Methods of Improving Boiler Efficiency

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.

Many years of working experience in Marine, Facilities, Construction has given the author material for writing e-books and articles related to engineering, and management. Subscribe to facworld ezine

More information at Marine Engineer and M & E Engineer

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