Wobbe index

Measured in MJ/m or Btu/ft, the Wobbe Index has an advantage over the calorific value of a gas (the heating value per unit volume or weight), which varies with the density of the gas. The Wobbe Index Is commonly specified in gas contracts as a guarantee of product quality. A customer usually requires a product whose Wobbe Index lies within a narrow range, since a burner will need adjustment to a different fuel air ratio if the fuel quality varies significantly. A sudden increase in heating value of the feed can cause a flame-out. 

When a customer agrees to purchase gas, product quality is specified in terms of the calorific value of the gas, measured by the Wobbe index (calorific value divided by density), the hydrocarbon dew point and the water dew point, and the fraction of other gases such as Nj, COj, HjS. The Wobbe index specification ensures that the gas the customer receives has a predictable calorific value and hence predictable burning characteristics. If the gas becomes lean, less energy is released, and if the gas becomes too rich there is a risk that the gas burners flame out . Water and hydrocarbon dew points (the pressure and temperature at which liquids start to drop out of the gas) are specified to ensure that over the range of temperature and pressure at which the gas is handled by the customer, no liquids will drop out (these could cause possible corrosion and/or hydrate formation). 

If the substitute fuel is of the same general type, eg, propane for methane, the problem reduces to control of the primary equivalence ratio. For nonaspiring burners, ie, those in which the air and fuel suppHes are essentially independent, it is further reduced to control of the fuel dow, since the air dow usually constitutes most of the mass dow and this is fixed. For a given fuel supply pressure and fixed dow resistance of the feed system, the volume dow rate of the fuel is inversely proportional to. ypJ. The same total heat input rate or enthalpy dow to the dame simply requires satisfactory reproduction of the product of the lower heating value of the fuel and its dow rate, so that WI = l- / remains the same. WI is the Wobbe Index of the fuel gas, and... 

Natural Gas—Calculation of Calorific Values, Density, Relative Density and Wobbe Index from Composition. ISO 6976 1995(E). 

The natural gas after appropriate treatment for acid gas reduction, odorization, and hydrocarbon and moisture dew point adjustment (ASTM D1142) would then be sold within prescribed limits of pressure, calorific value, and possibly, the Wobbe Index [(calorific value)/(specific gravity)]. 

The most important of these are the Wobbe index [or Wobbe number = calorific value/(specific gravity)] and the flame speed, usually expressed as a factor or an arbitrary scale on which that of hydrogen is 100. This factor can be calculated from the gas analysis. In fact, calorific value and specific gravity can be calculated from compositional analysis (ASTM D3588). 

Natural gas standards used for the determination of physical quantities of natural gas, including calorific values, Wobbe index, and density. 

Wobbe index This is a numerical value that is calculated by dividing the square root of the relative density into the heat content (kcal/m3 or Btu/scf) of the gas. 

In Europe and in other countries where the metric system is used, natural gas calorimeters are calibrated in megajoule units, and in the United States in Btu units. The output of the calorimeter may represent the gross calorific value (sometimes referred to as upper heating value or gross heating value), the net calorific value (sometimes referred to as lower heating value or net heating value), or the Wobbe index. 

The heat flow rate is measured by detecting the mass flow rate of the fuel gas and multiplying it by its heating value, which can be detected by Wobbe index sensors or calorimeters. Continuous and explosion-proof calorimeters are available for measurement of the heating value of any fuel gas (Section 3.2.4). Table 3.52 lists the compositions of various fuel gases. 

The heat flow rate (Q) of a gaseous fuel is calculated as the product of its volumetric flow rate at standard conditions (V0) and its calorific value (CV). The Wobbe index (WI) measures the ratio between the net CV and the square root of specific gravity (SG). With orifice-type flow sensors, the advantage of detecting the WI is that it eliminates the need to separately measure the specific gravity this is because the product of the WI and orifice pressure drop results in a constant times the heat flow rate (KxQ), without requiring a separate measurement of SG. 

Both the properties of the SNG produced by hydrogasification and the standard Dutch natural gas (Groningen quality) are presented in table 1, As can be seen, the quality of the produced SNG (calorific value and Wobbe-index) is comparable with Groningen natural gas. The Wobbe-index (MJ/Nin ) is defined as the ratio of the gross calorific value to the square root of the relative density of a gas ... 

Refinery gas specifications will vary according to the gas quality available and the end use (Rawlinson and Ward, 1973 Johansen, 1998). For fuel uses, gas as specified above presents little difficulty when used as supplied. Alternatively, a gas of constant Wobbe index, say for gas turbine use, could readily be produced by the user. Part of the combustion air would be diverted into the gas stream by a Wobbe index controller. This would be set to supply gas at the lowest Wobbe index of the undiluted gas. 

Fuel properties such as specific gravify heating value, elemenfal composition, viscosify, ash, moisture content, and autoignition temperature are important to be made available prior to testing. Wobbe index is an indicator that provides a basis for comparing differenf fuels. 

Carburetted water gas Is the result of combining the water gas and oil gas methods. Oil is sprayed into a brick lined chamber during the blow period of the BWG plant with air. This heats the bricks to around 1000°C. The ratio of the two determines the quality. Efficiency of cracking is of order 75%. Wobbe index of the resultant gas of order 24.9 MJ/m. ... 

A fired reboiler control S3rstem using the Wobbe index is shown in Fig. 17.3. The density meter should be installed directly in the fuel gas hne, downstream of the knockout drum and any points where fiiel gas streams are added, and where it would be unaffected by vibrations (370). 

Gases Calorific Value Typical Range Wobbe Index Calorific Value Specific Specific Gravity Specific Gravity... 

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