Boiler wall

Harvested and defivered whole, the trees are dried ia an air-supported fiber glass dome stmcture over a 30-d period by usiag waste heat from the combustioa process ia the adjaceat plant (Pig. 5). Trees leave the dome on the conveyor and, at the boiler wall, batches ate cut iato sectioas to fit the boiler. These sectioas are about 8.5 mloag for the 100-MW facility studied by EPRI and the Mianesota Power Light Company. 

Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive. 

A thermocouple well mounted through the wall of a boiler may be considered as a metal rod of 0.02 m outer diameter and 4 m length, with a thermal conductivity of 30 W/(m.K). The thermocouple reads 600°C, and the temperature of the boiler wall where the well is located is 200°C. If the gas heat transfer coefficient to the well is 200 W/(m2.K), find the average gas temperature. 

Figure 7. Mossbauer spectrum of boiler-wall slag deposits. The approximate phase distribution determined from CCSEM and Mossbauer results is shown in the inset.

Above bed over-fire air is strategically mixed with the volatiles, which consist primarily of CO, COj, CH4, HjO, Nj and tars. Combustion of the volatiles proceeds, the tars are burned out, and heat is transferred to the boiler walls and convective tubes. Reactions above the bed occur first in a reducing environment, and then excess air is added gradually to the upper combustion zone. 

Ash which deposits on boiler walls in the radiant section of a furnace is generally referred to as slag. Ash deposition on convection tube sections downstream of the furnace radiant zone is... 

ENRICHMENT OF IRON IN BOILER WALL DEPOSITS COMPARISON OF COMPOSITION OF ASH DEPOSITS AND AS-FIRED COAL ASHES... 

It is also important to control the chemistry in the bottom part of the radiant boiler. One must avoid condensation of insulating deposits on the boiler walls. For example, while operating with high reduction in the reaction shaft, lead sulphide can condense and deposit on the boiler walls. 

The molten phase produced in the smelting shaft falls to the bottom and separates from the gas phase, which passes under a dividing waU and into the vertical off-take shaft. The off-take shaft is lined with a membrane boiler wall, heated by radiation from the gas. Gas is normally cooled to around 7(X)°C, but preferably less than 8(X)°C before entering the convection section of the boiler. 

The already mentioned transformation of Fe(II) hydroxide into magnetite is particularly active betv een 120 and 570 °C [15]. All boiler and pipe valls become covered with a uniform protective scale of magnetite during exposure, which relative to the standard hydrogen electrode shows a very noble potential from +400 to +500 mV, while for bare iron a potential of -440 mV was measured [21]. The scale thickness on the pipes reached about 0.05 mm [22], on the boiler walls up to 0.2 mm. The interior of the vessel which is protected by magnetite is practically immune to corrosion when the following conditions are filled ... 

Stresses in boiler walls can cause the magnetite scale to spall. In the process the small exposed areas of the steel surface become anodic with respect to the large cath-odically acting undamaged surfaces so that rapid corrosion of the unprotected surface is likely to occur [53]. 

Pulverized coal (PC) boilers are the most common large combustion systems for the generation of electricity in the US and the industrialized economies of the world. PC boilers include both wall-fired boilers and tangentially-fired (T-fired) boilers wall-fired boilers include both front wall and opposed wall configurations. Like cyclone boilers, they have the potential to use petroleum coke as a fuel provided that a sulfur dioxide scrubber has been installed on the unit. The vast majority of the petroleum coke fired in the USA is burned in PC boilers due to their dominance of the industry. Again the low volatility in the petroleum coke limits its use in PC boilers the typical cofiring percentage is on the order of 20 - 30 percent (calorific value basis). 

Oil contamination of feed water causes a sludge that adheres to the boiler walls and is difficult to remove. The formation of sludge balls can be encountered when the binder is a corrosion inhibitor, a paint residue, a fuel oil, or a lubricant. These sludge balls can become very large under some turbulent conditions. Severe attack by sludges may result on carbon steel and even on Monel. 

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