Many coal-fired power plant operators are busy modifying bottom ash handling systems in response to changes in coal combustion residuals (CCR) management rules and Effluent Limitation Guidelines (ELG). To achieve compliance quickly and with minimal operational impact, plant operators are choosing to adopt new ash handling systems.
One system being considered is the remote dewatering system — an ideal system for plants with short outage windows or ones with multiple boilers. By combining an existing sluicing system with remote drag chain conveyors located away from the boiler house, these systems can be brought online relatively quickly and, if needed, shared among boilers for economies of scale.
How It Works
With remote dewatering systems, existing ash sluicing systems remain in service. The sluicing line that leads to the bottom ash impoundment, however, is rerouted to a submerged drag chain conveyor that diverts the ash sluice to a remote equipment island located away from the boiler.
After dewatering is performed and the ash settles out, the ash is dragged up a ramp at the site and dropped into a new bottom ash bunker. Sluice water overflows from the conveyor are then sent back to the plant and reused again to convey bottom ash.
Key Decision Criteria
Remote dewatering systems are often preferred when:
- Plants have existing sluicing systems for bottom and economizer ash that are in good working order.
- Plants contain multiple coal units that can be supported by a single remote equipment island.
- Operators wish to minimize installation outage time.
- Space constraints limit the types of retrofit equipment that can fit under or directly adjacent to existing boilers.
Benefits to This Approach
Cost avoidance — Because a single equipment island can support tie-ins from multiple boilers, these systems offer economies of scale to plants with more than one unit. Remote dewatering systems use existing ash sluicing systems, helping to minimize new equipment costs.
Minimal disruption — Because these solutions make use of existing conveyance equipment and dewatering systems are constructed away from the boiler, the outage requirements are minimal, limited only to the time needed to complete system tie-in. This approach is especially common in areas where coal-fired plants have a high utilization and the cost of an extended outage is significant.
An Important Consideration
Operators considering this approach must also take ELG bottom ash transport water regulations into account. This system uses water as the motive force for moving the ash, making the water in this system transport water.
The original ELG rule did not allow discharge of transport water unless it was being sent to your scrubber. However, the 2019 proposed rule has some accommodations for a typical blowdown. Regulations state that blowdown is allowed from bottom ash systems for stormwater, excess low-volume wastewater that cannot be segregated from the bottom ash system, maintenance activities that cannot be contained or to maintain system water chemistry. The maximum allowable blowdown rate is a 30-day rolling average of 10% of the primary bottom ash active wetted system volume. Careful consideration should be made before sending waste transport water to a scrubber due to potential fines and chemistry issues in the scrubber.
Every coal-fired plant has unique design criteria and arrangements. Selecting the right bottom ash management system requires careful consideration of the legacy system, schedule and budget limitations, environmental risks and short- and long-term goals.