Excess air consideration in FBC / AFBC & CFBC boilers

Excess air consideration in combustion for FBC / AFBC & CFBC boilers

My View

In FBC, AFBC, CFBC boilers the excess air is measured in the stack & often considered as excess air which can take care of proper combustion.

However based on my experiences in working with these boilers, the excess air determination has to be based on other assessments also along with Stack O2% for arriving to true excess air.

Methods for determining excess air

I) By given free O2 % value of the Boiler manufacturer
II) By calculation
III) By measurement 

I Value by Boiler Manufacturer

The free O2 % value for the stack given by the Boiler manufacturer is actually the design value by considering many parameters of fuel, air, excess air, flue gas velocity, thermal capture efficiency of the system etc.

II How does one calculate excess air :

After several studies upon boiler design, I present you with a simple formula which can help in calculating the free O2 % as per design

Free O2 % in Stack = ((100 / DE) -- 1) * 21.53 %

Here
DE is Design Efficiency of the Boiler
21.53 is a constant for O2 volume with 5% error (20.5*1.05)

The error of 5% is standard considering the ducts, air leakages etc.

For example for a 82% efficiency boiler by design, the free O2 can be calculated as

Free O2 = (100/82 -- 1) * 21.53 % = 4.73 %

Excess Air Calculation = Free O2 by Design / 20.5 * 100

Excess air = 4.73 / 20.5 * 100 = 23.05%

Inference 

1) The Boiler design efficiency is always a factor of excess air. Lesser the excess air, higher will be efficiency gain by design as per the above formula
(However the opposite is not true, if the excess air is reduced than design value, it does not increase efficiency but decreases it due to CO2 + C, reaction)

2) This also means that if free O2 is exceeded than design value, the extra volume of air removes the heat from the system & should lead to loss in efficiency

3) Achieving Design free O2% in the stack, means achieving design draft, design flue gas velocity, design air injection velocity, design turbulence & designed reactivity of C & O2 in the combustion chamber

4) Not achieving Design free O2 %, means the opposite

5) This also means that if free O2 is less than Design free O2 %, then draft will be compromised, flue gas velocity, air injection velocity, turbulence, designed reactivity of C & O2 in the combustion chamber are compromised

6) Running the boiler in Lesser than design O2 % is not a correct operation as it leads to lesser efficiency when checked in the Direct Method

My Experience :

1 % excess O2 (5% excess air) is increasing heat flight from the system approximately lowering the efficiency by 3.5 to 4% by Direct Method

1 % deficient O2 (5% deficient air) is increasing CO2 + C endothermic reaction in the system approximately lowering the efficiency by 3.5 to 4% by Direct Method

Which fan is giving excess air, when there are two or more fans ?

This has been my persistent question to many boiler users

I could deduce the following from my experience in working with the boilers

1) Boiler with PA fan & FD Fan
     a) The PA fan is giving maximum excess air in the boiler
     b) The FD fan is giving combustion air in the boiler with bare minimum excess air
     c) The PA fan air volume is approx. 15 to 18% which is of higher velocity & the balance air is from FD fan which is of lower velocity

2) Boiler with PA fan,  FD Fan & SA Fan

     a) The PA fan is giving maximum excess air in the boiler
     b) The FD fan is giving combustion air in the boiler
     c) The SA fan is giving minimum excess air in the boiler
     d) The PA fan air volume is approx. 15 to 18% which is of higher velocity & in the balance 82% to 85% air, the FD fan gives 82 to 85% & balance goes to SA fan

3) Boiler with FD Fan & SA Fan

     a) The FD fan is giving maximum excess air in the boiler
     b) The SA fan air volume is approx. 18% & the balance air is from FD fan

4) Boiler with FD fan only
 
     Excess air + Combustion air is given by FD fan

The above are average values & may vary by 1% to 3% as per boiler manufacturer.

PA Fan role : PA fan role is to ensure fuel feed + bed expansion is fully achieved + excess air in combustion due to its high pressure & velocity

Critical parameters in design :

Criticality about fuel density :
1) The PA fan's design is basically to drive a particular weight of the fuel as per its density
2) If the density increases due to either higher moisture content or higher fuel ash content in the fuel, the PA fan fails to deliver the fuel quantity per hour
3) Like wise if the density decreases either due to lower moisture content or lower fuel ash content in the fuel, the PA fan starts pushing excess fuel into the system

Criticality about Air Injection velocity into the fluidizing bed :
As the operating fans inject fuel & combustion air into the fluidizing bed, the air injection velocity plays a major role in determining turbulence in the system

Higher air injection velocity increases fuel flight from the system & the fly ash will contain unburnt fuel & the ash test will respond to VM% presence

Lower air injection velocity will lower turbulence & affect the C, O2 reaction increasing CO2 + C reaction & also unburnt carbon in the fly ash, the fly ash test will not or negligibly respond to VM test

Criticality about WBP :
WBP should be decided as per fuel density, base value from design & not by standard operation

Criticality about furnace draft or air resistance by fluidizing bed :
The PA air & FD air entering into the bed loses its velocity due to bed resistance & also suffers expansion in volume due to heat pick up upon conversion to flue gas.
The draft is the value obtained after both the above processes.
If the bed air resistance decreases, the flue gas velocity increases & causes flight of fuel particle & vice versa.

Achieving design Furnace draft is automatic if the right PA, FD, WBP are under operation.

Lower draft operation becomes necessary if the air injection velocity becomes higher than required.

Design flexibility for fuel, air, fans, operational parameters
As per my observation the design flexibility (after trouble shooting several FBC / AFBC boilers & careful calculations) is around + 3 to 4% at maximum


Question : If the boiler has been designed for 82%, can it be run at 89% efficiency by lowering free O2 in the stack

No. For solid fuel boilers atleast, it is not possible to achieve as portrayed above. Lowering the Air will increase CO2 + C, Boudouard reaction & will lower the Efficiency by Direct Method & wholly defeat the purpose.

It is advisable to run only on recommended O2 as per design.

Question : Whether the boiler design can include operation with number of fuels whose density is highly variable ?

In my experience, the boiler may be designed either at minimum fuel density or at mean or extreme fuel density with slight flexibility in density. It cannot be designed for all densities, viz., the air injection velocity will vary according to the fuel density & cannot be constant for all types of fuels.

Means that the boiler cannot be designed for either Husk & Coal, it can be one fuel as the density variation is too extreme.

The reason is PA or FD pressure has to change with the fuel density & its input volume.

How does one determine the right quantity or volume of air is being fed ?

As the knowledge of CO2 + C, Boudouard Reaction happening in oxyrich conditions is now a reality, it is important to give sufficient turbulence & bring back the C in the CO2 + C reaction to combustion.

I have developed the software which can perform this feature, for all sorts of operational loads.

III  By Measurement :

Measurement of free O2 % by O2 detector in the stack. However the value represented may not be factual as per the combustion reactions.

For example : When air volume is lesser, the CO2 + C reaction increases which consumes lesser oxygen than required, hence the free O2 will start showing an increase. I have checked this in few cases & found it to be correct

Case 1:
In a 100 TPH AFBC boiler the free O2 was showing drifting value of 4.9 to 6% & by Direct Method was working at 73 to 75% efficiency. However the Indirect Method showed an efficiency of 82.5%

After necessary calculations, I asked for an increase of 23 to 25% air volume to lower the CO2 + C reaction. This volume increase is equivalent to 5% O2.

The Boiler Incharge said that the free O2 will increase to 9.5 to 10% as already enough free O2 was indicated, but I insisted that free O2 would decrease as CO2 + C reaction consumes lesser O2 than C + O2 reaction & once proper turbulence could be created, the C + O2 reaction would increase & free O2 would decrease.

Upon increasing the air volume by 23% which was visible in the DCS, the free O2 started dropping & came to 4.1 to 4.2% stable.

Case 2:
In a 10 TPH, AFBC boiler similar situation existed, due to lower air volume. The same mantra was followed & the free O2 decreased from 7% to 4.3 to 4.5% when the air was increased by 25%

Case 3:
A 35 TPH AFBC boiler was operated at 3.1% O2 by controlling air volume. When excessive air control is made there will be more CO2 + C reaction happening & free O2 will drop down.

As per design the Free O2 was to be run at 6.8%, for 76% efficiency boiler. Here the Direct Method efficiency showed that the boiler was operating around 63 to 65% & the indirect method showed an efficiency of 84%.

Correction was made & air volume was increased by 35 to 40% & the free O2 increased from 3.1% to 6.5% -- 7%.

The efficiency of the boiler gained & the fuel consumption dropped by more than 10% in the above case.

NOTE : The % air increase can be deduced upon detecting the Boiler Efficiency by Direct Method & cannot be detected in the Indirect Method. % Air increase may vary upon case to case basis.

There are many examples where this correction gave a clear indication that CO2 + C reaction is occurring for low air volume systems.

Excess Air Conclusion for FBC / AFBC / CFBC boilers :
1) Run the boiler at excess air as per boiler design & not by excess air control
2) Calculate the excess air & ensure proper PA, FD, SA settings, furnace draft
3) Calculate the WBP required as per fuel density
4) Check what is the design fuel density considered
5) Achieve proper turbulence to avoid fuel over flow or unburnt carbon overflow in the fly ash
6) Do not depend upon measurement values & use necessary logic to deduce the actual excess air conditions vs. reaction condition in the furnace
7) Air volume has to be in sync with the load & fuel characteristics factors of the boiler

For any views on the subject, email me at sap@chargewave.in & visit our website www.chargewave.in

Regards
PS Anand Prakash