| FGD evaporation system at ENEL showing the forced circulation evaporator on the left, and the two 100% brine concentrators on the middle |
Meanwhile in the US, the Clean Water Act prohibits any entity from discharging "pollutants" through a "point source" into a "water of the United States" unless they have a National Pollution Discharge Elimination System (NPDES) permit.
The permit translates general requirements of the Clean Water Act into specific provisions tailored to the discharge of pollutants. NPDES permits are issued by states that have obtained EPA approval to issue permits or by EPA Regions in states without such approval. Of the states, only five have no authority to issue NPDES permits. Meanwhile, only five have full authority to issue permits.
Every five years, the permit must be renewed. The time required to renew the permits range from just a few weeks to more than a year based on the industry, discharge and objections filed. As a result, each power plant undergoes a period of uncertainty every five years – or sooner if a new pollutant discharge point source is identified.
For power plants, this can be catastrophic when new processes such as FGD systems are installed. This is especially true for regulated utilities requesting rate adjustments from Public Utility Commissions. Rates are adjusted at specific times. If the permit is not approved in time, a year may be lost before it is possible to begin recovering the costs of these improvements. FGD systems reduce the sulfur discharges to the air from power plants by reacting the sulfur dioxide created by the burning of coal. FGD systems remove a source of air pollution by converting it into a form of water pollution. For the first time since 1982, the limits of the pollutants arsenic, mercury, selenium and nitrates are being modified by the USEPA. It is estimated that 66 to 200 facilities will incur compliance costs or close due to the new regulations.
The new regulations that have been published indicate a preference to physical chemical treatment followed by biological treatment. The physical chemical treatment is designed to remove trace metals through pH adjustment, precipitation, clarification and filtration. Mercury and arsenic are removed using an ion exchanger resin, while selenium and nitrates are removed using an anoxic biological process. Nitrates are converted to nitrogen and oxygen and the selenium is converted.
EPA documentation concludes that physical/chemical/biological treatment will not reduce total dissolved solids which can be over 60,000 PPM. The physical treatment/biological process still results in a waste stream that must be discharged and requires an NPDES permit with all its uncertainties and risks. The ZLD process operates under a simple principal that a NPDES permit is not required if there is no discharge.
Water discharged from the FGD scrubber first may or may not be treated with a physical chemical treatment process. The selection of treatment and extent of pre-treatment is determined based on the method of final concentration of the discharge stream.
The FGD discharge is saturated in calcium sulfate and is high in chlorides. The water is first evaporated in a Brine Concentrator. This Vertical Tube Falling Film Evaporator is specially designed to concentrate brines containing compounds that would quickly scale the heat transfer surface of other types of thin film evaporators.
Equipment is optimized to minimize energy consumption through the use of a Mechanical Vapor Compressor. The compressor increases the pressure and the saturation temperature of the vapor produced in the evaporation process, and allows the large amount of residual energy in the vapor to be recovered. This has the added advantage of eliminating the need for cooling water and a steam condenser since the Brine Concentrator acts as its own condenser.
The concentrate can then be handled in one of three ways. The concentrate can be mixed with a solid to be discharged, typically sprayed onto fly ash for dust suppression before being discharged to a landfill, or undergo removal of the remaining water using a Spray Dryer. Both these processes eliminate the need for pre-treatment of the incoming brine.
Another option is to remove the water remaining in the concentrate using a Forced Circulation Evaporator. This equipment is designed to remove water from a circulating slurry. The Forced Circulation Evaporator can be powered either by steam, straight or with a Thermocompressor or a mechanical vapor compressor. The solids are then removed using either a centrifuge, belt filter press or other solids removal device.
Solids are disposed of in an approved landfill. Water recovered from the FGD wastewater is of high quality, typically less than 10 ppm, non volatile TDS for distillate from the Brine Concentrator and less than 30 ppm for mixed distillate from the combination of Brine Concentrator and Forced Circulation Evaporator. This high quality distillate can be recycled back to the scrubber or used as boiler feedwater or cooling tower makeup.
The issue of water conservation and pollution control in power plants is not limited to the US. For example, ZLD systems for FGD systems were installed in five power plants in Italy owned by Italian power company, ENEL.
In Italy, the FGD feed water is first treated with lime and then soda ash to reduce hardness. The treated FGD brine is then concentrated in a Brine Concentrator using a Mechanical Vapor Compressor to optimize energy consumption.
The concentration is completed in a Forced Circulation Evaporator using a thermocompressor to minimize steam consumption and cooling water usage. Solids are then removed with a belt filter press and moved to an approved landfill.
J. Michael Marlett is the process applications manager for industrial concentration for Aquatech International Corporation. Email: [email protected]
