Polk CCPs were originally expected to be directly suitable for use in other industries. However, upon plant startup in 1996, the large quantity of char mixed with the slag made the CCPs unsuitable for beneficial use. As an interim solution, the slag and unburned carbon was trucked to one of Tampa Electric’s pulverized coal burning stations and used as a fuel. This solution recovered the fuel energy content, but resulted in accelerated wear of fuel handling equipment due to the abrasive nature of the slag. Although a number of tons of slag were disposed of in a landfill, long-term on-site or off-site disposal in an open loop system was unacceptable. Because of these issues, a large quantity of slag had accumulated in the on-site temporary storage area.
In November of 2000, the use of the high carbon slag as a fuel at the pulverized coal units was halted due to a permitting issue. This left disposal at a landfill, at over $30 per ton, as the only option. According to Mark Hornick, General Manager of Polk Power Station, “we were facing a significant increase in expense to dispose of a material that expected to be a valuable byproduct”. The challenges were to (1) process the on-site CCP pile for recycling/beneficial uses and (2) produce CCPs directly from continuing operation that could be beneficially used.
Tampa Electric established an internal team to enhance the quality of slag produced at Polk so that disposal quantities could be minimized or eliminated and beneficial use would be maximized. The team included representatives from plant operations, engineering, marketing, and environmental personnel. The scope of their work was to first conduct an in-depth review of the beneficial use opportunities and specifications in light of current slag quality and processing operations. The scope then allowed for permanent fixes to be made rather than short term patches.
Louisville, Kentucky-based Charah, Inc. was approached by Tampa Electric’s team to see how Charah could assist with the growing IGCC slag problem at Polk. After an initial analysis of the slag samples, Charah contacted the University of Kentucky Center for Applied Energy Research and asked them to analyze the slag for characteristics that Charah felt were needed to determine if and how the slag could be processed and made into usable products. The resulting collaborative research determined that the slag could be processed and made into three usable products.
Charah designed a flow sheet detailing the equipment and processing needed to solve the power plant’s slag problem. The challenges were met by separating Polk’s CCPs into the two constituents, slag and char. Charah was then commissioned to develop an independent system to beneficiate the existing on-site CCP in December 2001 and immediately produced useful products. As a result, Charah designed and built a processing system which it owned and operated on-site. This patent-pending processing system separates unburned fuel from the slag materials and makes it available for reuse by the power plant. In addition, Charah has found marketing opportunities for the coarse and fine products that are left behind after fuel recovery is complete.
Beneficiation enabled Tampa Electric to reintroduce the char from the over 100,000 tons of stockpiled CCPs stored on-site into the fuel stream. The slag was sold for beneficial uses. As a result, Polk was able to process, use or market all the stockpiled slag plus all of the slag produced daily for nearly three years with a significant financial savings when compared with other methods of removal.
Polk now operates as a closed loop system, where the only CCPs leaving the site are those that are sold as raw materials for manufacturing. Charah now has a patent-pending on this IGCC slag beneficiation process.