Environmental implications of the use of agro‐industrial residues for biorefineries: application of a deterministic model for indirect land‐use changes

Abstract

Biorefining agro-industrial biomass residues for bioenergy production represents an opportunity for both sustainable energy supply and greenhouse gas (GHG) emissions mitigation. Yet, is bioenergy the most sustainable use for these residues? To assess the importance of the alternative use of these residues, a consequential life cycle assessment (LCA) of 32 energy-focused biorefinery scenarios was performed based on eight selected agro-industrial residues and four conversion pathways (two involving bioethanol and two biogas). To specifically address indirect land-use changes (iLUC) induced by the competing feed/food sector, a deterministic iLUC model, addressing global impacts, was developed. A dedicated biochemical model was developed to establish detailed mass, energy, and substance balances for each biomass conversion pathway, as input to the LCA. The results demonstrated that, even for residual biomass, environmental savings from fossil fuel displacement can be completely outbalanced by iLUC, depending on the feed value of the biomass residue. This was the case of industrial residues (e.g. whey and beet molasses) in most of the scenarios assessed. Overall, the GHGs from iLUC impacts were quantified to 4.1 t CO 2 -eq.ha À1 demanded yr À1 corresponding to 1.2-1.4 t CO 2 -eq. t À1 dry biomass diverted from feed to energy market. Only, bioenergy from straw and wild grass was shown to perform better than the alternative use, as no competition with the feed sector was involved. Biogas for heat and power production was the best performing pathway, in a short-term context. Focusing on transport fuels, bioethanol was generally preferable to biomethane considering conventional biogas upgrading technologies. Based on the results, agro-industrial residues cannot be considered burden-free simply because they are a residual biomass and careful accounting of alternative utilization is a prerequisite to assess the sustainability of a given use. In this endeavor, the iLUC factors and biochemical model proposed herein can be used as templates and directly applied to any bioenergy consequential study involving demand for arable land.

GHG emission factors for bioelectricity, biomethane, and bioethanol quantified for 24 biomass substrates with consequential life-cycle assessment

“…This is in accordance with the results of previous studies showing the benefits of avoiding conventional manure management through codigestion with manure (e.g. Tonini et al, 2015). Yet, as highlighted in Tonini et al (2015), the GHG savings of co-digestion may be completely determined by the share of manure itself.…”

“…Therefore, accounting for the effects of diverting them from their current use/function to bioenergy production is fundamental in order to avoid overestimating (or underestimating) the environmental savings induced in sectors other than the energy's (Tonini et al, 2015;. This refers to the so-called "lost opportunity", i.e.…”

“…The current use/function considered for each biomass is presented in Table 1. The lost opportunity for wild grasses and agro-industrial residues was modelled as described in Tonini et al (2015), and as described in for animal manure.…”

“…Bioethanol production was modelled following the approach of Tonini et al (2015). It was assumed to occur from C6-sugars hydrolyzed from cellulose.…”

“…The emissions following the use on-land of digestates (organic residues) from anaerobic digestion were modelled as in Tonini et al (2015) …”