(499a) Particles As Heat Transfer Medium to Drive Endothermal Chemical Reactions By Solar Energy

Keywords: particles, receiver, solar thermal, redox reaction, catalyst

Martin Roeb, Juan Pablo Rincon, Clarisse Lorreyte, Dimitrios Dimitrakis, Christos Agrafiotis, Dennis Thomey, Christian Sattler

High-temperature solar receivers with particles as a heat transfer medium have the potential to increase the operating temperature far above 900 °C and improve efficiency in comparison to technologies operating with molten salt at significantly lower temperature [1]. In addition to using the high-temperature heat for electricity generation, the heat can also be used directly for several industrial applications. One area of those applications is solar-chemical processes. In the particle receiver technology solid particles made from e.g. sintered bauxite, calcined flint clay, or quartz sand are heated directly by concentrated sunlight and act as a heat transfer medium. Various designs of particle receivers, e.g. free-falling particles, with braked particle flow, centrifugal receivers and fluidized-bed particle receivers are currently under development.

In order to further pursue the advantages of particle technology in concentrating solar technology, fundamental questions on material resistance, heat transfer processes, possible receiver alternatives and the use of chemically active particles have been scientifically investigated. In addition to the bauxite particles used to date as an inert material and as pure heat transfer medium, chemically active particles can also be used in solar receivers. Concentrating solar technology can be used not only in electricity generation, but also via generation of fuels and commodities in the industrial and transportation sectors. Chemically active means, for example, that they can be used as a catalyst material in gas-solid reactions or they can be reduced by means of a reduction-oxidation cycle at higher temperatures using solar radiation and oxidized at lower temperatures. As the reduction step is endothermic and the oxidation step is exothermic, they can be used for thermochemical storage. Other important fields of application are air separation, use as an oxygen pump, water splitting to produce hydrogen and CO2 splitting to produce CO.

Particle temperatures of up to 1500 °C are sometimes required for the chemical reactions described. This is well above the previous operating temperature range of the CentRec receiver [2]. Due to the operating temperature and the chemical reactions, different receiver-reactor concepts or a combination of several components such as receiver, heat exchanger and chemical reactor are used in these processes. Particles can be used both in receivers, where they directly absorb solar radiation, and as storage materials. In some cases, even the particles themselves represent the material to be treated and converted to the desired product like in the case of solar lime or cement production [3, 4].

A number of receivers and reactor concepts for thermochemical applications are described and analyzed with respect to their operation performance and scaling potential. A distinction is made depending on the application and its operating conditions. Different receiver concepts are compared. The advantages and disadvantages of those concepts are compared using an evaluation matrix. In addition to the technical aspects, the criteria of cost, scalability and sustainability are also considered.

In a first example the investigation of a novel thermochemical cycle in which the solid sulfur cycle is combined with DLR´s next generation particle receiver. The particles are heated in the centrifugal particle receiver CentRec with the help of solar irradiation and the hot particles are fed into the sulfuric acid splitting reactor. The heat energy of the particles is used to evaporate sulfuric acid to SO3 and finally decomposes into SO2 with the help of a catalyst (e.g. Fe2O3) at a temperature above 800 °C [5]. A lab scale particle heated reactor test setup for sulfuric acid splitting was built and tested as a proof of concept. The lab scale reactor is a new concept of counter-current flow shell-and-tube heat exchanger with particles and sulfuric acid mass flow rate of 10 and 1.8 kg/h, respectively. The particles which are electrically heated up to 900 °C acts as heat transfer medium and move downward on the shell side while the sulfuric acid flows upward inside acid resistant tubes. A one-dimensional heat transfer model was developed based on the correlations of flow boiling heat transfer coefficients, particle bed heat transfer coefficient and sizing of the shell-and-tube heat exchanger.

Another application example deals with an innovative approach of biomass conversion. The thermal energy derived either from particle heat carriers heated up in a solar receiver [6] or through electrical energy from renewable energy sources. The target products of the thermochemical process are bio-oil, biochar and syngas. The thermal energy required for the pyrolysis process is provided by the particles, which are heated using concentrated solar energy, so less biomass is wasted for the provision of energy. During periods of low or no solar irradiation, excess renewable energy from the grid can be converted into thermal energy for the pyrolysis process. To achieve the required temperature of approximately 800°C for the particles, a solar receiver is developed based on a rotary kiln concept.

And a third example is a particle based thermochemical process for the generation of another commodity: Nitrogen is required as a feedstock for the production of ammonia and nitrate fertilizers. Oxygen is removed from air by a thermochemical reaction to such an extent that the resulting nitrogen purity is sufficient for the generation of ammonia using the Haber-Bosch process. Key element of such process is the thermochemical reactor, where the air separation takes place and which contains a redox material that can reversibly incorporate and release oxygen without major changes of its crystal structure. When it is heated oxygen is released. This oxygen is flushed out of the reactor with a carrier gas. When then air is fed into the reactor, the oxygen oxides the oxygen-lean redox material and pure nitrogen remains as the gas phase. Lastly the material is heated again, the oxygen is released and the process starts all over again [7].

References:

[1] C.K. Ho, A new generation of solid particle and other high-performance receiver designs for concentrating solar thermal (CST) central tower systems, in: Manuel J. Blanco, Lourdes Ramirez Santigosa (Eds.), in Woodhead Publishing Series in Energy, Advances in Concentrating Solar Thermal Research and Technology, Woodhead Publishing (2017), Pages 107-128, ISBN 9780081005163.

[2] M. Ebert, L. Amsbeck, R. Buck, J. Rheinländer, B. Schlögl, S. Schmitz, M. Sibum, R. Uhlig. (2019) Operational Experience of a Centrifugal Particle Receiver Prototype. In: 24th SolarPACES International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2018, 2126 (030018), 030018-1. AIP Publishing. SolarPACES 2018, 2.-5. Oct. 2018, Casablanca, Morocco.

[3] G. Moumin, S. Tescari, P. Sundarraj, L. de Oliveira, M. Roeb, C. Sattler. (2019) Solar treatment of cohesive particles in a directly irradiated rotary kiln, Solar Energy 182, 480-490.

[4] J. P. Rincon Duarte, D. Kriechbaumer, B. Lachmann, S. Tescari, T. Fend, M. Roeb, C. Sattler. (2022) Solar calcium looping cycle for CO2 capturing in a cement plant. Definition of process parameters and reactors selection, Solar Energy, vol. 238, 189–202.

[5] V.K. Thanda, D. Thomey, L. Mevißen, H. Noguchi, C. Agrafiotis, M. Roeb, C. Sattler. (2022) Solar thermochemical energy storage in elemental sulphur: design, development and construction of a lab-scale sulphuric acid splitting reactor powered by hot ceramic particles. AIP Conference Proceedings (2445). American Institute of Physics (AIP) 2445, 130008.

[6] S. Rodat, S. Abanades, G.Flamant. (2010) Experimental Evaluation of Indirect Heating Tubular Reactors for Solar Methane Pyrolysis, International Journal of Chemical Reactor Engineering, 8 (1), Article A25.

[7] L. Klaas, M. Pein, P. Mechnich, A. Francke, D. Giasafaki, D. Kriechbaumer, C. Agrafiotis, M. Roeb, C. Sattler. (2022). Controlling thermal expansion and phase transitions in Ca1-xSrxMnO3-δ by Sr-content, Phys. Chem. Chem. Phys. 24, 27976-27988.