Glossary
Waste-to-energy and biomass
Alkali metals in ash
Also known as sodium in ash, potassium in ash, alkali loading.
Alkali metals — primarily sodium (Na) and potassium (K) — are the dominant drivers of low-melting fouling in biomass, waste-to-energy and certain coal boilers. Alkali compounds (KCl, NaCl, K₂SO₄, Na₂SO₄) melt or soften at temperatures (650–900 °C) lower than typical convective-pass tube-metal temperatures, so they arrive at the tube surface partly molten and bond tenaciously.
Where alkali concentration is high
| Fuel | Approximate alkali-in-ash range |
|---|---|
| Wood (clean stems) | Low (1–5%) |
| Bark, hog fuel | Medium (5–15%) |
| Straw and agricultural residues | High (10–25%) |
| Bagasse | Medium-high |
| MSW / RDF / SRF | High (variable) |
| Coal | Low |
Operational consequences
- Low-melt sticky ash bonding to superheater and economiser tubes
- Accelerated tube wastage from corrosive deposits
- SCR catalyst poisoning by alkali species
- Bed-material agglomeration in BFB and CFB boilers
Cleaning
Active sonic-horn cleaning prevents fresh alkali-rich deposits from consolidating into bonded slag, which is the only practical mitigation short of fuel substitution.
Related terms
Related terms
- Low-melt sticky ashLow-melt sticky ash forms when alkali-rich ash particles soften at typical convective-pass temperatures and bond to tube surfaces. Defeats steam sootblowers; primary target for sonic horns.
- Chloride-induced corrosionChloride-induced corrosion is the accelerated tube-wall thinning caused by chlorine-rich deposits on WtE and biomass boilers. The dominant tube-failure mechanism in WtE.
- Catalyst poisoningCatalyst poisoning is the chemical binding of trace species (arsenic, alkali metals, phosphorus, sulphur) to SCR active sites. Usually irreversible — the catalyst layer must be replaced.
- BagasseBagasse is the fibrous residue left after juice extraction from sugarcane. Burned in cogeneration boilers at sugar mills; silica-rich ash deposits aggressively.
- Straw and agricultural-residue firingStraw and other agricultural residues are burned in dedicated biomass boilers, primarily in Denmark, Germany, China and India. High potassium and chlorine produce aggressive fouling.