FGD Gypsum Byproduct Processing
An unusual byproduct is generated at fossil-fueled power plants. Specifically, when flue-gas desulfurization (FGD) is used to remove sulfur dioxide from pollutants, FGD gypsum is formed. Ironically, this gypsum can be used for many commercial uses, including cement, drywall, glass manufacturing, and more. But first, the water content needs to be reduced and even treated before moving to the next steps. WesTech solutions can help make that happen.
Gypsum Processing Solutions
Wet flue-gas desulfurization (FGD) systems generally operate in a pH of 5.0 or above and require feed of reagent to remove sulphur. Limestone slurry is the most commonly used reagent. The quantity of slurry depends mostly on the sulfur levels in the coal. It is common for modern FGD systems to achieve 99 percent removal of SO2. Scrubbers not only capture the SO2, but also capture up to 98 percent of mercury and 99 percent of fine particles associated with lung problems such as asthma.
This reaction produces gypsum (CaSO4) as a byproduct. The gypsum is removed either by thickeners or hydrocyclones and then dewatered. In general, gypsum is either disposed of in a landfill or sold for the manufacture of wallboard.
Disposal grade gypsum typically has a specified moisture of 15 percent. WesTech scraper discharge filters offer a simple, yet effective, means of dewatering disposal grade FGD gypsum.
Wallboard grade gypsum is specified as having less than 10 percent moisture. To achieve this low level of moisture, a horizontal belt filter is used. A major influence on obtaining this performance is the aspect ratio of the gypsum solids.
For a square cross-sectional area and crystals of equal volume, an 8:1 aspect ratio has 35 percent more surface area than a 2:1 aspect ratio. So, if crystals with 2:1 aspect ratio give 10 percent surface moisture, 8:1 aspect ratio will give 13.5 percent moisture under the same filtration conditions. This is approximately what has been observed in the field.
The system must start with limestone of high purity, generally greater than 95 percent CaCO3, which will then contribute less than 3 percent inert fines in the gypsum.
The oxidation step is run with long residence time (for gypsum growth), high solids concentration (for low aspect ratio), and low pump shear (for low mechanical nucleation rate). Keeping fly ash out of the gypsum and using makeup water with low silt content will also produce a lower moisture filter cake.
Horizontal Belt Filter Design for Gypsum Dewatering
The filter must be sized accurately for the gypsum specifications. Filtration rates vary from 150 to 400 lbs/h/ft2, depending on particle size distribution and desired moisture content of cake.
High feed solids concentration are required, typically 50 to 55 weight percent solids to avoid segregation of fine and coarse particles. High vacuum air flow rates of 15 to 25 cfm/ft2 are used to provide pressure drop to strip off surface moisture for 10 percent moisture or less.
Chloride removal is accomplished by two to three wash displacements to achieve the over 90 percent wash efficiency for required wallboard quality. Good feed distribution is essential. Even a slight flow bias gives cake thickness variation side to side with corresponding wash and dry differences.
Gypsum cake leaves a heel on the cloth. The cloth wash slurry contains about 1–2 percent of gypsum. This stream can be returned to the system as cake wash water. This allows the recovery of the water and the gypsum solids and reduces the total suspended solids (TSS) of the wastewater from the process.
Gypsum Disposal Circuit
Gypsum produced from flue-gas desulfurization (FGD) has been used as a valuable byproduct that was sold as a feedstock for wallboard manufacturing. Economic trends in housing starts affect the demand and price of gypsum. There is also a trend among utilities to close waste ponds at their power plants. These combined trends have fueled the need to find alternate gypsum disposal methods.
One method makes use of abandoned mine sites. This disposal method has the added advantage of neutralizing acid mine drainage (AMD). Gypsum has a high pH and can therefore neutralize AMD streams.
Flue-gas desulfurization effluent is dewatered to facilitate gypsum transportation. Mine disposal decreases the required level of dewatering. Wallboard grade gypsum requires a solids level greater than 90 percent. Disposal grade gypsum has a solids level of 85 percent. Mine disposal grade gypsum must only fail to have any free liquid. This substantially reduces dewatering costs, but results in more system water loss and increased shipping weight and bulk.
The gypsum is shipped to the disposal facility by barge, truck, or train and offloaded for feeding to a conveyor system. The conveyor transports the “dry” gypsum to the reclaim tank where it is mixed with water reclaimed from the process. Water may also be added to the effluent of this tank to ensure the proper solids content in the gypsum mix and storage tanks.
Solids settling is prevented in the gypsum mix and storage tanks by using mixers, while the other tanks are sequentially pumped to the mine. The slurry solids level is monitored to avoid slurry line plugging as well as excess water usage. Slurry barricades allow the deposited slurry to dewater via gravity.
Water from this phase is collected in a mine pool then pumped to an aboveground thickener. The thickener settles excess solids, while the water may be reclaimed as slurry makeup. If river water is used as makeup water, it may also be treated by the thickener.