Methods of Improving Boiler Efficiency

[jessie cheung]

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 boiler
efficiencies:



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

Reducing Excess Air
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.
Installing Economizer
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
operation.
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
oil.
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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