Rotary Kiln Particle Receiver Design and Performance for Solar-driven Biomass Pyrolysis

Decarbonisation through electrification is not sufficient in the chemical industry, as fossil feedstocks remain a major emissions source. Defossilisation by switching to renewable carbon sources such as second-generation biomass should thus be pursued. Conventional biomass pyrolysis, yielding high value bio-oil, biochar and pyrogas, is typically sustained by burning a fraction of these products, an economically and ecologically inefficient step. The PYSOLO project aims at developing an indirectly heated solar-driven biomass pyrolysis system which, thanks to the use of solar heat in the pyrolysis process, maximizes the products yield and achieves negative emissions associated to biochar production. This work focuses on the development of a 1-D optical-thermal model to predict the performance of the solar rotary kiln receiver selected for the project. Introduction System Description The rotary kiln 1-D optical-thermal model allows to predict optimal solar-to-thermal efficiencies in the range 60-65% depending on the SM. The constraint on the kiln “rolling mode” flow regime, described by the Froude number, poses a challenge in obtaining good solar field optical efficiencies due to the limited feasible tilt angles, which should range between 0.5-2°. Future works will consider possible paths to overcome this limit, for instance: (i) to introduce baffles/obstacles to increase the PHC residence time; (ii) to increase the PHC mass flowrate at constant thermal power, thus reducing the ΔT; (iii) to consider the adoption of PHCs with low bulk density and heat capacity. Results and Considerations This work is funded by the European Union under PYSOLO project (Grant Agreement n. 101118270). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them. Acknowledgements SolarPACES 2025, 31st International Conference on Concentrating Solar Power, 23-26 September 2025, Almeria (ES) In PYSOLO, solar-heated particles serve directly as heat carrier (PHC) in the pyrolyzer, removing the need for heat transfer surfaces and easing scale-up. Decoupling the solar receiver from the reactor improves flexibility and enables continuous operation due to the thermal storage. Rotary Kiln Particle Receiver Design and Performance for Solar-driven Biomass Pyrolysis Marco Colombi1, Matteo Romano1, Marco Binotti1 *mail: marco.co[email protected], www.pysolo.eu 1. Politecnico di Milano, Energy Department, via Lambruschini 4a, 20156 Milano, Italy Design Performance Thermal Model Optical Model The kiln is divided in n longitudinal parts to capture the temperature profile on lateral walls and PHC, accounting for radiative, convective and conductive losses as done by Gallo et al. (2019). Due to the solar flux peak on top of the kiln wall, the amount of radiation directly hitting the PHC is very limited. The PHC is mainly heated up thanks to conduction between particle bed and rotating lateral wall. The rotary kiln tilt angle is limited to few degrees to ensure proper PHC flow and fill ratio (no baffles / obstacles to increase residence time are considered). Consequently, the solar flux reaches its peak on top of the kiln’s lateral wall. A higher tilt also reduces the kiln diameter, thus reducing the intercept factor and lowering the optical efficiency. As the tilt angle grows, the kiln diameter and thus the kiln aperture decrease to ensure proper PHC flow. On one side, this guarantees higher thermal efficiencies, but on the other it reduces the solar field intercept factor and thus the optical efficiency. The compromise between these two opposite trends allows to identify the best solar-to-thermal efficiency. As SM increases: (i) the optimal HTower rises to limit cosine losses; (ii) the optimal tilt stays in the 0.5-2° range; (iii) DKiln increases, so to maintain a fixed Fr, ωKiln decreases. Design thermal power: 3.4 MWTH (SM=2). Tilt angle = 0.5°. Tower height = 60m. Overall optical efficiency: 72%. SM = 2, Tilt = 0.5°, Tower height = 60m). Overall thermal efficiency: 84%. SM = 2, Tower height = 60m Given the PHC thermal power and the inlet and outlet PHC temperatures, the kiln geometry is derived from residence time (𝜏𝑟𝑒𝑠), fill ratio (FR), and Froude number (Fr) equations, ensuring rolling particle motion, with bauxite as PHC. The solar field is generated in SolarPILOT and the flux distribution inside the kiln is obtained via SolTrace, from which an in-house 1-D thermal model computes the kiln’s temperature distribution and thermal efficiency. Finally, a parametric analysis on Solar Multiple (SM), tower height and tilt angle is carried out to maximise the solar-to-thermal efficiency. Methodology