Assessment of agricultural crops and natural vegetation in Scotland for energy production by anaerobic digestion and hydrothermal liquefaction

The current paper investigates the use of natural vegetation and agricultural crops commonly found in Scotland as a source of bioenergy. Such biomass is shown to have a high moisture content upon harvest (∼80%) which renders them suitable for wet conversion technologies such as anaerobic digestion (AD) and hydrothermal liquefaction (HTL). Experimental investigations are carried out on 16 different types of biomass to assess their bio-crude yields via HTL and theoretical methane potential via AD based on compositional analysis. The different types of biomass vary significantly in biomass yield upon harvesting from 1.1 t/ha (dry matter) for bracken to a maximum of 17.5 t/ha for winter rye. These area specific yields are the most influential factor in the final energy yield per area. Area specific energy yields are found to average at 67 GJ/ha for AD and 53 GJ/ha for HTL. The respective conversion efficiencies of HTL and AD for different biomass feedstocks are also shown to be an important factor on the overall energy potential. AD averages a mass to energy conversion of 9.1 GJ/t compared to 7.2 GJ/t for HTL. A combination of AD and HTL is investigated by liquefying digestate from rye, but the results suggest this is not a viable option due to low bio-crude yields. However, analysis of the water phase post HTL allowed the calculation of theoretical methane potential from the HTL process water and suggests that this can yield additional energy. Overall, the work shows that utilisation of natural vegetation is a promising approach for bioenergy production.