Evaluating Bioenergy Potential to Tackle Maritime & Air Transportation

Decarbonizing maritime shipping and air transportation would be an important step forward in moving the needle on climate change. These two transportation sectors, which are important backbones to trade and travel, are heavily dependent on fossil fuels and account for a significant portion of global greenhouse gas (GHG) emissions. Currently, both modes are relying on “drop-in” (renewable) hydrocarbon fuels with high energy density for the foreseeable future. The use of biomass – renewable organic material coming from plants and animals – to produce this “drop-in” and low-carbon liquid fuel presents an opportunity to reduce GHG emissions. A new MCSC white paper explores the potential total energy that may be supplied by energy crops and agricultural residues. The authors, MIT researchers Katie Daehn, Evan Coleman, Lama Aoudi, and Florian Allroggen review the wide-ranging estimates in the literature and explore key sources of uncertainty.

“Understanding the potential supply of biomass is important as multiple industries look to scale-up its use as a fuel, as well as a non-fossil carbon feedstock for plastics and chemicals,” said Daehn. There is a large body of literature exploring this question of global biomass availability under various scenarios and assumptions – and thus a range of outlooks on whether biomass could substantially contribute to displacing fossil fuels.

“We wanted to explore these underlying assumptions, particularly for agricultural residues, to understand what was driving these different versions of the future,” Daehn explained.

Agricultural residues are the parts of a crop plant that are not eaten – such as husks, stems and leaves. The potential supply of crop residues is massive (7.7Gt/yr globally, as reported by the United Nations Environment Programme), and there is interest amongst policy makers and industry whether these residues could be better utilized. Dedicated statistics on crop residues are not measured, however, which introduces uncertainty.

Today, agricultural residues are used mainly for animal husbandry (as feed and bedding) and on the field to provide nutrients to the soil and protect against erosion. Practices vary greatly farm to farm, and there is little data to infer an overall understanding, and how crucial these residues are in the current use.

Some questions that then arise include: Are residues an integral part of animals’ diets? What would substitute them if demand grows? In the context of regenerative agriculture, ‘no till’ practices to enhance the carbon content of agricultural soils require more residues to be left on the farm, though evidence on the best practices and conditions is not fully established. Further, the ratio of residues to the edible product varies greatly between crops (e.g. from potatoes to tomatoes). Cereals constitute the majority of crops grown globally and are the largest source of residues. The residue ratio can also change depending on growing conditions, and over time this ratio has been optimized to favor the edible product (e.g. cereal grain). Thus, the future supply of residues depends on the crops we choose to grow and their yields.

“Overall, there is a lot of uncertainty from multiple fronts: our current understanding, and projecting future trends,” said Daehn. “By exploring these sources of uncertainty, we hope to identify where future research effort and standardization would be most helpful.”

Looking at the overall system, one clear lever to increase the future supply of biomass (either with energy crops or residues) is to reduce meat consumption. About half of agricultural land (and 20% of total land on Earth) is used for livestock, and at least 6Gt/yr of crops are consumed by livestock. Globally, meat consumption is growing, but there is strong evidence that limiting meat consumption will be key to reduce agricultural emissions and enhance conservation. This would in turn reduce pressure on the global agricultural system and allow more crops and land to be used for energy and bio-materials.

As the authors of the white paper conclude, while using crops and residues may not cover anticipated demand in tough-to-decarbonize transportation completely, and there are several obstacles, it is important to continue to identify the conditions in which bio-feedstocks make sense – and establish frameworks and incentives to support the best practices.