Impact of Harvest Date and Cutting Length of Grass Ley and Whole-Crop Cereals on Methane Yield and Economic Viability as Feedstock for Biogas Vehicle Fuel Production

Thomas Prade,Emma Kreuger,Sven-Erik Svensson,Torsten Hörndahl

doi:10.1007/s12155-018-9952-x

Thomas Prade, Emma Kreuger + Show 2 more

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https://doi.org/10.1007/s12155-018-9952-x

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Abstract

Grass ley and whole-crop cereals used for biogas production are often finely chopped for subsequent ensiling and anaerobic digestion. Chopping can impact not only ensiling stability, digestibility and risk of process hick-ups in the digester but also harvesting capacity and fuel consumption. Based on field experiment data, the aim of this study was to investigate how three different nominal cutting lengths in the range of 3.5 mm to 12.5 mm impact methane yield and economic viability of grass ley and whole-crop cereals used as biogas substrate. A shorter cutting length affected the specific methane potential differently for the different crops, + 14 to − 25%. In biogas vehicle fuel production, balancing the additional energy and economic costs for shorter cutting length required an increased methane potential of less than 1% and 3%, respectively. As long as a decrease in cutting length increased the methane potential, the energy balance and economic result improved, despite higher energy inputs. However, mechanisms behind the impact on methane potential deserve further attention. In conclusion, we have shown that it is economically viable to produce methane gas, as a vehicle fuel, from several agricultural crops grown in the south of Sweden, i.e. grass ley and whole-crop rye and wheat, when they are harvested/chopped with a forager, ensiled as biogas feedstocks and processed to methane gas in a large-scale biogas plant.

Highlights

IntroductionBesides improving the ensiling process and facilitate handling, decreasing particle size may result in higher specific methane yields by increasing the reaction surface [2]
Crops such as grass ley and whole-crop cereals are oftenElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Besides improving the ensiling process and facilitate handling, decreasing particle size may result in higher specific methane yields by increasing the reaction surface [2]
We have shown that it is economically viable to produce methane gas, as a vehicle fuel, from grass ley and whole-crop rye and wheat grown in the south of Sweden, when they are ensiled as biogas feedstock and processed to methane gas in a large-scale biogas plant

Summary

Introduction

Besides improving the ensiling process and facilitate handling, decreasing particle size may result in higher specific methane yields by increasing the reaction surface [2]. The trend has gone to the use of forage harvesters with specific biogas chopping drums with increased cutting frequency, e.g. by an increased number of knives or increased chopping drum speed, resulting in minimum nominal cutting lengths of 2–4 mm. A finer cutting length generally increases fuel consumption and decreases capacity of the harvesting chain and adds to the costs of substrate supply [3]. (2019) 12:137–149 reduction of grass will increase cost without leading further improvements of the ensiling process, methane yield or handling properties. An economically optimal cutting length of in between some millimetres and some centimetres, dependent on substrate and its properties, has been suggested [5]

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