Combustion in a gas boiler is the process of burning a mixture of fuel gas and air. Hydrocarbon molecules in the fuel gas react with the oxygen molecules from the air and a lot of heat is released. The products of this combustion are carbon dioxide and water. The water is in the form of steam or water vapour which, if it is cooled enough, becomes liquid water. Carbon dioxide is one of the gases we breathe out all day and night. Carbon dioxide is needed for the growth of all green plants, which convert it back to hydrocarbons. Producing hydrocarbons takes a lot of energy which plants get from the sun. When plants are burned, or when the oil or gas formed originally from plants is burned, the energy is released again as heat.
Combustion of methane and propane
For methane (the main hydrocarbon in piped natural gas), 1 volume of methane combines with 2 volumes of oxygen. However, since air is only about one-fifth oxygen (21% of air is oxygen), about 10 volumes of air are needed to burn 1 volume of natural gas.
For propane (LPG), 1 volume of propane combines with 5 volumes of oxygen, so about 25 volumes of air are required to burn 1 volume of LPG.
These are the theoretical minimum amounts of air required. In reality, a certain amount of excess air is required to achieve complete (clean) combustion. In modern, high efficiency boilers this excess air may be as little as 10%. In older boilers the excess air may be over 100%. If there is insufficient excess air, clean combustion becomes impossible and poisonous carbon monoxide is produced.
Soot, carbon monoxide and CO alarms
Where carbon monoxide is produced there may also be soot produced which will eventually block the heat exchanger. In open-flued appliances the flue gases will then spill out into the room air carrying poisonous carbon monoxide with them. With all open-flued appliances we would recommend that an electronic carbon monoxide alarm is fitted in the room. These are freely available for less than £15 from DIY outlets, supermarkets and pharmacies. Most of them last for 5 years and should then be replaced.
Vehicles burn petrol or diesel and oil boilers burn kerosene. These are all hydrocarbon fuels which are combined with oxygen from air to produce carbon dioxide and water. In vehicles, the heat is converted to mechanical energy and the kinetic energy of movement.
Like all animals, humans burn hydrocarbon fuels (food), combining the hydrocarbons with oxygen to release energy. Our remarkable body chemistry allows this to be done at low temperatures. Interestingly, though we are all aware that we breathe in oxygen and release carbon dioxide as we breathe out, we forget that most of what we breathe in and out is nitrogen. About four-fifths (78%) of air is made up of nitrogen; nearly all the rest is oxygen. In a similar way to us breathing it in and out, the nitrogen in air passes into the boiler and out again via the flue, largely unchanged.
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