中文
EnglishI. Industry Challenges
In January 2024, five central authorities including the Ministry of Ecology and Environment, National Development and Reform Commission, Ministry of Industry and Information Technology, Ministry of Finance and Ministry of Transport jointly issued Document No. Huan Daqi〔2024〕5, specifying the following requirements:
(1) Organized Emission Control Limits
At a reference oxygen content of 10%, the hourly average emission concentrations of particulate matter, sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) in flue gas from cement kilns and kiln waste heat recovery systems shall not exceed 10 mg/m³, 35 mg/m³ and 50 mg/m³ respectively. For specific limits of other organized emissions, see Appendix 1. Pollutants such as ammonia not specified in the table shall comply with national or local standards. Cement plants that achieve ultra-low emissions shall maintain hourly average concentrations meeting the above criteria for no less than 95% of their monthly production runtime.
II. Implementation Roadmap for Stable Ultra-Low Emissions
- Roadmap for Stable Clinker Production
- Combined Denitrification Technical Package
- Ultra-Low Stable Emission Functional Formula
Economic & Stable Ultra-Low Emission Performance
Ex = f(Raw & Fuels, Solid Waste, Process Equipment, Output, Product Quality, SNCR, SCR, Intelligent Control)
III. Systematic Solution Package
1. Systematic Thinking & Comprehensive Scheme Evaluation
Multiple alternative technical routes available:
- Staged-fuel denitrification via multi-point fuel injection into calciner
- Staged-air denitrification via multi-path tertiary air feeding into calciner
- Precision ammonia injection SNCR technology
- Combined process: Optimized staged combustion + SNCR
- SCR technology: Deployed under the strictest emission limit requirements
2. Full Consideration of All Restrictive Constraints
- Baseline operating conditions of raw materials, fuels and burning equipment
- Operating cost budget
- Structural space constraints of the kiln back-end system
- Denitrification efficiency target
- System operability & maintainability
3. Selection of SCR Process Routes
The core of cement kiln flue gas SCR denitrification lies in denitrification catalysts and soot blowers. Commercially available SCR catalysts for cement kilns are mainly vanadium-titanium based, consisting of TiO₂ carrier, WO₃ additive and active V₂O₅ component. Additional dopants are incorporated to enhance catalyst anti-poisoning and anti-abrasion performance.
High-temperature catalysts operate within 300–400 °
注意里面的表格
I. Industry Challenges
In January 2024, five central authorities—the Ministry of Ecology and Environment, National Development and Reform Commission, Ministry of Industry and Information Technology, Ministry of Finance and Ministry of Transport—jointly issued Document Huan Daqi〔2024〕5 with the following requirements:
(1) Organized Emission Control Limits
At a reference oxygen content of 10 vol%, the hourly average emission concentrations of particulate matter, sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) in flue gas from cement kilns and kiln waste heat utilization systems shall not exceed 10 mg/m³, 35 mg/m³ and 50 mg/m³ respectively. For specific limits of other organized emissions, refer to Appendix 1. Pollutants such as ammonia without specified limits in the table shall comply with applicable national or local standards. Cement enterprises that realize ultra-low emissions must satisfy the above concentration limits for no less than 95% of monthly production hours.
II. Implementation Roadmap for Stable Ultra-Low Emissions
- Roadmap for Stable Clinker Production
- Integrated Denitrification Technology Portfolio
- Functional Formula for Stable Ultra-Low Emissions
Economical & Stable Ultra-Low Emission Performance
\(E_x=f(\text{Raw Materials & Fuels, Solid Wastes, Process Equipment, Output, Product Quality, SNCR, SCR, Intelligent System})\)
III. Systematic Solution Package
1. Systematic Thinking & Comprehensive Scheme Assessment
Multiple optional technical routes:
- Staged combustion denitrification with multi-point fuel injection into calciner
- Staged combustion denitrification with multi-channel air intake into calciner
- Precision ammonia injection SNCR technology
- Combined process: Optimized staged combustion + SNCR
- SCR technology: Adopted under the most stringent emission limits
2. Comprehensive Analysis of All Restrictive Factors
- Baseline status of raw materials, fuels and burning equipment
- Operating cost
- Structural constraints of kiln back-end system
- Denitrification efficiency
- System operability
3. Selection of SCR Technical Routes
The core of SCR denitrification for cement kiln flue gas lies in denitrification catalysts and soot blowers. Currently, SCR catalysts applied to cement kilns are predominantly vanadium-titanium based, composed of TiO₂ carrier, WO₃ auxiliary agent and active V₂O₅ component. Additional additives are doped to improve the catalyst’s anti-poisoning and anti-abrasion properties.
High-temperature catalysts have an active temperature window of 300~400 ℃ (expandable to 260~400 ℃ under low-sulfur conditions), while medium-low temperature catalysts work at 180~280 ℃. The final process selection shall be comprehensively determined based on catalyst maturity, site layout, flue gas dust load, SO₂ concentration and flue gas temperature.
According to differences in flue gas temperature and dust concentration entering the SCR reactor, three mainstream SCR processes for cement kiln tail flue gas are classified as High-Temperature High-Dust, High-Temperature Medium-Dust and Medium-Temperature Medium-Dust.
High-Temperature High-Dust (HTHD) SCR
For HTHD SCR, kiln tail flue gas directly flows from the C1 cyclone outlet into the SCR system. NOₓ in flue gas mixes with ammonia injected by the supplementary ammonia supply system and undergoes reduction reaction over catalysts to complete denitrification, after which flue gas proceeds to downstream processes. The flue gas temperature here ranges from 280 to 350 ℃, matching the reaction temperature of most catalysts, hence its wide application.
However, the extremely high dust concentration at C1 outlet risks catalyst blockage and accelerated abrasion, requiring a safe and reliable soot blowing system. HTHD process generally adopts 13-hole honeycomb catalysts with a maximum allowable dust load of 120 g/Nm³. Adding the SCR reactor and inlet/outlet flue ducts increases system pressure drop by approximately 800 Pa. Therefore, during retrofits, the motor power and structural capacity of the high-temperature fan must be verified simultaneously.
High-Temperature Medium-Dust (HTMD) SCR
In HTMD denitrification, kiln tail flue gas passes through the C1 cyclone separator first, then enters a high-temperature electrostatic precipitator (ESP) for pretreatment to reduce dust concentration below 30 g/Nm³ before flowing into the SCR reactor. This mitigates catalyst abrasion and blockage caused by dust. Nevertheless, the extra high-temperature ESP increases footprint, overall system pressure drop and operating cost. ESPs feature high failure rates, difficult construction and high upfront investment. The addition of an SCR reactor, high-temperature ESP and connecting flues raises pressure drop by around 1100 Pa, so the high-temperature fan’s motor power and body structure must be checked in retrofits.
Medium-Temperature Medium-Dust (MTMD) SCR
The SCR reactor is arranged downstream of the high-temperature fan or between the high-temperature fan and kiln waste heat boiler, with flue gas temperature of 180~230 ℃ and dust concentration of 50~60 g/Nm³. This setup alleviates catalyst blockage and abrasion to a certain extent, delivering a catalyst chemical service life of 16,000~24,000 hours. Installing the reactor after the waste heat boiler imposes no impact on power generation output. The flue gas volume under this working condition is relatively small, resulting in a compact SCR reactor footprint.
However, within this temperature range, SO₃ (originating from residual SO₃ in flue gas and SO₂ oxidized into SO₃ by SCR catalysts) reacts with NH₃ to generate ammonium bisulfate, which cannot be fully decomposed. The ammonium bisulfate adheres to fly ash particles and deposits on catalyst surfaces, clogging micropores and causing catalyst poisoning and deactivation. The reaction equation is shown below:
\(\ce{NH3 + SO3 + H2O = NH4HSO4}\)
Thus, the flue gas SO₂ concentration entering the SCR reactor shall be maintained at a low level. Stable long-term operation is generally achievable when inlet SO₂ concentration is controlled below 50 mg/Nm³.
Additionally, operating and under-construction MTMD projects remain limited, so the long-term stable reliability of this technical route requires further field observation and verification. The MTMD process adds roughly 800 Pa pressure drop after installing the SCR reactor and connecting flues. If the reactor is placed behind the high-temperature fan, the motor power and structural capacity of raw mill circulation fan and kiln exhaust fan must be verified during retrofits.
Comparison Table of Three Denitrification Technical Routes
表格
| Item | HTHD SCR | HTMD SCR | MTMD SCR |
|---|---|---|---|
| Layout Position | Between C1 cyclone and waste heat boiler | Between C1 cyclone and waste heat boiler | Downstream of high-temperature fan |
| Main New Equipment | New SCR reactor | New high-temperature ESP + SCR reactor | New SCR reactor |
| Flue Gas Temperature / ℃ | 280–360 | 280–360 | 180–250 |
| Dust Concentration / (g/Nm³) | 80–120 | 30–50 | 30–50 |
| SO₂ Concentration / (mg/Nm³) | No strict limit | No strict limit | ＜50 |
| Industrial Application Cases | Abundant | Few | Few |
| Key Equipment to Be Retrofit | High-temperature fan | High-temperature fan | Kiln exhaust fan, raw mill circulation fan, etc. |
| Impact After Retrofit | Waste heat boiler inlet temperature drops by 6–8 ℃ | Waste heat boiler inlet temperature drops by 20–35 ℃ | Risk of catalyst poisoning |
| Additional Pressure Drop / Pa | 600–800 | 1200–1400 | 600–800 |
| Catalyst Service Life / a | 2 | 2 | 2–3 |
| Denitrification Efficiency | High | High | Low |
| Catalyst Volume Consumption / m³ | High | Medium | High |
| Catalyst Unit Price / (10,000 CNY/m³) | 1.5–2.0 | 1.5–2.0 | 3.5–4.5 |
| Floor Space | Small | Large | Small |
Based on the above comparative analysis covering application track record, technical maturity, denitrification efficiency, number of practical engineering cases and the declining catalyst price year by year, the High-Temperature High-Dust SCR denitrification technology is the more reliable technical route at present.
IV. Customized Solutions from Sequoia
- Rationally design the rated flue gas flow rate to reserve sufficient margin for subsequent alternative fuel co-firing.
- Optimize the inlet NOₓ concentration of the SCR reactor, and coordinate with the routine operation of upstream SNCR to realize low-cost system operation.
- Systematically evaluate the comprehensive impacts of hazardous waste and alternative raw fuel co-processing, select catalysts rationally, and balance the performance of clinker production, solid waste co-disposal and carbon reduction.
- Optimize the technical renovation scheme for the burning system to strike a balance between waste heat power generation and denitrification performance. Reasonably control the incremental system pressure drop to cut overall production operating costs; the minimum ammonia water consumption can be controlled below 2.0 kg per ton of clinker.
If you require more detailed technical documents or wish to learn about Sequoia’s project track record in this field, please contact us promptly.
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