|The submerged flight conveyor shown here collects ash from the boiler and cools the ash in a water-filled trough. Photo courtesy of United Conveyor Corp.|
By Kevin L. McDonough, United Conveyor Corp.
Since the issue of the proposed Environmental Protection Agency (EPA) Coal Combustion Residual (CCR) rules in May 2010 and the Steam Electric Power Effluent Limitations Guidelines (ELG) in April 2013, utilities, power plants, technology providers, engineering firms and the EPA have been highly active in the review of technical alternatives and approximate costs to comply with the forthcoming regulations. The rules will establish new provisions to regulate the handling and disposal of coal combustion residuals, as well as establish limitations for the pollutants contained in various wastewater streams at steam electric power generating stations.
The response to the proposed rules has been significant with hundreds of thousands of comments and questions from environmental groups, utility groups, communities and companies active in the handling disposal and beneficial reuse of coal combustion residuals. The feedback has resulted in a very significant data collection effort from all stakeholders to qualify and quantify the implications of the new requirements. Utilities have the difficult task of balancing power price competitiveness with the additional costs associated with the design and installation of the new pollution control equipment. With an extended duration of depressed power prices relative to incremental domestic economic recovery, the proposed investments elevate the already intense pressure for profitability while still keeping power costs low. The EPA has the challenging task of properly identifying and mandating the best available technologies that will balance implementation costs with the desired reduction of environmental risk. At a minimum, the situation across the U.S. coal power fleet is highly complex with a multitude of technical and economic variables.
This complexity has resulted in inevitable delays, and corresponding market uncertainty, in the issue of the final rules. On a positive note, however, the extended rule development period has resulted in a more comprehensive risk assessment from the EPA, particularly where the two rules predictably overlap in achieving separate but common goals for environmental protection. This coordinated effort within the agency is clearly manifested in the following excerpt from the proposed ELG (CFR Vol. 78, p. 34442): “Although a final risk assessment for the CCR rule has not yet been completed, reliance on the data and analyses discussed above may have the potential to lower the CCR rule risk assessment results by as much as an order of magnitude. If this proves to be the case, EPA’s current thinking is that, the revised risks, coupled with the ELG requirements that the agency may promulgate, and the increased Federal oversight such requirements could achieve, could provide strong support for a conclusion that regulation of CCR disposal under RCRA Subtitle D would be adequate.” This conclusion, if enacted in the final rules, has a significant positive impact of greatly reducing implementation costs while still achieving the desired environmental goals.
In the meantime, a number of utilities have elected to move forward with fly ash, bottom ash and gypsum wet-to-dry conversion projects. These projects have been driven by various factors, including fuel switching, air quality control projects, existing CCR impoundment limitations and, in some cases, consent decrees with state and federal regulatory agencies. Some utilities have also placed value in the proactive benefits of addressing the future environmental requirements from the ELG and CCR regulations sooner rather than later. In these recent cases, the project activity has had the added benefit of potentially being “ahead of the curve” when the final rules are enacted.
|A PAX pneumatic ash extractor for wet to dry conversion of coal ash. Photo courtesy of United Conveyor Corp.|
Bottom Ash Wet-to-Dry Conversions
In the past few years, UCC has designed, installed and commissioned several bottom ash systems that have converted traditional wet sluice conveying systems to dry or zero liquid discharge systems. These projects have taken place at eight plants spanning sixteen operating units and have included a conventional dewatering bin system, multiple under-boiler Submerged Flight Conveyor (SFC) systems, several remotely-located submerged flight conveyor systems and two 100 percent dry bottom ash handling systems. Each technology selection was based on a unique set of criteria that included numerous variables. The following table lists some of the key variables.
In each case, the utility completed some variation of a Kepner-Tregoe (KT) analysis that took into account the numerous design criteria and relative importance values (“weighting”) of each criteria. As evidenced by the varied outcomes of the decision analyses, technology selection does not typically follow a “one size fits all” conclusion. Each plant, with its unique set of variables and priorities, came to an independent technology selection and coal combustion residual management solution.
|A PAX pneumatic ash extractor for wet to dry conversion of coal ash. Photo courtesy of United Conveyor Corp.|
Technology Selection and Ash Management Considerations
For several utility installations, plant owners elected to install UCC Submerged Flight Conveyor Technology (SFC). In addition to overall project cost, key decision criteria included outage time constraints, reduced maintenance and operations costs, physical space availability and the age/condition of the existing bottom ash hopper. For these projects where the existing bottom ash hoppers were in need of major repair or replacement, a new SFC replaced the existing bottom ash hopper/sluice conveying system, and the bottom ash pond was decommissioned. With ample outage time, project costs were highly favorable relative to other technical options.
In operation, the SFC collects ash from the boiler into a water-filled trough where it quenches and cools the ash. Horizontal flights move the ash continuously through the trough and up a dewatering ramp where it is then discharged into a load-out bunker or secondary transfer conveyor.
In the process, the bottom ash is dewatered to approximately 25-30 percent on the dewatering ramp, and then down to 15-20 percent prior in the temporary storage bunker. This has the tandem benefit of mitigating fugitive dust emissions during transport while also optimizing the material for compaction in the dry landfill. The bottom ash is also available for beneficial reuse, largely because of the consistency of the particle size distribution. For each of these systems, the overflow water from the SFC trough is recirculated to complete a zero liquid discharge (ZLD) system. This conversion has the net effect of eliminating nearly one million gallons per day of wastewater generation in the original sluice conveying systems.
At several other utility installations, plant owners elected to install UCC Continuous Dewatering and Recirculation (CDR) Systems with remotely located Submerged Flight Conveyors. Key decision criteria included limited outage time and physical space constraints for bottom ash hopper replacement. For these projects, the existing bottom ash hoppers and equipment remained in place, while the sluice conveying pipes were intercepted and routed to the remote SFC’s. After the particulate was removed from the sluice conveying water, the clarified water is returned to the boiler house to complete a zero liquid discharge system, while again decommissioning the existing bottom ash pond. This particular option is particularly cost effective when considering installations with two or more operating units due to the multiple unit synergies, and is also highly favorable by maintaining plant availability as it requires no major changes to existing bottom ash hoppers and minimal outage time with proper tie-in planning for piping, power and controls.
The CDR system combines the existing sluice conveying systems with the advanced SFC dewatering and clarifying technology. These systems have been designed to address the complexities of a bottom ash water balance, which addresses multiple flow sources, intermittent conveying cycles and variable flow rates. Now with an extended period of operation, the remote SFC’s have not only proven to be effective water clarifiers, but have also ensured that the water is sufficiently cooled for proper bottom ash hopper operation. These ZLD systems have also eliminated nearly one million gallons per day of wastewater generation. Regular water chemistry analysis has also confirmed no concentrating effects of fine particulates (TSS), chlorides or metal constituents (TDS) present in the bottom ash with the closed-loop system.
At another utility installation, plant owners elected to install a traditional dewatering system that utilizes Dewatering Bins, Settling Tanks and Surge Tanks. Key decision criteria included limited outage time and reuse of existing equipment. For this project, the plant retained existing Dewatering Bins, while providing additional particulate separation and water clarifying in the new tanks. Like the CDR system, the clarified water is returned to the boiler house to complete a zero liquid discharge system, while again decommissioning the existing bottom ash ponds. This particular option is particularly appealing if the plant has existing equipment that can be reused and is cost effective when considering installations with two or more operating units due to the multiple unit synergies. This technology can also be implemented with little to no outage time with proper tie-in planning. Many plant operators are familiar with this technology as it has been successfully used at dozens of installations throughout the U.S. coal generation fleet. With proper operation and maintenance, these traditional systems have proven to be effective water clarifiers, while eliminating a significant source of wastewater generation.
At another utility installation, plant owners elected to install two 100 percent dry UCC Pneumatic Ash Extractor Systems (PAX). Key decision criteria included elimination of wastewater generation and long-term life cycle costs. For this project, the existing water-impounded bottom ash hoppers were entirely replaced by dry, refractory-lined hoppers, vacuum conveying systems and a common dry bottom ash storage silo. The existing bottom ash pond was decommissioned. The patented UCC PAX™ system provides long-term operating cost benefits. While operating, forced draft fan air cools the ash to help complete combustion of unburned material and protection of ancillary equipment, resulting in greater boiler efficiency (≈0.5-1.5%). As the ash cools, it is crushed and fed into a pneumatic conveying line and transported to a storage bin prior to dry landfill disposal or beneficial reuse.
|Bottom ash recirculation system . Photo courtesy of United Conveyor Corp.|
Regulatory Flexibility for Other Waste Streams
At most coal-fired installations, mill rejects (“pyrites”) are collected independently from the combustion residuals, but are often combined with the bottom ash in the sluice conveying systems. As this material is not specifically mentioned in the ELG as a wastewater stream of concern, and also does not constitute a combustion residual (removed pre-combustion), the EPA appears to have given some flexibility to the plant operators on the management of mill rejects.
For those systems that share a common sluice conveying system to transport bottom ash and mill rejects, any of the dewatering technologies referenced above are suitable to dewater and prepare pyrites for dry landfill disposal. If the bottom ash is desired for beneficial reuse, the conversion technology must be designed to account for separation of the two unique sluice streams as the mill rejects are not commonly suitable for reuse applications. This can be accomplished using independent dewatering and particulate separating equipment and by alternating sluice conveying sequences. While this will add cost to the system, and some operational complexity, these costs are at least partially offset by the elimination of a portion of landfill disposal costs when reusing bottom ash for beneficial purposes.
If an under-boiler SFC is the preferred bottom ash technology, pyrites can readily be sluiced to the SFC for dewatering and landfill disposal preparation. In this instance, the SFC must be designed with either sufficient surge capacity to accommodate the volume of water used during a pyrites sluice conveying cycle or an ample overflow design to handle this low volume waste.
If a dry bottom ash hopper is the preferred technology, the mill reject system must be uniquely evaluated to confirm the appropriate disposal technology for the lowest possible cost.
In any case, UCC can provide sluice conveying systems, positive and negative pressure pneumatic conveying systems or mechanical systems to handle mill rejects depending on technical criteria and plant preferences.
Collection and disposal of Economizer Ash varies by plant site and boiler type. By comparison, economizer ash typically represents approximately 5 percent of the total fly ash generated. Typically, the economizer ash is collected in dry hoppers and then transferred to either the bottom ash or fly ash handling systems. In some instances, the collection hoppers are connected directly to the dry pneumatic fly ash systems. The economizer ash is then moved from the hoppers to a storage silo in a normal fly ash conveying cycle. Economizer ash can also be transferred by mechanical conveyors from the collection hoppers to temporary storage bins, and then connected to the dry pneumatic fly ash systems or can be discharged to under-boiler SFCs.
In other instances, the economizer ash is sluiced from the collection hoppers to the bottom ash hoppers or transfer tanks. From these points, the ash is sluiced to the ponds. Finally, Economizer ash can also be transferred by mechanical conveyors from the collection hoppers to temporary wet storage tanks or dry storage bins, and then connected to the bottom ash systems via sluice conveying lines.
The Agency has defined economizer ash as bottom ash when “it is collected with the bottom ash” (40 CFR 423.11f) and as fly ash when “it is collected with fly ash” (40 CFR 423.11e). These definitions should give the utilities a great degree of flexibility in managing economizer ash. Under either definition, the dry or closed-loop ash conversion technologies can be designed to facilitate economizer ash in either the fly ash or bottom ash systems. A key criteria in technology selection is beneficial reuse of the fly ash. In some cases, economizer ash can interfere with fly ash reuse due to its much wider variability in particle size distribution.
|Continuous dewatering and recirculation system for dry bottom ash. Photo courtesy of United Conveyor Corp.|
Next Steps and Likely Outcomes
For fly ash management, the regulations will likely result in the phase out of all remaining wet fly ash systems. The overwhelming majority of existing installations are already equipped with dry fly ash systems and most remaining plants have already initiated the process to make the conversion from wet to dry. UCC has numerous active projects and recently commissioned a number of dry fly ash systems for key base load units in the U.S. fleet.
For bottom ash management, the agency, under three of the four preferred ELG options, has indicated that the technology basis for bottom ash management is wet impoundments. To comply, the plants must meet water quality requirements that include Total Suspended Solids (TSS) and Oil and Grease discharge limitations, which can be achieved with a properly sized and designed impoundment. The proposed standards are consistent with NPDES discharge permit requirements that have been in place for a number of years. Under the fourth preferred option, compliance would require dry conversions or ZLD systems for units greater than 400MW. In tandem with the CCR regulations, however, which may necessitate impoundment upgrades including liners, leachate collection systems and groundwater monitoring systems, the utilities will have to weigh the costs and benefits of retrofitting existing ponds or constructing new ponds to meet the new requirements, or convert the systems from wet to dry. Impoundment stability, capacity and flood plain proximity will also be key factors in the evaluations.
Each scenario must again consider a wide variety of technical and economic criteria to determine the optimal solution for a given plant. Relative to the anticipated final issue of the proposed regulations, many utilities are now working diligently to investigate the various options and develop short and long range plans to achieve compliance in the most cost-effective manner.
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