Option 1: Increasing Sulfur Content of Jet Fuel in Commercial Fleet
This option involves increasing the sulfur content of jet fuel for the commercial fleet of jet aircraft (around 20,000 planes today) from 0.04% to 0.6 and increasing to 0.9% by 2050. Sulfur dioxide gas is emitted in the turbine exhaust and ideally, nearly all of it converted to sulfuric acid gas and then to sulfuric acid aerosol. The sulfuric acid aerosol floats around in the stratosphere for 1-2 years and reflects sunlight. The level in jet fuel is raised each year to match increased greenhouse gas emissions.
I realize that the idea of injecting sulfur dioxide gas into the stratosphere by jet fuel emissions is controversial and has been brought up before (26), but the implementation approaches mentioned by Crutzen and Wigley in their recent papers are poorly described and leave a misleading impression as to the practicality of the concept in general. Using potentially millions of balloons and 10,000-20,000 new, dedicated aircraft to deliver hydrogen sulfide or sulfur dioxide is infeasible. Likewise, developing new chemicals that would not exacerbate ozone depletion might take too long or not be possible at all.
This many aircraft would cost at least $1-2 trillion and would take 20 years to produce, effectively requiring Boeing and Airbus to double their output. Wigley did mention increasing sulfur fuel content, however (I haven’t seen his paper, so I’m piecing this together from news reports and if there is a more detailed presentation given that addresses this, my apologies).
As for the sulfur in fuel option, current jet fuel sulfur levels are around 400ppm, mostly due to the removal of sulfur by use of hydroprocessing to meet other parts of the jet fuel specification or the fact that low sulfur feedstock is available (27-29). Jet aircraft have a specification limit of 3000ppm in their fuel in the U.S. and similar elsewhere, so the level could be increased by a factor of almost 8 without any modification to the specifications (30).
I don’t know the origin of the 3000ppm limit, i.e., whether it is set for environmental or performance reasons and whether or not increasing it is feasible. Inquiries as to this have been made and will be reported in a revision to this commentary.
The refiners would have to reformulate the fuel, but ironically, since their recent problems have largely been how to remove sulfur, this would require modifying their refining process to add more, a job that might take 5 years or so to complete (31, 32) or reverse, locking in the strategy. Thus, if this strategy were to be adopted, it would have to be continued for some time, even if the results were unacceptable.
It also is not clear if the hydroprocessing step can be bypassed, i.e. higher sulfur feedstock used or whether the sulfur would have to be added as a separate step. Not all refineries use hydroprocessing, so a variety of options may be available to achieve the higher sulfur fuel content.
Effects of the higher sulfur content on corrosion of turbine blades and other engine parts would need to be understood. Also, since jets only fly in the lower stratosphere about 80% of the time (not sure if time is equivalent to fuel usage and 80% is my estimate), consideration would have to be given to environmental impacts during the takeoff and landing phases as well as to how much sulfur is actually burned while the aircraft are in the stratosphere.
