Solar Absorptance and Thermal Emittance of Particle Heat Carriers in a Solar Driven Biomass Process

Abstract

Particle Heat Carriers (PHCs) are an innovative solution for the storage and transport of solar thermal energy. Integrating PHCs into a biomass pyrolysis process, whereby the PHCs are heated in a solar receiver, then sent to a pyrolysis reactor to supply the thermal energy for the process, is a unique approach for creating a solar-driven biomass pyrolysis process. To properly characterize the heat transfer in a directly irradiated solar receiver, it is essential to understand the PHC optical properties at different stages of the process. In this study, the reflectance of five different PHCs was measured at different process conditions: i) materials as received, ii) materials heated to 950 °C in air (simulating conditions in the solar receiver), and iii) materials mixed with wood chips and heated to 800 °C in nitrogen (simulating the pyrolysis stage). Reflectance measurements were performed in the spectral ranges of 0.25-2.5 and 2.5-16 μm to calculate the solar absorptance and the infrared thermal emittance, respectively.

Similar optical characteristics were observed for each process condition for particles initially dark in color (ex: bauxite, silicon carbide). However, when taking the as-received samples as a reference, significant increases in the solar absorptance were observed for 1) olivine, magnesium oxide and sand samples after being exposed to the pyrolysis conditions (increase by 23, 36 and 39 percentage points, respectively), and 2) the olivine sample after being heated in air at 950 °C (increase by 10 percentage points).