Heat Transfer Modelling of Biomass Torrefaction

Abstract

The depletion of coal reserves and the increase of environmental problems urge the use of renewable energy sources. Biomass is a renewable energy source and is also used as one of the feeds for cofiring, a promising energy generation technology. However, the shortcomings of biomass, such as low calorific value, hygroscopic, and low grindability, limit its usage. Torrefaction is a mild form of pyrolysis of biomass that results in better solid fuel properties. This study aims to model a simple heat transfer numerical equation to predict the torrefaction of biomass in various operating conditions. The temperature distribution in biomass during the torrefaction process is also predicted, as well as the mass yield, energy yield, and HHV behaviour of the torrefied biomass. The modelling was conducted based on Calliandra wood biomass. The pellet biomass diameter was modelled to vary within a range of 1-3 cm and a length of 10 cm. The operating conditions used for this modelling were the temperature of torrefaction with a range of 230- 270 °C. Heat transfer modelling was carried out by using MATLAB. The modelling produces a numerical equation of transient heat transfer with a radial axis base with the kinetic model used as TPR (Three Parallel Reaction). The modelling produces an error of 1.424% compared to the experimental data by Felfli et al. The simulation shows that torrefaction that makes a higher and more uniform temperature distribution will result in a higher HHV value. Based on the simulation, the recommendation of torrefaction operating conditions for Calliandra wood with a diameter of 1-3 cm is at a torrefaction temperature of 270℃ and residence time of 1.5 hours. This will result in a calorific value of approximately 5300 kcal/kg.