Exothermic Events of Nut Shell and Fruit Stone Pyrolysis

Pyrolysis is carried out by means of a packed bed, heated along the lateral surface at a moderate temperature (about 585 K), of a significant number of nut shells (pine, macadamia and Brazil nuts, hazelnut, walnut, almond, coconut, chestnut, peanut, and pistachio) and fruit stones (olive, nectarine, peach, plum, cherry, and apricot). With the exception of pistachio shells, the residues own fixed carbon (and C) and lignin/extractive contents higher than those of wood. The SEM images of chars reveal a nonporous vesicular or foamed tissue generally crossed by a very few large hollow vessels surrounded by bundles of small void fibers and/or hosting elongated rolled elements again of much smaller size. The EDX analysis indicates that the deposits, scattered on the charred tissue, are generally rich in potassium. As a consequence, secondary reaction activity is enhanced and, compared with wood, pyrolysis-induced overheating is always higher. However, despite the similarities, feedstock differences during pyrolysis are remarkable, as testified by maximum temperature overshoots between 60-225 K (versus 50K for wood) and severe pyrolytic runaway established only for about half of the samples.