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

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.

Modelling the effects of particle size pretreatment method on biogas yield of groundnut shells

“…The effect of the particle size on oDMMY shows that smaller particle size produces the optimum yield, and it is a crucial parameter during the optimisation of the biogas yields. This result is in line with what was earlier reported in related research works (Ajayi-Banji et al, 2020; Prade et al, 2019). The optimum FMMY was recorded at the bigger particle size; this result complement what was reported by Herrmann et al (2012) earlier.…”

“…For oDMBY and oDMMY, their optimum yields were recorded at the same temperature and hydraulic retention time but with different particle sizes. Optimum oDMBY was recorded at a particle size of 4 mm, and this result agrees with what was recorded by earlier researchers when compared with 6 mm particle size (Ajayi-Banji et al, 2020; Kirby et al, 2020; Kulichkova et al, 2020; Prade et al, 2019). Compared with 2 mm particle size, it is contrary to what was earlier reported (Naegele et al, 2014; Nalinga and Legonda, 2016).…”

“…This result implies a specific particle size whereby further particle size reduction will not be beneficial to oDMBY. For the oDMMY, the optimum yield was recorded at 2 mm particle size, and this is in support of what was reported by the earlier researchers that smaller particles increase the yield of biogas (Kirby et al, 2020; Kulichkova et al, 2020; Nalinga and Legonda, 2016; Prade et al, 2019). In general, it can be deduced that higher temperature and hydraulic retention period favour the organic dry matter yields while lower temperature and hydraulic retention period enhance FM yields.…”

Ensiling for anaerobic digestion: A review of key considerations to maximise methane yields

“…The effect of storage time on methane yields has also been studied by [11,29,31,35]. All four studies reported that longer storage periods have a positive effect on methane yield.…”

Estimating the Methane Potential of Energy Crops: An Overview on Types of Data Sources and Their Limitations

“…Spence et al [7] examined substrates, including grass and triticale, at 38 • C for 20 days using a proprietary system with automated normalisation to STP (Anaero Technology, Cambridge, UK). Tests on the effect of harvest date and cutting length for grass and whole-crop rye and wheat were carried out by Prade et al [105] using the AMPTS II system, with values reported as dry gas at STP after 30 days of digestion. Allen et al [106] also employed AMPTS II for various substrates; the test duration is not explicitly stated but appears to have been 30 days.…”