Stainless Steel Water-Cooled Wall Coil Tube for Coal-to-Hydrogen Circular Gasifiers
1. Product Overview
The Water-Cooled Wall Coil Tube (also referred to as 盘管 / Coiltube) is a precision-engineered stainless steel tubular component designed specifically for integration into the internal water-cooled wall systems of coal-to-hydrogen circular gasifiers (Coal-to-Hydrogen Circular Gasifier). These coiltubes serve as the primary heat-transfer elements within the gasifier's combustion chamber, absorbing extreme radiant and convective heat loads while simultaneously protecting the refractory lining and structural shell from thermal degradation.
Developed and supplied by Wenzhou Jixing Metal Materials Co., Ltd. (Product Catalog No. 992), this coiltube product line represents a specialized solution for high-temperature, high-pressure, and high-corrosion environments encountered in coal gasification and hydrogen production processes. The coiltube is fabricated from austenitic stainless steel alloys, rolled into a continuous spiral or serpentine configuration, and welded into modular panels that form the complete water-cooled wall assembly.
Key Function: The coiltube operates as a closed-loop heat exchanger, circulating pressurized cooling water through its interior to maintain the gasifier shell temperature within safe operational limits (typically 400–600°C on the outer surface) while the inner wall faces temperatures exceeding 1,000°C during coal gasification reactions.
2. Material & Properties
2.1 Base Material Grades
The coiltube is manufactured from high-performance austenitic stainless steel grades selected for their exceptional resistance to thermal fatigue, oxidation, and carburization at elevated temperatures:
- SAE 304 (UNS S30400 / 06Cr19Ni10): General-purpose grade with 18% Cr and 8% Ni, suitable for moderate temperature and corrosion conditions.
- SAE 316L (UNS S31603 / 022Cr17Ni12Mo2): Enhanced grade with 2–3% Mo addition for superior resistance to pitting and crevice corrosion in chlorinated or sulfurous environments.
- SAE 321 (UNS S32100 / 06Cr18Ni11Ti): Titanium-stabilized grade offering excellent resistance to intergranular corrosion and thermal cracking during welding and high-temperature service.
- SAE 347 (UNS S34700 / 06Cr18Ni11Nb): Niobium-stabilized variant providing enhanced creep strength and thermal fatigue resistance for demanding gasifier applications.
2.2 Mechanical Properties (Typical)
| Property | SAE 304 | SAE 316L | SAE 321 | SAE 347 |
|---|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 485 | ≥ 515 | ≥ 515 |
| Yield Strength (MPa, 0.2% offset) | ≥ 205 | ≥ 170 | ≥ 205 | ≥ 205 |
| Elongation at Break (%) | ≥ 40 | ≥ 35 | ≥ 40 | ≥ 40 |
| Thermal Conductivity at 100°C (W/m·K) | 16.3 | 14.6 | 16.3 | 16.3 |
| Maximum Service Temperature (°C) | 870 | 870 | 925 | 925 |
| Coefficient of Thermal Expansion (20–100°C, ×10⁻⁶/K) | 17.3 | 16.0 | 17.3 | 17.3 |
2.3 Chemical Composition (Typical, wt%)
| Element | SAE 304 | SAE 316L | SAE 321 | SAE 347 |
|---|---|---|---|---|
| C | ≤ 0.08 | ≤ 0.03 | ≤ 0.08 | ≤ 0.08 |
| Cr | 18.0–20.0 | 16.5–18.5 | 17.0–19.0 | 17.0–19.0 |
| Ni | 8.0–10.5 | 10.0–14.0 | 9.0–13.0 | 9.0–13.0 |
| Mo | — | 2.0–3.0 | — | — |
| Si | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 |
| Mn | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 |
| Ti | — | — | 5×C to 0.70 | — |
| Nb | — | — | — | 10×C to 1.10 |
| P | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
3. Specifications
3.1 Coiltube Geometric Dimensions
| Parameter | Specification | Notes |
|---|---|---|
| Outer Diameter (OD) | Φ25 mm – Φ76 mm | Most common: Φ32 mm, Φ38 mm, Φ45 mm |
| Wall Thickness (t) | 2.0 mm – 5.0 mm | Dependent on pressure rating and thermal load |
| Coil Pitch (Center-to-Center Spacing) | 40 mm – 120 mm | Determined by heat flux density requirements |
| Coil Diameter (Loop Diameter) | 150 mm – 400 mm | Adapted to gasifier chamber geometry |
| Panel Length (Straight Segment) | 1,500 mm – 6,000 mm | Customized per gasifier design |
| Panel Height | 1,200 mm – 4,500 mm | Corresponds to gasifier shell height |
| Total Panel Area | 15 m² – 80 m² per panel | Modular assembly units |
| Weld Joint Type | Full-penetration butt weld / TIG weld | GTAW process with backing |
| Surface Finish | Pickled & passivated (Ra ≤ 1.6 μm) | Or as-surfaced per customer requirement |
3.2 Operating Conditions
| Parameter | Specification |
|---|---|
| Design Pressure (Internal Water Side) | 1.6 MPa – 4.0 MPa |
| Design Temperature (Hot Side) | Up to 1,200°C (gas/flame side) |
| Cooling Water Inlet Temperature | 80°C – 120°C |
| Cooling Water Outlet Temperature | 120°C – 180°C |
| Maximum Heat Flux | Up to 80 kW/m² (typical); peak up to 120 kW/m² |
| Design Life | ≥ 15 years continuous operation |
| Applicable Standards | ASME BPV Section I / VIII, EN 13445, GB/T 150, NB/T 47003 |
4. Manufacturing Process
The manufacturing of water-cooled wall coiltubes for coal-to-hydrogen gasifiers follows a rigorous multi-stage process designed to ensure dimensional accuracy, metallurgical integrity, and long-term service reliability:
4.1 Process Flow
- Raw Material Procurement & Inspection: Seamless stainless steel tubes are sourced from certified mills and subjected to spectroscopic analysis (OES), ultrasonic testing (UT), and hydrostatic pressure testing to verify chemical composition, internal soundness, and pressure integrity.
- Tube Straightening & Cutting: Tubes are cold-straightened to remove mill curvature, then precisely cut to required lengths using plasma or saw cutting, with deburring and chamfering at both ends to prepare for welding.
- Coil Forming (Rolling): The straightened tubes are cold-rolled into serpentine or spiral coil configurations using precision multi-roll forming machines. The coil pitch and loop diameter are controlled to within ±0.5 mm tolerance. Forming is performed at room temperature or with controlled low-temperature preheating to minimize work-hardening effects.
- Fixture & Positioning: Individual coiltube segments are positioned on precision welding fixtures (jigs) to maintain accurate spacing, alignment, and geometric configuration of the panel assembly.
- Welding Assembly: The coiltube segments are welded to inlet/outlet headers and interconnecting tubes using Gas Tungsten Arc Welding (GTAW / TIG) with argon shielding gas. For thicker sections, a hybrid approach combining GTAW for the root pass and Gas Metal Arc Welding (GMAW) for subsequent passes may be employed. Welding parameters (current, voltage, travel speed, gas flow rate) are optimized per WPS (Welding Procedure Specification) qualified under ASME Section IX or ISO 15614.
- Post-Weld Heat Treatment (PWHT): Welded assemblies undergo solution annealing (1,050°C – 1,150°C, water quench) to relieve residual stresses, restore full austenitic microstructure, and enhance corrosion resistance. Stabilization treatment (850°C – 900°C) may be applied for Ti/Nb-stabilized grades (321/347).
- Pickling & Passivation: The entire assembly is pickled using mixed acid solutions (HNO₃/HF or HNO₃/HCl) to remove weld oxide scales, followed by passivation treatment to form a protective chromium oxide layer (Cr₂O₃) on the surface.
- Non-Destructive Testing (NDT):
- Radiographic Testing (RT): Full-length RT inspection of all weld joints per ASME Section V, Article 2, with acceptance criteria per ASME Section VIII or Section I.
- Ultrasonic Testing (UT): Phased array UT (PAUT) for weld root inspection and volumetric flaw detection.
- Penetrant Testing (PT): Surface-breaking defect detection on all accessible weld surfaces.
- Helium Leak Testing: Leak rate verification ≤ 1×10⁻⁶ Pa·m³/s for water-side integrity.
- Hydrostatic Pressure Testing: 1.5× design pressure, held for minimum 30 minutes with zero pressure drop acceptance.
- Dimensional Inspection & Final QA: CMM (Coordinate Measuring Machine) or laser scanning verification of coil geometry, pitch uniformity, and panel flatness within ±1 mm/m tolerance.
- Marking, Packaging & Shipping: Each panel is marked with material grade, heat number, weld map, NDT report reference, and serial number. Panels are packaged with moisture-absorbent desiccants and protective wrapping for transportation.
4.2 Welding Repair Process (Welding Repair & Rework)
A critical aspect of coiltube manufacturing and field maintenance is the welding repair process (焊接返修工艺), which addresses defects identified during NDT or in-service failures:
- Defect Identification & Classification: NDT-identified defects are classified per ASME Section IX or relevant code (e.g., slag inclusion, porosity, undercut, incomplete fusion, cracks).
- Defect Removal: Defective weld material is mechanically removed using grinding, chiseling, or low-amperage DC arc gouging to expose the full extent of the defect, with a minimum overlap of 2× the defect length beyond visible boundaries.
- Pre-Heat & Interpass Temperature Control: Pre-heating to 100°C – 200°C (for thicker sections) to reduce cooling rates and minimize hydrogen-induced cracking risk. Interpass temperature maintained below 250°C.
- Repair Welding: GTAW repair welding using filler wire of matching or slightly higher alloy grade (e.g., ER308L for 304, ER316L for 316L). Minimum two-pass repair welding with full-penetration root preparation.
- Post-Repair NDT: Full re-inspection of the