Biomass such as agricultural and forestry residues, animal manure and municipal solid wastes can be a replenishable and widely available source of energy. Harnessing this resource can have significant energy and environmental benefits.
Messages for Policy
Maximizing either net energy or emissions reduction from waste-to-energy generation would lead to better utilization of wastes and residues relative to simply maximizing the quantity of renewable energy.
National-scale mandates on specific types of bioenergy or biofuels could lead to inefficient use of biomass resources.
Complementing renewable fuel targets with GHG taxes or life cycle emissions-based performance standards would lead to better outcomes in terms of clean energy production and emissions reduction.
based on B. Liu & D. Rajagopal Nature Energy https://doi.org/10.1038/s41560-019-0430-2 (2019).
The policy problem
Wastes and biomass residues can be used to derive a number of alternative useful products through different conversion pathways and are widely available and replenishable. The various conversion pathways are currently at different stages of technical and economic maturity. Identifying the optimum product to derive or pathway to use based on varying goals from emission reduction to economic efficiency is complex. From a policy perspective, it is important to understand what factors determine the energy and climate benefits of waste conversion, the potential aggregate energy and climate benefits of waste conversion, which pathways simultaneously maximize energy and climate benefits, and how this varies given the differing availability and composition of waste in differing locations (Fig. 1). Such information is needed to design policies that lead to the best use of biomass resources while mitigating unintended negative consequences.
We find that the type and quantity of energy consumed during biomass processing and the environmental footprint of the displaced products are key to determining the most environmentally beneficial use of any given biomass resource. We estimate that the utilization of all available wastes and biomass residues in the contiguous US can generate 3.1–3.8 EJ of renewable energy but deliver only 2.4–3.2 EJ of net energy gain if energy generation is maximized, and displace 103–178 million tonne CO2e of Greenhouse Gas (GHG) emissions. For any given waste feedstock, looking across all US counties where it is available, no single conversion pathway simultaneously maximizes renewable energy production, net energy gain and GHG mitigation except in rare instances. Maximizing the energetic and environmental benefits of waste conversion requires a life cycle assessment based of different technology pathways taking into consideration the spatial distribution of biomass resources and local conditions (such as the electricity mix).
This analysis compares the energy and climate benefits of a large set of feasible technology pathways for energy recovery from waste from a systems perspective — using a life cycle assessment with a consistent system boundary and region-varying inputs. This analysis quantifies life cycle GHG emissions and energy production for 15 conversion pathways and 29 waste feedstocks at both the county and national levels in the US. It also identifies the most efficient pathways for utilizing each distinct type of waste and biomass residue with respect to renewable energy production, net energy gain or GHG emissions reduction. In addition, it estimates the aggregate energy and climate benefits when all available wastes and biomass residues across the contiguous United States are dedicated for specific policy objectives.
Rajagopal, D. & Zilberman, D. Environmental, economic and policy aspects of biofuels. Found. Trends Microecon. 4, 353–468 (2008). Provides a comprehensive review of the environmental, economic and policy literature on biofuels at the peak of the biofuel renaissance of the 2000s.
Campbell, J. E. & Block, E. Land-use and alternative bioenergy pathways for waste biomass. Environ. Sci. Technol. 44, 8665–8669 (2010). Analyzes the alternative uses of biomass residues with respect to environmental and energy security outcomes and highlights the role of electricity inputs and avoided emissions in determining the best use of biomass.
Tonini, D., Hamelin, L., Alvarado-Morales, M. & Astrup, T. F. GHG emission factors for bioelectricity, biomethane, and bioethanol quantified for 24 biomass substrates with consequential life-cycle assessment. Bioresour. Technol. 208, 123–133 (2016). Estimates greenhouse gas emission factors for bioethanol, biomethane and bioelectricity derived from different biomass resources.
This study would not have been possible without financial support from the UCLA Grand Challenges—Sustainable LA programme.
The authors declare no competing interests.
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Rajagopal, D., Liu, B. The United States can generate up to 3.2 EJ of energy annually from waste. Nat Energy 5, 18–19 (2020). https://doi.org/10.1038/s41560-019-0532-x
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