Energy yield decline of Sida hermaphrodita harvested for biogas production

The perennial and herbaceous plant Sida hermaphrodita L. Rusby (Sida) has been investigated intensively with respect to its use as a solid biofuel for combustion and, to a lesser extent, for continuous biogas production. Since biogas production provides many opportunities for economic farm diversification, this study compared the biogas potential of Sida with common biogas crops silage maize (Zea mays L.) and cup plant (Silphium perfoliatum L.). To this end biomass samples were collected from a randomized field trial in Southwest Germany during the vegetative stage in five consecutive years starting in 2014. Biogas batchtests revealed constant specific-methane yields of 279 ± 8 LN kgVS−1 for Sida, significantly lower than for maize (335 ± 5 LN kgVS−1) and significantly higher than for cup plant (264.3 ± 3.7 LN kgVS−1). The annual Sida dry matter biomass yields declined significantly by 51% from 2015 (18.2 ± 2.6 Mg ha−1) to 2018 (9.2 ± 4.8 Mg ha−1) as a result of the summer harvest regime. This, in turn, led to a significant decline in Sida's total annual energy yields. Consequently, the average annual Sida energy yield (3549 ± 901 L heating oil equivalents (HOE) ha−1) was significantly lower than for maize (6611 ± 1186 L HOE ha−1) and cup plant (5261 ± 974 L HOE ha−1). These results clearly indicate that harvesting Sida during the vegetative stage for biogas production significantly decreases its energy yield over time. Therefore, Sida can be used economically and ecologically more sustainable for combustion using the dead plant biomass harvested in winter, while cup plant proved competitive to maize for biogas production.