Flue Gas Desulfurization Scrubber

Overview

A flue gas desulfurization (FGD) scrubber removes sulfur dioxide from the combustion gas of coal- and oil-fired boilers before it reaches the stack. The dominant design is the wet limestone forced-oxidation absorber, which contacts the flue gas with a fine spray of limestone slurry inside a tall vessel. Sulfur dioxide dissolves into the droplets, reacts with calcium carbonate, and is oxidized to calcium sulfate. The end product is gypsum, the same mineral used to make wallboard, so a well-run plant turns a regulated pollutant into a saleable solid. Removal efficiency runs from 95 to 99 percent, bringing outlet SO2 from inlet values of 500 to 3,000 mg/Nm3 down to 10 to 100 mg/Nm3.

The system handles very large gas volumes. A single absorber on a 600 MW unit treats on the order of 2,000,000 Nm3/h. The major equipment groups are the Absorber Tower Assembly where the reaction happens, the Spray Header and Nozzle System that contacts gas and liquid, the Recycle Slurry Pump Set that move the slurry, the Forced Oxidation Air System that finishes the chemistry, the Limestone Reagent Preparation that makes the limestone slurry, the Mist Eliminator System that cleans up the gas, the Gas Handling System ductwork, and the Gypsum Dewatering System that recovers gypsum.

How it works

Flue gas enters the absorber at 120 to 160 degC through the inlet duct and rises through the spray zone. Recycle pumps lift slurry from the reaction tank to several spray levels, where silicon carbide nozzles atomize it into a dense rain falling counter to the gas. The contact happens at a liquid-to-gas ratio (L/G) of 8 to 20 liters of slurry per cubic meter of gas, which is the main lever for removal efficiency: more spray levels in service raise both removal and pumping power.

The chemistry proceeds in stages. Sulfur dioxide first dissolves into the droplet and forms sulfurous acid, then reacts with dissolved limestone:

SO2 + H2O gives H2SO3, and CaCO3 + H2SO3 gives CaSO3 + CO2 + H2O.

The calcium sulfite is then oxidized in the reaction tank. Blowers in the Forced Oxidation Air System system sparge air through lances near the agitators, supplying 1.5 to 2.5 times the stoichiometric oxygen:

CaSO3 + 1/2 O2 + 2H2O gives CaSO4 dot 2H2O.

Forced oxidation drives the product fully to gypsum rather than a sulfite-sulfate mixture, which dewaters poorly and has no resale value. The reaction tank is held at pH 5.4 to 5.8. Below that the limestone dissolves fast but absorption falls off; above it the limestone is underused and gypsum scaling worsens. A Slurry pH Analyzer trims the reagent feed to hold the band.

Subsystems

The Absorber Tower Assembly is a rubber-lined or alloy-clad steel vessel with an integral lower reaction tank. Side-entry agitators keep gypsum crystals and limestone in suspension so the tank stays a uniform 10 to 15 percent solids. The Recycle Slurry Pump Set are the largest rotating equipment in the system, each a rubber-lined centrifugal pump of 250 to 700 kW; they carry ball-bearing assemblies and an oil-seal at the bearing housing, with a pressure-sensor on each discharge.

The Limestone Reagent Preparation system grinds limestone in a wet ball mill to 90 percent below 44 microns, fine enough to dissolve fast in the short droplet residence time. A hydrocyclone classifier returns coarse particles for regrinding. The Mist Eliminator System uses two stages of polypropylene chevron vanes to strip droplets from the saturated gas, with a wash system that prevents gypsum scale from blinding the vanes. Downstream, the Gypsum Dewatering System system concentrates the bleed slurry in hydrocyclones and dries it on a vacuum belt filter to roughly 50 percent solids, a stackable or saleable cake. The Control and Instrumentation System tie the loops together, with relay outputs to pumps and dampers, a wire-bundle to the field, and an lcd-panel at the local operator station.

Materials

Every wetted surface sees a saturated, chloride-bearing acid, so material selection drives both cost and reliability. The absorber shell is carbon steel protected by a glass-flake vinyl ester or rubber lining, or alternatively clad with nickel alloys such as C-276 in high-chloride service. Spray nozzles are silicon carbide for abrasion resistance. Recycle pump wetted parts are rubber-lined with high-chrome iron impellers. Chevron demister vanes are polypropylene. The wet stack downstream of the absorber needs an acid-resistant liner because the gas leaves the absorber saturated and below its acid dew point.

Variants

Limestone forced-oxidation is the most common design, but several alternatives exist. Magnesium-enhanced lime systems use a more reactive reagent and run smaller absorbers, at higher reagent cost. Seawater scrubbing skips the limestone entirely and uses the natural alkalinity of seawater, suited to coastal plants but producing no saleable byproduct. Spray dryer absorbers and circulating dry scrubbers are dry or semi-dry processes that produce a dry waste and reach lower removal efficiency, used where water is scarce or the unit is small. Among wet absorbers, designs differ by whether they add a Perforated Contact Tray for extra contact and by how many spray levels they carry.

Operation and maintenance

The two operating setpoints that matter most are reaction tank pH and slurry density. A Slurry Density Meter sets the gypsum bleed rate to the dewatering system so the tank inventory stays in range. Inlet and outlet SO2 analyzers report removal efficiency continuously for emissions compliance. The common maintenance items are spray nozzle erosion, recycle pump liner and impeller wear, demister scaling, and lining damage in the absorber. Nozzles and pump liners are designed as replaceable wear parts changed on a planned outage. Gypsum scale on the demister is managed by the wash sequence; if wash water flow drops, the vanes blind, pressure drop climbs, and droplet carryover contaminates the clean gas. Lining inspection through the manways at each outage catches pinholes before the carbon steel beneath is attacked.