Slag is the solid residue left after coal combustion, and it can adhere to furnace parts, like waterwalls, when it cools. This sticking of slag is referred to as “slagging.” In pulverized coal boilers, there’s a spacious furnace that can handle liquid slag on waterwalls, but the furnace exit temperature needs to be low enough to cool the slag below its softening point to prevent slagging issues.
The properties of coal ash are crucial for understanding slagging. Analyzing ash minerals and referring to ASTM standard D1857 (initial deformation, softening temperature, hemispherical temperature and fusion temperature) for fusion ash temperatures are vital steps in comprehending deposit formation. The chemical composition of ash varies due to factors like coal source, combustion conditions, and dust control equipment. It is useful to analize coal ash by ASMTD4326 to determine the major and minor components of Coal Ash. The major components are Si, Al, Fe, and Ca oxides, making up 95 to 99% of the ash. Minor elements such as Mg, Ti, S, Na, and K also impact ash behavior. Determining indexes based on coal ash characteristics, such as: slagging and fouling index, could suport the prediction of slagging and fouling issues.
The high iron content, mainly in the form of pyrite (FeS2), has a historical link to promoting boiler slagging. When pyrite is not pulverized sufficiently in the pulverizers, low-melting phases can be created, delaying complete oxidation into higher melting point oxides. Ash fusion temperatures typically range from 1000 to 1300 °C, with an average ash softening temperature of 1150 °C in an oxidizing atmosphere. Furnace exit gas temperatures at full load in the nose area consistently exceed 1100 °C. This combination of lower ash fusion temperatures and higher operational furnace exit temperatures likely contributes to more severe slagging in the boiler.
Slagging, a common issue, results in coal ash deposits forming on waterwall tubes, blockage the finishing superheater’s lower portions, and accumulating on the superheater regions and the slope of the boiler’s nose. These heavy ash deposits can lead to undesirable unit shutdowns, prompting costly practices like boiler derating and premature boiler tubes failures. Understanding slagging is vital to ensuring the safety of your personnel and assets.
Uncontrolled deposits on heat transfer surfaces can disrupt a coal-fired utility boiler’s normal operation, leading to reduced output, decreased efficiency, and forced outages. These deposits originate from the mineral content and inorganic elements present in coal. Specifically, when ash deposition occurs in the boiler’s radiant heat transfer section, it’s referred to as slagging. Slagging typically arises from the buildup of molten ash particles on the upper furnace tube surfaces, which solidify upon contact with the tube wall due to the surface’s lower temperature compared to the furnace exit gas temperature (FEGT). Once an initial deposit layer forms, it becomes sticky, causing further particle accumulation.
The slagging of waterwalls disrupts heat transfer as well as reduces local radiation heat transfer. As a consequence, flue gas temperatures at the furnace exit rise above the ash fusion temperature, exacerbating the slagging issue on the slope area and screen tubes in the boiler’s convection section. Additionally, corrosion may occur beneath developing deposits, all of which impact a power plant’s efficiency, availability, and operational cost. Increased flue gas temperatures also result in higher thermal NOx emissions.
Excessive slagging primarily occurs due to operating boilers at FEGT levels above the coal’s ash fusion temperature. Managing FEGT is crucial, as it affects the type of fly ash entering the convective sections. Factors influencing FEGT include excess air levels, boiler load, fuel and air distribution among burners, and coal feed variability. Sootblowing selectivity and frequency are also vital parameters for preventing severe slagging.
Temperature had a significant impact on the deposition process. Slagging occurred when the wall temperature exceeded 880 °C. Increasing the temperature difference between the gas and the wall resulted in the capture and subsequent deposition of fine particles onto cooler surfaces. This deposition, in turn, had an impact on the wall temperature, leading to a disruption in the heat flux.
The primary reason for slagging on high-temperature surfaces was the increased concentration of Ca, Si, Na, Fe, and S. Volatile substances like NaCl condensed on cooled surfaces, contributing to deposition. Ca underwent condensation and organized itself into a structured layer, forming a coating on the surfaces of ash particles, making them more adhesive.
The levels of mineral elements decreased as the gas flowed in one direction. The most significant changes in content occurred within the temperature range of 880–900 °C. These elements consistently had low levels at temperatures below 730 °C. However, gaseous Na, Cl, and K condensed into deposits along the gas flow path.
Deposits and ash exhibited distinct characteristics. Deposits contained a higher variety of mineral phases. Mineral elements like S, Ca, and Fe tended to accumulate more on surfaces with higher wall temperatures. It is important to be cautious about chlorinous corrosion in conditions involving air cooling.
As a result of adjusting boiler control settings and implementing furnace cleaning procedures, significant reductions in slagging have been achieved. A cleaning Boiler program during outages is good ally in controlling slagging. Notably, when a group of sootblowers is activated, substantial drops in furnace exit gas temperature indicate effective surface cleaning. Conversely, if activating sootblowers has a negligible impact on gas temperature, it suggests either the surface is already clean or the deposit cannot be removed by sootblowing.
Considering coal blend and purchasing, continuously monitoring bed temperatures, tracking spray reheater flow patterns, monitoring furnace temperatures, and assessing boiler efficiency all play a crucial role in mitigating the formation of slag in boilers.