March 15, 2008

Benefits of evaporating FGD purge water

William A. Shaw, PE, HPD

In coal-fired power stations, flue gas desulfurization (FGD) is used to “scrub” most of the sulfur dioxide (SO₂) from the gas. FGD is often performed using a wet process in which SO₂ is absorbed from the flue gas by spraying it with a slurry containing limestone (mainly calcium carbonate, CaCO₃). The SO₂ reacts to form calcium sulfite, CaSO₃, which is further oxidized to produce gypsum (CaSO₄ x 2H₂O) by introducing air into the scrubber. The gypsum product is often washed and sold for various uses; otherwise, it is disposed of in a landfill.

 

Scrubber wastewater chemistry

Water uses in a limestone-gypsum FGD scrubber are easily identified (Figure 1). The gypsum has to be removed continuously from the scrubber, and it is replaced by fresh limestone. In addition, a certain proportion of the circulating water is removed as effluent to control the buildup of chloride from coal combustion products in the scrubber water. The circulating scrubber water also contains other water-soluble impurities from the flue gas and the limestone.

 

 

FGD is an evaporative process, so it concentrates these impurities. The wastewater from gypsum dewatering will have a pollutant content that depends mostly on the type of coal burned, the efficiency of the unit's electrostatic precipitator (ESP), the level and type of impurities in the makeup water, the amount of heavy metals and impurities in the limestone, and the choice of gypsum dewatering equipment. A good understanding of these parameters is necessary to properly design an effective FGD purge water treatment system.

Coal is the primary source of the sulfur and chloride in scrubber wastewater and a contributor of heavy metals as well. The composition of the flue gas entering the FGD plant is closely related to the composition of the coal and the way it is burned. It's also determined by the type of upstream particulate-control and denitrification devices used. The flue gas contains SO₂, fine particles of flyash and their trace elements, volatile heavy metals such as mercury and selenium, and hydrogen chloride and fluoride.

The hydrogen chloride and fluoride are partially absorbed in the absorber and react with the limestone to form soluble calcium chloride and insoluble calcium fluoride. Flyash residues that have not been removed by the ESP are partially washed out in the absorber. Consequently, the amount of heavy metals entering the wastewater with the residues depends on the efficiency of the ESP. Nitrogen compounds such as nitrate, nitrite, and ammonia are also present in FGD wastewater. Most nitrogen compounds are created by coal combustion. The temperature of combustion and the nitrogen content of the coal affect the concentration of the nitrogen compounds. If a selective catalytic reduction (SCR) system is upstream of the FGD plant, unreacted ammonia also may be present in the wastewater.

Limestone is a widely variable material. It usually contains impurities such as compounds of magnesium, iron, and silica and traces of heavy metals. Dibasic acid (DBA)—a mixture of succinic, glutaric, and adipic acids—or another organic acid such as formic acid is sometimes added to buffer the pH to improve SO₂ absorption. These acids affect the solubility of calcium carbonate and also raise the biological oxygen demand of the effluent.

The purge water contains mainly calcium, magnesium, and sodium cations. Small amounts of potassium and manganese also are present, along with traces of ammonia and heavy metals. The main anions are chloride and sulfate. The remaining halogens (fluoride, bromide, and iodide), sulfur compounds such as dithionate and peroxodisulfate, sulfur-nitrogen compounds, and nitrate are present in smaller amounts.

Treatment of FGD wastewater must take into account that the stream has the chemistry above, is supersaturated with gypsum, has a pH between 4.5 and 5.5, and contains heavy metals, suspended solids, and a high (30,000 to 60,000 mg/l) chloride concentration. It also may have organic content if DBA is added to the scrubber to enhance its SO₂ removal efficiency.

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