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Importance of Air Distribution between Burners - Front
Wall Firing
written by: Dr V T Sathyanathan edited by: Lamar Stonecypher
updated: 3/17/2010
Coal, oil and gas burners performance will depend upon making
oxygen available forhydrocarbons being burned at the right quantity
at the right location. This makes the burner
design sensitive to air flow distribution.
In the case of any burner system, the total air is divided into
primary air, secondary, and tertiaryair. Each of this air plays a
very vital role in combustion. The primary air gives the
initial
ignition oxygen, the secondary air allows the higher hydrocarbon
or char to burn, and the tertiary
ensures the delayed combustion process purposely introduced by
designers to reduce NOx level.
While this distribution of total air will be normally 30%
primary, 55 to 60% secondary, andabout 10 to 15% tertiary air. The
burners are designed to ensure the intended ratio by actual
design of the resistance introduced in each section for the
required flow area. All these will befor a set register pressure or
windbox pressure depending upon wall firing or tangential
firing.
Airflow equalization in multi burnersCoal fired boiler
There are three important factors to be kept in mind when
airflow equalization is done among
burners.
o The primary air has to be kept at the minimum required level
with designed excess air
level
o The secondary and tertiary air combined together enter the
individual burner, and this
needs to be equal between all burnerso The proportioning of
secondary and tertiary air is done within the burner and also
needs
to be equal in all burners but the setting can change slightly
depending upon the variationin burners due to manufacturing
tolerances.
All burner and boiler designers provide air equalizing dampers
at inlet of each burner. Thisdamper or shroud is used to vary the
resistance offered by individual burners so as to achieve
equal flow of air at all loads. Many burner manufactures
nowadays prefer to factory-set the
secondary air swirl position and in low NOx burners the most
preferred setting is 30 degree open
for creating a moderate swirl. This keeps the flame long and
reduces the flame temperature. Thesecondary air flow damper is
normally kept full open. The damper or the shroud provided in
the
burner is all first set to full open to create the required
furnace to windbox differential pressure.Some manufactures of
burner- provide individual air flow measurement in each burner.
This can
be used to set equal air flow between burners. In absence of
this it is the practice to keep thestatic pressure equal in all
burners after the shroud or damper provided for flow equBurner
Management SystemsCombustex.com/BMS
After setting equal airflow when the burner is lighted up the
flame colour, flame temperature, the
overall CO level and NOx levels are checked at all loads
(Normally 50%, 60%, 80% and 100%)
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for proper performance. In case required a minor adjustment can
be done after identifying which
burner need to be adjusted.
Airflow equalization in multi burnersOil / Gas fired boiler
Oil and Gas fired boilers are more sensitive to air flow
equalization between burners for fewreasons.
o These burners are designed for much lower excess air
levels
o The primary, secondary and tertiary airs are all proportioned
inside the burner unlike coal
burner.
o Any minor deviation in proportioning can increase the CO level
very high. It can also
cause lot of soot formation which has its own effect in boiler
performance.
The procedure for equalizing air flow between all burners is the
same as coal fired boiler.
About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of
experience in various areas of high
pressure boiler trouble shooting. He holds a PhD in coal
combustion in boilers.
Related Reading
Tuning of Front Wall Coal Burners in Large Capacity Boilers- A
well tuned burner gives the
optimum performance of the boiler. This requires a careful
setting of air flow, fuel flow and the
swirl level which ensures air fuel mixing.
Reading a Coal Flame in Tangential Firing Boilers- In a
tangential coal fired boiler, the furnaceact as a single burner and
so it is required to look at and understand the quality of the
flame. It isnecessary to start from the control room of the boiler,
then go to the mill, furnace, bottom ash
and fly ash areas and study all in detail.
Combustion Tuning in High Ash Pulverized Coal Fired Boilers
having Direct Tangential Firing
System- Combustion tuning in boilers will lead to optimizing the
performance more so in high
ash coal fired boilers. In high ash coal fired boilers having
direct tangential firing systems,proper combustion will ensure
minimum water wall deposits and thus an optimal furnace outlet
temperature
http://www.brighthubengineering.com/power-plants/64336-tuning-of-front-wall-coal-burners-in-large-capacity-boilers/http://www.brighthubengineering.com/power-plants/64336-tuning-of-front-wall-coal-burners-in-large-capacity-boilers/http://www.brighthubengineering.com/power-plants/48616-understanding-flame-quality-in-tangential-firing-boilers/http://www.brighthubengineering.com/power-plants/48616-understanding-flame-quality-in-tangential-firing-boilers/http://www.brighthubengineering.com/power-plants/37755-combustion-tuning-in-high-ash-pulverized-coal-fired-boilers/http://www.brighthubengineering.com/power-plants/37755-combustion-tuning-in-high-ash-pulverized-coal-fired-boilers/http://www.brighthubengineering.com/power-plants/37755-combustion-tuning-in-high-ash-pulverized-coal-fired-boilers/http://www.brighthubengineering.com/power-plants/37755-combustion-tuning-in-high-ash-pulverized-coal-fired-boilers/http://www.brighthubengineering.com/power-plants/37755-combustion-tuning-in-high-ash-pulverized-coal-fired-boilers/http://www.brighthubengineering.com/power-plants/48616-understanding-flame-quality-in-tangential-firing-boilers/http://www.brighthubengineering.com/power-plants/64336-tuning-of-front-wall-coal-burners-in-large-capacity-boilers/
