The Challenge

Wastes and biomass residues from agricultural, dairy, forestry and household activities are sustainable energy resources that are widely available and replenishable. They can be used to derive alternative energy products like electricity, heat and bio-jet fuel. However, the transport and storage of waste are costly and processing it requires substantial energy. In order to consider wastes and biomass residues as potential alternative energy sources, there needs to be a holistic assessment of their potential energy production, net energy gain, and greenhouse gas emission reductions. This project aims to undertake this assessment in the contiguous United States.

The Solution

The research team incorporated a comprehensive, life-cycle assessment (LCA) perspective to analyze three distinct scenarios of optimal use of biomass wastes by waste type, at a much larger scale than any other previous studies. The three scenarios they looked at were MEP (max. energy production), MNE (max. net energy), and MER (max. emission reduction). The waste type was divided into four broad categories, namely agricultural residues, animal manure, forest residues and municipal solid wastes. As a result, the research team developed the first-ever database tool --Greenhouse Gases, Regulated Emissions, and Energy Production from Waste Resources (GREW) --to systematically compare the energy and emissions impacts of converting wastes and residues to biofuels in the U.S.

Results

• The use of all available wastes and residues in the contiguous U.S. can potentially generate 3.1 to 3.8 exajoules (EJ) of renewable energy each year. This is equivalent to 3.7% of total U.S. energy demand in 2016 or the amounts of energy consumed separately by Washington and Oregon in 2017.
• Additionally, the use of all available wastes and residues can potentially displace 103 to 178 million metric tons of carbon dioxide-equivalent greenhouse gas emissions. This is an amount comparable to taking 37 million passenger vehicles off the road, given typical passenger vehicle emissions of 4.6 metric tons of carbon dioxide each year.
• No one method of bioenergy production maximizes net energy gain, renewable energy gain, and climate benefits. Some create more renewable energy but require more energy to do so–resulting in less overall greenhouse gas emissions savings. Thus, identifying the optimal bioenergy application in any situation depends on the intended outcome(MEP, MNE, or MER).

Next Steps

While there are several policies in California (e.g. AB 32, LCFS, AB 1900, SB 1383) and at the federal level established to reduce greenhouse gas emissions and mitigate climate change, none target optimal use of waste biomass. This research project demonstrates there is a clear need for such a policy, given synergies and trade-offs between renewable energy and different environmental objectives for different resources, alongside spatial variations across the U.S.

Researchers envision the GREW database tool developed as part of this research can aid useful policy insights at a local, regional and national level, as it is also easily accessible to users for modification.

Publications and Reports

Liu, B., & Rajagopal, D. (2019). Life-Cycle Energy and Climate Benefits of Energy Recovery from Wastes and Biomass Residues in the United States. Nature Energy, 4(8), 700-708. doi:10.1038/s41560-019-0430-2

Rajagopal, D., & Liu, B. (2020). The United States can Generate up to 3.2 EJ of Energy Annually from Waste. Nature Energy, 5(1), 18-19. doi:10.1038/s41560-019-0532-x

Science Journal for Kids
Waste to Energy: How can we get the most from our waste?