Coal combustion residuals (CCR) management rules and effluent limitation guidelines (ELG) have been issued by the Environmental Protection Agency (EPA) in recent years, prompting coal-fired power plants to make changes to their bottom ash management systems. Now is the time to start implementing technologies to achieve compliance.
Transport water was commonly used to convey bottom ash from boilers to ponds or surface impoundments. In these scenarios, the bottom ash has the potential to leak into groundwater, prompting the latest regulations. Under these regulations, bottom ash transport water is no longer eligible to be discharged. These new rules force operators to consider handling and treatment of ash transport water in a new way.
One option for bottom ash management is a pneumatic ash removal system. This solution replaces wet bottom ash hoppers and a wet transport system traditionally used at coal-fired power plants with a dry alternative.
Pneumatic bottom ash solutions were introduced in the 1920s and were widely used through the 1950s on smaller boilers. Today, pneumatic systems offer large-scale operators a solution to eliminate task water generation and optimize long-term life cycle costs.
The Pneumatic Approach
How pneumatic bottom ash solutions work:
- Bottom ash falls from the boiler into the dry hopper, landing on a set of large grate doors.
- The ash sits on the door grates for a period of time to cool, with the support of an inlet stream of a forced draft fan air. Expect 1% to 1.5% of total forced draft air to be redirected here. It is unlikely to require an additional source of air.
- Once the ash is sufficiently cooled, the doors cycle open, allowing the ash to fall through the hopper to feed a clinker grinder. Each boiler system has multiple hoppers, each with its own clinker grinder.
- Then, the ash is fed into a vacuum conveying system, similar to that utilized on a typical fly ash conveying system.
- Vacuum exhausters are used to create airflow to pull the ash through the conveying piping lines. This airflow conveys the ash from the new bottom ash feeders to filter separators, separating the ash from the conveying air, and then depositing the ash into a dry storage silo below.
- The ash flows out via the silo’s sloped hopper outlet(s). Wet ash conditioners (pug mills), located in the floor of the silo, load the ash into open-top trucks for disposal. The ash can also be loaded into a closed-top truck through a telescoping dry unloading chute.
- Finally, the ash is transported to a landfill or beneficial reuse facility. Bottom ash may be used as construction aggregates, for example.
What Pneumatic Solutions Take
Pneumatic solutions require replacement of the existing bottom ash hoppers with new dry hoppers. A vacuum conveyance system and storage silo with ash unloading equipment must also be installed.
During the detailed design, perform a forced draft fan study to confirm there is adequate capacity available to support this new system. Fluidizing air is generally not required for the storage silo. The top of the silo is equipped with a bin vent filter to control emissions from air displaced during the silo loading process.
The conveying length of the system should also be considered in the design phase. There are physical limits on conveying via vacuum. Piping line losses due to friction drive how much ash can be conveyed. It is advised to keep the length of piping between a boiler and the destination silo short, with as few directional changes as possible. It is recommended to keep the conveying length less than 1,000 feet as a starting point for system layout.
For a completely dry system, the existing boiler seal trough system will be replaced with a new dry seal system. This is a multilayer seal similar to an expansion joint made of high-temperature fabrics and chain mesh. Although many plants are accustomed to wet seals for existing boilers, a dry seal would mean that absolutely no water is required in the pneumatic bottom ash system. However, a wet seal can still be utilized with this technology, since the water for the seal is not transport water.
Dry bottom ash is an abrasive particle to convey. This, compounded with the velocities required for dilute phase conveying and the weight of the particles, can lead to excessive pipe wear, even in the abrasion-resistant alloy piping required by these systems. It is best to eliminate as many elbow and wear locations as possible in the design. After the system is running, the plant will need to periodically check the system for spots that may have worn more than anticipated. This can be remediated with ceramic-lined directional-change fittings and piping.
Benefits of Pneumatic Bottom Ash Solutions
Among the benefits of using a pneumatic solution for bottom ash:
- Similar equipment. For plants with pneumatic fly ash systems, there is little new equipment to get familiarized with. The vacuum conveying system has the same type of components as a typical fly ash system — wear-resistant piping/fittings, a vacuum producer, filter separators, storage silos and ash conditioners/loadout spouts. The only significant differences in systems are the bottom ash hoppers, ash clinker grinders and screw feeders, all of which are located below the boiler. All other major equipment is remote from the boiler.
- Small footprint. When space is tight, a pneumatic solution may be a good choice. There is little new equipment to be installed directly below the boiler hoppers. Dry hoppers are similar in size to water-impounded hoppers, and piping is more easily routed than mechanical conveyors, enabling a pneumatic system to fit within an existing plant footprint.
- Economies of scale. Operators can often achieve economies of scale by applying pneumatic technology at a multiunit site. Ash from multiple boilers can often be conveyed to a single silo.
- Risk and cost avoidance. This completely dry pneumatic system reduces the risks and project costs associated with transport water and transport water treatment. Wet-to-dry system conversions can also help reduce costs by potentially saving millions of gallons of water per year.
The new regulations are making their way through courts and facing environmental groups, pro-coal groups and policy makers, resulting in delays and modifications in rule language and enforcement. In the meantime, plant owners and operators are weighing options for how to comply without certainty of what the final rule will state. The completely dry aspect of pneumatic bottom ash systems is one avenue that can help minimize outage duration, risk and project costs.
Coal-fired plant operators pursuing bottom ash conversion projects have several ash management technologies to choose from. Selecting the right option requires careful consideration.
