With air preheaters the boiler’s total efficiency will increase significantly:
roughly 2%for every 50 K orC(90F)decrease in temperature of flue gas or 2%for every 50 K orC(90F)increase in that of combustion air.
A high combustion air temperature seems to increase the emission of nitrogen oxides, particularly in the case of natural gas. The reduction of excess air, however, may moderate the NOxincrease.
Maximum combustion air temperature does not generally exceed 200 C (392 F), depending on fuel, type of burner, and boiler condition. Higher temperatures are required if pulverized coal is burned. The lowest flue-gas temper- ature at the outlet of the preheater is limited by potential corrosion, as mentioned in Sect.6.8.
Air preheaters can be classified as either recuperative or regenerative.
In recuperative units, heat is transferred directly from the hot medium such as hot flue gas on one side of the stationary surface to air on the other side.
Regenerative units transfer heat indirectly from the hot flue gas to combustion air through an intermediate heat storage medium. They are used mainly in retrofitting existing boiler plants.
An alternative to the above-mentioned preheaters is the heat-pipe heater (see Sect.15.5). They are not currently in use in boiler units, but they may become competitive with the traditional ones.
Economizers provide additional heating to the feedwater by recovering heat from stack flue gas. In this way the boiler’s firing rate needed to produce the same amount of steam is reduced and the overall efficiency is increased. Economizers can also be installed to heat hot water for users apart from the boiler water cycle.
The advantage of economizers is closely related to the existing feedwater temperature and condensate return system. Roughly, an increase in feedwater temperature of 5 K orC(9–10F)will result in an efficiency increase of 1%.
In case of significant blowdown (over 10 % of output steam) or of low conden- sate recovery, heat recovery can be installed to preheat the incoming feedwater.
6.9.2 Water Treatment Systems
Water treatment is one of the main factors in boiler reliability. It is necessary because impurities in the feedwater can lead to scale deposits on the boiler heating surfaces and to corrosion. Suspended solid matter, colloids, dissolved minerals, atmospheric gases, and impurities coming from the steam cycle are always present in varying degrees in boiler feedwater.
The main equipment for water treatment can be summarized as follows:
demineralization, pretreatment systems to remove suspended solids such as clarifiers, softeners, and media and precoat filters.
Demineralization removes dissolved salts from pretreated water, where they are present in the form of positive and negative ions (cations and anions). The goal is reached by means of ion-exchange resins exchanging harmless ions with impurities such as sodium cations and chloride anions. Exhausted resins can be regenerated for reuse; after a certain number of regenerations the resin bed must be discarded. The two-column system, which is one of the commonest installations, includes an acid cation exchanger followed by a base anion exchanger with a degasifier interposed to strip out the CO2produced in the first exchanger. Boilers operating at high pressure and temperature require feedwater of very high purity (roughly 5–10 parts per billion of sodium and silica). In this case other systems such as mixed cation–anion- resin beds and semipermeable membranes are generally installed.
Filters and clarifiers are used to remove solid particles and some dissolved substances. Clarifiers work with chemical coagulants which help to gather finely divided matter into larger masses. In addition, calcium and magnesium can precipi- tate depending on the chemicals added. Calcium and magnesium hardness can also be removed by separate softeners.
Softeners are used to remove hardness and also to remove silica and reduce carbonate alkalinity. They generally work at ambient temperature and are effective for low-pressure boilers that do not require complete feedwater demineralization.
Hot-process softeners operating at above 373 K (100C; 212F) can be used for higher-pressure boilers.
Notice that blowdown is commonly used to remove boiler water impurities. The amount depends on many factors, particularly on boiler water treatment performances, and it can be as much as 5–10 % of the total boiler steam. It is always possible to use heat recovery from blowdown water to preheat the incoming feedwater water. Low-pressure boilers generally use heat exchangers; high- pressure and high-capacity boilers use flash systems where part of the blowdown wastewater is flashed into high-purity steam, then condensed and recycled into the feedwater system. The remaining waste water exchanges heat with the makeup water and then is discarded.
Table 6.7 synthesizes the main features of the commonest types of water treatment equipment.
6.9.3 Condensate Return Systems
The recovery of condensate from process steam furnishes a great amount of usable energy and clean feedwater. The degree of recovery and its economic validity depend on many factors such as contamination from process, layout of steam end users and pipelines. The quantity of energy contained in the saturated liquid can be estimated as roughly 20 % of the original energy contained in the steam at the same pressure.
86 6 Boiler Plants
The main systems used to recycle the condensate into the boiler are: the atmospheric system, pressurized systems, and deaerators.
The atmospheric system consists of an open tank where condensate is received downstream of steam traps or drains. At atmospheric pressure the condensate temperature is reduced to 373 K (100 C; 212F): a portion of the condensate, about 10–15 %, will flash to steam and will be lost in the atmosphere; the remaining hot water will feed the boiler together with as much treated makeup water added as the boiler requires.
The semi-pressurized and fully pressurized systems work with a receiver at intermediate pressure below the lowest process steam pressure or at process steam pressure. The flashed steam is piped to a low-pressure steam main where it can be utilized for process purposes; the lost flashed steam is conveyed to the outlet through relief valves or automatically operated vents which are installed to prevent abnormal pressure surges. This loss, too, can be reduced if some additional provisions are made for steam recovery.
The condensate is generally cooled by the makeup water to at least 283 K (10C;
18F) below the saturation temperature to prevent vapor generation in the boiler feedwater pump system.
Notice that although the use of flash steam in low-pressure mains seems to offer a significant heat recovery, its practical application involves a number of problems, essentially economic, that must be carefully considered. In addition to inherent factors which must be taken into account when planning a condensate recovery system, the quantity of flash steam must match potential needs at any time since steam cannot be stored economically for later use.
Spray deaerators, also called barometric condensers, consist of a tank where steam rises through a cold water spray which condenses it (see Fig. 6.13). This system, which achieves complete flash steam recovery to heat boiler feedwater, has the additional advantage of producing a deaerating effect: if the temperature of the tank is kept above about 363 K (90C; 194F) by a proper metering of the spray, dissolved gases in the condensate and feedwater, such as oxygen and CO2, will come out of the solution and will be released through the atmospheric vent.
Table 6.7 Effect of treatment techniques on makeup water contaminants under ideal operating conditions
Treatment techniques
Suspended
Solid Alkalinity Hardness
Dissolved silica
Dissolved solids
Filtration Excellent No
change
No change
No change No change
Cold-process softening Good Fair Fair Good Fair
Hot-process softening Good Good Good Good Fair
Sodium-cycle cation exchange
Excellent No change
Excellent No change No change Demineralization Excellent Excellent Excellent Excellent Excellent Reverse osmosis Excellent Excellent Excellent Excellent Excellent