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Changes in Fuel

Petroleum has changed over the past few decades and is a far cry from its original early forms of the 19th century. Since the early 2000s, gasoline and diesel have both gone through a number of changes. In 2005 the Renewable Fuel Standard (RFS) program was created. There are four renewable fuels standards that nest within each other:

  • Biomass-based diesel – a diesel fuel substitute not derived from petroleum often from animal wastes or fats, solid waste materials or sludge and wastewater treatment waste.

  • Cellulosic biofuel – biofuel derived from grasses, woods, algae or other plants. The main biofuel on the U.S. market is corn based ethanol.

  • Advanced biofuel – a second-generation biofuel made from various types of biomass. This is made from non-food crops like switchgrass or left over parts of a food-crop such as the stems, leaves or husks left behind from crops grown for food.

  • Total or conventional renewable fuel – includes those made from feedstocks like sugarcane. Refer to the EPA website https://www.epa.gov/renewable-fuel-standard-program/renewable-fuel-annual-standards which shows the fuel nesting scheme of the RFS.

Understanding the different types of fuel, where they are derived from and how different each is helps us to appreciate the potential difficulty in managing them. With the introduction of biofuels come potential quality issues as well as water contamination problems due to its water solubility.

Today fuel quality issues can now be associated with corrosion. Diesel and gasoline are not problems in and of themselves. A combination of factors from the addition of biofuels to the removal of sulfur have all attributed to at least a few unintended consequences.

Legislation

Globally there are legislative bodies and organizations that require or recommend initiatives that define fuel quality standards. The previously mentioned RFS is one. The EU and the U.S. are the two major driving bodies impacting fuel quality and cleanliness. In an effort to meet higher air and emissions standards engines are being designed to require cleaner fuels. The Worldwide Fuel Charter (WWFC) in step with engine manufacturers have continued to recommend higher quality, cleaner fuels. Mostly due to the emissions requirements, the automotive and engine manufacturing industries are required to meet a higher standard. Unfortunately, the real world is often miles apart from the need.

Legislating lower emissions of nitrogen oxides (NOx), sulfur dioxides (SO2), soot, carbon monoxide (CO) and volatile hydrocarbons (high CO2) the industry has had to develop ultraclean fuels and add biofuels. The WWFC, established in 1998 has played an significant role in the process. They have been a driving force in the world to create and promote a fuel quality standard, establishing the global fuels harmonization effort to develop global fuel quality recommendations. At present the 5th Edition of the WWFC recommends the standard for fuel quality and cleanliness and can be found at www.oica.net/wp-content/uploads//WWFC5-2013-Final-single-page-correction2.pdf

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Fuel Basics

Legislation has changed refining. Most of the conventional diesel fuel being used is now ultra-low sulfur diesel (ULSD). A decade ago, it was not uncommon to find diesel fuel with a sulfur content in excess of 500 parts per million (ppm). Today, ULSD is no higher than 15 ppm sulfur in the U.S. In the UK the requirement is 10 ppm.

The refining process starts with fractional distillation which involves the separation of crude into component parts (fractions) through boiling and condensation as seen in the simple diagram of a distillation unit.

The refining process changed in order to meet clean fuel requirements and the wide range of feedstocks and products today. Hydrodesulphurization (HDS), oxygenated additives and biofuels have presented challenges and opportunities for the industry.

HDS is the process of removing sulfur from refined petroleum products in order to reduce SO2. Previously, sulfur acted as a lubricity agent in fuel. Now with its removal, fuel is very dry resulting in engine component wear and failure. As a result, lubricity additives must be used in ULSD to compensate. Biodiesel is often used to increase the lubricity of ULSD. In the U.S. and other areas of the world, up to 5 percent biodiesel can be added to conventional diesel without labeling or notice. As a result, biodiesel is often blended and distributed to the end user as conventional ULSD. It is important to note since biodiesel presents some maintenance challenges. Biodiesel is generally more hygroscopic (water soluble) than conventional diesel. Biodiesel molecules are primarily fatty acid methyl esters (FAME) usually acquired from vegetable oils. These oils love water and are mildly acidic.

Oxygenates are compounds that contain oxygen. Ethanol is a common oxygenated additive in fuel, used to reduce CO and soot. When the EPA mandated the removal of MTBE - an oxygenated additive - ethanol became its replacement. Unfortunately, ethanol brought a host of other issues with it. Ethanol is both hydrophilic – easily dissolves in water and hygroscopic – water absorbing. Water is always present in fuel at some level, often suspended in the ethanol blended fuel until it becomes heavy enough to drop out. Ethanol and water are also miscible. That is, they dissolve in each other. Ethanol is lighter than conventional gasoline so it remains suspended in the fuel unless enough water bonds to it. Because water is heavier than fuel, with enough volume, it will drag the ethanol to the bottom once enough is present.

Research shows it only takes .398% water in fuel for phase separation to occur. At saturation the water and much of the ethanol separate from the fuel and drop to the bottom of the tank. This is phase separation. Because ethanol is used to raise octane, the fuel above the phase layer at the bottom of the tank is now substandard. Fuel can also remain partially phase separated. This happens when enough water enters the fuel but has not reached saturation. During this stage, fuel can become hazy often clogging filters and shutting down engines. It does not take much water to cause fuel to phase separate. Less than 20 gallons of water in 5,000 gallons of fuel will result in phase separation.