Industrial Steam Boiler Chemical Descaling Technology

1. Introduction

Industrial steam boilers are core power equipment in the chemical, textile, food, and power industries, playing a key role in providing high-temperature, high-pressure steam for process heating and turbine drives. China has over 500,000 industrial boilers in service, consuming approximately 500 million tons of standard coal annually. As operating time accumulates, hard scale primarily composed of CaCO₃, CaSO₄, and silicates forms on boiler heating surfaces. Scale thermal conductivity is only 1/30 to 1/50 that of steel; 1mm of scale can increase fuel consumption by 3%–5%, and 2mm by 8%–10%. Estimates indicate that a 20t/h boiler can waste ¥200,000–500,000 in annual fuel costs due to scaling, and in severe cases can cause tube overheating, burst failures, or even boiler scrap, resulting in safety incidents and massive economic losses. Chemical cleaning is the most effective means of restoring boiler thermal efficiency, but boiler cleaning differs from ordinary heat exchanger cleaning — the high-temperature, high-pressure operating environment, complex scale layer structure, and stringent passivation requirements necessitate a systematic process. This article starts from scaling mechanisms and systematically explains the Sulfamic Acid formulation system, alkaline boil-out conversion principles, acid cleaning descaling control, and NaNO₂ passivation protection for the complete process technical solution.

2. Technical Analysis

2.1 Boiler Scale Formation Mechanism

Although industrial boiler feedwater undergoes sodium ion exchange softening treatment, residual Ca²⁺ and Mg²⁺ combine with CO₃²⁻ produced by thermal decomposition of HCO₃⁻ under high-temperature (160–250°C) and high-pressure conditions, precipitating CaCO₃ scale layers. CaSO₄ scale formed from sulfate hardness actually decreases in solubility as temperature rises — solubility at room temperature is approximately 2.0 g/L, dropping to approximately 0.7 g/L at 100°C, with more severe deposition in superheater zones. Additionally, SiO₂ colloids in feedwater undergo dehydration condensation at high temperatures to form dense silicate scale with extremely high hardness (Mohs hardness 5–7) and slow acid dissolution rates. The three scale types typically form a layered composite structure — outer CaCO₃ is loose and porous, middle CaSO₄ is dense and hard, and the inner silicate layer bonds firmly with the metal substrate. This "loose outer, hard inner" composite scale structure is the difficulty in boiler cleaning: acid solution first rapidly dissolves the outer CaCO₃, producing large amounts of CO₂ bubbles, but the reaction rate drops sharply when reaching the middle and inner layers, easily leading to a "false endpoint" misjudgment — where acid concentration has stabilized but the inner dense scale has not yet been removed.

2.2 Key Parameters

3. Cleaning Solutions

3.1 Alkaline Boil-Out (Pre-Treatment)

For composite scale containing significant CaSO₄ and silicates, alkaline boil-out treatment must precede acid cleaning. This is one of the core steps that distinguishes boiler cleaning from ordinary heat exchanger cleaning. CaSO₄ has extremely low solubility in acidic media (only about 0.2g/100g water in 0.1mol/L HCl at room temperature), with direct acid cleaning typically achieving only 40%–60% scale removal. Alkaline boil-out uses NaOH to convert CaSO₄ into Ca(OH)₂ and soluble Na₂SO₄, while simultaneously disrupting the silicate Si-O framework, enabling more thorough acid penetration in subsequent steps. After alkaline boil-out, the system must be flushed with clean water until pH ≤9 before entering the acid cleaning stage — residual alkaline solution will neutralize the acid cleaning solution, reducing effective acid concentration and causing cleaning failure.

3.2 Chemical Cleaning Formulation

3.3 Process Flow

1. Water Flushing: High-pressure water flushing to remove loose deposits inside the boiler and discharge sludge. Check tube flow conditions to eliminate mechanical blockage risks.
2. Alkaline Boil-Out (8~12h): Prepare 3~5% NaOH solution, circulate and heat to 100~130°C, maintain constant temperature for 8~12 hours. Test alkaline concentration every 2 hours and replenish chemicals to maintain concentration ≥2%. After boil-out, discharge waste liquid and flush with clean water until effluent pH ≤9. The reaction equation for NaOH and CaSO₄ is: CaSO₄ + 2NaOH → Ca(OH)₂ + Na₂SO₄; the resulting Ca(OH)₂ is loose and easily detached, while Na₂SO₄ dissolves in water and is discharged from the system. Meanwhile, NaOH's ability to dissolve SiO₂ at high temperatures far exceeds that of ambient-temperature acid solutions — the Si-O-Si framework undergoes hydrolytic cleavage in strong alkaline environments, converting to soluble sodium silicate (Na₂SiO₃), completely dismantling the silicate scale structure.
3. Acid Cleaning (4~8h): Inject mixed solution of 5~8% Sulfamic Acid + 1~2% Citric Acid + corrosion inhibitors, circulate and heat to 50~65°C. Sample every 30 minutes to test acid concentration and Fe³⁺ concentration. When the acid concentration difference between two consecutive tests is ≤0.1% and no CO₂ bubbles are generated, the endpoint is reached. Note during acid cleaning: Sulfamic Acid begins to hydrolyze above 70°C, generating NH₄HSO₄, which not only reduces effective acid concentration but also causes corrosion to copper components — therefore the temperature upper limit must be strictly controlled.
4. Water Flush to Neutral: Discharge acid cleaning solution, circulate with clean water and flush until effluent pH is 6~7. During flushing, inject 0.1% Citric Acid to prevent Fe(OH)₃ precipitation (rinse step).
5. Passivation (4~6h): Prepare 1~2% NaNO₂ solution, adjust to pH 9.5~10.5 with NaOH, circulate and heat to 60~80°C, maintain constant temperature for 4~6 hours to form a dense Fe₃O₄ protective film on boiler metal surfaces. After passivation, discharge passivation solution and allow natural ventilation drying for 24 hours.
6. Acceptance Inspection: Visual inspection: inner walls free of residual scale and pitting; passivation film uniform steel gray. Wipe inner walls with clean white cloth — no rust color residue. Test post-cleaning water sample for total iron concentration ≤50mg/L for acceptance. Copper sulfate spot test: apply 1 drop of acidic CuSO₄ solution to the tested surface; no copper (red) precipitation within 30 seconds confirms the passivation film is dense and intact. Headers and lower drum bottoms free of residual scale deposits; blowdown pipes unobstructed.

4. Engineering Case Study

Project Background: A chemical group's 20t/h steam boiler (model SHL20-2.5-AII), natural gas-fired, operated for 4 years without comprehensive chemical cleaning. Boiler outlet temperature dropped from the design value of 225°C to 213°C, exhaust gas temperature rose from 155°C to 180°C, and daily gas consumption increased from 2,800 m³ to 3,050 m³ (9% increase). Shutdown inspection revealed average scale thickness of 2.1mm on tube inner walls, with maximum 3.5mm; scale composition analysis showed CaCO₃ 62%, CaSO₄ 21%, SiO₂ 12%, Fe₂O₃ 5% — typical high-temperature steam boiler composite scale.

Cleaning Solution: Considering scale thickness and composition, a four-step process of alkaline boil-out → acid cleaning → rinse → passivation was adopted. Alkaline boil-out used 5% NaOH solution circulated at 115°C for 10 hours with NaOH replenished twice to maintain concentration; acid cleaning used 6% Sulfamic Acid + 1.5% Citric Acid + 0.3% Urotropine + 0.05% Surfactant composite formulation, circulated at 55°C constant temperature for 6 hours to endpoint; passivation used 2% NaNO₂ solution at pH 10.0±0.2, circulated at 70°C constant temperature for 5 hours, followed by 24 hours of natural ventilation drying.

Cleaning Results: Scale removal rate 98.5%, tube inner walls fully restored to bare metal appearance, borescope inspection confirmed no dead-zone residue. After boiler recommissioning, evaporation capacity recovered to 98.5% of design value, exhaust gas temperature decreased to 158°C (22°C lower than pre-cleaning), daily gas consumption reduced to 2,780 m³ (8.8% reduction from pre-cleaning), equivalent to annual gas cost savings of approximately ¥320,000. Passivation film passed copper sulfate spot test with no copper precipitation at 35 seconds, showed no secondary rust after 72-hour wet storage, and all indicators passed first-time acceptance.

5. Summary and Recommendations

Industrial steam boiler chemical descaling is a systematic engineering task; single-step acid cleaning alone cannot thoroughly remove composite scale. Alkaline boil-out pre-treatment is the key step for addressing CaSO₄ and silicate scale — though time-consuming (8~12 hours), it cannot be omitted. Skipping alkaline boil-out and proceeding directly to acid cleaning typically achieves only 70%–80% scale removal for CaSO₄-rich scale layers, far below the 95%+ achievable with alkaline boil-out + acid cleaning. Sulfamic Acid, with its low carbon steel corrosion rate (≤2 g/(m²·h)), solid powder form at room temperature for easy storage and transport, reaction with CaCO₃ producing soluble calcium salts without precipitation, and stable descaling performance, has become the preferred acid agent for small and medium-sized industrial boiler cleaning. It is recommended that users schedule preventive chemical cleaning every 1~2 years based on feedwater hardness and operating load, combined with boiler water pH and conductivity online monitoring data to determine the optimal cleaning window. The post-cleaning NaNO₂ passivation step is the non-negotiable final quality defense line — inadequate passivation will result in widespread secondary floating rust within 48 hours of boiler startup, undoing all previous work. An acceptable passivation film appears uniform steel gray or silver gray, free of rust spots and pitting, and passes the copper sulfate spot test with no copper precipitation within 30 seconds.