Slagging deposits form on the water-wall tubes, or on superheaters when the platen type is used. When the ash layer depositing on these surfaces is soft and friable they may self-shed or be easily removed by soot-blowing. Sticky sintered or molten deposits are the troublesome ones. These may accumulate on the tubes in a stage-wise manner (Figure 22). The ash that sticks to the clean tube may be powdered because the tube metal is relatively cool (Tw) compared with the combustion gases (Tf). Because this layer insulates the tube, the ash that deposits on top of it gets progressively hotter (Td) as the layer thickness grows. Ultimately the temperature may rise high enough for the surface of the deposit to become molten.
Figure 22: Progressive accumulation of ash deposits: (a) thin layer of adhering powder, (b) sintered deposit, (c) molten deposit
If the deposit layers are soft while they are still thin, they may be prevented from growing to the troublesome stage by frequent soot-blowing, but only if the soot-blowers are situated in the correct locations.
The effects of slagging may include:
· High furnace exit temperature. This may lead to damage from high superheater metal temperature
· High boiler exit temperature, associated with poor boiler efficiency
· Large lumps falling to the bottom of the furnace, causing damage or blockages
· Interference with burner flow patterns
· Excessive soot-blowing (steam wastage, erosion).
As is the case with most adverse coal-related behaviour, slagging occurs as a result of the combination of coal properties, boiler design and boiler operation. Boilers that are physically small relative to their output, or those with the burners situated in a relatively small zone tend to operate at higher flame temperatures and are prone to slagging problems. Boilers that are operated as base-load stations are more prone than those with cycling loads.
Relevant Coal Properties
Ash Fusion Temperatures
The AFT test was designed specifically to predict the formation of sintered or fused deposits. It is often unreliable because it does not account for the selective deposition of the more troublesome components of the ash, not does it account for the stage wise growth of deposits.
Composition of Ash
The presence of significant quantities of Fe2O3 and CaO in the ash elemental analysis may suggest that ash deposition problems would be expected, although this is not always the case, and the influence of other properties especially flame temperature, boiler design and mode of boiler operation often dominate the influence of ash composition.
Organic Composition and Moisture Content of Coal
These parameters influence flame temperature. Some coals produce ash that is fusible at high temperatures but do not cause slagging difficulties because flame temperatures are relatively low. This applies particularly to low rank/high moisture coals.
Predictive Indices
Numerous predictive indices based on ash composition have been derived by several authors (Tables 2 and 3) to better describe ash deposition behaviour, however, the fact that there are so many indices demonstrates their limited application. Some of these are successfully used for a particular type of coal (eg. eastern USA coals) but, because they are not based on sound physical principles, may fail for other coals.
*) Even when soot-blowing is effective, too frequent application is not desirable because (a) it uses steam which is not therefore used to drive turbines and (b) it may cause the removed ash to erode the boiler tubes.
Table 2: Slagging Indices
Table 3: Fouling Indices
Pilot-Scale Results versus Coal PropertiesFigure 23 shows a correlation related to ash deposition, and shows the relationship between coal ash constituents (in this case Fe2O3 + CaO) and the flue gas temperature at which the deposited ash will form sintered and/or molten deposits, which will cause problems in operating boilers. This correlation was developed from the pilot-scale testing of a wide range of coals, and includes both coal property data and a plant design parameter in terms of flue gas temperature.
The figure shows a direct correlation of actual boiler performance (flue gas temperature at which deposits become troublesome) with coal properties. The temperature at which the formation of sintered and/or molten deposits occur can be predicted and the relevant temperatures in a full-scale boiler can be designed for a particular coal. The scatter of results in the Figure demonstrates the limited reliability of predictions based on calculated indices.
Figure 23: Nature of Slagging Deposits (Powder or Molten versus (Fe2O3 + CaO) in Ash for Variable Furnace TemperatureFurnace Ash Deposition: Fouling
Fouling is the deposition that occurs when certain volatile constituents (usually sodium compounds) condense on fly ash particles which stick to surfaces in areas where the temperature regime is such that the constituents remain liquid (i.e. do not evaporate). These constituents react with fly ash, other deposits and the flue gas to form bonded deposits.
Fouling is usually restricted to the lower temperature parts of the boiler including the convective section, and the air heaters. Though the temperature regime and chemistry of the deposits are different than those for slagging, the same considerations apply regarding stagewise increase in strength as the deposits grow and the effects of soot-blowing.
The effects of fouling may include:
· High boiler exit temperature leading to poor boiler efficiency
· Low steam temperature
· Excessive soot-blowing (steam wastage, erosion)
· Poor gas flow distribution leading to erosion of tubes