This PhD project investigates whether calcium-rich ash can improve iron-based oxygen carriers for sustainable hydrogen production. Combining thermodynamic analysis, laboratory experiments, fluidised-bed testing and process modelling, it will assess effects on oxygen transfer, hydrogen yield and carrier stability. Linked to the EU-funded HELIA project, the research is based at TalTech and co-supervised by Professors Oliver Järvik and Abhishek Agarwal.
The project will address the following key areas:
The behaviour of iron-based oxygen carriers depends not only on their initial composition but also on how their structure and phase assemblage evolve during repeated redox cycling. Fe2O3/Al2O3 materials can provide improved structural and mechanical stability, but under reducing conditions, interactions between iron oxide and alumina may promote the formation of FeAl2O4, which can exhibit lower reactivity towards steam. Calcium provides a potential means of influencing these phase transformations. CaO formed from CaCO3-rich ash may interact with iron- and aluminium-containing phases, potentially promoting calcium-containing phases such as CaFe2O4 and Ca2Fe2O5 and altering the redox behaviour of the carrier. The central question is whether calcium entering with the feedstock can be used as a beneficial in-situ modifier, rather than being treated only as an unwanted ash component.
The research will examine how calcium is transferred from ash to oxygen-carrier particles, which phases are formed, and how these changes affect oxygen-transfer capacity, hydrogen production and long-term oxygen-carrier stability. Particular attention will be given to the effects of silicon, sulphur, chlorine and alkali metals, which may interact with calcium and the oxygen-carrier components and influence phase formation and particle behaviour.
Experimental observations will be incorporated into an Aspen Plus process model of the chemical-looping process. The model will describe the relevant reactors and mass and energy balances while accounting for experimentally observed changes in oxygen-carrier conversion, calcium accumulation and activity.
Responsibilities and (foreseen) tasks:
Thermodynamic analysis of relevant Fe-Al-Ca-O-C-H phase equilibria under chemical-looping conditions.
Preparation and characterisation of Fe2O3/Al2O3 oxygen carriers and CaCO3-rich ashes.
Investigation of reduction, steam oxidation and air regeneration using thermogravimetric analysis.
Study of calcium modification using controlled CaCO3/CaO additions.
Investigation of calcium transfer from synthetic and real ash samples to oxygen-carrier particles.
Characterisation of phase formation, oxygen-carrier structure and material degradation using techniques such as XRD, SEM-EDS and BET.
Long-term cyclic testing in a laboratory-scale fluidised-bed reactor.
Evaluation of hydrogen production, oxygen-carrier stability, attrition and agglomeration.
Development and validation of an Aspen Plus process model.
Evaluation of mass and energy balances and identification of suitable operating conditions.
Collaboration with researchers and PhD students working on chemical looping, fluidised-bed systems and energy conversion.
Publication of research results in peer-reviewed journals and presentation at international conferences.
Collaboration with other PhD students and colleagues in the department.
Supervision of BSc and MSc students, where appropriate.
Applicants should fulfil the following requirements:
A master's degree in chemical engineering, energy technology, materials science, chemistry, mechanical engineering or another closely related subject.
Proven experience in high-temperature experimental research, particularly combustion, gasification or chemical-looping processes.
A clear interest in chemical looping, hydrogen production, thermochemical conversion or oxygen-carrier materials.
Experience in thermodynamic calculations and process modelling.
A good understanding of thermodynamics and chemical reaction processes.
Strong analytical and problem-solving skills.
The ability to work independently and as part of a multidisciplinary research team.
Good written and spoken English (at least B2 level according to the Common European Framework of Reference for Languages).
Additional general requirements can be found at PhD Admission | Tallinn University of Technology (TalTech) .
The following experience is beneficial:
Thermogravimetric analysis, including interpreting reaction kinetics and cyclic reduction-oxidation behaviour.
Materials characterisation, using techniques such as X-ray diffraction (XRD) and scanning electron microscopy with elemental analysis (SEM-EDS).
Fluidised-bed operation and gas analysis, including experimental planning, reactor monitoring and safe handling of reactive gases.
Technical report writing, presenting results and collaborating in international research teams.
We offer:
A 4-year PhD position in the Department of Energy Technology at Tallinn University of Technology. This is a full-time paid employment in accordance with TalTech's doctoral employment conditions, including social security.
The opportunity to work on an interdisciplinary research topic combining oxygen-carrier materials, chemical looping, hydrogen production and process modelling.
A research environment with access to relevant experimental and computational facilities.
Opportunities for collaboration with international research partners associated with the HELIA project.
Opportunities for professional development, scientific collaboration and international networking.
Support for conference travel and research dissemination.
Supervisors:
Main supervisor: Tenured Associate Professor Oliver Järvik, School of Engineering: Department of Energy Technology: Research Group of Environmental, Energy and Chemical Technology
Co-Supervisor: Tenured Associate Professor Abhishek Agarwal, School of Engineering: Department of Energy Technology: Research Group of Sustainable Energy and Fuels
Tallinn University of Technology (TalTech) is an international scientific community with approximately 9,000 students and 2,000 employees; it is one of the largest universities in Estonia, the leading EU country in digitalisation. The university's strengths are broad multidisciplinary study/research interests, a modern research environment, and strong collaboration with international educational and research institutions. TalTech is aiming to be an organisation leading the way to a sustainable digital future.
The Department of Energy Technology at TalTech provides a strong research environment for the proposed research, combining expertise in thermochemical conversion, chemical looping and circulating fluidised-bed (CFB) systems. The department has access to pilot-scale CFB combustion and conversion facilities, thermochemical processing and gasification systems, and advanced equipment for gas and emissions analysis, supporting experimental development and validation of oxygen-carrier materials and hydrogen-production processes. This experimental infrastructure is complemented by expertise in redox materials, reactor operation, thermodynamic analysis and process modelling.
For information about the admission process, please visit the PhD Admission homepage
Applications can be submitted from 16.09.2026 to 07.10.2026
For further info, please contact:
Associate Professor Oliver Järvik oliver.jarvik@taltech.ee; Associate Professor Abhishek Agarwal abhishek.agarwal@taltech.ee, regarding the admission process, please contact the doctoral admissions team at docstudy@taltech.ee.
