Residual Fuels
A residual fuel oil is any petroleum-based fuel which contains the undistilled residue from atmospheric or vacuum distillation of crude oil and may be called Bunker Fuel Oil, No. 6 Fuel Oil, or heavy Fuel Oil. When diluted with distillate to reduce viscosity, it may be called Marine Diesel Intermediate Fuel, No. 5 Fuel Oil, No. 4 Fuel Oil, or Light Fuel Oil) (Table 1). It is higher in viscosity than a distillate fuel, so it normally requires preheating for pumping and atomization. Asphaltenes, which are high molecular weight condensed aromatics in residual fuel, are difficult to burn completely, so combustion tends to give particulate emissions. Residual fuel usually contains organometallic compounds of vanadium, which leave an ash residue that may cause deposits and corrosion in boilers and engines because residual fuels are higher in sulfur and nitrogen than distillates. The emission of sulfur and nitrogen oxides during combustion is much greater. Carbon content is also higher than for distillate fuels or gas, so it emits more CO2 than they do, but less than coal. Basically residual fuels are more difficult to handle and tend to pollute more than distillate or gaseous fuels, but less than solid fuels such as coal.The major advantage of residual fuel is that it is the cheapest liquid fuel available, in large part because it is a byproduct of the refining of gasoline and diesel fuel. As such, it sells for less per gallon than crude oil, the raw material from which it is made. Since heavy fuel prices are usually 50 percent to 75 percent of the price of crude, refiners try to minimize production of heavy fuel oil and maximize the production of higher-value gasoline and distillate fuels.
In order to use this low-priced fuel, consumers must make a significant investment in handling equipment. Heavier grades need preheat equipment to maintain 50°C (122°F) for pumping in the distribution system, and 135°C (275°F) for atomization in boilers or diesel engines. A relatively long residence time is needed in the boiler or engine to give complete burnout of the fuel asphaltenes, which tend to
produce particulates. Thus, residual fuels are normally used only in relatively large installations.
In the earliest days of refining, kerosene was the principal refinery product. All the crude heavier than kerosene was sold as residual fuel and used initially for industrial and commercial steam generation. Residual fuel soon began to displace coal in steamships and railroad locomotives because of its portability and relatively low ash content. In the 1920s, residual fuel underwent a drastic change in composition because of the installation of thermal cracking (a refinery process in which a heavy distillate or residual fraction of petroleum is subjected to high temperature, which causes large, high boiling molecules to crack into smaller ones which boil in the gasoline or heating oil range) to meet gasoline demand. Most heavy fuels contained large amounts of thermal tars and visbreaker products (products of a thermal racking process). Great care had to be taken when blending these fuels to avoid sediment formation due to incompatibility.
With the development of catalytic cracking in the 1930s and 1940s, residual fuel composition changed again. Vacuum distillation came into use to provide additional clean feed for catalytic cracking. Vacuum distillates, which had previously been part of residual fuel, were now converted to gasoline and heating oil in the catalytic cracker.
The residuum from vacuum distillation became, and still is, the basic component of residual fuel oil. It contains the heaviest fraction of the crude, including all the ash and asphaltenes. It is extremely high in viscosity and must be diluted with light distillate flux (a low viscosity distillate or residual fraction which is blended with a high viscosity residual fraction to yield a fuel in the desired viscosity range) to reach residual fuel viscosity. The lowest value distillates, usually cracked stocks, are used as flux. In some cases the vacuum residuum is visbroken to reduce its viscosity so that it requires less distillate flux.
By the end of World War II the use of residual fuel oil in the United States had reached about 1.2 million barrels per day. The bulk of this use was in industrial/commercial boilers, railroad locomotives, and steamships. Shortly thereafter, railroad use declined rapidly as diesel engines, which used distillate fuel, replaced steam locomotives. In the 1950s and 1960s residual fuel oil use for marine and industrial applications, as well as for electric power generation, continued
to grow. During this period, the use of fuel oil for marine propulsion gradually shifted from steamships to low and medium speed diesel engines, which were gradually displacing steam turbine propulsion because they were more efficient.
By 1973 about 1.4 million barrels per day of residual fuel oil were used for electric power generation in the United States. This accounted for 16.8 percent of U.S. electricity generation, mostly in areas where cheap, foreign heavy fuel could be delivered by tanker. That same year, another 1.4 million barrels per day of heavy fuel oil were used in the United States for industrial and commercial applications. Worldwide during 1973 about 2.6 million barrels per day of residual fuel oil were used in marine diesel engines, and another 1.1 million barrels per day were used for steamship propulsion.
After the energy crisis of the 1970s, the price of heavy fuel oil rose dramatically and demand dropped drastically. By 1983, heavy fuel oil for electric power generation in the United States fell to 650 thousand barrels per day, providing just 6.2 percent of the total generated. Industrial/commercial demand fell almost as far, to 770 thousand barrels per day. Worldwide demand for marine diesel fuel had dropped to 1.4 million barrels per day and steamship demand to 245 thousand barrels per day. Since 1983, the demand in each of these sectors except marine diesel fuel has continued to decrease. Diesel engines, which are more efficient than steam turbine systems, have become the dominant means of ship propulsion. By 1997, the demand for marine diesel fuel was back to about 2.4 million barrels per day. It is forecast to stay at about that level or slightly higher.
Refiners have coped with this decrease in demand for residual fuel oil by making as little as possible and by shifting their production to heavier grades. They no longer make lighter grades, such as No. 4 or No. 5 fuels, except by special arrangement. Consumers who previously used these grades now use No. 6 fuel oil or gas. Similarly, most marine diesel fuel oil is now supplied as IF 180 (180 Cst @ 50°C) or IF 380 (380 Cst @ 50°C). Prior to 1973, many marine diesel engines were operated on lighter grades. Switching from IF 60, which is roughly 29 percent distillate and 71 percent bunker fuel (the residual fuel burned in boilers on steamships), to IF 380, which is 2 percent distillate and 98 percent bunker fuel, would increase bunker fuel consumption by 38 percent and reduce distillate consumption by 93 percent. Such switches have been beneficial to both refiners and consumers since the energy crisis. Refiners supply more residual and less distillate to a given consumer, and the consumer pays a lower price for the heavier grade of fuel. In most cases, the diesel engine operates properly on the heavier fuel.
While some refiners have reduced residual fuel production by supplying heavier grades, others have eliminated residual fuel completely by installing cokers or hydrocrackers. These are process units that convert residua to gasoline or distillate. They are very expensive to install and operate, but can be justified when there is an oversupply of residual fuel.
| No. 6 Fuel Oil, Bunker Fuel Oil |
| Viscosity, Cst @ 50C | 500 |
| Density, Kg/Cu.Meter | 985 |
| Flash Point, Deg C | 60 |
| Energy Content, KJoule/Kg. | 43,000 |
| Chemical Composition | |
| Carbon, Wt.% | 86 |
| Hydrogen, Wt.% | 11.5 |
| Sulfur, Wt.% | 2.5 |
| Ash Content, Wt.% | 0.08 |
| Vanadium, PPM | 200 |
| Lower Viscosity Blends |
| Marine Diesel Fuel | |
| Intermediate Fuel, IF 180 Cst @ 50 C | 180 |
| Intermediate Fuel, IF 380 Cst @ 50 C | 380 |
| No. 5 Fuel Oil, Cst @ 50 C | 40 |
| No. 4 Fuel Oil, Cst @ 40 C | 20 |
| | United States | Worldwide |
| | Electric Power Generation | Industrial & Commercial | Diesel Ships | Steam Ships |
| 1973 | 1406 | 1421 | 2591 | 1085 |
| 1978 | 1612 | 1411 | 2101 | 700 |
| 1983 | 652 | 769 | 1418 | 245 |
| 1988 | 646 | 732 | 1803 | 240 |
| 1993 | 424 | 656 | 2171 | 227 |
| 1997 | 301 | 496 | 2451 | 210 |
Bibliography
Guthrie, V. B. (1960). Petroleum Products Handbook. New York: McGraw-Hill.
Energy Information Administration. (1998). Monthly Energy Review(October). Washington, DC: U.S. Department of Energy.
Siegmund, C. W. (1997). Marine Fuels: Specifications, Testing, Purchase, and Use. ASTM Technical & Professional Training Course Notes. West Conshocken, PA: ASTM.
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