Thermal inertia effects of the structural elements in heat losses during the charcoal production in brick kilns

Abstract

Brazil is the largest producer of charcoal from planted forests with 5.5 million tons in 2016. The Brazilian steel industry consumes 85% of the national production of charcoal from eucalyptus. The walls and floor of industrial brick kilns are built using isolation materials that minimize heat losses during the wood carbonization stage. However, the thermal inertia of these components represents additional heat that must be removed during the charcoal cooling stage, as reflected in the extended process time. This study aims to evaluate the effect of the thermal inertia of the kiln structural elements for the charcoal production. A CFD (Computational Fluid Dynamics) analysis was performed to simulate the heating and cooling of the system composed of wood, carbonization gases, brick walls and floor. A typical industrial kiln with capacity of 700 m3 was modeled and validated using a set of experimental measurements of temperatures during a 4-day carbonization stage with final temperature of 400 °C and an 8 day cooling stage. The temperature profile in the walls was linear, corresponding to a pseudo-steady state, where the thermal load increases with the pyrolysis time. The heat transfer at the floor is extensive; therefore, the adiabatic boundary condition cannot be imposed at the wood bed–floor interface. Our findings provide important information for the improvements in the kiln operation and allow establishment of consistent initial conditions of temperature and heat flux for kinetics models for charcoal cooling in kilns.