In actual practice, gas, oil, coal
burning and other systems do not
do a perfect job of mixing the fuel
and air under the best achievable
conditions. Additionally, complete
mixing may be a lengthy process.
Figure 1 shows that in order to
ensure complete combustion and
reduce heat loss, excess air has to
be kept within a suitable range.
The regulation of excess air provides:
A better furnace heat transfer rate
An 'advance warning' of flue gas problems (excess air coming out of the zone of maximum efficiency)
Substantial savings on fuel
Oxygen trim
When a measurement of oxygen in the fuel gas is available, the combustion control mechanism can be vastly improved (since the
percentage of oxygen in the flue is closely related to the amount of excess air) by adding an oxygen trim control module, allowing:
● Tighter control of excess air to oxygen setpoint for better efficiency
● Faster return to setpoint following disturbances
● Tighter control over flue emissions
● Compliance with emissions standards
● Easy incorporation of carbon monoxide or opacity override
Burner modulation
Modulation control is a basic improvement in controlling combustion.
A continuous demand signal is generated by a controller monitoring
the furnace atmosphere.
Reductions in temperature lead to an increase in firing rate. The
advantages of introducing burner modulation in combustion control
include:
● Fuel and air requirements are continuously matched to the
combustion demand
● Furnace temperature is maintained within closer tolerances
● Greater furnace efficiency
● Weighted average flue gas temperature is lower
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