Feedwater treatment

Fixed investment includes cooling tower, boiler feedwater treatment, raw water ammonia storage as minimum off-sites requirement. 

Feedwater treatment is designed to protect the feedwater system and, to some extent, the boiler. Most systems contain carbon steel piping. Carbon steel corrosion (Fig. 23a) is considerably slower at a pH between 9.0 and 11.0. In aH-ferrous feedwater systems, the preferred pH range is therefore 9.2 to 9.6, although some systems are operated at a pH as high as 10. In systems where copper alloys are present, high concentrations of ammonia accelerate corrosion of the copper alloys. In those systems the preferred pH is 8.8—9.2. 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

In the case of boilers operating at low pressure, organic materials such as natural and modified tannins, starches, or alginates are added to aid blowdown. For boilers operating at high pressure, synthetic materials such as polyacrylates and polymethacrylates have been developed. The most commonly used chemicals for boiler feedwater treatment are phosphates and hydrazine. 

The choice of boiler steam inlet conditions is usually dictated by the desire to achieve maximum output from the process steam flow. This requires high boiler steam pressure and temperature. However, there are practical considerations to observe. Above 40 bar more exacting feedwater treatment is necessary, and therefore it may be advantageous to maintain pressures below this figure. High steam and temperatures can also influence selection of boiler materials such as alloy steels. The upper limit for industrial applications is around 60 bara and 540°C. 

Stewart Richard, G. Partial Oxygenated Feedwater Treatment at the Paradise Fossil Plant Unit I. Tennessee Valley Authority. Ultrapure Water, Tall Oaks Publishing, Inc., USA, November 1997. 

Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. Raw water from a reservoir, river, lake, borehole or a seawater desalination plant is fed to the steam system. However, it needs to be treated before it can be used for steam generation. The treatment required depends both on the quality of the raw water and the requirements of the utility system. The principal problems with raw water are1,2 ... 

Figure 23.2 Boiler feedwater treatment. (From Smith R and Patela EA, 1992, The Chemical Engineer, No 523 16, reproduced by permission of the Institution of Chemical Engineers.).

The deaerated treated boiler feedwater then enters the boiler. Evaporation takes place in the boiler and the steam generated is fed to the steam system. Solids not removed by the boiler feedwater treatment build up in the boiler, along with products of corrosion. These are removed from the boiler by taking a blowdown (purge) from the boiler. The steam from the boiler goes to the... 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

There are many types of steam boilers, depending on the steam pressure, steam output and fuel type. Blowdown is required to remove the dissolved solids not removed in the boiler feedwater treatment. The efficiency of the boiler depends on its load. 

Improving the energy efficiency to reduce steam demand and hence reduce the wastewater generated by the steam system through boiler blowdown, boiler feedwater treatment and condensate loss (see Chapter 23). 

Increasing condensate return for steam systems to reduce makeup water requirements, reduce aqueous waste from boiler feedwater treatment and boiler blowdown (see Chapter 23). 

Waste from steam systems. If steam is used as hot utility, then inefficiencies in the steam system itself cause utility waste. Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. The constant loss of condensate from the steam system means that there must be a constant makeup with freshwater. This makeup causes utility waste ... 

Severe Screen tubes Overheating following partial plugging of tubes from inadequate feedwater treatment... 

Hydrogen and oxygen are released separately and the gases are manifolded from the cell batteries into separators, where water vapor and excess electrolyte are removed. The system costs included feedwater treatment and hydrogen compression to 400 psi. 

PART OF feedwater (treatment chemical) omitted. Water treatment fault. Same as above. 

Hybrid systems have attracted interest in recent years due to their potential for reducing costs, decreasing environmental impact from discharge streams, and expanding the envelope of source waters that can be treated [52], For example, electrodeionization (EDI) is a hybrid process involving ED and IX. In an EDI system, the space between the IX membranes is packed with IX resins. The addition of the resin improves the conductivity across the cell and allows the production of highly deionized water. EDI systems have attracted interest for boiler feedwater treatment applications [53]. Hybrid systems using combinations of RO and IX have also been piloted and commercialized [54]. 

The economics of boiler feedwater treatment are strongly dependent on local conditions, including the water quality... 

One development that may alter the technology landscape for cooling water and boiler feedwater treatment is the increased competition for water from municipal, agricultural, and industrial users. Combined with pressures on supply, more power plant operators are considering the use of lower-quality impaired water resources [59]. Examples of impaired water include brackish aquifers, produced water from hydrocarbon recovery operations, municipal waste, and... 

G. Navitsky and F. Gabrielli, Boiler water treatment, feedwater treatment, and chemical cleaning of drum-type utility steam generators. Combustion 52(2), 19 (1980). 

Spent resins from boiler feedwater treatment plants may be returned to the resin producer or burned in high-temperature kilns. 

Modem power utility boiler feedwater treatments commonly include the addition of organic amines to inhibit the corrosion of iron components of the system by scavenging oxygen and increasing the pH of the feedwater. These chemi-Cctls such as morpholine and hydrazine decompose in service to yield ammonia, which can be corrosive to some copper alloys. However, elevated oxygen levels are generally more deleterious than elevated ammonia levels. 

There are two major methods of boiler feedwater treatment, external and internal. 

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