A method is described to find the effective thermal conductivity due to radiation (k(rad)) for groups of particles at packed and less than packed states. Unlike most previous studies, the method does not rely on the assumption of a unit cell or absorption and scattering coefficients to derive k(rad). In this method, radiation is modeled with a 3D Monte Carlo ray tracing code, steady state particle temperatures are found with a particle-particle heat exchange simulation, and k(rad) is found with a comparison to heat conduction in an isotropic solid of the same geometry. This leads to the dimensionless Exchange Factor (F-E), allowing k(rad) to be calculated at any temperature and particle radius. The key result is a model for F-E over the entire range of emissivities from 0.3 to 1 and solid fractions from 0.25 to the fully packed state of 0.64. F-E results are compared to previous models, with agreement shown in some cases but a large disagreement found for low solid fractions. The k(rad) results are combined with the Zehner and Schlunder model for solid and fluid conduction, providing an equation for the full effective thermal conductivity. (C) 2020 Elsevier Ltd. All rights reserved.