Free & Total Glycerin (processing specification):

Perhaps the most important quality test for biodiesel is the measure of glycerin content. The key reaction in biodiesel production, transesterification, involves converting oils and fats to fatty acid methyl esters (FAME). Glycerin is the major byproduct and is higher in density and more polar than the FAME. Glycerin may be present in free or bound (mono, di and triglycerides) form. Measurement of both is necessary to determine how the conversion reaction proceeded. For producers, high free glycerin content indicates poor separation and high glycerides indicate that the reaction has not proceeded to completion or the catalyst concentration is low. For distributors and end-users high glycerin may separate out in storage, plugging pumps and filters. It can also contribute to dirty injectors or the formation of deposits on nozzles, pistons, and valves causing poor combustion conditions. ASTM D6751 Requirement: a maximum for free glycerin of 0.020% by mass and a maximum for total glycerin of 0.240% by mass.

Back to Top

Flash Point (processing specification):

The lowest temperature at which the vapor of a combustible liquid can be made to ignite momentarily in air is identified as the flash point and correlates to the ignitability of the fuel. For producers and blenders a low flash point can indicate residual methanol remaining from the conversion process. For distributors and end-users, a low flashpoint in biodiesel can result in premature ignition, causing irregular timing, excessive fuel blow by into the crankcase oil, and excessive emissions. ASTM D6751 Requirement: minimum 93°C.

Back to Top

Total Acid Number (quality specification):

The acid number is a sum parameter for all acidic components to indicate the level of free fatty acids (FFA) present in biodiesel, as well as the presence of process acids and degradation by-products. The acid number correlates to the fuel’s long-term stability and corrosivity; the smaller the value, the higher the quality. Regulators require non-aqueous potentiometric acid-base titration for determining the acid number. Typical problems associated with a high acid number include poor process control, such as methanol carryover, when converting the oils and fats to fatty acid methyl ester (FAME) fuel. For distributors and end-users, a high acid number fuel can have a strong solvency effect on rubber seals and hoses in the engine, thereby causing premature failure. It may also leave deposits, which can clog the fuel filter or drop fuel pressure. ASTM D6751 Requirement: a maximum total acid number of 0.50 mg KOH/g.

Back to Top

Cloud and Pour Point (property specification):

A fuel property that is particularly important for the low temperature operability of biodiesel fuel is the cloud point, defined as the lowest temperature at which wax crystals begin to form in the fuel. Therefore, it is an index of the lowest temperature of the fuel’s usability for certain applications. Operating at temperatures below the cloud point of a biodiesel fuel can result in fuel filter clogging due to the wax crystals. Pour point is the temperature at which the fuel ceases to flow. Therefore, the pour point provides an index of the lowest temperature of the fuel’s utility during cold temperature handling. Cloud point and pour point determinations are also useful information for blending purposes. No ASTM D6751 requirement.

Back to Top

Water & Sediment (quality specification):

The water and sediment test relies on a centrifuge to vigorously separate out free water and any trace sediment present in the fuel. Water can be detected visually if gross contamination is present (cloudy appearance). For producers and blenders, excessive water may indicate the water wash/removal process is not performed correctly. For distributors and end-users, improper storage may be the reason for elevated water and sediment percentages resulting in poor ignition, filter clogging, and fuel pump problems. Sediment in storage tanks can obstruct the flow of fuel from the tank to a combustor. Water in middle distillate fuels can cause corrosion and growth of microorganisms. ASTM D6751 Requirement: a maximum for water and sediment of 0.050% by volume.

Back to Top

Visual Inspection (quality specification):

Visual Inspection is a standardized test that supports the requirement that fuel is clear and bright and free of visible particulate matter. Biodiesel fuels with these physical qualities will increase the confidence of distributors and end-users. No ASTM requirement, yet this is a BQ9000 expectation for each production lot.

Back to Top

Total Sulfur by UVF (quality specification):

Sulfur content impacts engine wear, creates deposits, and may affect emissions control system performance. The effects vary with operating conditions. The Environmental Protection Agency (EPA) has imposed limits on sulfur emissions for environmental reasons and ultra-low sulfur fuels are preferred. B100 contains low quantities of sulfur. ASTM D6751 Requirement: a maximum sulfur content of 15ppm for Grade S15 and 500ppm for Grade S500.

Back to Top

Moisture by coulometric kf (processing specification):

The content of water in petroleum products plays an important role in predicting quality and performance of the product. If moisture is present, premature corrosion and wear, diminished lubrication, filter plugging, decreased effectiveness of additives, and bacterial growth may ensue. No ASTM D6751 Requirement yet many distributors require 500ppm or less.

Back to Top

Oxidation Stability (property specification):

As a fuel oxidizes, it may form acids and polymers that may cause fuel system deposits leading to fuel filter clogging and fuel system malfunctions. ASTM D6751 Requirement: a minimum oxidative stability of 3 hours.

Back to Top

Calcium & Magnesium by ICP (processing specification):

Calcium and Magnesium may form abrasive solids or be present as soluble metallic soaps. If present as solids, excessive injector, fuel pump, piston, and ring wear may result. Soluble metallic soaps may contribute to filter plugging and engine deposits. High levels of these elements may accumulate in exhaust particulate removal devices and may increase back pressure that may require shorter maintenance intervals. ASTM D6751 Requirement: a maximum of 5ppm (wt/wt) combined calcium and magnesium content.

Back to Top

sodium & potassium by ICP (processing specification):

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. ASTM D6751 Requirement: a maximum of 5ppm (wt/wt) combined sodium and potassium content.

Back to Top

Phosphorus by ICP (processing specification):

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. ASTM D6751 Requirement: a maximum phosphorus content of 10ppm (wt/wt).

Back to Top

copper strip corrosion (quality specification):

Acids and sulfur-containing compounds have the potential to cause corrosion in an engine system. The Copper Strip Corrosion test 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.

Back to Top

Kinematic Viscosity (processing specification):

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 must have suitable flow characteristics to ensure that an adequate supply reaches injectors at different operating temperatures. For producers and blenders, the kinematic viscosity verifies that the transesterification reaction is adequate; high viscosity may require producers to reprocess the batch. For distributors and end-users, high viscosity can cause fuel flow problems and lead to stall out or fuel pump failure. ASTM D6751 Requirement: viscosity value from 1.9 to 6.0 mm²/s (cSt).

Back to Top

sulfated ash (quality specification):

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. ASTM D6751 Requirement: a maximum sulfated ash content of 0.020% by mass.

Back to Top

distillation temperature (property specification):

Vacuum distillation 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 fundamentally different than petroleum-based diesel, and it is especially evident with distillation. Biodiesel has a fairly homogenous consistency of straight chain hydrocarbons, all with 16 to 18 carbons. Consequently it exhibits a boiling point rather than a distillation curve. For producers, high distillation values may indicate the conversion reaction has not gone to completion (flash point may also be high); low values indicate methanol carryover. For users, high distillation values lead to poor starting, especially in cold weather; low values can lead to poor timing and seal material failure. ASTM D6751 Requirement: 90% distilled at a maximum temperature of 360˚C.

Back to Top

micro-carbon residue (quality specification):

Carbon residues contribute to engine deposits. The Micro-Carbon Residue test provides an estimation of the the carbon-depositing characteristics of the fuel. ASTM D6751 Requirement: maximum carbon residue of 0.050% by mass.

Back to Top

Methanol Content (processing specification):

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 byproduct as a commodity rather than manage it as a waste. ASTM D6751 Requirement: maximum methanol content of 0.2% by mass in biodiesel product.

Back to Top

cetane number (property specification):

The 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. ASTM D6751 Requirement: minimum cetane number of 47.

Back to Top

soap & Catalyst (processing specification):

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. No ASTM requirement.

Back to Top