Learn & Understand

Fire Makes Water: The Chemistry of Combustion Moisture

It seems paradoxical that burning a dry fuel produces water, yet every hydrocarbon flame does exactly that. The hydrogen in fuel combines with oxygen to form water vapor, so fire is, quietly, a water-making process — a fact with consequences from corroding chimneys to the contrails streaming behind aircraft high overhead.

Hydrogen Becomes Water

Fuels are made of carbon and hydrogen, and when they burn, the hydrogen combines with oxygen from the air to form water — the same reaction that makes water from its elements. This water leaves the flame as invisible vapor mixed into the hot exhaust. Every kilogram of hydrogen in the fuel produces several kilograms of water, a fixed ratio set by chemistry.

Dry Fuel, Wet Exhaust

This is why even bone-dry fuel yields a wet exhaust. The water is not moisture that was in the fuel; it is created fresh in the burning, from the fuel's hydrogen and the air's oxygen. A natural gas flame, rich in hydrogen, produces especially large amounts of water vapor — which is why gas appliances can drip condensation and clean gas stacks show pronounced white plumes.

The Corrosion Risk

The water in flue gas is not always benign. If the exhaust cools below the point where its vapor condenses, liquid water forms on ductwork and stacks — and if the fuel contained sulfur, that water turns acidic and corrosive. This is why conventional systems keep the gas warm enough to stay above condensation, while condensing appliances are specially built to handle the water they deliberately capture.

Contrails in the Sky

The most visible sign of combustion water is high above us. The white streaks behind jet aircraft — contrails — are largely the water vapor from burning jet fuel, condensing and freezing in the frigid air of high altitude. Water vapor is itself a greenhouse gas, and these persistent ice clouds are one way aviation's combustion touches the sky far from any stack.

Calculating Water Vapor

To find water vapor produced from fuel hydrogen, use the Flue Gas Water Vapor Calculator. See its effect on the plume with the Wet Flue Gas Calculator, and its condensation risk with the Flue Gas Dew Point Calculator.