CLADDING TECHNOLOGY SHANXI CO., LTDCladding of Pulverized Coal Slurry Pressurized Gasifier Nozzles Technical Review
Literature Overview
This 1998 publication by Yang Shanglei and Zhang Wenhong, published in the context of Shandong Lunan Chemical Industrial Group, addresses one of the most demanding weld overlay challenges in the coal-to-chemicals industry — the cladding of pressurized gasifier nozzles used in pulverized coal slurry gasification systems. The gasifier nozzle operates under a uniquely severe combination of high temperature, high pressure, chemical erosion, and abrasive wear from coal particles suspended in slurry. This document represents an early engineering effort to extend the service life of these critical consumable components through appropriate overlay selection and welding process optimization.
Operating Environment and Failure Analysis
The pulverized coal slurry pressurized gasifier nozzle operates under the following conditions:
| Parameter | Typical Range |
|---|---|
| Operating temperature | 1200–1400 °C at flame tip |
| Operating pressure | 2.0–4.0 MPa |
| Wear mechanism | Erosion + corrosion + thermal cycling |
| Medium | Coal-water slurry (40–60 wt% solids) |
| Typical service life (unc clad) | 200–400 hours |
| Target life after cladding | 1000–2000 hours |
The primary failure mode of uncladded nozzles is erosive-corrosive wear at the throat and exit region, where the coal slurry impinges at high velocity on the refractory-lined metal surface. Thermal fatigue cracking also occurs at the weld boundary between the cladding layer and the base material due to thermal cycling between startup and shutdown.
Overlay Material Selection
Based on the severe service conditions, the overlay material selection follows the following logic:
- Hardfacing alloys with chromium-carbide structure: Materials such as Stellite 6 (Co-Cr-W) or high-chromium cast iron provide excellent erosion resistance and moderate corrosion resistance in oxidizing environments.
- Nickel-based alloys: Inconel 625 or Incoloy 800HT provide superior hot corrosion resistance at elevated temperatures, particularly in the presence of sulfur and alkali metals from coal ash.
- Refractory metal overlays: For the highest temperature zones, tungsten or molybdenum-based overlays may be considered, though weldability is significantly reduced.
The typical overlay design employs a multi-pass strategy: a transition layer of nickel-based alloy (e.g., Ni-27Cr) to reduce the dilution effect and prevent cracking, followed by 2–3 passes of the final hardfacing or nickel-based wear-resistant alloy.
Welding Process Considerations
The geometry of the gasifier nozzle presents significant welding access challenges:
- Nozzle geometry: Cylindrical or conical shape with internal refractory lining
- Weld access: Limited to the outer surface or through machined channels
- Residual stress: High due to the combination of cladding heat input and the thick base material (typically 20–40 mm wall thickness)
The recommended welding process is submerged arc welding (SAW) for the transition layer and multi-wire SAW or flux-cored arc welding (FCAW) for the final overlay passes. The process parameters typically include:
| Parameter | Transition Layer | Final Overlay |
|---|---|---|
| Process | SAW | FCAW / Multi-wire SAW |
| Current | 350–450 A | 400–550 A |
| Voltage | 28–32 V | 30–36 V |
| Travel speed | 100–150 mm/min | 120–200 mm/min |
| Preheat temperature | 150–200 °C | 150–200 °C |
| Interpass temperature | < 250 °C | < 250 °C |
| Post-weld heat treatment | 600 °C × 2h | As required |
Key Technical Challenges and Countermeasures
Dilution Control
The dilution rate from the carbon steel base material can exceed 30% in the first overlay pass, significantly degrading the corrosion and wear resistance of the final overlay. Countermeasures include:
- Using a nickel-based transition layer with low carbon content
- Applying a back-arc gas shielding to reduce dilution from the root side
- Increasing the number of overlay passes to progressively dilute the base material influence
Cracking Prevention
Hot cracking in the nickel-based overlay is a major concern due to the formation of low-melting-point eutectics at grain boundaries. The following measures are recommended:
- Maintaining the carbon content of the overlay alloy below 0.05%
- Adding trace amounts of titanium or boron to refine the grain structure
- Controlling the interpass temperature to prevent excessive grain growth
Residual Stress Management
The thermal cycling during operation can cause fatigue cracking at the cladding-to-base interface. Post-weld stress relief at 600–650 °C for 2 hours per 25 mm of thickness is essential. The stress relief temperature must be carefully controlled to avoid sensitization of any stainless steel or nickel-based components.
Engineering Practice Insights
From a practical standpoint, the most critical lesson from this work is the recognition that nozzle cladding is not a standalone operation but must be integrated into the overall gasifier design philosophy. The refractory lining and the metal cladding must be designed to work together, with the cladding providing the first line of defense against erosion while the refractory absorbs the bulk of the thermal load. The interface between the refractory and the cladding must be designed to accommodate differential thermal expansion without delamination.
The 1998 publication predates modern simulation capabilities and comprehensive materials databases, which means that the overlay selection was largely based on empirical experience and trial-and-error. Today, engineers can leverage computational fluid dynamics (CFD) to predict the impingement zone and erosion pattern, allowing for targeted cladding application rather than uniform coverage. This represents a significant evolution in the approach to nozzle protection.
Summary and Implications
The cladding of pulverized coal slurry pressurized gasifier nozzles represents a classic example of the intersection between materials selection, welding process engineering, and process design. The key takeaway is that successful cladding requires not only the right alloy but also the right process parameters, the right sequence of operations, and the right integration with the overall component design. The multi-pass strategy with a transition layer remains the industry standard approach, and the principles established in this 1998 work remain fundamentally applicable to modern gasifier nozzle cladding, even as materials and simulation tools have advanced considerably.