Fueling the future of flight
The following article appears in the 2012–2013 issue of AeroAstro, the annual report/magazine of the MIT Aeronautics and Astronautics Department. © 2013 Massachusetts Institute of Technology. Also available as a PDF.
Today’s air transport system depends on having a plentiful, secure supply of liquid hydrocarbon fuel. These are typically kerosene-type jet fuels such as Jet A, Jet A-1 or JP- 8, which have a high energy density both by volume and mass. Jet fuels must satisfy a number of important safety-related constraints including thermal stability (to avoid thermal deposits building up in hot components) and freezing point (given the low temperature at high altitude).
Jet fuels in use today are almost exclusively petroleum-derived. While petroleum-derived jet fuel has proven reliable and available at a cost that enables ever-greater sections of society to travel, it has two key disadvantages. First, combustion of fossil fuels results in net CO2 emissions (just over 3 kg of CO2 per kg of jet fuel burned directly, and more if indirect emissions associated with production and refining are considered). CO2 is the primary greenhouse gas, although others are significant too. Additionally, other emissions including SOx, NOx, and particulate matter are a risk to human health and affect the climate. Second, energy security and independence is an increasing concern for a number of countries. This issue is most acute for petroleum-derived fuels, where there is a significant mismatch between producer and consumer countries.
Both of these concerns—environment and energy security—motivate assessing the potential for alternative feedstocks for jet fuel production, and technologies for converting alterna- tive feedstocks to jet fuel. AeroAstro’s Laboratory for Aviation and the Environment (LAE) is researching a broad set of possible feedstocks and feedstock-to- fuel conversion technologies with the goal of quantifying their net environmental impacts and costs. The purpose of this research is to provide independent and impartial information to policy-makers, the aviation and fuel industries, and NGOs on the environmental and economic impacts of alternative jet fuels.
The current civil fleet and associated refueling infrastructure is worth trillions of dollars, and the aircraft that comprise the fleet have a lifespan of some 30 years once in service. Given the length of aircraft design, production, and service lifecycles, it is likely that some current aircraft designs will be in service well beyond the mid-21st century. Furthermore, Jet A and other kerosene-type jet fuels have proven reliable over a long period, and a global transport and handling infrastructure exists for these fuels. This means that the fuels that have greatest potential to significantly impact the environ- mental performance of aircraft for the coming decades are likely to be “drop-in” jet fuels; that is, those that function in existing aircraft and fueling infrastructure and typically meet the current Jet A specification. LAE’s alternative fuels work has a focus on drop-in fuels, but we are also assessing other fuels that would require more major aircraft design and refueling infrastructure changes, particularly cryogenic fuels such as liquefied natural gas.
From feedstock to fuel
In addition to petroleum the current “baseline” fuel against which we compare alternatives, jet fuel feedstocks can be other fossil sources or biomass. These are then converted to drop-in jet fuels through different possible processes.
The Fischer-Tropsch process for converting a “syngas” (carbon monoxide and hydrogen) to a fuel has been around for the better part of a century. It is still very much relevant today and can be used to convert various types of gasified biomass such as switchgrass and willow, or fossil fuels like coal and natural gas, to drop-in jet fuel.
The hydroprocessed esters and fatty acids (HEFA) process converts oils like palm, rapeseed, or soy, or waste animal fats into HEFA jet fuel. For example, hydroprocessing 100 tons of soybean oil can result in 49 tons of jet fuel as well as other products such as diesel and naptha.
Energy-rich sugars can be extracted from sugary (e.g., sugarcane), starchy (e.g., corn grain), or lignocellulocic (e.g., switchgrass) feedstocks. These can then be converted to alcohols via conventional fermentation, which then needs significant additional processing to create jet fuel. Alternatively, advanced fermentation using engineered microorganisms can directly yield a hydrocarbon that is relatively similar to jet fuel. Thermochemical methods to extract sugars in aqueous phase are also being developed.
Finally, oils suitable for hydroprocessing into a component of jet fuel can be created by pyrol- ysis (oxygen-free “burning”) of feedstocks such as forest residues. Pyrolysis tends to result in aromatic hydrocarbons, which are a necessary component of jet fuels today. As such, for a fully synthetic jet fuel, pyrolysis could be used to create the aromatic component with other processes being used for the bulk of the fuel.
When an alternative jet fuel is burned it results in direct emissions of CO2 that are almost iden- tical to conventional jet fuel. This is to be expected; because the fuel is “drop-in” it is chemically similar to conventional fuel. However, biomass-derived alternative jet fuels have the potential to reduce lifecycle CO2 emissions. This is because the carbon in the fuel came from the atmosphere originally (via photosynthesis), thus this component of the carbon is “closed-loop.” While this does offer the potential for a carbon-neutral fuel in theory, in practice the situation is more complex. In particular, converting biomass into jet fuel requires energy and materials often resulting in CO2 emissions. For example, hydroprocessing requires hydrogen, which is created from natural gas (mainly methane, CH4). Conversion of methane to hydrogen entails the C in the CH4 being vented to the atmosphere as CO2, with the result being H2. Also, processes that require electricity or heat can result in CO2 emissions, as does the transportation of the biomass feedstock and resulting jet fuel.
To assess the environmental and economic performance of each feedstock-to-fuel pathway, the material and energy flows associated with each step of the fuel’s lifecycle is tracked. This is known as “lifecycle assessment,” where the aim is to determine the net CO2 emissions associated with the lifecycle of a fuel including initial land use change for cultivation, biomass cultivation, biomass transport, conver- sion to a jet fuel, credits or debits for co-products of this process, fuel transport, and fuel production. Additionally, indirect changes in land use induced by increased biomass cultivation for fuel can be estimated. This competition for land gives rise to concerns over potential impacts on food supply and prices.
Lifecycle assessments typically consider CO2-equivalent greenhouse gas (GHG) emissions. Our results show that use of alternative fossil feedstocks always result in greater GHG emissions than conventional jet fuel. For example, coal- derived F-T jet fuel has more than double the carbon intensity of conventional jet fuel, while natural gas as a feed- stock still results in a quarter more CO2 emissions than Jet A. On the other hand, many biomass-derived jet fuels have lower lifecycle GHG emissions than conventional fuels. For example, switchgrass to F-T jet has a carbon intensity about 80% lower than regular Jet A.
More recent LAE lifecycle assessment work has focused on feedstock-to-fuel pathways including waste oils and fats to HEFA jet fuel, advanced fermentation of sugary, starchy and lignocellulocic feedstocks, and thermochemical (aqueous phase) processing of feedstocks. These new assess- ments indicate significant potential for reduced lifecycle GHG emissions.
Lifecycle assessments typically focus on “biogeochemical” effects; that is, net fluxes of green- house gases to and from the atmosphere. We are now moving towards including “biogeophysical” effects in lifecycle assessments. These are the changes in the radiation balance of the planet due to physical changes. For example, large-scale biomass cultivation for fuel production results in changes to the average planetary albedo. Our research indicates that this effect is usually cooling, and of a similar order to the biogeochemical effects. A second example relates to contrails — the line-shaped clouds that aircraft sometimes leave in their wake. These are thought to significantly contribute to the climate warming attributable to aviation. Our latest research indicates that combustion of alternative fuels may result in contrails that are optically thinner, so may result in less warming than contrails produced by combustion of conventional jet fuel. We are also assessing the air quality and human health implications of alternative jet fuels, which typically burn cleaner than their conventional counterparts. Beyond the potentially significant benefits of many alternative jet fuel options, there are also a range of downsides, some of which we are investigating. In particular, fresh water use in biomass cultivation and fuel production is a potential concern. Our calculations indicate that use of unirrigated biomass for alter- native fuel production results in a water intensity about the same as conventional jet fuel. However, maximizing biomass yield would result in many times more fresh water consumption than regular Jet A—in some cases hundreds of liters of water per liter of fuel. A second potential disadvantage is the “environmental opportunity cost” of using biomass for alternative jet fuel production; that is, the extent to which more GHG emissions could have been offset if the biomass were used for a different purpose. For example, rather than expending energy on converting biomass to a tightly specified high quality fuel like Jet A, it could be directly burned in a combined heat and power plant, potentially offsetting CO2-intensive coal-fired generation. Finally, we are assessing the production costs of different feedstock-to-fuel pathways. This is critical for both economic and environmental reasons — a fuel that is too expensive will not be adopted and, thus, will result in no environmental benefit.