Methods to Improve Boiler Efficiency
By: Thomas Yoon
With the rising cost of fuel prices, industries that use steam
boilers for heating or power generation are hard pressed to
operate at peak efficiencies.
While steam consumption, leakages, and other heat transmission
losses can contribute to the overall energy bill, this article
focuses on the heart of the steam generator - the boiler.
Controlling the boiler is of utmost importance in any steam
generation energy saving program. Below are some ways to improve
Reducing Excess Air
- Reducing excess air
- Installing economizer
- Reducing scale and deposits
- Reducing blow down
- Recovering waste heat from blow down
- Stopping dynamic operation
- Reducing boiler pressure
- Operating at peak efficiency
- Preheating combustion air
- Switching from steam to air atomization
- Switching to lower cost fuel
By far the most common reason for energy inefficiencies in a
boiler can be attributed to the use of excess air during
combustion at the burners. When there is more air than is
required for combustion, the extra air becomes heated up and
is finally discharged out to the atmosphere. However, there
are reasons for putting in some extra air for combustion - to
compensate for imperfect burner fuel-air mixing conditions, air
density changes, control system "slop", burner maintenance, fuel
composition and viscosity variation, and imperfect atomizing
steam or air controls for burners.
Adjusting the fuel-air ratio for combustion can be quite tricky.
If the fuel is too much as compared to the air, incomplete
combustion occurs. This will give rise to carbon soot deposits
inside the combustion chamber or even over the boiler tubes.
The consequences of having soot deposits over the heat transfer
surfaces and the potential of having explosive flue gases inside
the boiler are much worst than losing a slight amount of energy
through the exhaust stack. Therefore, many boiler operators
choose to adjust their burners to be slightly on excess air.
This is only appropriate if there are insufficient heat transfer
surfaces in the boiler. The economizer tubes may contain either
circulating boiler water or circulating feed water. Because the
temperature of the exhaust gases can be quite high, the economizer
tubes may be fitted with safety valves to avoid over-pressure
damage. Also temperature control of feed water is required to
prevent pump airlock. To avoid corrosion, careful design is needed
to ensure that the exhaust flue gas temperature does not drop
below the dew point.
Reducing Scale and Deposits
For any boiler operation, this is a must. The safety of the boiler
is at stake. Any scale or deposits will lead to reduced heat
transfer that will eventually lead to overheating, reduction of
mechanical strength of the steel and finally to bursting.
This should already be in the normal daily procedure of boiler
Reducing Blow down
Blow down of boiler water is discharging hot water into the drains.
However, blow down is necessary to maintain the boiler water
concentration of dissolved solids that are necessary for
conditioning the boiler water. The dissolved solids are necessary
for preventing boiler corrosion and scaling.
As steam is generated from the evaporation of water, the remaining
water in the boiler becomes more and more concentrated. This must
be drained away during blow down.
The challenge is to control the draining to the minimum.
Recovering Waste Heat from Blow down
Since it is necessary to blow down to control the total dissolved
solids in the boiler water, methods can be adopted to recover back
some of the heat from the drained hot water.
Blow down tanks, heat exchanger tubes and pumping arrangements can
be fabricated to recover some of the heat back into the boiler.
Stopping Dynamic Operation
Whenever a boiler starts or stops, a few minutes are spent running
the forced draft fan for purging the combustion chamber of unburnt
gases. This is a necessary step for the safe operation of a boiler.
During this time the heat from the boiler water in the shell or
tubes will be lost to the purging air.
To avoid this type of losses, it is better to maintain a steady
firing condition in the boilers.
Reducing Boiler Pressure
By reducing the boiler pressure, some of the heat losses through
leakages or transmission may be reduced slightly. However there can
be problems with the boiler with reduced pressure. The boiler
circulation may be upset and the steam lines may have insufficient
capacity and flow to transport the low pressure steam.
Operating at Peak Efficiency
When operating two or more boilers, improved efficiency can sometimes
be obtained by unequal sharing of the load so that the combined load
operates at peak efficiency.
Preheating Combustion Air
Any heat loss from the skin of the boiler to the boiler room can be
utilized back for combustion. By preheating the intake air the
combustion in the furnace becomes more efficient.
Switching from Steam to Air Atomization
For burners with steam atomization, switching to air atomization will
naturally result in less steam consumption overall and better boiler
efficiencies. This is only applicable for heavy fuel oil burners.
Switching to Lower Cost Fuel
When comparing natural gas and fuel oil, if the cost is the same or
more per BTU delivered, switch over to fuel oil.
The reason for this is that in the combustion process, hydrogen
combines with oxygen to form water. The latent heat of vaporization is
lost when water vapor leaves the boiler stack.
Fuels like natural gas with higher hydrogen to carbon ratio will lose
this heat more than those with lower hydrogen-carbon ratio like fuel
However one must also recognize that there will be increased
maintenance, operating costs and greater need for more excess air in
order to achieve complete combustion for fuel oil. In addition, soot
deposits and incomplete combustion might also affect the overall costs.
Some of the ways mentioned above may not be feasible at all for your
plant. Each of them may result in only a few percentage points of
boiler efficiency improvement. However, if carried out carefully and
with the proper tools and instruments, they do add up to huge savings.
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