Biodiesel represents a popular, renewable alternative to traditional petroleum-derived fuels. Its carbon neutral footprint, sustainability and economic viability have brought it sharply into consideration as a substitute and supplement for middle distillate fuels. Biodiesel is synthesised by transesterification of triglycerides into mono-alkylated, fatty esters and glycerine.
In the early 2000s, insufficient regulation on biodiesel production led to multiple mechanical issues, such as engine clogging. As biodiesel is less stable than petrodiesel, it is more vulnerable to oxidation. The products of this oxidation have a waxy, viscous texture which can block engines. However, it’s important to note that this is usually only a concern with low quality biodiesel.
The American Society for Testing and Materials (ASTM) D6751 is the Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. ASTM international test methods exist to standardise product quality across industries. Method D6751 is comprised of several chemical analyses, which we will explore here, that must be performed on biodiesel samples to meet the properties and quality benchmark.
There are multiple forms of flash point testing, which is conducted to determine the risk when transporting and storing the fuel. The sample is placed into a closed cup container to which an ignitor is introduced periodically as the temperature increases.
The minimum flash point for biodiesel in ASTM D6751 is 93°C, which overlaps with non-hazardous categories set by the National Fire Protection Agency (NFPA).
Pensky-Martens Closed Cup Tester
While methanol is a common precursor for biodiesel, it has associated health hazards, environmental detriment and engine wear. Therefore, there is an accepted limit of methanol concentration in biodiesel: 0.2% by percentage mass. The method EN14110 uses gas chromatography to determine the alcohol content.
Dissolved sulphur can lead to degraded engine performance and increased, environmentally damaging sulphur emissions. To measure sulphur content, a biodiesel sample is combusted in an oxygen rich environment, which produces SO2. The SO2 is then exposed to UV light, exciting it to a higher energy level. When this absorbed energy is lost through molecular collisions, the SO2 molecule emits light at characteristic frequencies which can be measured by a fluorometer. This measured fluorescence describes the total sulphur content in the sample.
There are four established grades of sulphur-containing biodiesel: No.1-B 15 ppm S; No.1-B 500 ppm S; No.2-B 15 ppm S; and No.2-B 500 ppm S.
Abrasive solids or soluble metallic soaps composed of sodium, potassium, magnesium or calcium in biodiesel can collect in vehicle exhausts and lead to increased back pressure, clog filters, and cause excessive wear on engine components. The limit of these species in biodiesel is 5 ppm.
The metal content can be measured using inductively coupled plasma optical emission spectroscopy. After exciting atoms or ions with ionised argon plasma, the excited species relax from the higher energy state back to their ground state, emitting photons of wavelengths which describe the identity and concentration of that element.
The PlasmaQuant® PQ 9100 is the ideal instrument for this analysis.
PlasmaQuant® PQ 9100
Water content can cause fouling and corrosion of systems and sediments can collect in containers and filters and cause obstruction to flow: limits must be applied to both to ensure fuel quality. Biodiesel samples are tested using a centrifuge to separate the water and sediments from the fuel for measurement. The maximum percentage by volume of these contaminants is 0.05%.
Metals such as barium, calcium, magnesium, sodium, potassium, zinc and tin can appear in biofuels. As well the ICP-OES testing mentioned above, sulphated ash analysis can reveal the concentrations of these metals if they are present. This is done by burning the diesel and analysing the remaining ash, which cannot contain more than 0.02% metal contamination.
Some substances that are soluble in biodiesel at room temperature will precipitate out at lower temperatures, leading to problems with aggregated solids at filters. A cold soak filtration is performed by cooling a 300 ml fuel sample to 4°C and pulling it through a glass fibre patch filter by a vacuum. The time of filtration can be used to predict levels of filter plugging; maximum allowed time is 200 seconds for No.1-B grade and 360 seconds for No.2-B.
Corrosive impurities can cause complications with distribution, storage and operation of renewable diesel. A polished Cu strip is placed into a pressure vessel containing the biodiesel sample, which is sealed and placed inside a heating bath for three hours. The corrosive effect on the Cu strip can then be measured by comparison to a standardised plaque.
Compounds in the biofuel can oxidize and form acids and polymers that can cause problems in fuel systems. Resultantly, it is important to test the oxidation stability of the fuel to see if potentially damaging chemicals can form. The accelerated oxidation method involves heating at 110°C and passing air through a biodiesel sample and, by adding an electrode, volatile organic acid production can then be measured. The minimum stable duration under these conditions is three hours
Fuels operating outside of the acceptable viscosity range set by engine manufacturers can lead to diminished performance or engine damage. The kinematic viscosity can be determined by measuring the time it takes for the biofuel sample to pass through a glass viscometer held at a set temperature. The range for kinematic viscosity is between 1.9 and 6.0 mm2 s-1.
The acid number measures the acid content of a biofuel sample, but not the corrosiveness. To find the acid number, the biofuel is dissolved in a solvent and titrated using alcoholic potassium hydroxide, while using electrodes to record a titration graph.
High levels of free and total glycerin content in biodiesel can have multiple negative effects: lesser fuel quality, degraded storage stability and system performance, clogged filters and fuel injectors which impact engine function. The maximum permitted free glycerin and total glycerin levels are 0.02% and 0.24% respectively, as measured by gas chromatography.
The carbon residue number is a measurement of the tendency of a sample to form carbonaceous residue under certain conditions. To obtain this number, a biodiesel sample is heated to 500°C in an inert atmosphere. The coking reaction that takes place produces a non-volatile carbon residue, which can be weighed and compared against the original sample mass. The acceptable limit is 0.05% mass.
The chains of methylated esters in biodiesel are typically between 16-18 carbon atoms long and have boiling ranges of 330-357°C. The standardised boiling point of 360°C was instituted to account for high boiling point contaminants. ASTM D1160 can be performed by distilling a fuel sample at pressures between 0.13-6.7 kPa and conditions that would yield a single theoretical plate fraction.
To speak to one of SciMed’s team about how we can help with your biodiesel testing needs, or to find out more about ASTM methods for renewable fuels, contact us below:
