Despite the relevance of physical properties on the fluid dynamic behavior of binary beds, the physical transformation of feedstock particles during pyrolysis is often overlooked. The present work aims at studying the variation of physical properties of sewage sludge during biochar production and assessing the impact of such change on the fluid-dynamic behavior of binary mixtures with silica sand. Sewage sludge was obtained from the municipal wastewater treatment plant of the city of Bogotá, D.C. Biochar samples were produced through pyrolysis in a thin layer fixed bed reactor at 450 °C, 550 °C and 650 °C under heating rates of about 5 K/s. Three different particle sizes of the initial sewage sludge samples were studied (106 μm to 150 μm, 150 μm to 212 μm, and 300 μm to 500 μm). Particle size and particle density of the original sewage sludge and the produced biochar samples were measured. It was found that the reduction in particle mean diameter at the highest temperature was of 19% and for the particle density the reduction was of 29%. The fluidization behavior of the biochar samples mixed with silica sand (approximately 20% char to 80% sand in volume) was studied in a cold flow model and the minimum fluidization velocity of the mixtures (umf) was experimentally determined. The ability of different correlations reported in the literature to accurately predict the minimum fluidization velocity of the obtained biochars – sand mixtures was evaluated. Even though some qualitative changes in the fluid dynamic behavior of the mixtures were identified as a result of the change in properties, no clear quantitative change in the minimum fluidization velocity could be observed. On the one hand, the reduction in particle mean diameter and in particle density does not present relevant changes in the Geldart classification of the fuel particle, and on the other hand, the fuel particles are prone to segregate creating an overlap in the minimum fluidization velocity of the two different solid phases.
Martínez et al. (Wed,) studied this question.