Diesel Fuels Technical Review

Chevron |
2007

Diesel fuel technical overview, including driving performance, air quality and how emissions regulations impact the engine, refining and chemistry, diesel and biodiesel specifications and test methods, how engines and fuel work together, and fuel additives

Review of diesel fuel driving performance – Review of engine/fuel properties affecting: starting ease, power, noise, fuel economy, low wear (high lubricity), low-temperature operability, fuel stability – filter life, smoke
Diesel fuel and air quality
- Who regulates emissions, why and how they are regulated, types of vehicle emissions and how they are formed, how emissions are affected by diesel fuel characteristics and how diesel is being reformulated to reduce emissions
  - Air quality standards: Volatile Organic Compounds, carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter, air toxics
  - Sources of emissions: oxides of nitrogen (as a result of nitrogen in the air), particulate matter (as a result of unburned or partially burned fuel)
  - Table of heavy-duty engine emissions standards for U.S., Europe & Japan
Diesel Fuel Dyeing
- IRS and EPA both require addition of red dye to certain classes of diesel, but at different concentrations, for different reasons and to different classes of fuel
  - EPA – requires high-sulfur fuels to have red dye when they leave the refinery
  - IRS – requires tax-exempt fuels to have red dye (in amount of 5x more than EPA amount)
Refining Process
- Separation processes – feed is separated into 2 or more components based on physical property, usually boiling point – most common separation process is distillation
  - Distillation
    - Hot oil pumped into distillation column & lightest hydrocarbons rise to top (propane and butane) and are removed, gasoline is a little heavier – removed from the side of the column, kerosene and diesel are next heaviest and drawn off from lower points on column
    - Products obtained directly from crude oil distillation are called straight-run products (e.g., straight-run diesel); material too heavy to vaporize (atmospheric bottoms) are removed from bottom of column
- Upgrading processes – improve quality of a material using chemical reactions to remove compounds present in trace amounts that give the material an undesirable quality – most commonly used upgrading process is hydrotreating to remove sulfur
- Conversion processes – fundamentally change molecular structure of feedstock, usually by “cracking” large molecules into small ones (e.g. catalytic cracking and hydrocracking)
  - Applied to hydrocarbons with higher boiling points (larger molecules in distillation bottoms)
  - Fluid catalytic cracking very popular – most of liquid product from FCC eventually goes into gasoline, but one product stream, light cycle oil (LCO) is often blended into diesel
  - Hydrocracking also popular, utilizes catalyst and high pressure of hydrogen – primary feed into hydrocracking unit is VGO (vacuum gasoil), most of what is produced is kerosene and diesel
- Blending – diesel produced by a refinery is a blend of straight run product, FCC light cycle oil, and hydrocracked gas oil. May be acceptable as is or may need minor upgrading for use in diesel fuel for off-road use
  - To meet 15 ppm sulfur limit, all the streams used to prepare diesel need hydroheating to lower sulfur content
Hydrocarbons
- Organic compounds composed entirely of carbon and hydrogen atoms
- 4 major classes: paraffins, naphthenes, olefins, and aromatics (classes differ in number of carbon atoms, geometry, ration of hydrogen to carbon atoms, and way carbon atoms are bonded together); there are several variations of compounds in each hydrocarbon class
Biodiesel
- Vegetable oils and animal fats consist of 3 fatty acids – hydrocarbon chains of varying lengths, bonded to a glycerol molecule (triglyceride); fatty acids vary by hydrocarbon chain length and the number of carbon-carbon double bonds
- Transesterification = process by which triglycerides react in presence of a base chemical (sodium or potassium hydroxide) with an alcohol (methanol), resulting in 3 fatty acids bonded to the methyl group; the chemicals are referred to as fatty acid methyl esters (FAME) with alkyl chain lengths of 12-22 carbons
Standards and Test Measurements
- ASTM International’s D-2 committee (Petroleum Products and Lubricants) is responsible for diesel fuel specifications and test methods (also rely on SAE International and CRC for additional reliable technical data to help establish a specification or develop a test method)
- Specification = prescribes the required properties of diesel and sets the limits (requirements) for the values of these properties
- Additional requirements beyond ASTM’s:
  - State governments: CARB (to reduce emissions)
  - Pipelines: some limit density and pour point (ASTM does not limit those)
  - Consumers: can specify additional requirements by contract
- Review of EU, Canadian, Japanese specifications
- Premium Diesel = diesel with properties modified beyond the minimum ASTM D 957 specifications
- Unlike premium gasoline (which means higher octane to avoid knock in engines with higher compression ratios), premium diesel related to multiple fuel properties (usually cetane number, low-temp operability, stability, lubricity, detergency, and heating value)
- National Conference on Weights and Measures (NCWM)
  - Develop and propose national model laws and regulations for products sold in the U.S.
  - Standards carry no legal authority, become law only after officially adopted by relevant governmental authorities
  - NCWM Premium Diesel Working Group – focuses on functionality and practicality
- Biodiesel Fuel Standards
  - ASTM D 6751 = specification for B100 Grades S15 and S500
  - Review of European Biodiesel Fuel and Blend Stock Standard
- Review of ASTM standard test methods for diesel properties
Diesel Engines
- High torque output, durable, exceptional fuel economy, ability to provide power under a wide range of conditions = dominant engines for trucking, construction, farming, mining, stationary power generation and marine propulsion
- 4 fundamental differences from gasoline engines
  - Gas engines inject fuel into air as it is drawn into cylinder; diesel engine draws air into cylinder and injects fuel after air has been compressed
  - Gas engines ignite fuel air mixture with spark; diesel engines rely on high temp generated by compressing air above the piston as it travels upward
  - Power output of gas engine controlled by throttle which varies amount of fuel-air mixture drawn into cylinder; diesel engine controls power output by varying amount of fuel injected into the air (varying the fuel-air ratio)  why diesel engines are more fuel efficient
  - Conventional gas engine runs stoichiometrically (fuel-air ration is fixed so there is just enough air to burn all the fuel); diesel engine runs lean – always more air than is needed to burn the fuel
- Main advantage of diesel engine is its thermal efficiency (excess of 50% vs. 30-33% for gasoline and only for wide throttle openings)
- Emissions = nitrogen oxides, hydrocarbons, carbon monoxide, particulates
- Emission reduction technologies = higher injection pressures, careful injection targeting, charge shaping (rate of fuel injection), more air motion, charge cooling (forced air injection), exhaust gas recirculation, exhaust aftertreatment systems (particulate traps, catalytic converters)
Additives
- Engine and fuel delivery system performance
  - Ignition improvers (cetane number improvers), lubricity additives, smoke suppressants
- Fuel handling
  - Antifoam additives, de-icing additives, low-temperature operability additives
- Conductivity (to prevent accumulation of charge)
- Fuel stability (to prevent formation of gums)
  - Antioxidants, stabilizers, metal deactivators, dispersants
- Contaminant control
  - Biocides, demulsifiers, corrosion inhibitors
California
- Restricts aromatics content to less than 10 wt.% to reduce emissions – can be accomplished by low aromatics diesel and/or alternative low aromatics diesel that gives an equivalent reduction in emissions