Boiler Efficiency vs Thermal Efficiency
Direct Method vs. Indirect Method
Which one is reliable ?
My ViewI have been researching into Boilers Design & Studying Boiler Design parameters for various types of Boilers viz. Manual Fired Boilers, Stoker Fired Boilers, FBC Boilers, AFBC Boilers, CFBC Boilers & Pulverized Fuel Boilers.
The basic question always remains as which method is more accurate for measuring Thermal Efficiency in a Boiler.
Two methods are available
A) The Direct Method
- Measures Output Energy / Input Energy
- Considers Energy converted is real & the rest are losses
- No losses are measured
- No assumptions, this is a straight method
- Efficiency of the Boiler is Output / Input * 100
Salient Points
* Fuel consumption has to be measured
* Steam output has to be measured
* Steam Temperature, Pressure has to be monitored
* Fuel quality viz., GCV has to be closely monitored
* Bunker Level deviations may be present in the order of 1 to 1.5%
* GCV deviations may be present due change in fuel quality, which will impact the operations
* Boiler Input Excess Air, Furnace temperatures have to be recorded & have impact on boiler operations
* Furnace draft has to be monitored
* Efficiency depends upon Boiler Design
* Boiler Efficiency practically never reaches the Design Efficiency
* Boiler Design Fuel parameters, Design Excess Air, Design Air Injection Velocity, Design Draft, Design flue gas velocity, Design Reactivity of C, O2 etc. have role to play in Boiler Efficiency
* Efficiency depends upon Operation Steam Load
* Thermal zones play a major role in realizing operating efficiency
Boudouard Reaction Tracking
* CO2 + C, Boudouard Reaction occurrence % can be tracked, whether it is higher or lower than stipulated values
B) The Indirect Method
- Measures the losses from Stack, Specific heat lost in Ash, Blowdown losses, Radiation Loss, Moisture Loss, H2 Loss, LOI losses
- Assumes energy lost is real & the rest has been converted to Steam
Salient Points
* No need to measure fuel consumption
* No need to measure Steam output
* Steam Temperature, Pressure need not be monitored
* Fuel quality viz., GCV need not be monitored
* Bunker Level deviations need not be monitored
* GCV deviations do not have any impact on the system
* Boiler Excess Air, Furnace temperature need not be monitored & have no measurable impact
* Furnace draft has no reference in the method
* Efficiency does not depend upon Boiler Design & only Indirect Method measured parameters depict the Efficiency
* Boiler Efficiency can over reach the Design Efficiency, by control of excess air & LOI
* Boiler Design Fuel parameters, Design Excess Air, Design Air Injection Velocity, Design Draft, Design flue gas velocity, Design Reactivity of C, O2 etc. have no role to play in Thermal Efficiency
* Efficiency is independent of Steam Operation Load
* Thermal zones do not play a major role
Boudouard Reaction Tracking
* CO2 + C, Boudouard Reaction cannot be tracked
My Experience with Boilers
Having been associated with products which enhance energy generation & absorption above input GCV in Boilers through modification of Reaction Efficiency parameters of C & O2, I have been monitoring the above two methods for over 14 years.
I would like to tell you my experience on the above
THE INDIRECT METHOD IS LESS RELIABLE THAN THE DIRECT METHOD
Example 1 : 20 TPH, imported coal fired AFBC boiler, with Coal GCV as fired of 3100 Kcal / Kg, was taken as a case study
Operating Steam Load of 18 TPH (90% of design TPH) was audited by a reputed company & certified that the Boiler was operating at 81.4% efficiency by the Indirect Method
Direct Method showed the Boiler was working @ 70% Efficiency in no. of tests
Example 2 : A 175 TPH, CFBC boiler was operating at 87% efficiency as per Indirect Method. The Efficiency by Direct Method was 73%
Example 3 : A 100 TPH AFBC boiler was operating at 84% efficiency as per Indirect Method. The Efficiency by Direct Method was 76%
Example 4 : A stoker fired boiler was operating with 81% Efficiency as per Indirect Method. The Efficiency by Direct Method was 56%
Example 5 : A Pulverized Fuel Boiler was operating with 91% Efficiency as per Indirect Method. The Efficiency by Direct Method was 65%
Example 6 : A 25 TPH AFBC boiler was running at 50% load & operation was only in 2 out of 4 compartments. The Indirect Method showed an efficiency of 81.7% & Direct Method showed 52%
There are scores of examples. I had observed that only 1.5 to 2% boilers are under operation where Direct & Indirect Methods closely match with deviation of < 2 to 3%, in all other cases the deviation is average of 8 to 10% & maximum deviation is around 20%.
Question : Why do both the methods show different values ?
The main reason for low efficiency in boilers was traced to improper combustion air pressure, lower furnace draft operation, lower loading of thermal zones, coal ash% deviation from design to operation, coal GCV deviation from design to operation, leakages in air ducts, improper design fuel mix selection by the user leading to increase in Boudouard Reaction.
Till now it has been assumed that there is only one reaction type happening during C, O2 oxidation conditions. Now the new research says the following
There are 3 reactions, not 1
1) C + O2 ---> CO2 is 72 to 78% of the total reaction, Exothermic
2) 14 % C equivalent weight of CO2 will react with 14% C equivalent weight to form 28% CO, this is called the Boudouard Reaction, Endothermic
3) 28% CO + O2 ---> 28% CO2, far less Exothermic than the first reaction
Boudouard Reaction is reaction of CO2 + C --> 2 CO, which is endothermic in nature.
Conditions that support this reaction are
1) Operation Air pressure / Air Velocity / Air Quantity / Excess Air / Flue gas velocity / Furnace draft is lower than Design Values
2) Over feeding of fuel
3) Improper settings of PA, FD, SA, WBP
4) Higher bed or furnace temperature operation
5) Mismatch of Operating fuel vs. Design fuel
6) Combustion time variance between operating fuel & design fuel
7) Ignition temperature variation between operating fuel & design fuel
Corrections
Correction can be applied in any Boiler, once the Efficiency is first tracked in the Direct Method. Deviation between Design Efficiency & Efficiency by Direct Method can be narrowed to less than 3.5% by applying corrections in operations, air settings & fuel selection.
I have corrected scores of Boilers, having identified the Working Efficiency &
Design Efficiency, by correction of Air parameters.
Example 1 : A 3 TPH manual fired boiler working at 2.3 TPH & consuming 14 tons of Coal per day. Coal GCV 4200 Kcal / kg @ Efficiency of 53%. Indirect Method Efficiency was around 76%. Here the lower efficiency was due to induced condition where Boudouard Reaction would increase in the furnace due to low FD pressure.
Correction was applied by increasing FD air pressure, by correction of Ducts & Duct dampers
The coal consumption reduced to 11 TPD from 14 Tons with a saving of 30%, with the same GCV.
Example 2 : A 8 TPH FBC boiler working at 5.5 TPH boiler was consuming 41 tons of Coal per day. Coal GCV 3100 Kcal / kg @ Efficiency of 52%. Indirect Method Efficiency was around 76%. Here the lower efficiency was due to induced condition where Boudouard Reaction would increase in the furnace due to low ignition time of the fuel in use vs. ignition time of the fuel by design. (ignition time mismatch)
Correction was applied by keeping all settings constant & only by decreasing FD air pressure & volume by 18%.
The boiler started consuming 25 tons to 26 tons of the same coal by this correction, for the same steam output, saving of more than 25%
Example 3 : In one installation where the FD, PA were identified, to be running lower than Design values & Direct Method efficiency was indicating 75 to 76% & Indirect Method showing 84%, the case was taken up for correction.
Having identified that Boudouard Reaction has increased due to the settings, I had asked for increasing 20 to 25% air by FD & PA, by volume. The free O2 started to decrease from 6 to 7% to 4.2 to 4.3%.
The coal feeder RPM's started to decrease by 8 to 10% lowering the coal consumption.
I have scores of these examples, where fuel saving was achieved by studying Design vs. Operation parameter deviation & conducting efficiency tests by Direct Method.
Direct Method Advantages over Indirect Method
Boudouard Reaction over flow can be identified
Air corrections can be applied by pressure or volume or both
Actual deviating condition can be easily identifed
Correction is straight & simple
Fuel saving is achievable than current values & even in indications of Highest Thermal Efficiency
Software
With all the experience & solutions I could conclude that DIRECT METHOD GIVES ADDITIONAL SCOPE FOR OPERATIONAL PARAMETERS CORRECTION & TO LOWER FUEL CONSUMPTION FIRST.
I have now developed a software to estimate the deviations & directly deduce the operating efficiency of the Boiler by Direct Method.
The software works with an efficiency of 97 to 98%.
I have corrected many boilers using this software.
You can contact me at sap@chargewave.in
My Advice to Boiler Users
Direct method gives you power & understanding what is wrong with the operation especially the occurrence of Boudouard Reaction which cannot be tracked in the Indirect Method
Fuel Consumption can be saved by 5 to 10% minimum in any type of Boiler when Direct Method is followed & deviations are corrected
Changing the Efficiency method to Direct Method & taking proper corrections itself will save India an annual consumption of 1 to 5 million tons of coal
In many Boiler installations, the GCV or moisture or coal quantity is freely corrected to adjust this aberration of Low Efficiency by Direct Method & High Efficiency by Indirect Method
The facts are visible, so take the step today & seek advice to what to be corrected to save energy
Energy saved is Energy Generated, change the assessment method & see the potential of saving
for more information on Boudouard Reaction, visit this Link from
Institute of Combustion, Stuttgart, Germany
Boudouard Reaction in Oxidation conditions published by Institute of Combustion
Heat Balancing Direct Method vs Indirect Method
When heat balance is conducted between Direct Method Output Energy vs. Input Energy & the Indirect Method, the bet is that over 98% of the cases there will be mismatch & in almost every case Direct Method Efficiency will be less than Indirect Method Efficiency.
Conclusion
Tuning the boiler to really run at high efficiency in Direct Method is a challenge once it becomes known that improper operations can lead to increase in Boudouard Reaction & increased fuel consumption leading to increased Steam generation costs.
Advice through our combustion projection software is available with us, which can project the severity of Boudouard Reaction in a particular operation.
The conclusion is that fuel consumption can be lowered by proper tuning of the Boiler resulting in lower Boudouard Reaction once the Direct Method Efficiency is known.
Fuel saving potential by adopting Direct Method & Boiler tuning
Through proper tuning itself, Indian boiler users have an opportunity to save more than 5 to 7 million tons of coal, just by switching the efficiency method & correcting the parameters.
Our Scope
We manufacture chemicals which severely limit Boudouard Reaction occurrence during combustion returning C to the system & generating GCV over & above the established value
Thank you very much for your attention
PS Anand Prakash
Director Technical
Chargewave Energykem Pvt. Ltd.
sap@chargewave.in
