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

Tricks for lowering tube failures in FBC, AFBC boilers

Tricks for lowering tube failures in FBC, AFBC boilers

My Experience & view

Boiler not yet delivered : 

Go in for bigger steam drum size, which should be 50 to 60% of boiler capacity

Boiler already running :

Now the case becomes complicated.

In many installations, the boiler drum size is small which has major implication on the tube life as follows

Low Boiler Drum size creates the following condition

1) Low natural water re-circulation rates

2) Increased exposure of tubes to thermal stress

3) Tube surfaces receive more heat & become red hot & soft

4) Increased erosion due to point no. 3

5) Increased erosion due to increased recirculation rate of the bed material

What is bed material recirculation rate :

Imagine a coin which is flipped in air for HEADS or TAILS. Similar way for FBC & AFBC the fuel particle & bed material particle is flipped.

The maximum average height the bed material has to reach is called Bed Expansion Height

When due to low PA or low FD or high WBP or due to high bed material density, the bed expansion height reduces, it is known as a condition which promotes increased bed material recirculation.

Consider this

When bed material reaches a particular height it takes time & then returns back to same starting position & then again returns to the expanded height earlier

One cycle where it starts & ends at the same point is called recirculation & time take is the recirculation time

Now recirculation is inversely proportional to the time i.e. more the time, lesser the recirculation & vice versa

When bed expansion is lesser the recirculation rate increases & bed expansion is more the recirculation rate decreases

Heat retention in the bed is also directly proportional to recirculation rate, i.e. heat retention increases which higher recirculation rates & reduces with lower recirculation rates due to heat trapping

Tube failures occurrence the causes :

1) KEY or 90% reason is tube surface gets more heated due to smaller drum sizes, low drum level or low water recirculation rate

2) balance is increased recirculation rate of the bed which increases erosion

How to know, whether the current settings are increasing the bed recirculation ?

If the bed material density is over & above the stipulated value of 1000 to 1100 grams per liter then bed recirculation rate has increased

Higher bed recirculation increases mutual bed particle friction leading to increase in density

How to avoid tube failures :

1) Change smaller drum size to bigger drum size if possible

2) Run drum level at 70 or 75%, there will be no water hammering (explained in Steam dryness blog)

3) Set the right conditions for bed expansion by proper setting of PA, FD, SA, WBP etc.

4) Check drained bed density values frequently, once in a day

5) Avoid bed over draining

6) Drain the bed when the level reaches 20 mm above the set point & stop the drain at the set point

For example : WBP is say 500 mmwc, then Start drain at 520 mmwc & Stop draining at 500 mmwc

7) Operate at design furnace draft to keep the exact heat retention time by design (lower furnace draft increases retention time & therefore the thermal stress on tubes)

Results :

I have changed several boiler operations & reset the PA, FD, WBP, draft & drum levels, viz., all the above 

The result is there are no tube failures in AFBC or FBC boilers for over 5 years now & tubes display excellent characteristics to last another 3 years

The tube life has increased even in tubes which are not studded

The trick is to ensure to take care that tubes do not get thermally stressed due to excess heat retention in the bed, low bed expansion & low water recirculation rates, low drum levels.

Wish you happy increase in tube life

for any questions please contact me at

sap@chargewave.in

PS Anand Prakash

Director Technical

Chargewave Energykem Pvt. Ltd.

The SECRET is OUT regarding Fuel GCV

The SECRET is OUT regarding Fuel GCV

The Institute of Combustion & Power Plant Technology research upon Carbon Combustion confirms Boudouard Reaction occurring in Oxyrich & CO2 enriched conditions

Link :http://www.ifk.uni-stuttgart.de/allgemeines/Veroeffentlichungen/Diss2009/DissAl-Makhadmeh.en.html

There are 3 reactions occurring during Combustion

1) C + O2 --> CO2 ( 72% to 78% )

2) CO2 + C --> 2 CO ( 11% to 14% ), The Boudouard Reaction

3) 2 CO + O2 --> 2 CO2 ( 22% to 28% )

Out of the above the 2nd reaction is endothermic & the 3rd reaction releases around 1/4th of the energy released by the 1st reaction

Implications of this research

Energy generation potential is much higher than what has been previously thought, if the 2nd reaction is blocked or stopped from occurring

This reaction is occurring in all conditions where O2 is excessive or CO2 is excessive

The excessive energy available for tapping is around 30%, which is not available due to occurrence of Boudouard Reaction & Less exothermic reactions

The Secret is Out that Fuel GCV is 30% greater if the Boudouard Reaction is blocked

This energy can be extracted by Activiser chemical application, which blocks the Boudouard Reaction.

Visit us at www.chargewave.in

or write to me at sap@chargewave.in

Improving Steam Dryness Factor

Improving Steam Dryness factor

In Boilers the steam dryness constitutes a major milestone which cannot be measured online as no measurement systems are present

We have to buy the argument from the Boiler manufacturers, that the steam is 100% dry due to moisture separator efficiency in the drum

My view

Prologue :

1) In many power plants on identical turbine load the Steam / MW varies between 1% to 7%
2) In process boilers the steam trap losses account to 8% to 14% based on the distance travelled by Steam

The Steam velocity properties are closely associated with its dryness.
Increased dryness makes steam density lower & contributes to higher velocity & vice versa.

Look at the following questions
 
q1) Whether Steam velocity is relative to its enthalpy ? 
q2) Can the Steam velocity be increased for the same enthalpy ?
q3) What are the factors effecting the Steam velocity in the Boiler ? 
q4) Whether Steam is really 100% dry as portrayed ?
q5) What is the proof ?

If the Steam Velocity is relative to its enthalpy then at what dryness factor & if the dryness is reduced what would be the effect on steam properties ?

If the dryness factor varies, then Steam Velocity also should relatively change ?

Does Water pump, drum level, super heaters etc. the entire water circuit has a role to play regarding Steam velocity or it is just an assumed or concurred output ?

How does one come to knowledge instead of assumptions that Steam is Dry & at what % dryness ?

The drum is basically a cylindrically shaped construction with water injecting into it from the water pump, with a steam separator mounted at 80% level or height.

Drum Condition 1
The Drum level say considering average of 50%, the condition would be 

50% space for Steam
50% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 25% height of the drum at mean value (space between water & dry steam) which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is more due to 50% occupation of space
2) Water residence time is less due to its occupation of 50% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is higher since higher residence time of steam will lead to its condensation i.e. increase its wetness

Drum Condition 2
The Drum level say considering average of 40%, the condition would be 

60% space for Steam
40% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 35% height of the drum (space between water & dry steam) at mean value which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is more than condition 1 due to 60% occupation of space

2) Water residence time is less due to its occupation of 40% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is higher since higher residence time of steam will lead to its condensation i.e. increase its wetness
4) If Steam condensation increases due to not drawing the steam or any other reason, boiler stoppage etc., the steam residence time will further increase increasing the water vapour & water temperature & the possibility exists that all water, steam will convert into water vapour leading to water hammering

Drum Condition 3
The Drum level say considering average of 75%, the condition would be 

25% space for Steam
75% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 5% height of the drum at mean value which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is less due to 25% occupation of space

2) Water residence time is more due to its occupation of 75% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is lesser since lesser residence time of steam will lead to its negligible condensation i.e. increase its dryness


Let us look into the physical equilibrium condition that exists in the Steam or water drum

Steam, Water Vapor & Water are all H2O, the same chemical substance but in different physical forms, all existing under one roof the Drum.

Equilibrium condition is when all the 3 phases merge into single phase either steam or water vapour or water. 

Of these possibilities, only water vapour possibility exists as the others are not possible to be achieved.

There are 3 conditions for the water drum

Water Quantity + Water vapor  Quantity > Steam Quantity
Water Quantity + Water vapor  Quantity = Steam Quantity
Water Quantity + Water vapor  Quantity < Steam Quantity

The 1st Condition : Water Quantity + Water vapor  Quantity > Steam Quantity
Steam is dry or with higher dryness factor due to its low residence time for condensation

This will improve its velocity & hence forth its kinetic energy driving the turbine
Increased velocity will lead to fewer losses in the steam traps

Equilibrium condition of Steam, Water vapour & water not achievable

The 2nd Condition : Water Quantity + Water vapor  Quantity = Steam Quantity
Steam is less dry or with higher wetness factor due to its increased residence time for condensation

This will decrease its velocity & hence forth its kinetic energy driving the turbine, leading to increased consumption of steam


Decreased velocity will lead to higher losses in the steam traps


Equilibrium condition of Steam, Water vapour & water in critical condition, which means it can swing either way


The 3rd Condition : Water Quantity + Water vapor  Quantity < Steam Quantity
Steam is much less dry or with increased higher wetness factor due to its maximized residence time for condensation

This will lower its velocity & hence forth its kinetic energy driving the turbine, leading to maximized consumption of steam


Lower velocity will lead to maximum losses in the steam traps

If steam condensation is high, then water vapour content will increase due to heat transfer between steam & water & the entire drum content will convert to water vapour, leading to water hammering.


Equilibrium condition of Steam, Water vapour & water is achievable


Now the key question is how to improve Steam Dryness ?

Simple, keep the steam residence time in the drum as mean or as low as possible

Example 1 :

A 6 TPH manual fired boiler was operating at 10.5 kg pressure & drum level of 40 to 50% in water pump auto mode, having reported Steam trap losses ranging from 9% to 12%

Correction was taken in the water level in the drum to run at 73% Max. & 63%. The Steam trap losses lowered by 75%

Example 2 :

A 90 TPH Stoker boiler operating at 65 kg pressure & 450 deg C temperature & drum level 35% to 40%, was having a unique problem that if the Steam load increased to > 50% i.e. 45 TPH, the steam temperature started to increase upto 490 deg C & pressure would drop to 56 kg. The boiler was never loaded to > 50% capacity in over 30 years of its installation

Correction was taken by raising the drum level to 75% & Steam Load achieved up to 100% of the Load, without any raise in Steam temperature or lowering of its pressure


Example 3 :


A 33 TPH with 42 Kg pressure rating, Stoker boiler was delivering Steam max. up to 22 to 27 TPH with 33 Kg pressure operation. The drum level was 40%, when the Steam load would increase, the boiler would tipsy turvy & compromise on pressure leading to higher demand. This problem was present for over 22 years of installation


Correction was taken by raising the drum level to 65% & boiler never had a problem in delivering load or pressure


Example 4 :

A client was complaining that their 8 TPH boiler was always over loaded & was never able to deliver steam properly when steam was in demand. The drum level was operated continuously at 30%

Analysis showed that they had demand of only 3.5 TPH, but the pressure dropped from 10 kg to 4 to 5 kg max. 


Correction was taken by raising the drum level to 75%, the pressure improved to 10 kg & steam demand dropped by 1 TPH for the highest production. The trap losses almost dropped by over 80%


There are scores of such cases, where correction of drum level improved the Generation as well as steam dryness properties.


Example 5 : A 70 TPH AFBC boiler had frequent tube failures & complained excessive erosion of bed coils even though the bed coils were studded


Reason was informed that they maintained very low drum level of 35% to 40% which had to be increased to 70 or 75%


What is happening ?
 
Where the drum level is low the Steam demand as well as steam parameters are unable to be achieved well. Even if achievement was possible, steam was wet & condensation losses also had to be produced increasing steam demand.


Most recent Boiler installations have witnessed reduction in Boiler Drum Sizes which are aggravating the above situation.


Small drum boiler installations are the easiest target for high erosion of tubes.


What are the dangers of small drum sizes, operating in Drum Condition 1 & Drum Condition 2 ?


The small drum size brings alive the situation where the water recirculation rate in the Boiler is low.

In small drum boiler installations the steam consumption per MW is higher & keeps wavering than prescribed values & such installations are more prone to tube failures.


Low water recirculation rate increases the possibilities of heat reception by the tubes & tube surfaces getting overheated & further opening up the possibilities of increased Erosion, Departure from Nucleate Boiling (DNB), leading to early tube failures ? Why ?


When Steel is in hot condition or tube surface is very hot, it becomes very soft & hence very vulnerable to erosion.


Alternatively if the tube surface is cold, it is always strong & has negligible vulnerability to erosion.

In almost all the cases where early tube failures have registered, they all lead to one common observation, low drum level operation or smaller size of the drum.

I myself have suggested many FBC & AFBC boilers to increase the drum level to 75% & operate. The result is they have no tube failures over past 5 to 7 years. 


The trick is to keep the tube surface cold i.e. enable increased water recirculation i.e. water should evacuate the heat & the tube should not be the recipient of the heat.


Increased Steam dryness


Ensure the drum size is large enough to hold 50 to 60% of generating capacity then @ 50% drum level 25 to 30% water will hold weight in the drum.


Operate drum level at 70 or 75% to increase water recirculation rates


Steam consumption will also lower & tube life also will be saved.

Hope you got some answers.

Thank you, for the attention
 
If you have any questions please write to me at  sap@chargewave.in

 

PS Anand Prakash

Director Technical

 

Chargewave Energykem Pvt. Ltd.