Flue Gas Desulfurization Processes

The combustion of fossil fuels to power manufacturing and refining facilities occurs across a wide range of industrial sectors. Fossil fuel plants power chemical and oil refineries, as well as those facilities that produce cement, paper, glass, steel, iron, copper, and other metals. The SO₂ gases emitted as a result of that process are a primary contributor to acid rain and has long been regulated by every industrialized nation in the world due to their harmful effects on human health and the environment. In the U.S., ELG rules regulate wastewater discharges from coal-fired power plants and IGCC power plant wastewaters utilized in flue gas air pollution control systems or solid waste handling systems. Flue-gas desulfurization was an important development to meet environmental challenges.

Flue Gas Desulfurization Processes

The process of FGD is designed to absorb the sulfur dioxide in the flue gas before it is released. This is accomplished through either a wet or a dry process.

Dry FGD

In the process of dry scrubbing injection systems, lime is used as a reagent to react and remove gaseous pollutants. There are two common dry methods, the dry injection system and spray drying systems. Each process injects lime into flue gas to remove SO₂. A dry injection process injects dry hydrated lime directly into the flue gas duct, whereas the spray dry system injects atomized lime slurry into a separate vessel. Both methods yield a dry final product, collected in particulate control devices for further treatment.

Wet FGD

The process of wet scrubbing typically utilizes an alkaline-based slurry of lime to scrub gases. A shower of lime slurry is then sprayed into a flue gas scrubber, where the SO₂ is absorbed into the spray and becomes a wet calcium sulfite. One by-product of that sulfite is it can be converted to salable gypsum. Wet scrubbing provides high-efficiency sulfur dioxide removal capacity, in addition to reducing any scaling potential.

FGD Wastewater Treatment

The circulating water used during the wet scrubbing process ends up with many contaminants and pollutants, and the composition can vary significantly, plant to plant, influenced by factors such as the coal and limestone composition. In order for operating conditions to be properly maintained, this FGD wastewater needs to be discharged constantly from the scrubber system through a purge stream consisting of contaminants from coal, limestone, and the makeup water. The water is typically supersaturated with gypsum, is highly acidic with high concentrations of total dissolved solids (TDS) and total suspended solids (TSS), chlorides, fluorides, nitrites and nitrates, along with trace metals such as arsenic, mercury, selenium, boron, cadmium, and zinc.

Flue gas desulfurization wastewater can be effectively and efficiently treated using large filter presses or large vacuum belt filters for very large sludge production. Recess chamber filter presses are used effectively in coal-fired power plants to separate the solids from liquids in the purge stream. The FGS sludge dewatering process results in formation of large volumes of dry filter cake for proper disposal in a landfill. In some instances, additional treatment of the wastewater may be necessary if, for instance, lower levels of nitrates or selenium are required.

FGD sludge dewatering is only one step – yet an important step – in the overall FGD process. But when it comes to dewatering, proper filter media selection will play a critical role in effectively treating the wastewater from the purge stream. This will be a big contributor to the overall process of eliminating SO₂ from the flue gases.