Experimental determination of the optical properties of walnut shell particles

Abstract

The index of refraction (IOR) is required to model thermal radiation interaction with pulverized solid fuels. In this work, the complex index of refraction of biomass (walnut shell) is therefore determined using pulverized particles. Single particles are irradiated, and the scattered radiation is measured in different directions. To avoid falsification of the scattering pattern (phase function), the particles are kept contactless in an acoustic levitator. Here, over 1000 different phase functions are measured. The measured scattering patterns are evaluated using an inverse evaluation procedure to determine the IOR. Mie theory serves as the basis for the mathematical modeling of the radiation properties of the particles. The measured IOR is then compared to data from the literature on coal. For the wavelength range $\lambda =2000\text{–}4000\mathrm{nm}$ no distinct differences are noticed between the coal and biomass IOR. For $\lambda >\text{4000}\text{nm}$ the real part of the biomass IOR is larger and the differences increase with increasing wavelength. However, the order of magnitude still matches that of coal IOR, and thus, only minor differences in the radiative properties of coal and biomass are expected.