While many power generation companies have moved away from coal due to an increasingly stringent regulatory environment and/or concerns with climate change, many countries continue to rely on their vast coal resources to maintain a low cost to consumers while insuring a level of energy security. Hence, there is an ongoing need for efficient, effective flue gas desulfurization (FGD) systems.
According to Transparency Market Research, expanding global demand for power has led to a huge and growing power generation market. Government regulations in some regions have mandated the use of FGD systems, the result of which is estimated growth in flue gas desulfurization market valuation from US$14.9 billion in 2012 to US$22.2 billion in 2019.
Power plants that utilize coal release some amount of sulfur dioxide (SO2), depending on the type of coal, which has adverse effects on the environment. FGD systems are used to remove sulfur from flue gas to make it less environmentally harmful. Though many variations in the FGD process exist, those employing wet scrubber technology, with an SO2 removal efficiency of up to 99%, represent the most practical method for treating boiler gas generated from the combustion of medium to high sulfur coal.
The Wet Flue Gas Desulfurization Process
Flue Gas Desulfurization is a widely adapted technology for removing sulfur dioxide from the exhaust flue gas in coal- and oil-fired power plants. In the “wet limestone-gypsum” form of FGD, an aqueous slurry of finely ground limestone is introduced into an absorber tower and contacted with the flue gas. The interaction results in the conversion of SO2 in the gas stream into calcium sulfate, or gypsum, with carbon dioxide (CO2) being routed up the stack.
As diagrammed below, a typical FGD system consists of four units operating simultaneously: (1) Limestone Processing; (2) Gypsum Recovery; (3) Wastewater Treatment; and (4) SO2 Scrubbing (or Absorption).
Magnetrol® produces a wide range of instrumentation to regulate and measure levels in these units. The level and flow control applications found in these units will be discussed in our blog series, beginning with this article.
Limestone is an extensively used reagent in today’s utility industry. Limestone arrives by truck and train and is transferred from the limestone bunker to the ball mill where it is ground into a fine powder. The ground limestone is then transferred to the hydrocyclone feed tank where makeup water is added. The slurry proceeds to the limestone slurry storage tanks from where it enters the SO2 absorber tower. There the flue gas is contacted by the limestone slurry resulting in the conversion of SO2 in the flue gas to calcium sulfate, or gypsum. The gypsum is typically recovered for resale.
Level and Flow Applications for Limestone Processing
- Limestone Store
Limestone is delivered either as crushed stone or pre-ground powder. Unground limestone from road or rail hoppers is conveyed to the limestone store for storage. A limestone reclaim machine operates within this vessel.
Continuous Level: Eclipse® Guided Wave Radar (GWR)
Point Level: Solitel® Vibrating Rod Level Switch for bulk solids
- Limestone Bunker
Unground limestone is transferred from the limestone store to a bunker prior to its conveyance to the ball mill for grinding. The bunker is a day silo containing a 16- to 24-hour supply.
Continuous Level: ECLIPSE GWR
Point Level: SOLITEL Vibrating Rod Level Switch for bulk solids
- Pump Protection
Process water is added to limestone between the bunker and the grinding mill and again in the hydrocyclone feed tank. A flow switch positioned along the pump’s discharge piping will actuate an alarm and shut down the pump when liquid flow drops below the minimum flow rate.
Flow Alarm: Thermatel® TD1/TD2 Switch for low flow cutoff
- Hydrocyclone Feed Tank
Hydrocyclones separate solid-liquid suspensions through centrifugal sedimentation. An agitated hydrocyclone feed tank supplies the separator.
Continuous Level: Pulsar® Thru-Air Radar; ECLIPSE GWR; or Echotel® Ultrasonic Transmitters
Point Level: THERMATEL TD1/TD2 Switch with Spherical Tip for Hi/Low Alarm
- Limestone Slurry Feed Tank
Hydrated lime stored in steel or poly tanks is kept in suspension with a slow speed mechanical agitator. The slurry fed to the absorber serves as the scrubbing reagent. Continuous Level: PULSAR Thru-Air Radar; ECLIPSE GWR; or ECHOTEL Ultrasonic Transmitters
Point Level: THERMATEL TD1/TD2 Switch for Hi/Low Alarm