Lisa Fouts

Effect of low-level impurities on low-temperature performance properties of biodiesel

Biodiesel, a renewable fuel consisting of fatty acid methyl esters and made from lipid feedstocks, has presented persistent cold weather operability problems that are not predicted using the standards tests common in the petroleum refining industry. These problems have been referred to as “precipitate formation above cloud point” and are known to be caused by minor impurities. This study investigates the fundamental causes of this issue. To this end, water, steryl glucosides (SG), and saturated monoglycerides (SMGs) were spiked into 100% biodiesel (B100) from various feedstock sources. Only SMGs were found to have a significant effect on the cloud point (CP) and final melting temperature (FMT) in four B100 samples with a range of CP. A large difference between FMT and CP indicates that a metastable phase of SMG forms initially and can transform into a more stable, less soluble polymorph over time or upon heating. This occurred for SMG content above approximately 0.2 to 0.3 wt%. For more saturated B100, a large FMT–CP difference was only observed at slower heating rates, suggesting a slower rate of phase transformation. β-monostearin solubility (the most stable phase) in B100 was measured as a function of temperature. CP measurements suggest the metastable phase is as much as 10 times more soluble than the β phase. Differential scanning calorimetry experiments suggest that the metastable phase is a hydrated α-gel. SMGs at concentrations above 0.24 wt% caused failure of the cold soak filtration test (ASTM D7501); however, at higher water concentrations (∼1200 ppm), the effect of SMGs was significantly reduced. Addition of SGs had no effect on cold soak filterability.

Quality Parameters and Chemical Analysis for Biodiesel Produced in the United States in 2011

Samples of biodiesel (B100) from producers and terminals in 2011were tested for critical properties: free and total glycerin, flash point, cloud point, oxidation stability, cold soak filterability, and metals. Failure rates for cold soak filterability and oxidation stability were below 5%. One sample failed flash point due to excess methanol. One sample failed oxidation stability and metal content. Overall, 95% of the samples from this survey met biodiesel quality specification ASTM D6751. In 2007, a sampling of B100 from production facilities showed that nearly 90% met D6751. In samples meeting D6751, calcium was found above the method detection limit in nearly half the samples. Feedstock analysis revealed half the biodiesel was produced from soy and half was from mixed feedstocks. The saturated fatty acid methyl ester concentration of the B100 was compared to the saturated monoglyceride concentration as a percent of total monoglyceride. The real-world correlation of these properties was very good. The results of liquid chromatograph measurement of monoglycerides were compared to ASTM D6751. Agreement between the two methods was good, particularly for total monoglycerides and unsaturated monoglycerides. Because only very low levels of saturated monoglycerides measured, the two methods had more variability, but the correlation was still acceptable.

Effects of Heat of Vaporization and Octane Sensitivity on Knock-Limited Spark Ignition Engine Performance

Knock-limited loads for a set of surrogate gasolines all having nominal 100 research octane number (RON), approximately 11 octane sensitivity (S), and a heat of vaporization (HOV) range of 390 to 595 kJ/kg at 25°C were investigated. A single-cylinder spark-ignition engine derived from a General Motors Ecotec direct injection (DI) engine was used to perform load sweeps at a fixed intake air temperature (IAT) of 50 °C, as well as knock-limited load measurements across a range of IATs up to 90 °C. Both DI and pre-vaporized fuel (supplied by a fuel injector mounted far upstream of the intake valves and heated intake runner walls) experiments were performed to separate the chemical and thermal effects of the fuels’ knock resistance. The DI load sweeps at 50°C intake air temperature showed no effect of HOV on the knocklimited performance. The data suggest that HOV acts as a thermal contributor to S under the conditions studied. Measurement of knock-limited loads from the IAT sweeps for DI at late combustion phasing showed that a 40 vol% ethanol (E40) blend provided additional knock resistance at the highest temperatures, compared to a 20 vol% ethanol blend and hydrocarbon fuel with similar RON and S. Using the prevaporized fuel system, all the high S fuels produced nearly identical knock-limited loads at each temperature across the range of IATs studied. For these fuels RON ranged from 99.2 to 101.1 and S ranged from 9.4 to 12.2, with E40 having the lowest RON and highest S. The higher knock-limited loads for E40 at the highest IATs examined were consistent with the slightly higher S for this fuel, and the lower engine operating condition K values arising from use of this fuel. The study highlights how fuel HOV can affect the temperature at intake valve closing, and consequently the pressure-temperature history of the end gas leading to more negative values of K, thereby enhancing the effect of S on knock resistance.