Combustion and carbon monoxide glossary

Fuel gases

• Natural gas — mostly methane, with small amounts of ethane and other gases. It is not an LPG.
• Gas families — first family: manufactured (town) gas; second family: natural gas; third family: LPG, such as propane and butane.
• Relative density — how dense a gas is compared with the same volume of air at the same temperature and pressure. Natural gas is about 0.6, so an escape tends to rise and disperse. Propane vapour is about 1.5 and butane vapour about 2, so they can collect in low places, although airflow also affects where gas travels.
• Flammability limits — a gas burns in air only when its concentration lies between its lower and upper flammability (explosive) limits, the LEL and UEL. Below the LEL there is too little gas; above the UEL there is too little air.
• Calorific value — the heat released when a unit of gas is burned: the figure that turns a measured gas rate into a kW heat input.

Combustion

• Combustion — burning: fuel gas combines with oxygen from the air and releases heat. It uses oxygen up; it does not produce it.
• Complete combustion — the fuel and oxygen mix and react fully. For natural gas the main products are carbon dioxide and water vapour, and the flame is normally steady and mainly blue. The water vapour lets a condensing boiler recover heat from its flue gases.
• Incomplete combustion — some carbon in the burning fuel does not finish reacting with oxygen to form carbon dioxide. Carbon monoxide can result; some faults also leave soot. Too little air at the flame is one cause, but poor mixing or a flame cooled against a surface can also interrupt the reaction. A damaged burner may mix badly even where room ventilation is open. A smaller, correctly adjusted flame can still burn fully; a flame going out is a separate fault.
• Excess air — air supplied above the theoretical (stoichiometric) amount. A margin is normally provided because mixing is imperfect, but plentiful air overall does not prove that every part of the flame burns completely.
• Aerated burner — mixes air with the gas before it burns, giving a hotter, cleaner blue flame. A non-aerated burner draws its air in only at the flame, giving a cooler, more luminous yellow flame.
• Flame impingement — contact with a cooler object, such as a heat-exchanger surface or pan base, chills part of a burner flame. The reaction may stop short there and release carbon monoxide; soot can also form. Any deposits impede heat transfer and need investigation.
• Flame lift and light-back — flame lift is the flame lifting off the burner ports; light-back is the flame burning back towards the injector.
• Vitiated air — air whose oxygen has already been used up by combustion. Re-using it makes combustion worse and can raise carbon monoxide production.
• Air requirement — each volume of methane needs two volumes of oxygen to burn completely, and air is only about a fifth oxygen, so natural gas needs roughly ten volumes of air for every volume of gas.

Carbon monoxide and analysis

• Carbon monoxide (CO) — a poisonous gas produced by incomplete combustion, with no colour, taste or smell. It is not the odorant added to natural gas and not the fuel gas.
• Soot — unburnt carbon deposited by incomplete combustion. A build-up lowers efficiency, can disturb combustion further and is linked to carbon monoxide.
• Room-sealed appliance — combustion is sealed off from the room by the appliance case and its seals. A perished case seal can let products of combustion into the living space.
• CO/CO₂ ratio — carbon monoxide divided by carbon dioxide from a flue gas analyser, with both in the same units. 10.0% CO₂ is 100,000 ppm, so 400 ppm CO gives 400 ÷ 100,000 = 0.004. In a valid, undiluted sample, a lower ratio means more complete combustion at that test point; it does not prove the whole installation safe.