Steam system boiler feedwater treatment

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 ... 

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... 

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 ... 

Steam systems. Copper and copper alloys resist attack by pure steam, but if carbon dioxide, oxygen, or ammonia is present, condensates can be quite corrosive to copper alloys. 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 chemicals tend to release ammonia, which can be corrosive to some copper alloys. 

In ammonium phosphate production and mixed and blend fertilizer manufacturing, one possibility is the integration of an ammonia process condensate steam stripping column into the condensate-boiler feedwater systems of an ammonia plant, with or without further stripper bottoms treatment depending on the boiler quality makeup needed. 

Power Production. Steam cycles for generation of electric power use various types of boilers, steam generators, and nuclear reactors operate at subcritical or supercritical pressures and use makeup and often also condensate water purification systems as well as chemical additives for feedwater and boiler-water treatment. These cycles are designed to maximize cycle efficiency and reliability. The fuel distribution of sources installed in the United States from 1990—1995 are as follow coal, 45% combined cycle, 27% miscellaneous, 14% nuclear, 11% solar, oil, and geothermal, 1% each and natural gas, 0.3%. The 1995 summer peak generation in the United States was 620 GW (26). The combined cycle plants are predominantly fired by natural gas. The miscellaneous sources include bagasse, black liquor from paper mills, landfill gas, and refuse (see Fuels frombiomass Fuels fromwaste). 

The wastewater from a hydrogen plant typically consists of only the blowdovm from the boiler system. The boiler feedwater that feeds the steam generation system has small amounts of impurities such as sodium, chlorides, silica, and organic carbons. These impurities will accumulate within the boiler system and create sludge, scaling of the boiler tubes, and possible carryover of solids into the process steam. Blowdown of the boiler water is performed to prevent these issues from affecting the operation of the steam system. The blowdown is typically sent to the sewer or the on-site waste treatment plant for treatment and disposal. 

The steam system is an area that requires constant attention to ensure proper operation of the hydrogen plant. If the steam quality decreases, it can lead to solids carryover from the steam drum. These solids will plate out in the feed preheat coil of the convection section and subsequently lead to an equipment failure and plant shutdown. To reduce the probability of an upset, the steam drum should be manually blowndown on a regular basis to reduce the amount of dissolved solids and other impurities in the steam drum. Next, the boiler feedwater should be analyzed on a regular basis to ensure that the treatment it is receiving is adequate for the desired steam generation conditions. 

The steam drum is the entry point for boiler feedwater and internal chemical treatment. It is also the withdrawal point for continuous blowdown, purging from the solute of suspended solids to maintain the minimum level of dissolved and suspended solids. If blowdown is not done deposits of solids would form continuously on the boiler tubes and/or contaminate the system. The solid contents of steam should, in principle, be kept to zero. 

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

Recommended limits should be low because all solids in the feedwater will either deposit in the boiler or be earned over with the steam to the turbine. Consequently, water-treatment chemicals must be volatile. All cycles should have condensate-polishing systems to meet the limits show ll in Table 3. A schematic diagram is shown in Fig. 11. Laboratory tests as well as field studies show that high-flow-rate condensate-polishing systems 25 to 50 gal per min per sq ft (1015-2030 liters/minute/square meter) of cross-sectional bed area] perform as filters of suspended material and ionized particles. Ammonia is added to control the pH in die system. Fig. 12 indicates the amount of ammonia required, in terms of ppm or solution conductivity, to give a certain pH in the system. Hydrazine is added to the cycle for oxygen scavenging. 

---