Delayed harvest of miscanthus—influences on biomass quantity and quality and environmental impacts of energy production

Abstract

One important aim of this paper is to investigate the effect of delayed harvest of miscanthus on biomass yield and quality. Miscanthus field trials were therefore carried out at three locations in south Germany. The plants were harvested at different dates, and the yields and chemical composition of the biomass were measured. Another aim is to investigate the effects of delayed harvest of miscanthus on the net primary energy balance, total emissions of greenhouse gases, and total emissions of acidifying gases in the case of a large-scale heat generation from this biomass. Thus, life cycle assessment (LCA) for a theoretical large-scale heat generation from this specially grown and harvested miscanthus was carried out. Since no standard data set was available, two different model approaches (A and B) were applied within LCA to describe the energy consumption of biomass drying. Results of the field trials show an energy yield of 187–528 GJ ha −1 harvest from miscanthus in December. With delayed harvest, bioenergy yields decreased by 14–15% between December and February and by a further 13% between February and March. This was accompanied by a significant decrease in water content and in the concentrations of ash, nitrogen, chloride and sulphur in the biomass. The decline in energy yield, however, cannot be traded off by a reduced total primary energy consumption due to a reduced need for drying. The amount of total CO 2 equivalent emissions of theoretical heat generation from miscanthus varies between 10.3 and 36.1 kg CO 2 equivalents GJ −1 depending on the harvest time, production site and drying process model used. A clear effect of delayed harvest on total CO 2 equivalent emissions per GJ was not found. The amount of total SO 2 equivalent emissions varied between 44 and 213 g GJ −1. Here, results of both drying models show a clear decrease of SO 2 equivalents with delayed harvest. In addition, energy generation from miscanthus emits less CO 2 and SO 2 equivalents than conventional heat generation from light heating oil. It can be concluded that an early harvest of miscanthus maximises energy yield and finite primary energy savings per hectare. It may also maximise CO 2 equivalent savings per energy yield when energy consumption of the drying processes can be kept low. To gain certainty on this point, more robust data bases for the LCA are needed. On the other hand, late harvest reduced total SO 2 equivalent emissions of an energetic use of miscanthus and it also recommended for economic reasons.