One of the most studied topics over the biomass valorization from a life cycle assessment perspective is the waste-to-energy (WtE) approach. This topic appears as a relevant point because it addresses a critical issue related to the final disposal of wastes towards a circular economy of resources and energy recovery. WtE is part of the modern management of wastes and can reduce the dependence on fossil fuel, as well as the minimization of landfilling treatment. In this regard, thermal WtE technologies, such as pyrolysis and gasification are proposed as improved energy efficiency techniques, which also reduce environmental burdens compare to classical incineration processes (Panepinto et al. 2014; Zheng et al. 2016). Nevertheless, all these processes should need to be analyzed systematically and holistically, and it is here where the LCA appears as a tool to measure all inputs and outputs of materials and energy "from cradle to grave", including all up- and downstream activities. According to (Zhou et al. 2018), who performed a review analysis of available LCA WtE technologies (i.e. landfill with energy recovery, incineration, pyrolysis, gasification and pyrolysis-gasification), stated that LCA is sufficiently developed and widely accepted for WtE environmental evaluation. In brief, the main conclusions found were:
- The data inventory is every day more concrete and detailed from a mass and energy flows point of view;
- Sensitivity analyses are widely considered to decrease uncertainty;
- Allocation and characterization are grown into different methods;
- The environmental impact results of WtE techniques are lower than those of the conventional municipal solid waste treatment methods.
On the other hand, studies of biomass residues transformation into renewable energies are quite studied from an LCA standpoint. This is the case of the analysis made by Neri et al. (2016), who focused on the assessment of a small Italian municipality to treat wood residues to produce renewable energy. They evaluated the potential environmental impacts linked to resource depletion and human health within the whole biomass handling chain, from the wood collection, transportation, and utilization to produce wood chips. According to the results found, the global environmental impacts (e.g. GHG emissions and non-renewable fuel depletion) reduce if the district heating system in the municipality change from fossil resources to the waste biomass system; however, biomass combustion resulted in the worst effects in terms of toxic substances emitted. Furthermore, transportation contributes to the global impact by 98%, even if distances are limited to a 30 km roundtrip.
Another recent example of the LCA applied to biomass residues valorization can be found in the work done by Kopsahelis et al. (2019). They calculated the environmental impacts of end-of-life dairy products (EoL-DPs) managements throughout co-treatment with agro-industrial wastes (AgW) in a centralized biogas plant in Cyprus. They analyzed two scenarios:
- EoL-DPs co-treatment with different AgW in one-stage mesophilic anaerobic digestion, and;
- The same amount of EoLDPs acidified before methanogenesis with AgW to improve biogas production. According to the LCA results, EoLDPs showed better environmental performance before acidification, compare to the direct co-digestion in a mesophilic digester. Additionally, biogas production upon acidification, and energy yield, was higher compared to the case where no pretreatment was carried out. Nevertheless, further studies must be performed from an environmental point of view in order to extend the system boundaries of the analysis (i.e. they only analyze from a gate-to-gate approach).
Regarding environmental assessments applied to food waste valorization, it is possible to find several scientific references. For instance, Woon et al. (2016) evaluated the valorization of food for 3 types of energy use: i) electricity and heat; ii) city gas; and, iii) biogas fuel as petrol, diesel, and liquified petroleum gas substitute for vehicle use. They based this analysis on data extracted from reports of government and industrial sectors in Hong Kong. One of the main conclusions was that biogas fuel as a petrol substitute for vehicle use shows benefits over the other type of energy use regarding human health and ecosystems. Transforming 1080 tons per day of food waste into biogas vehicle fuel can reduce 1.9% of the GHG emissions in the transport sector in the Hong Kong context.
On the other hand, the waste valorization technologies applied to poultry production was also lately studied from an LCA perspective (Kanani et al. 2020). Poultry industry (including both meat and eggs), is considered one of the industries with the highest growth rates among livestock sectors in the following decades. Primary methods for handling these industry wastes are currently either landfill or rendering for spent hens and mortalities, a landfill for egg-shell sand direct land application of manure as organic fertilizer. This review study identified, categorized, and described current and emerging waste valorization technologies for livestock biomass and assesses their possible applicability for key poultry waste streams from a theoretic viewpoint. As an outcome, this review identified 4 well-developed technologies as potentially suitable for the valorization of key poultry waste streams: i) anaerobic co-digestion; ii) anaerobic mono-digestion (biological technologies); iii) pyrolysis; and; iv) gasification. From an LCA angle, the analyzed studies recommended to systematically calculate the potential net sustainability benefits and impacts of these technologies compared to conventional alternatives, as well as cost comparisons to assess their viability for commercial applications.
Waste biorefineries are also widely studied under a circular bioeconomy approach. Ahmad et al. (2020) performed a critical review of the state-of-the-art biorefinery opportunities beyond traditional methods as a solution in the grape wine industry. They analyzed the current challenges in this sector, such as waste minimization, stems, seed, pomace, wine lees, as well as the biosynthesis of different high-value bioproducts (e.g. phenolic compounds, hydroxybenzoic acids, hydroxycinnamic acids, lignocellulosic substrates, etc). The study was focused on the valorization of winery waste (i.e. solid, liquid, or gaseous) and the LCA was used to find a sustainable solution with value-added energy products in an integrated biorefinery approach, maintaining the environment and circular economy emphasis.
Finally, another interesting research paper linked to LCA applied to waste and biomass valorization was published by Bellon-Maurel et al. (2013). This scientific document summarizes the main issues encounter during the application of the LCA methodology in the before mentioned topic. They identified issues related to: i) goal and scope: the difficulty of choosing the functional unit due to the highly multifunctional nature of such systems, as well as the allocation selections and the need for spatial differentiation; ii) inventory analysis: the prickly issue of modeling complex systems and properly estimating field emissions; iii) impact assessment: the lack of suitable impact categories in LCA (e.g. odor indicator); iv) interpretation: efforts must be set to facilitate the way actor can deal with multicriteria results in LCA.
Information about LCA applied to biomass valorization, as well as its limitations can be found in more 4.2.4.