Modeling the effects of cultivar and harvest on the decomposition kinetics of potato crop residues

Thermogravimetry and kinetic analysis is carried out of the decomposition of potato crop residues, integrated by model compound measurements. Compared with other herbaceous by-products, conversion takes place at lower temperatures, gives rise to higher char amounts and, apart from the usual well evident absolute peak, the rate curves generally exhibit two additional local maxima, at low and high temperatures. Multiple integral and differential data modeling requires six parallel reactions (one, two, one and two steps for the pseudo- extractives, pectin-hemicellulose, cellulose-starch, and lignin-protein, in the order). Residue variability (five samples of different cultivar and harvest year) can be associated with increasing average plant ageing, essentially resulting into variable product yields and, in a few cases, activation energies, describing variations in macro-component contents and properties, respectively. With ageing, the activation energies (180–206 kJ/mol) and volatile yields (19–27 wt%) of the pseudo-cellulose-starch progressively increase (higher cellulose content and crystallinity). Moreover, although the decomposition of pseudo-pectin-hemicellulose gives approximately constant volatile yields (17–19 wt%), the decrease in the activation energy (163–127 kJ/mol) of the first reaction indicates progressively lower contributions of pectin (and extractives). Also, volatile yields from pseudo-lignin-protein decomposition decrease (21–14 wt%), owing to successively lower contents of the second component. Peculiar features of the six-step scheme are originated from pectin, starch and protein components and very high alkali metal contents.