If the optimization of boiler efficiency is one of the Engineer’s KPA’s, one would expect him/her to know in more than superficial detail where and how energy losses occur in the boiler, their magnitudes, their relative priorities and remedies. For those who would like to know more our next issues of Boiler Bits will touch on the many ways in which efficiency can be calculated and expressed, as well as realistic targets for efficiency.

In this issue however I would like to discuss the origins and magnitudes of energy loss in a typical pea coal fire tube boiler. The diagram above shows the most common heat losses from the boiler. Most significant are the stack losses (4 of them in total), not only in number, but also in magnitude. And then we also have shell loss (convection and radiation), bottom ash loss and blow down loss. Steam distribution and plant losses are not indicated. Tramp air loss (not shown here) may be significant; its effect is similar to that of added excess air and contributes to increased dry heat loss.

Most heat losses can be calculated fairly accurately, however I am not going to indulge in any formulas and calculations to demonstrate how one arrives at the losses mentioned here and elsewhere. The purpose of Boiler Bits is after all to gain an understanding of specific boiler and combustion related principles, rather than presenting an academic dissertation on topics covered.

So, let us take a closer look at the heat losses of boiler operations :

Dry Heat Loss: this is the energy carried in the flue gas and is dependent on its quantity (mass) and temperature. It is thus obvious that increased excess air, as well as tramp air ingress (higher flue gas mass flow), increases this heat loss. Fouled boiler tubes, and operating the boiler at high capacity, also achieve increased dry heat loss on account of higher flue gas temperature. Installing flue gas heat recovery equipment reduces heat loss due to the lower temperature of flue gas discharged to atmosphere. Typical dry heat loss varies between 10% and 20% of gross fuel energy. 

Wet Loss is the name given to latent energy lost in water vapour entrained by the flue gas. Water vapour is formed in two different ways:

  • through the evaporation of surface and inherent moisture contained in the fuel, and
  • the chemical combination of hydrogen contained in the fuel with oxygen to form water vapour.

Both processes absorb heat from the combustion process, which it can only release again during condensation, i.e. at flue gas temperature below some 60 ⁰C. This heat is in most instances unrecoverable and for all practical purposes energy lost. Burning coal containing 3,5% hydrogen (from ultimate analysis) and 3% inherent moisture (from proximate analysis) the following losses are experienced:

  • evaporation of inherent moisture = 0,3%, and
  • hydrogen combustion = 3,3%.

Total wet loss = 3,6%. It is interesting to note that inherent (and surface) moisture plays a relatively insignificant role in total wet heat loss.

Combustibles Loss is normally associated with dark smoke from the stack and typically manifests where the fuel is starved of combustion air (oxygen). The dark colour of the smoke comes from carbon molecules in the flue gas, but the flue gas may also contain other products of incomplete combustion, such as carbon monoxide (CO). The magnitude of combustibles energy loss is mostly relatively low; typically less than 1%. Its greater significance is with air pollution and agitating the public and authorities at large because of the visibility of the smoke.

Carbon Loss is associated with fine particles of unburned fuel carried along with the flue gas and being expelled through the stack, or caught up in a grit collector or flue gas filtration system. Because this heat loss depends on so many different factors it is not easy to calculate. As a general rule carbon loss is assumed to be between 0,2% and 0,5%. 

Shell Loss represents a more or less constant heat loss from the boiler due to the temperature difference between the boiler shell and the operating environment. So whether the boiler is operating at 10% load or at 100% load the rate of energy loss through the shell remains the same. It is generally accepted that shell loss = ±2% of the fuel energy at full load. A boiler operating at half load will thus waste some 4% of the fuel energy through its shell.

Bottom Ash Loss depends on the coal characteristics and can be calculated from the carbon in the ash, as well as the temperature of the ash. However, the temperature of the ash makes a minor contribution to the ash loss; it is the carbon in the ash that really counts. If we assume 22% carbon in the ash at a temperature of 350 ⁰C the corresponding bottom ash energy loss will typically be as follows:

  • carbon energy loss = 3,6%
  • ash temperature loss = 0,18%
  • Total bottom ash loss = 4,78%.

Blow down Loss: this loss depends on the volume of condensate returned, as well as the TDS of the make-up water. Note that the percentage blow down volume is significantly more than the percentage blow down heat loss. This is so because we are blowing down water and not steam. Blow down volume normally varies between 2% and 5% of steam produced, whilst blow down energy loss is typically less than 1% of total fuel energy.

What is of particular interest regarding the above heat losses are the following:

  • Excess air has the greatest influence on heat losses, namely dry heat loss, combustibles loss and to some extent bottom ash loss. Close control of the combustion process (excess air) is key to high boiler efficiency.
  • Coal is an integral part of the combustion system; it dictates the excess air requirements and determines the condition of the bottom ash. It also causes fire side scale which impacts on the rate of heat transfer to the boiler water.
  • Returning as much condensate as possible reduces plant, as well as blow down energy losses.
  • Operating the boiler at higher capacity is advantageous in terms of shell loss percentage, but increases the flue gas temperature and hence dry heat loss.
  • Oversized boilers tend to be inefficient.
  • On average adding 10% excess air to the fire reduces overall thermal efficiency by 1%. Increasing flue gas oxygen from 8% to 9% adds 22% to the excess air percentage and will increase the fuel bill by 2,6%. Compelling numbers to make us look at combustion optimization from a new perspective!

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about combustion control systems and combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

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