Will slow and steady win the race for alternative jet fuels?
Bioenergy can solve some of our climate problems, but not all of them at once
Bioenergy has become a popular decarbonization strategy—so popular that policy makers and researchers in several sectors are claiming biofuels, bio-electricity, and bio-heat can deliver a large portion of the greenhouse gas (GHG) reductions needed to meet climate goals. Biofuels have been promoted for GHG reductions in the road sector in countries around the world, mostly notably the European Union, United States, and Canada. The EU and Canada incentivize the use of biomass in heat and power to meet climate goals in those sectors. The International Civil Aviation Organization (ICAO) has presented alternative fuels as a key part of their strategy to eliminate GHG emissions from the aviation sector over the long-term. In addition, some researchers count biofuels as a large part of the decarbonization strategy for the marine sector.
A clear-eyed look at the physical, environmental, and technological constraints on production of biofuels, and at the likely demand for them, suggests that we need to temper our enthusiasm. Bioenergy can be a key part of a global strategy for reducing GHG emissions, but it’s not the panacea for all our climate ills. Putting too much hope in bioenergy may well be counterproductive.
The maximum plausible amount of low-carbon biomass that could be supplied for energy is around 90 exajoules (EJ) per year, or roughly 50-60 EJ in delivered transport fuel, heat, and electricity, assuming future improvements in conversion efficiencies. This falls far short of the expected global energy use of 650 EJ per year by 2050. Even with efficiency improvements and rapid deployment of wind, solar, and other renewables, bioenergy can’t eliminate global GHG emissions. If we only have so much bioenergy available, how will it be divvied up?
In earlier research, we projected that most future biomass would be used for heat and power by extrapolating from European renewable energy plans and a U.S. energy forecast. Recent trends in biomass use across sectors show a different picture. Data on biomass use in transport, centralized electricity, and heat production from Eurostat and the U.S. Energy Information Administration show the share of bioenergy in transport rose from 39% in 2007 to 47% in 2016 in the EU and U.S. combined. If we assume a linear increase in biomass consumption by each of these sectors to 2050, transport would consume 54% of the total bioenergy available.
While there are some reasons to expect continued growth in the use of biomass in heat and power, there are even better reasons to be skeptical. Biomass can supply baseload power to counter the variability of wind and solar electricity supply. However, improving long-distance electricity transmission capacity, something currently being explored in the EU, can effectively do the same thing by allowing excess wind and solar production in one region to supply another with a local shortfall. Importantly, while wind and, especially, solar electricity prices are expected to fall drastically over the coming decades, there are much higher barriers to technology improvements in biomass production, making it less likely those costs will fall comparably. Bio-electricity may thus only be competitive in areas with high local biomass supply. Biomass use in heat generation, on the other hand, is cost effective in many regions and may remain competitive with other heat sources for some decades to come. With this in mind, it seems likely that the use of biomass in electricity in particular may not continue to grow at the current rate. We thus might assume 75% of biomass produced for energy will be available for transport, amounting to 70 EJ (40 EJ post-conversion) in a maximum-production scenario.
There’s one other non-transport use of biomass to consider: bio-plastics, the production of which has been increasing in recent years as petroleum consumption declines. Half of all projected plastics production in 2050 would consume approximately 10 EJ worth of biomass in 2050. Now we’re down to 60 EJ per year in available biomass for transport, which could supply roughly 35 EJ per year in delivered fuel.
How will the 35 EJ per year be divvied up among the three main transportation sectors: road, aviation, and marine? Currently, virtually all transport biofuel is used in the road sector for a simple reason: it’s the cheapest. Aircraft can only burn a few types of fuel, and the only widely-available type that it can use, hydroprocessed esters and fatty acids (HEFA), is more expensive than the ethanol and biodiesel used in the road sector today. Because airlines are more sensitive to fuel price than most drivers, they are less willing to pay the incremental costs of clean fuels. Marine engines might be able to accept a wider range of biofuel types, but it’s hard for any kind of biofuel to compete economically as long as low-priced bunker fuel remains the norm. Therefore, biofuels will likely continue to flow into the road sector as long as there is liquid fuel demand for cars and trucks.
We expect road fuel use to decline due to improving vehicle fuel efficiency and increasing market penetration of electric vehicles. In one of the International Energy Agency’s (IEA) most optimistic (“improvement”) scenarios, around 75% of new vehicles sold globally in 2050 will be electric or plug-in hybrids. Even in that scenario, the road sector would still require around 25 EJ per year of liquid fuel. In less optimistic projections, the amount of liquid fuel demand from the road sector would far outstrip the 35 EJ per year of biofuel we estimate could be available. Given the cost advantages, such scenarios could leave no biofuel for aviation and marine.
If, on the other hand, we are optimistic about vehicle electrification, a maximum of 10 EJ per year of liquid fuel could be available for aviation and marine. Whatever the uncertainties concerning fuel types and economics for marine biofuels, it seems likely that this sector will consume some of the remaining biofuel resource—say half, for simplicity. This leaves around 5 EJ per year of biofuel for aviation. This amount could offset a substantial fraction of energy demand in aviation, reducing GHG emissions from fuel burn by around 20% after efficiency improvements are made. But while that would represent a substantial dent in aviation emissions, it does not represent a solution even to that comparatively small part of the GHG problem.
Even if we can maximize sustainable bioenergy production, cross-sectoral competition is high, as summarized in the figure below. There’s simply not enough bioenergy to decarbonize all these sectors together. We should expect to continue consuming biomass in many different uses across the economy. It’s very unlikely that any single sector will be the sole consumer of bioenergy in 2050 or beyond – so no policy target should count on the entire resource.