Interchangeability is the question of whether one batch of gas can replace another without upsetting the equipment that burns it, and there is no single number that answers it for every end use. Two very different metrics are in common use, and they measure different things. The Wobbe index describes how much heat a burner or appliance will release when it is fed a gas at a given pressure, so it governs whether furnaces, boilers, and domestic appliances fire correctly. The methane number describes how resistant a gas is to knocking when it is burned in a reciprocating engine, so it governs whether gas-fuelled engines run without damaging detonation. A gas can satisfy one of these while failing the other, which is why anyone blending or delivering gas to mixed end uses has to watch both. This guide explains what each metric captures, why they diverge, and how a monitoring system tracks both from live composition.
Methane number vs Wobbe in one line: The Wobbe index and the methane number are two separate interchangeability metrics that answer different questions. The Wobbe index measures how much heat a burner releases at a fixed supply pressure, so it governs the correct operation of appliances, burners, and furnaces. The methane number measures a gas mixture's resistance to knock in a reciprocating engine, so it governs whether an engine can burn the gas without damaging detonation. A gas can pass one and fail the other, so applications with both burners and engines must monitor both, which SCADA can compute continuously from live chromatograph composition.
The Wobbe index is defined so that two gases with the same Wobbe index will deliver roughly the same heat rate through a given orifice at a given pressure. It is the higher heating value of the gas divided by the square root of its relative density, and that combination is exactly what matters at a burner nozzle, because the flow through a fixed orifice depends on density while the energy carried by that flow depends on heating value. When the Wobbe index matches, an appliance that was set up for one gas will fire at about the same rate on another without its air-fuel balance drifting into sooting, flame lift, or incomplete combustion. This is why Wobbe is the master interchangeability parameter for appliances and burners and why pipeline tariffs put upper and lower Wobbe limits on delivered gas.
The methane number answers a completely different question that a burner never asks. In a reciprocating engine the fuel-air charge is compressed before it is ignited, and if the fuel is prone to auto-igniting under compression it will knock, meaning it detonates ahead of the flame front in a way that hammers the engine and can destroy it. The methane number rates a gas on a scale where pure methane, which is very knock-resistant, sits at one hundred, and it tells you how much compression the gas will tolerate before it knocks. Heavier hydrocarbons such as propane and butane lower the methane number sharply because they auto-ignite far more readily than methane, while hydrogen lowers it too and inerts such as nitrogen and CO2 raise it because they dilute the knock-prone components.
So the two metrics look at the gas through different lenses. Wobbe is about heat release through an orifice and cares about heating value and density together. Methane number is about detonation resistance under compression and cares about how much heavy hydrocarbon, hydrogen, or inert the gas carries. There is no fixed relationship between them, because the composition changes that move one do not move the other in any consistent way. That independence is the whole reason both are needed.
The clearest way to see the divergence is to imagine a gas getting richer in heavy hydrocarbons, picking up more propane and butane. Those heavies raise the heating value and the density, and the Wobbe index may stay comfortably inside its limits or move only modestly, so from a burner's point of view the gas looks acceptable. But the same heavies are exactly what a reciprocating engine hates, because they slash the methane number and make the charge prone to knock. That gas can be perfectly interchangeable for a furnace and simultaneously dangerous for an engine running at high compression. A single Wobbe check would pass it and miss the problem entirely.
The divergence runs the other way too. Adding inerts such as nitrogen or carbon dioxide dilutes the heating value and pulls the Wobbe index down, potentially below its lower limit, so the gas may fail interchangeability for burners because it no longer releases enough heat through the nozzle. Yet those same inerts raise the methane number, because they take up space that would otherwise hold knock-prone hydrocarbons, so from an engine's point of view the gas has become more tolerant, not less. A gas that an engine would happily accept can be off-spec for appliances, which is the mirror image of the heavy-hydrocarbon case.
This is why an operation that feeds both burner loads and engine loads from the same gas cannot rely on one metric. A lease that runs a gas-fired engine for a compressor or a generator while also feeding heaters, or a pipeline blending point that delivers gas to industrial burners and to engine-driven equipment alike, has to satisfy the Wobbe limits for the burners and the methane number floor for the engines at the same time. Rich gas is the classic trap, because it flatters the burner metric while quietly starving the engine metric, and an operator watching only Wobbe can send knock-inducing gas to an engine without any warning from the number they were watching.
Both the Wobbe index and the methane number are computed from the gas composition, which means a custody or fuel-gas chromatograph already measures everything needed to calculate them on every cycle. The Wobbe index falls straight out of the heating value and relative density that the analysis already produces. The methane number is derived from the full component breakdown through one of the established methods, so that the fractions of methane, the heavier hydrocarbons, hydrogen, and the inerts each weigh into the knock-resistance rating. Because both are functions of the same composition, a control or analysis system can update both metrics every time the chromatograph reports a new result, giving a continuous live view of interchangeability for burners and for engines together.
Bringing both into a cloud SCADA platform such as Merobix lets an operation put a limit and a trend on each metric independently, which is exactly what the two-sided nature of the problem demands. Wobbe carries an upper and lower limit for the appliance and burner side, and the methane number carries a lower limit for the engine side below which knock risk becomes unacceptable. Trending the two side by side against a shifting composition makes it visible when the gas is drifting rich, because the operator sees the methane number sliding down toward its floor even while the Wobbe index still looks healthy, and that early divergence is precisely the warning a single metric would hide.
For engine-fuel and pipeline-blending applications this continuous, dual monitoring is the practical safeguard. At a blending point, watching both metrics as the blend ratio changes lets the operation trim the mix to keep burners and engines in spec at once rather than optimising for one and tripping the other. At an engine site, an alarm on the methane number crossing its floor can prompt a load reduction or a fuel change before knock damages the engine, well ahead of any audible knock or engine trip. Because the platform derives both metrics from the live chromatograph rather than from a spot lab sample, the protection tracks the gas in real time and does not lapse in the hours between manual samples, which is when a rich-gas excursion is most likely to catch an operator out.
The Wobbe index measures how much heat a gas releases through a burner orifice at a given pressure, so it governs whether appliances and burners fire correctly, and it is the higher heating value divided by the square root of relative density. The methane number measures a gas mixture's resistance to knock in a reciprocating engine on a scale where pure methane is one hundred. They describe unrelated behaviours, one for burners and one for engines, so both are needed for an operation that uses both.
Yes, and rich gas is the classic example. When a gas picks up heavy hydrocarbons such as propane and butane, its Wobbe index may stay within limits while its methane number drops sharply, because those heavies auto-ignite readily and promote knock in an engine. A burner would accept the gas while a high-compression engine could be damaged by it, which is exactly why watching only Wobbe is not enough where engines are fuelled.
Nitrogen and carbon dioxide dilute the heating value, which lowers the Wobbe index and can push it below its limit for burners, so they can make a gas off-spec for appliances. At the same time those inerts raise the methane number, because they take up space that would otherwise hold knock-prone hydrocarbons, so they make the gas more knock-tolerant for engines. The two metrics move in opposite directions, which is why they must be tracked separately.
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