Torrefaction of the Black Lilac (Sambucus nigra L.) as an Example of Biocoal Production from Garden Maintenance Waste

The paper presents the conversion of the black lilac (Sambucus nigra L.), which is an example of garden plant residues coming from garden maintenance works into valuable energy carriers by using torrefaction technology. A laboratory reactor was built for the torrefaction process, which allowed black lilac chips to be roasted at a temperature ranging from 250 to 300 °C for several dozen minutes. The black lilac’s properties and structure were investigated before and after material torrefaction to identify the effect of this process on the plant. The average higher heating value (HHV) of the raw black lilac increased from 17.2 to 24.0 MJ/kg after torrefaction. The average mass yield amounted to 39–65%, while the energy yield amounted to 58–96%. The moisture of the black lilac after cutting and grinding was up to 50%, while after torrefaction it did not exceed 4%. Analysis using a scanning electron microscope and an optical microscope revealed the black lilac’s fibrous and annular structure with spherical inclusions, which changed following the torrefaction process into a flatter, more even structure with fewer inclusions. Elementary analysis revealed a significant decrease in the O/C ratio as a result of the torrefaction process. It was also found that the spherical inclusions were composed to a high degree of Ca, Al or Si elements. TGA analysis showed high volatile matter content in the raw black lilac, which decreased significantly after torrefaction. The study shows that torrefaction technology is a valuable process for the production of biocoal from garden maintenance residues.