Comparative analyses of three olive mill solid residues from different countries and processes for energy recovery by gasification

Biomass is a renewable energy source which may provide a significant contribution to the reduction of fossil fuels consumption and the associated environmental impacts. The use of agricultural or agro-industrial waste such as solid residues from olive oil production is particularly relevant since it may combine several benefits. Gasification is a promising waste-to-energy technique for this type of lignocellulosic residues. The technology however is adapted to a relatively limited panel of solid waste fuels of defined specifications, which must therefore be characterized properly to assess their adaptation. The purpose of this research was to analyze and compare three different olive mill solid residues by complementary techniques such as Fourier transform infrared spectroscopy (FTIR) and thermochemical methods, in order to characterize these residues as potential fuels for gasification. The results obtained underlined the complex nature of the residues and indicated that they were mainly organic, with very little mineral matter. In addition to the major organic components (cellulose, hemicelluloses and lignin), the presence of several minor organic constituents was shown by thermogravimetry coupled to differential scanning calorimetry and FTIR. The gas produced from pyrolysis was analyzed by gas chromatography and mass spectrometry. It was found to contain several degradation products from lignocellulosic material and olive oil, such as hydroxyacetone, furfural and methoxyphenols. The influence of the olive oil extraction process (two-phase or three-phase) was also demonstrated. It was shown that the thermochemical degradation of olive mill residues followed a complex pathway but the composition of the residues met the requirements for gasification for most parameters.