Download: Additives.xlsm

Blending to the EN-590 specification requires severe hydrotreating to achieve less than 10 ppm sulphur. At this level, the fuel will have lost natural lubricity, polar sulphur, nitrogen and oxygen containing compounds having been removed. Wear scars by the HFRR technique are usually above 550 microns and the fuel has to be treated with lubricity additive to achieve the 460 micron maximum specification. The untreated wear scar result is an indicator for the level of residual polar compounds and values above 600 microns indicate the fuel is seriously depleted in compounds that form a boundary film.

Lubricity Improver is effective in increasing the wetting of metal surfaces by the fuel and strengthening the barrier film between two pressured moving metal surfaces. The mechanism is a ‘boundary film’ formation due to the polar carboxylic acid groups of the TOFA (tall oil fatty acid) adsorbing on to metal surfaces.

The HFRR test is a severe test of lubricity which evaluates the effectiveness of the boundary film in reducing wear. To form a boundary film, the fuel must contain polar compounds attracted to the metal surfaces. The loading of the test specimen is too high for hydrodynamic lubrication and floating of the two surfaces with minimal metal on metal contact.

The Excel Download Additives.xlsm (link above) estimates the required dosage and effect of TOFA, also taking in to account the effects of FAME, Cetane improver and the Cold Flow additives.

FAME

BioFuels contain polar ester groups and add to the fuel lubricity. Fatty Acid Methyl Esters (FAME) at 1.5% in a blend will generally bring the fuel below the 460 micron specification and negate the requirement to dose lubricity improver.

Cetane Improver

Cetane Improver, 2-ehn is a strong solvent and works against the additives used to enhance lubricity in low sulphur blends. The effect is significant in fuels hydrotreated down to sulphur levels below 10 ppm but can be compensated by increasing lubricity additive dosages when using high levels of 2-ehn. Additives.xlsm correlates the wear scar with dosage and fuel analysis.

Cold Flow

Cold Flow additives are relatively high molecular weight co-polymers of ethylene and vinyl acetate. The vinyl acetate ester groups have polarity and a moderate benefit to the fuels measured lubricity. This effect can be observed between the winter and summer seasons, the cold flow dosages in summer usually being low or zero.