Energy-Tech Magazine

It is obvious that even with a high-efficiency reverse osmosis unit, some wastewater is still produced. For this reason, or for power plants that operate in water-scarce locations, air-cooled condensers are an option (ACC).

Unfortunately, there are some major downsides to using air instead of water. The density of air is, of course, much lower than water, so an ACC and its footprint must be much larger than a water-cooled condenser. Secondly, an ACC can only cool the steam within an approach to dry bulb temperature. In a cooling tower, water evaporation lowers the circulating water temperature to below dry bulb, considerably improving condenser heat transfer. Loss of critical megawatts during summertime operation is a distinct possibility with an ACC. Finally, ACC inlet headers and piping are notorious locations for flow-accelerated corrosion (FAC). FAC has been a deadly problem in some steam generator feedwater systems, and although the safety issue is less serious in ACC’s, the corrosion mechanism will still cause significant equipment damage. Furthermore, release

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of corrosion products in the ACC increases the potential for carryover and deposition of these products in the steam generator. Waterwall deposits increase the possibility of tube overheating and under-deposit corrosion.

Eliminating ash handling water in coal-fired plants
Another issue that will definitely impact water conservation at many coal-fired plants, for more than one reason, is wet ash handling. The failure of an ash pond in Tennessee in 2008, and subsequent inundation of the surrounding countryside, brought intense focus to wet ash handling and storage.

These concerns also include possible leakage of ash-induced water impurities into groundwater.

Anticipated regulations may soon prohibit wet ash pond storage at many locations, which will force plants toward dry ash handling. One technology for bottom ash removal in dry form is already in place at a number of plants, and consists of quenching the ash in water, as has been done for years, but removing it by drag-chain conveyors so that the quench water drains back to the conveyor basin. The ash is deposited in a stack-out pile, and then is trucked to a repository on plant grounds. Totally dry conveyor designs also are emerging – look for a report on this subject in a future Emissions Control Expert column in Energy-Tech.

Wet flue gas desulfurization (WFGD) wastewater
For those coal plants with WFGD systems, the liquid purge stream from the process can be a major headache. The stream might contain unacceptable concentrations of arsenic, boron, mercury and selenium, among others. The schematic outlined below shows a viable treatment method.

The following processes are important to operation.

• Lime [Ca(OH)₂] injection to the influent stream, which is typically saturated with sulfate ions (SO₄^-2), to precipitate the ions as gypsum (CaSO₄•2H₂O) in the desaturation tank. This treatment minimizes scaling problems downstream. Some of the metals in solution will precipitate as hydroxides.
• Suspended solids removal in the primary clarifier.
• Addition of ferric chloride to the partially treated water. The iron oxides that form will initiate co-precipitation of other compounds, including arsenic.
• Injection of a sulfide-active-group polymer to capture mercury for removal as sludge in the secondary clarifier.
• Secondary clarification to remove precipitated metals.
• Conversion of the primary and secondary clarified sludge to a solid material in a filter press, such that the material can be disposed in a properly-designed and permitted landfill.