Waste Heat Boiler Tube Bundle Cleaning Technology

1. Introduction

Waste heat boilers recover thermal energy from high-temperature exhaust gases to generate steam or hot water, playing a critical role in industrial energy conservation and carbon reduction. China's steel industry alone operates over 3,000 waste heat boilers, while cement kiln WHB installations continue to grow. However, harsh operating conditions—flue gas temperatures of 300-500°C with high dust loading and complex chemical composition—cause progressive fouling of heat exchange tube bundles. Industry data shows that an unmaintained waste heat boiler operating for 3 years loses 15-30% of its thermal efficiency, equivalent to hundreds of tons of standard coal waste annually.

In terms of equipment types, waste heat boilers fall into three main categories: duct-type, shell-and-tube, and heat-pipe designs. Shell-and-tube WHBs present the greatest cleaning challenge due to their densely packed tube bundles—a 150 t/h unit may contain 800-1,200 tubes with over 3,000 m² of total heat exchange area. Once severe fouling sets in, routine soot blowers alone cannot restore design performance.

Tube bundle fouling occurs on two fronts: externally, flue gas deposits form hard sulfate-silicate scales bonded with unburned carbon particles; internally, water-side scaling from Ca²⁺ and Mg²⁺ precipitation combines with dissolved oxygen corrosion to produce Fe₂O₃ rust layers. The combined effect reduces heat transfer coefficients, raises exhaust temperatures, and can lead to localized tube overheating and rupture. A properly designed cleaning program not only restores thermal efficiency but also extends equipment service life by 3-5 years.

2. Fouling Mechanisms and Scale Analysis

2.1 Flue Gas Side Ash Deposition

Flue gas side fouling occurs through physical deposition and chemical condensation. Physical deposition involves inertial impaction and thermophoretic settling of suspended particles on tube surfaces. Chemical condensation occurs when SO₂ and SO₃ in the flue gas react with water vapor to form sulfuric acid mist, which condenses on tube walls below the acid dew point and bonds with fly ash to form hard sulfate scales. These scales are dense and strongly adherent, resisting removal by conventional mechanical soot blowers.

2.2 Water-Steam Side Scaling

Water-side deposits are predominantly CaCO₃ and Mg(OH)₂, accompanied by iron oxides (Fe₂O₃, Fe₃O₄). When feedwater hardness exceeds limits or blowdown is inadequate, carbonates precipitate on high-temperature heat transfer surfaces. Dissolved oxygen causes electrochemical corrosion producing Fe₂O₃ rust layers that further thicken the deposit. In severe cases, tube inner diameter reduction can exceed 20%, significantly impairing circulation flow.

2.3 Scale Analysis Protocol

3. Chemical Cleaning Process

3.1 Cleaning Formulation System

For the composite scale characteristics of WHB tube bundles, an acid pickling + inhibitor + cleaning aid system is deployed. The flue gas side, dominated by sulfate hard scales, requires a combination of HCl and Sulfamic Acid; the water side, dominated by carbonate scales, responds well to HCl as the primary agent. A BTA + Urotropine inhibitor combination ensures base metal protection for both carbon steel and stainless steel tubes.

3.2 Process Flow

1. Water Flush: High-volume industrial water flush to remove loose ash and surface deposits until discharge runs clear.
2. Alkaline Degreasing: Na₂CO₃ + Surfactant solution circulation for 2-3 hours at 60-70°C to remove oil and organic residues.
3. Acid Pickling: HCl + Sulfamic Acid combined acid circulation for 4-6 hours at 50-60°C, monitor Fe³⁺ concentration until stable.
4. Neutralization Rinse: Displace residual acid with clean water, then neutralize to pH 6-7 using Na₂CO₃ solution.
5. Passivation: Na₃PO₄ solution circulation passivation for 2-3 hours, forming a dense phosphate protective film to prevent flash rust.
6. Drying and Inspection: Drain and blow-dry with compressed air, visually inspect tube cleanliness, perform quality acceptance.

4. High-Pressure Water Jetting Assistance

For flue gas side sulfate hard scales that resist chemical dissolution alone, high-pressure water jetting serves as an essential complementary technique, particularly for external ash deposits and localized hard scale zones on tube bundle exteriors.

Recommended jetting parameters: operating pressure 50-80 MPa (adjusted based on scale hardness), flow rate 40-60 L/min, fan nozzle for uniform coverage. For finned tube bundles, pressure should be reduced to 30-50 MPa to prevent fin damage. Operations follow tube row alignment, proceeding row by row to ensure complete coverage. Wire brushing combined with high-pressure water improves removal efficiency on dense hard scales.

The typical combined workflow: chemical cleaning first dissolves internal water-side scale and external soluble salts, followed by high-pressure water jetting to remove residual hard scales and ash deposits from tube exteriors, and finally passivation treatment. This "chemical + physical" combined approach achieves tube cleanliness exceeding 95%.

5. Engineering Case Study

Project Background: A steel enterprise's sintering plant waste heat boiler, model QC150/400-6.5-2.5, operated for 4 years without comprehensive cleaning. Tube bundle construction: 20G carbon steel + 304 stainless steel composite, 860 tubes total. Operating data showed exhaust temperature rising from 180°C (design) to 235°C, with steam output dropping approximately 22%.

Scale Analysis Results: External ash deposit thickness 2-4 mm, gray-white hard lumps. XRF analysis showed CaSO₄ ~55%, SiO₂ ~18%, Fe₂O₃ ~10%. Internal scale thickness 1-2 mm, predominantly CaCO₃ (~70%) with minor iron corrosion products.

Cleaning Approach: Combined HCl + Sulfamic Acid circulation (internal) + 50 MPa HP water jetting (external). Tube bundles partitioned into zones for sequential circulation, 5 hours acid pickling per zone. BTA + Urotropine inhibitor system with temperature controlled at 55±5°C.

Results: Post-cleaning, tube internal surfaces revealed bright metal finish; external ash deposit removal exceeded 95%. After restart, exhaust temperature dropped to 188°C, steam output recovered to rated capacity, saving approximately 420 tons of standard coal annually. No tube wall thinning or leakage issues reported in 2 years of subsequent operation.

6. Quality Acceptance Standards

7. Summary and Recommendations

Waste heat boiler tube bundle cleaning is a critical maintenance measure for restoring thermal efficiency and ensuring safe operation. Based on extensive field experience, we recommend industrial users establish a preventive cleaning program: comprehensive chemical cleaning every 2-3 years based on flue gas dust loading and operating conditions, with annual high-pressure water jetting for ash removal. For newly commissioned waste heat boilers, pre-operational chemical cleaning with pre-filming treatment creates a protective layer that significantly slows initial fouling rates.

From an economic perspective, a complete chemical + high-pressure water jetting cleaning program typically costs 15-20% of the annual WHB maintenance budget while restoring 15-30% thermal efficiency, with payback periods under 6 months. For continuous-process steel and cement plants, the steam output recovery and fuel savings far exceed cleaning costs.

Water quality management is the foundation for extending cleaning intervals. Strictly control feedwater hardness (<0.03 mmol/L), dissolved oxygen (<15 μg/L), and ensure continuous and periodic blowdown procedures. Online water quality monitoring that tracks real-time parameter changes can reduce tube bundle scaling rates by over 50%. When selecting a professional cleaning service provider, prioritize inhibitor formulation expertise and proven track record on similar equipment to ensure effective scale removal while protecting the tube bundle substrate.

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