Slagging in biomass boilers: why it happens and what you can control
Plants that have just moved to biomass tend to hit the same problem: ash starts building into hard deposits on the furnace walls and grate, which never happened on coal. The cause is in the ash chemistry.
Photo: Adhirej J R Nair on Unsplash
Why wood ash mixed with soil, sand or bark fuses at lower temperatures
Clean stemwood ash is calcium-rich, and its fusion temperatures are close to coal ash (white wood ash softens at about 1,075°C and flows at about 1,290°C, against about 1,018°C and 1,320°C for coal fly ash).
The trouble is potassium. Biomass ash averages about 19.4% K2O against about 1.6% for coal. When that potassium meets silica from soil, sand, bark and twigs, it forms potassium silicates that start melting at roughly 600–980°C. Potassium acts as a flux, lowering the melting point of the whole mixture.
Ash therefore starts softening and sticking at temperatures below what the furnace was designed around for coal. Once it sticks, it builds into the hard layer known as slag.
Slag is not a housekeeping issue. It insulates the heat transfer surfaces, so efficiency falls steadily, and in bad cases it blocks air paths or damages the grate.
Variables you can control
Furnace temperature. The most direct lever. If temperature exceeds the ash softening point, slag forms. Air distribution and feed rate are the main tools.
Soil and sand contamination. Silica from soil combines with the potassium in wood to form compounds that melt more readily still. Well-screened fuel is visibly less trouble.
Moisture consistency. Swinging moisture swings temperature, which makes the furnace hard to hold steady. A drier-than-usual batch burns hotter than the setting assumes.
Chip size. Excess fines burn fast and create local hot spots.
What usually gets missed
Slagging is normally blamed on the boiler when a large share of the cause is the fuel.
If contamination varies batch to batch, you will spend every shift re-tuning and never settle, because the upstream variable keeps moving.
Asking a supplier for ash content as a percentage, and for it to stay put, is worth more than negotiating a few baht off the ton.
Ash percentage alone does not predict slagging, though. Ask for potassium and silicon in the ash as well.
Checklist when slagging starts
- Compare the batch burning when it appeared against the previous one. What changed?
- Is collected ash per ton rising?
- Is there visible soil or sand in the pile?
- Review furnace temperature and air distribution over that period.
- Ask the supplier whether the batch came from the usual source.
Well-screened fuel genuinely helps
We screen soil and foreign matter before chipping every batch. That is not an optional step; it is what keeps the spec stable between deliveries.
If you are seeing slag and suspect the fuel, send us the details. Call +66 92 392 2292.
Sources
- Miles et al., Alkali Deposits Found in Biomass Power Plants (NREL/TP-433-8142)
- L. J. Roberts, PhD thesis, University of Leeds (2018)
Reviewed by
Morse · Plant Manager, NTC Biomass
Runs the chipping yard in Ratsada, Trang, and takes customer calls directly. Outside figures in this article are cited in the sources at the end.