Case Study · Industrial Emission Control
How a leading pre-baked anode producer achieved 99.5% desulfurization and 95% dust removal from combined calcination and sintering furnace off-gas — deploying an integrated limestone-gypsum FGD system (L/G=29.7, 5-layer spray) plus BLWESP-540 wet electrostatic precipitator to treat 400,000 Nm³/h of highly corrosive high-SO₂ off-gas while managing the critical CO explosion risk inherent in carbon materials processing.
Limestone-Gypsum FGD
SNCR Denitrification
Wet Electrostatic Precipitator
Carbon Anode Sintering
01 — Industry Background
Carbon Materials Production: A Strategically Critical Sector With Demanding Emission Challenges
Carbon materials are indispensable to the global industrial economy. Pre-baked anodes serve aluminium electrolytic smelting as the primary consumable electrode material; graphite electrodes serve electric arc furnace steelmaking; carbon-carbon composites serve aerospace, high-performance braking systems, and semiconductor manufacturing; and new carbon materials including graphene-based composites, carbon nanotubes, and carbon fibre are increasingly central to new energy vehicle components, energy storage systems, and lightweight structural materials.
The growth of renewable energy — solar panels, wind turbines, and grid-scale batteries — is driving sustained demand growth for high-quality carbon materials, particularly for storage electrode applications and lightweight structural components. The global carbon materials sector is simultaneously expanding its market scope and facing increasing regulatory pressure on its production processes, particularly on the high-SO₂ and high-particulate emissions from the calcination and sintering furnaces that are central to carbon material production.
The enterprise in this case study is a specialist pre-baked anode production enterprise, covering a 70,000 m² site with 8 calcination furnaces, 48 sintering furnaces, 2 lines of 150,000 t/year forming equipment plus associated environmental protection equipment (waste heat power generation waste heat power generation included), and annual production capacity of 300,000 pre-baked anodes. The facility is a provincial-level leading enterprise in the aluminium pre-baked anode sector, serving aluminium smelters as a critical supply chain component. With tightening environmental regulations, the facility’s flue gas purification system has become a strategic investment priority: limestone-gypsum wet FGD combined with wet electrostatic precipitation is now the standard configuration being deployed across the sector to address the multi-pollutant emission challenge from carbon material sintering furnaces.
The context of wet FGD for this application: limestone-gypsum FGD is one of the most widely applied flue gas desulfurization technologies globally. Its principal characteristics are: high desulfurization efficiency; wide applicability; relatively low limestone-to-calcium ratio; technically mature; and by-product gypsum can be sold as a commercial product. The system includes a flue gas system, SO₂ absorption system, absorbent preparation system, and gypsum treatment system. Wet electrostatic precipitation (WESP) is a high-efficiency flue gas purification technology primarily for treating fine particulate and acid mist in the post-FGD gas stream, reducing combined outlet pollutant concentration to below 5 mg/Nm³ in the best cases.
02 — Pollution Profile
Calcination + Sintering Combined Off-Gas: Extreme SO₂ at 6,000 mg/Nm³ Plus CO Explosion Risk
This project treats mixed off-gas from both the calcination furnaces and the sintering furnaces. After cooling the calcination furnace off-gas to a suitable temperature and capturing coke particulates, all furnace off-gas is combined and directed to the new desulfurization system and wet electrostatic precipitator for desulfurization and dust removal treatment. With the existing sintering furnace off-gas system also combined into the new system, cleaned flue gas is discharged directly from the stack via the induced draft fan. The treatment system uses one DCS control system and shares the fan system, slurry system, slurry preparation system, gypsum dewatering system, and slurry treatment system.
Two furnace types contribute to the combined flue gas stream: the calcination furnace (calcination furnace) and the sintering furnace . The combined standard flue gas volume is 230,000 Nm³/h; at process conditions (200°C), the volume is 400,000 Nm³/h. Natural gas fuel consumption is 4,500 m³/h. The critical emission challenge is the SO₂ concentration at 6,000 mg/Nm³ at the FGD inlet — one of the highest SO₂ inlet concentrations in any of the 30 case studies in this brochure. This extreme SO₂ loading drives the very high L/G ratio (29.7) and 5-layer spray configuration required in the FGD absorber.
CO explosion risk is the unique safety dimension of carbon materials processing that does not appear in other industrial off-gas treatment applications. Carbon calcination and sintering processes generate CO as a combustion by-product; if the CO concentration in the combined flue gas stream rises above the lower explosive limit (≤250 mg/Nm³ interlock threshold), there is a risk of explosion in the wet electrostatic precipitator where the high-voltage electrical field could ignite a flammable CO-air mixture. This requires: continuous CO monitoring at the wet ESP inlet linked to an automatic wet ESP shutdown interlock when CO exceeds the threshold.
| Parameter | Initial Concentration | Designed Outlet | EU IED / NER Limit |
|---|---|---|---|
| NOx | 50–100 mg/Nm³ | ≤100 mg/Nm³ | IED 2010/75/EU ≤100 mg/Nm³ |
| SO₂ (at FGD inlet) | 6,000 mg/Nm³ | ≤35 mg/Nm³ | Dutch Activities Decree ≤35 mg/Nm³ |
| Particulate matter (PM) | 100 mg/Nm³ | ≤5 mg/Nm³ | Dutch NER ≤5 mg/Nm³ |
| CO (wet ESP interlock) | Variable; explosion risk above 250 mg/Nm³ | Wet ESP auto-shutdown at 150–250 mg/Nm³ | Safety interlock required |
| Standard flue gas volume | 230,000 Nm³/h | — | — |
| Process flue gas volume | 400,000 Nm³/h at 200°C | — | — |
| Furnace exit temperature | 200°C (calcination); 170°C (sintering/desulfurization) | — | — |
| O₂ content | 12–15% actual (11% baseline) | — | — |
| Moisture content | 100 g/Nm³ | — | — |
03 — Treatment Solution
Limestone-Gypsum FGD + BLWESP-540 Wet ESP: Combined System Exploiting the Synergy Between Wet Scrubbing and Electrostatic Precipitation
The combination of limestone-gypsum wet FGD and wet electrostatic precipitation was selected because the two technologies are complementary and mutually reinforcing for this application. The FGD stage primarily removes SO₂ acid gas at high efficiency, with secondary co-capture of fine particulates in the spray droplets. The WESP stage primarily removes fine particulates and acid mist that pass through the FGD mist eliminators, achieving the sub-5 mg/Nm³ PM outlet that cannot be reliably achieved by FGD alone. The combination delivers ultra-low emission compliance for both SO₂ and PM that neither technology can achieve individually in this application context.
The project constructs one new desulfurization tower and one new wet electrostatic precipitator. The control system uses one DCS system shared across the two unit operations, with shared fan, slurry, slurry preparation, gypsum dewatering, and slurry treatment systems. The process flow subsystems are: fan system; CO monitoring system; slurry absorption system; slurry preparation system; gypsum dewatering system; process water system; and electrical system.
FGD Absorber Tower (φ8.4–6.4 m, 400,000 Nm³/h)
The limestone-gypsum FGD absorber is specified for the full combined flue gas volume and the extreme SO₂ inlet. Key parameters: flue gas volume 400,000 m³/h; flue gas temperature 200°C at inlet; SO₂ inlet concentration 6,000 mg/Nm³; SO₂ outlet concentration 35 mg/Nm³; calcium-to-sulfur ratio 1.03; gas velocity <3.5 m/s; tower internal diameter φ8.4/6.4 m (stepped); absorption tower height 31.5 m; liquid-to-gas ratio 29.7; spray layers 5; single pump flow 1,400 m³/h; slurry settling time 5 h; limestone operating consumption 2,150 kg/h (maximum); gypsum production 3,850 kg/h (maximum, i.e. approximately 3.85 t/h); gypsum moisture content ≤15%; mist eliminators: 2-layer screen type; intermediate limestone storage capacity 180 m³ (7-day autonomy at 180 m³). The FGD slurry material is 2205 duplex stainless steel, selected for its corrosion resistance to the high-chloride, high-sulfate slurry environment of carbon materials processing off-gas.
Wet Electrostatic Precipitator (BLWESP-540, 320,000 Nm³/h)
Post-FGD gas at approximately 60°C enters the BLWESP-540 wet electrostatic precipitator. The WESP captures fine particulates, acid mist, and sub-micron aerosols not removed by the FGD mist eliminators. Key parameters: WESP model BLWESP-540; tower-external configuration; gas flow bottom-entry, top-exit (direct through-flow); purification efficiency ≥95%; inlet mixed pollutant concentration 100 mg/m³; outlet mixed pollutant concentration 5 mg/m³; body resistance 300 Pa; treatment flue gas volume 320,000 m³/h; flue gas temperature <60°C; tube panel dimensions 360×6,000 mm; anode tube height 6 m; anode tube count 540; field-enhanced gas velocity 1.46 m/s; device dimensions 11,500×7,500×13,000 mm; device height 18,000 mm; design pressure ±5,000 Pa; power supply model BLEMG-2K; power supply count 2 units; average power 200 kW.
Process Flow Summary
Furnaces
8 units
Coke Dust
Capture
Furnaces
48 units
FGD ⭐
99.5% SO₂
BLWESP-540
≥95% PM
→ Stack
⭐ New equipment in this project. CO monitoring interlock on wet ESP (auto-shutdown at 150–250 mg/Nm³ CO) protects against explosion risk throughout the system.
Key Equipment and Operating Cost Summary
| Item | Specification |
|---|---|
| FGD absorber tower | φ8.4/6.4 m; H=31.5 m; L/G=29.7; 5 spray layers; 1,400 m³/h pump; 2205 duplex SS slurry material |
| FGD limestone consumption (max) | 2,150 kg/h; annual cost approx. 672 ten-thousand RMB (400 RMB/t) |
| FGD gypsum production (max) | 3,850 kg/h (≈3.85 t/h); moisture ≤15% |
| Wet ESP | BLWESP-540; 320,000 m³/h; ≥95%; 540 anode tubes φ360×6,000 mm; 11,500×7,500×13,000 mm; BLEMG-2K |
| Circulating pumps (FGD) | 5 units (A/B/C/D/E); 132/160/185/185/200 kW; total installed approx. 862 kW for circulation alone |
| Induced draft fans | 350×2 kW (1 duty + 1 standby); 6,000 Pa; φ3,220 mm duct |
| Max system running power | 1,664.95 kW actual; 1,959.45 kW total installed |
| Annual electricity cost (8,000 h) | Approx. 479.5 ten-thousand RMB equivalent (0.36 RMB/kWh) |
| Annual limestone cost | Approx. 672 ten-thousand RMB (2,150 kg/h at 400 RMB/t) |
| CO interlock threshold (wet ESP) | Auto-shutdown at CO 150–250 mg/Nm³ at wet ESP inlet (explosion prevention) |
04 — Core Advantages
Five Reasons Why Limestone-Gypsum FGD + Wet ESP Is Optimal for Carbon Anode Sintering Off-Gas
- ✓
FGD + Wet ESP Combination Achieves What Neither Technology Can Alone: Wet FGD at 99.5% efficiency reduces SO₂ from 6,000 mg/Nm³ to 35 mg/Nm³ — but the FGD also generates residual fine calcium sulfate crystallite mist that carries through the mist eliminator and would give a PM outlet of 20–50 mg/Nm³ at the stack without further polishing. The wet ESP captures these fine crystallites and acid mist droplets to deliver the ≤5 mg/Nm³ PM outlet that the EU IED BAT limit demands. The FGD does the heavy SO₂ removal; the wet ESP does the final PM polishing. Each stage would fail to meet the full compliance requirement if operating alone, but together they achieve ultra-low compliance across both parameters. - ✓
L/G=29.7 and 5-Layer Spray Are Correctly Specified for 6,000 mg/Nm³ SO₂ Inlet at 99.5% Removal: The liquid-to-gas ratio of 29.7 — among the highest of any FGD system described in the 20 case studies reviewed — is the direct consequence of the 6,000 mg/Nm³ SO₂ inlet concentration combined with the 99.5% removal requirement. At standard power plant FGD L/G ratios of 8–15, the SO₂ partial pressure in the gas phase at 6,000 mg/Nm³ inlet would exceed the absorption capacity of the liquid phase before the outlet target was reached. The 5-layer spray and L/G=29.7 provide the extended gas-liquid contact residence time needed to achieve the thermodynamic SO₂ removal duty. A system designed for power plant conditions and simply enlarged in size would not work correctly for this application without specifically reoptimising the L/G ratio and spray layer count. - ✓
2205 Duplex Stainless Steel for FGD Slurry Wetted Parts Addresses Carbon Processing Off-Gas Corrosivity: Carbon anode sintering off-gas carries organic compounds, chloride residues, and high sulfate concentrations that create an exceptionally aggressive corrosion environment for the FGD slurry loop. Standard 316L stainless steel used in power plant FGD slurry systems would experience accelerated corrosion and premature failure in this environment. 2205 duplex stainless steel, with its higher chromium (22%), molybdenum (3.1%), and nitrogen content compared with 316L, provides superior resistance to pitting, crevice corrosion, and stress corrosion cracking in the chloride-rich, high-sulfate FGD slurry environment of carbon processing applications. This materials upgrade adds to the capital cost but is essential for achieving the designed service life. - ✓
CO Interlock on the Wet ESP Provides Essential Safety Protection Against Explosion Risk: The wet electrostatic precipitator operates at high voltage (BLEMG-2K generator, 200 kW average power). Carbon processing off-gas contains CO at concentrations that can approach or exceed the lower explosive limit in the wet ESP chamber if the furnace combustion becomes unstable. The CO monitoring system at the wet ESP inlet, linked to an automatic wet ESP shutdown interlock at 150–250 mg/Nm³ CO, is the primary safety barrier between a CO accumulation event and an explosion in the wet ESP. This interlock must be treated as a life-safety-critical system, maintained and tested on the same schedule as fire suppression and gas detection systems. - ✓
Gypsum By-Product at 3.85 t/h Generates Significant Commercial Value: At 3,850 kg/h maximum gypsum production, this FGD system generates approximately 30.8 t of gypsum per 8-hour operating day — a commercially significant volume. If the gypsum quality meets the construction material specification under EN 13279-1 (CaSO₄·2H₂O purity ≥90%, chloride ≤0.01%, moisture ≤15%), sales revenue from gypsum delivery to wallboard manufacturers or cement producers can substantially offset the 2,150 kg/h limestone reagent cost. Establishing a gypsum supply agreement before commissioning, and implementing a gypsum quality monitoring programme from startup, is as important commercially as the SO₂ compliance programme.
05 — Operational Results
Verified Compliance Data and Annual Cost Summary
Annual operating costs: electricity at 1,664.95 kW actual (0.36 RMB/kWh, 8,000 h/year) = approximately 479.5 ten-thousand RMB; limestone at 2,150 kg/h (400 RMB/t, 8,000 h) = approximately 672 ten-thousand RMB; limestone is by far the dominant reagent cost item. Gypsum production at 3,850 kg/h at 8,000 h/year = approximately 30,800 tonnes/year, which can generate substantial sales revenue to offset reagent cost depending on local gypsum market prices.
06 — Implementation Cautions
Six Critical Engineering and Safety Considerations for Carbon Anode Off-Gas Treatment
- 🚫
CO explosion risk in the wet ESP is a life-safety hazard — the CO interlock is not optional and must never be bypassed: Carbon processing off-gas contains CO at concentrations that can approach explosive levels in the wet ESP if combustion becomes unstable. The high-voltage field of the wet ESP provides an ignition source. When CO at the wet ESP inlet reaches 150–250 mg/Nm³, the automatic wet ESP shutdown interlock must activate reliably every time. This interlock must be: tested at the specified frequency (at minimum monthly); maintained by a qualified electrical instrument technician; never bypassed for any operational reason; and connected to the facility’s central safety monitoring system with alarm notification to on-duty management. The response measures include: linking the flue gas desulfurization system inlet monitoring CO concentration to the wet ESP operating control system, shutting down the wet ESP when gas CO concentration reaches 150–250 mg/Nm³; and utilising the surrounding embankment, dikes, and collection pools for emergency recovery as secondary containment. - ⚠️
Flue gas corrosivity combined with equipment service life shortfalls require proactive materials management: The second documented risk is that flue gas corrosivity is strong and equipment service life does not reach design requirements. The 2205 duplex stainless steel specification for FGD slurry wetted parts is a direct response to this risk. However, material specification alone is insufficient: corrosion monitoring (wall thickness measurement at representative locations, at minimum annually from year 2 onward), pH management of the FGD slurry loop (maintaining pH within the specified window to prevent under-pH acid attack and over-pH scale deposition), and chloride concentration control in the slurry loop (bleed-and-dilute to prevent chloride buildup above the stress corrosion cracking threshold) are all required operational disciplines. - ⚠️
Production process pipe leaks due to pipe cracking cause wastewater overflow and environmental contamination of the circulation environment: The third documented risk is pipe cracking leading to wastewater overflow. The combination of high-sulfate, high-chloride, high-temperature slurry cycling through pipes at up to 1,400 m³/h pump flow creates significant mechanical stress. Implement weekly visual inspection of all slurry pipework; include FGD slurry lines in the annual planned maintenance scope for non-destructive thickness testing; maintain a spare parts inventory for standard pipe sections and fittings; and ensure all secondary containment (drip trays, bund walls, emergency collection pools) are maintained in serviceable condition to capture any overflow before it reaches the environment. - ⚠️
Very high limestone consumption (2,150 kg/h) requires robust supply chain and storage management: At 2,150 kg/h maximum limestone consumption with 180 m³ storage (7-day autonomy at full load), limestone supply must be managed as a production-critical input. The supply contract must guarantee delivery frequency. Maintain a minimum stock trigger level (3-day remaining supply) that initiates automatic purchase orders. For any unplanned supply interruption, have a documented contingency procedure that includes production throughput reduction proportional to available limestone stock. - ⚠️
Gypsum quality must be proactively managed to maintain commercial reuse classification — carbon process contaminants can affect gypsum purity: Carbon anode sintering off-gas may carry organic compound residues and coke particulates that absorb into the FGD slurry, potentially contaminating the gypsum product with organic compounds, heavy metals from electrode raw materials (petroleum coke), or elevated chloride content. Monthly gypsum quality testing covering CaSO₄·2H₂O purity, moisture, chloride, and heavy metal content is required to confirm the gypsum remains within the commercial reuse specification. If carbon-related contamination is detected, the gypsum must be reclassified as industrial waste and disposed of through licensed contractors, eliminating the revenue credit and adding disposal cost. - ⚠️
The DCS control system shared between FGD and wet ESP must have independent safety interlocks that cannot be overridden by the process control logic: Because the FGD and wet ESP share one DCS system, there is a risk that a DCS failure or software logic error simultaneously affects both treatment stages. The CO interlock in particular must be implemented as a hardware safety relay (not a software PLC logic path) to ensure it operates independently of any DCS state. Similarly, the wet ESP high-voltage power supply shutdown on CO alarm must be a hardwired interlock that activates regardless of the DCS status. Both interlocks must be verified by the electrical safety commissioning team before any production operation begins.
07 — Engineering Takeaways
Four Lessons from This Carbon Materials FGD + Wet ESP Project
- !
CO explosion risk in wet ESP is the unique and critical safety differentiator for carbon materials applications — it must be treated as a life-safety issue, not a compliance issue. The wet ESP CO interlock is the single most important safety system in this installation. Carbon materials processing is unique among the twenty case studies in generating CO at concentrations that can cause explosion in the high-voltage wet ESP environment. Engineers designing wet ESP systems for carbon processing applications who fail to implement the CO interlock as a hardwired life-safety system are creating an unacceptable explosion risk. This is not a question of regulatory preference — it is a question of preventing a potentially fatal explosion. - 2
6,000 mg/Nm³ SO₂ is not simply a “higher concentration” version of the 2,800 mg/Nm³ steel kiln case or the 4,645 mg/Nm³ lithium carbonate case — it requires a fundamentally different FGD design with L/G=29.7 and 5 spray layers. Each doubling of SO₂ inlet concentration with the same outlet target requires approximately a 20–30% increase in L/G ratio to maintain the thermodynamic absorption driving force. At 6,000 mg/Nm³ inlet with 35 mg/Nm³ outlet target (99.4% removal), the system has effectively reached the upper practical limit of limestone-gypsum FGD process parameters. Any future increase in SO₂ inlet beyond 6,000 mg/Nm³ would require either a two-stage absorber system or a different desulfurization technology entirely. - 3
2205 duplex stainless steel for FGD wetted parts in carbon processing applications is not a premium upgrade — it is the minimum viable specification for adequate service life. The combination of high SO₂ (producing sulfate), high organic compounds from carbon sintering, and high chloride from raw material impurities creates a slurry environment that attacks 316L stainless steel through stress corrosion cracking within 2–3 years. 2205 duplex stainless steel — specified throughout this installation for all slurry-wetted FGD components — is the material grade that provides adequate resistance to this specific corrosion environment. Accepting a lower-grade material specification to reduce initial capital cost will result in premature equipment failure within 2–3 years, creating replacement costs far exceeding the initial saving. - 4
Gypsum at 3.85 t/h is a major revenue opportunity that justifies investment in gypsum quality management from day one. Most FGD system operators treat gypsum as a compliance by-product — something to be disposed of at minimum cost. At 3.85 t/h production, this installation generates approximately 30,800 tonnes of gypsum per year. If this qualifies as commercial-grade FGD gypsum (which requires active quality management to confirm and maintain), the revenue from gypsum sales can generate returns that materially offset the dominant limestone reagent cost of 672 ten-thousand RMB per year. Treating the gypsum quality programme as a commercial enterprise, not just a waste characterisation obligation, is the difference between an FGD system that pays for part of its own operating cost and one that is a net cost centre.
08 — Frequently Asked Questions
Carbon Anode Sintering Off-Gas FGD + Wet ESP Treatment: Ten Questions Answered
Questions from environmental permit managers, process engineers, and HSE teams at carbon materials, graphite electrode, and pre-baked anode manufacturing facilities planning FGD and wet ESP emission control upgrades under EU IED / Dutch Activities Decree requirements.
Ready to Solve Your Carbon Materials High-SO₂ Emission Challenge?
Explore the Full Range of Industrial Emission Control Solutions
From limestone-gypsum FGD and wet electrostatic precipitation for carbon anode sintering furnaces to regenerative thermal oxidation systems for industrial VOC abatement, our engineering team delivers EU IED–compliant solutions for the most demanding carbon materials emission control requirements.
