By Michael Roush, PE, MBA, Burns & McDonnell
Utilities with coal fired assets are making and implementing their plans for compliance with the new Effluent Limitation Guidelines (ELG) and Coal Combustion Residual (CCR) rule. The final version of the ELG Rule bans the discharge of bottom ash transport water and the final version of the CCR rule puts new requirements on existing ash ponds. The combination of these two regulations will force many facilities with wet bottom ash handling to convert to dry ash handling. Planning for dry ash conversions for many facilities began when the original proposed CCR rule was released in 2010. For these early planning studies, the technology review assessments and recommendations for conversion were made based on a power generation market that is significantly different than what we are living with today.
When the CCR rule was originally proposed, most coal plants were base loaded and running 24/7. Since outages were expensive they were only planned when absolutely necessary. Consequently, most facilities placed an extremely high importance on short outage time for implementation and a remote drag chain conveying system became the default solution. The remote system can be installed in an open area of the site before the outage, with final pipe tie-ins completed during a short one to two week outage. This is not as big of a factor in today’s electric markets. The low cost of natural gas as well as a significant increase in new renewable energy has caused many utilities to begin cycling their coal units or keeping them in standby mode, ready to run when the other sources of power aren’t available. Outages today are not nearly as costly, and many utilities are now investigating other compliance alternatives.
The final ELG rule defines ash transport water as water that is in direct contact with the ash and is used convey the ash. The rule makes a point to dictate that an under-boiler (or local) drag chain system, which doesn’t sluice ash, contains quench water rather than transport water. Thus, the water in a local drag chain conveyor can be discharged under a facility’s NPDES permit limits for low volume waste, while water in a remote conveyor (with a sluicing system) cannot be discharged.
In general, bottom ash contained within the boiler is viewed as fairly benign and water chemistry in a closed-loop bottom ash system is anticipated to not be a large hurdle for the industry as a whole. However water chemistry issues at numerous facilities are beginning to surface as the industry moves towards closed-loop bottom ash handling. Issues within the bottom ash system range from extremely low pH which causes corrosion to extremely high pH and scaling issues. The problems are difficult to pin point and require a detailed investigation and customized solution to keep from creating new problems when cycling the water.
While ELG restricts discharge of bottom ash transport water and the anti-circumvention provisions prevent mixing of the six major waste streams covered by the rule, one caveat is extremely helpful for utilities that choose closed loop systems. This exception allows ash transport water to be taken to a scrubber. This blowdown stream will be particularly helpful to keep the chemistry and fines in these closed loop systems in check. Note that just because EPA says you can take a stream to your scrubber doesn’t mean that your scrubber can handle the makeup of the water. It is highly recommended to do an analysis of your scrubber and ash system to better understand the effects of this water on the scrubber operations. Also, any bottom ash water introduced into the flue gas desulfurization (FGD) system will have to be cleaned up to the new FGD blowdown standards.
With these new environmental regulatory drivers in place, there is a renewed sense of vigor for under-boiler systems at many plants. Laser scanning technology is becoming more cost effective and is an excellent new tool to help design for a large drag chain conveyor under a boiler with otherwise tight space requirements. In addition, many facilities are discovering that remote systems will have a higher operating costs due to the original sluice system remaining in place as well as a significantly higher up front capital cost to build out the infrastructure for the remote system. Not having or creating ash transport water removes a large compliance hurdle and the ability to discharge the water is a huge benefit when considering the water chemistry concerns and varying operating conditions. Remote systems are still a great fit for many plants, and these new hurdles are not insurmountable. But if your facility performed an ash conversion study several years ago, you may want to revisit that study to make sure that the decision factors used before are still valid today.