By Kevin L. McDonough
Since 2015, with the implementation of the Coal Combustion Residual (CCR) rule and the Effluent Limitations Guidelines (ELG), numerous utilities have moved forward with plans and projects to address the new groundwater and surface water regulatory requirements, with particular attention to bottom ash transport water and bottom ash impoundment closure.
In the recent past, UCC has been contracted to provide wet-to-dry ash conversion and wastewater management/treatment technologies on 53 plants covering 114 operating units. As of the date of this publication, approximately half of the U.S. coal fleet has now converted traditional wet bottom ash systems to either dry handling systems or closed-loop recirculation systems.
Relative to the unique operating conditions and design aspects of each plant, UCC has pursued a technological approach of “one size does not fit all,” and each plant must be evaluated for its own particular set of operating parameters, physical conditions and design criteria. Table 1 includes a typical list of design criteria commonly used for evaluating bottom ash conversion and ash wastewater technology selection.
The combination bottom ash dewatering and transport water clarifying system features remotely-located Submerged Flight Conveyors (R-SFC) and circular clarifiers with internal rake mechanisms.
For a given plant in the southern region of the U.S., a utility was faced with the challenge of converting a wet bottom ash sluicing conveying system that covered multiple operating units. The existing system combined conveying lines from the different operating units into two sluice lines that directed the bottom ash slurry to a receiving pond, where the bottom ash dropped out of suspension via gravity settling.
The transport water was further clarified over the remaining area of the pond, allowing finer particulate to settle out relative to Stoke’s Law principles. In between sluice conveying cycles, the bottom ash was excavated from the pond and allowed to dewater prior to transport to a dry landfill facility.
A primary driver in the technology selection was the cost benefit of a system with multiple-unit synergies.
Given the complexity of the multi-unit sluice system and associated water balance, along with some uncertainty relative to legal challenges and possible modifications to the ELG, the owner wanted to implement a technology solution that gave them maximum operating flexibility for current regulatory requirements and potential future revisions. As with any project, the technology options were reviewed relative to the design criteria noted in Table 1, with particular emphasis on available budget and desired performance requirements.
In particular, this project required the new bottom ash dewatering and wastewater treatment technologies to meet the following operating criteria.
• Receive and process 2,700 gpm under typical conditions (normal flow)
• Receive and process up to 5,400 gpm (maximum flow)
• Dewater bottom ash to approximately 15-20 percent moisture (target is generally based on Paint Filter Test requirements and ensures sufficient moisture to limit potential fugitive dust emissions and for optimal landfill compaction)
• Achieve 30 ppm Total Suspended Solids (TSS) on a 30-day rolling average
• Achieve 100 ppm daily maximum TSS
• Maintain a pH between 6 and 9
• Utilize existing sluice pumps for bottom ash conveying from the hoppers
In addition, the system had to:
• Meet current discharge requirements (assumes final ELG allows discharge of bottom ash transport water, subject to ongoing U.S. EPA review with expected confirmation by December 2018);
• Be readily capable of being converted to a closed-loop, zero liquid discharge system by producing a water quality suitable for feeding existing high-pressure, clean water sluice pumps;
• Ideally produce water quality that would allow for potential reuse/makeup in the plant’s wet flue gas desulfurization (WFGD) operations.
The system has produced an effluent water quality that is suitable for discharge (under current permit requirements), can be readily recirculated in a closed-loop, zero liquid discharge system or be utilized for FGD makeup/process water.
Summary of System and Process Equipment
After evaluating several dry conversion alternatives, including traditional dewatering technologies and dry handling options, the plant selected a combination bottom ash dewatering and transport water clarifying system that featured remotely-located Submerged Flight Conveyors (R-SFC) and circular clarifiers with internal rake mechanisms. In particular, the selected system included two processing trains, one primary and one fully redundant standby, that could receive the bottom ash slurry (ash and transport water) from the two operational conveying lines with crossover capabilities to either process train.
A primary driver in the technology selection was the cost benefit of a system with multiple-unit synergies, whereby more than three operating units could be directed to this fully redundant system without needing to make changes within the powerhouse and under each operating unit. In addition, all construction activities could be executed in a remote location and without the need for a planned outage, thereby providing greater schedule flexibility and reduced installation costs.
Clean water ultimately exits the R-SFC through internal troughs that overflow into the secondary phase of the system — the clarifier.
The first phase of the system featured the patented technology known as the Continuous Dewatering Recirculation (CDR) System with Integral Coal Combustion Residual High Flow Plate Separator (Ref. US Patent No. 9,776,106 B2), which is now being used widely throughout the U.S. coal fleet for the management of bottom ash dewatering and bottom ash transport water treatment.
In this first phase, the bottom ash sluice lines are directed into the R-SFC where the flow enters a baffle-ring assembly that impacts on abrasion-resistant receiving plates to dissipate the energy and force the larger and coarser particulate to readily drop out of suspension. The collected solids are transported via a dual-chain scraper operation along the length of the submerged bottom of the R-SFC, up an inclined dewatering ramp before discharging into a dry, three-walled concrete bunker.
The partially clarified water is then directed under a series of chambers where the water is directed upward through sets of narrowly spaced lamella plates. As the water flows in between each pair of plates, the particulate with a specific gravity greater than water will settle onto the top surface of each lower plate. After settling onto the lower plate, the particles will slide down the surface of the plates and settle out in the lower section of the R-SFC.
The enhanced settling performance of the lamella plate design has the following direct benefits on system performance:
• Reduced solids carryover to the clarifier, thus maximizing its settling efficiency;
• Reduction or elimination of particle neutralizing coagulant in secondary phase (clarifier) to enhance fine particulate settling performance;
• Reduction of flocculent consumption in secondary system phase (clarifier) to remove fine particulate.
The clean water then continues to travel upward and exits at the top of the lamella plates where it is directed to a series of internal troughs that ultimately exit the R-SFC and overflow into the secondary phase of the system — the clarifier.
The UCC clarifier design approach is largely based on client specifications of material properties, laboratory testing and analysis of representative samples, and the target TSS outlet concentrations. Material samples (e.g., bottom ash/economizer ash) are tested for particle size distribution and specific gravity. Settling velocities are determined mathematically and through experimental testing to size the R-SFC with lamellas and the clarifier. Detailed study is conducted to achieve a clear water overflow from the top of the clarifier, while preventing undue compaction of agglomerated solids in the clarifier bottom to mitigate the risk of plugging the slurry outlet and/or tripping the internal rake.
The system also includes chemical injection skids that introduce flocculent for fine particle agglomeration and enhanced settling. This polymer is injected into the drain piping between the R-SFC and the clarifier and then fully mixed with the water in the clarifier center feed well. As particulate settles in the clarifier, the agglomerated solids are directed via a low-velocity raking mechanism to the discharge outlet at the bottom center of the clarifier.
The outlet size and associated piping is uniquely engineered to remove the slurry at a reduced velocity that does not upset the agglomerated solids (i.e., limits the breaking of flocculent bonds). Pump sizes and pipe diameters are specifically selected to reduce the risk of plugging in the slurry lines. The slurry is then pumped from the clarifier outlet to the idle R-SFC, where it is added to the coarse bottom ash and then conveyed up the dewatering ramp and discharged into the dry concrete bunker for final transport to a landfill.
After an extended period of operation, the effluent water quality has consistently remained below TSS targets and within a pH range of 7.5 to 8.5.
After an extended period of operation, including periods of varying fuel types and boiler loads, the system has met all performance guarantees. In particular, the effluent water quality has consistently remained well below the daily maximum and monthly average TSS target. In addition, the discharge water has remained slightly basic with a pH range of 7.5 to 8.5, and an average of 8.0.
In addition, the system has shown favorable performance on dissolved solids (TDS) concentrations, with the clarifier system showing no influence on inlet constituent levels, and in some cases reducing constituent levels (e.g., chlorides, sulfates; see Table 2). As a once-through system, this performance was expected relative to minimizing the amount of time the bottom ash particulate and clarifier slurry remains in contact with the transport water. By design, the solids collected in the system are intended to remain in contact with water for no more than 24 hours, and typically less than 12 hours for most of the material, thereby reducing the risk of solids dissolution into the transport water. If the system is modified to operate as a full time closed-loop, zero liquid discharge system, the TDS levels would need to be monitored on a consistent basis to confirm any potential risk of the cycling up of constituent concentrations.
With the successful implementation of the combined bottom ash dewatering and transport water clarifying system utilizing remotely located SFCs and clarifiers, this facility is a proven case study of state-of-the art technologies that can readily meet new and pending regulatory requirements while providing for expanded operating flexibility. In particular, the system has produced an effluent water quality that is suitable for discharge (under current permit requirements), can be readily recirculated in a closed-loop, zero liquid discharge system or be utilized for FGD makeup/process water. IWW
About the Author: Kevin L. McDonough is vice president of sales and marketing for United Conveyor Corporation (UCC), a provider of combined heat and power (CHP) solutions for industrial, commercial and agriculture applications.
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