Combustion efficiency is a calculation of how well your equipment is burning a specific fuel, shown in percent. Complete combustion efficiency would extract all the energy available in the fuel. However 100% combustion efficiency is not realistically achievable. Common combustion processes produce efficiencies from 10% to 95%. Combustion efficiency calculations assume complete fuel combustion and are based on three factors:
1. The chemistry of the fuel.
2. The net temperature of the stack gases.
3. The percentage of oxygen or CO2 by volume after combustion.
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COMBUSTION PROCESSES AND THEIR COMBUSTION EFFICIENCY RANGES
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PROCESS
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TYPICAL COMBUSTION EFFICIENCY RANGE
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| HOME FIREPLACE |
10-30 %
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| SPACE HEATER |
50-80 %
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| COMMERCIAL GAS BOILER |
70-82 %
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| RESIDENTIAL GAS BOILER WITH ATMOSPHERIC BURNER "LOW EFFICIENCY" |
70-82 %
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| OIL BURNER HEATING SYSTEM |
73-85 %
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| INDUCED DRAFT FURNACE "MEDIUM EFFICIENCY" |
74-80 %
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| BOILER WITH GAS BURNER |
75-85 %
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| CONDENSING FURNACE (GAS & OIL) "HIGH EFFICIENCY" |
85-93 %
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If your calculation shows that your equipment is losing 20 % of the heating energy of the fuel through stack losses, your equipment is running at 80 % efficiency.
Combustion efficiency relates to the part of the reactants that combine chemically. Combustion efficiency increases with increasing temperature of the reactants, increasing time that the reactants are in contact, increasing vapour pressures, increasing surface areas and increasing stored chemical energy. One way of increasing the temperature of the reactants and their vapour pressures is to preheat them by circulating them around the combustion chamber and throat before being injected into the combustion chamber. The specific heat of combustion is a chemical property that refers to the amount of energy that can theoretically be extracted from a fuel at 100 % combustion efficiency. The heating value is a more realistic term and does not include the condensation of the water vapour produced. It is thus more easily applied to real combustion processes.
Air preheating is one method used in steel works, for instance, to increase combustion efficiency. This uses the heat in the flue gases to heat one of a pair of chambers and the inlet air passes through the other one. The use of the chambers is switched as soon as the one chamber has reached temperature, so the air passes through the heated chamber. This is one of the simplest and best methods of increasing combustion efficiency in this kind of process. Such preheaters are standard euipment these days for larger systems.
The introduction of the condensing burner has led to the strange situation where combustion efficiencies in excess of 100 % are reported. These devices also use the specific heat of vaporisation as a surce of energy and therefore have an increased yield, provided they are operated in the appropriate temperature range.