Changes in coal combustion residuals (CCR) management rules and Effluent Limitations Guidelines (ELG) are driving modifications of bottom ash management systems at coal-fired power plants.

To understand the future of ash management, let’s look at how it is typically treated and handled today.

After combustion, ash that accumulates in the bottom of the boiler — known as bottom ash — is captured in water-impounded bottom ash hoppers located directly beneath the boiler. Bottom ash is then crushed into small pieces by boiler hopper ash clinker grinders and sluiced by jet pumps to existing ponds or dewatering bins.

During coal pulverization, not all of the coal can be crushed. The objects rejected from the coal mill — mill rejects or, more commonly, pyrites — are also typically commingled with bottom ash and sluiced together into the bottom ash hopper or existing sluice lines to the existing pond. Because pyrites are not combusted, they do not fall under CCR guidelines but are typically handled with the bottom ash system.

Ash that accumulates in the bottom of the boiler’s economizer sections — economizer ash — is captured in hoppers at the bottom of the boiler ductwork. Economizer ash is typically handled pneumatically and commingles with the fly ash system or sluiced together into the bottom ash hopper or existing sluice lines to the existing pond.

Due to past ash pond failures and leaks, the latest CCR regulations and ELG guidelines place closer scrutiny on ash management. To achieve compliance quickly and with minimal operational impact, some plant operators are choosing to make changes to their existing ash handling systems rather than replace them completely with other alternatives.

What’s Changing

The upgraded systems — known as compact submerged conveyor systems — reuse most of a plant’s existing ash management infrastructure. That includes the existing water-impounded hoppers, clinker grinders, ash gates, crushers and other existing equipment found beneath the boiler. Only the wet sluice pipe is replaced with compact, fully submerged conveyors.

How It Works

This system consists of under-boiler compact submerged conveyors that replace the existing bottom ash sluice piping for bottom ash transportation. The existing bottom ash hopper remains in place, while the boiler hopper ash jet pumps and some piping are removed and replaced with a new compact submerged conveyor system.

The bottom ash is reduced in size through grinders and transferred, via the compact submerged conveyors, to a new ash bunker. Pyrites and economizer ash transfer lines, meanwhile, are rerouted to the bottom ash hopper or conveyor. Ash is dewatered and transported on these conveyors to a storage bunker or directly to ash hauling trucks for transport to the existing ash landfill. Unlike current systems, the “quench water” used in the process does not require a closed loop and can be discharged as low-volume waste under the currently stayed version of the ELG.

Key Decision Criteria

Operators typically choose compact submerged conveyor systems when one of these is true:

  1. Operators wish to reduce initial capital cost spend
  2. There is limited space around the boiler bottom
  3. Plants have a relatively short expected life

Benefits to This Approach

Unchanged footprint — The system’s footprint is also small since the new conveyors are smaller than traditional equipment — due to the use of existing crushers and conveying smaller ash pieces — and with minimal new auxiliary equipment required. Compact submerged conveyors can also be oriented at a relatively high incline, minimizing the need for modifications to or relocation of other equipment.

Cost avoidance — Costs for system modifications are typically lower than those for a completely new system installation. Because most existing equipment stays in place, outage time is also short.

 

Every coal-fired plant has unique design criteria and arrangements. Selecting the best bottom ash management system requires careful consideration of the legacy system, schedule and budget limitations, environmental risks and short- and long-term goals. Learn more about current innovations in bottom ash conversion technologies.

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by
Steven Hibbard, PE, is a senior mechanical engineer and CCR handling specialist at Burns & McDonnell. His experience spans CCR and ELG conversion planning, technology assessments, budgeting, scheduling and establishing critical plant objectives.