Boiler fouling

Some additional methods of classification are under development that center on the use of lignite for combustion in utihty boilers or electric power generation. Correlations based on the sodium concentration in the lignitic ash (10), or soluble A1 concentration (11) are used. The classifications are often given in terms of the severity of boiler fouling. 

Coal deposits from east of the Mississippi River generally have acidic mineral constituents, ie, they are richer in siUca and alumina and tend to produce higher melting ash mixtures. These materials do not soften until above 1000°C and have limited problems with deposition on the inside walls of the boiler (slagging) or on the superheater tubes inside the boiler (fouling). 

The concentrations of specific elements can be useful indicators of some coal quality characteristics. Huggins et al. (5) and Reid (6) demonstrated that the aluminum, silicon, potassium, calcium, magnesium, and sodium values of a coal ash can be used to estimate ash fusion temperature. The Si/Al ratio of coal ash has been used as an indicator of the abrasiveness of a coal. Sodium is a major contributor to boiler fouling and metal corrosion and contributes to agglomeration in fluidized-bed reactors. Trace elements are generally defined as those elements with concentrations below 0.1 wt. % (1000 ppm). Despite concentrations in the parts-per-million range, certain trace elements can have a significant impact on coal... 

Several bituminous coals have been employed during the course of this study. The method can be applied to relatively unprepared coals and seems to be very effective for the removal of alkali and alkaline earth metals which are related to boiler fouling. With modification, other mineral groups could be removed. Coal treated with CO2/H2O occasionally crumbles during processing. The system is flexible, may be modified both chemically and physically, and may be integrated into, or used to modify, an overall coal preparation... 

Combustion (poor combustion causes boiler fouling, poor heat transfer and stack solid emission). 

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

Off-Design Performance—This is an important eonsideration for waste heat reeovery boilers. Gas turbine performanee is affeeted by load, ambient eonditions, and gas turbine health (fouling, ete.). This ean affeet the exhaust gas temperature and the air flow rate. Adequate eonsiderations must be given to bow steam flows (low pressure and high pressure) and superheat temperatures vary with ehanges in the gas turbine operation. 

Evaporators—These usually utilize a fin-tube design. Spirally finned tubes of 1.25 in to 2 in outer diameter (OD) with three to six fins per ineh are eommon. In the ease of unfired designs, earbon steel eonstruetion ean be used and boilers ean run dry. As heavier fuels are used, a smaller number of fins per ineh should be utilized to avoid fouling problems. 

Compensation is made for variations in ambient air pressure and temperature, calorific value, boiler resistance due to fouling and burner performance drift by trimming the air damper with a separate servo motor. 

The potential to recover heat from the gases of an oil- or coal-fired boiler is therefore limited to a temperature drop from 240°C to 170°C. This results in a 3 per cent saving. The average saving would be somewhat lower than this since fouling of the economizer surface is inevitable from the carbonaceous emissions of the firing equipment. 

Unlike RO, which is essential for producing ultra-pure water, there is little experience of ED in this field. The process has some potential advantages over RO it is less liable to fouling and it can be engineered to waste much less water. Like RO, its costs fall sharply at higher temperatures, but the prospects of improved engineering making this a reality are better than for RO. It offers some prospects particularly where the product water has to be heated in any case (e.g. boiler make-up). 

Where relatively clean by-product gas is available, WH boilers of both horizontal and vertical FT design are often used. For larger applications WT boilers are preferred, and in fact most WH boilers are of the WT type. However, if the gas is dirty, special boiler designs are required to prevent fireside fouling of the furnace tubes, superheaters, and burners. 

Feedwater supply duties include checking and recording the demand for FW makeup and maintaining correct FW temperatures to prevent risks of pump cavitation and boiler thermal shock. Periodically checking deaerator performance and inspecting the FW pumps and lines for any signs of fouling or corrosion is also required. 

Crystalline scales and other minerals may form deposits that reduce the effectiveness of the boiler as a heat transfer device or cause fouling that can impede the flow of water and cause overheating by reducing the level of furnace cooling below design specifications. The presence of process contaminants also leads to fouling. 

Most waterside problems develop insidiously. Over time, scale and other types of deposit are gradually formed on internal heat transfer surfaces, which gradually raises the cost of providing heat energy. Some types of deposition can be very difficult and costly to remove. Corrosion wastes away the fabric of the plant (sometimes very quickly) and may produce an unexpected and untimely boiler plant shutdown, with a consequential loss of space heating, electricity, or process manufacturing capability. Likewise, fouling reduces the size of waterways and increases boiler operational problems. 

In practice, both fouling and deposition actions are likely to occur simultaneously to some degree or other in a boiler system. Furthermore, corrosion processes and the entry of contaminants into the steam-water system usually results in some form of deposition occurring elsewhere in the system. 

To maintain all boiler surfaces and other waterside surfaces in the HW and steam-system cycles in a structurally sound and clean condition, properly protected against the operational and economic problems associated with deposition, corrosion, fouling, and contamination... 

Thus, the proper control of deposition, corrosion, and fouling and boiler structural integrity are interdependent functions, and all these phenomena are directly related to boiler design and real-time operating conditions. 

Where corrosion takes place, the origins of the metal oxides and salts formed from corroded boiler system metals should be traced in a systematic fashion to establish cause and effect and avoid misclassify-ing the fundamental waterside problem. Occasionally however, it is difficult to positively confirm the starting point of a corrosion problem because it is common for corrosion products to be transported from their point of origin and deposited elsewhere in the steam-water circuit, or alternatively to act as binders and contribute to fouling and contamination of the overall boiler plant system. 

Fouling produces dirty and inefficient steam-water systems, impeding the natural or forced circulation within a boiler, limiting the flow in... 

Fouling, like deposition and corrosion, also takes many forms but generally involves the physical adherence to waterside surfaces of materials such as oils, process contaminants, corrosion products, and settled boiler sludges. 

Contaminants such as oil, magnesium phosphate, and hematite have a natural binding action that exacerbates the fouling problem and may result in the rapid agglomeration of tube deposits in WT boilers and fire-tube bridging in FT boilers. 

Innocuous sludges such as those resulting from using phosphate precipitation programs may cause severe fouling problems, especially when oil, saponifiable fats, or other deposit binders are present in the boiler. 

Oxygenation treatment also reduces the risk of erosion-corrosion problems and limits iron transport to other parts of the boiler system where fouling could take place. 

Apart from the oxygen corrosion that results in HW and LP steam heating systems where water losses occur as a result of leaking pump mechanical seals, excess BD, faulty steam traps, and other sources, a subsequent effect is the development of fouling. This effect stems from the production of corrosion debris and (high iron content) sludge that eventually settle out in the boiler. This corrosion debris, sludge, and other foulants must be periodically removed from the boiler by BD, which merely adds to the water loss, and the cycle perpetuates. 

Higher rates of sludging also take place in the boiler vessel. In turn, this potential fouling problem requires additional maintenance time because more frequent internal surface cleaning, wash-down, and boiler vessel sludge removal usually is required. Carryover of contaminants into the steam also is more likely. 

The effect of carryover and after-precipitation is that solids settle out and cause pre-boiler system fouling and result in reduced flow and equipment waterway blockages. Check valves are especially prone to blockage. 

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