Minimising dust in blown wood-pellet deliveries

Some wood-pellet systems are more sensitive to dust than others, but all systems will benefit from minimising the fines content in the delivered pellets.

Dust in air creates an unpleasant and hazardous environment, and is likely to find its way out of the store and into the surrounding area, if measures are not taken to prevent it. Dust in the store will increase the coefficient of friction and prevent wood pellets from flowing as well as intended. Differential settlement will (if not tackled) lead to build-ups of dust in the store, which will block augers. Dust in the boiler will become entrained in the exhaust air and cause poor emissions and increased rates of slagging and clinkering.

The techniques for minimising dust are tried-and-tested, but not always applied in an immature wood-pellet market like the UK. The key techniques are:

  1. Clean the pellets (by screening and suction) at each point where the pellets are handled, and particularly at the final point where the pellets are loaded for delivery to the customer.
  2. Use pressurised tankers to reduce the generation of additional dust during delivery.
  3. Use low-abrasion pipes (i.e. smooth internal surfaces).
  4. Minimise the distance and number of angles in the pipes from the truck to the store.
  5. Use an impact mat (or other device) in a suitable position to minimise damage to the pellets when they enter the store.
  6. Use suction systems on the outlet to extract some of the remaining dust and (more significantly) reduce the pressure in the store to minimise the escape of dust into the surrounding area.

These elements are standard good practice for blown wood-pellet deliveries on the continent, where they have had more experience to refine the techniques. Forever Fuels is one of only a small number of wood-pellet suppliers who offer this continental-standard of delivery (for points 1, 2, 3, and 6 - points 4 and 5 are the responsibility of the customer and their installer).

Designing for dust-minimization

It is important to be aware that the fines content depends not only on the measures that the supplier takes, but also on the circumstances of the delivery.

The further we have to blow, the more fines are produced in the process. The more sharp angles that the pellets encounter in the inlet pipework, the greater the impacts and the more fines are generated during delivery. The harder the impact when the pellets are blown into the store (e.g. if there is no impact mat or if there is only a short distance between the inlet and the point of impact), the more dust you will get in your store.

These are largely the responsibility of you and your equipment supplier, and out of our control. That is one reason why the fines content is measured, for the purposes of quality testing, at the point of loading, not within your store.

See our Design Guide for a useful tool to help you work out whether your installation is better or worse for minimising the generation of fines during delivery.

Clean wood pellets before a blown delivery

Where we deliver direct from the factory (e.g. from Verdo Renewables in Andover and Grangemouth), we only work with producers who clean the wood pellets so that they contain less than 1% fines before loading onto our trucks.

All of our stores and depots are equipped to clean the wood pellets to less than 1% fines before loading on our trucks. We inspect the pellets during loading. We take samples from every load, so we can test that the cleaning is working to keep the fines below 1%.

Pressurised tankers for gentler blowing

Tippers use gravity and the weight of pellets in the tipped body to push the pellets into the blower pipe. The pellets would naturally flow too fast and block the pipe, so tippers control the flow with a rotary valve. This is effectively a mincing machine that damages the pellets as they flow through it.

Pressurised tankers use air pressure to push the pellets into the blower pipe. There is no need for a rotary valve. The pellets encounter no moving parts in the process.

Tippers also have dead corners on either side of the body, where the wood pellets would get stuck if they weren't moved mechanically towards the outlet. Typically, tippers include augers running from these corners to the outlet in the centre of the rear of the body. These augers cause more damage to the wood pellets.

Tippers are not as gentle as tankers, but the extent of the damage depends on the way that the driver operates them. You need skilled, trained operators for both types of truck. An unskilled driver operating a tanker may do more harm than a skilled driver operating a tipper. However, a skilled driver operating both will get better results with a tanker than a tipper. We only use skilled, trained drivers for our deliveries.

Suction systems to reduce dust-escape during delivery

Wood pellets are delivered under significant pressure. That pressure feeds through to the store. If the store is not perfectly sealed (and most are not), the pressure will force the dust towards the cracks and out into the surrounding area. This is unpleasant, messy and hazardous (if the dust is inhaled or if it settles and is then ignited, causing an explosion).

Pressurised tankers are typically equipped with a suction system, which can be attached to an outlet to reduce the pressure in the store during delivery. All Forever Fuels' tankers are so equipped. Tippers are often not equipped with suction systems, although it's not impossible to do so - the supplier just has to find the space to store them in transit.

The suction system connected to the outlet by a Storz 110A connector. The vacuum sucks from the store and blows into a "sock", which catches the dust. It does not extract all or even most of the dust blown into the store. That is not its main purpose, although it does extract some of the dust. Its main purpose is to reduce the pressure in the store and therefore reduce the amount of dust that finds its way out into the surrounding area.

It is highly recommended to use suction on deliveries to most (but not all) types of store. If you want suction during delivery, you must (a) provide an outlet with a Storz 110A connector, (b) provide an electric socket within a practical distance of the parking place for the truck, to plug the suction system into, and (c) check that your supplier will send a truck equipped with a suction system.