Carbon Residue
The carbon residue of a fuel oil indicates its coke forming tendency and can be used to determine the tendency to form deposits in the combustion chamber and gasways. The higher the carbon residue value, the higher the fouling tendency.Some changes in the combustion process, requiring adjustment of the maximum pressure, may also be attributed to a high carbon residue content The value is measured by standardized tests, such as the Conradson or Ramsbottom tests which give similar results. The non-vaporized residue from the carbonizing test consists of carbonaceous material and inorganic impurities and is expressed as percentage weight of the fuel sample tested. Carbon residue and asphaltenes content generally move in parallel, both in relation to the carbon-to-hydrogen ratio, with in-creasing values for a higher ratio.The carbon-to-hydrogen ratio and thus also the carbon residue depends on the source of the crude oil and the type of refinery processing used. The effect of carbon residue is impossible to counteract by pre-treatment of the fuel oil, as centrifuging only influences solid inorganic contaminants and hard asphalts, which are only small amounts of the percentage weight called carbon residue.
Asphaltenes
Asphaltenes is defined as the part of a fuel oil sample which is insoluble in heptane. The content of asphaltenes is expressed as percentage weight of the fuel oil sample tested. Asphaltenes, which is aromatic, stow-burning, semi-
solid hydrocarbon compounds dispersed in the fuel oil, has a similar effect on the combustion process to the carbon residue, the main impact being fouling of gasways. The stability of the fuel oil is related to the asphaltenes content.
Asphaltenes also influences the lubricating properties of the fuel oil and, in extreme cases, high asphaltenes content may lead to fuel injection pump sticking.Fuel oils with a high asphaltenes content will have a tendency to form sludge, especially if the water content is also high. The asphaltenes content of a fuel oil is influenced by pretreatment. The heaviest semi-solid asphaltenes, and asphaltenes bound to water as sludge, can be separated by centrifuging.
Diesel Index
Diesel index is a calculated value to determine the ignition quality of a fuel oil. The ignition quality is related to the hydrocarbon composition, paraffin being of high quality, n-heptanes of moderate quality and aromatics of low quality.
With certain exceptions the properties of the aniline point and the specific gravity reflect the hydrocarbon composition of a fuel oil, and are therefore used in the following simple formula as an expression of ignition quality:
Diesel index = (aniline point x API gravity) x 0.01.
The aniline point is the lowest temperature at which equal volumes of the fuel and aniline become just miscible. The test relies on the fact that aromatic hydrocarbons mix completely with aniline at comparatively low temperatures, whereas paraffins require considerably higher temperatures before they are completely miscible. A high aniline point thus indicates a highly paraffinic fuel, and consequently a fuel oil of good ignition quality. Similarly, a high API gravity number denotes a low specific gravity and high paraffinicity, and again a good ignition quality. The diesel index provides a reasonable idea of the ignition quality, but generally gives figures slightly above the cetane number. Fuel oils with poor ignition quality and a low diesel index might in particular cause problems in starting diesel engines and running at low load. In addition to starting difficulties, a prolonged ignition delay may give rise to alternations in the maximum pressure, leading to increased mechanical or thermal load.Furthermore, fuel oils with poor ignition quality may cause retarded combustion and subsequent fouling of gasways.
Sulphur
Sulphur is present in fuel oil, mainly as organic compounds, the amount present being expressed as percentage weight of an oil sample tested. If free sulphur is present it may cause corrosion in the fuel system. The main problem caused by sulphur is low temperature corrosion. During combustion, sulphur oxides are produced in the form of gases. Since humidity is also present sulphur and sulphuric acid may be formed on components in the combustion chamber and in the gasways , where the temperature is below that of the dew point for sulphuric acid. The detrimental effect of sulphur in fuel oil is counteracted by maintaining an adequate temperature of the combustion chamber components and by using alkaline lubricating oil to neutralize the sulphuric acid produced during combustion.
Vanadium and Sodium
Vanadium and sodium are constituents of the ash content. The amounts of these are measured by analyzing the residue from the combustion test used for determination of the ash content. The amount of vanadium and sodium present is expressed in ppm, parts per million, by weight in relation to the fuel oil sample being tested for ash content. Vanadium derives from the crude oil itself and, being oil soluble, cannot be removed from the fuel oil by conventional pretreatment Sodium derives from the crude oil, and also from contamination with salt water during storage and transport of the fuel oil. Sodium is water-soluble and, regardless of derivation, tends to combine with the water present in the fuel oil.
Owing to its water solubility, it is possible to remove or reduce the amount of sodium present in the fuel oil. During combustion: corrosive ash is formed from vanadium and sodium. Especially if the weight ratio of sodium to vanadium exceeds 1:3, ash with a very low melting point and suction temperature is formed, giving rise to high temperature corrosion of exhaust valves and deposit formation in turbochargers. It is possible to reduce the tendency for formation of
detrimental vanadium-sodium ash by effective centrifuging, which will remove sodium salts together with water. Ha very low content of sodium is ensured, a relatively high vanadium content might be acceptable.
Water
The water content of fuel oil is measured by a standardized distillation test and is expressed as percentage volume of the sample tested. Water in the fuel oil may lead to several detrimental effects on the fuel oil system, and corrosion and cavitation of fuel injection pumps and fuel valves, and cause fouling of exhaust systems and turbochargers.Due to its content of sodium, salt water in combination with vanadium contributes to the formation of low-melting corrosive ash, which attacks exhaust valves and turbochargers. When it disturbs the fuel atomization, water will lead to poor combustion, resulting in higher heat load on the combustion chamber components.It is possible to reduce the water content of a fuel oil pi manly by centrifuging, and this should be done to the widest possible extent in order to avoid the
detrimental effects of water in the fuel oil.
Ash
Ash content is a measure of the non-combustible material present in the fuel oil. The ash content is determined by a combustion test and Otis expressed as a percentage weight residue from complete combustion of the oil sample tested.
Ash-forming materials are present in the fuel oil as natural components of crude oil and due to external contamination of the fuel oil. Solid contaminants such as sand, rust; certain metal oxides and catalyst fines can be removed by centrifuging, and the same goes for water-soluble salts such as sodium. Some of the components included in the ash content have been found to be particularly harmful and are therefore stated individually in the analysis data.
Silicium and Aluminium Oxides
Residual fuels produced by refineries using fluid catalytic cracking may be contaminated by catalyst panicles in the form of silicium and aluminium oxides. Any catalyst particles are shown by the ash content value. Separate values for silicium oxide content and aluminium oxide content are measured by analyzing the ash content. The amount of silicium and aluminium oxides is expressed in ppm in relation to the weight of the original fuel oil sample being tested for ash content.
As catalyst particles are very hard and abrasive, they can cause extreme mechanical wear of the fuel injection system, cylinder liners and piston rings.
Density
Density is defined as the mass of a unit volume and is expressed in g/cm3 at a temperature of 15°C (59°F).Specific gravity is the ratio of the mass of a given volume of liquid at 15.6°C and the mass of an equal volume of water at the same temperature. For a given liquid, the specific gravity will generally give the same numerical value as the density. Density is an important parameter in the centrifuging process, where separating water and water dissolved impurities from the fuel oil is based on the difference in densities. If the density of the fuel oil approaches that of water, centrifuging thus becomes less effective, necessitating a reduced flow rate and therefore increased centrifuge capacity.
Viscosity
Basically viscosity is a measure of the internal friction or resistance of a liquid to flow. Adjustment of viscosity to adequate values is possible by taking advantage of the interdependence between the temperature and viscosity index of the fuel oil.The nominal viscosity of a fuel oil is the factor determining the preheating temperatures necessary to obtain adequate viscosity for pumping, centrifuging and injection of the fuel oil, and thus also the factor determining the capacity of the preheating equipment in the fuel oil system. Viscosity of Marine Gas Oil (MGO) and Marine Diesel Oil (MDO) are expressed in centistokes (cSt) at 44°C. Viscosity is an important parameter in connection with pumping, pre-treatment and injection of fuel oil, since the possibility and efficiency of these processes to a Large extent depend on adequate viscosity.The water separation ability of fuel oil is increased by preheating the fuel oil prior to centrifuging since the densities of fuel oil and water change with the temperature at different rates, thus making it possible to optimize density differences. To some extent the quality of a fuel oil can be judged by the density, since this is directly proportional to the carbon-to-hydrogen ratio, which again is indirect proportion to aromaticity, carbon residue and asphaltene content, but in reverse ratio to calorific value.
Pour Point
The pour point is the lowest temperature at which an oil will flow or can be poured. The pour point is
measured under specific test conditions. Fuel oil must be stored, handled and pumped at temperatures above the pour point to avoid wax crystallizaiton, which may result in precipitation in storage tanks, blocking of filters and pipe lines and prevention of pumpability. Normally, the pour point of residual fuel oil does not create any problems, since the temperature needed to reduce the viscosity to pumpable levels will be adequately in excess of the pour point.
Flash Point
The flash point of an oil is defined as the temperature at which it gives off sufficient vapour to create an
inflammable mixture with air. This mixture will ignite or flash under the influence of an open flame, but will
not support combustion itself. The flash point of fuel oil is normally tested by the Pensky-Martens closed-
up method. In order to provide a Comments on Analysis Data for Fuel Oils Carbon Residue. The carbon residue of a fuel oil indicates its coke-forming tendency and can be used to determine the tendency to form deposits in the combustion chamber and gasways. The higher the carbon residue value, the higher the fouling tendency. Some changes in the combustion process, requiring adjustment of the maximum pressure, may also be attributed to a high carbon residue content. The value is measured by standardized tests, such as the Conradson or Ramsbottom tests which give similar results.The non-vaporized residue from the carbonizing test consists of carbonaceous material and inorganic
impurities and is expressed as percentage weight of the fuel sample tested. Carbon residue and
asphaltenes content generally move in parallel, both in relation to the carbon-to-hydrogen ratio, with in-
creasing values for a higher ratio. The carbon-to-hydrogen ratio and thus also the
carbon residue depends on the source of the crude oil and the type of refinery processing used.The effect of carbon residue is impossible to counteract by pre-treatment of the fuel oil, as centrifuging only influences solid inorganic contaminants and hard asphalts, which are only small amounts of the percentage weight called carbon residue.