Influence of the harvest time and the airflow rate on the characteristics of the Arundo biochar produced in a pilot updraft reactor

The thermal degradation (pyrolysis) of renewable biomass (as crop residues or energy crops) into a carbon-rich byproduct has been proposed as a practice to fight soil degradation and to foster the mitigation of climate change. The pyrolysis conditions and the characteristics of the feedstock influence the quality of the biochar. The present study used a pilot updraft reactor to make a pyrogasification of the biomass of giant reed (Arundo donax L.). The temperature varied in the range 300–400 °C and the process lasted 60–90 min, conditions specific to slow pyrolysis. The work aimed to compare three airflow rates (0.001 m3 s−1, 0.0007 m3 s−1, and natural ventilation) and the Arundo biomass collected in winter (February) or summer (June), on the base of known harvesting time. The biochar yield was comparable between February (19.8%) and June (21.8%) but the biochar composition in terms of CHNSO was depending on the airflow rate. The values of the molar ratios indicated that the biochar produced with Arundo collected in winter (February) in presence of the air (0.001 and 0.0007 m3 s−1) may have the highest level of aromaticity (H/C ratio), hydrophilicity, and polarity. Germination test on lettuce seeds showed different results: the winter biochar produced at 0.0007 m3 s−1 gave germination comparable to the control up to a concentration in the substrate up to 2% in weight; the summer biochar reduced the growth of the seedlings already at 0.5% when produced under natural ventilation.