Case Study · Industrial Emission Control
How a waste salt resource recovery facility treating 50,000 t/year of hazardous industrial salts achieved 87% desulfurization, 80% denitrification, and 98.8% dust removal compliance — deploying dynamic closed-loop adaptive control technology to manage the extreme complexity and variability of SPI incineration furnace off-gas containing acid gases, heavy metals, dioxins, and corrosive alkali compounds simultaneously.
Dry + Wet Desulfurization
SNCR 反硝化
Hazardous Waste Emission Control
Adaptive Closed-Loop Emission Control
01 — Industry Background
Waste Salt Treatment: An Emerging Sector With Complex Multi-Pollutant Incineration Challenges
The global chemical industry — encompassing salt manufacturing, chlor-alkali production, fine chemicals, and specialty chemicals — generates substantial volumes of industrial waste salt as a by-product of chemical synthesis reactions, electrolytic processes, and wastewater treatment operations. These waste salts contain diverse impurities: heavy metals, organic compounds, residual reagents, and complexing agents that classify them as hazardous waste streams in most regulatory jurisdictions.
Waste salt treatment has emerged as an independent industrial sector focused on converting hazardous waste salts into reusable industrial salt or safely managed residues. The driving principle is “reduction, recycling, and harmlessness” — minimising waste volume, recovering resource value where possible, and eliminating toxicity through controlled high-temperature incineration before resource recovery or disposal. Thermal incineration in SPI (Spinning Pyrolysis Incinerator) furnaces at temperatures exceeding 1,100°C is the primary processing technology, with residence times of at least 2 seconds at temperature to ensure destruction of dioxins, furans, and other persistent organic pollutants.
The flue gas produced by SPI waste salt incineration is among the most chemically complex off-gas streams in industrial manufacturing: simultaneously containing acid gases (HCl, HF, SO₂), heavy metals (from metal-contaminated waste salts), organic micropollutants (dioxins, furans from incomplete combustion of organics), fine particulates, NOx from high-temperature air reactions, and CO from combustion chemistry — all at concentrations and variability levels that challenge conventional single-technology treatment approaches. The Hazardous Waste Incineration Pollution Control Standard (EU Waste Incineration Directive 2000/76/EC, now incorporated into IED 2010/75/EU Chapter IV) applies, imposing stringent multi-pollutant limits and requiring continuous emission monitoring.
“Waste salt incineration off-gas is not simply a more complex version of industrial boiler flue gas. It is a fundamentally different pollution control problem: the pollutant concentrations change dramatically across each incineration batch cycle, the chemical composition shifts depending on which waste salt feedstock is being processed, and the combination of HCl, dioxins, heavy metals, and high-SO₂ simultaneously requires every major treatment technology to work in coordination. Static control parameters cannot cope — only dynamic closed-loop adaptive control succeeds.”
— Engineering Technical Summary, Waste Salt Treatment Industry Dust Removal / Desulfurization / Denitrification Project
02 — Pollution Profile
SPI Incineration Furnace Off-Gas: Six Simultaneous Pollutant Categories with Extreme Concentration Variability
The facility operates a waste salt treatment production line with SPI incineration furnace capacity for 50,000 t/year of hazardous waste salt. The operational scope includes production and sales of 32% sodium hydroxide solution, liquid ammonia, fluorine gas, salt acid, hypochlorous acid sodium, dimethyl sulfoxide, methylene chloride, carbon tetrachloride, and other high-risk chemical products (excluding dangerous chemical products), as well as chemical industrial products (non-hazardous chemical). The enterprise also operates steam generation, power supply, water purification, softened water, and industrial water production, alongside sales of coal ash, gypsum, fly ash, slag, and stone gypsum.
The waste salt incineration off-gas is fired on a combination of natural gas and waste salt feed. Raw flue gas exits the SPI furnace at 150–180°C and enters the pre-treatment tower for NaOH solution spray absorption, cooling, and mist elimination, before being directed by a booster fan to the absorption tower for further NaOH solution spray absorption and mist elimination, entering the stack through online monitoring for discharge. This first-generation treatment was supplemented by the integrated dust removal, desulfurization, and denitrification upgrade described in this case study.
The six simultaneous pollution challenges of waste salt SPI incineration off-gas are:
- Complex composition, high variability: Waste salt off-gas simultaneously contains NOx, fine particulates, CO, dioxins, and other pollutants. Flue gas is highly corrosive. Processing technology is complex, and all aspects of each processing stage temperature must be controlled precisely.
- High dust loading with high alkali metal content: SPI furnace off-gas carries significant fine particulate matter with elevated potassium and sodium salt content, simultaneously high corrosivity, requiring a combined dual-combustion chamber + waste heat boiler + quench cooling + dry desulfurization + bag filter + wet acid desulfurization treatment chain.
- Secondary combustion chamber temperature control critical for dioxin destruction: The secondary combustion chamber temperature must be precisely controlled; the waste heat boiler design must control outlet temperature, adjusting equipment operating parameters and process parameters based on monitored flue gas temperature.
- SO₂ at 600 mg/Nm³ inlet: High SO₂ concentration requiring combined dry + wet desulfurization. Target outlet: ≤80 mg/Nm³ under EU IED / WID framework limits. Desulfurization efficiency: 87%.
- NOx at 500 mg/Nm³ inlet: SNCR denitrification with urea reagent achieves 80% efficiency, reducing to ≤80 mg/Nm³ outlet (actual measured: ≤80 mg/Nm³).
- PM at 1,500 mg/Nm³ inlet: Bag filter achieves 98.8% dust removal, reducing to ≤20 mg/Nm³ outlet (actual measured: ≤20 mg/Nm³). Additional concern: high-temperature corrosivity requires careful bag material selection (PTFE+PTFE membrane).
| 范围 | 初始浓度 | Outlet (Design) | EU IED / WID Limit |
|---|---|---|---|
| 氮氧化物 | 500 mg/Nm³ | ≤80 mg/Nm³ | IED WID: 80 mg/Nm³ |
| SO₂ | 600 mg/Nm³ | ≤80 mg/Nm³ | IED WID: 80 mg/Nm³ |
| Particulate matter (PM) | 1,500 mg/Nm³ | ≤20 mg/Nm³ | IED WID: 20 mg/Nm³ |
| 一氧化碳 | 15,000 mg/Nm³ | ≤80 mg/Nm³ | IED WID: 80 mg/Nm³ |
| 高频 | 2 mg/Nm³ | ≤50 mg/Nm³ (HCl+HF) | IED WID HCl+HF combined |
| 盐酸 | 30 mg/Nm³ | ≤2 mg/Nm³ (HF) / ≤50 mg/Nm³ (HCl) | IED WID |
| Process flue gas volume (industrial) | 28,200 Nm³/h | — | — |
| Flue gas temperature (furnace exit) | 150–180°C | — | — |
| Corrosive substances at inlet | 30 mg/Nm³ NaCl (alkali salts) | — | — |
| Humidity (at desulfurization inlet) | 15% | — | — |
03 — Engineering Requirements
Why Standard Static Control Parameters Fail for Waste Salt Incineration Off-Gas Treatment
The engineering requirements for this project reflect the fundamental difference between waste salt incineration off-gas and the stable, well-characterised flue gas streams of conventional industrial boilers or power plants for which most pollution control equipment is designed.
Dynamic Closed-Loop Adaptive Control
The system must implement dynamic response control — based on real-time monitoring of key gas parameters especially SO₂ concentration — that continuously adjusts reagent dosing, fan speeds, and process set-points to compensate for batch-to-batch and intra-batch variability. Static set-points optimised for average conditions will create compliance exceedances during peak concentration periods.
Secondary Combustion Chamber at ≥1,100°C
The secondary combustion chamber must maintain gas temperature above 1,100°C for at least 2 seconds to achieve dioxin/furan destruction per EU IED Chapter IV (Waste Incineration) requirements. Temperature monitoring with automatic fuel gas rate adjustment is mandatory; any dropout below 1,100°C triggers immediate alarm and corrective action to prevent dioxin breakthrough.
Quench Cooling to Below 200°C in Under 1 Second
After secondary combustion, gas must be quenched from approximately 550°C to below 200°C in under 1 second by water spray. This rapid cooling prevents dioxin/furan re-synthesis in the 250–450°C temperature window (the de-novo synthesis zone). The quench tower design must achieve this cooling rate reliably under all operating conditions.
Combined Dry + Wet Desulfurization
Single-stage wet NaOH scrubbing cannot achieve 87% SO₂ removal from 600 mg/Nm³ with the reliability required. A combined dry lime injection stage followed by wet scrubbing provides the necessary treatment depth and redundancy. The dry stage also provides partial HCl and HF removal, reducing the load on the wet stage.
PTFE+PTFE Membrane Bag Filter for Corrosive Gas
Standard polyester or even P84 filter bag materials are attacked by the combined HCl / HF / SO₂ / alkali salt environment of waste salt incineration off-gas at 200°C operating temperature. PTFE (polytetrafluoroethylene) membrane-on-PTFE fabric bags are specified throughout, with a 3-year service life guarantee under the full-corrosion-exposure operating condition.
One-Button Automatic Restart
All process zones must provide real-time temperature and reagent flow feedback to the control system, with automatic valve and pump interlock. One-button automatic restart capability must be implemented for the urea solution preparation and urea thermal decomposition systems after planned or emergency shutdown events, reducing the startup sequence time and operator error risk.
Comprehensive Hazardous Waste Management
All solid waste from the incineration process (furnace ash HW18, fly ash HW18, wastewater treatment sludge HW18, spent activated carbon HW49, spent bag filter cloth bags HW49, chemical lab reagents HW49, spent wipes HW49, and others) must be characterised and handled in compliance with hazardous waste classification standards. Slag from lime filtration during slurry makeup must be classified and managed as potentially hazardous waste.
Self-Adaptive Ultra-Low Emission Technology
The facility has pioneered a self-adaptive ultra-low emission technology specifically developed for the waste salt treatment sector. This technology uses dynamic closed-loop control of reagent injection rates based on real-time pollutant monitoring to achieve and maintain ultra-low emission performance despite the inherent variability of waste salt feedstock composition.
04 — Treatment Solution
Seven-Stage Integrated Treatment: From High-Temperature Incineration to Compliant Stack Discharge
The integrated treatment system addresses all regulated pollutant categories in a coordinated seven-stage sequence. Each stage handles a specific set of pollutants while conditioning the gas stream for optimal performance of the next stage:
Stage 1: Dual Combustion Chamber
Waste salt is incinerated in the primary combustion chamber. Off-gas then passes through the secondary combustion chamber where temperature is maintained above 1,100°C for ≥2 seconds, ensuring complete dioxin destruction. Temperature monitoring feedback automatically adjusts natural gas fuel rate to maintain the required temperature window.
Stage 2: Waste Heat Boiler
Hot gas at secondary combustion chamber outlet temperature is directed through a waste heat boiler where thermal energy is recovered as steam for facility use. Gas temperature is reduced significantly, enabling more controlled conditions for downstream quench cooling.
Stage 3: Quench Cooling Tower (φ4.2×12 m)
The quench tower reduces gas from approximately 550°C to below 200°C within 1 second using a dual-fluid nozzle spray system (3+1 nozzle configuration) with average spray droplet size of 85 µm and evaporation time of approximately 1 second. Compressed air system outlet pressure: 0.6 MPa; spray water flow: 0.1–1.2 m³/h per nozzle. This rapid cooling prevents dioxin re-synthesis in the de-novo synthesis temperature window.
Stage 4: SNCR Denitrification
Urea solution is injected into the secondary combustion chamber at the outlet temperature window of 850–1,050°C, where thermal NOx decomposition is most efficient. Urea consumption: 10 kg/h (urea granules). Denitrification efficiency: 80%. The urea solution preparation and thermal decomposition systems include one-button automatic restart capability with valve and pump interlock feedback.
Stage 5: Dry Desulfurization (Lime Injection)
Dry lime (slaked lime, purity >99%, consumption 12 kg/h) is injected into the cooled gas stream upstream of the bag filter. The high surface area lime particles react with SO₂, HCl, and HF in the gas stream, partially neutralising these acid gases before the bag filter stage. The lime injection and reaction also pre-coats the bag filter fabric surface, enhancing the filter’s acid gas capture capability through the dust cake layer.
Stage 6: Bag Filter (BLCC-1627, 76,000 m³/h)
The bag filter removes fine particulates and captures lime reaction products carrying absorbed acid gases. Four filter units in parallel treat 76,000 m³/h total flow. Technical specifications: 1,627 m²/unit filtration area, filtration velocity 0.78 m/min, 540 filter bags per unit, bag dimensions φ160×6,000 mm, bag material PTFE+PTFE membrane, operating temperature ≤260°C, service life 3 years. Inlet concentration: ≤1.5 g/Nm³; outlet: ≤20 mg/Nm³. Pulse-jet cleaning system with 36 cleaning valves, 100,000-cycle service life, cleaning pressure 0.20–0.40 MPa.
Stage 7: Two-Stage Wet NaOH Scrubbing
Two wet scrubbing towers in series (both φ2.8 m diameter, 8 m absorption height, 2-layer spray) complete the SO₂, HCl, and HF removal. Liquid-to-gas ratio: 3 L/Nm³; 2 recirculation pumps per tower (50 m³/h rated capacity); tower-internal recirculation. The combined dry + wet desulfurization chain achieves the target 87% total SO₂ removal efficiency.
Salt Furnace
Chamber
≥1100°C
Boiler
Tower
<200°C/1s
FGD
Filter
聚四氟乙烯
NaOH
Scrubber
→ Stack
Key Equipment and Reagent Consumption Summary
| Item | Specification / Consumption |
|---|---|
| Quench tower | φ4.2×12 m; inlet 550°C → outlet ≤200°C; evaporation time <1 s |
| Bag filter model | BLCC-1627 ×4 units; 76,000 m³/h total; PTFE+PTFE membrane bags |
| Bag filter inlet / outlet PM | ≤1,500 mg/Nm³ inlet; ≤20 mg/Nm³ outlet |
| Wet FGD towers | 2× φ2.8 m, H=8 m, 2-layer spray; L/G 3 L/Nm³ |
| Sodium hydroxide (NaOH) | 108 kg/h (20% solution) |
| Hydrochloric acid (HCl, for pH) | Facility self-supplied |
| Slaked lime (dry FGD) | 12 kg/h; <600 d storage; purity >99% |
| Activated carbon | 20 kg/h (dioxin adsorption) |
| Urea (SNCR) | 10 kg/h (urea granules) |
| Nitrogen (N₂) | 5,200 m³/h |
| Process water | 13.5 m³/h (soft water) |
| Max system running power | 438 kW (actual operating: approx. 147.5 kW) |
| Annual electricity cost (8,000 h) | Approx. 126.1 ten-thousand RMB/year equivalent |
05 — Core Advantages
What Makes This System Design Uniquely Effective for Waste Salt Incineration Off-Gas
- ✓
Dynamic Closed-Loop Adaptive Control — First Application to the Waste Salt Sector: The core innovation of this installation is the “dynamic response and precision regulation” control technology, which operates on real-time SO₂ concentration feedback to continuously adjust reagent dosing across the dry lime, SNCR urea, and wet NaOH stages simultaneously. By monitoring key gas parameters in real time and dynamically adjusting the coordinated reagent injection strategy, the system achieves simultaneous co-efficient removal of all pollutants and stable ultra-low emission performance despite the inherently variable waste salt feedstock. This self-adaptive approach was pioneered in the waste salt treatment sector through this installation. - ✓
PTFE+PTFE Membrane Bags Provide 3-Year Service Life in an Aggressive Corrosive Environment: The combination of HCl at 30 mg/Nm³ NaCl alkali metal content, SO₂, HF, and operating temperature of 200°C creates a bag filter environment that destroys conventional filter bag materials within months. The PTFE+PTFE membrane specification used in this installation provides both the chemical inertness and the surface release properties needed for the high-alkali, high-acid operating environment, achieving a 3-year service life that makes the maintenance interval compatible with annual planned shutdown schedules. - ✓
Sub-1-Second Quench Cooling Reliably Prevents Dioxin Re-Synthesis: The φ4.2×12 m quench tower with dual-fluid nozzle spray achieves the sub-1-second cooling from 550°C to below 200°C that is the physical prerequisite for preventing dioxin/furan re-synthesis in the de-novo synthesis temperature window of 250–450°C. The average 85 µm spray droplet size provides sufficient evaporation surface area for complete and reliable cooling within the 1-second residence time, verified by the evaporation time data confirming average evaporation at 1 second and maximum at 1.5 seconds. - ✓
Existing Process Infrastructure Leveraged — Minimal Footprint Addition: The integrated system was designed to build on the facility’s existing process infrastructure and technology framework, using the existing technology framework as the foundation while adding targeted upgrades. This approach minimised the capital cost and installation disruption compared with a greenfield treatment system design. The computer simulation design optimises the system layout for low resistance and energy-efficient flow design within the available site footprint. - ✓
Gypsum By-Product from Wet FGD Enables Resource Recovery: The wet NaOH scrubbing stage produces a sodium sulfate / sodium chloride solution by-product. With appropriate concentration and crystallisation treatment, this stream can be returned to the facility’s salt manufacturing process or disposed of as a recoverable industrial by-product, contributing to the circular economy objectives of the waste salt treatment operation. - ✓
Sector-First Technology Providing Replicable Template for Waste Salt Industry: As the first application of this integrated adaptive control approach to the waste salt treatment sector, this installation has provided a replicable technology template that has since been applied to comparable facilities. The approach demonstrates that ultra-low emission compliance is technically achievable for hazardous waste incineration off-gas, even at the extreme complexity and variability levels characteristic of industrial waste salt incineration.
06 — Operational Results
Verified Compliance Data: All Parameters Below EU IED / WID Limits
The system achieved the following verified compliance data across all regulated parameters, with actual emissions well below the applicable EU Industrial Emissions Directive Waste Incineration Chapter limits:
Annual operational costs: electricity at 438 kW maximum (daily run cost 3,784.32 RMB at 0.36 RMB/kWh; annual at 8,000 h: approx. 126.1 ten-thousand RMB); water at 13.5 t/h (annual cost approx. 43.2 ten-thousand RMB at 4 RMB/t); urea at 10 kg/h for SNCR (annual cost approx. 8.8 ten-thousand RMB at 1,100 RMB/t); lime at 12 kg/h for dry FGD (annual cost calculated separately).
07 — Implementation Cautions
Critical Engineering and Operational Lessons for Waste Salt SPI Incineration Off-Gas Treatment
- ⚠️
Flue gas temperature and pollutant concentration fluctuations are the primary operational risk — the system must be designed for the worst-case scenario, not the average: The documented primary risk is that flue gas temperature and NOx / SO₂ concentration fluctuations cause system discharge instability. These fluctuations arise from variations in waste salt feedstock composition between batches, and intra-batch variations as the incineration chemistry evolves. The control system’s adaptive response must be validated against the maximum rate-of-change of SO₂ concentration during the most aggressive feedstock transitions, not only against steady-state average conditions. Include a formal stack test programme during the first 3 months of operation covering multiple feedstock batches to confirm compliance across the full operating envelope. - ⚠️
High dust concentration with high alkali metal content accelerates bag filter fouling — do not use standard pulse-jet cleaning intervals: The 1,500 mg/Nm³ inlet dust loading with 30 mg/Nm³ of NaCl alkali salts creates a hygroscopic, sticky dust cake that adheres to bag surfaces more aggressively than typical industrial dust. Standard pulse-jet cleaning intervals from general industrial bag filter practice will result in progressive bag blinding, rising pressure drop, and loss of filtration velocity control. Calibrate the cleaning interval from first-month operating data on the actual waste salt dust, not from analogous industrial references. - ⚠️
High system temperature variability and high corrosivity require comprehensive temperature-based corrosion management: The system operates across a wide temperature range from 1,100°C (secondary combustion chamber) to approximately 60°C (wet scrubber outlet). Different corrosion mechanisms apply at different temperature zones. At temperatures above the acid dew point (approximately 130°C for HCl-containing gas), dry acid corrosion dominates; below the dew point, wet acid condensate corrosion is the primary mechanism. Material specification must account for both regimes for every section of the treatment train, and enhanced temperature monitoring with real-time corrosion management alerts should be incorporated into the SCADA system. - ⚠️
All solid waste streams from the incineration process are potentially hazardous and must be managed accordingly: Furnace ash (HW18), fly ash (HW18), wastewater treatment sludge (HW18), spent activated carbon (HW49), and spent bag filter cloth bags (HW49) are all classified hazardous waste under applicable regulations. Transfer, storage, and disposal of each stream must comply with hazardous waste classification requirements. Lime filtration slurry by-product must be individually characterised before any disposal or reuse pathway is confirmed. Failure to classify and manage these streams correctly creates regulatory liability that can result in operating permit suspension. - ⚠️
Close operational integration between the incineration furnace team and the gas treatment control room is mandatory: When flue gas temperature or pollutant concentrations fluctuate, advance notification from the furnace team allows the treatment system control room to pre-position reagent dosing before the concentration spike enters the treatment train. Without this communication, the adaptive control system responds reactively, with a lag time that can result in brief compliance exceedances during transitions. A formal communication protocol with a minimum 15-minute advance notice requirement for any planned furnace operating parameter change must be established and enforced from commissioning day. - ⚠️
Pipe leaks during operation are the secondary risk and require proactive inspection protocols: The high-corrosivity environment and wide temperature cycle range create significant mechanical stress on pipework. All slurry lines, acid solution lines, condensate drain lines, and expansion joints must be included in weekly visual inspection rounds during the first year of operation. Maintain a spare parts inventory for all pipework sections exposed to the corrosive gas stream — emergency pipe section replacement should be achievable within 4 hours under any planned maintenance scenario.
08 — Engineering Takeaways
Four Lessons from This Pioneering Waste Salt Incineration Emission Control Project
- 1
Dynamic adaptive control is not a premium option for waste salt incineration — it is the only viable architecture. Static control parameters optimised for average conditions will produce compliance exceedances during peak SO₂ concentration periods of each incineration batch cycle. The “dynamic response, precision regulation” approach that continuously adjusts all reagent dosing rates based on real-time online measurement is the technical foundation that makes reliable compliance achievable for this inherently variable pollution source. Any project specification for waste salt incineration off-gas treatment that does not explicitly require dynamic closed-loop control should be questioned before procurement. - 2
The sub-1-second quench cooling requirement is non-negotiable for dioxin compliance — the quench tower is the most safety-critical equipment item in the system. The temperature window from 550°C to 200°C must be traversed in under 1 second to prevent dioxin/furan re-synthesis. This requires a quench tower specifically designed for the required cooling rate, not an adapted industrial cooler. The spray nozzle system, water flow rate, droplet size distribution, and tower residence time must all be validated against the quench duty calculation before equipment procurement. The quench tower is the piece of equipment where an under-specification has the most severe regulatory consequence. - 3
PTFE+PTFE membrane bag specification is the minimum acceptable standard for hazardous waste incineration bag filters — cost-down to lower specification bags will result in early failure. The combined acid gas, alkali salt, and elevated temperature environment of waste salt incineration off-gas destroys polyester, polypropylene, and P84 bag materials within weeks to months. PTFE+PTFE membrane is the minimum specification that delivers a 3-year service life under full-exposure conditions. Accepting a cheaper bag specification to reduce procurement cost will result in a replacement cost and production interruption cost that far exceeds the initial saving within the first year of operation. - 4
Hazardous waste stream management for treatment system by-products must be planned before commissioning, not resolved post-commissioning. All solid waste streams from the incineration treatment system — fly ash, spent bags, spent carbon, wastewater sludge — are potentially classified as hazardous waste. Establishing the hazardous waste classification for each stream, identifying approved disposal routes and contractor agreements, and obtaining any required hazardous waste transfer approvals must all be completed before the facility begins processing waste salt. Discovering post-commissioning that a by-product stream does not have an approved disposal route creates a production stoppage risk.
09 — Frequently Asked Questions
Waste Salt Incineration Emission Control: Ten Questions Answered
Questions from environmental permit managers, hazardous waste facility engineers, and compliance teams at industrial waste salt processing and chlor-alkali chemical facilities planning SPI incineration off-gas treatment upgrades.
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