Common FGD Scrubber Problems and Immediate Symptoms
SO₂ removal efficiency dropping below 90% often indicates poor reagent contact from a low liquid-to-gas (L/G) ratio or compromised spray nozzle distribution. This can also be caused by a drop in slurry alkalinity, often due to inconsistent limestone feed or a change in the sulfur content of the fuel. A sudden increase in system pressure drop, specifically readings exceeding 250 Pascals (Pa) across the absorber tower, is a primary and immediate indicator of mist eliminator fouling or plugging. In practice, a pressure drop increase of just 50 Pa can signal the beginning of a fouling event that requires attention. A visible white plume from the stack is not merely a visual nuisance; it signifies entrained slurry droplets due to mist eliminator failure, poor demisting, or excessive gas velocity. This plume can also indicate sub-cooling of the gas stream, which leads to condensation and the formation of a visible aerosol. Pump cavitation noises and reduced slurry flow rates frequently point to scaling in pipework or pump wear from abrasive slurries with solids content consistently above 18–20% by weight. The characteristic sound of cavitation—a loud rattling or grinding noise—is a clear auditory clue that requires immediate investigation to prevent severe pump damage. Recognizing these symptoms together allows for quick diagnosis before minor issues become forced outages. Correlating a slight efficiency drop with a minor pressure drop increase can help schedule a water wash during a minor load reduction, avoiding a full shutdown.
Mist Eliminator Fouling: Causes and Field-Tested Fixes
Mist eliminator fouling occurs when the suspended solids concentration in the recirculation slurry exceeds 18–20% wt, leading to particulate deposition and plugging of the vane passages, as noted in the EPA Limestone FGD Handbook. The geometry of the mist eliminator also influences fouling; chevron-style designs with tight passages are more prone to plugging than more open designs. High chloride levels, typically above 20,000 mg/L, drastically promote tenacious scale formation on vane surfaces by reducing gypsum solubility; counter this by maintaining a consistent blowdown rate of 5–8% of the total recirculation flow. Verify the blowdown system itself is not clogged, as a failed blowdown valve can quickly lead to a chloride concentration spike. For offline cleaning, a high-pressure water wash protocol using 30–50 bar pressure with 15° fan spray nozzles mounted on 300 mm spacing effectively removes most soft deposits without damaging the eliminator blades. The spray pattern should be systematically moved across the entire face of the eliminator to ensure complete coverage and avoid leaving hardened deposits in unwashed zones. In severe scaling cases, a low-concentration (3–5%) citric acid wash circulated for 4–6 hours can dissolve calcium sulfite scales, but this requires careful pH monitoring and material compatibility checks. Always perform a water rinse to neutral pH before returning the unit to service to prevent accidental corrosion. For persistent issues, consider a Zhongsheng's integrated FGD scrubber system with low-scaling design that optimizes flow distribution to minimize fouling. Proactive online monitoring, such as trending pressure drop versus unit load, can provide an early warning and allow for planning corrective actions during scheduled maintenance windows.
Scaling in Limestone Slurry Systems: Prevention and Removal
Scale forms aggressively when slurry temperature exceeds 65°C or when pH fluctuates outside the optimal 5.2–5.6 range for gypsum crystallization, a critical threshold from EPRI FGDSolver data. Temperature excursions are common during unit startup, shutdown, or sudden load increases when the gas bypass system is not properly managed. The use of organic scale inhibitors like EDTA, dosed at 50–100 ppm into the reagent makeup water, can reduce scaling by up to 60% in systems with high calcium hardness. These inhibitors work by sequestering calcium ions and distorting the crystal structure of scale, making it softer and less adherent to surfaces. For existing scale, an effective chemical cleaning procedure involves circulating a 5% citric acid solution with a corrosion inhibitor at 50°C for 4–6 hours, followed by a thorough water rinse and neutralization before returning the system to service. The efficacy of the cleaning process can be monitored by tracking the concentration of calcium in the cleaning solution, which will rise as scale is dissolved. Implementing an PLC-controlled lime dosing system for stable FGD pH management is the most reliable method to prevent the pH excursions that cause scale. This system should be integrated with real-time flue gas SO₂ analyzers to create a feedforward control loop, adjusting the limestone feed rate in anticipation of load changes to maintain a stable chemical environment.
| Scale Type | Common Location | Primary Cause | Corrective Action |
|---|---|---|---|
| Gypsum (CaSO₄·2H₂O) | Spray headers, pumps | pH > 5.8, low oxidation | Acid wash, improve oxidation |
| Calcium Sulfite (CaSO₃) | Mist eliminators, tanks | O₂ < 3.5% in slurry, pH < 5.0 | Increase air sparging, adjust pH |
| Calcium Fluoride (CaF₂) | All wetted surfaces | High fluoride in limestone | Improve reagent quality, inhibitor |
Corrosion in Wet Stack and Absorber Sections
Stack lining blistering and failure occur due to moisture penetration into FRP or rubber linings under continuous wet operation, leading to underlying metal corrosion, a well-documented issue in EPA Nepis reports. This is exacerbated by thermal cycling, which causes the liner to expand and contract, creating micro-fissures for corrosive agents to penetrate. For zones experiencing consistently low pH (< 4) and elevated temperature (> 50°C), specify corrosion-resistant alloys like C-276 or non-metallic materials such as fiberglass-reinforced vinyl ester (FRVE) for new installations or replacements. The initial higher capital cost of these materials is almost always justified by their dramatically extended service life and reduced maintenance requirements. A quarterly borescope inspection protocol for the absorber tower outlet, mist eliminator supports, and upper stack sections allows for early detection of lining cracks, blisters, or pitting before they necessitate extensive repairs. Documenting these inspections with photos creates a valuable timeline for tracking the rate of degradation and planning for repairs during scheduled outages. Maintaining chloride levels below the 20,000 mg/L threshold and controlling cyclical wet-dry conditions are operational keys to maximizing the service life of all internal components. Installing improved insulation and heat tracing on stack exteriors can help minimize the internal condensation that accelerates corrosive wear, particularly in colder climates.
Optimizing Oxidation and Gypsum Quality
Poor oxidation, indicated by dissolved oxygen (DO) levels below 3.5% v/v in the sparger zone, leads to gypsum byproduct containing more than 10% unreacted limestone, rendering it unsellable and creating dewatering problems. Inadequate oxidation air pressure or a partially clogged sparger ring are frequent mechanical root causes for this issue. Maintain slurry density within a range of 1.12–1.15 g/cm³ and ensure a residence time greater than 6 hours in the reaction tank to facilitate optimal gypsum crystal growth and ease of dewatering. Larger, well-formed gypsum crystals (100–200 microns) have a lower surface area and release water more easily in the dewatering equipment, resulting in a drier cake. For accurate DO monitoring, place the probe at a depth of approximately 1.5 meters above the tank floor to avoid false readings from the air sparger plume or settled solids. These probes require regular calibration and maintenance to ensure they provide reliable data to the control system. Consistent oxidation is critical for producing marketable gypsum and can be managed with equipment like a high-capacity filter press for byproduct handling. Furthermore, optimizing the oxidation air flow rate based on the real-time SO₂ load can result in significant energy savings, as the oxidation air blowers are major consumers of power within the FGD system.
Frequently Asked Questions
What is a mist eliminator in FGD?
A mist eliminator is a critical device installed in the absorber tower designed to remove entrained liquid droplets from the cleaned flue gas, typically achieving > 99% droplet capture efficiency at design gas velocity. They are most commonly constructed from polypropylene or other durable plastics and are configured in a chevron or mesh style to maximize surface area for droplet impingement.
What removes sulfur dioxide from flue gas?
In wet limestone FGD systems, SO₂ is absorbed into a slurry of calcium carbonate (CaCO₃), forming calcium sulfite, which is then oxidized into gypsum (CaSO₄·2H₂O), a solid byproduct.
What is the optimal pH range for limestone FGD scrubber operation?
The optimal pH range for limestone slurry in a wet FGD absorber is 5.2 to 5.6. This range balances high SO₂ absorption efficiency with low limestone consumption and minimizes the risk of calcium sulfite scaling on wetted surfaces.
How often should mist eliminators be inspected?
Borescope or visual inspections of mist eliminators should be performed quarterly, with pressure drop trend monitoring done continuously. A sustained increase in pressure drop across the absorber is the leading indicator that the mist eliminator requires offline cleaning or replacement.
What causes high chloride levels in FGD scrubber slurry?
High chloride levels result from the combustion of chlorine-containing fuels, infiltration of cooling water leaks, and inadequate blowdown. Chlorides above 20,000 mg/L increase the risk of mist eliminator scaling, accelerate stainless steel pitting, and reduce overall gypsum purity.
How is oxidation air optimized in a wet FGD system?
Oxidation air is optimized by adjusting the air-to-SO₂ molar ratio to roughly 2:1, maintaining dissolved oxygen above 3.5% v/v, and verifying that sparger rings are clean and free of calcium carbonate deposits. Modern systems use SO₂ load feedback to trim blower output and reduce parasitic power consumption.
What is the difference between forced oxidation and natural oxidation in FGD?
Natural oxidation relies on residual dissolved oxygen in the slurry and produces calcium sulfite-rich waste, while forced oxidation uses sparged air to convert sulfite to gypsum, producing a stable, marketable byproduct and reducing scaling within the absorber vessel.
Can FGD scrubber scaling be prevented without chemical inhibitors?
Operational controls can reduce but not eliminate scaling. Maintaining slurry pH between 5.2 and 5.6, controlling slurry temperature below 65°C, and ensuring consistent oxidation are baseline practices. In high-hardness makeup water conditions, however, organic inhibitors such as EDTA at 50–100 ppm remain the most cost-effective prevention method.
What materials are recommended for FGD absorber internals?
Alloy C-276, alloy 625, fiberglass-reinforced vinyl ester (FRVE), and dual-laminate constructions are widely specified for FGD internals, including spray headers, tank walls, and outlet ducts. Material selection should weigh chloride concentration, temperature, pH swings, and expected erosion rates from slurry particulates.
How is gypsum quality measured for wallboard-grade sale?
Wallboard-grade gypsum typically requires purity above 95%, chloride content below 100 ppm, moisture content below 10%, and a consistent particle size distribution with a mean diameter between 100 and 200 microns. Filter press cake from a well-tuned absorber can meet these specifications with minimal additional processing.
