Changes in coal combustion residuals (CCR) management rules and the EPA’s Effluent Limitation Guidelines (ELG) are driving coal-fired power plants to modify their bottom ash management systems. Plant operators seeking to eliminate transport water from their ash management process and significantly decrease their operations and maintenance costs should consider a dry belt/tray conveyance system. These robust, proven conveyors offer a solution that can bring significant benefits to the plant.
Designed for use on pulverized coal (PC) boilers, this dry ash technology removes water from the bottom ash handling equation. Commonly used in Europe, this system captures bottom ash on a steel belt/tray system. The ash is cooled as it is conveyed from the boiler and taken to a new bottom ash silo. Depending on site constraints, operators can also wet-condition the ash and place it directly at grade or pneumatically convey the material to a remote bottom ash silo.
What It Takes
With these conveyance systems, legacy bottom ash hoppers must be completely removed and replaced with the new dry conveyors. This type of conveyor typically allows the existing seal trough to be replaced with a new dry boiler seal. These conveyors consist of a stainless steel mesh belt with steel/alloy trays that are similar to the pans used on apron feeders, the heavy-duty belt systems used in mining and other heavy industries.
How It Works
Coal ash is transported on these relatively slow conveyors from a boiler to a loadout bin, silo or bunker.
After falling to the bottom of the boiler and landing on the conveyor, the ash is cooled by ambient or forced draft air as it is conveyed to the end of a ramp section. From there the ash is crushed and placed on a second conveyor for further conveyance and cooling. Miscellaneous ash streams (e.g., economizer, air heater) are retrofitted with steel belt or dry flight conveyors, as required, to move the material to the new bottom ash conveyor. The plant’s pyrites system is retrofitted with a wet sluicing system that handles this material separately from the bottom ash. Because pyrites are not considered CCR materials, they are not subject to zero discharge requirements in the CCR rule. Some applications can utilize a steel belt system for handling pyrites.
When discharged at the final conveying section, the ash is either placed in a storage silo or bin, or it is wetted and stacked on the ground. If the latter, conditioning equipment wets the ash to minimize dusting.
Key Decision Criteria
Operators typically choose dry belt/tray conveyor systems under these three circumstances:
- They wish to eliminate transport water from their systems and reduce regulatory uncertainty.
- They wish to reduce long-term operational costs.
- Their plants have a relatively long expected life.
Benefits to This Approach
Reduced long-term O&M costs — Dry belt/tray systems utilize rollers and a steel belt/tray system. This equipment avoids the wear and friction caused by a scraper-type system. Components subject to wear and tear typically have extended operating lives and are designed to be replaced while the system is online. The belts commonly last 10+ years, reducing outage time and work required by other systems.
Economic benefits — Dry belt/tray systems can reduce overall fuel consumption by reducing the heat lost to overflow water in a wet bottom ash system. Additionally, the reduced O&M and long-term lives of the belt systems have a significant impact in future cost savings. The reliability of the belt systems can also decrease unplanned outages and increase uptime and profitability for the plant.
Regulatory risk reduction — This system eliminates the use of transport water within the bottom ash system and thus avoids the headaches that come with National Pollution Discharge Elimination System (NPDES) permits and regulatory compliance. Additionally, eliminating transport water from the system may protect plant operators from future regulatory changes that could further restrict or eliminate water use in bottom ash systems.
Every coal-fired plant has unique design criteria and arrangements. Selecting the appropriate 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.