The assessment of Virginia mallow (Sida hermaphrodita Rusby) and cup plant (Silphium perfoliatum L.) productivity, physico–mechanical properties and energy expenses

• Growing year, liming and N rate influenced plant mass yield and energy potential.
• Energy expenses of investigated plant growing technologies – 8.6–29.2 GJ ha−1.
• Net calorific value of Virginia mallow DM reached 18.5 MJ kg−1, cup plant −17.6 MJ kg−1.
• Main parameters of Virginia mallow satisfy the requirements for solid biofuels.

Field and laboratory trials of the energy crop species Virginia mallow (Sida hermaphrodita L. Rusby) and cup plant (Silphium perfoliatum L.) were conducted to evaluate the effects of liming and nitrogen application on biomass and energy productivity and the suitability of the biomass for biofuel production. Independent of the growing year, the average DM (dry mass) yields of cup plant and Virginia mallow ranged from 6746 kg ha−1 to 13,485 kg ha−1 and from 4675 kg ha−1 to 7451 kg ha−1, respectively. The Virginia mallow and cup plant pellet humidity ranged from 9.6% to 11.6%, and the Virginia mallow pellet density reached 969.3 kg m−3 DM. The ash contents of the Virginia mallow and cup plant pellet were high, reaching 10% and 6.1%, respectively. The net calorific values of the Virginia mallow and cup plant dry masses were 17.49–18.44 kJ ha−1 and 17.19 to 17.48 kJ kg−1, respectively, and were correlated with the biomass crude fibre content with energy outputs of 79–105 GJ ha−1 (for Virginia mallow) and 200–236 GJ ha−1 (for cup plant). The energy expenses of the Virginia mallow and cup plant growing methods (including planting and harvesting year) reached 8630–29,264 MJ ha−1.