Waste Heat Boiler Descaling Case at a Steel Enterprise

1. Project Background

Waste heat boilers are critical equipment in steel plants for recovering thermal energy from high-temperature flue gas, directly contributing to energy savings and emission reduction. During a routine inspection in June 2026, a steel enterprise found that the operational parameters of a flue-type waste heat boiler associated with its steelmaking workshop had been deteriorating continuously. The boiler had been running for approximately 18 months since its last major overhaul. Although the feedwater underwent softening treatment, occasional fluctuations in the makeup water system caused intermittent hardness spikes. Combined with long-term exposure to dust and acidic gases in the flue gas, scaling developed on both the water-side and gas-side surfaces. The plant reported that steam output had been declining month by month for three consecutive months, with natural gas supplementary firing increasing accordingly, seriously impacting steam supply stability and energy costs.

2. Equipment Parameters and Operating Conditions

The waste heat boiler is a vertical flue-type natural circulation unit that recovers heat from converter off-gas (inlet temperature approximately 850°C) to generate saturated steam for the plant's production steam network. Key parameters before and after scaling are shown below:

3. Scaling Diagnosis

After shutdown, borescope inspection and scale sample analysis were conducted. A grayish-white to yellowish-brown hard scale layer was found on the inner walls of the heat exchange tubes, averaging 3–5 mm in thickness and exceeding 6 mm at certain elbow sections. Chemical analysis of the scale samples revealed:

• CaCO₃ (calcium carbonate) — approximately 62%, deposited from hardness constituents in the feedwater due to high-temperature precipitation;
• SiO₂ (silicon dioxide) and complex silicates — approximately 18%, from concentrated dissolved silica in the feedwater;
• Iron oxides (Fe₂O₃/Fe₃O₄) — approximately 15%, from high-temperature oxidation corrosion of the tube metal;
• The remaining ~5% comprised sulfates and trace oily organic matter.

The scaling caused a chain of problems at three levels: first, significantly increased thermal resistance raised the exhaust gas temperature from 180°C to 230°C, wasting large amounts of heat energy; second, reduced effective flow cross-section increased circulation resistance, causing a ~20% drop in evaporation; third, elevated metal wall temperature beneath the scale layer posed a risk of localized overheating and tube rupture. The comprehensive assessment concluded that chemical cleaning was imperative.

4. Chemical Cleaning Program

Based on the scale composition dominated by carbonates with silicates and iron oxides, DanYang LanXing Cleaning designed a three-stage circulating cleaning program: acid descaling → rinsing → passivation. A temporary cleaning pump station was connected to the boiler's bottom header blowdown port and the drum riser port, forming a closed circulation loop to ensure uniform distribution of cleaning solution through all heat exchange tubes.

4.1 Cleaning Formula

4.2 Process Flow

1. Water Flushing: High-volume industrial water flush to remove loose deposits and confirm circuit integrity — approximately 2 hours.
2. Alkaline Degreasing: Circulation of an alkaline solution formulated with Na₂CO₃ and Surfactant to remove oily residues and organic fouling from tube inner walls, at 60–70°C for 4 hours.
3. Acid Descaling: Injection of the acid cleaning solution with HCl and Citric Acid, combined with Urotropine for inhibition and NH₄HF₂ for silicate dissolution, circulating at 50–60°C. Acid concentration and iron ion levels were sampled every 30 minutes; the endpoint was determined when both stabilized. Main acid cleaning lasted approximately 8 hours.
4. Acid Displacement Rinse: Demineralized water flush after acid cleaning until effluent pH ≥ 5. Waste liquid was discharged to a neutralization pit for treatment.
5. Rinsing: Dilute Citric Acid solution rinse to remove residual iron ions.
6. Passivation: Na₃PO₄ solution circulation at 80–90°C for 6 hours to form a dense iron phosphate protective film on tube walls, preventing flash rusting. After passivation, the system was drained and air-dried naturally.

5. Cleaning Results Comparison

After cleaning, borescope re-inspection confirmed that the scale layer on the heat exchange tube inner walls had been essentially removed, with the metal surface showing a uniform passivation film color. The boiler was recommissioned and monitored continuously for 72 hours, with all operating parameters recovering to near-design levels:

The 52°C reduction in exhaust gas temperature means a substantial portion of heat previously carried away by flue gas is now recovered for steam generation. At 8,000 operating hours per year, this alone saves hundreds of thousands of RMB in natural gas supplementary firing costs, with the cleaning investment recovered within two months.

6. Key Takeaways

From this waste heat boiler descaling project, we summarize the following insights for similar applications:

1. Prioritize feedwater quality management. Feedwater hardness is the root cause of carbonate scale formation. Even with softening equipment in place, resin exchange capacity must be verified regularly and regeneration performed on schedule. In this case, occasional hardness exceedances accelerated scaling; adding an online hardness monitor with alarm thresholds is recommended.

2. Time the cleaning intervention correctly. The exhaust gas temperature of a waste heat boiler is the most direct indicator of scaling severity. When exhaust temperature exceeds the design value by 40–50°C, or evaporation drops by more than 15%, descaling should be scheduled to prevent excessive scale buildup and tube overheating risks. Operating with heavy scaling wastes energy and accelerates material creep due to elevated metal wall temperatures.

3. Customize formulas and procedures. Scale composition varies significantly across different steel plants due to differences in feedwater quality, flue gas composition, and operating temperature. A one-size-fits-all acid cleaning formula should not be applied — the cleaning agent combination must be tailored based on scale sample analysis. Scale with higher silicate content requires co-solvent NH₄HF₂ for thorough removal. The passivation step is critical for long-term corrosion protection and demands strict control of temperature and circulation time to ensure a complete and uniform protective film.

4. Manage spent solution disposal for environmental compliance. Acid cleaning wastewater contains heavy metal ions and high COD, requiring neutralization, flocculation sedimentation, and filter-pressing before compliant discharge. On-site waste liquid collection and treatment facilities must be in place before the project starts to avoid delays due to environmental issues.

All acceptance criteria follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment.

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