The handling of hot slag from many types of coal fired combustors has been an integral part of any stations infrastructure for many years and continuously demands high levels of personnel attention to ensure proper and efficient operation.

Historically, the method adopted to remove furnace ash was the water-impounded hopper type. The water-impounded hopper system is and old system used for removing furnace bottom ash from pulverized coal boilers. A typical impounded-hopper system receives, quenches, stores, crushes and removes furnace bottom ash using hydraulic means. This type of technology collects ash over a predetermined period before discharging on a batch basis.

Due to the nature of this type of system ash is allowed to build up in the hopper resulting in ash removal every 6 or 8 hours. This alone means that pumps and pipe work have to be designed for 6 to 8 times the normal hourly continuous rate.

More modern systems adopt a continuous removal philosophy by means of the Submerged Scraper Conveyor. Essentially a heavy duty chain conveyor, the SSC is submerged in a water trough below the furnace which quenches hot ash as it falls from the combustion chamber and removes the wet ash continuously up a de-watering slope before onward discharge into mechanical conveyors or directly to storage silos.

Some of the key elements of this continuous removal technology include:

	Automatic chain tensioning unit   Based on an oil/inert gas differential system which provides a fully automatic system. This feature correctly tensions the chain under all load conditions and has the facility to send data back to the PLC for visual or audible indication in the control center.
	Hydraulic Drive & Power Pack   While it is possible, and in some cases more economical, to provide electro-magnetic drives. We rely on a hydraulic system due to the inherent torque characteristics of this technology. Typically, twin-motors are used providing good start-up characteristics and better flexibility during upset conditions. Power packs are installed close to the motors to minimize hydraulic interconnecting pipe work.
	Sprockets & Chain Wheels    Essentially there are three types: - tooth sprocket for drive & tail, transition wheel, and submerged transition wheel. All components are procured from leading suppliers such as HEKO & RUD, however these could be manufactured locally depending on project requirements.
	Drive Chain   To meet the demanding duties placed on the SSC, CBM use proprietary heavy duty case hardened round link chain.

	The major advantages of the SSC, over   water impounded hopper systems are:     ● Reduced water usage (no transport water required) ● Reduced power consumption (by eliminating the high pressure sluicing water required by jet pumps) ● Reduced complexity of dewatering bins when used ● Reduced operational and maintenance costs
When considering the type of technology for your furnace bottom ash removal system you should always think about:	a) water availability     b) environmental restrictions     c) capital & maintenance costs     d) existing plant layout when applicable