The phrase "high sulfur" refers to something that contains a large amount of sulfur, which is a yellow chemical element found in various substances. Full definition
Acid rain effects would be greatly spread out not concentrated as in the case of the use of high sulfur coal. (volokh.com)
Burning high sulfur jet fuel when at altitude might be one option. (realclimate.org)
The new hydrogen plant, which will allow the company to increase fuel production, will remain, as well as equipment to more efficiently handle higher sulfur - content crude oil. (scientificamerican.com)