Improved fuel properties of whole table olive stones via pyrolytic processing

Abstract

This paper describes the thermochemical transformation of residual whole olive stones from the industrial production of pitted and stuffed table olives by using a rotary reactor. This experimental investigation describes the chemical, physical and fuel properties of the resulting solids and liquids obtained in the temperature range between 200 °C and 900 °C. Optimum torrefaction conditions, intended to maximize mass and energy yields, were obtained at 278 °C and resulted in a solid product with 68 wt% volatile matter, 29 wt% fixed carbon, 58 wt% elemental carbon, 0.55 O/C ratio, 23.4 MJ/kg of HHV, 11.25 GJ/m3 apparent energy density for an energy yield of 89%. The carbonized solids obtained at temperatures between 500 °C and 900 °C exhibited LHV and apparent energy density up to 57–66% higher than the original biomass. The carbonization process generates a condensable liquid that represents 50–53 wt% of the original biomass and contains between 57 and 61 wt% water and 39–43 wt% organic products. The carbon content (up to 25 wt%) and heating value (HHV and LHV up to 5.2 MJ/kg and 2.8 MJ/kg, respectively) of this liquid is limited. A model has been tested and a series of equations have been produced which allow us to predict the chemical and energy properties of the solid fraction derived from the torrefaction and carbonization process. This model has found linear correlations between the solid yield and elemental/proximate composition of the solids, and exponential correlations between solid and energy yields.