Eight Issues Behind Boiler Slag Discharge Difficulties

Eight Issues Behind Boiler Slag Discharge Difficulties

Causes and Solutions of Slag Discharge Problems

The issue of difficult slag discharge in boilers can generally be attributed to several key causes, including: blockage of the air distribution plate slag outlet, blockage or leakage of the slag-falling pipe, clogging inside the slag cooler, blockage inside the slag discharger, obstruction at the outlet of the slag removal device, malfunction in the slag conveying system, and discharge problems at the slag silo. These problems are commonly encountered by boiler users and maintenance teams, including China pressure seal gate valve suppliers, who often deal with related system failures. This section provides a brief explanation and analysis of each issue.

1.Blockage at the Air Distribution Plate Slag Outlet
Slag outlets on the air distribution plate are generally of two types: side slag discharge and bottom slag discharge. Though the mechanisms of blockage are largely similar, there are slight differences. The bottom slag outlet is located at the active region of fluidization in the center of the air distribution plate. It is characterized by vigorous fluidization, a thin viscous layer, a high proportion of large particles, and a relatively high content of fresh fuel. Conversely, side slag outlets typically have fewer large particles, weaker fluidization, a thin viscous layer, and less fresh fuel.

Blockage at the bottom outlet is often caused by large particles squeezing together and forming a barrier at the slag discharge port. This can be alleviated by adding a directional surrounding wind cap facing the slag outlet and a high-temperature-resistant ring-shaped short tube slightly higher than the surface of the air distribution plate, which helps disperse large clumps and improves slag discharge. If unburned fresh fuel accumulates and forms a sticky layer near the outlet while there is air leakage in the wind chamber, localized coking may occur, blocking the slag path. The solution is to eliminate local air leaks and improve fuel distribution to prevent direct fresh fuel accumulation at the outlet.

For side outlets, the causes and solutions differ based on their characteristics. Blockages may still occur due to large particles jamming the outlet. In such cases, local expansion or the addition of baffles may help, and directional fluidization nozzles can also be considered. Common causes also include a stuck cone valve, blocked slag pipe, or fallen refractory materials, all of which can restrict or stop slag discharge.

2.Blockage or Leakage in the Slag Pipe
Compared to bottom discharge, side slag discharge carries fewer fine fresh fuel particles and thus results in lower combustion loss. Additionally, it has less impact on bed uniformity, leading to better temperature consistency, making it a preferable option for large air distribution plates. However, side discharge performs less effectively in removing large, unburned particles. The slag pipe for side discharge often lacks a vertical angle, which reduces its discharge efficiency. Therefore, installing a manual cleaning port outside the furnace is recommended for emergency handling.

Slag pipes, if not properly water-cooled, may suffer from long-term high-temperature corrosion. Once metal failure occurs, air leaks can form, resulting in weak fluidization, localized coking, and reduced slag flow. Severe leakage may not affect slag dropping but can create dangerous flying sparks, posing safety and environmental hazards. In some CFB boilers, this issue is common. Applying refractory castables externally to protect the pipe may accelerate corrosion.

Unlike vertical bottom discharge pipes, side discharge pipes have an angled design. If this angle is insufficient, slag flow is restricted. Many users have adjusted the angle and added water-cooled jackets after removing cone valves to solve this problem.

3.Blockage Inside the Slag Cooler
The air-water combined internal cooling slag cooler, typically a bubbling or pneumatic type, struggles with large-sized particles, making it unsuitable for low-grade fuels like oil shale or coal gangue commonly used in Chinese CFB boilers. Even when burning raw coal, the fuel preparation system often cannot maintain a particle size of 0–8mm, with a cut size of 1.8–2.5mm, making it difficult for the cooler to operate properly.

Common causes of blockage include: remote control failure or sensor errors, unexpected tripping of the slag cooler fan, oversized particles, degraded coal quality, sudden ash surges due to return feeder collapse, leakages causing wet ash, blocked slag inlet/outlet, cone valve failures, insufficient air supply, air chamber dust leakage, poorly designed internal flow paths, damaged or blocked wind caps, or leaks in the fan outlet duct.

4.Blockage Inside the Slag Discharger
If internal cooling water leaks inside the slag discharger, bottom ash becomes wet and sticky, hindering discharge. Other causes include insufficient rotational speed, remote control failure, inadequate cooling water flow, poor equipment design, poor coal quality, return feeder collapse, damaged spiral blades, motor faults, gear jamming, transmission issues, and inlet/outlet pipe blockage. All require targeted handling, and daily inspections should focus on identifying and resolving these problems early.

For drum-type slag dischargers, maintaining negative pressure inside the drum is beneficial. This is usually achieved by drawing negative pressure from the economizer ash hopper via a main pipe and balanced across multiple dischargers using branch pipes with control valves. Stable negative pressure improves particle flow, cooling efficiency, and sealing performance.

5.Blockage or Leakage at Slag Discharge Outlets
Common causes include foreign objects entering the slag outlet, cracks near the outlet, residual refractory castables, too-small outlet diameter, or downstream conveying equipment failures.

6.Malfunction of Slag Conveying Equipment
Most conveying systems comprise scraper conveyors, vertical or inclined elevators, and feed hoppers. The most frequent issues are conveyor or elevator jamming, caused by poor alignment, weak or deformed parts, motor faults, hard particles jamming the path, high-temperature deformation, or misalignment of the entire system. Many of these issues are caused by low-quality equipment from unbranded manufacturers aiming to reduce costs.

7.Difficulties in Discharging from Slag Silo
This is often due to poor equipment installation or selection, resulting in quality defects. Typical issues include stuck telescopic pipes, malfunctioning slag gates, faulty displacement sensors, infrequent emptying, rough or improperly angled silo surfaces, excessive moisture, and unremoved original plugs at the pipe end. These can be resolved step by step by following standard maintenance practices.

8.Other Causes of Slag Blockage
Additional causes include: fallen refractory castables disrupting fluidization, oversized fuel or bed materials, foreign objects near the outlet, and coking caused by poor operation. In some cases, poor cleaning of wind cap holes or leftover refractory materials in the air chamber during installation also leads to incomplete fluidization or severe blockage.