Sustainability assessment of agro-bioenergy systems using energy efficiency indicators
The sustainability of agro-bioenergy systems is dependent on many factors, some local or regional in implementation, some others global in nature. While previous sustainability assessments of agro-bioenergy systems focus on the effects of alternative biomass feedstock (e.g. grass, manure, wood, straw and crops); the impacts of alternative conversion technologies (e.g. fermentation, anaerobic digestion, gasification), and the influence of indirect energy investments in the production of farming inputs (e.g. fertilizer, pesticides, lime, machineries); this study focus on the effects of local and regional factors. The local and regional factors whose effects were examined by this study include alternative agronomic factor options, alternative agricultural production systems and alternative biomass flows.
Alternative agronomic factor options whose effects on the sustainability of agro-bioenergy systems were considered under this study include alternative farm power options (Tractors VS. Humans VS. Animals); alternative fertilizer options (Synthetic fertilizers vs. Animal manure vs. Biogas digestate); alternative tillage options (Conventional tillage vs. Reduced tillage vs. No tillage); alternative seed sowing options (Native seeds vs. hybrid seeds vs. Genetically modified seeds), and alternative irrigation options (Rain-fed vs. Surface vs. Sprinkler vs. Drip). Alternative agricultural production systems whose impacts on the sustainability of agro-bioenergy systems were assessed by this study are the low-intensity eco-agricultural systems and the high-intensity industrialized agricultural systems. Alternative biomass flows whose influence on the sustainability of agro-bioenergy systems were examined by this study are the harvested grain, extracted residues and the unextracted residues. This study assessed the effects of these local and regional factors on the sustainability of agro-bioenergy systems using energy efficiency indicators. The energy efficiency indicators employed for assessing the effects of local and regional factors within this study include the net energy gain (NEG) and the energy return on energy invested (EROEI) indicators. NEG and EROEI energy efficiency indicators measure the capacity of energy sources to support continuous socio-economic functions e.g. food production, domestic and industrial heating, commercial and industrial services, lighting of homes and so on.
Within the frame of this study, maize ethanol and maize biogas production systems across generic agro-climatic zones (i.e. tropics - 0-23.5o N and S, sub-tropics - Latitude 23.6-40 o N and S, and temperate - Latitude 40.1-60 o N and S) were chosen as case studies. This is because they are widespread globally. Generic agro-climatic zones were chosen as spatial scale for this study in order to factor in the productivity of agricultural biomass systems globally.
The methodology used for assessing the effects of alternative agronomic factor options on the sustainability of agro-bioenergy systems involve a life cycle assessment substitution approach. This life cycle assessment substitution approach considered the relative substitution effects of changing from certain agronomic factor options to the others. The impacts of alternative agricultural production systems on the sustainability of agro-bioenergy systems were assessed by undergoing a broad classification of agricultural production systems. These broad classifications of agricultural production systems were based on the likely patterns of adoption of alternative agronomic factor options, as observed in literatures. The individual effects of adopting different agronomic factor options (based on the agricultural production system classifications) were summed up to obtain the overall effects of the different agricultural production systems on the sustainability of agro-bioenergy systems. The influence of alternative biomass flows on the sustainability of agro-bioenergy systems was examined by conducting a human appropriation of net primary production (HANPP) based biomass flow inventory of agricultural biomass flows that may be available for bioenergy production. This biomass flow inventory is followed by a life cycle inventory of energy inputs and energy outputs associated with using such agricultural biomass flows for bioenergy production.
In order to calculate the effects of local and regional factors on energy efficiency of agro-bioenergy systems (using NEG and EROEI indicators), we obtained data on energy conversion factors from various literature sources. Conversion factor data for the estimation of different biomass flows (i.e. harvested grains, extracted residues and unextracted residues) across the generic agro-climatic zones considered were obtained from Scarlat et al., 2010 and IFF, 2014. Maize yield data across generic agro-climatic zones were obtained from IIASA, 2000a, IIASA, 2000b and FAO, 2016. The data on the best N, P and K fertilizer management practices for maize production were obtained from Heisey & Mwangi, 1996, Msarmo & Mhango, 2004, Belfield & Brown, 2008 and IFA, 2011.
Results from this study suggests that conservative choices of agronomic factor options will enhance the energy efficiency and sustainability of agro-bioenergy systems. Such conservative choices of agronomic factor options include the use of lower HP tractors (10-20 HP), human and animal labour, fertilizers from waste sources (animal manure and biogas digestates), native seed sowing options, reduced and no tillage options, as well as rain-fed and surface irrigation systems. In comparison to large scale, high-intensity industrialized agricultural production systems, which are by far more energy intensive, adopting a low-intensity, eco-agricultural production system on a small scale can help improve the energy efficiency and sustainability of agro-bioenergy systems. The use of unextracted residues is not only a very energy inefficient (with low EROEI and negative NEG values) option, it also conflicts with biodiversity conservation concerns e.g. food provision for primary consumers and decomposer who contribute to the improvement of soil structure for better aeration and drainage. The use of harvested grain for bioenergy can be quite energy efficient with EROEI values of above 2 after bio-refinery plant stage, and positive NEG values. It may however compete with human and livestock food chains. The reuse of extracted residues for bioenergy is also quite energy efficient with EROEI values of above 2 after bio-refinery plant stage, and positive NEG values. It also do not conflict with food provision and biodiversity conservation concerns. However, higher demand for the use of crop residues as soil cover (especially in arid regions or areas with increasing trends of aridity) might eventually still affect the sustainability of the reuse of extracted residues for bioenergy adversely. This is because the use of crop residues as soil cover is important for preserving soil carbon and maintaining crop yields. Limited recovery and reuse pathways for extracted crop residues also casts some doubts on the feasibility of its continuous use for bioenergy production.
In conclusion, the way upward is looking downwards. In other words, the key to enhancing the energy efficiency and sustainability of agro-bioenergy systems is paying attention to local and regional factors such as alternative agronomic factor options, alternative agricultural production systems and available biomass flows.