Slow pyrolysis of olive stones in a rotary kiln: Chemical and energy characterization of solid, gas, and condensable products

The aim of this work is to investigate the slow pyrolysis of olive stones in a rotary kiln as a means to increase the fuel properties and potential use of this renewable solid fuel. The pyrolysis process takes place primarily at temperatures between 300 and 500 °C resulting in the transformation of the solid biomass into a biochar, a pyrolysis liquid (up to 38.1 wt. %) and a non-condensable gas fraction (up to 35.4 wt. %). This thermal treatment has a positive influence in the fuel properties of the solid fraction in terms of increased C content (up to 75.9 wt. %), reduced O/C and H/C ratios (down to 0.28 and 0.03), reduced volatile matter and moisture content (down to 6.9 wt. % and below 1.0 wt. %, respectively), increased fixed carbon (up to 90.2 wt. %), increased Lower Heating Value (LHVo up to 37.1 MJ/kg) and energy density (26.7 GJ/m3). The process also involved changes in the surface chemistry (increasingly hydrophobic nature) and textural properties of the solid (formation of cracks and internal voids, resulting in the development of a pore structure of up to 0.193 cm3/g and a surface area up to 507 m2/g). The condensable and gas fractions resulting from the pyrolysis process may also be used for their fuel properties. Thus, the pyrolysis liquid exhibited a high water content (62.5 wt. %), a mass density of 1.063 kg/m3, a viscosity of 1.33 cSt, and a Higher Heating Value (HHVo) of 16.9 MJ/kg. The gas fraction resulting from the pyrolysis of olive stones contains high concentrations of combustible gases like CO and H2, and lower proportions of light hydrocarbons. The gas fraction exhibited HHV up to 6.83 MJ/Nm3 due primarily to CO and H2, while the formation of light hydrocarbons was very limited. The energy distribution resulting from the pyrolysis of olive stone at 700 °C (following completion of the thermal degradation) is as follows: solid fraction 48.2%; oil fraction 11.0%; and gas fraction and energy losses (by difference) 40.8%.