The impact of conversion process parameters in pyrolysis (maximum temperature, inert gas flow rate) and hydrothermal carbonization (maximum temperature, residence time and post-washing) on biochar and hydrochar properties is investigated. Pine wood (PW) and corn digestate (CD), with low and high inorganic species content respectively, are used as feedstock. CD biochars show lower H/C ratios, thermal recalcitrance and total specific surface area than PW biochars, but higher mesoporosity. CD and PW biochars present higher naphthalene and phenanthrene contents, respectively, which may indicate different reaction pathways. High temperatures (>500 • C) lead to lower PAH (polycyclic aromatic hydrocarbons) content (<12 mg/kg) and higher specific surface area. With increasing process severity the biochars carbon content is also enhanced, as well as the thermal stability. High inert gas flow rates increase the microporosity and wettability of biochars. In hydrochars the high inorganic content favor decarboxylation over dehydration reactions. Hydrochars show mainly mesoporosity, with a higher pore volume but generally lower specific surface area than biochars. Biochars present negligible availability of NO − 3 and NH + 4 , irrespective of the nitrogen content of the feedstock. For hydrochars, a potential increase in availability of NO − 3 , NH + 4 , PO 3− 4 , and K + with respect to the feedstock is possible. The results from this work can be applied to "engineer" appropriate biochars with respect to soil demands and certification requirements.
In order to promote the transformation of a burnt Mediterranean forest area into a // / dehesa system, 10 t ha À1 of dry matter of the same sewage sludge in three different forms: fresh, composted and thermally-dried, were added superficially to field plots of loam and sandy soils located on a 16 per cent slope. This application is equivalent to 13Á8 t ha À1 of composted sludge, 50 t ha
À1of fresh sludge and 11Á3 t ha À1 of thermally-dried sludge. The surface addition of a single application of thermally-dried sludge resulted in a decrease in runoff and erosion in both kinds of soil. Runoff in thermally-dried sludge plots was lower than in the control treatment (32 per cent for the loam soil and 26 per cent for the sandy soil). The addition of any type of sludge to both soil types also reduces sediment production. Significant differences between the control and sludge treatments indicate that the rapid development of plant cover and the direct protective effect of sludge on the soil are the main agents that influence soil erosion rates. Results suggest that the surface application of thermally-dried sludge is the most efficient way to enhance soil infiltration.
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