Reduced Boiler Fuel Costs Through Improved Feedwater - Part II

Introduction:

The last Technical Tip discussed the relationship of feedwater quality to the boiler blowdown rate. We pointed out that by increasing the amount of condensate returned, feedwater quality could be improved, thereby allowing a deduction in the blowdown rate. Increased condensate returns tend to dilute make-up water impurities as a means of improving the quality of the feedwater. Other methods can be employed to reduce or even eliminate many impurities, and these methods involve the treatment of the make-up water itself. Several of the more commonly employed treatment systems will now be discussed.

Determining the Limiting Impurities:

There are four basic types of impurities in most make-up waters which directly affect feedwater quality and boiler blowdown rate. These are:

1. Total hardness
2. Total alkalinity
3. Silica
4. Total dissolved solids

Using the information in the previous Technical Tip of the Week, we can determine which impurities impose the lowest limit on our feedwater concentrations and hence, must be reduced or removed for more efficient operation.

Hardness (Suspended Solids) Limiting:

Modern water tube boilers cannot be operated satisfactorily with feedwater containing appreciable hardness. Fire tube units and older water tube boilers will tolerate hardness in the feedwater, subject to the suspended solids limits shown in Table I of last week’s Technical Tip. Of course, in any case, proper internal chemical treatment must be used.

In most cases, the preferred method of hardness removal is by means of sodium Zeolite softening. In this process, cation-exchange (zeolite) resin is used to exchange all calcium and magnesium ions for sodium, thus reducing hardness to nearly zero. Sodium zeolite softening has no effect, however, on dissolved solids, silica or alkalinity. In fact, if the make-up water alkalinity is high, the installation of a sodium zeolite softener can result in excessive boiler water alkalinity, hence little or no reduction in blowdown. To see why this happens, let’s look at the following make-up water analysis:

 

Total hardness as CaCO3	38 mg/l
Total alkalinity as CaCO3	26 mg/
Dissolved solids	80 mg/l

Before installing a sodium zeolite softener, all of the hardness would be removed by precipitation within the boiler with chemical treatment, and a portion of the alkalinity would also be precipitated. After installing the softener, little or no alkalinity removal will take place. Thus practically all of the feedwater alkalinity will concentrate in the boiler water, and in some cases, alkalinity will then become the limiting feedwater constituent.

Alkalinity Limiting:

If total alkalinity is presently the limiting feedwater constituent, or if the installation of a sodium zeolite softener will cause alkalinity to become the limiting factor, several methods of alkalinity reduction can be considered.

Split-stream dealkalization: This method employs two cation exchange units, operated in parallel. One unit is a conventional softener, regenerated with salt, while the other unit contains the same type resin, but regenerated with acid. In operation, a portion of the make-up water passes through the sodium zeolite for the hydrogen (acid) zeolite for both hardness and alkalinity removal. By adjusting the ratio of sodium zeolite effluent to hydrogen zeolite effluent, any desired alkalinity reduction can be obtained. In practice, the ratio is determined by the boiler water alkalinity level desired. Split stream dealkalization reduced hardness to nearly zero, reduces alkalinity, reduces dissolved solids to the extent of alkalinity reduction, but does not reduce silica. The chief disadvantage of split stream dealkalization is the need to handle acid regenerant.

Chloride Dealkalization:

In this process, two ion exchange units are operated in series. The first unit is a conventional sodium zeolite softener containing cation exchange resin. This is followed by the dealkalizing unit, which contains anion exchange resin. Both units are regenerated with salt, thus avoiding the handling of acids. In operation, the sodium zeolite unit reduces hardness by exchanging the calcium and magnesium for sodium. The anion unit then exchanges bicarbonate, sulfate and other anions for chloride. Alkalinity of the final effluent is reduced to nearly zero by removal of bicarbonate. This process, however, will not reduce dissolved solids or silica. Although exchanging the bicarbonate for chloride reduces alkalinity, sulfate and all other anions are also unavoidably exchanged. Therefore, high sulfate waters are costly to treat by chloride dealkalization. Also, because alkalinity is almost completely removed, caustic soda must be added to the boiler system in order to obtain the necessary hydroxide (OH) alkalinity in the boiler water.

Silica Limiting:

When silica is the limiting feedwater constituent, it can be removed by a strongly basic anion exchange resin regenerated with caustic soda. Either of the following two systems can be employed, depending upon whether or not a reduction in dissolved solids is also required.

Desilicization:

The strongly basic anion resin unit follows a conventional sodium zeolite softener. Hardness is reduced in the cation unity while the anion resin reduces all anions and silica. Silica will be reduced to nearly zero. However, because all cations are exchanged for sodium and all anions are exchanged for hydroxide, the hydroxide alkalinity of the final effluent must be partially neutralized in order to be suitable for boiler make-up.

Demineralization:

When it is desired to remove silica and dissolved solids to a very low level, complete demineralization is employed. A cation exchanger in the hydrogen form (regenerated with acid) is followed by a strong base anion exchanger (regenerated with caustic soda). All cations and anions are exchanged for hydrogen and hydroxide respectively, thus removing all dissolved solids including silica to nearly zero. While demineralization results in a very pure effluent, the cost of operation is quite high and may not be justified for low to moderate pressure boilers.

Summary:

Ion exchange processes offer a means of reducing make-up water impurities, which improve boiler feedwater quality, and therefore reduce blowdown requirements. The choice of a particular method depends upon several factors, namely:

1. Boiler type and operating pressure
2. The impurities which limit feedwater concentrations
3. Characteristics of the make-up water
4. Handling of corrosive regenerants
5. Equipment first cost and operating cost versus advantages to be gained

In addition to the methods described above, other variations of ion exchange technology may have application for a particular water or steam system. Chemical precipitation methods, such as cold lime or hot lime soda, may also be suitable for certain situations.

1. Reduce load gradually. Be sure the rate of cooling does not exceed the boiler manufacturer’s recommendations. The rate of cooling suggested by a major boiler manufacturer is not more than 150°F (66°C) per hour.
2. Cut fuel off gradually.
3. Maintain normal water level until the boiler is ready to be drained.
4. When the steaming rate has reached 20% of normal load, go to full manual operation of fuel and feedwater controls.
5. Before cutting off the last burner, open drain valves at steam, non-return, and superheater outlet header valves. Be sure the bypass around the non-return valve is closed. Operate drain valves as necessary to maintain prescribed rate of cooling.
6. Operate draft fans until all fuel has been purged.
7. Shut down fans.
8. Close all dampers.
9. When boiler pressure starts to drop, close stream-line stop and non-return valves.
10. When the non-return valve is closed, open valve in economizer recirculation connection (if present).
11. When drum pressure is less than 25 psig (1.75 kg/cm², open drum vent valves.
12. Empty the boiler only after boiler water temperature is below 200°F (93°C).
13. Follow manufacturer’s/insurance carrier’s instructions regarding fireside inspection and cleaning.