How to Judge Whether a Gas will Be Ignited Efficiently? This Depends on Its Heating Value, Wobbe Index and Methane Number.

Natural gas can be ignited immediately, while blast furnace gas is difficult to ignite, has the problem of burning instabilitye, and it will knock easily. Both are combustible gases — so why such a big difference? The key isn't how clean the gas is, but the Heating Value, Wobbe Index, and Methane Number — these three indicators determine whether a gas can burn well and generate power stably.

01

Heating Value: How much Energy Is Hidden in a Cubic Meter of Gas

Heating value refers to the heat released by the complete combustion of a unit volume of gas. To generate the same amount of electricity, the lower the gas heating value, the larger the intake volume required.

Heating value of natural gas is about 37.78 MJ/Nm³, while of blast furnace gas is only 3.3 – 3.8 MJ/Nm³, so its intake volume must be magnified tenfold or even more.

02

Wobbe Index: Will the "Firepower" Change, When Switching Gases?

Wobbe Index = Heating Value ÷ √Specific Gravity

It measures whether the firepower of the same equipment remains stable after switching gases. According to GB 50028 and GB/T 13611, taking 12T reference gas as an example, the standard value of Wobbe index is 50.73 MJ/Nm³, with an allowable fluctuation range of 48.19 – 53.27 MJ/Nm³ (±5%). Beyond this range, direct interchange is generally not possible.

Wobbe index of natural gas is about 50.7 MJ/Nm³, while blast furnace gas is only about 3.5 MJ/Nm³ — a difference of about fifteen times. Direct interchange will inevitably "fail to burn."

03

Methane Number: Will the Gas Knock?

The Methane Number (MN) is an indicator measuring the anti-knock performance of gas fuels.

Simply put, by comparing the test gas with methane-hydrogen reference gas, when the anti-knock performance is the same, the volume percentage of methane in the reference gas equals the MN.

Pure methane = 100, hydrogen = 0. The lower the MN, the more prone to knock. Knock can cause unstable power generation or, at worst, damage to the unit.

04

For Gas-fired Power Generation, How to Consider the Aforementioned Three Indicators

Natural gas is easy to use because these three indicators are stable. Industrial exhaust gas is exactly the opposite: its composition fluctuates greatly, and the three indicators change at any time, so the generator set must adapt in real time. For exhaust gas power generation, these three indicators govern three things:

Heating value: determines intake volume and output power. For a gas whose heating value is only one-tenth that of natural gas, the intake volume must be ten times or more, and the turbocharging and mixing systems must be configured according to this gas volume.

Wobbe index: affects gas source interchangeability. When switching gases, confirm that the Wobbe index is within the allowable range of the unit, and recalibrate if necessary.

Methane number: determines the knock control strategy. For gases with low MN, knock detection and lean-burn control must be provided, and load reduction operation is required when operating conditions fluctuate greatly.

05

Conclusion

Once you understand these three indicators, the next time you encounter "why is this gas hard to burn," you can judge whether the problem lies in energy, matching, or combustion stability.

Of course, the three indicators are only core references. Actual engineering projects also need to evaluate gas volume stability, medium impurities, and pressure and temperature conditions. The prudent approach is to collect gas samples and carry out a professional assessment.