Cleaning methods, boiler

Excessive us of high-pressure steam soot blowers is a common source of tube erosion-corrosion. Other boiler cleaning methods less threating to boiler tubes are available such as mechanical rapping, shot cleaning, and compressed air soot blowing. 

Continuous conductivity measurement controlled with the electrode placed in the boiler. This method is not recommended because of potential safety and liability issues. In addition, there are difficulties with cleaning and maintaining the electrode, and the intense heat to which the electrode is constantly subjected may cause failure. FT boiler installations generally provide for the electrode to be placed above the first set of tubes but 4 to 6 inches below the waterline. 

Several methods of carbon dioxide production are in commercial use. These include the reaction between sulphuric acid and sodium bicarbonate, the combustion of fuel oil, the extraction of carbon dioxide from the flue gas of a boiler or similar heating facility, the distillation of alcohol and the fermentation of beer carbon dioxide is also a byproduct of fertiliser manufacture. Following manufacture the gas must be cleaned to ensure it is free from impurities and is fit for purpose. Two typical processes are described below. 

All of these boilers lend themselves to flexibility of arrangement. The length of tubes, numbers in a row (horizontally and vertically), etc., may be varied to suit economical or other conditions. Boilers of this type are generally supported independently of brickwork. Standard methods of cleaning have been developed, but a large number of joints must be made after Babcock Wilcox or Heine boilers have been completely opened for cleaning. 

ACID SOAK - A method of acid cleaning, in which the acid is pumped into the boiler and rests there for a period of time. 

One rather ingenions method has been used recently to dispose of the ammoniated EDTA solutions nsed for cleaning high-pressnre boilers. The used solvent is pumped into the fnmace of an adjacent operating boiler. The water and ammonia evaporate, the organic molecnles are destroyed, and the dissolved metals are converted to finely divided solids that will be captnred by the electrostatic precipitator. The basic ideas may have other applications. 

The principles for flue gas cleaning will be similar to those for pulverized coal combustor boilers, except that in addition to the use of a selective catalytic reduction (SCR) system for NOx reduction, various other combustion methods can be used, such as two-stage combustion, and a rebum concept in which 15%-35% of the coal is pulverized, and added into the main boiler chamber, bypassing the cyclone furnace. 

Postcombustion cleaning involves the use of processes that remove pollutants from the flue gases exiting the boiler (Kuhr et al., 1988 Frazier et al., 1991). Finally, cleaning by coal conversion (which is a departure from traditional coal-burning methods) involves the conversion of coal into a gas or liquid that can be cleaned and then used as fuel. 

A foolproof way to make sure you are not badly oxygen deficient is to connect a piece of tubing to the back end of the high-pressure boiler. Then attach the other end of the tubing to a bottle filled with clean, damp cotton. Observe the cotton. If it starts turning black after a few minutes, you are running oxygen deficient. Personnel who have been unable to master other analytical techniques find this method useful. 

In 1958, Loucks at al. described the use of citric acid as a cleaning agent. Reich developed a formulation for removing iron oxide from steel surfaces that consisted of a mixture of citric acid and formic acid. He claimed that the mixture of the two acids would hold more iron in solution than either of the acids alone. Citric acid used alone precipitated iron after several hours of contact. A process for cleaning iron oxide and copperfrom utility boilers was described by Alfano. This method uses monoammo-nium citrate at pH values between 3 and 5 to remove the iron oxide. The ammonium citrate solved the precipitation problems encountered with the use of citric acid alone. The pH is then raised to 9 with ammonia or an amine, and an oxidizer is introduced. The oxidizer dissolves the copper. The iron oxide removal stage is conducted at 150°F... 

Experience in the chemical cleaning industry has shown that no two iron oxides are alike. The heat history, density, and impurities may have a very large effect on the actual rate of deposit removal. Azuma and Kametani made systematic studies of the effect of sample preparation on oxide dissolution. The formation temperature of the oxide markedly influenced the dissolution rate. The physical properties apparently controlling the reaction were the surface area (calculated by a permeability method) and a normalizing factor that is dependent on the aggregate crystal structure. Baud and Perrier and Fields also demonstrated the importance of the scale structure (layers and microcracks). McPherson showed that the presence of chromium-containing spinels has a marked influence on deposits found in the superheat or reheat sections of power boilers. 

While chemical cleaning solvents usually are applied by pumping solvents through process equipment or power boilers, several other methods of applying them have been used. These include foams, "vapor phase" solvents, and special methods for cleaning pipelines. Because of the literature available, only foam cleaning and pipeline cleaning procedures will be reviewed. 

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