Coal and Ash Handling: In Search of Cost Savings

The following excerpt is courtesy of Power Engineering April 2003

Electrostatic Separation

Fly ash processing via electrostatic separation is an entirely different animal. Rather than combusting the fly ash to get rid of the carbon, a physical separation takes place. Differential charging enables carbon-rich particles to be separated from the low-carbon ash particles, creating two potentially valuable product streams: low LOI fly ash for use in the ready mix concrete market and a carbon-rich material for use as a supplemental boiler fuel or as feedstock for cement plants.

In late February, JEA began installing an STI electrostatic separator at St. John's River Power Park to beneficiate carbon from fly ash. The separator is expected to go on-line in April.

Separation Technologies Inc. (STI), which patented the electrostatic process for power plant fly ash, currently has three plants in operation in the U.S., one in operation in Scotland, and a fourth U.S. plant scheduled for on-line operation in April. The units in operation have an on-stream factor of 90 percent, according to STI. During short periods of downtime for required equipment maintenance, fly ash simply accumulates in silos. STI has identified at least 50 coal-fired plants in the U.S., and another 50 worldwide, as prime candidates for fly ash separation technology, based on an analysis of regional markets for concrete-quality fly ash.

Constellation Energy Group's Brandon Shores Plant has relied on a 35 tph electrostatic separation system (owned and operated by STI) since 1999 to process its fly ash. The system handles a wide range of feed LOI levels, from the low teens in the summer ozone season to around five percent in the non-ozone season. Brandon Shores works very closely with STI. "We share information so that we can supply sufficient ash to STI so they can meet their commercial commitments," said John Strauch, General Manager at Brandon Shores. "In turn, STI works within our maintenance and scheduling constraints, and provides data – including LOI and foam index – that enable us to improve combustion tuning and airflow." Further, although the system only processes the fly ash from Unit 2, Constellation sometimes asks STI for an LOI reading on samples from Unit 1 since STI can measure LOI more quickly.

When JEA's St. John's River Power Park began firing a mix of petroleum coke and coal in 1997 to maintain lower power costs, a higher fly ash LOI level resulted than when burning coal alone. The high LOI level, and high ammonia levels resulting from the ammonia injection system used to buffer the SO3 plume, rendered the fly ash unmarketable in the Jacksonville area. JEA decided to have STI install an electrostatic separation system to upgrade the quality and value of the 300,000 tons of ash it produced each year. "The key benefit to JEA, apart from recycling another plant byproduct, is the avoidance of landfill costs and the extension of the life of our existing landfill capacity," said Paul Smith, plant manager. "For this plant, that savings can amount to more than $1 million annually. In fact, the STI system will almost eliminate all of our landfill costs."

Although several power plants using STI technology have performed extensive testing to demonstrate the viability of recovering the fuel value of the carbon-rich stream, none have allocated the necessary capital to install feed systems to utilize the carbon on a continuous basis. "At Brandon Shores, because of engineering differences introduced when the boilers were converted from oil to coal-fired, there is some concern that the higher ash content of the carbon-rich stream could increase boiler tube erosion," said Strauch. "Further in-depth analysis to measure the impact of this increased ash content, as part of the overall project cost and benefit, would be necessary prior to full-scale implementation." A portion of the carbon-rich material from Brandon Shores, however, is used as feedstock in cement kilns, and JEA is exploring the use of the material at a fluidized bed plant.

The electrostatic carbon separation process is not affected by the presence of ammonia on the fly ash. Two of the plants with STI operations are using SCR systems to control NOx emissions, and no impact on ash quality has been observed. To mitigate potential concerns, however, STI has developed an ammonia removal process that can be integrated with the carbon separator or operated as a stand-alone unit. Small quantities of alkaline compounds and water are added to the fly ash, creating a thin-film solution of high pH on the ash particles that cause the ammonia to be rapidly released under controlled conditions. The final chemistry of the ash is insignificantly changed from the untreated ash.

JEA's St. John's River Power Park will house the first such ammonia removal system, capable of handling up to 40 tph. "If fly ash with elevated ammonia slip is used in concrete, the curing concrete releases strong ammonia fumes that are unacceptable," said JEA's Smith. "Removing the ammonia certainly is a customer requirement. We felt STI had the only fly ash separation technology that could handle the ammonia removal cost effectively and at the high volumes we needed."

The complete article is available at www.power-eng.com.

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