boiler operating costs with #6 oil and natural gas
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Boiler Operating Costs with Natural Gas and #6 Oil
Dr. Herbert EckerlinCenter for Boiler Studies
Raleigh, NC
March 27, 2002
Why Measure Parasitic Losses?
• Cost comparison based on “stack loss” only ignores parasitic losses and costs associated with oil-firing
• Important to quantify thermal losses and parasitic losses to determine the “true bottom line” efficiency and steam cost
• Bottom line efficiency helps in choosing the most economic fuel
Fuel
Stack Loss
Tg
% O2
Fire Tube Boiler
Effects of Excess Air on Flue Gas Composition
15
10
5
0-50 0 50 100 150 200
Excess Air
CO
CO2
CO2
O2
CO
2, C
O, o
r O
2(%
)
IncompleteCombustion
Excess Air (%)
+
+
+
Typical Oil Firing SetTypical Oil Firing Set--upup
+
+
+
Q StackThermal Losses Common forThermal Losses Common forNatural Gas and #6 Oil Natural Gas and #6 Oil –– stack lossstack loss
Thermal Losses Common for Natural Gas and #6 Oil
• Stack Loss– Largest single loss– Depends on flue gas temperature, ambient air
temperature, excess air and type of fuel– To determine stack loss, measure all above
parameters
+
+
+
Q Stack
Q CO
Q CH
Thermal Losses Common for Natural Thermal Losses Common for Natural Gas and #6 Oil Gas and #6 Oil –– CO and UnCO and Un--burnt CH burnt CH
Thermal Losses Common for Natural Gas and #6 Oil
• CO and un-burnt CH – Caused by incomplete combustion– Lack of air to support complete combustion– Poor mixing of fuel and air– Poor atomization of fuel oil
+
+
+
Q Oil Storage Tank
Q Stack
Q CO
Q CH
Parasitic Losses for #6 OilParasitic Losses for #6 OilTank HeatingTank Heating
Parasitic Losses for #6 Oil
• Oil Tank Heating– Fuel oil is viscous and requires heating to
facilitate pumping– Generally heated to 140 F– Heat loss can be quite high for single wall un-
insulated oil storage tanks
+
+
+Q Oil Heater
Q Oil Storage Tank
Q Stack
Q CO
Q CH
Q Oil Pumping
Parasitic Losses for #6 OilParasitic Losses for #6 OilPumping & PreheatingPumping & Preheating
Parasitic Losses for #6 Oil
• Oil Pumping– Pumps are required to circulate oil from storage
tank to the burner• Oil Preheating
– To ensure proper atomization, oil must be heated 200 – 220 oF
– Generally heated by steam or hot water– In some fire tube boiler application, steam/hot
water heating is augmented by electric heating
+
+
+Q Oil Heater
Q Oil Storage Tank
Q Atom
Q Stack
Q CO
Q CH
Q Oil Pumping
Parasitic Losses for #6 OilParasitic Losses for #6 OilAtomizationAtomization
Parasitic Losses for #6 Oil
• Oil Atomization– For proper combustion, oil must be broken up
into small particles– Done by injecting steam or compressed air into
small oil supply at the burner
+
+
+Q Oil Heater
Q Oil Storage Tank
Q Atom
Q Stack
Q SootBlowers
Q CO
Q CH
Q Oil Pumping
Parasitic Losses for #6 OilParasitic Losses for #6 OilSoot BlowingSoot Blowing
Parasitic Losses for #6 Oil
• Soot Blowing– Soot is created by oil burning– Soot leaves deposits on boiler tubes– Reduces heat transfer to boiler fluid– Generally cleaned by steam or compressed air– Fire tube boiler generally do not use soot blowers– Soot removal must be done by punching the tubes when
stack temperature gets too high– Should be done at least quarterly to prevent high stack
temperature resulting in high stack loss– Boiler must be shut down when tubes are punched
Flue Gas Temperature Over Time
200
250
300
350
400
450
0 5 10 15 20 25 30Days
Deg
F
#6 Oil High Fire
Gas High Fire
Gas Low Fire#6 Oil Low Fire
+
+
+Q Oil Heater
Q Oil Storage Tank
Q Atom
Q Stack
$ Water$ Chemical $ Preheating
$ Maintenance$ Fuel Oil Additives
$ Oil Inventory
$ Corrosion
Additional Makeup Water
Q SootBlowers
Q CO
Q CH
Q Oil Pumping
Parasitic Losses and Cost for #6 OilParasitic Losses and Cost for #6 OilInventory, Additives, Maintenance etc.Inventory, Additives, Maintenance etc.
Parasitic Losses and Cost for #6 Oil
• Oil Storage Inventory– Storing oil on site ties up capital– This investment yields no return
• Oil Additives– Purposes
• Reduce sludge in oil storage tank• Improve combustion• Reduce soot deposits• reduce cold end corrosion
Parasitic Losses and Cost for #6 Oil
• Corrosion and Maintenance– Sulphur in oil causes cold end corrosion– Adds to maintenance costs– Reduces boiler life
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+
+
Q Oil Heater0.35%
Q Oil Storage Tank8.40%
Q Atom0.93%
Q Stack 12.4%
Q SootBlowers
Q CO
Q CH
Q Oil Pumping1.15%
220oF
Radiation 1.0%
$ Water$ Chemical $ Preheating
$ Maintenance3.41%
$ Fuel Oil Additives
$ Oil Inventory0.30%
$ Corrosion
Additional Makeup Water
140oF
Summary of Thermal and Summary of Thermal and Parasitic LossesParasitic Losses
Bottom Line Efficiency forOil Firing
100
Stack Loss 12.4
Radiation 1.0Oil Pumping 1.15
Oil Atomization 0.93
Corrosion & Maintenance 3.41Oil Inventory 0.3
Oil Heating 0.35
Oil Tank Heating 8.4Bottom Line Efficiency 72%
Bottom Line Boiler Efficiency Comparison
82.590
72
0102030405060708090
100
N.G. StandardBoiler
N.G. HighEfficiency Boiler
#6 Fuel Oil
Effect of Parasitic Losses on Cost of Delivered Steam
$7.3230.6
$/GJcents/litre
Cost of Delivered Steam(considering stack and parasitic losses)
$1.238.6
$/GJcents/litre
Incremental Cost Increase(due to parasitic losses)
$6.09$/GJCost of Delivered Steam(considering stack and radiation losses)
22.0$5.27
cents/litre$/GJ
Average Fuel Cost
Conclusions
• On average, parasitic losses can add about 9 cents per litre to #6 fuel oil price
• Take into account all parasitic losses and costs associated with #6 fuel oil firing while comparing costs with natural gas
• Keep boilers properly tuned for maximum efficiency
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