Slagging, Sooting, and Erosion [1,7,15,26]

The solid portions of the products of combustion (refuse) are a source of opera-tional and maintenance problems. They may stick to the heat-transfer surfaces;

they may be deposited in areas of low gas velocity, clogging the gas passages;

they may cause corrosion and erosion; or they may help keep the heating surfaces clean by a scrubbing action.

The refuse—which varies according to the type, composition, and tempera-ture of the fuel—may be classified in the following manner.

1. Flue dust: the particles of gas-borne solid matter carried into the prod-ucts of combustion, including (1) fly ash, the fine particles of ash; (2) cinder, particles of partially burned fuel that are carried from the fur-nace and from which the volatile gases have been driven off; and (3) sticky ash, ash that is at a temperature between the initial deformation and softening temperatures.

2. Slag: molten or fused refuse, including (1) vitreous slag, a glassy slag;

(2) semifused slag, hard slag masses consisting of particles that have partly fused together; (3) plastic slag, slag in a viscous state; and (4) liquid slag, slag in a liquid state.

3. Soot and smoke: unburned combustibles formed from hydrocarbon va-pors that have been deprived of oxygen or adequate temperature for ignition.

1. Slagging

Slagging is the formation of molten, partially fused or resolidified deposits on furnace walls and other surfaces exposed to radiant heat. In the furnace the molten fly ash sticks, or plasters itself, as softened slag to the walls. This accumulation reduces the heat transmission of the walls and increases the surface temperature.

The slag becomes molten and runs down the walls or drips from the roof. As it runs down the walls (an action called washing), a chemical reaction occurs that causes erosion or slag penetration. This slagging is one of the major causes of high refractory maintenance. Metal walls give the least difficulty from adherence of fly ash, although slag flowing over them will, in time, cause destruction through erosion. If the furnace temperature is not high enough, solidified fly ash may deposit on the walls to a thickness such that the surface temperature equals the ash fusion temperature. Variations in furnace temperature will cause the fly ash to melt or build up until equilibrium is reached. Burning particles of fuel will become embedded in the sticky mass, further increasing the temperature. Around cool openings in the hot zones, the slag may harden and build up into large masses, such as burner ‘‘eye brows.’’ Burning particles of fuel may be carried in suspension into the boiler passes. The slagging action may move along with the gas stream as far back into the convection sections as the gas temperature remains above ash-softening temperature. This fusibility (property of the ash to melt, fuse, and coalesce into a homogeneous slag mass) depends on the tempera-ture and ash-softening characteristics of the fuel.

2. Fouling

Fouling is defined as the formation of high-temperature–bonded deposits on con-vection heat-absorbing surfaces, such as superheaters and reheaters, that are not exposed to radiant heat. In general, fouling is caused by the vaporization of vola-tile inorganic elements in the fuel during combustion. As heat is absorbed and temperatures are lowered in the convective section of the boiler, compounds formed by these elements condense on ash particles and heating surface, forming a glue that initiates deposition.

3. Clogging

Deposits from burning coal or oil choke the gas passages, reduce the heat trans-mission rate, and effectively limit the steaming rate. The accumulation may take many forms, including the following:

1. Sponge ash: agglomeration of dry ash particles into structures having a spongy appearance

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2. Bridging: agglomeration of refuse and slag that partially or completely blocks the spaces or apertures between heat-absorbing tubes

3. Fouling: agglomeration of refuse in gas passages or on heat-absorbing surfaces that results in undesirable restrictions to the flow of gas or heat

4. Bird-nesting: agglomeration of porous masses of loosely adhering re-fuse and slag particles in the first tube bank of a watertube boiler 5. Segregation: the tendency of refuse of varying compositions to deposit

selectively in different parts of the unit

Beyond the hot zones the ash begins to cool and has a less agglomerate nature. In the rear passes the ash has the flaky, soft characteristics of soot and is easily blown from the tubes.

4. Erosion

Ash erosion usually occurs wherever ash concentrates in streams, such as at the baffle turns of the boiler banks of watertubes and the entrance to firetubes. To prevent this erosion, the gas velocity must be kept low or the gas baffling elimi-nated as far as possible. Because of the high concentration of fly ash, dry-bottom, pulverized coal furnaces are particularly susceptible to erosion.

5. Corrosion

Deposits tend to set up in the cold-end equipment (air heater, economizer, dust collector) where gas temperatures drop close to, or below, the dew point. Soot deposits have an affinity for absorbing moisture. Coal soot has traces of SO2and SO3; oil soot has sodium and potassium sulfates in addition. These react with moisture to form a dilute, but very corrosive, sulfuric and sulfurous acid, adding to the normal rusting action. Fuel oil slag may contain vanadium pentoxide, which will attack and corrode steels, including those of high chromium content.

6. Effect of Delayed Combustion of Slag Deposits

When active combustion extends into the boiler and superheater, very trouble-some deposits of slag on these heating surfaces may result. This slag deposit is caused by higher temperature and partly reducing conditions, both of which make the ash sticky. The delayed combustion is caused by general or local deficiency of air. The local air deficiency is due to insufficient mixing of combustibles with air in the combustion space. The combustibles consist of gas, as well as hot carbon particles. Some of these hot carbon particles are deposited, along with ash, on the boiler tubes and superheater, where they continue to burn, generating heat in the slag deposit. They can be seen as bright red-hot specks that continue to glow for several seconds, until completely burned. New burning particles are continually deposited, and keep the surface of the deposit hot and sticky. The

burning particles not deposited can be seen as red streaks in the gas passages of the boiler. The higher the velocity of the gas in these passages, the faster the slag and burning carbon particles are deposited on the surfaces.

7. Furnace Design

Furnaces may be designed to maintain the ash below or above the ash-fusing temperature. When the ash is below the fusing temperature, it is removed in dry or granular form. When low-fusing–ash coal is burned, it is difficult to maintain a furnace temperature low enough to remove the ash in the granular state. Cyclone and some pulverized–coal-fired furnaces are operated at temperatures high enough to maintain ash in a liquid state until it is discharged from the furnace.

These units are referred to as ‘‘intermittent’’ or ‘‘continuous’’ slag-tapped naces, depending on the procedure used in removing the fluid ash from the fur-nace. Because this process permits high furnace temperatures, the excess air can be reduced and the efficiency increased.

8. Iron in Coal Ash

Compounds of iron are responsible for much of the misbehavior of coal ash.

Therefore, coals with ash high in iron are always under suspicion as causing trouble. If the ash, and particularly the iron, is uniformly distributed through the coal, difficulties are more likely to occur than if the iron compounds are in large pieces, separated from the coal. The large pieces are likely to drop quickly through the reducing zone of the fuel bed, with little reduction of the high oxides to lower oxides of iron. When the coal is pulverized, the larger and heavier pieces of ash are rejected by some pulverizers and do not go through the furnace. In screenings having a high percentage of fines, the ash is uniformly distributed through the coal; therefore, compounds of iron are likely to cause trouble. In some coals, the iron is in the form of pyrite (FeS2) which, while passing through the furnace, undergoes various changes. Both the iron and sulfur may combine with oxygen; iron forming the lower oxides, and sulfur, SO2or SO3. Sulfur may also combine with the alkaline metals, Na and K, and form sulfur compounds that have a very low fusion temperature.