Changes in Coal Combustion Residual (CCR) management rules and Effluent Limitations Guidelines (ELG) in recent years have resulted in coal plant operators needing to make changes to their bottom ash systems. While some have evaluated and chosen technologies for compliance, others have delayed because of expected compliance costs. Until now, many of the available technology options have been complex, costly solutions that were difficult to justify.
What’s Changed?
Impoundment breaches, spills, environmental stewardship and increased public awareness led the Environmental Protection Agency (EPA) to issue stronger CCR regulations and ELG guidelines in recent years. However, those regulations continue to be challenged in court by environmental groups, pro-coal groups and policymakers, resulting in delays and modifications in rule enforcement.
For plants evaluating an approach to ash management and compliance, the available options have also not been ideal. Under boiler and remote systems are available to meet requirements, but it isn’t always possible to justify the cost and logistical challenges. In some cases, plant closure is considered the most reasonable option.
During this period of uncertainty, manufacturers and installers have worked on new technologies that can achieve compliance faster, with less disruption and at a lower cost.
Evaluating Approaches
The CCR and ELG rules work together to stipulate zero-discharge of ash transport water and groundwater monitoring for surface impoundments, including corrective action that often includes pond closure. These rules force operators to consider water handling and treatment in a new way.
Traditionally, bottom ash removal is based on a simple system design that uses hoppers and crushers under a furnace and a sluice system that conveys the ash to a surface impoundment or dewatering system. Compliance with CCR and ELG means bottom ash transport water can no longer be discharged — a closed-loop system. Operators must adopt a cost-effective retrofit system with minimal outage time.
Numerous technologies have been developed to help achieve compliance, but they all come with the cost to retrofit systems and shut down operations for upgrades. Converting traditional wet sluice conveyors, existing hoppers and crushers to pump ash to a remote dewatering system is an effective but complex solution. This remote solution has often been the only option for operations where other retrofit equipment cannot fit under the existing boiler.
Traditional under-boiler systems are another alternative but require the removal of hoppers and crushers, which must be replaced with new equipment. While this costs less than a remote system, the solution has limitations for many operators.
Conveying Innovation
New technologies are being developed to help operators work toward compliance with less disruption and greater return.
The Babcock & Wilcox Submerged Grind Conveyor (SGC) system is one such new technology that cleverly utilizes existing infrastructure to help plants upgrade faster and with less operational impact. Using existing hoppers, clinker grinders, ash gates, crushers and other current equipment on the bottom of the boiler, the SGC helps meet operational goals by replacing the sluice pipe with small, fully submerged conveyors. In the simple SGC design, the system conveys bottom ash released from the hopper through compact SGCs into a storage bunker, achieving compliance with CCR and ELG requirements.
As plants consider their existing footprint and space challenges, the SGC may be an option because it allows conveyors to be oriented at a higher angle of incline to minimize modifications, avoid increasing the operational footprint and prevent the need to move other structures or equipment. Installation does not require demolition of existing hoppers and can significantly reduce outage durations for an under-boiler retrofit system installation, saving operators time and cost in achieving compliance.
The Burns & McDonnell team has helped clients evaluate suitability of SGC system technology and, where appropriate, installed and commissioned systems. For plants with limited options, SGC technology can be a good fit and cost half as much as a traditional system and about a quarter the cost of a remote system. Burns & McDonnell is working on the first three of these installations in the U.S., including one that is currently operational.
For utilities that have already decided on technology or are delaying compliance strategy decisions, considering new technologies and systems may be the less obvious — but correct — pathway toward achieving compliance and improving safety while justifying capital spend.
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