Controlling Slagging and Fouling in Coal-Fired Boilers

Ash deposits on heat transfer surfaces can change heat absorption patterns and increase boiler cleaning requirements. Controlling slagging and fouling requires an understanding of fuel minerals, combustion conditions, and deposit characteristics. Anti-slagging additives can form part of the program, but their selection should be supported by ash analysis and operational assessment. This article focuses on fireside deposits, which require a different approach from waterside scale control.

Deposit characteristics determine the approach

Slagging involves the adhesion and consolidation of ash in high-temperature zones, including furnace surfaces. Fouling involves deposit accumulation on surfaces exposed to combustion gases, including convection sections. Mineral composition and temperature affect the tendency of ash to become sticky, sinter, or form a liquid phase. Assessment based solely on total ash content cannot adequately explain deposit behaviour. [1,2]

Coal trade names and visible physical properties cannot replace mineral analysis. Changes in fuel blends should be reviewed alongside ash composition, temperature distribution, and combustion conditions to identify the cause of problems before selecting an additive program.

How anti-slagging additives work

Magnesium-, aluminium-, or aluminosilicate-based additives can modify mineral reactions and ash characteristics. A 2023 study by Hariana and colleagues evaluated these three groups in blended coal using a drop tube furnace, ash fusion temperature (AFT) analysis, and assessment of deposits, emissions, and mineral morphology. The findings support evaluating additives for specific fuels and combustion conditions. [1]

Kaolin has also been studied as a means of changing deposit behaviour in high-alkali coal. Liu and colleagues used a 30 kW circulating fluidized bed system at bed temperatures of 820, 870, and 920 °C. These conditions formed part of an experimental design; the results therefore require validation before application to commercial boilers with different configurations. [2]

Data to examine	Technical purpose
Ash and mineral composition	Identify the minerals that require treatment
AFT and sintering characteristics	Assess additive-related changes in ash behaviour
Deposits at critical locations	Examine adhesion, thickness, and ease of removal
Temperature and combustion profiles	Relate deposit changes to operating conditions

An increase in AFT can provide a useful indication, but a single AFT value does not establish successful boiler performance. Assessment should include sintering characteristics and actual deposits. A study of an MgO-based additive in Indonesian coal combustion showed changes in AFT and sintering behaviour; these responses remain specific to the samples and test conditions. [3]

Validating performance in boiler operation

Establish a baseline before a trial, including fuel blend, unit load, fuel consumption, gas temperatures, sootblower activity, and cleaning events. During the trial, record the additive injection rate on a clear basis, such as kilograms of product per tonne of fuel. Injection location, additive distribution, and feed continuity should be included in operating records.

Compare trial periods at similar loads and fuel quality. Monitor furnace exit gas temperature, flue gas temperature, differential pressure across convection sections, sootblowing requirements, and deposit conditions when inspection is possible. If these parameters change alongside unit load or coal quality, the effect of the additive should be distinguished from operating changes.

Reduced sootblower frequency provides a benefit only when surface cleanliness and heat absorption are maintained. Economic assessment can include additive cost, cleaning steam consumption, downtime, and maintenance cost. Savings percentages should be derived from comparable trial data. In technical discussions with Harvest Chemical, fuel and deposit analysis provide the starting point for designing these trials.

References

[1] Hariana, Prismantoko, A., Prabowo, Hilmawan, E., Darmawan, A., and Aziz, M. (2023). Effectiveness of different additives on slagging and fouling tendencies of blended coal. Journal of the Energy Institute, 107, 101192.

[2] Liu, Y., Cheng, L., Ji, J., Wang, Q., and Fang, M. (2018). Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin. RSC Advances, 8, 33817–33827.

[3] Effect of MgO-based additive on the sintering behavior for slagging-fouling mitigation in Indonesian coal combustion. (Published online in 2022). International Journal of Coal Preparation and Utilization.

All reference links were checked on 10 October 2026.