Sawdust Drying: Equipment Selection

The difficulty with sawdust is that the moisture is not uniform, not that it is high. Moisture in stacked wood chips is very uneven: the surface has already dried in the air to somewhere in the teens while the core is still in the twenties, and if the feed is sized on a single average the output is inevitably a dry shell around a wet core that goes mouldy within two days of stacking. Our approach is to put a screen at the feed end, separate coarse from fine and even out the size distribution, because with a wide spread the coarse material travels slowly and the fines travel fast, so the bed splits into two paths inside the drum and the moisture separates with them.

The difficulty with sawdust is that the moisture is not uniform, not that it is high. Moisture in stacked wood chips is very uneven: the surface has already dried in the air to somewhere in the teens while the core is still in the twenties, and if the feed is sized on a single average the output is inevitably a dry shell around a wet core that goes mouldy within two days of stacking. Our approach is to put a screen at the feed end, separate coarse from fine and even out the size distribution, because with a wide spread the coarse material travels slowly and the fines travel fast, so the bed splits into two paths inside the drum and the moisture separates with them. The advantage of the rotary route shows up directly here: the flights lift and drop the material continuously, so large and small pieces keep mixing and the outlet moisture is far more consistent than with continuous feeding. Biomass pellets or coal are the most economical heat sources because the fuel is easy to get and the cost per tonne of water is low, but direct firing carries ash and sparks in the flue gas, and ash in the wood chips contaminates the product. The heat source side therefore needs flame isolation, spark capture and separate ash settling, with the ash kept out of the drum. Do not raise the speed early: once the fibres tangle, a higher speed only makes the mat worse.

Why does dried sawdust go mouldy?
Almost always uneven moisture: the surface is dry, the inside is still wet, and the inside takes moisture back and grows mould within two days of stacking. The remedy is a screen at the feed end to even out the size distribution, so the material travels one path inside the drum and the outlet moisture is consistent.

What has to be watched with a biomass heat source?
Biomass pellets and coal are the most economical, with fuel easy to get and a low cost per tonne of water, but direct firing carries ash and sparks in the flue gas, and ash in the wood chips contaminates the product. The heat source side needs flame isolation, spark capture and separate ash settling, with the ash kept out of the drum.

Does sawdust need disintegration?
Stockpiled sawdust that has balled up does, and a disintegrator at the feed end opens the lumps. But watch the speed: once the fibres tangle, a higher speed only makes the mat worse, so the flight pattern and the speed have to be set together.