This study analysed the effects of Salix variety on energy and mass flows co-digested with DaM to produce biomethane. The results from the literature, laboratory experiments and process modelling were useful in identifying factors and parameters affecting energy output and performance of the anaerobic digestion process. Overall, the results showed good potential for biomethane production and can serve as a guideline for future assessments to determine biomethane output in relation to amount of Salix processed. Site-specific data that include spatial and temporal aspects are needed to refine the results further to provide exact figures for real biogas applications.
The results in the present case showed that the energy output was higher than the energy demand of the Salix-to-biomethane systems, but with differences between varieties, highlighting the importance of including varietal effects in such analyses. The wide system boundary chosen in the study (Fig. 1) also provided a more holistic picture of the performance of the system, as all steps from cultivation of Salix to digestate disposal were included.
There were large variations in energy demand of the Salix production chain between the different varieties, due to fertilisation and differences in yield. Fertilisation increased the energy demand per unit mass of biomass produced, but also gave higher biomass yield in most cases. The productivity of Salix crop varieties is an important parameter, as there is reported to be a 3.5-fold difference in land requirement between the lowest- and highest-producing varieties [28]. Arable land is a scarce resource in the majority of countries worldwide, so it is important to strike a balance between the amount of land needed for production and the energy input per unit of biomass. The productivity level of unfertilised crops is also questionable in the long run, as it is very likely that the soil nutrients will deplete over time. Thus, fertilisation is beneficial to ensure a steady and secure supply of Salix biomass.
The biomethane facility studied comprised steam pre-treatment, hygienisation of manure and anaerobic digestion, and biogas upgrading (stages 2, 3 and 4 in Fig. 1). The hygienisation process had the highest energy demand, for heating liquid dairy manure to deactivate pathogens. This increased the energy input of the system, but rendered the digestate safe as a fertiliser. Although optimisation of energy performance is important, it is not always the main objective of biogas plants. Use of digestate on fields reduces the need for mineral fertilisers and can contribute to increased soil carbon sequestration. This is favourable from the perspective of climate change mitigation and waste management. Climate impact studies on the system scale (e.g. LCA) are needed to calculate the climate benefit of such processes. Based on the N-P-K content of Salix biomass and DaM reported in the literature (Table S6 in SM), annual application of 30 tons/hectare of digestate can add about 60 kg, 12.5 kg and 59 kg of N-P-K per year (Table S7 in SM).
Co-digestion of the Salix varieties Gudrun and Tordis with DaM gave the highest biomethane output in this study. In both fertilised and unfertilised form, these two varieties produced more than 100 kg/h of biomethane from co-digestion of 300 kg/h of Salix feedstock with DaM in a 1:1 VS ratio. The biomethane output was modelled in reactors in Aspen Plus, using stoichiometric reactions and BD ratio calculated from laboratory-scale BMP studies. Different approaches in modelling biogas reactors can lead to varying results and there is uncertainty regarding how biogas production in industrial-scale plants compares with laboratory-scale experiments. Anaerobic digestion is a simple process but has complex dynamics, as it involves intricate microbiological interactions, so it is difficult to upscale laboratory-scale BMP values to methane production in large-scale plants. In this study, RMY was conservatively assumed to be 80% of the BMP value. Depending on anaerobic digester conditions and management practices, RMY can be higher. Pilot-scale studies are needed to identify the reaction dynamics and interactions, which will allow more accurate modelling and extrapolation of such processes to industrial scales.
Due to the lack of a standardised method for measuring energy performance, it is challenging to make direct comparisons of different systems. Output-input ratio is one of the most common indicators used in energy performance calculations for biogas production [51], but differences in system boundaries between studies determine what are included as input and output energies in respective systems. To conduct an accurate energy balance analysis, direct and indirect energy requirements should be established for all stages of the crop-based energy production cycle. The R values in this study ranged from 1.46 to 1.83 for the base scenario without heat recovery, and from 2.26 to 2.76 in the heat recovery scenario. These values are at the lower end of the range of R values reported in the literature, e.g. for Salix biogas production in Denmark values of 7.3 without pre-treatment and 12.3 with pre-treatment have been reported [52]. Those higher R values can be due to omission of biomethane upgrading and manure hygienisation processes in their system. The R values in that study were higher for Salix than for maize and miscanthus, although total energy output was higher from maize without pre-treatment [52]. The perennial energy crops (Salix and miscanthus) had significantly lower energy inputs for cultivation and harvest than maize, and pre-treatment improved biogas yield [52]. A similar analysis of biomethane production from untreated hemp (Cannabis sativa L.) in Sweden reported a R value of 2.6 [53], which is comparable to the R values of Salix in the HRE scenario in our study. Another study analysing biomethane production from maize, fodder beet, lupin and perennial ryegrass, with heat and electricity demand fulfilled from the biogas produced, reported R values of 2.0 to 2.9 for crops in the system [54]. However, the results of such systems analyses are dependent on site-specific conditions and modelling choices such as system configuration, secondary feedstock selection, and pretreatment conditions. Hence, it is important to consider these factors when interpreting the results.
The heating demand for the pretreatment and manure hygienization processes is one of the main energy consumers in the system. Reduction of the pretreatment energy consumption for pre-treatment, while maximising the release of sugars, is critical for improving the energy performance of biomass to biofuel systems [55]. Achieving such an improvement could make lignocellulosic materials such as Salix an efficient and attractive feedstock for sustainable production of biofuels and biogas [56]. The lower energy demand in the HRE scenario improved the energy performance of the system in this study. Process design to maximise heat recovery while balancing the economic costs of a more complex set-up is necessary to ensure the success of industrial-scale production.
The heating value of the raw materials was not considered in this study, as the energy performance of different energy carriers other than biomethane or other conversion pathways (e.g. combustion or gasification) for the feedstocks were not compared. The focus was on biomethane production and the system performance of different feedstock combinations. Differences in energy conversion efficiency must be included when comparing different conversion pathways.
Upgrading and compression of biogas to biomethane had a high demand for electricity and cooling, which negatively affected the overall energy balance, as these steps did not increase the net energy output of the system. The increased energy demand for upgrading biogas can be justified, as it improves fuel quality and enables direct use of biogas as a vehicle fuel or injection into gas grids as biomethane. If biogas is to replace natural gas as fuel, upgrading is necessary to remove the non-combustible CO2 fraction from biogas. Reducing the energy demand for upgrading would greatly benefit the energy performance of the system, but might not be as relevant from an economic standpoint if cheap electricity and cooling are available on-site. Various upgrading technologies (in addition to water scrubbing) are undergoing constant improvement in their energy and environmental performance, but their actual performance will depend on site-specific and economic conditions, which must be taken into account when selecting the best technique [57].
In addition to replacement of fossil natural gas by biomethane, potential for soil carbon sequestration by Salix cultivation [25] and digestate application [58] make co-digestion of Salix an interesting strategy to mitigate climate change. In future work, we will extend the mass and energy analysis to a LCA to evaluate and compare the climate performance of biomethane production from Salix varieties.