Identifying and designing the critical zones in a combustion chamber

Why zoning makes the difference

A combustion chamber is not a homogeneous system. Completely different combinations of loads act in different areas: temperature, chemical attack, erosion, thermal cycling and mechanical loading vary greatly depending on the position and function of each lining area. Whoever lines the whole combustion chamber with a single material either wastes money in uncritical areas or risks premature failure in the high-load zones. Deliberate zoning — assigning the optimum material to each load zone — is the key to an economical and long-lasting lining.

The typical weak points

• Burner surround: extreme temperature peaks and cyclical thermal loads from direct flame contact call for high-temperature-resistant, thermal-shock-resistant materials
• Slag zone: chemical attack by aggressive slag melts combined with erosion demands corrosion-resistant and dense materials
• Grate-to-wall transition: mechanical loading from grate cleaning and falling ash combined with high temperature makes this area particularly vulnerable
• Constriction and boiler inlet: accelerated flow and fly ash loading cause heavy erosion, which calls for abrasion-resistant materials
• Joints and transitions: changes of material and expansion joints are potential weak points that have to be carefully resolved in the design

Designing the critical zones

Correctly designing a critical zone involves more than choosing the right material. The wall thickness has to be dimensioned generously enough to ensure both the wear allowance over the planned campaign length and the required thermal insulation. The anchoring has to absorb the thermal expansion of the material without causing stress cracks. And the structural connection to the adjacent areas has to compensate for relative movement between different material zones.

In practice, the design often has to balance material costs against service life. High-grade special materials increase the investment cost per square metre, but can double or triple the service life in critical areas. The commercial assessment has to take the full life cycle costs into account: material costs, installation costs, downtime and lost production. In the vast majority of cases, the higher-grade material in the critical zones is the commercially better choice.

Preventive monitoring and acting in good time

Even with optimum design, critical zones are subject to natural wear. What matters is monitoring this wear and initiating countermeasures in good time. Regular inspections, temperature monitoring on the outer furnace wall and systematic documentation of wear patterns make it possible to plan repairs specifically and to avoid unplanned shutdowns. Care spraying at the right moment can defer a costly complete relining by a further campaign.