Thanks for the additional information.
I recognize MGO as (or was, at least, when I was involved with RFO) a component of fuel oil additives specifically formulated for RFO. The reason for it being added to RFO was to make the ash deposits soft and friable, so that they could be easily cleaned from the boiler/economizer tubes with soot blowers, etc.
Perhaps someone thinks that dosing on the air side will convey the MGO to the furnace where it will get involved with the combustion process, and carried to the boiler tube banks, rather than admitting it directly as a fuel oil additive. Or, maybe they are trying to add quantities that would be prohibitive as a fuel oil additive. I can't say.
I am not familiar with this method of dosing, but I think I recognize the end result that is trying to be obtained, ash deposition minimization, and control.
rmw