In an effort to reduce acid emissions from the aviation industry, preventing an annual number of between 1,000 and 4,000 deaths, it is planned to burn very low-sulphur jet fuel in planes. However, although better air quality is anticipated, such low sulphur fuels might also reduce the formation of sulphate aerosols, particles of which reflect solar energy back into space and help cool the planet.
Such ultra-low sulphur jet fuels (ULSJ) contain just 15 ppm of sulphur, to be compared with a high of 3,000 ppm for some jet fuels. Indeed, the sulphur content of aviation fuel has been increasing of late, thought due to an increasing reliance on high-sulphur crude oil obtained from the Middle East and Venezuela.
There are different means for removing sulphur from crude oil and fuels, of which the industry standard in hydrodesulfurization (HDS). In HDS, the liquid oil or fuel is contacted with a catalyst under a relatively high pressure of H2 gas, in the presence of a catalyst, removing the sulphur from compounds like bezothiophene and it s higher homologues in the form of hydrogen sulphide, H2S. However, to deal with higher sulphur contents, a greater pressure of H2 must be used, and in general the contact time with the catalysts also needs to be longer, thus slowing the production process and increasing the amount of energy required to run it.
There are methods for removing sulphur from fuels, e.g. by absorption onto metal oxides such as zinc oxide (ZnO) onto which are supported various transition metals. Another route is called "oxidative desulphurisation", in which the benzothiophenes are converted to sulphones, which contain the SO2 functional group and are more easily removed. The sulphur compounds can also be removed using microporous adsorbents such as activated carbon and zeolites. Most refineries prefer to use HDS, and the other options are best regarded as fall-back, Plan-B strategies.
While there is no real disagreement that removing sulphur from jet fuel leads to better air and public health benefits, it can be argued that an additional 2% of CO2 emissions is incurred, because energy derived from fossil fuels is needed to drive process, which it is thought will add between 2 and 7 cents per gallon to the cost of the fuel. It is thought that desulphurizing the fuel might recover a health benefit of perhaps one quarter, an increased climate change impact of maybe one tenth would be incurred. While there remains some doubt as to the accuracy of the relatively simple models used to estimate health issues which are underpinned by complex mechanisms, all evidence is that to remove sulphur from fuels is a positive course of action.
Interestingly, rather than the expected cooling effect of sulphate aerosol particles, a study by Mark Jacobson, made at Stanford University, suggests that there might actually be an increase in warming because sulphate becomes coated onto carbon black particles in the exhaust and increases the warming effect of the carbon. By reducing the concentration of sulphate from the low sulphur fuels, the effect is diminished and cooling is experienced relative to the higher sulphur containing fuels, although this may refer to an uncertainly in the model used. Presumably, any such warming effect must be to some degree counterbalanced by the reflection of solar energy from sulphate particles per se, generated from free SO2, liberated into the atmosphere.
It seems most likely that it is emissions from the combustion of fuels with low sulphur content at ground level, rather than at cruising altitude that will provide the greatest health improvement, while the model and analysis made using it have revealed various factors that are likely to be of importance to mechanisms and issues of global warming and public health.