Source: B&W
Typically exported coal is crushed to a 2″ top size, with approximately 18% of the material retaining pieces larger than 1″. In practice, it is recommended to reduce the coal size to ¾” top size before feeding it into the power plant to ensure mill requirements. This size reduction helps prevent large coal chunks or frozen pieces from entering the coal bunkers that supply the pulverizers (or mills) and thus prevents blockages. Some power stations find this crushing step unnecessary and can effectively feed 2″ coal to their equipment.
Reducing the coal size to ¾” enables the pulverizers (or mills) to operate smoothly, producing a more uniform particle size, resulting in fewer rejects compared to when larger pieces are fed. Additionally, the same applies when coal becomes wet during storage, as crushing increases the surface area, improving coal flow through the bunkers. An interruption or blockage of the coal feeders can lead to a shutdown of the pulverizers and a decrease in boiler efficiency.
Power stations employ magnets and metal traps in the coal supply chain to protect their crushing and grinding equipment. Various types of pulverizers (mills) are available in the market, each with different operational advantages and disadvantages, depending on the design conditions of each power plant. The most crucial factor is that they have sufficient capacity to handle the required coal quantity to operate the power plant at full load. Generally, when the quality specifications of the coal are used for the original unit design, there are minimal issues with pulverizer capacity. However, for units initially designed for coals with higher calorific values, the pulverizer capacity may be limited when using different coals or blended fuels.
Most power plants operate their boilers satisfactorily by pulverizing coal to pass through a 75% mesh 200 sieve. Power plants should periodically assess the coal fineness from their pulverizers and adjust their classifiers and/or grinding pressure to achieve the desired fineness. The percentage retained on the 50-mesh sieve should not exceed 1.5%, as excessively coarse coal contributes to poor combustion, slag formation, and a high content of unburned material in the ash.
The ash content is a parameter that can affect the performance of pulverizers, which is why some power stations are concerned about certain ash components. For instance, cilica content can be highly abrasive and accelerate the wear of the moving components of the pulverizers and coal ducts. So, pyritic sulfur can also cause wear on the pulverizers and increase rejections. In general, Colombian coal has low abrasiveness and a low pyrite content, enhancing its quality.
One of the most critical conditions for a power plant is the temperature of both the coal and the air-fuel mixture entering the boiler, which is generally recommended to be at 75°C. However, increasing fuel moisture poses the risk of not achieving this condition, as there may not be sufficient hot air from the air heater. This low-temperature condition in the coal duct worsens with increasing coal moisture, making handling and storage more challenging. This low-temperature condition contributes to decreased combustion efficiency and increased slagging in the boiler. It has been observed that both operational performance and low slagging conditions are achievable with air-fuel mixture temperatures as low as 60°C, provided that coal fineness is excellent. However, to ensure optimal operating conditions, a temperature between 65-75°C is recommended using colombian coals.
From a mining perspective, coal mining can assist the power stations and their pulverization operation by keeping the material inert out of the coal. This means extracting the cleanest coal possible while maintaining low ash and moisture content. However, there isn’t much that can be done to control the Hardgrove Grindability Index (HGI).