Answering questions about SAF benefits
Sustainable Aviation Fuels (SAF) are claimed to reduce jet CO2 emissions by up to 80%. Yet it is also argued that SAFs are not a “clean” aviation solution. How can these contradictory claims be reconciled?
A common (but incorrect) assumption is that SAF emits less greenhouse gas than fossil jet fuel when burned. During combustion, SAF emits the same amount of CO2 as fossil jet fuel! The proposed benefit of SAF is that CO2 is drawn from the atmosphere during its production, which, in principle, cancels out the amount emitted when burned and therefore hypothetically results in net-zero emissions.
Massport presents the comforting assertion that SAF technologies “have the potential to dramatically reduce lifecycle aviation emissions to near zero by 2050.” Yet no published analysis supports this statement. The International Civil Aviation Organization projects that even with a major effort to develop a SAF market, global CO2 aviation emissions will increase by 2050.
There are three known ways to make SAF, and each has intractable problems that prevent it from realizing its hypothetical climate benefit. In fact, most types of SAF today, and for the foreseeable future, create MORE CO2 than fossil jet fuel does. Each of the three types of SAF will be examined, and the issues explained in the following sections.
Cooking oil and waste-based SAF can produce only a tiny fraction of SAF requirements
This type of SAF requires the least effort to produce and represents the majority of production today. Waste cooking oil and animal fats can be refined into SAF using the so-called “HEFA” process. SAF represents less than 0.1% of jet fuel today; any fuel claimed to be “SAF” today is primarily fossil jet fuel, with a few percent of oil-based SAF added.
The problem with these waste feedstocks is that the amount available in the world is tiny in comparison with aviation fuel requirements. These feedstocks are also used to make biodiesel for ground transportation, which consumes much of the available supply. Due to the limited supply, oil-based SAF represents a dead-end technology because it cannot scale. To put this into perspective, the global demand for fuel is approximately 4,500 Megatons, while the global supply of waste cooking oil is around 15 Megatons. Furthermore, many of these waste products are today sold for other industrial and food purposes. As they are diverted to the production of SAF, the existing uses of these wastes need to find alternative feedstocks, most of which would be responsible for additional CO2 emissions.
Due to limited feedstock supply, SAF based on waste oil and fats cannot be considered a significant contributor to a climate solution for aviation.
Crop-based SAF competes with food production and forests
Another proposed source of SAF feedstock is crop-based. Crops like corn and soybeans can be harvested and processed to make biofuel, including SAF. The two principal processes are named “Alcohol to Jet” and “Fisher Tropech.” These methods have three major problems.
The first major problem is that its production directly competes with food crops. A single Boeing 737 using crop-based SAF would consume approximately 10,000 Tons of SAF per year, requiring 40,000 acres of cropland per year. All commercial flights together are projected to consume nearly 100 billion gallons of fuel per year; if they all used SAF, it would require nearly 2 billion acres of arable land, an area representing nearly ½ of the world’s available cropland. Such a use of dwindling farmland would displace and disrupt food supplies. The only feasible solution to this problem is to create extensive new cropland for SAF by cutting down forests.
The second major problem arises from the need to cut down forests to obtain the necessary cropland. SAF from cropland created by displacing forest is much worse for the environment than fossil jet fuel. Forests are natural absorbers of CO2, removing approximately 2 Tons of CO2 per acre per year. Cutting down the amount of forest to supply a single Boeing 737 for a year causes a permanent increase of 80,000 tons of CO2 per year. The same Boeing 737 using fossil jet fuel only generates about 30,000 tons of CO2 per year. Therefore, any aircraft supplied from SAF that displaces forests contributes more than twice as much CO2 to the planet as an aircraft powered by fossil jet fuel! It is better for the environment if we leave the forest alone and continue burning fossil jet fuel!
The third major problem with crop-based SAF is that even if the above two serious problems were overcome, this fuel cannot reach the theoretical ideal of CO2 reduction. Fertilizer production, harvesting, soil changes, and transport are just some of the CO2 contributors that offset a significant part of the potential gain, and the resulting fuel still has CO2 emissions of approximately half of fossil jet fuel.
The European Union understands these issues, and specifically excludes the use of crop-based fuels for aviation because the effects on land-use and food competition are so devastating. Unfortunately, this type of fuel is often proposed as a possible solution for the next decades, yet any attempts to increase its use are worse for the environment than fossil jet fuel.
Synthetic SAF uses an enormous amount of electricity, which has major GHG effects
Synthetic SAF, also known as e-fuel, is the only type of SAF that can scale to meet global aviation requirements because it requires only air, water, and energy. Through a multi-step process, CO2 is removed from the air, combined with hydrogen extracted from water, and assembled into hydrocarbon jet fuel using heat and catalysts. This type of fuel is not yet produced or available for purchase, and has only been demonstrated in laboratories. Many proposals for reducing future GHG emissions through SAF have recognized that crop-based and waste-based SAF are impractical, relying on the assumption of a scale-up of e-fuel SAF. The International Civil Aviation Organization projects that, under the most aggressive scenario, this fuel will only begin to be produced around 2050. However, it has a major problem, which will likely prevent it from providing any carbon benefit before 2100.
The problem with e-fuel is that it requires about three times more electrical energy to produce than the energy contained in the resulting e-fuel. If energy were free and zero carbon, this would not be a problem. But energy will never be free, and our electrical energy sources are unlikely to be zero-carbon before 2100.
In Massachusetts, our electrical use generates 0.4kg of CO2 for every kWh we use. If we were to apply power from the current grid to make e-fuel to meet the Commonwealth’s current jet fuel consumption, we would need over 50 TWh of electricity. This additional amount is more than twice the total of what we generate today! The power required to make this SAF would be responsible for over 21 Megatons per year of CO2. The resulting e-fuel would save only 6.0 Megatons of CO2 from the fossil jet fuel it replaced. The increase in emissions associated with the required electricity generation greatly exceeds any emissions savings from using the e-fuel in aircraft! The result is that conversion to e-fuel today makes no sense because it would more than triple the CO2 emissions from fossil aviation.
Most proposals regarding future use of e-fuel attempt to avoid this issue by saying that dedicated new clean electrical generation will be used for e-fuel. The amount of electrical generation required to supply Massachusetts with e-fuel is equivalent to ten of the retired Pilgrim Nuclear Plants. If we did develop such a supply of clean electricity, and devoted it to SAF production, it turns out that we would have much larger CO2 emissions than if we had used that clean energy to offset fossil generation on the grid! Using clean energy to displace natural gas generation on the grid results in MUCH greater CO2 reductions than applying the same energy to making SAF! This condition continues until clean energy displaces all the baseload natural gas generation on the grid. There is currently no chance that this will occur by 2050, and it is unlikely to occur before 2100.
As we develop clean electrical generation, we must apply it where it causes the highest CO2 reduction, such as displacing gas generation, electric cars, heat-pumps, etc. These applications, and many others, provide MUCH larger CO2 reduction per kwhr than SAF production. This is because SAF production is so inefficient.
An additional problem is cost. E-fuel is forecasted to cost about $4 per gallon more than fossil jet fuel. Converting Massachusetts’ jet fuel consumption to e-fuel would require making it competitive with fossil jet fuel, which would require an investment of between 1 and 5 billion dollars per year. Conversion to e-fuel requires either mandates, subsidies for e-fuel, or carbon fees on fossil jet fuel. The amount of money needed is much greater than the total amount the Commonwealth spends on transportation, and almost certainly unworkable at the state level; such subsidies would need to be implemented at the national level, which is very difficult in today’s policy environment.
It is clear that e-fuel cannot reduce aircraft emissions by any significant amount this century; it cannot provide an emissions benefit until the entire electrical and industrial heat system is decarbonized, which is unlikely before 2100.
Summary
SAF, of any type, does not reduce CO2 effects due to processing-related emissions. If today’s crop-based SAF were produced at a sufficient quantity to supply the aviation industry, it would require a huge amount of cropland, which would either directly replace forests or push some other food crop to replace forests. The loss of carbon sequestration caused by the destruction of forests completely overwhelms any theoretical benefit of SAF. Synthetic SAF, or e-fuel, only begins to provide a climate benefit when the electrical grid is decarbonized, which is unlikely before 2100. Any representations that e-fuel or any type of SAF is environmentally beneficial today, or in the near future, are false and misleading.