In a power plant, the cover of insulation material for pipes and tanks is aluminum. Soot caused by fuel oil burning is clinging on to the surface of aluminum. What is the best solvent to remove the soot? (The best in view of economic and efficiency)
Paint thinner or rubbing alcohol may work, but use with caution because of toxicity and fumes. Wear rubber gloves, and use ventilation while working.
It may be easier to just replace the insulation covers or paint the covers. After several years, the aluminum is probably already oxidized on the surface.