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 in tandem allows for rapid diagnosis before minor issues escalate into 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.

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.