Production of biodiesel using an alkali catalyst always produces some amount of soap. There will be more soap with recycled restaurant waste and animal fats and less with refined vegetable oils. Catalyst and soap tend to concentrate in the glycerol phase; however, some may be found in the biodiesel phase even after the washing process. Soap and Catalyst are evaluated to determine the presence of residual contaminants. The presence of soap is attributed to saponification of free fatty acids present in the feedstock. Residual catalyst may remain if the reaction is incomplete or may be attributable to the excessive use of initial caustic reagent material.
ASTM D6751 Requirement: None.
The Derived Cetane Number relates to the readiness of the fuel to self-ignite when exposed to the high temperatures and pressure in the diesel engine combustion chamber. The number is also indicative of the relative fuel stability. On-highway diesel fuels in the U.S. are required to have Cetane numbers of 40 or higher. Derived Cetane Number is evaluated in accordance with ASTM D6890.
ASTM D6751 Requirement: minimum cetane number of 47.
Methanol is a primary raw material reactant used in the production of biodiesel. Methanol content of biodiesel fuel is an important factor in determining the tendency of the fuel to exhibit flammable characteristics. Additionally, a key quality factor for the alcohol reactant material is water content; methanol is hygroscopic to some degree. Water affects the extent and rate of the esterification reaction. Producers will value methanol purity tests to assure that water content is negligible at the start of production. Additionally, excess alcohol used during the production reaction can be recovered to minimize operating costs and environmental impacts. Evaluating the purity of recovered methanol enables the determination of reuse suitability. Lastly, the evaluation of methanol in a glycerin byproduct enables producers to market this fuel by-product as a commodity rather than manage it as a waste. Methanol content in biodiesel is performed in accordance with EN 14110.
ASTM D6751 Requirement: maximum methanol content of 0.2% mass. If the Flash Point is greater than 130° C, methanol analysis is not required.
Carbon residues contribute to engine deposits. The Micro-Carbon Residue test, performed in accordance with ASTM D4530, provides an estimation of the the carbon-depositing characteristics of the fuel.
ASTM D6751 Requirement: maximum carbon residue of 0.050% by mass.
Vacuum distillation in accordance with ASTM D1160 is used to determine the boiling range characteristics of a hydrocarbon sample. The boiling characteristics have an important effect on the performance, storage, and safety of the fuel. Biodiesel is produced from oils with fatty acid chains that are mostly straight-chain hydrocarbons containing 16 to 18 carbons. Because biodiesel is relatively homogenous and not composed of hydrocarbon fractions with various volatility properties, biodiesel is characterized by a boiling point rather than a boiling range distribution (i.e., distillation curve).
ASTM D6751 Requirement: 90% distilled at a maximum temperature of 360˚C, Atmospheric Equivalent Temperature (AET).
The biodiesel may contain materials that are in the form of abrasive solids, soluble metallic soaps, and unremoved reaction catalysts. The Sulfated Ash determination indicates gross levels of ash-forming compounds in the biodiesel. The primary ash-forming materials that may be present in biodiesel are calcium, magnesium, sodium, and potassium. Sulfated ash is determined in accordance with ASTM D874.
ASTM D6751 Requirement: a maximum sulfated ash content of 0.020% by mass.
Kinematic Viscosity is the resistance of a fluid to flow under gravity. The viscosity is important in determining optimum handling, storage, and operational conditions. Fuels with a very low viscosity may cause power failure due to leakage at the injector and injector pump. Kinematic viscosity is determined in accordance with ASTM D445.
ASTM D6751 Requirement: viscosity value from 1.9 to 6.0 mm2/s (cSt).
Acids and sulfur-containing compounds have the potential to cause corrosion in an engine system. The Copper Strip Corrosion test, performed in accordance with ASTM D130, indicates the potential of the biodiesel to affect copper and brass fuel system parts. Polished copper strips are immersed in the biodiesel sample and placed in a sample tube in a heated bath for several hours. The sample test strip is then compared to a standard test strip to determine the effect of the biodiesel on the copper.
ASTM D6751 Requirement: a maximum copper strip corrosion Number 3.
Phosphorus content must be kept as low as possible because of its detrimental effect on catalytic converters. Phosphorus coats the catalyst in the converter and renders it ineffective in treating the exhaust. Phosphorus content is measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) in accordance with ASTM D4951.
ASTM D6751 Requirement: a maximum phosphorus content of 0.001% mass (10mg/kg).
Sodium and Potassium are used as reaction catalysts in the production of biodiesel. If not removed from the finished product, Sodium and Potassium may be present as abrasive solids or soluble metallic soaps. Sodium and Potassium may have the same effects as Calcium and Magnesium. Sodium and Potassium analysis is performed in accordance with EN 14538.
ASTM D6751 Requirement: a maximum of 5ppm (wt/wt) combined sodium and potassium content.