Abbreviations 0%DFAME saturated with moisture at 40 °C 20% D 0%D FAME diluted to 20% v/v DDMF (100%D) 40%D0%D FAME diluted to 40% v/v DDMF (100%D) 60%D0%D FAME diluted to 60% v/v to DDMF (100%D) 80%D 0%D FAME diluted with 80% v/v to DDMF100%D 100% DDMF, moisture <50 ppm 95CI 95% Confidence interval 95CI1 95% Confidence interval, n = 1 95CI3 95% Confidence interval, n = 3 AG Concentration of FG after agitation AGQM Arbeitsgemeinschaft Qualitätsmanagement Biodiesel, e.V. ASTM American Society for Testing and Materials ASUB An approximately 100.0 mg subsample of B100 ASUBINT Time interval in seconds between ~100.0 mg subsamples B100 Commercial Grade of FAME biodiesel BSTFA Bis-(trimethylsilyl)trifluoroacetamide CDC US Center for Disease Control CEN Comité Européen de Normalisation-European Committee for Standardization CV Coefficient of variation DDMF Distilled, dry, methanol free EI Electron ionization EN European normalization EU European Union FAME Fatty acid methyl esters: here refers to aAbstract Residual free glycerin (FG) is a critical marker of fatty acid methyl ester (biodiesel) fuel quality. Yet, routine determinations by standard methods display excessive imprecision, and the uncertainties undermine confidence in the data. Various degrees of agitation are used here to evaluate the possibility of heterophase formation as a contributor to imprecision and potential bias. Statistical markers from the analysis of 13 biodiesel samples reveal that seven contain unexpected heterophases, which, due to settling, cause underestimates of 15-68%. Agitation alone resuspends heterophases for brief periods, easing potential biases, but also increases imprecision. A promising new sample pretreatment is presented, which reduces the deleterious effects of heterophases. Solubility limitations are explored as possible contributing factors, and measurements made at 23 °C in purified soy FAME reveal an equilibrium solubility which is more than 80% below previously published estimates. Strong interactions of liquid FG with small amounts of moisture reduce solubility to below international quality limits, while interactions of initially dissolved forms of moisture and FG produce bias-inducing heterophases at 0.02% FG. The unexpected low solubility of FG, exacerbated by interactions with impurities, is seen as an important contributor to underestimates and imprecision, and a strong influencing factor on the fate of residual FG in commercial biodiesel fuels.

Newer, improved high pressure common rail diesel engines require increased protection from damaging particles in fuel, resulting in tighter, low-micron filtration requirements. The unobstructed flow of fuel through filters underlies the well-known efficiencies and reduced exhaust emissions of diesel engines, and when flows are hindered, these efficiencies become severely compromised. Biodiesel fuels frequently contain saturated monoglycerides, glycerol (GLY) and/or steryl glucosides, systemic impurities that become problematic when low concentrations exceed ill-defined solubility limits, usually unexpectedly at low temperatures. Here, filters and blended fuels from seven flow obstruction incidents were subjected to detailed compositional analyses. With one exception the fuels contain estimated levels within international specifications, yet exhibit heterophase (HP) formation in the laboratory at 2 C, well above measured cloud points. Feedstock composition, pretreatment and/or purification processes are found to modulate insolubility risks. Interferences in standard gas chromatography methods used without modification can hide or complicate estimates of residual levels of GLY. The data show that classical variables controlling solubility, such as concentrations and temperatures, play important roles in setting the risks associated with in situ formation of HPs from these common impurities. Further, such risks are likely attenuated considerably by reductions in critical impurities beyond currently specified limits.

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