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. Particle-size segregation 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 the UK. The key techniques are:
- 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.
- Use pressurised tankers to reduce the generation of additional dust during delivery.
- Use low-abrasion pipes (i.e. smooth internal surfaces).
- Minimise the distance and number of angles in the pipes from the truck to the store.
- Use an impact mat (or other device) in a suitable position to minimise damage to the pellets when they enter the store.
- 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.
Clean wood pellets before a blown delivery
The ENplus accreditation scheme requires that the pellets should contain less than 1% fines when the truck is loaded. Fines are defined as particles smaller than 3.15mm.
We only load out from sites equipped with screening equipment that is capable of reducing the fines level to less than 1%. The pellets are visually inspected at loading to check that the screening is working properly. A reference sample of every load is taken and stored for 9 months, for testing later if there is a query about the quality of the load.
This is the minimal ENplus requirement. In practice, ENplus is not strict enough and not adequately enforced. A load of 3.5mm particles would represent a valid ENplus delivery of wood pellets. And enough latitude is permitted in the sampling methodology that it is easy for a supplier to cheat by taking a reference sample that is not representative of the load.
We know that (permitted) 4mm particles will have the same effect on your system as (unpermitted) 3mm particles. All our screens have a 5mm aperture to remove most of the borderline particles, so that what is loaded on the truck is mostly whole pellets (6mm diameter, so a wider aperture is not feasible).
By screening with a 3mm or 4mm aperture, competitors can allow for less wastage. But the truth about screens is that most particles of approximately the size of the aperture do not go through. So the wood pellets loaded out with screens like this will often contain many particles in the 2-4mm range.
Representative reference samples
They avoid being caught supplying out-of-spec pellets by taking an unrepresentative reference sample. The proper way to take a sample is multiple increments across the flow of the pellets (e.g. when they are falling into the truck). But ENplus also permits the increments to be taken with a scoop off the conveyor belt after the screen. On a belt, the fine particles fall to the surface of the belt and the whole pellets sit above them. When a scoop is taken, it consists mainly of the whole pellets on the surface and rarely captures a proper cross-section of the pellets on the belt. When you ask for one of our competitors' reference samples to be tested, it was probably taken in this way, skewing the result in their favour and giving a false impression of how clean the pellets were at loading.
Forever Fuels always takes samples from the flow. Our depots and our screening systems at the ports are all equipped to take multiple increments from across the flow of wood pellets as they fall into the truck. There is no way to skew the sample to collect mainly the whole pellets, because the fines and pellets are mixed together as they fall.
We also go beyond the requirements of ENplus by analysing an extra sample every week, to make sure that the screen is working efficiently. Although the fines limit is 1%, our trigger level is 0.7% - in other words, if our test sample is over 0.7% fines, we check to make sure that the screen is not blocked and is working properly, and keep checking and improving until the fines level is back below 0.7%. ENplus does not specify any measures like this to ensure that problems with the screen are spotted before they become a problem, other than recommending regular visual inspections (which we do weekly).
The extent to which a screen removes fines depends not only on the aperture size but also on its capacity. If you put a thick layer of wood pellets on a screen and they move across it quickly, many of the smaller particles will never get a chance to fall through the apertures. A screen that cleans adequately at 20 tonnes per hour may be completely inadequate at 40 tonnes per hour.
When we specified our screening systems for the ports, we considered the centrifugal system used by many of our competitors. We contacted the manufacturer, and they refused to guarantee that it could remove more than 2% fines at 60 tonnes per hour. After wood pellets have been handled several times and sat in storage for a while, they normally have more than 2% fines. In our experience, you should allow for 5% fines.
So we went with the only screen manufacturer that would guarantee to remove 5% fines on a 5mm mesh (which is more testing than a finer mesh because a higher proportion of the material needs to be removed) at 90 tonnes per hour. We can achieve thorough cleaning at an operationally-efficient loading-out rate.
Given the limitations of the centrifugal screens (or worse) that most of them use, our competitors have a choice of cleaning thoroughly and loading out slowly, or loading out faster with incomplete screening. Most of them shouldn't really be loading out faster than 20 tonnes per hour. Trucks cannot be loaded like a continuous convoy - there are gaps between trucks and pauses for repositioning the trucks, checking the correct loaded weight, etc.
A flow rate of 20 tonnes per hour would equate to loading one rigid delivery truck per hour, and one bulker every hour and a half. If there are several trucks queuing to load out in the morning, it means the last one to load out may be delayed by several hours. Few operators can afford that kind of delay, so most of them will load out faster and use the sampling technique described above to disguise their inadequate screening.
The screens at our depots have a more limited capacity, so we only ever send screened pellets to the depots. This is effectively a "polishing" process, removing the last of the fines that were not removed in the original screening. This gives us confidence that we are loading out with clean pellets even if the depot was fed from a site with a more limited screen than our own.
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.
Tankers are able to use higher pressures than tippers, depending on the circumstances of the delivery. In many circumstances, higher pressures reduce degradation by reducing the speed that the pellets travel and increasing their concentration in the pipe (where they shield each other from impact). People who tell you that higher pressures mean higher velocities and more degradation are confusing the behaviour of liquids (for which that may be true) with the behaviour of gases (such as the blowing air) for which the exact opposite is true. In some circumstances, the advantages of venting air may exceed the benefits of higher pressure, and in those circumstances we will vent some of the delivery air and accept lower pressures for the delivery.
These are some of the reasons why the vast majority of high-quality wood pellet deliveries on the continent (where they have decades more experience than the UK) are done with pressurised tankers. Unpressurised tipper-blowers are rarely used. They are more common in the UK, because they have been used widely for delivering animal feed and can be sub-contracted or bought secondhand more cheaply than the proper pressurised tankers can be obtained.
Forever Fuels is unique in the scale of its investment in the proper technology. It has more than two-thirds of all the pressurised tankers used for delivering wood pellets in the UK.
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.
Active venting (suction) on delivery
Large volumes of air are required to transport wood pellets - typically around 700 m3/hr at normal temperature and pressure. That is the amount of air in a small room every minute. That air has to get out, or the pressure in the store will increase, and the air will look for routes to get out.
Some fabric silos are designed to release the air through pores that are small enough that the dust can't escape. The dust will tend to stick to the inside of the store and gradually block the pores, so you need to clear the dust from the fabric occasionally by beating it like curtains. For these systems, suction is not required and is often not advised (because the roof of the store needs to be inflated during delivery).
For all other systems, the air needs a route to escape. It the store is not equipped with large enough vents for the air to escape without assistance (passive venting), it needs to be sucked out (active venting). The rule of thumb for passive venting is that the cross-sectional area of the vents must be at least double the area of the inlet pipe. The outlet on most stores is the same size as the inlet. For these, active venting is required to avoid pressurisation of the store.
If the store becomes pressurised and is not perfectly sealed, the air will escape through any gaps and out into the surrounding area. If there are no gaps to start with, they may be created as the pressure increases in the store. Escaping dust 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 is not impossible to do so - the supplier just has to find the space to store them in transit.
The suction system connects to the outlet by a Storz 110A or Camlock 4" male connector. The vacuum sucks from the store and blows into a "sock", which catches the dust. Although some people refer to this as "dust extraction", in reality, the fan 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, to prevent damage to the store, reduce the amount of dust that finds its way out into the surrounding area, and ensure a good pressure gradient along the pipes to minimise degradation.
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 suitable connector, (b) provide a 13A 240V electric socket within 20 metres of the outlet, to plug the suction system into, and (c) check that your supplier will send a truck equipped with a proper suction system - one designed to extract at least as much air as is blown in to the store.
Many suppliers are not suitably equipped and will either tell you that suction is not required or stick an inadequate suction system (such as an adapted hoover) on the passenger seat to pretend to offer the facility. That is not what the ENplus accreditation scheme is referring to when it says that all accredited trucks must carry proper suction systems. But there is minimal enforcement of the international ENplus rules in the UK, because the majority of suppliers do not want to spend the money to comply fully.