Residual fuels properties

Liquid fuels for ground-based gas turbines are best defined today by ASTM Specification D2880. Table 4 Hsts the detailed requirements for five grades which cover the volatility range from naphtha to residual fuel. The grades differ primarily in basic properties related to volatility eg, distillation, flash point, and density of No. 1 GT and No. 2 GT fuels correspond to similar properties of kerosene and diesel fuel respectively. These properties are not limited for No. 0 GT fuel, which allows naphthas and wide-cut distillates. For heavier fuels. No. 3 GT and No. 4 GT, the properties that must be limited are viscosity and trace metals. 

Detailed analysis of residual products, such as residual fuel oil, is more complex than the analysis of lower-molecular-weight liquid products. As with other products, there are a variety of physical property measurements that are required to determine that residnal fnel oil meets specifications. But the range of molecular types present in petrolenm prodncts increases significantly with an increase in the molecular weight (i.e., an increase in the number of carbon atoms per molecule). Therefore, characterization measurements or studies cannot, and do not, focus on the identification of specific molecular structures. The focus tends to be on molecular classes (paraffins, naphthenes, aromatics, polycyclic compounds, and polar compounds). 

The significance of the measured properties of residual fuel oil is dependent to a large extent on the ultimate uses of the fuel oil. Such uses include steam generation for various processes, as well as electrical power generation and propulsion. Corrosion, ash deposition, atmospheric pollution, and product contamination are side effects of the use of residual fuel oil, and in particular cases, properties such as vanadium, sodium, and sulfur contents may be significant. 

Viscosity is an important property of residual fuel oils, as it provides information on the ease (or otherwise) with which a fuel can be transferred from storage tank to burner system under prevailing temperature and pressure conditions. Viscosity data also indicate the degree to which a fuel oil needs to be preheated to obtain the correct atomizing temperature for efficient combustion. Most residual fuel oils function best when the burner input viscosity lies within a certain specified range. 

Fuels used in marine applications are quite diverse in their properties. Low-viscosity distillate fuels and high-viscosity residual fuels can both be considered marine fuels. The applications, though, would differ and could include use in direct injected diesel engines, boilers, and gas turbines. Also, high-speed, medium-speed, and slow-speed engines can be found in marine applications. 

This is one of the more important properties of residual fuel oil. It is an indication of both the pumpability characteristics of the fuel and the fuel atomization quality. 

The pour point of residual fuel is not the best measure of the low-temperature handling properties of the fuel. Viscosity measurements are considered more reliable. Nevertheless, residual fuels are classed as high pour and low pour fuels. Low-pour-point fuels have a maximum pour point of 60°F (15.5°C). There is no maximum pour point specified for high-pour-point residual fuels. A residual oil paraffin carbon number analysis is provided in FIGURE 3-1. 

On occasion, the performance of an EVA copolymer can be enhanced by blending with a wax crystal modifier of a different chemical type. Wax crystal modifiers used to modify the crystal structure of lubricant, residual fuel, and crude oil waxes can be blended at low concentrations with EVA copolymers to improve their performance. However, the performance enhancement is usually fuel specific and not broad ranged. Also, the low-temperature handling properties of the EVA may be impaired when blended with other wax crystal modifiers. 

The pour point test is used to determine the lowest temperature at which a fuel can be effectively pumped. However, the pour point value can be misleading, especially when it is used to determine the low-temperature handling characteristics of residual fuel oil and other heavy fuels. Low-temperature viscosity measurements are considered more reliable than pour point values for determining the flow properties and pumpability of these oils. 

Fuel-water emulsion technology has been utilized for several years to improve the combustion properties of heavy residual fuel oils. In the high-temperature combustion environment, water droplets that are finely dispersed in fuel begin to boil and explode into vapor within the fuel drops. As a result, a highly atomized... 

Physical properties of the three test fuels are presented in Table I. Except for the surface tension of No. 6 fuel oil, which was a typical value, all properties were measured for the specific samples tested. The primary differences between the SRC-II middle distillate and the No. 2 fuel were the higher specific gravity, surface tension, and viscosity of the SRC-II. The No. 6 grade fuel, a residual fuel oil, had a much higher viscosity than either of the distillate fuels. Both the SRC-II and No. 2 fuel oil were sprayed at a nominal temperature of 80°F to simulate usage in a non-preheat combustion system. The No. 6 fuel oil was sprayed at temperatures ranging from 150° to 240°F in order to assess spray formation processes and spray quality over a broad range of viscosities. 

Biomass and coal have fundamentally different fuel properties that can lead to benefits or deterrents to co-firing. For instance, biomass is a more volatile fuel than coal and has higher oxygen content. Coal, on the other hand, has more fixed carbon than biomass. Wood fuels tend to contain very little ash (on the order of 1% ash or less) and consequently increasing the ratio of wood in biomass/coal blends can reduce the amount of ash that needs to be disposed. A negative aspect of biomass is that it can contain more chlorine than coal. This is particularly true for some grasses, straws, and other agricultural residues. 

As well as naphtha, some operations use gas-oil as the feedstoek. Gas oil is the crude oil fraction boiling typically at 220°C to 360°C, and some processing vacuum gas oils boiling typically at 360°C to 550 C. However, in some instances these crackers have been revamped to use the atmospheric column bottoms (sometimes called long residua) where the crude oil being processed has the appropriate properties of high wax (linear paraffin) content and low metal content (which otherwise promotes excessive coke formation). This material is often referred to as Low Sulphur Waxy Residual Fuel Oil (LSWR). 

The reduction in viscosity of residua tends to reach a limiting value with conversion, although the total product viscosity can continue to decrease but other properties will be affected. Sediment (which is predominantly organic but may contain some mineral matter) may also form—a crucial property for residual fuel oil—and conditions should be chosen so that sediment formation is minimal, if it occurs at all. When shipment of the visbreaker product by pipeline is the process objective, addition of a diluent such as gas condensate can be used to achieve a further reduction in viscosity. Recovery of the diluent after pipelining is an option. 

Test methods of interest for hydrocarbon analysis of residual fuel oil include tests that measure physical properties such as elemental analysis, density, refractive index, molecular weight, and boiling range. There may also be some emphasis on methods that are used to measure chemical composition and structural analysis, but these methods may not be as definitive as they are for other petroleum products. 

The refractive index is the ratio of the velocity of light in air to the velocity of light in the measured substance. The value of the refractive index varies inversely with the wavelength of light used and the temperature at which the measurements are taken. The refractive index is a fundamental physical property that can be used for the determination of the gross composition of residual fuel oil and often requires its measurement at elevated temperature. In addition, the refractive index of a substance is related to its chemical composition and may be used to draw conclusions about molecular structure. 

Negus, C.R., Dale, B.W., Stenhouse, LA. and McNiven, A.I. (1987) An investigation of the confined combustion properties of residual fuels used in marine and industrial engines. CIMAC Paper D78, Warsaw, June 1987. 

---