Fallstudie · Industrielle Emissionskontrolle
How a leading solid waste resource recovery enterprise achieved 99.85% desulfurization, 50% SCR denitrification, and 98.4% dust removal from 48,000 Nm³/h of highly variable multi-source rotary kiln off-gas — deploying SDS sodium-bicarbonate dry desulfurization, low-temperature SCR, and pulse-jet bag filter technology adapted for the challenging high-HCl, high-HF, high-SO₂ composition of contaminated soil and industrial solid waste incineration off-gas.
SDS-Trockenentschwefelung
Low-Temperature SCR Denitrification
Pulse-Jet Bag Filter
Contaminated Soil Thermal Treatment
01 — Branchenhintergrund
Large-Scale Solid Waste Comprehensive Processing: A Growing Sector With Complex Multi-Pollutant Emission Challenges
Developing resource utilization of large-scale solid waste is a core component of sustainable development strategy. Large-scale solid waste encompasses an exceptionally diverse range of materials: construction waste, coal ash, tailings rock, coal gangue, industrial by-product gypsum, desulfurization waste, smelting slag, and industrial waste residue. The scale of this challenge is significant — annual new large-scale solid waste accumulation continues to grow while comprehensive utilization rates remain below 60%, with existing historical stockpiles representing a major land resource and ecological safety challenge in many industrial regions.
The facility in this case study specialises in environmental remediation and solid waste resource utilization, with primary business encompassing contaminated soil remediation, hazardous waste treatment, and wastewater treatment technology services. As a leading enterprise in the solid waste treatment sector, it has built an integrated production line covering contaminated soil treatment (annual capacity: 1.1 million m³ of industrial solid body contaminated soil), sludge treatment (annual capacity: 360,000 m³ of sludge including heavy metals), and resource utilization of construction materials and road materials (annual capacity: 730,000 m³ of construction material bases and road material bases). After processing, annual output includes approximately 600,000 m³ of construction engineering base materials and road materials.
The rotary kiln thermal treatment of contaminated soil generates off-gas at 170°C carrying a highly variable multi-pollutant load that reflects the diverse and unpredictable chemical composition of the contaminated soil and industrial waste feedstocks. Unlike purpose-built industrial waste incinerators with fixed feedstock specifications, the solid waste processing rotary kiln must handle feedstocks whose composition can vary dramatically between batches — from lightly contaminated construction demolition waste to heavily contaminated industrial process residues. This compositional variability is the defining engineering challenge for the off-gas treatment system.
“The initial data provided for this project was inaccurate — the actual HF, HCl, and SO₂ concentrations in the rotary kiln off-gas proved significantly higher than the pre-design characterisation indicated. The desulfurization system was consequently operating under overloaded conditions from commissioning, and equipment wear during operation was severe. This experience demonstrates that for contaminated soil and mixed solid waste processing applications, conservative design margins are not optional — they are essential insurance against the inherent unpredictability of feedstock composition.”
— Engineering Experience Summary, Large-Scale Solid Waste Comprehensive Processing Dust Removal / Desulfurization / Denitrification Project
02 — Verschmutzungsprofil
Contaminated Soil Rotary Kiln Off-Gas: Unpredictable Multi-Pollutant Composition Demands Conservative Design
The rotary kiln operates on sulphur-containing fuel (sulphur). The standard flue gas volume is 48,000 Nm³/h; process flue gas volume 80,000 Nm³/h at operating conditions (170°C). The oxygen content varies between 12–15% actual (11% baseline). Two induced draft fans provide 200×2 kW at 6,000 Pa, with 1 m pair operating. The initial pollutant profile from the design characterisation was as follows:
- SO₂ at 500–600 mg/Nm³: High variability. Target outlet: ≤80 mg/Nm³ (design), actual achieved 50 mg/Nm³. The wide inlet range — and the subsequent discovery that actual concentrations exceeded the design characterisation — means the SDS dry desulfurization system was designed with insufficient capacity for the actual operating condition, necessitating post-commissioning upgrades to the desulfurization system and use of high-efficiency calcium-based desulfurization reagent.
- Particulate matter (PM) at 20 g/Nm³ (20,000 mg/Nm³): Extremely high dust loading from contaminated soil particulates and combustion ash. After heat exchanger pre-cooling and SDS injection, the bag filter inlet concentration is substantially reduced. The bag filter achieves 98.4% dust removal, delivering outlet PM of 3 mg/Nm³ (actual) against a 20 mg/Nm³ design target.
- HCl at 15 mg/Nm³: From chloride compounds in the contaminated soil and waste feedstocks. Target outlet: ≤6 mg/Nm³. Actual: 2 mg/Nm³ — captured partially by the SDS sodium bicarbonate injection (which reacts with HCl as well as SO₂) and the bag filter.
- HF at 30 mg/Nm³: Elevated HF from fluoride-bearing waste components in the contaminated soil feed. The actual HF concentration proved higher than the design characterisation, contributing to the overload condition discovered post-commissioning. Target outlet: ≤60 mg/Nm³ (design); actual achieved: 6 mg/Nm³ (under normal operating conditions).
- NOx (unspecified initially, treated by SCR): Low-temperature SCR denitrification at 220–260°C inlet temperature achieves 50% denitrification efficiency. SCR inlet temperature 220°C; outlet 200°C.
- Temperature points: Kiln off-gas exit at 380–450°C; after heat exchanger, temperature reduces to approximately 260°C before SDS injection zone; temperature at desulfurization inlet approximately 250°C; temperature at bag filter inlet approximately 260°C; SCR denitrification inlet 220°C (after bag filter).
| Parameter | Anfangskonzentration | Designed Outlet | Actual Outlet | EU IED Limit |
|---|---|---|---|---|
| NOx | — | ≤180 mg/Nm³ | ≤180 mg/Nm³ | 200 mg/Nm³ (IED WID) |
| SO₂ | 500–600 mg/Nm³ | ≤80 mg/Nm³ | 50 mg/Nm³ | 80 mg/Nm³ (IED WID) |
| Feinstaub (PM) | 20 g/Nm³ (20,000 mg/Nm³) | ≤20 mg/Nm³ | 3 mg/Nm³ | 20 mg/Nm³ (IED WID) |
| HCl | 15 mg/Nm³ | ≤6 mg/Nm³ | 2 mg/Nm³ | 10 mg/Nm³ (IED WID) |
| HF | 30 mg/Nm³ | ≤60 mg/Nm³ | 6 mg/Nm³ | 1 mg/Nm³ (IED WID) |
| Sichtbare weiße Rauchfahne | Gegenwärtig | Keine (unsichtbar) | None — confirmed | Keine sichtbare weiße Rauchfahne |
| Standard flue gas volume | 48,000 Nm³/h | — | — | — |
| Prozessabgasvolumen | 80,000 Nm³/h at 170°C | — | — | — |
| Kiln exit temperature | 380–450°C | — | — | — |
03 — Behandlungslösung
Four-Stage Dry Treatment System: Heat Exchange → SDS Dry FGD → Bag Filter → Low-Temperature SCR
The treatment approach uses an entirely dry process chain, avoiding the wastewater generation that would result from wet scrubbing of a gas stream this heavily contaminated. The four treatment stages address the pollutant profile in sequence, exploiting the high-temperature window before the bag filter for SDS dry desulfurization and reserving the lower-temperature post-filter zone for low-temperature SCR denitrification.
Stage 1: Flue Gas Cooling Heat Exchanger (380–450°C → 260°C)
Hot kiln off-gas at 380–450°C enters the cyclone pre-duster for coarse particle removal, then passes through the water-cooled heat exchanger to control flue gas temperature to not more than 260°C. Key parameters: flue gas volume 48,000 m³/h; heat exchange area 284 m²; device pressure drop 429 Pa; hot side inlet 350°C; hot side outlet 250°C; device dimensions 1,989×2,170×3,150 mm. This pre-cooling step brings the gas within the operating temperature window of the SDS dry desulfurization system and the bag filter, and prevents the anti-corrosion materials and bag filter fabric from exceeding their rated temperatures.
Stage 2: SDS Dry Desulfurization (Sodium Bicarbonate Injection)
The cooled gas then enters the SDS (Spray Dry Scrubbing / Sodium Bicarbonate Dry Sorbent) dry desulfurization tower. SDS uses pulverized sodium bicarbonate (NaHCO₃) as the sorbent, which when injected into the gas stream thermally decomposes to produce sodium carbonate (Na₂CO₃) and then reacts with SO₂, HCl, and HF to form sodium sulfite/sulfate and sodium chloride/fluoride salts. Key SDS parameters: flue gas volume 78,000 m³/h; flue gas temperature 250°C; SO₂ inlet 250 mg/Nm³ (design) / 500–600 mg/Nm³ (actual); SO₂ outlet 80 mg/Nm³ (design) / 50 mg/Nm³ (actual); calcium-to-sulfur ratio 1.1; limestone storage capacity 5 m³; 3-day autonomy. High-efficiency calcium-based desulfurization reagent at 0.03 t/h consumption; annual desulfurization reagent cost approximately 21.6 ten-thousand RMB equivalent. The SDS process simultaneously removes HCl and HF in addition to SO₂, achieving the multi-acid-gas removal required in a single injection stage without generating any liquid waste.
Stage 3: Pulse-Jet Bag Filter (2,712 m² Filtration Area)
After SDS injection, the gas and the SDS reaction products enter the pulse-jet bag filter for particulate removal. The bag filter captures both the original kiln off-gas particulates and the sodium salt reaction products from the SDS stage, achieving effective PM and acid gas salt removal simultaneously. Key parameters: filtration area 2,712 m²; bag count 900; bag diameter φ160 mm; filtration velocity ≤0.7 m/min; outlet PM concentration ≤10 mg/Nm³ (design) / 3 mg/Nm³ (actual); body resistance 300 Pa; flue gas temperature ≤260°C; device dimensions 8,300×7,140×13,360 mm; device height 13,360 mm; design pressure ±5,000 Pa. Overall system dust removal: 98.4% design / 90% actual (the actual performance reflects the overloaded operating condition due to higher-than-expected inlet pollutant concentrations). The bag filter is the critical compliance component for PM — ensuring the filter bags remain within temperature limits and maintaining pulse-jet cleaning effectiveness are the primary operational priorities.
Stage 4: Low-Temperature SCR Denitrification (220°C → 200°C)
The post-bag-filter gas, now substantially cleaned of particulates and acid gases, enters the low-temperature SCR reactor at approximately 220°C for NOx reduction. The SCR is positioned downstream of the bag filter (cold-side SCR) to protect the catalyst from the high dust loading of the kiln off-gas, which would otherwise rapidly foul and mechanically abrade the catalyst surface. Key SCR parameters: device outer dimension 85,000 mm (plan); device outer height 1,308 mm; 15 catalyst modules; catalyst volume 17 m³; device pressure drop 500 Pa; SCR inlet temperature 220°C; SCR outlet temperature 200°C. The cold-side SCR configuration requires a catalyst formulation designed for operation at 200–260°C, which is outside the typical 350–400°C window of standard SCR catalysts. Low-temperature SCR catalysts use modified formulations that maintain adequate NOx reduction activity at 200–260°C while resisting deactivation by the sodium and calcium salt residues carried over from the SDS stage that pass through the bag filter in very fine form. Denitrification efficiency: 50% (design and actual).
380–450°C
→260°C
NaHCO₃
SO₂/HCl/HF
2,712 m²
98.4% PM
220 °C
50% NOx
→ Stack
Key Equipment and Reagent Summary
| Item | Spezifikation |
|---|---|
| Cooling heat exchanger | 48,000 m³/h; 284 m² area; 429 Pa pressure drop; 350→250°C; 1,989×2,170×3,150 mm |
| SDS dry desulfurization | 78,000 m³/h; 250°C; SO₂ inlet 250 mg/Nm³; outlet 80 mg/Nm³; Ca/S ratio 1.1; limestone storage 5 m³ (3-day) |
| Bag filter | 2,712 m² area; 900 bags; φ160 mm; ≤0.7 m/min; ≤10 mg/Nm³ outlet; 300 Pa; 8,300×7,140×13,360 mm |
| Low-temperature SCR | 85,000 mm (plan); 15 catalyst modules; 17 m³ catalyst volume; 500 Pa; 220→200°C; 50% NOx efficiency |
| Induced draft fans | 90,000 m³/h per unit; 6,000 Pa; 200–250°C operating temperature; 200 kW per unit; 1 duty + 1 standby |
| High-efficiency calcium desulfurization reagent | 0.03 t/h; 900 RMB/t; annual cost approx. 21.6 ten-thousand RMB equivalent |
| Ammonia water (SCR reductant) | 0.06 t/h; 600 RMB/t; annual cost approx. 28.8 ten-thousand RMB equivalent |
| Max system running power | 326.21 kW (actual); 534.46 kW (total installed) |
| Annual electricity cost (8,000 h) | Approx. 93.9 ten-thousand RMB equivalent at 0.36 RMB/kWh |
04 — Kernvorteile
Why Dry Process SDS + Bag Filter + Low-Temperature SCR Is the Right Architecture for Mixed Solid Waste Off-Gas
- ✓
SDS Dry Process Avoids Secondary Liquid Waste From a Gas Stream Containing Contamination From Unknown Sources: For contaminated soil and mixed solid waste processing, the chemical composition of the off-gas is inherently unpredictable. Wet scrubbing of this off-gas would generate heavily contaminated wastewater containing heavy metals, organic micropollutants, and all the acid gas absorption products in a single liquid stream that would be exceptionally difficult to treat and dispose of. The SDS dry process converts all acid gas pollutants (SO₂, HCl, HF) into solid sodium salt reaction products that are collected by the bag filter as dry solid waste, classified, and disposed of through the facility’s existing hazardous waste management chain. Zero liquid waste is generated from the treatment process itself. - ✓
SDS Sodium Bicarbonate Removes SO₂, HCl, and HF Simultaneously in a Single Injection Stage: Unlike limestone FGD (which primarily removes SO₂), SDS sodium bicarbonate reacts effectively with all three acid gases simultaneously: SO₂ to form sodium sulfite/sulfate, HCl to form sodium chloride, and HF to form sodium fluoride. For a gas stream with simultaneous high concentrations of all three acid gases — as characterises solid waste rotary kiln off-gas — SDS provides a single injection stage that addresses all three pollutants rather than requiring separate desulfurization and acid gas treatment stages. This multi-pollutant simultaneous capture is a key operational simplification for variable-composition off-gas streams. - ✓
Cold-Side SCR After Bag Filter Protects the Catalyst From the Extreme Dust Loading of Contaminated Soil Off-Gas: At 20 g/Nm³ initial particulate loading, placing the SCR reactor upstream of the bag filter (hot-side SCR) would result in rapid catalyst channel blocking and mechanical erosion by the abrasive dust particles. Cold-side SCR placement (after the bag filter reduces PM to ≤10 mg/Nm³) protects the catalyst from these mechanisms and enables the catalyst to deliver its rated 50% NOx removal efficiency without the accelerated degradation that would occur in a high-dust environment. The tradeoff of requiring a low-temperature catalyst formulation for 200–260°C operation is outweighed by the catalyst protection benefit for this specific application. - ✓
Limestone-Based Reagent Advantages: Widely Available, Low Cost, No Secondary Pollution: The SDS process specification for this installation incorporates several design principles drawn from limestone-gypsum FGD practice: (1) low energy consumption and operating cost; (2) by-products (sodium salts) can be properly managed without secondary pollution; (3) small footprint and rational flow design; (4) system design through computer simulation for optimized performance; (5) appropriate gas flow velocity design; (6) absorption reagent (calcium-based high-efficiency desulfurization reagent) is widely sourced and price-competitive. These principles are directly transferable from limestone FGD to SDS applications and represent established design practice for acid gas dry desulfurization systems. - ✓
Modular Architecture Accommodates Future Desulfurization Upgrades Without System Replacement: The documented project experience includes the honest assessment that the initial feedstock characterisation data was inaccurate, leading to an undersized desulfurization system that operated under overloaded conditions from commissioning. The modular SDS injection system architecture allowed the facility to address this by upgrading to a higher-efficiency calcium-based desulfurization reagent and improving the SDS system capacity within the existing framework, without requiring replacement of the bag filter, SCR, or heat exchanger. Modular design is not only an environmental compliance feature — it is an insurance policy against the inevitable uncertainty of feedstock characterisation for variable mixed waste applications.
05 — Betriebsergebnisse
Compliance Data After Post-Commissioning System Upgrade
Following the post-commissioning upgrade to the desulfurization system (higher-efficiency calcium-based reagent and improved system capacity), the treatment system achieved the following compliance data:
Annual operating costs: electricity at 326.21 kW actual running power (0.36 RMB/kWh equivalent, 8,000 h/year) = approximately 93.9 ten-thousand RMB equivalent; water (cooling water, system make-up, heat exchanger cooling) approximately 4.8 ten-thousand RMB equivalent; high-efficiency desulfurization reagent approximately 21.6 ten-thousand RMB equivalent; ammonia water (SCR reductant) approximately 28.8 ten-thousand RMB equivalent.
06 — Hinweise zur Umsetzung
Critical Lessons from This Project — Including What Went Wrong and How It Was Fixed
- 🚫
CRITICAL LESSON: Initial feedstock characterisation data was inaccurate — actual HF, HCl, and SO₂ concentrations were significantly higher than the design basis, causing immediate system overload and severe equipment wear: The project experience summary explicitly documents that the initial data provided was inaccurate, with the actual HF, HCl, and SO₂ concentrations proving significantly higher than the design characterisation indicated. This caused the desulfurization system to operate under overloaded conditions from commissioning, with high pollutant concentration fluctuations and severe equipment wear during operation. For any contaminated soil, mixed industrial waste, or variable-composition solid waste processing application, the design SO₂ and acid gas concentrations must incorporate a conservative upward margin (minimum 50% above the characterisation measurement) to account for feedstock variability. A single spot measurement of the feedstock composition does not represent the operational range; a statistical characterisation over at least 30 batch cycles is needed before fixing the design basis. - ⚠️
Raw material source instability and complex composition create chronically unstable system discharge — strengthen source control before investing in additional treatment capacity: The primary documented risk is that raw material source instability and complex composition cause system discharge fluctuations. The first response measure is to strictly control the raw material source and ensure stable system operation. Before upgrading the treatment system, the facility must implement feedstock acceptance testing that characterises the key pollutant-generating compounds (sulfur, chloride, fluoride) in each batch before it enters the rotary kiln. Batches that exceed the design characterisation basis should be rejected or blended with lower-concentration feedstocks to bring the combined composition within the treatment system’s rated capacity. - ⚠️
High-corrosivity gas causes premature equipment wear — the desulfurization system must be upgraded and improved to increase desulfurization capability: The second documented risk is that the high-corrosivity gas causes premature equipment wear that reduces service life below specification. The response measures are: (1) upgrade and improve the desulfurization system to increase desulfurization capability (implemented through the switch to high-efficiency calcium-based reagent); (2) use high-efficiency calcium-based desulfurization reagent to improve desulfurization efficiency, replacing the original reagent; (3) strengthen personnel inspection rounds and maintain normal equipment operation; (4) continuously improve related personnel safety awareness and technical skills. For any future installation in this application category, specifying corrosion-resistant materials throughout the SDS injection zone and bag filter housing (rather than bare carbon steel) will significantly reduce the wear rate. - ⚠️
Bag filter operating temperature must be actively managed — temperature excursions above the bag fabric rated temperature are the primary bag failure mode: At 380–450°C kiln exit temperature, any failure of the pre-cooling heat exchanger (reduced cooling water flow, heat exchanger fouling, or valve failure) will result in elevated gas temperature entering the bag filter. The bag filter temperature limit (≤260°C) provides only a modest margin above the normal 250°C operating temperature. Implement continuous temperature monitoring at the bag filter inlet with a high-temperature alarm at 250°C and automatic kiln shutdown or bypass at 270°C, to prevent bag fabric damage during cooling system upset events. - ⚠️
Low-temperature SCR catalyst is susceptible to poisoning by SDS reaction product sodium salts that carry over from the bag filter in very fine form: Sodium compounds from the SDS process (sodium sulfite, sodium chloride, sodium fluoride) that pass through the bag filter as sub-micron particles will deposit on the low-temperature SCR catalyst surface over time, progressively blocking catalyst pore channels and reducing NOx conversion efficiency. Monitor SCR pressure drop continuously — rising pressure drop at constant gas volume is the primary indicator of catalyst fouling. Implement periodic soot blowing of the SCR catalyst bed (frequency to be established from first-year operating data), and include catalyst activity testing as part of the annual maintenance scope. - ⚠️
All solid waste from the treatment system must be classified as potentially hazardous before any disposal route is confirmed: The SDS process produces sodium salt reaction products (sodium sulfate, sodium chloride, sodium fluoride) collected in the bag filter hoppers. These solid wastes must be classified by laboratory testing (TCLP leachate testing under EN 12457) to confirm whether they meet the criteria for non-hazardous industrial solid waste or must be managed as hazardous waste. In a contaminated soil processing context, the reaction products may also contain absorbed heavy metals and organic micropollutants from the feedstock, potentially classifying them as hazardous waste under EU Waste Framework Directive category codes. Confirmation of waste classification and approved disposal route must be obtained before commissioning.
07 — Wichtigste Erkenntnisse aus dem Ingenieurwesen
Four Hard-Won Lessons From This Solid Waste Rotary Kiln Off-Gas Project
- !
Never accept a single-point feedstock characterisation as the design basis for a mixed solid waste treatment system. The entire engineering failure in this project — overloaded desulfurization system, severe equipment wear, post-commissioning emergency upgrade — stemmed directly from using inaccurate initial characterisation data as the design basis without any conservative margin. The minimum acceptable characterisation programme for a variable mixed waste application is: 30 representative batch samples, full acid gas analysis (SO₂, HCl, HF, NO, NO₂) for each sample, and design basis set at the 95th percentile concentration, not the mean. The cost of this characterisation programme is a tiny fraction of the cost of a post-commissioning emergency upgrade. - 2
SDS dry desulfurization is the right technology for contaminated soil and mixed solid waste off-gas, but it requires accurate inlet characterisation to be correctly sized. The SDS process advantages — no secondary wastewater, simultaneous SO₂/HCl/HF removal, dry solid waste output, zero liquid effluent — are fully applicable and appropriate for this application. The failure was not in the technology selection but in the system sizing. Had the design basis reflected the actual 500–600 mg/Nm³ SO₂ range rather than the underestimated initial characterisation, the SDS system would have been sized appropriately from the start and the post-commissioning overload would not have occurred. - 3
Cold-side low-temperature SCR (after the bag filter) is the correct SCR architecture for high-dust contaminated soil rotary kiln off-gas — do not place the SCR upstream of the bag filter. The 20 g/Nm³ initial PM loading is 100× the typical power plant SCR inlet dust loading. Hot-side SCR at this dust level would block and erode the catalyst within weeks. Cold-side SCR at 200–260°C after the bag filter reduces the PM to ≤10 mg/Nm³ before catalyst contact, delivering the 50% NOx efficiency target with manageable catalyst maintenance requirements. The lower operating temperature requires a specifically formulated low-temperature SCR catalyst, but this technology is commercially available and the specification cost is fully justified by the catalyst protection benefit at extreme dust loading. - 4
The experience of this project — including its post-commissioning failure and subsequent recovery — is more valuable than a project that succeeded from day one. The honest documentation of the characterisation data inadequacy, the overloaded desulfurization system, the severe equipment wear, and the remediation approach provides engineering teams at other solid waste processing facilities with a direct template for what to avoid and how to respond when it happens. Projects that document only their successes deprive the industry of the learning that comes from documented failures. This project is a valuable reference precisely because its engineers were transparent about what went wrong and how it was fixed.
08 — Häufig gestellte Fragen
Solid Waste Rotary Kiln Off-Gas Treatment: Ten Questions Answered
Questions from environmental permit managers, remediation engineers, and compliance teams at contaminated soil treatment, hazardous waste management, and solid waste resource recovery facilities planning off-gas treatment upgrades under EU IED / Dutch Activities Decree requirements.
Ready to Design a Reliable Solid Waste Off-Gas Treatment System?
Entdecken Sie das gesamte Spektrum an Lösungen zur industriellen Emissionskontrolle
From SDS dry desulfurization and low-temperature SCR for solid waste rotary kilns to regenerative thermal oxidation systems for industrial VOC abatement, our engineering team delivers EU IED–compliant solutions with the conservative design margins that complex waste applications demand.
