Water is treated properly before feeding boiler. External Boiler Feed water treatment is the decrease or elimination of contaminants from water outside the boiler. It involves removal or reduction of some undesirable characteristics and addition of desirable characteristics to make water suitable and less troublesome for a proposed application before it is delivered to the point of use.
With object to prepare boiler feed water, the water after conventional treatment is further treated before using as boiler feed water. The following steps are associated in external boiler Feed water treatment.
- Demineralization to remove dissolved minerals, salts or acids.
- Dearation to remove dissolved oxygen
Demineralization of boiler feed water
The following steps consist in demineralization section:
– Cation exchanger
– Anion exchanger
– mixed Bed polisher
In demineralization process, hydrogen cation exchanger is generally employed to remove all cations from water. Filtered water is passed though the cation tower, which is filled with hydrogen cation exchanger. In cation exchanger the following reactions are taken place. Therefore the scale forming cations such as Ca, Mg ions are replaced by the labile hydrogen, and water is set free from those ions.
Ca(HCO3)2 + 2HR = CaR2 + 2H2O + 2CO2
Mg(HCO3)2 + 2HR = MgR2 + 2H2O + 2CO2
NaHCO3 + HR = NaR + H2O + CO2
Here the symbol R stand for the organic radical.
The following reactions are taken place between the resins and sulfates or chlorides ions.
CaSO4 + 2HR = CaR2 + H2SO4
MgSO4 + 2HR = MgR2 + H2SO4
NaCl + HR = NaR + HCl
Regeneration with sulfuric acid is the most widely used and most economical method of regeneration. The reactions, in condensed form, may be indicated as follows:
CaR2 + H2SO4 = CaSO4 + 2HR
MgR2 + H2SO4 = MgSO4 + 2HR
2NaR + H2SO4 = Na2SO4 + 2HR
Carbonic acid is produced by the reactions with bi-carbonate salts. In sequence which is removed as CO2 gas. Sulphate and chloride ions in water give rise to mineral acids. The resins are turn into corresponding salts and the feed water becomes acidic soft water.
Most of the CO2 is removed from acidic soft water in degasifier. This is obtained by spreading water over raching rings and air that flows there in counter current direction by the blower.
Then water is passed though the anion exchanger. Which is filled with two types of organic synthetic resin, i.e highly basic or weakly basic. Both types can remove strongly ionized acids for example sulfuric acid, hydrochloric acid or nitric acid. But only highly basic anion exchange resin can remove weakly ionized acids such as silicic and carbonic acids. For the anion exchange of a strongly ionized acid the following reaction may take place, where R4N represents the complex anion-exchanger radical.
H2SO4 + 2R4NOH → (R4N)2SO4 + 2H2O
Highly basic anion exchangers are regenerated with caustic soda, and weakly basic anion exchanger may be regenerated with caustic soda, soda ash or sometimes ammonium hydroxide.
(R4N)2SO4 + 2NaOH → 2R4NOH + Na2SO4
Mixed Bed Polisher
Lastly the water is passed though the mixed bed polisher containing cation exchange resins and anion exchange resins to remove the ions left in water after passing though anion exchange resins demineralized water thus produced is used as make-up for boiler feed water.
The simplest process for removing dissolved gases like oxygen is by the use of an open tank containing the water and dissolved oxygen, where most of the dissolved oxygen is stripped of by means of low pressure steam.
The Deaerator consists of a horizontal tank provided with a packed tower. During normal operation, the demineralized water is introduced above the packed bed & the stripping steam is introduced under the bed, controlled by the pressure controller. During start up steam can be introduced at the bottom of the tank, to heat up the stored quality of water.
The deaerator is operated at approx. 132⁰C in order to ensure proper shipping of the water, it is significant that an additional of steam be available in the bed, as a result the inlet temperature of the water to the Deaerator is kept (5-20)⁰C bellow the temperature level in the deaerator. It is checked that a reasonable flow of steam leaves the deaerator through the flow orifice. The deaerated water is called as the boiler feed water. The quality of stripped demineralized water is determined by analyzing samples taken from sample line. The boiler feed water from the deaerator pumped by the BFW pumps to the auxiliary boiler.