1. Project Background
In carbon product manufacturing, calciner flue gas at 850-1100°C passes through a waste heat boiler to generate steam for power generation or process heating. The WHB is the core energy-efficiency asset in a carbon plant — its heat transfer efficiency directly impacts the plant's overall energy costs.
A carbon plant operating a 150,000 tpa prebaked anode production line has two shaft calciners and two WHBs (model QCF57/850-22-3.82/450). The calciners use petroleum coke as feedstock. The No.1 WHB had been running for 3 years since commissioning when it began showing a steady rise in exhaust temperature and declining steam output over a 6-month period, severely curtailing the waste-heat power generation unit. Online chemical dosing had little effect — the gas-side scale was dense and the spray could not uniformly cover all tube bundles. After a technical and economic evaluation, the plant decided to perform a full chemical cleaning during the annual maintenance shutdown.
2. Equipment Parameters
| Parameter | Design | Pre-Cleaning |
|---|---|---|
| Rated Evaporation | 22 t/h | 13.2 t/h |
| Working Pressure | 3.82 MPa | 3.82 MPa |
| Steam Temperature | 450°C | 410°C |
| Exhaust Temperature | 180°C | 245°C |
| Tube Specification | Φ51×3.5 mm, 20G boiler steel, 1,520 tubes | |
| Heat Transfer Area | 1,850 m² | |
The exhaust temperature was 65°C above design, meaning substantial thermal energy was being vented to atmosphere — an estimated 18% thermal efficiency loss. The steam output at only 60% of rated capacity forced the power generation unit to operate at low load year-round, with significant revenue loss from reduced electricity generation.
3. Deposit Analysis
During the shutdown inspection, grey-brown hard scale with thickness of 3-8 mm was found on the gas side (external surface) of the tube bundles, with the hot-end sections being the most heavily fouled. Laboratory analysis results:
| Component | Content (wt%) | Source |
|---|---|---|
| Carbon powder & organics | 18.5 | Entrained coke fines from calciner flue gas |
| CaSO₄ | 32.7 | SO₂ + CaO reaction and deposition |
| SiO₂ + Silicates | 21.3 | Aluminosilicate ash from petroleum coke |
| Fe₂O₃ + Al₂O₃ | 15.8 | Metal oxides in petroleum coke |
| Na, K salts | 8.6 | Alkali metal residues in petroleum coke |
| Others | 3.1 | Trace V, Ni heavy metals |
The deposit is a classic carbon-industry gas-side composite scale: carbon powder acts as a structural skeleton adsorbing sulfates and silicates, which sinter into a dense, hard layer at tube wall temperatures of 550-750°C. The CaSO₄ content of 32.7% makes it a difficult-to-dissolve scale (solubility only 2.0 g/L), rendering simple water washing completely ineffective.
4. Cleaning Methodology
4.1 Process Strategy
A single cleaning method alone cannot address this composite scale. Chemical cleaning alone penetrates dense silicate scale too slowly; pure high-pressure water jetting cannot effectively strip sulfate scale shielded by the carbon skeleton. Our approach uses a synergistic two-stage process: chemical softening and penetration, followed by high-pressure water jetting for mechanical removal.
4.2 Chemical Cleaning Formulation
| Chemical | Concentration | Function |
|---|---|---|
| Sulfamic Acid | 8-10% | Dissolve CaSO₄ and carbonate scale |
| Citric Acid | 3-5% | Chelate Fe₂O₃ and Al₂O₃ |
| NH₄HF₂ | 2-3% | Dissolve silicate components |
| BTA (corrosion inhibitor) | 0.3% | Protection of 20G boiler steel |
| Surfactant (penetrant) | 0.1-0.2% | Lower surface tension, penetrate carbon layer |
| Sodium Sulfite | 0.05% | Oxygen scavenger, prevent oxygen corrosion |
4.3 Process Control Parameters
| Parameter | Control Range | Monitoring Frequency |
|---|---|---|
| Solution Temperature | 55-65°C | Every 30 min |
| pH | 1.5-2.5 | Every 30 min |
| Fe³⁺ Concentration | ≤300 mg/L | Every 60 min |
| Circulation Time per Section | 4-6 hours | — |
| HP Water Jetting Pressure | 500-700 bar | Before each section |
5. Execution
Day 1 — Preparation: WHB cooled naturally to below 50°C (forced ventilation assisted). Inlet and outlet flue ducts isolated with blind flanges. Low-pressure water (3-5 bar) wash to remove surface ash; tube wall corrosion and fin integrity inspected. Circulation piping, temperature probes, and flow meters installed at superheater, evaporator, and economizer sections. Three sets of 20G steel corrosion monitoring coupons (3 per set) suspended, each weighed to 0.1 mg accuracy.
Day 2 — Sectional Chemical Cleaning: Cleaning executed in hot-end (superheater) → mid-section (evaporator) → cold-end (economizer) order, each section with dedicated solution preparation and independent circulation to prevent temperature-gradient non-uniformity. Sulfamic Acid + NH₄HF₂ solution circulated for 4-6 hours per section at 55-65°C. The surfactant reduced surface tension from 72 mN/m to approximately 30 mN/m, enabling the acid to penetrate the carbon powder skeleton and dissolve the sulfate deposits. Fe³⁺ concentration remained below 300 mg/L throughout, avoiding pitting risk from iron ion overload. Endpoint determined when acid concentration and Fe³⁺ readings stabilized (Δ < 0.2%) over two consecutive measurements.
Day 3 — Citric Acid Rinse: Spent acid solution discharged after neutralization (pH 6-9). Citric Acid rinse circulated for 2 hours to chelate residual iron oxides and form a temporary passive layer on tube surfaces. Rinse endpoint confirmed at pH 4.5 with Fe³⁺ < 50 mg/L.
Day 4 — High-Pressure Water Jetting: Chemical cleaning softened approximately 70% of the hard scale. Remaining adherent silicate deposits removed via high-pressure water jetting (500-700 bar), tube by tube. Chemically loosened scale stripped effectively — each tube required only 30-60 seconds. All 1,520 tubes completed. Jetting wastewater neutralized before discharge. Protective shielding and impact-resistant PPE used throughout; jet direction carefully controlled to avoid damaging tube bends and welds.
Day 5 — Inspection & Recommissioning: All tube bundles visually inspected; 50 tubes randomly selected for borescope examination, with emphasis on hot-end bend welds and tube-sheet joints. Corrosion coupons retrieved and weighed for rate calculation. Temporary piping and blind flanges removed; flue duct flanges reconnected with gasket integrity verified. Hydrostatic test at 1.25× working pressure passed. WHB reignited and returned to service.
6. Results
| Indicator | Pre-Cleaning | Post-Cleaning | Acceptance Criterion |
|---|---|---|---|
| Exhaust Temperature | 245°C | 185°C | ≤195°C |
| Steam Output | 13.2 t/h | 21.2 t/h | ≥20 t/h |
| Descaling Rate | — | 96.8% | ≥95% |
| Corrosion Rate | — | 0.35 g/(m²·h) | ≤1.0 g/(m²·h) |
| Tube Wall Loss | — | < 0.05 mm | ≤0.1 mm |
Post-cleaning, tube surfaces showed bare metallic finish with no residual scale or pitting on borescope inspection. Existing light oxidation scale was removed by the Citric Acid rinse. Exhaust temperature stabilized at just 5°C above design — thermal efficiency fully restored. Steam output at 21.2 t/h (96.4% of design) enabled the waste-heat power generation unit to return to full load, with estimated annual incremental generation of ~1.2 million kWh.
7. Key Takeaways
Carbon plant WHB gas-side fouling has a distinct industry signature: a composite scale with a carbon powder skeleton filled with sulfates and silicates, fundamentally different from the pure fly-ash deposition in coal-fired boilers and significantly harder to remove. Lessons from this project:
(1) Surfactant is the process enabler: The carbon powder layer is hydrophobic — conventional acid solutions cannot wet or penetrate it. Adding 0.1-0.2% surfactant reduced surface tension from 72 mN/m to ~30 mN/m, tripling the penetration rate. This was the single most critical factor in the chemical cleaning stage.
(2) Chemical + HP water jetting synergy is optimal for carbon WHBs: Chemical cleaning alone would require 72+ hours to fully dissolve dense CaSO₄ scale; high-pressure water jetting alone is inefficient against hard scale shielded by carbon skeleton. The two-stage synergy — chemical softening of the skeleton followed by water jetting of residual scale — compressed the total schedule to 5 days with minimal tube damage risk.
(3) NH₄HF₂ is indispensable: With silicate content at 21.3%, the Sulfamic Acid + Citric Acid system had limited silicate dissolution capacity. Adding 2-3% NH₄HF₂ increased silicate dissolution from 18% to 89%, dramatically reducing the high-pressure water jetting workload. The HF generated by NH₄HF₂ requires strict concentration and temperature control to prevent excessive corrosion of 20G boiler steel.
(4) Establish a regular cleaning schedule: Based on observed fouling rates (2-3 mm/year scale growth), we recommend chemical cleaning every 18-24 months, aligned with annual maintenance windows. Regular cleaning not only sustains thermal efficiency but prevents tube overheating, oxidation spalling, and burst risks from excessive scale accumulation. An upgrade to multi-cyclone dust collectors at the calciner outlet is also recommended to reduce carbon carryover at the source and extend cleaning intervals.
All acceptance criteria follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment.
Waste Heat Boiler Chemical Cleaning · Free Technical Consultation
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