Miscanthus for biogas production: Influence of harvest date and ensiling on digestibility and methane hectare yield

Abstract

The 8,000 biogas plants currently in operation in Germany are mainly fed with biomass from annual crops. However, feedstock from perennial crops such as miscanthus is expected to be more environmentally benign. If miscanthus is to be used in greater amounts as a substrate for anaerobic digestion, storage will become a relevant topic, as a continuous supply of biomass throughout the year is necessary. The objective of this study was to identify the miscanthus harvest time that best balances the simultaneous achievement of high silage quality, high digestibility and high methane hectare yields. For this purpose, biomass from four miscanthus genotypes with varying senescence characteristics was harvested on three different dates in autumn 2017. Part of the biomass was ensiled, and the methane yield of both ensiled and non‐ensiled biomass was analysed in a biogas batch test to assess the effect of ensiling on the methane hectare yield and digestion velocity. The ensiled biomass was found to have an up to 7% higher substrate‐specific methane yield and also showed a higher digestion velocity than the non‐ensiled biomass. The silage quality was best when miscanthus was harvested in mid‐October, due to highest lactic acid content (average: 3.0% of DM) and lowest pH (average: 4.39) compared to the harvests in mid‐September and beginning of October. Mass losses during ensiling (as high as 7.6% of fresh matter for the M. sinensis genotype Sin55) were compensated for by a higher substrate‐specific methane yield (up to 353 Nml CH4 (g oDM)−1) in ensiled miscanthus. This resulted in non‐significantly different methane hectare yields for non‐ensiled (average: 4.635 Nm3 CH4/ha) and ensiled miscanthus biomass (4.803 Nm3 CH4/ha). A comparison of the four genotypes suggests that Miscanthus x giganteus is the most suitable genotype for ensiling as it had the best silage quality.

Economic and environmental performance of miscanthus cultivated on marginal land for biogas production

“…In addition, Mangold et al. (2018b) showed that the ensiling of miscanthus biomass reduces the necessary intensity of pretreatment. Besides the economic performance, this would also improve the environmental performance of miscanthus‐based biogas production.…”

Status quo and perspectives of biogas production for energy and material utilization

“…So far, ensiling is the best‐known preservation technique for biomass. (Mangold et al, ) demonstrated the ensiling ability of miscanthus biomass and showed positive effects of ensiled miscanthus on digestion‐velocity compared to nonensiled miscanthus. Mass losses of up to 7.6% of fresh matter for the Miscanthus sinensis genotype Sin55 were compensated by a higher substrate‐specific methane yield (up to 353 ml CH 4 (g oDM) –1 ), which resulted in a methane hectare yield of 4757 m 3 CH 4 ha –1 for ensiled miscanthus (Mangold et al, ).…”

“…(Mangold et al, ) demonstrated the ensiling ability of miscanthus biomass and showed positive effects of ensiled miscanthus on digestion‐velocity compared to nonensiled miscanthus. Mass losses of up to 7.6% of fresh matter for the Miscanthus sinensis genotype Sin55 were compensated by a higher substrate‐specific methane yield (up to 353 ml CH 4 (g oDM) –1 ), which resulted in a methane hectare yield of 4757 m 3 CH 4 ha –1 for ensiled miscanthus (Mangold et al, ). To enable a large‐scale use of perennial crops such as miscanthus, implementation barriers must first be removed for farmers who are afraid of a 20‐year fixation of the land to one crop with above‐average initial investments.…”

“…They reduce the risks of biomass production because they deliver more stable yields over the years than annual crops, which are more influenced in their establishment and growth by precipitation and temperature conditions (Ehmann, Bach, Laopeamthong, Bilbao, & Lewandowski, 2017;Lewandowski, 2016). Both Wagner et al, (2019) and Mangold, Lewandowski, & Kiesel (2019a) take up this point of positive ecological effects of alternative substrates in their contributions and exemplify it with miscanthus. Although miscanthus has so far mainly been used for heat production and material applications, targeted use in biogas plants can also be important in the context of taking ecological effects into account, especially as the methane hectare yields of miscanthus can keep up with those of silage maize .…”

Biobased value chains for a growing bioeconomy

“…The challenge is dealing with the ensiling and the lower specific methane yield of the lignocellulosic miscanthus biomass. Mangold, Lewandowski, Hartung and Kiesel () show that ensiling miscanthus biomass is possible and that the specific methane yield can be improved by green harvesting in October. Another advantage of miscanthus over maize is its ability to grow on land with biophysical constraints to food crop production.…”