The objective of a boiler is to burn the hydrogen contained in the fuel with oxygen from the atmosphere to produce heat.
Combustion efficiency analysers exploit the fact that by knowing the fuel (and its chemical composition) and measuring the flue gas temperature and either the oxygen or carbon dioxide level the efficiency of the boiler can be calculated.
On some boilers the settings can then be adjusted to maximise the efficiency.
In a perfect world the maximum efficiency would be achieved with 0% oxygen in the flue and the lowest flue gas temperature. In the real world allowance must be made for variations and uncertainties and so 0% oxygen is not practical. The settings on a boiler must allow for differences in fuel composition, atmospheric pressure, wind direction, boiler demands etc.
If the oxygen level is set too low and something changes the combustion process can become ‘fuel rich’ as there is insufficient oxygen for all the fuel to burn. This can cause high levels of CO to be generated and in the extreme enough fuel to enter the boilers flue and ignite (explode) outside the combustion chamber.
Typically for a natural gas boiler oxygen readings may be in the range 3% to 5%, for an oil boiler 5% to 8% and for a coal fired boiler 8% to 10%.
The efficiency of modern condensing gas boiler can be over 100% as heat is extracted from the incoming air. A traditional brick built coal fired boiler may only be 50% efficient.
Combustion Efficiency v Boiler Efficiency
A combustion or flue gas analyser is used to measure the efficiency of the combustion process within a boiler. This is not the same as the boiler efficiency as it does not take account of, for example, the heat losses from the case of the boiler. So generally the efficiency stated on the rating plate of the boiler will always be lower than the measured efficiency of combustion.
Net Combustion Efficiency v Gross Combustion efficiency
Net combustion efficiency calculations assume that the energy contained in the water vapour which is formed as a product of combustion is recovered and is not exhausted from the flue or stack.
Gross combustion efficiency calculations assume that the energy contained in the water vapour is not recovered.
Typically the difference between the value of Net combustion efficiency and the value of Gross combustion efficiency for a natural gas fuelled boilers is around 8% with the net value being higher than the gross value.
Most boiler manufacturers now quote their boiler efficiencies based on the Net combustion efficiency.
Some modern boilers are now described as being condensing boilers. The combustion efficiency calculation must be modified to properly reflect the efficiency of the combustion process within these boilers. The practical difference is that a condensing boiler utilises an additional heat exchanger just before it exhausts the flue gases. This extracts additional heat from the flue gas and further reduces energy losses. Under certain circumstances this can lead to net condensing combustion efficiencies of greater than 100%.
Typically for a natural gas fuelled boiler the temperature of the flue gas being exhausted needs to be less than 50oC for the condensation process to recover additional energy. Above 50oC the normal Net combustion efficiency calculation operates.
There is a theoretical amount of fresh air that when mixed with a fixed amount of fuel, and burnt will result in perfect combustion. In this situation all of the fuel will have been properly burnt and all of the oxygen in the air will have been consumed. In this circumstance there will be no excess air and combustion efficiency will be maximised.
In the real world perfect combustion is not possible. The theoretical amount of fresh air would provide insufficient oxygen for complete combustion and some of the carbon in the fuel would be converted into carbon monoxide rather than carbon dioxide. A lack of air can lead to dangerous levels of carbon monoxide being formed and smoke being produced.
Therefore it is usual to adjust the combustion process so that a level of excess air is present to give a margin for safety. This level is set to account for any likely process variable, e.g. the variability of the fuel supply, changes in atmospheric pressure, changes in wind direction etc.
where O2 m% = the measured value of oxygen in the exhaust.
NB - Consult the manufacturer's handbook for details of the recommended excess air levels for each particular appliance.
This information is supplied in good faith for use by trained professionals after due consideration of any risks involved.
Always check with the equipment manufacturer and with local regulations before making any adjustments to combustion processes.
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