Materials technology in biomass power plant combustion chambers

The particular challenges of biogenic fuels

Burning biomass differs fundamentally from burning fossil fuels. Wood chips, waste wood, bark and other biogenic fuels have widely varying compositions. Moisture content, ash content and chemical composition vary not only between different types of fuel, but also between individual deliveries of the same type. This variability presents the refractory lining of the combustion chamber with particular challenges that cannot be adequately mastered with standardised material concepts.

Biomass ashes contain high proportions of alkalis such as potassium and sodium, which form aggressive melt phases at combustion temperatures. These melt phases attack the refractory lining chemically and lead to accelerated wear. The combination of high alkali contents and low ash softening points, as occurs when burning straw or cereal residues, is particularly critical.

Material selection as the key factor

• High-alumina refractory materials are highly refractory and abrasion-resistant, but under alkali attack they form volume-increasing phases such as kalsilite or leucite, which can spall the microstructure – in alkali-laden zones, dense, alkali-resistant fireclay or andalusite grades are usually the better choice
• Silicon carbide-based materials combine high wear resistance with good thermal shock resistance, but are more expensive to purchase
• Chromium oxide special materials are used in extreme areas, but require particular disposal routes
• Castables based on low-cement bonding systems make jointless linings possible in complex geometries

Zoning of the combustion chamber

A modern combustion chamber lining is not executed with a single uniform material, but divided into zones, each of which is lined with the optimum material. The grate and feed area is subject to heavy mechanical and thermal loading and requires abrasion-resistant, thermal-shock-resistant materials. The afterburning zone is exposed to the most intense chemical attack from ash melts and needs corrosion-resistant materials. The radiant section and the constriction are primarily thermally loaded and are designed with highly refractory materials.

When planning a combustion chamber lining we work closely with the operators to establish the specific load profiles. Which fuels are used? How high is the ash content? Are there seasonal variations in fuel quality? On the basis of this information we develop a tailor-made materials concept that maximises service life and minimises maintenance costs.

Design details as drivers of service life

Besides the choice of material, design details decide the success or failure of a combustion chamber lining. The layout of the expansion joints, the execution of the anchoring, the avoidance of thermal bridges and the connection to adjacent components all call for years of experience and a deep understanding of thermomechanical relationships. A frequently underestimated detail is the design of the transition areas between different material zones. Here materials with differing thermal expansion behaviour meet. Without suitable expansion compensation, stress cracks form at these points that serve as an entry route for aggressive media and lead to premature failure.

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