Effect of particle size distribution, drying and milling technique on explosibility behavior of olive pomace waste

Abstract

The Mediterranean area is responsible for about 98% of the olive oil worldwide production, with 900 million olive trees occupying 10 million hectares. However, the processing of 100 kg of olives leads to the production of 40 kg of wastes, mainly constituted by olive pomace, which is potentially recoverable as energetic or material source. In general, in the past 20 years, the exploitation of olive pomace has increased, but along with it, the need for further information about its chemical-physical characterization and the related hazard in industry. Thus, a risk analysis assessment was conducted. When pelletized or in chunks, olive pomace does not pose any greater hazard than a pile of woody material, but when pulverized, it might become dangerous. Two parallel series of experiments were carried out at Dalhousie University (Lab 1) and at Polytechnic of Turin (Lab 2) using the same olive pomace sample, according to slightly different experimental procedures. Olive pomace dust explosibility and flammability parameters were measured: minimum ignition energy (MIE), minimum ignition temperature (MIT), maximum pressure rise rate ((dP/dt)max and KSt), maximum pressure (Pmax), and minimum explosible concentration (MEC). Moreover, the chemical and physical characterization of olive pomace was carried out: moisture content, particle size analysis, Scanning Electronic Microscope (SEM) investigation, thermo-gravimetric analysis (TGA), solid-state Nuclear Magnetic Resonance (NMR), mass spectrometry, calorific value, and bulk density estimation. Different thermal behaviors were observed according to the sieving/grinding pre-treatment. As concern flammability tests, samples seemed not to be sensitive to electric arc ignition (a value of MIE could not be measured), while coarser samples demonstrated higher ignition sensitivity to hot environment sources (MIT furnace) than finer ones. On the other hand, explosion violence parameters were enhanced by decreasing the particle size, while peak pressures were significantly influenced by the heat of combustion and the moisture content. Finally, a new test was developed to quantify the propensity of the raw material to produce fines by abrasion. It is defined “Abrasion by Rolling Test” (ART). The properties of the fines produced were measured as well.