Wobbe number

Wobbe Number. Wobbe Number is an indicator of the characteristics and stability of the combustion process. 

Increasing the Wobbe Number can cause the flame to burn closer to the liner. Decreasing the Wobbe Number can cause pulsations in the combustor. 

The Wobbe number of gas is defined as The heat release when a gas is burned at a constant gas supply pressure . It is represented by ... 

The relative density (with respect to air) has no units, so that the units of the Wobbe number are the same as calorific value. The value for gas is 49.79 MJ/m. ... 

The Wobbe number is of interest because it represents the heat released at a jet. A gas of varying CV but constant Wobbe number would give a constant heat release rate. 

Fuel gas to fuel gas purging The purge may be carried out by burning the gas under supervision on an appliance or appliances. If the two fuel gases have different burning characteristics and Wobbe numbers, correct burner selection is important. 

The effect of the air is to depress the vapor dew point temperature. A further advantage is that the physical properties of the gas can be made to simulate another gas, e.g. natural gas or manufactured town gas. Such a simulated gas will produce the same heat release through a burner if the supply pressure is the same. This is characterized by a term known as the Wobbe number W) ... 

The gas-air ratio is therefore chosen to achieve the desired Wobbe number. However, the use of a simulated gas for plant conversion may not entirely avoid burner... 

Several methods have been investigated to find correlations between physical properties of fuel gas mixtures and the excess air ratio to optimize the combustion procedure. In spite of the varying composition of natural gas it is said to be possible to control a heater system by measurements of the dynamic viscosity of the gas [7]. One explanation could be the correlation between Wobbe number and viscosity With increasing Wobbe numbers the viscosity decreases, and if the Wobbe number of a gas is known, the excess air ratio can be adjusted, resulting in an open loop control. 

Wittig reaction, 2 64-65, 19 65 Wobbe Number, 12 378-379 Wohl-Aue synthesis, 2 787 Wohlwill refining process, 22 647 Wolff-Chaikoff effect, 14 373 Wolff-Kichner reduction, 13 569-570,... 

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). 

Another important combustion criterion is the gas modnlns, M = P jW, where P is the gas pressnre and W the Wobbe number of the gas. This must remain constant if a given degree of aeration is to be maintained in a preaerated burner nsing air at atmospheric pressure. 

Three properties of the simulated test fuels that need to be matched are the isentropic coefficient, molecular weight, and lower heating value (LHV). These properties need to be closely approximated to reproduce the fuel pressure versus heat release relationships. The Wobbe number may also be calculated to determine similarity in fuels. The chemical properties that need to be matched are the adiabatic flame temperature, inert content, olefins and hydrogen content, and the LHV. These properties need to be closely approximated so that the combustion process produces similar flame heights, pollutant emissions, and flue gas temperatures. 

For better burner performance analysis, it is necessary to have the composition of the fuel, its density, heating value, and Wobbe number (WN). All those characteristics can be obtained from the local analytical laboratory services or measured using gas chromatography techniques followed by the simple calculations ... 

If a fuel gas is substituted on the same burning installation (i.e., area and pressure drop remain the same) the new heat release rate may be calculated as the ratio of the GCV multiplied by the square root of gas density Q1/Q2 = (GCV/Vpi)/(GCV/Vp2) = (GCV/VSPi)/ (GCV/VSG2) (where SG is the specific gas density relative to air density). Thus, a number may be derived that gives an indication of the interchangeability of the gases, the Wobbe number (Wo = GCV/VSG). In practice, the specific gravity with relation to air is used instead of density. 

In general, the flame speed depends upon the kinetics of combustion and hence varies from gas to gas. The H2 flame is the fastest with the normal velocity having the maximum value of about 2.5 m/s. The CO flame has a lower normal velocity in the range of 0.3 to 0.4 m/s. The flame speed rises if the initial temperature of the combustible mixture is increased or the pressure of the system decreased. If the Wobbe number and Weaver flame speed factor are identical for two gases they are completely interchangeable. 

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