Thermal Oil Boiler and Pipeline Cleaning | Organic Heat Carrier Furnace Decoking

I. Causes and Hazards of Coking in Thermal Oil Boilers

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 prolonged 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°C and 350°C. At such elevated temperatures, thermal oil molecules undergo carbon-carbon bond cleavage, generating free radicals that subsequently trigger polymerization reactions to form high-molecular-weight gums and asphaltenes.

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

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

II. Chemical Cleaning Methods for Thermal Oil Systems

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

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

2. Alkaline Boiling Method. High-temperature alkaline solutions formulated with sodium hydroxide and surfactants are used for cleaning. The alkaline solution concentration is 2%–5%, with 0.3%–0.5% penetrant and dispersant added, circulated at 100–120°C for 12–24 hours. The alkaline solution saponifies some organic matter, loosening and detaching the coke layer. The alkaline boiling method is suitable for moderate coking levels, offering good cleaning results but generating larger volumes of waste liquid requiring treatment.

3. Chemical Decoking Agent Method. This is currently the most widely used 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 cross-linked structures between coke molecules, softening, dispersing, and stripping the coke from metal surfaces. The cleaning temperature is controlled at 80–100°C, with circulation time generally 12–48 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: Cleaning of a 12 Million kcal/h Thermal Oil Boiler at a Chemical Enterprise. This boiler had operated continuously for 5 years without thorough cleaning, resulting in severe furnace tube coking. The exhaust gas temperature had risen from the design value of 250°C to 380°C, with fuel consumption increasing by 30%. We adopted a staged chemical cleaning approach: first, a specialized decoking agent was circulated at 90°C for 48 hours to dissolve and disperse the bulk of the coke layer; then alkaline boiling 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%, yielding annual fuel cost savings of approximately RMB 1.2 million.

Case 2: Thermal Oil Pipeline System Cleaning at a Textile Printing and Dyeing Enterprise. This system included 500 meters of thermal oil piping and multiple heat-consuming devices. Long-term operation had caused coking on the pipe inner walls, resulting in a 40% increase in circulation pump current. An online circulation cleaning method was adopted, injecting the decoking agent into the system and using 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 the thermal oil flow rate recovered to the design value. The entire cleaning process required no pipe disassembly and resulted in only 3 days of downtime.

IV. Cleaning Precautions

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

2. For heavily 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. The temperature during the cleaning process must be strictly controlled and must not exceed the design operating temperature of the thermal oil, to avoid thermal damage to the equipment.

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

5. The waste liquid contains organic matter and coke residues and must be collected and disposed of by 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.

Contact: +86 18952832843