditions. Therefore, it is indispensable to study these issues under in situ under SCWG conditions with promising metal oxides (e.g. RuO2, SnO2, ZrO2) and, for purpose of comparison, e.g. also with CdO and with CeO2, because CdO shows not, and CeO2 much less, enhanced acidic activity after calcination. The technique of choice here is synchrotron-based X-ray absorption spectroscopy, because XAS is very sensitive, atom type specific, metal oxidation state- and local structure-sensitive, and suited for solid, wet and gaseous samples as it does not require long-range structural order. We will employ an in situ critical water-dedicated circulating flow reactor that had been improved recently by our laboratory and now allows probing Ru, Zr, Sn, and Cd selectively during gasification of model biomass (methanol, ethanol) in supercritical water by recording at the appropriate K-edge absorption energies (Ru~22.1 keV, Zr~18.0 keV, Sn~29.2 keV, Cd~26.7 keV) over both, metal oxidation state- (XANES) and local structure-sensitive (EXAFS), regions. But, these XAS experiments are also high risk, because the challenge is to distinguish bulk atom from surface atom signal contributions. Therefore, we use sub-nano-sized metal oxide particles in view of their beneficial surface-to-volume atom ratios. Some metal oxides can be purchased; others will be synthesized by our in-house expert (Dr. Andrea Testino). Recently, we showed that no sulphur is present on the carbon carrier after complete deactivation of Ru/C during SCWG of 7.5% ethanol with dimethyl sulfoxide as sulphur-agent. Therefore, the nano-sized metal oxide either can be deposited on the carbon carrier or synthesized by as individual particles by means of organic stabilisers (surfactants). We may study the impact from several sulphur-containing agents, i.e., sulphuric acid (H2SO4) in analogy to the preparation method for metal superacids, inorganic (sulphide, sulphate), aromatic (thiophene), and organic sulphur (DMSO). Possible acidic activity of the metal oxides during SCWG conditions will be tested using model biomass (methanol, ethanol) by means of gas-phase (GC, MS) and solid-phase (SEM) analyses (to check for deposits on the Ru/C catalyst. The preliminary stability and S-catching performance experiments are performed in a new, re-designed hydrothermal continuous flow reactor. Promising SCW(G)-treated suspensions resulting from these preliminary S-catching tests are also studied ex situ at room temperatures using the softer, sulphur-specific K-edge absorption energies (E~2.5 keV) after their transfer into much better X-ray penetrable “liquid-XAS” measuring cells. These investigations allow to check whether the S oxidation state and neighbouring atom distances are consistent with (complementary to) those “seen” in situ by the surface metal atoms of the metal oxide nano-carrier surfaces involved. The second goal of this project is to test other metal oxides that reportedly do not develop enhanced acidic activity with sulphate after calcination, such as ZnO, NiO, MgO, Fe2O3, TiO2, CeO2, and MnO2. For example, preliminary tests at PSI showed high stability for ZnO in SCW. And also here, these materials should be small (nano-dimensions) and can either be bought or should be synthesized at PSI. However, these oxides can only be studied with XAS ex situ with the “wet-XAS” cells due to constraints set by the in situ XAS-reactor wall with the much lower X-ray energies involved. These metal oxides are also to be prepared in the hydrothermal batch reactor and studied thereafter at RT at both, sulphur and metal-specific, absorption edges. Here the challenge is to transfer the SCW-treated suspension from the batch reactor to the “wet-XAS” cell without reacting with O2 from air or loosing possibly formed H2S, as they may change the sulphur chemistry or phases with the suspensions. Therefore, a batch-reactor must be constructed with special valves. Immobilised metal oxides will be pre-screened regarding thermal stability (solubility) in SCW by ICP-OES metal content analysis of the effluent. The XAS experiments are preferentially performed at the Swiss Light Source (PSI) in view of its high-quality photon sources, tuneable beam spot area, and in-house experience and infrastructure for performing the unprecedented in situ SCW(G) experiments. This project aims at gaining insights into the sulphur-adsorbing propensity of promising metal oxides in supercritical water near to it’s critical point (Tc=647 K; Pc=22.1 MPa), because this information is neither available nor obvious from the literature. The findings are also important for dealing with S-rich biomass wastes, such as sewage sludge or manure, when wanting to recover their energy potential by catalysed methanation under supercritical water conditions.
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