Abstract
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 4 3 − , 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.
Highlights
The definitions of biochar provided by the European Biochar Certificate (EBC) [1] and the IBI (International Biochar Initiative) Biochar Standards [2] address biochar properties requirements for its application, as well as methodological standards to assess them, but they do not describe in detail the production process and its influence on biochar properties
corn digestate (CD) biochars present higher yields than Pine wood (PW) biochars, due to the significantly higher inorganic species content, which remain in the solid product after pyrolysis
The influence of process conditions and feedsctock on several biochars and hydrochars properties has been studied in a comprehensive way and analyzed from mechanistic perspectives
Summary
The definitions of biochar provided by the European Biochar Certificate (EBC) [1] and the IBI (International Biochar Initiative) Biochar Standards [2] address biochar properties requirements for its application, as well as methodological standards to assess them, but they do not describe in detail the production process and its influence on biochar properties. The EBC considers mainly pyrolysis as the process to produce biochar, including gasification as part of the pyrolysis technology spectrum. The IBI Biochar Standards consider all forms of thermochemical conversion with limited presence of oxygen as appropriate technologies to produce biochar. As Abiven et al [3] pointed out, it is necessary to develop tailor-made biochar systems according to individual applications, taking into account soil type, climate and social environments, instead of considering biochar as an universal soil enhancer
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