What These Standards Govern
Biodiesel sold as a fuel blendstock must meet published product specifications before it enters the distribution chain. In the United States, the governing document is ASTM D6751, which covers 100% biodiesel (B100) intended for blending with middle distillate fuels. The European equivalent is EN 14214, which covers fatty acid methyl esters (FAME) for diesel engines and heating applications. The ASTM definition covers mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats and is not restricted to methyl esters. EN 14214 specifically concerns FAME. Their scopes and requirements should not be treated as identical. These standards govern the finished blendstock, not the feedstock or production process used to make it.
ASTM D6751 Grade Structure and Key Limits
The AFDC public table lists four grade combinations distinguished by sulfur tier and Grade No. 1-B or No. 2-B. The ASTM catalogue entry for D6751-24 also describes low-metals variants. The numerical values below refer to the grades shown in the AFDC table, which is a selected summary rather than the complete standard. Grade No. 1-B is intended for applications sensitive to partially reacted glycerides and requiring good low-temperature operability, Grade No. 2-B is a general-purpose blendstock. Within each grade, S15 and S500 designations cap sulfur at 0.0015% mass (15 ppm) and 0.050% mass (500 ppm) respectively. Cold soak filterability limits differ by grade in the AFDC summary: 200 seconds maximum for Grade No. 1-B and 360 seconds maximum for Grade No. 2-B, both applying to B100 blendstock under D6751. Requirements applying to all grades include a minimum flash point of 93°C (closed cup), total glycerin maximum of 0.240% mass, free glycerin maximum of 0.020% mass, acid number maximum of 0.50 mg KOH/g, and oxidation stability minimum of 3 hours measured by EN 15751. Alcohol control must satisfy one of two alternatives: methanol content at or below 0.20% mass, or a flash point at or above 130°C.
Why Individual Parameters Matter
Each specification parameter addresses a specific failure mode in fuel system or engine performance. Elevated acid number—arising from free fatty acids or oxidative degradation—has been associated with fuel system deposits and shortened pump and filter life. High total or free glycerin indicates incomplete conversion or inadequate glycerin separation during production, and fuels exceeding these limits are liable to plug fuel filters and contaminate storage and engine components. Glyceride testing should be performed at the B100 stage before blending, as the test method is sensitive to the presence of diesel fuel and is not appropriate for use on blends. The T90 distillation limit exists to detect contamination with high-boiling materials such as raw fats or used motor oil. Oxidation stability, measured as an induction period under an accelerated aging test using EN 15751, reflects the fuel's oxidation reserve, a declining induction period can warn of reduced oxidation resistance before other quality changes become apparent, making it a useful early-warning metric.
B6–B20 Blend Specifications
The AFDC summary of ASTM D7467 covers B6–B20 blends. It lists a minimum cetane number of 40, maximum acid number of 0.30 mg KOH/g, maximum water and sediment of 0.05% by volume, and minimum oxidation stability of 6 hours by EN 15751. Its viscosity range is 1.9–4.1 mm²/s at 40°C, with a lower minimum of 1.3 mm²/s when Grade No. 1-D or blends of No. 1-D and No. 2-D diesel are used. The general flash-point minimum is 52°C, with a 38°C exception for those base-fuel cases or where a cloud point below -12°C is specified. These conditions are part of the table, not optional details. The biodiesel component must meet D6751 before blending, and cold-operability requirements depend on the intended application and conditions.
EN 14214 and the Scope of Available Sources
EN 14214 is the European standard for FAME used in diesel engines and heating applications, published by national standards bodies including BSI as BS EN 14214:2012+A2:2019. The full numeric tables of EN 14214 are not available from the public sources reviewed here, and specific EN 14214 limits should be verified against the current edition of the standard through a standards body such as BSI. What the available sources do confirm is that EN 15751 is an oxidation stability test method listed in ASTM D6751, ASTM D7467, and ASTM D396. Those needing EN 14214 limits for their application should consult the current edition of the standard directly.
Quality Assurance Beyond the Specification Table
Product quality also depends on handling and storage after manufacture. AFDC notes that specifications are updated and describes BQ-9000 as a voluntary programme combining ASTM requirements with a quality-systems approach. Its current detailed accreditation requirements should be checked with the programme itself. Oxidation stability and acid number provide different information about stored fuel. Antioxidants can help preserve oxidation resistance but do not remove acids already formed or establish that an aged fuel meets all requirements. Monitoring should therefore use the applicable specifications and the actual storage and distribution conditions.