Improve Fuel Efficiency with Economizers

Boilers, fired furnaces, and kilns are extremely inefficient and are often the major consuming of energy in many different processes. The ability to recover wasted heat is critical to achieving optimal energy efficiency, capacity and profitability.

The majority of heat and energy losses in a furnace can be split into the following categories:

  • Waste gas
  • Shell losses
  • Heat in the product - i.e. steam

Waste gas energy losses can then be further divided:

  • Quantity of waste gas - excess air
  • Temperature - enthalpy
  • Composition - CO content, humidity, incomplete combustion

The amount of heat lost through the waste gas is a significant portion of every unit operation involving combustion, however it can be harnessed. This will improve the furnace’s thermal efficiency and reduce total plant fuel consumption. There are two main ways this can be done

  1. Reduce the quantity of waste gas through excess air control
  2. Reduce the temperature of the waste gas by installing an economizer

Simply put an economizer is a heat exchanger which preheats a boiler’s water feed by using the hot waste gas.

For furnaces other than boilers, for examples calciners or kilns, the same concept can be used, however another liquid stream may be required to reduce total plant energy requirements.

The economic justification for upgrading a boiler by installing an economizer can be made by simply looking at the potential in temperature reduction on heat loss. The potential enthalpy that can be recovered by reducing the stack temperature by 10 degrees can often be significant, and the temperature drop is usually much greater than this.  From this the design of an economizer in terms of heat exchange area, and liquid flows can be made. From this it is virtually the same as designing a heat exchanger, although the conditions particularly for high sulfur fuels can be extreme.

General rule of thumb’s suggest that more than 5% fuel consumption reductions can be achieved by installing an economizer, with additional 5% improvement possible with a condensing economizer - which condense the stack gases to additionally recover the latent heat of vaporization.

The re-use of heat is the primary method for improving fuel efficiency, and an economizer is the re-using heat in the most basic sense. Looking further afield synergy between unit operations can be taken into consideration when looking for even greater energy gains.

More Reading:

Consider installing a condensing economizer - US Department of Energy

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