Coupling sorghum biomass and wheat straw to minimise the environmental impact of bioenergy production

The reform of the European sugar market in 2006 paved the way for the development of new agricultural value chains in the Po Valley (Italy). A value chain based on the use of biomass sorghum (Sorghum bicolor (L.) Moench) to produce electricity in a medium-scale power plant was investigated. A Life Cycle Assessment was carried out to explore the environmental impact and energy performance of power generation from three biomass sorghum genotypes characterized by different earliness (early, medium-late and late) in the Po Valley (Italy). To fully cover the plant needs, sorghum was complemented by winter wheat straw. Productivity and losses of sorghum for the past 39 years as simulated in Serra et al. (2017) were used to produce a probability distribution of environmental impacts. Soil organic carbon change relative to the straw removal and sorghum incorporation in soil as well as indirect land use change CO₂ emissions for the substitution of sugar crops with energy crops were also accounted for. To test the influence of the assumptions an extensive sensitivity analysis over several parameters was performed. The lowest average GHG emissions (68.9 g CO_2eq.MJ⁻¹) were achieved with the late genotype while medium-late and early genotypes emitted 73.5 g CO_2eq. MJ⁻¹ and 76.8 g CO_2eq.MJ⁻¹, respectively. Despite the conservative assumptions, the bioenergy system contributed on average 47.7% less GHG than a natural gas power plant. In the lowest productivity years the sorghum based energy system emitted 52% less GHG than the Italian electricity mix.

Overall, when harvesting and bailing failed due to unfavourable weather conditions, the lowest GHG emissions were found, thanks to the increased replacement of sorghum with straw. In fact, soil incorporation of sorghum biomass resulted in more nutrients added to the soil than with incorporation of wheat straw. Considering that GHG emissions decreased linearly when sorghum biomass yield increased, the highest reductions of GHG were found with late genotypes, that produced the highest yields. The lowest GHG emissions were found when harvesting failed, as the fertilizer debit of straw is lower than the fertilizer credit of sorghum. However, since carbon and nutrients storage in the soil is not rewarded monetarily, this option will not correspond to an optimal profit as the risk of failures are highest with late genotype.

All other environmental impacts assessed were higher for the sorghum based system than for the fossil alternatives. It was found that the presence of DeNO_x SNCR (Selective Non-Catalytic Reduction) technology achieved the expected mitigation of acidification potential and photochemical oxidant formation but at the expenses of an increased climate change impact, due to additional N₂O emissions.