Boiler feed-water preheating in sugar production

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Description

The efficiency of a boiler’s steam production may be improved by increasing the efficiency of heat transfer by the heat carrying medium. The feed-water absorbs the heat produced from combustion and steam is produced. The higher the temperature of the feed-water when it enters the boiler, the larger quantities of high quality steam will be produced, because little energy is lost to increase the feed-water temperature from ambient to evaporation conditions (EU, 2005).

Usually the boiler feed water preheating is performed by Indirect Heating using Heat Exchanger Tanks and Solar collectors depending on the requirement. Evacuated Tube Collectors (ETC) to heat the feedwater from 25 deg C to 70 deg C (avg.) are better than Flat plate collectors because of its lower cost, No scalign issues and ability to use diffusion radiation too for heating the water.


Applications

In the food industry sector, steam is generated using boilers with working pressures up to about 30bar. For power generation by steam turbines, higher pressures are needed. Steam is used for the sterilization of pipes and pipelines. Another application is UHT treatment with direct steam injection. Sometimes steam injection is used for heating the product or for adjusting the water content of the raw material. In all these cases, more or less direct contact between steam and the food product is possible, so drinking water quality is required. The quality of the boiler water is controlled by the quality of the boiler feed-water (EU, 2005).


Flowsheet

A typical Diagram of the Boiler Feed-water Preheating process:


Boiler Feed-water Preheating Process.jpg


Typical parameters of the process

Typical process Temperature range (°C) Collector based on temperature level
Boiler feed water preheating <150 Evacuated Tube Collector / Flat plate


Technologies and energy saving potentials

The most common system for preheating the boiler feed-water includes a heat exchanger and an auxiliary stream with relatively high thermal content which will transfer energy to the boiler feed-water. The type of heat-exchanger can differ according to the size and water needs of the boiling system, but careful considerations should be taken for efficient heat transfer (EU, 2005). It’s important that the boiler water-feed does not cause scaling in the boiler or corrosion of the steam system. This means that boiler feed-water has to have very low hardness and be de-aerated (EU, 2005).


Changes in the heat supply system

  • Heat recovery from flue gas

Heat recovery from the flue gas of the ovens can be used to preheat boiler feedwater in an economiser. the limiting factor is that the economiser wall temperature must not drop below the dew point of acids contained in the flue gas.In fact, the economiser wall temperature is much more dependent on feed water temperature than on flue gas temperature because of the high heat transfer coefficient of water. As a result it makes sense to preheat feed water to close to the acid dew point before it enters the economiser.Typically 1% of fuel use is saved for every 25 degrees celsius reduction in exhaust gas tewmperature.(Ganapathy,1994)


  • Heat recovery from waste streams: (Joanneum Research)

Waste streams which are produced during the process and contain adequate thermal content can be recycled and re-used for preheating the boiler feed-water. A system of heat exchangers will be used to allow the heat transfer between the waste streams and the feed-water.


  • Re-use of condensates: (EU, June 2005)

Condensates produced during evaporation may be re-used in the process depending on their quality, e.g. content of organic and/or inorganic matter and SS. Steam condensate may be used as boiler feed-water. This leads to recovery of a considerable amount of heat, as well as savings in the use of chemicals for the treatment of boiler feed-water. If condensate is reused this can be optimised by maximising the condensate return and avoiding losses of flash steam from condensate return.


References

  • Ganapathy, 1994.Understand Steam Generator Performance in Chemical Engineering Progress
  • European Commission, 2006. BREF Reference document on Best Available Techniques in the Food, Drink and Milk industries



  • Applications

Boilers are widely used in sugar industries to provide energy. Preheating the boiler feed-water is a technology typically applied in most production lines.


  • Example flowsheet

Boiler feed-water preheating is not a processing step when manufacturing sugar products, therefore it's not included in process flowsheets.


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