science

Wobbe index

Wobbe index is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wobbe index rather than just read about it. In short: The Wobbe index (WI) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities. If V C {\displaystyle V_{C}} is the higher heating value, or higher calorific value, and G S {\displaystyle G_{S}} is the specific gravity, the Wobbe index, I W {\displaystyl…

Key takeaways

Wobbe index belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wobbe index to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wobbe index from memory before moving on to harder problems.

Reference excerpt

The Wobbe index (WI) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities. If V C {\displaystyle V_{C}} is the higher heating value, or higher calorific value, and G S {\displaystyle G_{S}} is the specific gravity, the Wobbe index, I W {\displaystyle I_{W}} , is defined as:

I W = V C G S . {\displaystyle I_{W}={\frac {V_{C}}{\sqrt {G_{S}}}}.}

G S = ρ S T P ρ a i r , S T P = M M a i r {\displaystyle G_{S}={\frac {\rho _{STP}}{\rho _{air,STP}}}={\frac {M}{M_{air}}}}

ρ S T P {\displaystyle \rho _{STP}} is the density of the gas at standard conditions, the definition of which changed in 1982. Published Wobbe data may be using 0 °C, 15 °C, 15.56 °C, 20 °C or 25 °C. EU directives on gas quality use 15 °C in accordance with ISO 13443 and ISO 6976.

ρ a i r , S T P {\displaystyle \rho _{air,STP}} is the density of air at standard conditions, M {\displaystyle M} is the molar mass of the gas and M a i r {\displaystyle M_{air}} is the molar mass of air which is about 28.96 kg/kmol. The Wobbe index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.). If two fuels have identical Wobbe indices then for given pressure and valve settings the energy output will also be identical. Typically variations of up to 5% are allowed as these would not be noticeable to the consumer. The Wobbe index is a critical factor to minimise the impact of the changeover when analyzing the use of substitute natural gas (SNG) fuels such as propane-air mixtures. The Wobbe index also requires the addition of propane to some upgraded biomethane products, particularly in regions where natural gas has a high calorific value such as Sweden. The Wobbe index has its origins in the 1920's with Italian physicist and engineer Goffredo Wobbe.

Wobbe index of common fuel gases Note that these Wobbe numbers below are not calculated at 15 °C and so are not correct according to ISO 13443, see Standard temperature and pressure.

Note: 1 Joule = 2.3901×10−4 kcal.

Usage The Wobbe index is expressed in MJ/Nm³ (where 'Nm³' indicates 'm³ in Normal conditions'), or sometimes in BTU/scf. In the case of natural gas (molar mass 17 g/mol), the typical heating value is around 39 MJ/Nm³ (1,050 BTU/scf) and the specific gravity is approximately 0.59, giving a typical Wobbe index of 51 MJ/Nm³ (1,367 BTU/scf). There are three ranges or "families" of fuel gases that have been internationally agreed based on Wobbe index. Family 1 covers manufactured gases, family 2 covers natural gases (with high and low ranges) and family 3 covers liquefied petroleum gas (LPG). Combustion equipment is typically designed to burn a fuel gas within a particular family: hydrogen-rich town gas, natural gas or LPG.

Other flame characteristics and composition limits may determine the acceptability of the replacement gas, e.g. flame speed, "yellow tipping" due to incomplete combustion, sulfur content, oxygen content, etc.

Limitations In spite of its usefulness, Wobbe index alone is not a good indicator of the interchangeability of two or more gases, or mixtures of them. It is necessary to bear in mind other criteria while determining the plenty substitution of a fuel by other, different of the one used to adjust the burning system.

References "The Wobbe Index and Natural Gas Interchangeability" (PDF). Emerson Process Management. 2007-07-30. Archived from the original (PDF) on 2012-03-24. Retrieved 2010-08-26.

Worked examples

Example 1 — a first encounter with Wobbe index

Start with the simplest possible case. Write down what Wobbe index claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wobbe index before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wobbe index ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wobbe index

In research: Wobbe index appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wobbe index in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wobbe index is common in secondary-school and first-year university syllabi. It links to neighbouring topics Eponymous indices, Fuel gas, Gas technologies, so understanding it makes those chapters shorter.
In everyday life: Look for Wobbe index outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wobbe index in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wobbe index means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wobbe index out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wobbe index in simple terms?

Why does Wobbe index matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wobbe index?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wobbe index.