I. Introduction: The Core Role of Gas Primary Coolers in Coking Processes
The gas primary cooler is the first cooling stage in a coking plant's gas purification system, receiving raw coke oven gas at 650–750°C from the carbonization chamber and cooling it to 30–35°C via circulating cooling water, thereby creating the process conditions required for subsequent desulfurization, benzol scrubbing, and ammonia removal. The operating condition of the primary cooler directly determines the production efficiency of the entire by-product recovery plant—once cooling efficiency declines, the gas outlet temperature rises, causing not only a significant drop in chemical product yield but also a surge in exhauster load and power consumption, potentially forcing the coke oven to reduce production in severe cases.
Because raw coke oven gas entrains large quantities of tar mist, naphthalene vapor, H₂S, HCN, and coal dust, these substances continuously condense and deposit on heat exchanger tube surfaces during the cooling process, forming a composite scale layer with tar as the matrix. Unlike ordinary water scale or rust scale, the scale in gas primary coolers is characterized by high viscosity, strong adhesion, flammability, and corrosive media content. Conventional mechanical cleaning is ineffective, and targeted chemical cleaning solutions must be employed.
II. In-Depth Analysis of Gas Primary Cooler Scaling Mechanisms
Scaling in gas primary coolers is a complex multi-component co-deposition process. Scale composition varies significantly across different zones and must be addressed separately. Horizontal-tube primary coolers are divided into upper, middle, and lower sections, each with different operating temperatures and condensing media, leading to varying scale characteristics.
Tube-Side Scaling (Inside Tubes): Cooling water flows inside the tubes, where Ca²⁺ and Mg²⁺ react with HCO₃⁻ under heated conditions to form CaCO₃ and Mg(OH)₂ scale deposits on tube walls. Suspended solids and microbial slime from the cooling water also adhere to surfaces. Although this water scale is conventional, it reduces the tube wall thermal conductivity, raising the outer tube wall temperature, which paradoxically accelerates the carbonization and hardening of shell-side tar scale.
Shell-Side Scaling (Outside Tubes): This is the core difficulty in gas primary cooler cleaning. Tar mist droplets from the raw coke oven gas condense on tube walls, initially as a viscous liquid but gradually hardening and coking as light fractions evaporate over time. Simultaneously, naphthalene vapor crystallizes when the temperature drops below 80°C, embedding as white flake crystals within the tar matrix, while coal dust and coke breeze act as a "skeleton" filling the tar scale, forming a dense, hard, three-dimensional composite scale layer. During a shutdown inspection at one coking plant, the shell-side scale thickness of a primary cooler that had operated for only 8 months already reached 3–5mm, with localized blockages exceeding 40% of the tube bundle cross-section.
Furthermore, H₂S and HCN in the gas dissolve in condensate to form an acidic environment, causing uniform corrosion and pitting on carbon steel heat exchanger tubes. Under-deposit corrosion rates can reach 3–5 times those in a scale-free state. Therefore, timely scale removal not only restores cooling efficiency but is also an essential measure for extending equipment life.
III. Chemical Cleaning Agent System Design
The composite nature of gas primary cooler scale means that a single cleaning agent cannot meet requirements; a multi-component synergistic compound cleaning system must be constructed. Scientific cleaning solution design begins with scale sample analysis.
Scale Sample Analysis: Collect scale samples from different locations for loss-on-ignition testing and solvent extraction testing to determine tar content (typically 40%–70%), naphthalene content (10%–25%), and inorganic ash (5%–15%). Use X-ray diffraction to analyze the proportions of inorganic components such as CaCO₃, CaSO₄, and SiO₂ in the ash, providing a basis for selecting the inorganic acid cleaning stage.
Organic Solvent Stage: Tar scale and naphthalene scale are primarily organic in nature and must first be dissolved and removed by solvents. In practice, a compound system of aromatic solvents (xylene/heavy aromatics) and ketone co-solvents is used at a working concentration of 10%–15% and a circulation temperature of 60–80°C. This solvent system penetrates the scale layer via the principle of like-dissolves-like, softening the tar and dissolving naphthalene crystals. Immersion and circulation alternate, with a single-stage duration of 6–12 hours, adjusted based on scale thickness.
Surfactant Enhancement Stage: After solvent dissolution, residual asphaltenes and carbonized layers still require further treatment. Non-ionic surfactants (such as fatty alcohol ethoxylates) are added to reduce oil-water interfacial tension, emulsifying and dispersing residual tar into the cleaning solution for removal. Surfactant concentration is controlled at 0.5%–1.0%, with a circulation temperature of 50–60°C, combined with intermittent nitrogen agitation to enhance mass transfer.
Inorganic Acid Cleaning Stage (Optional): If the water scale layer is thick, acid cleaning is added after organic cleaning. Typically, 5%–8% sulfamic acid with corrosion inhibitor or a 3%–5% citric acid system is circulated for 2–4 hours to remove residual CaCO₃ scale. The acid cleaning stage must strictly control temperature (≤60°C) and corrosion inhibitor dosage to prevent corrosion of carbon steel tube bundles.
Neutralization and Passivation Stage: After cleaning, neutralize residual acid with Na₂CO₃ solution to pH 7–8, then passivate with 0.5%–1.0% sodium nitrite to form a protective film on metal surfaces and prevent flash rusting.
IV. High-Pressure Water Jetting as an Auxiliary Cleaning Process
Chemical cleaning dissolves the bulk of the tar scale, but dead zones such as tube bends, baffle plate backsides, and under-deposit hard spots may still retain residues. High-pressure water jetting serves as a supplementary measure to chemical cleaning, significantly improving final cleanliness.
For horizontal-tube primary coolers, perform tube-by-tube high-pressure water flushing after chemical cleaning. Operating pressure is 1500–2800 bar with a flow rate of 40–60 L/min, using rotating nozzles to provide 360° water curtain coverage inside the tubes. For shell-side areas, use a flexible lance inserted through the manway, flushing inter-tube spaces segment by segment at 1800–2200 bar, with emphasis on clearing baffle plate scale accumulation zones.
Beyond physical stripping, high-pressure water jetting generates instantaneous high-pressure water hammer effects that can fracture residual scale structures, particularly effective on dense carbonized layers formed by coal dust and tar. During flushing, observe changes in effluent turbidity; flushing is deemed complete when discharge water becomes clear and transparent with no visible suspended solids. High-pressure water jetting for a single primary cooler typically takes 4–8 hours, depending on tube count and blockage severity.
V. Standardized Cleaning Process Flow
A complete gas primary cooler cleaning, from site survey to acceptance and handover, typically requires 3–5 working days. The standardized process flow is as follows:
1. Site Survey and Pre-Work Preparation: Confirm the primary cooler model, tube material (carbon steel/stainless steel), scale distribution, and measure heat exchange area and tube count. Isolate gas inlet and outlet valves and install blind flanges. Drain residual liquid and purge combustible gases with nitrogen to below the lower explosive limit. Set up a temporary circulation cleaning system, including cleaning pump station, solution preparation tank, heating device, and piping connections.
2. Water Flushing and Pressure Test: Flush tube-side and shell-side with industrial water to remove loose dust and deposits. Conduct a hydrostatic pressure test of the circulation system at 0.4–0.6 MPa; confirm no leaks before beginning chemical cleaning.
3. Organic Solvent Cleaning (Main Cleaning Stage): Inject the prepared solvent cleaning solution, start the circulation pump, and heat with steam to 60–80°C. Use an alternating mode of 2 hours circulation → 1 hour immersion, sampling every 30 minutes to test changes in cleaning solution color and viscosity. When color no longer deepens over two consecutive samples and scale is visually largely detached, drain the spent solvent.
4. Surfactant Enhancement Cleaning: Inject surfactant cleaning solution and circulate at 50–60°C for 2–3 hours to emulsify and remove residual tar and naphthalene. Rinse twice with 60–80°C hot water after draining.
5. Acid Cleaning for Scale Removal (As Needed): If water scale is evident, inject sulfamic acid or citric acid cleaning solution and circulate for 2–4 hours, monitoring acid concentration and iron ion content. When acid concentration no longer decreases, water scale dissolution is complete; drain the spent acid solution.
6. Neutralization and Passivation: Neutralize with Na₂CO₃ solution to pH 7–8, circulate for 30 minutes, and drain. Inject sodium nitrite passivation solution and circulate at 50°C for 1–2 hours. Drain after a gray passivation film forms on the tube walls.
7. High-Pressure Water Jetting Fine Cleaning: Immediately after draining chemical cleaning solutions, perform tube-by-tube high-pressure water flushing (1500–2800 bar), focusing on residual deposits in dead zones until discharge water is clear. For the shell side, flush baffle plate zones segment by segment with a flexible lance.
8. Acceptance and Restoration: Visually inspect tube bundle cleanliness (surface free of visible scale, bright metal appearance). Spot-check tube inner walls with a white cloth wipe—no stains permitted. Remove blind flanges and restore piping connections. Purge air with nitrogen before restoring gas pathways. Issue an acceptance report documenting key parameter comparisons before and after cleaning.
VI. Cleaning Effectiveness Evaluation and Economic Benefits
Scientific cleaning effectiveness must be measured with quantitative indicators. The following parameters should be recorded and compared before and after cleaning: gas outlet temperature, cooling water temperature difference, heat transfer coefficient, and system pressure drop.
Taking a cleaning case at a large coking plant's horizontal-tube primary cooler as an example: before cleaning, the gas outlet temperature was as high as 42–45°C (design value: 30–32°C), the heat transfer coefficient was only 55% of design, and coke oven production had to be reduced by 10%–15% in summer. After the comprehensive chemical cleaning + high-pressure water jetting solution described above, the gas outlet temperature recovered to 31–33°C, the heat transfer coefficient improved to 96% of new equipment levels, and the system pressure drop decreased from 18 kPa before cleaning to 9 kPa. Annual electricity savings for the exhauster amounted to approximately 800,000 kWh. The benzene yield in chemical products increased by 0.05 percentage points, with crude benzene production increasing by approximately 200 tons per year. The comprehensive economic benefit exceeded RMB 3 million.
Restoring cooling efficiency to 95% or more of new equipment levels is the basic benchmark for cleaning compliance. For heavily scaled primary coolers, a cleaning interval of 6–12 months is recommended; units operating under favorable conditions (fixed coal type, low dust content in raw gas) may extend intervals to 12–18 months. Establishing a regular cleaning regime ensures more consistent and stable coking production than post-failure repair cleaning.
VII. Precautions and Daily Maintenance Recommendations
Safety Management: Gas primary cooler cleaning involves work in flammable gas environments, requiring strict compliance with hot work and confined space management protocols. Before cleaning, confirm effective gas isolation and qualified nitrogen purging (combustible gas concentration below 10% of LEL). The organic solvent cleaning stage requires explosion-proof electrical equipment and adequate ventilation; operators must wear gas masks and chemical-resistant gloves. Waste liquid contains tar, naphthalene, aromatics, and other hazardous substances and must be collected for disposal by qualified hazardous waste treatment facilities—direct discharge is strictly prohibited.
Online vs. Offline Cleaning Selection: Online cleaning (without production stoppage) is only suitable for mild blockages in the early scaling stage, using specialized dispersant injection to slow scale growth, but its descaling effectiveness is limited. Moderate to severe scaling requires scheduled offline cleaning during coke oven maintenance windows, with each primary cooler typically cleaned in 2–3 days. We recommend incorporating primary cooler cleaning into the annual overhaul plan and executing it concurrently with coke oven maintenance.
Routine Operational Maintenance: Monitor gas outlet temperature and cooling water inlet/outlet temperature differential daily. If the outlet temperature exceeds the design value by more than 5°C and continues to rise, accelerated scaling is indicated. Apply water treatment chemicals (scale inhibitor and corrosion inhibitor) to the circulating cooling water regularly, controlling the concentration factor to no more than 3×, thereby mitigating water scale formation at the source. Open the manway quarterly to inspect shell-side scaling conditions with photographic documentation, building a trend archive to enable predictive cleaning rather than reactive repair.