Wobbe index

The Wobbe index , also Wobbewert , Wobbekennzahl , Wobbe index (after the gas engineer and inventor Goffredo Wobbe , which it developed in 1926), is used to characterize the quality of fuel gases (eg. As natural gas , town gas ).

The measurement takes place through controlled combustion of the test gas. A test flame heats a mantle , the temperature of which is measured. The mantle material must not oxidize . A calibration is carried out with calibration gases.

definition

Upper wobbe index

${\displaystyle W_{s}={\frac {H_{s}}{\sqrt {\frac {\varrho }{\varrho _{0}}}}}}$ ("s" from superior, formerly W _o )

With

• Calorific value ${\displaystyle H_{s}}$
• relativ density ${\displaystyle {\frac {\varrho }{\varrho _{0}}}}$
  • Density of the fuel gas ${\displaystyle \varrho }$
  • Dense dry air under the same pressure and temperature conditions. ${\displaystyle \varrho _{0}}$

Lower Wobbe index

${\displaystyle W_{i}={\frac {H_{i}}{\sqrt {\frac {\varrho }{\varrho _{0}}}}}}$("i" from inferior, formerly W _u )

With

• Calorific value . ${\displaystyle H_{i}}$

The unit of the Wobbe index is  J / m³ or  kWh / m³, like the unit of the calorific value.

Wobbe index of various fuel gases

	Lower Wobbe index Wi	Upper Wobbe index Ws
Propane , C 3 H 8	74.74 MJ / m³	81.18 MJ / m³
Methane , CH 4	48.17 MJ / m³	53.45 MJ / m³
Russian natural gas	47.97 MJ / m³	53.21 MJ / m³
Verbund Erdgas Nord	46.54 MJ / m³	51.55 MJ / m³
Biomethane (96% CH 4 )		51.03 MJ / m³
Hydrogen , H 2	40.90 MJ / m³	48.34 MJ / m³
Biogas (65 vol .-% CH 4 )		28.44 MJ / m³

application

The Wobbe index is required to assess the exchangeability of fuel gases.

If z. B. If natural gas is to be replaced by propane , it is not enough to produce a mixture with the same calorific value. Since this mixture would have a higher density than the original natural gas, a larger mass flow would result through the nozzles of the burner (but a smaller volume flow with the same nozzle diameter) and this would result in a higher energy conversion .

Only when the density is included in the Wobbe index does the volume flow , which is necessary to implement the same amount of energy and thus achieve the same heat load in the burner (e.g. a heating system) , result with the same nozzle pressure . If the mass or volume flow cannot be regulated appropriately, the burner nozzle must be exchanged for a larger or smaller one.

Where blast furnace gas is also available for underfiring a coke oven battery , blast furnace gas and coke oven gas are mixed according to the Wobbe index.

units

• 1 kWh / Nm³ = 3.6 MJ / Nm³ = 3.6 · 10 ⁶  J / Nm³ (1  Nm³ = 1 m³ under standard conditions : 1,013.25 mbar, 0 ° C)
• 1 Btu / scf = 0.03525 MJ / Nm³ (1 Btu = 1.05506 kJ: British thermal unit ; 1 scf = 0.02679 Nm³: Standard cubic foot = 1  cft at standard conditions: 1,013.25 mbar, 15.56 ° C = 60 ° F)

literature

• DIN 51857 (03.97): "Gaseous fuels and other gases - calculation of gross calorific value, calorific value, density, relative density and Wobbe index of gases and gas mixtures"

Individual evidence

1. ↑ Ing. Goffredo Wobbe, La definizione della qualita del gas, in the specialist journal L'industria del gas e degli acquedotti, Volume XV (15), nº 11, Roma, November 30, 1926, pp. 165-172