USA: Fuels: Diesel Fuel

US Federal Diesel Fuel

Regulatory Requirements

In parallel with the 1994 emission standards for heavy-duty diesel engines, the EPA introduced new requirements for highway diesel fuel. Effective October 1, 1993, a new sulfur limit of 0.05% (500 ppm) superseded the earlier ASTM specification of 0.5%. The 0.05% sulfur fuel was termed “low sulfur” diesel. In addition, the EPA required either a minimum cetane index of 40 or a maximum aromatic content of 35%. These requirements applied to all areas of the USA except Alaska. Diesel fuel not intended for on-road use was required to be dyed red.

Further significant reductions of sulfur levels in highway diesel fuel were legislated by the EPA as a part of the 2007-2010 emission regulations for heavy-duty engines [66 FR 5135-5193, 18 January 2001]. Fuel of maximum sulfur content of 15 ppm (wt.)—called “ultra low sulfur diesel” (ULSD)—was required to be available beginning in mid-2006. Other fuel properties remain unchanged. The ULSD fuel was legislated as a “technology enabler”, to facilitate the use of sulfur-sensitive catalyst-based emission technologies on MY2007 and later heavy-duty engines, as well as on Tier 2 compliant light duty diesel vehicles.

The ULSD implementation schedule was as follows:

• 1 June 2006: Refinery level and importers
• 1 September 2006: Terminal level (downstream of the refinery/importer, except for retail outlets and wholesale consumers)
• 15 October 2006: Retail level (including wholesale consumers)

To ease the transition, diesel fuel meeting a 22 ppm sulfur limit was allowed as ULSD at the pump until October 15, 2006. A number of additional flexibility provisions were included to lessen the impact of the specification change on refiners:

• Temporary compliance option: Refiners are allowed to produce up to 20% of their output of highway diesel fuel at 500 ppm until 31 December 2009 (31 May 2010 if the refiner banked sufficient credits). The 500 ppm and 15 ppm S fuel batches must be kept separate in the distribution system.
• Credits: Early credits can be generated for manufacturing 15 ppm S fuel ahead of the regulatory schedule. Credits may be generated until 31 December 2009, and must be used by 31 May 2010. Early batches of the 15 ppm S fuel must be segregated in the distribution system and sold at retail as 15 ppm diesel to be eligible for credits.
• Averaging, Banking and Trading (ABT) program
• Alternative requirements for Alaska and Hawaii
• Increased flexibility for small refineries

Other Federal Regulatory Provisions

Fuel Additives. Diesel fuel additives for use in motor vehicle fuel subject to the 15 ppm sulfur standards must have a maximum sulfur content of 15 ppm. Additives with a sulfur content exceeding 15 ppm are allowed provided that (1) the additive is used in the diesel fuel in a quantity less than 1% (vol.), (2) the additive is not used or intended for use by the ultimate consumer and (3) product transfer documentation meets EPA requirements.

Lubricating Oil. The EPA has not mandated any sulfur limits in engine lubricating oils. EPA estimated that the sulfur contribution from normal oil consumption is less than 2 ppm on an equivalent fuel sulfur basis.

Used Motor Oil. Introducing used motor oil, or used motor oil blended with diesel fuel, into the fuel system of MY2007 or later diesel motor vehicles is not allowed, unless (1) the engine manufacturer has received a Certificate of Conformity and the engine was emission certified based on emissions data with the addition of motor oil, and (2) the oil is added in a manner and rate consistent with the conditions of the Certificate of Conformity.

ASTM D975

In addition to the federal regulatory requirements, diesel fuel must meet a number of other requirements in order for it to be used successfully in diesel engines. In the US, these other properties are specified in ASTM D975, Standard Specification for Diesel Fuel Oils. While diesel fuel is not mandated at the federal level to meet this standard, many states have legislation in place that does.

Since 2004, the D975 standard has covered seven grades of diesel, Table 1. Heavier fuel oils Grade 5 and 6 (residual), which are used primarily for heating purposes, are described by ASTM D396.

The Sxxx designation was first adopted in the D975-04 edition of the standard to distinguish grades by sulfur content. The S5000 grades correspond to the “regular” sulfur grades, the previous No. 1-D and No. 2-D. S500 grades correspond to the previous “Low Sulfur” grades (D975-03). S15 grades are commonly referred to as “Ultra-Low Sulfur” grades or ULSD.

Fuel grade 1-D is a special purpose volatile distillate fuel. Fuel grade 2-D is a general-purpose, middle distillate fuel for automotive diesel engines. Table 2 lists the detailed requirements of the No. 1 and No. 2 grades.

A number of changes to D975 are a result of concerns over the introduction of ULSD. In 2005 a diesel fuel lubricity limit of 520 µm wear scar diameter (HFRR test, ASTM D6079) was added to ASTM D975 to minimize lubricity-related problems with ultra low sulfur fuels. Meeting this requirement has forced the widespread use of lubricity additives. In 2008, a conductivity requirement was added to minimize the risk of static discharge when transferring fuel into a tank and has increased the use of static dissipater additives.

In 2008, the standard was modified to allow up to 5% biodiesel. No specific labelling of the fuel is required so that the purchaser may not know whether or not the fuel contains biodiesel.

Premium Diesel Fuel

Specifications for Premium Diesel fuel are defined by the National Institute of Standards and Technology (NIST). It is up to individual states to adopt the NIST requirements. In addition to meeting the requirements of the most recent version of ASTM D975, several additional requirements must be met before a diesel fuel can be described with terms such as premium, super, supreme, plus, or premier. The requirements are [NIST Handbook 130 - 2008]:

1. Cetane Number—A minimum cetane number of 47.0 (ASTM D613).
2. Low Temperature Operability—Low temperature operability as defined by either cloud point (ASTM D2500) or LTFT (ASTM D4539) must meet the ASTM D975 10^th percentile minimum ambient air temperatures between October 1^st and March 31^st of each year. [N.B. compliance with the ASTM D975 low temperature operability guidance is normally voluntary for fuels produced to meet only the mandatory requirements of ASTM D975.]
3. Thermal Stability—A minimum reflectance measurement of 80% (ASTM D6468) when tested for 180 min at 150°C).
4. Lubricity—A maximum HFRR wear scar diameter of 520 µm (ASTM D6079). If a single enforcement test of more than 560 µm is measured, a second test is required. The average of the two must be less than or equal to 560 µm.

Nonroad Fuels

Background

Historically, the maximum sulfur content in nonroad and highway diesel fuels was the same at 0.5%, as specified by ASTM standards. Since the introduction of low sulfur fuel (500 ppm) for highway use by the EPA in 1993, the specifications for highway and nonroad fuels became different. Sulfur in nonroad fuels remained unregulated at the ASTM specification of 0.5% (max) and an average in-use sulfur level of about 0.3% = 3,000 ppm (with the exception of California, where low sulfur CARB diesel has been widely used for nonroad engines).

EPA Tier 4 Fuel Program

Fuel Specification. In May 2004, EPA adopted the Tier 4 nonroad emission standards and diesel fuel rule, which mandates the following sulfur limits in nonroad fuels:

• 500 ppm effective June 1, 2007 for nonroad (NR), locomotive and marine (LM) fuels
• 15 ppm (ULSD) effective June 1, 2010 for nonroad fuel, and June 1, 2012 for locomotive and marine fuels

Heavy fuel oils (HFO) used in Category 2 and Category 3 marine diesel engines are exempt.

The Tier 4 program also extends the cetane index/aromatics specification for highway fuel—a minimum cetane index of 40 or a maximum of 35% (vol.) aromatics—to NRLM diesel fuel.

Implementation and Flexibilities. The above dates applied to the refinery/importer level. Terminals had to meet the 500 ppm deadline by August 2007 and retailers/wholesale customers by October 2007. Likewise, for the ULSD deadlines in June 2010/2012, the deadline for terminals was two months later (August), and retailers four months later (October).

A number of flexibility provisions existed during the transitional period, such as:

• Diesel sulfur credit banking and trading program—Sulfur credits could be accumulated for early introduction of low sulfur fuels.
• Small refinery flexibility—Small refiners and suppliers who accumulated sulfur credits were allowed a delay; they had to convert all NRLM fuel to 500 ppm in June 2010, and to 15 ppm in June 2014. An exception was the Northeast/Mid-Atlantic region, where all NRLM fuel had to be ≤ 500 ppm sulfur by June 2007.
• Interface/transmix fuel provisions—Material generated within the distribution system could be sold into the NRLM diesel fuel markets as follows:
  1. High sulfur NRLM diesel fuel or heating oil from June 1, 2007 through May 31, 2010
  2. 500 ppm sulfur NRLM diesel fuel or heating oil from June 1, 2010 through May 31, 2014
  3. 500 ppm sulfur LM diesel fuel or heating oil after June 1, 2014 (i.e., indefinitely)
• Special provisions for Alaska and the Territories

Applicability. The Tier 4 program applies to the following fuels:

1. ASTM No. 1 and 2 distillate fuels used in nonroad, locomotive, or marine diesel engines and DMX and DMA marine fuels.
2. No. 1 distillate fuels (e.g., kerosene) added to No. 2 diesel fuel (e.g., to improve its cold flow properties).
3. Other fuels used in nonroad, locomotive, or marine diesel engines or blended with diesel fuel for use in such engines (such as biodiesel and special distillates used by the military such as JP-5, JP-8, and F-76).

The Tier 4 sulfur standards do not apply to:

1. No. 1 distillate fuel used in aircraft.
2. No. 1 or No. 2 distillate fuel used for stationary source purposes, such as to power stationary diesel engines, industrial boilers, or for heating.
3. No. 4, 5, and 6 fuels (e.g., residual fuels or residual fuel blends, IFO Heavy Fuel Oil Grades 30 and higher), used for stationary source purpose.
4. Distillate fuels with a T90 distillation point above 370°C (700°F), when used in Category 2 or 3 marine diesel engines. This includes No. 4, 5, and 6 fuels (e.g., IFO Heavy Fuel Oil Grades 30 and higher), as well as DMB, DMC, RMA-10 and heavier.
5. Fuels for which a national security or research and development exemption has been approved or fuel that is exported from the USA.

Other Provisions. The Tier 4 fuel rule adopts the same fuel additive and used motor oil provisions as specified by the 2006 highway fuel rule. The prohibition against mixing of used motor oils with diesel fuel applies to MY2011 or later nonroad diesel engines (not including locomotive or marine diesel engines).

EPA Marine Fuels

The United States and Canada established an emission control area (ECA) along their shorelines in 2010 that triggered international and US EPA sulfur limits in Category 3 marine engine fuels:

• International IMO limits applicable in ECAs are 1% (10,000 ppm) sulfur beginning in 2010, and 0.1% (1,000 ppm) sulfur from 2015. SOx aftertreatment, such as SOx scrubbers, are allowed in lieu of low sulfur fuel.
• US EPA 2009 EPA Category 3 marine engine rule established a sulfur limit of 1,000 ppm for marine fuels produced and/or sold for use within an ECA. SOx aftertreatment can be used in lieu of low sulfur fuel. Additional flexibilities apply to vessels operated on the Great Lakes and Saint Lawrence Seaway: the low sulfur requirements can be deferred—subject to fuel availability and economic hardship provisions—and are not applicable to steamships.

An ASTM standard (D2069) once existed for marine diesel fuels, but it has been withdrawn. It was technically equivalent to ISO 8217. While some marine diesel engines use No. 2 distillate, D2069 covered four kinds of marine distillate fuels: DMX, DMA, DMB, and DMC and residual fuels (see also ISO marine fuel specifications):

• DMX is a special light distillate intended mainly for use in emergency engines.
• DMA (also called marine gas oil, MGO) is a general purpose marine distillate that must be free from traces of residual fuel. DMX and DMA fuels are primarily used in Category 1 marine engines (< 5 liters per cylinder).
• DMB (marine diesel oil, MDO) is allowed to have traces of residual fuel, which can be high in sulfur. This contamination with residual fuel usually occurs in the distribution process, when using the same supply means (e.g., pipelines, supply vessels) that are used for residual fuel. DMB is produced when fuels such as DMA are brought on board the vessel in this manner. DMB is typically used for Category 2 (5-30 liters per cylinder) and Category 3 (≥ 30 liters per cylinder) engines.
• DMC is a grade that may contain residual fuel, and is often a residual fuel blend. It is similar to No. 4-D, and can be used in Category 2 and Category 3 marine diesel engines.
• Residual (non-distillate) fuels are designated by the prefix RM (e.g., RMA, RMB, etc.). These fuels are also identified by their nominal viscosity (e.g., RMA10, RMG35, etc.).

California (CARB Diesel)

Effective October 1, 1993, the California Air Resources Board adopted a diesel fuel specification limiting the sulfur and aromatics in commercially available diesel fuel. Starting in January 2005, California diesel fuel was required to meet a minimum lubricity specification as well. This fuel, commonly referred to as the CARB diesel, is mandatory for use in a variety of applications including both highway and off-road vehicles. The limits and applicability have evolved as follows:

• 1993.10—Sulfur limited to a maximum of 500 ppm and aromatics to 10% or equivalent. Applicable to on- and off-road vehicles but not stationary engines, locomotives and marine vessels.
• 2004.12—CARB diesel requirement extended to stationary sources (applies to on- and off-road motor vehicles and nonvehicular sources other than locomotives and marine vessels).
• 2005.01—CARB diesel required to meet a lubricity requirement of a maximum wear scar diameter of 520 microns by ASTM D6079, the High Frequency Reciprocating Rig (HFRR).
• 2006.06—Sulfur in CARB diesel limited to 15 ppm.
• 2007.01—CARB diesel requirement extended to intrastate locomotives and marine engines (within the SCAQMD, CARB diesel must be sold to harborcraft operators beginning 2006.01).

The California fuel regulations allow significant flexibility in meeting the limit on aromatics. These options are:

• Limit aromatics to a maximum of 10% (vol.).
• Designated Alternative Limit. Limit aromatics to a designated alternative limit that can exceed 10% (vol.). The volume of aromatic hydrocarbons in the fuel with an aromatic limit above 10%, however, must be fully offset with fuel having an aromatic content of less than 10% within 90 days.
• Certified Diesel Fuel Formulation. Certify a fuel formulation by demonstrating that the exhaust emission reduction of a candidate fuel is equivalent include a provision allowing for alternatives to the 10% aromatic hydrocarbon specification, provided the supplier demonstrates that emissions are equivalent to those with of the reference fuel shown in Table 3. The emission equivalency is determined on a 1991 HD diesel engine tested over a hot start FTP transient test. These “low emission” fuels typically have much higher cetane numbers, lower sulfur, but higher aromatics, higher polycyclic aromatics and higher nitrogen than the reference fuel.
• Designated Equivalent Limits. Produce a fuel that meets the designated equivalent limits shown in Table 4. These limits are based on the average properties of certified formulations.
• Small Refiner Exemptions. Small California refiners (≤ 55,000 bbl/day crude oil capacity) have additional flexibility that allows them to limit aromatics to 20%, use a designated alternative limit only if aromatics exceed 20% or certify a diesel fuel formulation with a reference fuel having properties different from those for larger refiners.

Given the flexibility options, almost all California diesel fuels are certified alternative formulations or fuels that meet the designated equivalency limits.

By 2004, about 90% of diesel fuel sold in California was Low Sulfur 2-D complying with the CARB diesel requirements. Average properties of diesel fuel in California, before and after 1993 are compared with those of the federal diesel in Table 5.