1.4. Characterisation of heavy oil properties

The properties listed below provide a qualitative indication of the oil's performance, and most of them are of common interest in any other liquid fuels.

1.4.1. Physical properties

1. Density, specific gravity (ASTM Designation D 1298-85) : Mass per unit volume. It is important for metering of fuels and energy liberated per unit volume. Typical values of specific gravity in heavy fuel oils lie around 0.97 kg/m³ (Williams (1976)) .

2. Viscosity and pour point (ASTM Designation D 97-96a , ASTM Designation D 445-92) : Viscosity is defined as the resistance of a liquid to flow. It is dependent upon temperature and measured in centistokes, cSt. It indicates both ease of oil flow and of atomisation. For instance, optimum oil viscosity is required for good flow, atomisation and pumping. Flow and atomisation will be difficult to achieve if the viscosity is too high and a fuel will not be pumped if it is too low.

   The pour point is defined as that temperature 2.8 °C above which the oil will not flow when the test vessel is held in a horizontal position for 5 seconds. It indicates the appearance of wax crystals. In combination with ambient temperature it determines the extent of heating requirements to facilitate fuel flow.

   The atomisation of a heavy oil is improved by lowering its viscosity. Deficient atomisation results in poor ignition, smoking, unsatisfactory temperature distribution, low combustion efficiency and formation of carbonaceous particulates.

3. Thermal stability: A measure of a fuel's resistance to breaking down to form deposits of tar, asphalt and sludge. Polymerisation is enhanced by excessive heat, intermittent heating and cooling cycles and agitation (especially with air). Since heavy oils require pre-heating in order to meet the viscosity limits, stability is most critical.

4. Compatibility (ASTM Designation D 4740) : Some fuels form stringy deposits when mixed or after mixing and heating (see previous paragraph). Such oils are said to be incompatible. Deposits thus formed may cause blocking and clogging of handling facilities and atomisation installations. For instance, compatibility turns out to be an important feature in gas turbine start-up and shut-down procedures as both processes are accomplished by successive shift from light distillate to heavy oil or viceversa.

5. Emulsion stability (Marcano (1992)) : The stability of an emulsion is an important parameter. Long-term stability is required in some cases, whereas in other cases, for instance to recover the initial crude oil, limited stability is wanted. This parameter is controlled by variables such as the type and amount of surfactant and non-surface active components, temperature, mechanical agitation, physical properties of the oil, the age of the emulsion...

   There are five ways in which the structure of an emulsion can change. The first four may appear concurrently:

   1. Accumulation of droplets on one end of the system, with no change in droplet size or size distribution. This is the effect of external forces: gravitational, centrifugal, electrostatic... Creaming is the formation of a layer at the top of a emulsion.

   2. Flocculation: Aggregation of droplets within the emulsion, with no change in droplet size. This is the result of attractive forces between droplets.

      Droplets resulting from the former two processes aggregate to form larger droplets.

   3. Increase of the average droplet size as a result of the emulsion liquids not being totally immiscible (Ostwald ripening)

   4. Inversion of the emulsion, ie a water:oil emulsion becomes oil:water.

1.4.2. Chemical properties

1. Ash and metallic elements (see also section "4.4.4. Ash") (ASTM Designation D 482-91) : Ash is formed by mineral inorganic compounds present in the crude oil that concentrate in residual oils during the refining process. Ash residua left after combustion generally increases with the asphaltene content. Residual fuels may contain up to 0.1 % ash.

   Ash compounds may contain elemental metals and alkaline earth elements (Al, Fe, Ni, V, Ca...) in the form of oxides and sulphates, non metals (Si) as oxides and alkaline elements (Na) as sulphur and vanadium compounds (Harman (1981)) . They all form corrosive combinations (Foster (1970)) .

   The effect of ash deposition on components located in the hot gas path is to reduce the cross-sectional areas for flow, to increase surface roughness and to alter surface profiles leading to reduced performance and corrosion of metallic materials. Other effects include the erosive and abrasive action of ash particles.

2. Sulphur content (see also section "4.2. SO_X") (ASTM Designation D 4294-90) : It has an obvious influence on the formation of harmful sulphur compounds. Although sulphur causes corrosion and deposits, the removal of sulphur by desulphurisation is an expensive process. No limit is generally imposed in specifications, although a proposal to reduce the S content in fuels is being reviewed by the EU at the time of editing this thesis (Commission of the European Communities (1997)) . The effects of sulphur can be minimised by restricting the metallic elements in oil which may form harmful combinations. The sulphur content in heavy fuel oils can sometimes exceed 3 %, mainly in organic form.

   Sulphur is oxidised in flames to form SO₂, and subsequently SO₃ in small amounts. SO₃ represents a serious problem if a heat recovery unit is placed downstream from exhaust, as it may work at a metal temperature below 120 °C, where acid corrosion may take place if SO₃ reacts with water vapour to form H₂SO₄.

3. Nitrogen content (see also section "4.1. NO_X") (ASTM Designation D 3228-96) : Nitrogen in fuel normally concentrates in the fractions of higher boiling point during distillation. As a result, the nitrogen content in fuel varies considerably. The following contents can be found, according to the type of fuel (Bowman (1979)) :

   Fuel type	Average N (% by weight)
   Crude oil	0.65
   Asphaltenes	2.30
   Heavy distillates (nos. 4, 5, 6)	1.40
   Light distillates (nos. 1, 2, 3)	0.07

   Table 2: Typical nitrogen content of standard fuels

   The nitrogen content increases with the asphaltene content in heavy oils. Fuel-NO_X, originated by the oxidation of fuel-N, can then become important. NO_X emissions from Orimulsion are comparable to those of heavy oils.

   Nitrogen oxides are regarded as environmental pollutants, and all sources of NO_X are subject to environmental regulations.

4. Water and sediment: They tend to cause fouling of the fuel-handling facilities. These impurities may be incorporated during sea transportation in unclean tanks and because of careless storage practices. They must be kept as low as possible, although they are not likely to exceed 0.5 % or 0.005 % respectively. Filtration is required if these limits are exceeded.

5. Carbon residue: A measure of the quantity of solid deposits formed when medium or heavy oils are heated so that evaporation and pyrolysis take place. Carbon residue tests provide an indication of the extent of carbon formation which may be expected in real operation. Carbon residue formation can be estimated by two tests:
   1. Ramsbottom Carbon Residue of Petroleum Products (ASTM Designation D 524-88) : A weighed sample in a glass bulb with a capillary opening is placed inside a metal furnace at approximately 550 °C. The sample is quickly evaporated from the bulb, leaving the heavier residue behind to undergo cracking and coking reactions in the presence of air. The Ramsbottom Carbon Residue is reported as the weight percentage of original sample remaining and therefore it includes ash in the value.

   2. Conradson Carbon Residue of Petroleum Products (ASTM Designation D 189) : Porcelain or silica crucibles are the sample container. The heat treatment temperature is less well controlled than in the Ramsbottom test, although heating times are specified.

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