PURPOSE: To reduce the emission of air pollutant and improve the cetane number of a fuel oil by adding, to the oil, a compd. which is an org. peroxide and simultaneously a perbasic compd. of a metal carboxylate and is obtd. by oxidizing a recovered waste edible oil, etc.

CONSTITUTION: A recovered edible oil contg. 80% or higher soy bean oil, corn oil, etc., is pref. as the raw material. A perbasic compd. is such a salt compd. that the amt. of the salt-constituting metal contained is higher than equivalent to that of the salt-constituting carboxylic acid contained. Such a compd. is obtd. e.g. by putting specified amts. of a recovered oil, a diluent (e.g. n-hexane), a metal hydroxide (a Ca salt is esp. pref.), and a perbasic accelerator (e.g. an alcohol) into a reactor, raising the temp. of the reaction system, blowing CO2 into the system to convert it into a stable colloidal dispersion of a perbasic compd., and blowing oxygen into the resulting soln. Thus obtd. perbasic compd., which is simultaneously an org. peroxide, is added as a fuel additive to a fuel oil (e.g. light oil) in a metal concn. of 20-5,000ppm, reducing the emission of soot, etc., and improving the cetane number.