Coal ash corrosion

In a study by Koripelli et al. (2010), one finishing superheater tube sample, from a coal-fired unit, was received for metallurgical analysis. The tube was specified as a 1.875-in. outer diameter (OD) x 0.330-in. medium wall tubing (MWT), SA-213 T22 Cr-Mo steel. It had been in service for 23 years. Figure 3.4 illustrates the as-received tube sample and the internal diameter (ID) view. Significant wall thinning was observed on the flanks of the tube, characteristic of coal ash corrosion. Very hard scale was observed on the tube OD. On the steam side, there was a thick oxide. 

Tube wastage will often be evident and manifested as flat spots on either side of the upstream of flue gas flow, as can be seen in Figure 3.6a. A ring sample (Figure 3.6b) was sectioned from the tube for dimensional and hardness measurements. The tube had thinned significantly at two positions due to coal ash corrosion. The thinnest section of the ring (0.291 in. at the 8 00 position) was at 88% of MWT. The hardness, averaging 73 Rockwell B (RB), showed that the tube had softened after 23 years of service. 

Originally, it was thought that coal-ash corrosion was confined to boilers burning high alkali coals. However, combustors burning medium to low alkali coals also encounter the same problem. In cases where there was no corrosion, either the complex sulfates were absent or the tube wall temperatures were below 595°C (1100°F). 

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

Fig. A.12 Effect of SO2 content on coal ash corrosion loss of stainless steel tubes (T aken from ref. [ 1 ])...

Knowledge of the composition of coal ash is usehil for estimating and predicting coal performance in coke making and, to a hmited extent, the folding and corrosion of heat-exchange surfaces in pidverized-coal-fired furnaces. 

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... 

The flame imprinted characteristics of pulverized coal ash relevant to boiler slagging, corrosion and erosion have been discussed previously (1,2). Silicate minerals constitute between 60 and 90 per cent of ash in most coals and boiler deposits are largely made up from the silicious impurity constituents. This work sets out first to examine the mode of occurrence of the silicate mineral species in coal followed by a characterization assessment of the flame vitrified and sodium enriched silicate ash particles. The ash sintering studies are limited to investigations of the role of sodium in initiating and sustaining the bond forming reactions to the formation of boiler deposits. 

Often coal ash deposit effects are inter-related. For example, slagging will restrict waterwall heat absorption changing the temperature distribution in the boiler which in turn influences the nature and quantity of ash deposition in downstream convective sections. Ash deposits accumulated on convection tubes can reduce the cross-sectional flow area increasing fan requirements and also creating higher local gas velocities which accelerate fly ash erosion. In-situ deposit reactions can produce liquid phase components which are instrumental in tube corrosion. 

Additives for high-temperature oil and coal-ash fouling and corrosion can be applied as powders or slurries. The form of the additive determines the... 

Materials problems in newer coal gasification processes accrue generally from operating temperatures of 1500 -2800°F and pressures of 150-1200 psi. Added to these conditions are low oxygen activity and high sulfur activity in the product gas atmosphere. Finally, coal ash and sulfur sorbents present in the system can cause materials failure by corrosion and/or erosion-corrosion. Current metallic alloys that were developed... 

Oxidation-corrosion data obtained from the pilot plants generally compare well with laboratory data in ranking of high-temperature alloys. Pilot plant results, however, indicate more severe corrosion than laboratory oxidation-corrosion data. This should be expected because of cyclic operation of pilot plants and additional variables comprising the pilot plant environments. The contribution of erosion and erosion-corrosion by coal ash, char, and sulfur sorbents to the corrosion process in the pilot plants has not been defined. 

This chapter deals with a specific type of corrosion that occurs in dry and rather high-temperature environments. Hot ash corrosion accounts for about 60% of the failure occurring in coal-fired power generation stations in addition to the erosion by ash (Prakash et al. 2001). Fundamentally, high-temperature corrosion is caused by the action of molten species produced in situ on the construction materials of the heating system. 

It is impossible to prevent some degree of interaction between a metallic component and high-temperature gases, so the common term prevention is not strictly applicable. What is meant is a reduction of the interaction to a very low value. The use of cheap low-grade fuels limits the options of improving the corrosion environment. About 30% of annual corrosion costs may be saved with the use of preventive measures. Three common methods are employed to prevent hot ash corrosion for both coal- and oil-fired furnaces. The use of highly resistant surfaces, the ranoval of the undesired contaminants in the fuels, and the use of additives are the most practical methods in this respect. The first two methods are identical for both types of furnaces. The type of preventive additives used in coal-fired furnaces differs from those utilized in oil-fired furnaces. 

The greatest single problem in operation of coal-fired units is the accumulation of coal ash on boiler heat transfer surfaces. Coal ash causes three main problems in large furnaces (1) buildup of ash on furnace wall tubes, (2) accumulation of small, sticky, molten particles of ash on superheater and reheater tube banks, and (3) corrosion. 

Buckets filled with coal ash, lime powder, or inert material to prevent spreading of spilled corrosive liquids. 

Service testing to simulate ash/salt deposit corrosion is of importance to a number of industries. The fossil-fired power generation industry must deal with what is called "fuel ash corrosion fixrm sulfur- and vanadium-containing fuels and alkali, chlorine, and sulfur in coal. The gas turbine industry must deal with "hot corrosion" problems arising fixjm sulfur in fuel and sodium salts from ingested air. Waste incineration environments can become even more complex with refuse containing sulfur, chlorine, phosphorus, and numerous metallic elements. 

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