I. Causes and Hazards of Thermal Oil Boiler Coking

Thermal oil boilers (organic heat carrier furnaces) are widely used heating equipment in industrial production, offering advantages such as low-pressure high-temperature operation and precise temperature control. However, during long-term high-temperature operation, thermal oil undergoes thermal cracking and polymerization reactions, gradually forming coke and carbon deposits on the inner walls of furnace tubes. Thermal oil typically operates at temperatures between 280 and 350°C. At such high temperatures, thermal oil molecules undergo carbon-carbon bond cleavage, generating free radicals that subsequently trigger polymerization reactions, forming high-molecular-weight gums and asphaltenes.

The thermal conductivity of the coke layer is extremely low, approximately 0.1 to 0.3 W/(m·K), only 1/150 to 1/50 of carbon steel's thermal conductivity. When the coke thickness on the inner wall of furnace tubes reaches 1 mm, the tube wall temperature will rise by 80 to 120°C. This not only causes a significant increase in fuel consumption (typically 15% to 25% increase) but more seriously leads to localized overheating of furnace tubes, potentially causing tube bulging, deformation, or even tube rupture. Statistics indicate that accidents caused by coking account for over 60% of all thermal oil boiler incidents.

Furthermore, coking increases system circulation resistance, reduces thermal oil flow rate, and results in insufficient heating capacity, affecting normal production line operation. Therefore, regular chemical cleaning and decoking of thermal oil boilers and pipeline systems is an essential measure to ensure safe and energy-efficient equipment operation.

II. Thermal Oil System Chemical Cleaning Methods

Chemical cleaning of thermal oil systems is a technically demanding operation, requiring targeted cleaning solutions based on the degree of coking, equipment materials, and system characteristics. The commonly used chemical cleaning methods are primarily the following three:

1. Solvent Dissolution Method. Organic solvents are used to clean systems with light coking. Common solvents include diesel, kerosene, and aromatic solvent oils. The solvent is injected into the system and circulated at 80 to 120°C for 8 to 24 hours, utilizing the principle of like-dissolves-like to dissolve and disperse coke deposits. This method is suitable for mild contamination during the early stages of coking, causing minimal equipment damage, but with relatively low cleaning efficiency.

2. Alkaline Boiling Method. High-temperature alkaline solution prepared with sodium hydroxide and surfactants is used for cleaning. The alkaline solution concentration is 2% to 5%, with 0.3% to 0.5% penetrant and dispersant added, circulated at 100 to 120°C for 12 to 24 hours. The alkaline solution saponifies some organic matter, loosening and detaching the coke layer. The alkaline boiling method is suitable for moderate coking levels, providing good cleaning results, but generates a relatively large volume of waste liquid.

3. Chemical Decoking Agent Method. This is currently the most widely applied and most effective method. Specialized chemical decoking agents contain multiple components including organic solvents, penetrants, dispersants, and corrosion inhibitors. The decoking agent penetrates into the interior of the coke layer, disrupting the cross-linked structure between coke molecules, softening and dispersing the coke layer, and stripping it from the metal surface. Cleaning temperature is controlled at 80 to 100°C, with circulation time generally 12 to 48 hours depending on the degree of coking. This method achieves significant results for heavy coking without damaging the metal substrate.

III. Typical Cleaning Case Studies

Case 1: 12 Million kcal/h Thermal Oil Boiler Cleaning at a Chemical Enterprise. This boiler had operated continuously for 5 years without thorough cleaning, with severe furnace tube coking. The exhaust gas temperature had risen from the design value of 250°C to 380°C, and fuel consumption had increased by 30%. We adopted a staged chemical cleaning approach: First, a specialized decoking agent was used at 90°C for 48 hours of circulation cleaning to dissolve and disperse the majority of the coke layer; then the alkaline boiling method was applied at 110°C for 12 hours to thoroughly remove residual coke; finally, water flushing and drying treatment were performed. After cleaning, the exhaust gas temperature recovered to 260°C, fuel consumption decreased by 28%, and annual fuel cost savings were approximately ¥1.2 million.

Case 2: Thermal Oil Pipeline System Cleaning at a Textile Printing and Dyeing Enterprise. This system comprised 500 meters of thermal oil pipelines and multiple heat-consuming devices. Due to long-term operation, coking on the inner walls of pipes caused the circulation pump current to rise by 40%. The online circulation cleaning method was adopted, injecting decoking agent into the system and utilizing the system's own circulation pump for cleaning. After 36 hours of circulation cleaning, the system circulation resistance returned to normal, the circulation pump current dropped to normal levels, and thermal oil flow rate recovered to the design value. The entire cleaning process required no pipe disassembly, with downtime of only 3 days.

IV. Cleaning Precautions

1. Before thermal oil cleaning, the system must first be completely drained of thermal oil and purged with nitrogen to ensure no combustible gases remain in the system, preventing fire or explosion during the cleaning process.

2. For severely coked systems, strong acid cleaning must not be used, as acid may penetrate the coke layer and directly corrode the metal substrate, causing localized perforation.

3. During cleaning, temperature must be strictly controlled and must not exceed the design service temperature of the thermal oil, to avoid thermal damage to equipment.

4. After cleaning, the system must be thoroughly dried. Residual moisture will affect the performance of new thermal oil and equipment safety.

5. Waste liquid contains organic matter and coke residues and must be collected and entrusted to qualified units for harmless treatment. Direct discharge is prohibited.

For thermal oil boiler cleaning services, please contact Danyang Blue Star Anti-corrosion Cleaning Co., Ltd.

Tel: +86 18952832843

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