Charting a pathway to next-gen biofuels

These next-generation biofuels are emerging as a more sustainable substitute for fossil fuel-derived gasoline and diesel that, if carefully managed, could remove more greenhouse gases from the atmosphere than they emit over the course of their lifecycle. And unlike conventional or first-generation biofuels, which are produced from crops like corn and sugarcane that could otherwise be used for food, second-generation biofuels are derived from agricultural waste or non-food crops grown on low productivity or recently abandoned land.

Yet as a still-nascent technology, these next-generation fuels must contend with considerable uncertainty about their role in a low-carbon energy future.

Previous studies on biofuels tend toward two extremes, either focusing on the 'bio' -- incorporating crop growth, productivity, and land use data without considering downstream supply chain concerns in detail -- or the 'fuels' -- mapping out a supply chain and biorefinery design using overly simplistic land and crop data.

The Princeton study unites the two perspectives to provide a more comprehensive forecast of a supply chain for biofuels across an eight-state region in the Midwest, grounded in highly detailed data. Their findings were published May 22 in Nature Energy.

"What we're doing with this study is bringing together two different approaches to studying biofuels," said Christos Maravelias, the Anderson Family Professor of Energy and the Environment and professor of chemical and biological engineering. "A lot of high-quality data at fine spatial scales went into our analyses, giving us a much more holistic view of these systems."

Optimization from crop growth to sequestration site

Supply chains for biofuels are complex. Feedstocks for biofuels must be grown and harvested from a fragmented network of land. Those feedstocks must then be transported to a centrally located refinery. At the refinery, several different technologies could convert the plant matter into liquid biofuel, and any carbon emissions produced through the conversion process can be captured and subsequently sequestered offsite.

Consequently, decisions made at every point along the supply chain could result in systems with widely diverging costs and emissions impacts, from the crop chosen as a feedstock to the distance between field and refinery and the technology used to convert the plant into biofuels.

"Even seemingly isolated or unrelated decisions, like how much incentive you plan to provide for carbon capture or which conversion technology you favor, can have dramatic impacts on the landscape design of a bioeconomy," said co-author Caleb Geissler, a graduate student in chemical and biological engineering.

Thus, Geissler said, the optimal landscape design depends on the starting goals: what quantity of biofuels should be produced, at what cost, and at what carbon intensity?

While the researchers cautioned that their model was not designed specifically as a decision-making tool, Maravelias said it provides valuable insights into the economics and environmental impacts of a future bioeconomy. And since second-generation biofuels have yet to achieve widespread commercialization, proactive research now can inform efforts to ensure the fuels are meaningfully implemented into the future energy system.

"The model accounts for all the components of the system, so we can use it to answer many different types of questions," said Maravelias. "We can use it to identify the optimal way to produce a certain quantity of biofuels while minimizing economic costs. We can use it to identify the system that produces the same amount of fuel while minimizing environmental impacts. Or we could have it design a system that strikes some balance between the two."

Highlighting the impact of policy

Using their model, the research team could also probe the role of policy incentives in shaping the preferred technologies and emissions impact of a biofuels supply chain.

For instance, the team found that the 45Q tax credit for carbon capture, which provides $85 per ton of sequestered carbon, sufficiently incentivized carbon capture across the system. However, tax credit values below $60 per ton of carbon -- the 45Q tax credit was only worth $50 prior to the Inflation Reduction Act of 2022 -- were insufficient to drive investment in carbon capture and sequestration. In this case, the system generated rather than removed carbon emissions, though it still produced far fewer emissions compared to today's fossil fuels.

"Even if the value of an incentive changes, we still wanted our results to be informative," said Geissler. "It's also a way to inform policymakers about how varying incentives support different technologies and configurations for the system."

And while current incentive schemes only assign a monetary value for the carbon captured at the refinery itself, the researchers also modeled alternative scenarios that sought to minimize emissions from the entire supply chain, including both direct emissions from transportation and indirect emissions embodied in the electricity used to power the system.

These alternative scenarios highlighted even more tradeoffs. The tax credit would have to be worth at least $79 a ton to begin incentivizing carbon capture at the refinery and worth around $100 per ton for carbon capture to be installed at every refinery. Below those values, it would often be more cost effective to reduce transportation and offset emissions from purchased electricity before investing in carbon capture.

The researchers even charted pathways that mitigated carbon emissions beyond financial incentives, using site-specific soil carbon sequestration potentials and management decisions, such as whether to fertilize, to yield a landscape design with the greatest overall environmental benefits.

"Because these next-generation biofuels are still emerging as a technology, the model we developed allows us to make sure we're designing these systems properly," Maravelias said. "It's important to have as much information as possible now, before we lock ourselves into less-than-ideal technologies and system configurations."