Level and Flow Instrumentation for Flue Gas Desulfurization Systems

The expanding global demand for energy has led to growth in the power generation market. Many countries rely on coal resources to maintain low energy costs and ensure a level of energy security. In these regions of the world, there is an ongoing need for flue gas desulfurization systems (FGD systems) to reduce the environmental impact of coal-based power plants. Magnetrol® produces a wide range of instrumentation to measure and control level and flow in FGD systems. Last week, we discussed the wet flue gas desulfurization process and instrumentation for limestone processing. In this week’s blog post, we explore level and flow solutions for the remaining three steps of the FGD process.

Gypsum Recovery

Although the FGD process was originally established for environmental purposes, a large amount of high quality, synthetic gypsum is created as a by-product of the scrubbing process. Gypsum is widely used as a source material for manufacturing wallboard and as a conditioner for agricultural soils. Because of its commercial value, most FGD systems incorporate equipment to recover, dewater and store gypsum for resale. The gypsum slurry pumped from the absorber is fed to the centrifuge feed tank. The centrifuge reduces the gypsum slurry to a dewatered cake containing less than 10 percent moisture by weight.

 

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Level and Flow Applications for Gypsum Recovery

  1. Centrifuge Feed Tank
    The gypsum slurry is pumped from the absorber to the centrifuge feed tank. Centrifuge operation is initiated by a level controller located in the centrifuge feed tank.
    Continuous Level: Eclipse® Guided Wave Radar (GWR); or Pulsar® Thru-Air Radar Transmitters
  1. Centrate Tank
    The gypsum slurry stream (centrate) produced in the gypsum dewatering operation is routed for residence in a centrate tank prior to its introduction into the limestone slurry feed tank to serve as an additive.
    Continuous Level: ECLIPSE GWR; PULSAR Thru-Air Radar; Echotel® Ultrasonic; or E3 Modulevel® Displacer Transmitters
    Point Level: ECHOTEL Ultrasonic Switch; or MAGNETROL Float and Displacer Switches
  1. Centrifuge Pump Protection
    Process water is added to the gypsum dewatering centrifuge. A flow switch along the pump’s discharge piping will actuate an alarm and shut down the pump when liquid flow drops below the minimum flow rate.
    Point Level: Thermatel® TD1/TD2; or F10 Mechanical Flow Switch
  1. Gypsum Storage
    Dewatered gypsum is transferred to a gypsum storage vessel equipped with a gypsum reclaim machine. The gypsum may later be transferred to a storage shed.
    Point Level: Solitel® Vibrating Rod Level Switch for bulk solids

Wastewater Treatment

A dedicated wastewater facility properly integrated with the FGD scrubber operation is typically required to meet the very strict wastewater discharge requirements that ensure water returned to the source will meet quality standards set by the regulatory authority.

A well-designed treatment facility includes a number of technologies such as lime neutralization, gypsum desaturation, clarification, filtration, biological treatment, and sludge thickening and dewatering.

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Level and Flow Applications for Wastewater Treatment 

  1. Water Equalization Tanks
    Wastewater is held in the equalization tank to allow solids to begin settling and to help maintain a more constant flow rate through the treatment unit.
    Continuous Level: ECLIPSE GWR; PULSAR Thru-Air Radar; ECHOTEL Ultrasonic; or E3 MODULEVEL Displacer Transmitters
    Point Level: ECHOTEL Ultrasonic Switch; or MAGNETROL Float and Displacer Switches
  1. Chemical Storage Tanks
    An array of chemicals are added from feed tanks to facilitate processing, adjust water quality and chemistry, and inhibit corrosion, scaling, or other conditions.
    Continuous Level: ECLIPSE GWR; PULSAR Thru-Air Radar; ECHOTEL Ultrasonic; Enhanced Jupiter® Magnetostrictive; or E3 MODULEVEL Displacer Transmitters
    Point Level: ECHOTEL Ultrasonic Switch; or Float and Displacer Switches
  1. Clarifier Level
    Wastewater influent slowly passes through clarifiers where sludge settles and floating material can rise to the surface to be skimmed off. The sludge is pumped away for further treatment.
    Continuous Level: ECHOTEL Ultrasonic; or ECHOTEL Non-Contact Transmitters
  2. Sludge Output Pump Protection
    A flow switch along the pump’s discharge piping will actuate an alarm and shut down the sludge pump when liquid flow drops below the minimum flow rate.
    Flow Alarm: THERMATEL TD1/TD2 Switches
  1. Sludge Tank Level
    Sludge that settles at the bottom of the clarifier is pumped to a sludge tank where level is monitored to control against incomplete discharge or dilution of the sludge.
    Continuous Level: ECLIPSE GWR; or ECHOTEL Ultrasonic Transmitters
  1. Treated Water Output
    Flow measurement is accomplished by a transmitter programmed to convert a level reading into “units of volume per time” as liquid passes through a flume or weir in an open channel.
    Continuous Level: ECHOTEL Non-Contact Ultrasonic Transmitters

SO2 Absorption System

The SO2-laden boiler flue gas stream passes through the electrostatic precipitators and into the absorber column. Inlet wash water used to prevent a buildup of solids in the inlet duct cools the flue gas as it enters the absorber.

The SO2 absorption process begins as the flue gas is scrubbed by the recirculating limestone slurry that also washes out the remaining pulverized fuel ash. (When particulate content of the gas is very high a venturi scrubber may be necessary to remove residual particles). The reagent suspension used in the scrubbing process is water containing approximately 20% fine-grained limestone. The slurry is pumped from the bottom of the absorber and is sprayed downward from nozzles arranged at separate levels in the absorber tower. Process chemistry transforms the recirculating slurry into gypsum and a portion of it is continuously pumped away to the primary and secondary hydrocyclones for separation and dehydration. Water removed from the scrubber purge stream is then pumped to the water treatment unit.

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Level and Flow Applications for the SO2 Absorption System

  1. Sump Overflow and Leak Detection
    Small sumps are positioned under tanks plant-wide to contain leaks or spills. Sumps also contain leaks from pumps, valves, and pipelines.
    Point level: Top Mounting Displacer Switch; ECHOTEL Ultrasonic Switch; Float-actuated Flood Level Switch; or THERMATEL TD1/TD2 Switch with a Spherical Tip
  1. Dibasic Acid (DBA) Storage Tank
    Organic acid additives improve scrubbing efficiency and reliability, and reduce tower operating cost. DBA is typically stored in a heated, agitated tank from where the acid is fed into a limestone slurry tank or directly into the scrubber.
    Continuous Level: PULSAR Thru-Air Radar with a 4-inch horn; or ECLIPSE Guided Wave Radar
  2. Oxidation Air Compressor
    The FGD absorber is equipped with an oxidation air system to provide oxidation air for the absorber vessel.
    Continuous Flow: THERMATEL TA2 Mass Flow Transmitter
    Flow Alarm: THERMATEL TD1/TD2 Switch; or F10 Mechanical Flow Switch for low flow cutoff
  3. Absorber Reaction Tank
    The recycle slurry falls from the spray zone into the reaction tank located at the base of the absorber. This tank is sized to provide sufficient residence time for all of the gypsum precipitation and limestone dissolution reactions to take place.
    Continuous Level: PULSAR Thru-Air Radar equipped with a 4- or 6-inch horn and mounted within a 30- to 40-foot stilling well; or ECLIPSE GWR with a flexible probe
  4. Demister Water Feed
    Process water flow to the demister affects the continuous and reliable running of the FGD system. Interrupted water feed can not only stop the FGD system but also shut down the power generating unit.
    Flow Alarm: THERMATEL TD1/TD2 Switch or F10 Mechanical Flow Switch for low flow cutoff
  5. Flue Gas
    Treated gas passes from the absorber through outlet dampers and into the chimney where the off-gases are discharged directly to the atmosphere. A mass flow meter may be used to measure the treated gas flow.
    Continuous Flow: THERMATEL TA2 Mass Flow Transmitter
  6. Absorber Purge Tank and Wastewater Outlet
    After the slurries pass through the hydrocyclones, wastewater is purged in the absorber purge tank to avoid accumulation of impurities in the system. The wastewater is then routed to water treatment for processing.
    Continuous Level: ECLIPSE GWR; PULSAR Thru-Air Radar; ECHOTEL Ultrasonic; Enhanced JUPITER Magnetostrictive; or E3 MODULEVEL Transmitters.
    Point Level: MAGNETROL Float and Displacer Level Switches
    Flow Alarm: THERMATEL TD1/TD2 Switch; or F10 Mechanical Flow Switch

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