Soot blowers

Near drum wastage is the most prevalent type of wastage documented. It cart be found anywhere in the generating bank but is most common on the wall tubes, in tlic row immediately next to soot blower lanes and in the hot and cold row s. Three types of near drum w astagc have been documented. 

A related property is the viscosity of coal ash. Ash viscosity affects the rate at which ash deposits may flow from the walls, and thus the requirements for ash removal equipment such as wall blowers and soot blowers. The preferred coal ash has a narrow temperature range through which it passes the plastic range, ca 25, 000-1,000, 000 mPa-s (=cP) (62). 

Oil additives have been used for a number of years. They are normally introduced as suspensions of metallic oxides or other salts such as Mg (OFI)2, Ca (OH)2, AI2O3, etc., in fuel oil or water. Use of these compounds has been shown to be uneconomic, ineffective and to cause problems, i.e., tube fouling, increased soot blower usage, solids disposal, etc., rather than cure them. 

The fuel management system includes the various types of fuel preparation and fuel delivery equipment, burners, ignitors, stokers, soot blowers, and ash handling equipment. 

NOTE Soot blowers are typically employed only on WT boilers and may be regarded as appurtenances because boiler safety and reliability is directly related to the cleanliness of the heat transfer surfaces. Boiler performance and efficiency also depend on the same heat-transfer cleanliness factors. 

The furnace areas of WT boilers require the periodic deployment of retractable soot blowers to remove the buildup of soot and combustion products from water-wall tubes to maintain heat transfer and furnace cooling efficiency, as well as to minimize the interference of flue gas pathways. 

Soot blowers also are used to clean particular boiler tube bundles, such as superheaters and economizers. Steam or compressed air normally is used, and operating practice may require the use of manual boiler control and increased furnace draft or boiler loading during sootblowing periods to avoid the risks of loss of furnace flame (flameout) or small furnace explosions (furnace puff). 

High-pressure steam from steam soot blowers can easily cut through tubes, so correct alignment of soot blowing equipment is critical. 

Operators are required to maintain correct fuel pressures and ensure that the combustion equipment (burners, stokers, soot blowers, ash handling, etc., as appropriate) is in good working condition. 

Prior to shut down, switching to higher grade fuels for a week or so and increasing the use of soot blowers helps remove deposits from fireside surfaces. Additionally, fuel treatments such as combustion additives, slag modifiers, and anticaking agents may prove very useful. 

Boiler fireside walls, baffles, tubesheets, tubes and drums should be cleaned to remove ash and soot before examination. Where soot blowers are installed, they should be utilized as a means of cleaning fireside surfaces before the boiler load has dropped to 50% and should not be operated again, especially if the fire has been extinguished because a risk of explosion exists. 

Excessive soot or deposit buildup and acidic cold-end corrosion may occur on the fireside of economizers. Soot blowers can fail due to mechanical wear. 

Misaligned soot blowers may cause cutting or erosion of tubes. 

The degree of slagging, is, in turn, closely related to the concentration of vanadium, nickel, and sodium compounds present in the fuel, and the types of low melting-point oxides and complex sticky deposits formed under specific boiler temperatures and prevailing conditions. These deposits are difficult to remove online with soot blowers, but... 

The damage continued to the top of the furnace, but screen and superheater tubes were not damaged. One soot blower was blown onto the roof of an adjoining building and another knocked loose. 

A potential drawback of this technology is the potential formation of ammonium sulfate salts and their resulting fouling. This can be mitigated through either use of high pressure soot blowers or on-line water washing in the economizer. These salts will exist as small particulates that will increase opacity. 

The catalyst must be designed to handle the abrasive environment where catalyst hnes are always present in the flue gas yet still perform with a low pressure drop typically below 5 inches of water column. It must also maintain activity continuously for a 5 year cycle, yet be selective enough to limit undesirable reactions like SO2 oxidation. The catalyst must also be able to withstand periodic blasts of steam or pressurized air coming from the soot blower system found in many of the newer FCCU SCR units. 

There are two design options when considering an SCR unit for the FCCU upstream or downstream of an ESP or TSS. If the SCR unit is placed upstream of an ESP or TSS, then the refiner has to incorporate soot blowers for catalyst fines removal from the catalyst surface and use a wider pitch catalyst to handle the higher levels of catalyst particulates. The wider pitch catalyst contains more void volume and thus will directionally increase the catalyst bed dimensions since the NO reduction is based on the total amount of surface area, not just catalyst volume. 

If the SCR is placed downstream of an ESP or TSS, the design can take advantage of a cleaner flue gas. This would allow for smaller catalyst volumes using finer pitch catalyst and thus smaller SCR reactors. Problems occur when the ESP or TSS collection efficiency no longer removes the particulates from the flue gas. Not only does the SCR catalyst bed foul, requiring increased run frequency on the soot blowers, the stack opacity will also increase. 

Several units with a PM collection device located upstream of the SCR have seen increased pressure drop from fine particulates accumulating on the catalyst bed. Soot blowers have been partially successful in this application. When the SCR is applied to a CO boiler with limited pressure drop, the SCR has typically been located upstream of PM removal to avoid this problem. Some refiners have chosen to install a spare SCR reactor to provide redundancy due to pressure drop concerns. Others have used a bypass where local regulations allow. 

ABS formation on a FCCU SCR is a major concern because it is a sticky foulant. Once formed, it traps catalyst fines to its surface. Once these particles are no longer moving within the flue gas stream, it is very difficult to reentrain them even with the use of soot blowers. 

Figure 17.18 shows catalyst modules installed inside the SCR reactor. The modules are loaded at the same time the sealing gutters are installed. A small space is allotted between adjacent modules to slide in a narrow steel gutter that creates the seal between itself and the modules pedestal frame at the base. Adjacent modules are not touching rake soot blowers. In this configuration, the distance from the rake soot blowers to the top grid of the catalyst modules is approximately 3 feet. 

Figure 17.22 shows the sections where the catalyst layers are located during assembly. The layers are spaced approximately 10 feet apart to accommodate the modnles and soot blowers. The volnme of catalyst reqnired to achieve the high levels of NOx redaction is snbstantial, typically aronnd 150 cnbic meters. This drives the catalyst bed design to be split into mnltiple layers. 

To reduce deposition on boiler tubes, the soot blowers should be used just prior to shutdown. The tubes farthest from the stack should be blown first, followed sequentially... 

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. 

A number of physical methods are available for handling fouling in combustion systems. So-called soot blowers that consist of directing a jet of air or steam at the heat-transfer surfaces, where fouling is likely to occur, have been employed for many years. The force created by the jet on the deposit weakens it and knocks it off the surface. Because of the need for the jets to be close to the surface, and the extent of the heat-transfer surfaces usually involved, it is necessary to use a number of soot blowers in a particular system. To be effective, the device has to be able to rotate or move relative to the heat-transfer surfaces so that they are fully covered movement may be manual or motorized automatic. 

Can stand high sulfur and high particulate loadings. A soot blower can be provided for particulate control (the particulates are stirred up and flushed through the honeycomb). 

Soot blowers may be required for services with high particulate loadings. Down flow designs can also help in flushing any solids through the system. 

Magnesium orthovanadate has a melting point of around 1190°C, which is considerably higher than that of sodium-vanadium compounds. This means that magnesium orthovanadate has a lesser tendency to deposit and, when it does, it is loose and powdery and easily removed using the soot blowers (Salooja, 1972). 

Application of aqueous MgO-based slurry via the soot blowers should theoretically result in lower treatment rates than other modes of application. However, data suggests that dosage rates are in the same range as oil-based MgO dispersions. However, with this mode of application, inadequate dosage or poor distribution can result in an increased ash burden, thus exacerbating the problem. 

The principal designs of soot blower fall into three types [ESDU 1992]. 

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