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Why materials fail in hot, dirty places
From a boiler tube to a single grain of ilmenite: what heat, ash and alkali do to materials, and how understanding it makes biomass energy work.
Based on the research of Pavleta Knutsson, Chalmers University of Technology.
Burning wood instead of coal sounds like a simple swap. It is not. Biomass carries its own chemistry: potassium, sodium, calcium, chlorine and sulfur that end up in the ash. At 600–900 °C these elements do not sit still. They travel with the hot gas, condense on surfaces, and react with whatever they touch.
The things they touch are expensive: superheater tubes, coated components, and the tonnes of hot, sand-like bed material that keep a fluidized-bed boiler or gasifier running. Whether a plant lasts for decades or needs constant repair depends on what happens at the scale of micrometres.
The skin that keeps metal alive
High-temperature alloys survive because they grow a thin oxide skin, a scale of Al2O3 or Cr2O3, that slows further attack. The scale only protects while it holds on. Early work in this research line looked at exactly that grip: how platinum changes the interface between NiAl and its oxide (Svensson et al., Corrosion Science, 2008), and how voids form at, and can heal from, that interface.
When the gas itself is corrosive
Corrosion is usually pictured as a salt deposit eating into metal. But alkali compounds such as Na2SO4 and KOH can attack directly from the gas phase. Studying that requires an experiment that delivers the salt as a vapour, not as a coating, which is what a laboratory method for hot corrosion by gaseous Na2SO4 set out to do (Knutsson et al., Corrosion Science, 2013).
The bed that fights back
In a fluidized bed, the bed particles meet the ash first. For a long time they were treated as inert sand. Replacing up to 40 wt% of the sand in Chalmers' 12 MW boiler with the natural ore ilmenite showed otherwise: iron moved to the particle surfaces, titanium concentrated in the core, and potassium diffused all the way into the core (Corcoran et al., Energy & Fuels, 2014). Potassium locked inside a particle is potassium that cannot glue the bed together or attack the boiler tubes.
Sampling the same material after 5 to more than 300 hours revealed how this happens over time: potassium migrating inward, a calcium layer building up around each particle, and calcium also moving inward (Corcoran et al., Energy & Fuels, 2018).
In the labPlaceholder
Photo essay: placeholders for real photographs of samples, micrographs and the facilities.
From a research gasifier to a commercial route
The same understanding scales up. A decade of operating the Chalmers research gasifier, together with the GoBiGas demonstration plant, showed that constituents of the biomass ash can act as a catalyst for the process, and that with coated heat exchangers ordinary fluidized-bed boilers could be retrofitted into gasifiers (Thunman et al., Energy Science & Engineering, 2018). The paper estimates that Sweden's existing fluidized-bed boilers alone correspond to a jet-fuel capacity of about 10% of current global consumption.
Waste as a resource
The newest work asks which residues can do useful work. Copper slag turns out to capture potassium in silicates whose melting point is raised by the magnesium and aluminium it contains, making it a promising oxygen carrier for biomass fuels (Störner et al., Fuel Processing Technology, 2025). And pine wood, activated with potassium compounds at 800 °C, becomes a carbon with a surface area above 2,000 m2/g that captures CO2 selectively (Dziejarski et al., Chemical Engineering Journal, 2025).
The common thread is the same question that started it all: what does a hot, aggressive environment do to a material, and how can that knowledge be turned to our advantage?