To minimise degradation, wood pellets should be blown at a low velocity with a high solids loading. That is best achieved with a high delivery pressure. Many people confuse pressure, velocity and flow rate, and think that wood pellets should be delivered with a low pressure and flow rate. They have it back to front. The gentlest blown delivery will have a high pressure and flow rate, and a low velocity.
The following information is an amateur summary of some principles of bulk solids handling. Understanding these factors is critical to good design and operation of wood pellet heating systems. Any organisation that intends to supply wood pellet heating equipment or fuel should ensure that it has a solid grounding in these principles. Experts like the Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich offer courses to provide that solid grounding.
The UK Pellet Council organises courses for the industry in cooperation with the Wolfson Centre. If you are installing a wood pellet heating system or purchasing wood pellets, you should check that your supplier has a certificate of completion of one of these course, to be sure that they understand the principles that affect their designs and operations.
Velocity of the delivery air
The velocity (i.e. speed) of wood pellets in a pneumatic delivery system is closely related to the velocity of the delivery air. The wood pellets are dropped into the delivery pipe at zero velocity along the pipe, and are accelerated by the delivery air, tending towards (but never reaching) the velocity of the delivery air, in the absence of obstacles. The longer that wood pellets travel down a straight, unobstructed pipe, the closer they will get to travelling at the same velocity as the air in the pipe.
The velocity of the delivery air depends on two main factors:
- The flow rate of the air delivered by the blower, and
- The cross-sectional area of the pipe.
The air flow rate (m3/s) divided by the cross-sectional area (m2) gives the air velocity (m/s). For a given air flow rate, the air will travel four times as fast if you halve the diameter of the pipe used to carry it.
The diameter of the pipes for a blown delivery of wood pellets is usually not a variable. Standard delivery pipes have an internal diameter of 100mm (approx. 4").*
If the cross-sectional area of the pipe is fixed, the main factor that determines the velocity of the air in the delivery pipe is the flow rate of the air delivered by the blower.
Flow rate of the delivery air
The blowers used on pneumatic delivery trucks typically have a fixed flow rate at normal temperature and pressure. The intake air is typically close to normal temperature and pressure, so in effect, the blowers have a fixed rate of air intake.†
Boyles' Law states that the volume of a gas is inversely proportional to the pressure of the gas. Double the pressure and you halve the volume.
The flow rate of a gas is the volume of the gas that is transported per unit of time. If you halve the volume of the gas, you halve the flow rate of the gas. If you double the pressure of the gas, you halve the volume and the flow rate of the gas.
The blower on a pneumatic delivery truck compresses the air. Consequently, the volume of the air on the delivery side is lower than the volume of the intake air. The higher the pressure on the delivery side, the lower the volume and the lower the flow rate of the air. Higher delivery pressure means lower air flow rate at the pickup point (i.e. where the wood pellets enter the delivery pipe).
Units of pressure
Pressure can be measured in terms of absolute pressure or gauge pressure. Absolute pressure is the pressure above zero pressure (i.e. the pressure in a perfect vacuum). Gauge pressure is the pressure above normal atmospheric pressure (i.e. the pressure in the air under normal weather conditions).
A bar is a unit of pressure. bar(a) is a unit of absolute pressure. bar(g) is a unit of gauge pressure. Delivery pressures are normally measured in bar(g) (or equivalent units of gauge pressure).
Atmospheric pressure is roughly 1 bar(a). A delivery pressure of 1 bar(g) is a pressure of 1 bar more than atmospheric pressure, which itself is 1 bar(a). A delivery pressure of 1 bar(g) is roughly equal to 2 bar(a).
In other words, a delivery pressure of 1 bar(g) is roughly double the pressure of the intake air at atmospheric pressure. The volume and flow rate of the delivery air at 1 bar(g) is roughly half the volume and flow rate of the intake air.
Delivery pressures for blown wood pellets
Delivery pressures for blown wood pellets are rarely as high as 1 bar(g). There is a trade-off between the weight, energy and economic costs of a higher-pressure delivery system, and the benefits of reduced degradation from a higher-pressure delivery.
For a truck to use higher pressures, it would need a larger blower, and a thicker tank, and it would consume more energy in the delivery to produce the higher pressures. All of these cost money, and the additional weight of the blower and tank also sacrifice payload (i.e. the truck can carry fewer wood pellets). A lower payload means that the truck is less energy- and carbon-efficient.
For tankers, the trade-off is commonly struck at about 1 bar(g). The tank is tested to 2 bar(g) for safety, but a pressure-release valve ensures that the tank will never be pressurised beyond 1 bar(g). This margin of error is always required for safety. A system that was intended to operate at upto 2 bar(g) would be pressure-tested to at least 3 bar(g), requiring a thicker, heavier tank.
The abrupt venting of pressure as the pressure-release valve is triggered destabilises a blown delivery. Drivers therefore aim to set the pressure at the highest level that allows them to maintain steady pressure and flow without triggering the pressure-release valve. Unless there are special circumstances, that will typically be 0.8 - 0.9 bar(g). It is often expressed in millibar (mbar), i.e. 800 - 900 mbar.
Tipper-blowers (e.g. animal-feed blowers) typically operate at lower delivery line pressures. In both tippers and tankers, the pressure in the line tends to push the wood pellets back up into the truck. In a tanker, the pressure in the line is balanced by the equal and opposing pressure in the tank, which allows the wood pellets to fall into the line even when the line pressure is high. In a tipper, the wood pellets in the body are at atmospheric pressure (i.e. much lower than the line pressure). The wood pellets in a tipper are therefore fed into the line via a rotary valve, to prevent the pressure from forcing the pellets back upwards.
There has to be some air leakage in a rotary valve, or it would not be able to turn. The degree of leakage depends on the age and condition of the rotary valve. The more air that leaks through the rotary valve, the fewer wood pellets will be fed to the pipe. Fewer wood pellets in the pipe means less shielding. A worn rotary valve also causes more degradation to the pellets, as some particles are trapped and ground between the valve and its case. The quality of a tipper-blower delivery is very dependent on the condition of the rotary valve.
Even with a rotary valve in good condition, tipper-blowers are usually more comfortable at a lower line pressure than tankers. 0.5 bar(g) is a common setting. A higher line pressure would have the benefit of reduced velocity, but it would also reduce the solids loading in the pipe as fewer wood pellets made it through to the line. Lower solids loading means less of a benefit from shielding. That trade-off (between reduced velocity and reduced shielding) means that higher pressures do not have the same benefit in a tipper-blower delivery as they do in a tanker delivery.
Pickup air velocities for tankers and tippers
If a standard blown delivery from a tanker is at 0.8 bar(g) and from a tipper is at 0.5 bar(g), the pickup air velocity (i.e. the speed of the air at the point where the pellets drop into the delivery line) is 20% higher for a tipper than for a tanker.
The force of impact is proportional to the square of the velocity. The wood pellets will accelerate towards the velocity of the delivery air. If the wood pellets were travelling at the velocity of the delivery air when an impact occurred (e.g. at a bend in the pipes or when they hit the impact mat), and if the differential in the air velocity in a tipper and a tanker were 20%, the force of the impact would be 44% higher from a tipper than from a tanker.
However, the wood pellets will not be travelling at the speed of the delivery air. And the differential in the delivery air pressures will not remain 20% all the way to the mouth of the inlet pipe. At the mouth of the inlet pipe, the line pressure is roughly atmospheric pressure, unless the store is markedly pressurised or depressurised (which would normally be a sign of a problem). The line pressure at the far end of the delivery line is therefore very similar for tankers and tippers. The difference is that the wood pellets accelerate from a slower starting velocity in a tanker at 0.8 bar(g) than in a tipper at 0.5 bar(g). The velocity differential is reducing along the line.
If the pickup pressure is roughly 20% higher in a tanker than in a tipper and the outlet pressure is similar in a tipper and a tanker, it is a rough but reasonable approximation to say that the average difference in the velocity along the line is 10%.
The impact force is proportional to the square of the velocity, so it is a reasonable approximation to say that (all other things being equal) the degradation expected during delivery from a tipper at 0.5 bar(g) is roughly 20% higher than the degradation during delivery from a tanker at 0.8 bar(g).
In a high-quality blown delivery (e.g. an ENplus accredited delivery), the fines level should start at less than 1% and finish at less than 4% (typically 2-3%). The expected degradation from a high-quality delivery therefore equates to an increase of only a couple of percentage points in the fines concentration. 20% of 2% is 0.4%. All other things being equal, one might therefore expect a delivery from a tipper-blower in good condition to result in a delivered fines level that is 0.4% higher than the delivered fines level from a pressurised tanker.
0.4% may not seem like a significant difference in the fines content, and in some applications it is not. But segregation is likely to magnify this difference, so that the fines concentrations that reach the boiler will often be 1-2% higher if the delivered fines content was 0.4% higher. An increase of 1-2% in the fines content reaching the boiler will have a noticeable but not normally critical effect on the performance of the boiler.
All other things are not equal. The rotary valve on a tipper will cause additional degradation, to varying degrees depending on the condition of the valve. And the higher solids loading that can be achieved with a higher pickup pressure will result in a greater benefit from the shielding effect.
Solids loading and pressure gradients
The pressure gradient for a blown wood pellet delivery is the difference between the pickup pressure (pressure where the wood pellets drop into the line) and the outlet pressure (where the wood pellets emerge from the pipe into the store), divided by the distance from one point to the other.
In a sense, the wood pellets "roll down the pressure gradient". Without a sufficient pressure gradient, the wood pellets will not move. With a higher pressure gradient, more wood pellets can be moved at a given velocity.
Given that the outlet pressure is normally atmospheric pressure, the pressure gradient is proportional to the pickup pressure divided by the equivalent length of the delivery pipes.‡ If the pickup pressure is higher in a tanker than in a tipper, more wood pellets can be loaded into the delivery pipe for a given pickup air velocity.
More wood pellets in the pipe is known as a "higher solids loading". A well-trained wood pellet delivery driver should aim to achieve the highest solids loading feasible without stalling the delivery and blocking the pipes. They will aim to set the pressure as high as possible for the circumstances, and then open the valve as wide as possible to release as many pellets into the line as is consistent with maintaining a steady flow.
A higher solids loading reduces degradation through the shielding effect. It also increases the flow rate of wood pellets down the line. A higher solids flow rate may be achieved with wood pellets running thick but slow down the delivery line (thanks to a high pickup pressure) than with the pellets running thin but fast (the result of a low pickup pressure). A high flow rate of wood pellets means that the delivery can be completed in a shorter time.
Counter-intuitively, high pressure and a quick delivery is consistent with low velocity and low degradation, and is a sign of a good-quality delivery. Low pressure and a long delivery time is a warning of greater degradation during delivery.
High equivalent length, low pressure gradient
The lower the pressure gradient along the line, the lower the solids loading must be to keep the pellets flowing. If the pressure gradient is too low, it may be impossible to maintain a flow of wood pellets, even if they are loaded thinly into the pipe. That is why there are limits to the distance that a wood pellet delivery truck can blow.
Bends cause a significant pressure drop, equivalent to a long length of straight pipe. Narrow-radius bends cause a greater pressure drop than wide-radius bends. A delivery pipe with many bends is equivalent to a long delivery pipe in its effect on the pressure gradient.
A high pressure at the pickup point may make it possible to maintain flow down a delivery pipe with a high equivalent length (i.e. long or with several bends), so long as the solids loading is kept low. The higher pressure differential between one end and the other of the pipe compensates for the long length over which the pressure is spread.
The ability to maintain a flow does not diminish the degrading impact of a blown delivery with low solids loading over long distances and through many bends. The delivery may be possible, but you should expect high fines at the end of it, particularly if bends were a significant factor in the equivalent length.
Wood pellet stores designed for low pressures
Some wood pellet silos are designed to withstand only low pressures in the store. The installer puts up a label stating that deliveries should be carried out at a pressure above that limit. Not understanding the principles, they may believe that the necessity for a lower pressure delivery will result in gentle deliveries, although the main driver is probably cost-saving (as the store does not need to be constructed so well) rather than pellet quality.
Besides the confusion over the basic principles, such an installer is also displaying their ignorance of pressure drops along the delivery pipe. If the pressure at the truck is 0.8 bar(g), the pressure in the store will not be 0.8 bar(g). The pressure falls along the delivery line. If the store has sufficient active venting or passive venting, the pressure in the store will be close to atmospheric pressure, even if the pressure at the truck were 0.8 bar(g). By requiring a lower delivery pressure, all that the installer is achieving is an increase in degradation during delivery.
The store may not have sufficient active or passive venting, in which case pressure will increase in the store. A positive pressure in the store means a lower pressure gradient along the line and a lower delivery air velocity. That means that the wood pellets have to be delivered with a lower solids loading. The result is a longer delivery time with greater degradation of the wood pellets. Usually, the strain of the pressure on the store will result eventually in damage to the structure of the store and leaks of dusty air during delivery.
Surge at the end of a pot on the delivery truck
When the last wood pellets are released from one of the outlets on a pressurised tanker, there are no longer any pellets to act as a plug, holding back the pressure in the tank. If the driver does not close the valve in time, the air in the tank rushes out down the delivery line for a fraction of a second. This can result in a momentary surge in the pressure in the delivery line, possibly as high as three times the gauge pressure. The pressure will reduce along the line, but there will be a momentary increase in the pressure in the store. The blower on an active venting suction system is unlikely to be large enough to exhaust the additional air immediately.
The driver should try to prevent this surge by closing the valve in time, but it is a hard phenomenon to anticipate, and there will be occasions when the driver doesn't quite shut the valve in time. A wood pellet store should be designed to withstand the brief pressures on these occasions.
Some wood pellet silos may be equipped with explosion pressure relief valves. They should not be set to release at too low a pressure, or they may be triggered by a surge of delivery air. A risk assessment should be carried out to determine the risk of an explosion in a store, and a pressure relief valve should only be installed if the risk justifies it. If necessary, the store should be designed to withstand the pressures during a delivery surge, so that the pressure relief valve can be set at a pressure that will not be triggered by a surge. If the pressure relief valve has to be set at a low pressure, sufficient active or passive venting should be installed to keep the pressure in the store below the trigger level, even during the course of a surge from the delivery truck.
If an installer wants to minimise (a) degradation during delivery and (b) future problems from a damaged silo, they should install a wood pellet store that can withstand substantial pressures. Pre-fab silos should be designed to cope with high pressures. If your store splits or leaks during delivery, the manufacturer or installer has not constructed a sufficiently robust system.
To recap, the impact force is proportional to the square of the relative velocity, the mass, and the inverse of the slow-down distance. If the impact is not too abrupt, the energy may be dissipated through vibrations and deformation of the colliding objects, whereas the objects will tend to fragment in the case of abrupt impacts.
If wood pellets travelling fast along a pipe hit the wall of the pipe, for example at a bend, the relative velocity (i.e. the difference between the velocity of the pellets and the velocity of the pipe) is high, the mass is relatively high (i.e. the pipe is much heavier than individual wood pellets), and the slow-down distance is very small (i.e. there is minimal "give" in the pipe wall). The abrupt impact tends to fragment the wood pellets.
If wood pellets travelling fast along a pipe hit each other, the relative velocity (i.e. the difference in the velocity of the pellets) is low, the mass (of two pellets) is relatively low, and the slow-down distance is higher than when wood pellets hit the wall of the pipe. The gentler impact tends to dissipate the energy through vibration and deformation rather than fragmentation.
When wood pellets are flowing densely along a pipe, most of them will collide with each other rather than with the wall of the pipe at a bend. When wood pellets are flowing thinly along a pipe, most of them will collide with the wall of the pipe at a bend.
The protection against impacts that particles get from each other in a pneumatic delivery pipe is known as the shielding effect. The higher the solids loading (i.e. the thicker the flow of wood pellets in a pipe), the greater the shielding effect.
Higher pressures at the pickup point and steeper pressure gradients along the line allow for a higher solids loading in the delivery pipe. Higher pressures therefore have two complimentary benefits:
- Lower average delivery velocity, and
- The shielding effect.
The skills to optimise a blown wood pellet delivery
The benefits of a pneumatic tanker are not automatic. The driver must understand how to operate the truck to maximise these benefits. An unskilled driver may operate a tanker in a way that does not make the most of its potential.
The most common mistake is to operate at a lower solids loading than could be achieved, by not opening the valve sufficiently on a tanker or running the rotary valve too slowly on a tipper. The benefit to the driver is that the system is less likely to stall. A thinly-loaded delivery requires less care by the driver, but the average delivery velocity will be higher and the shielding effect will be lower than if the driver were taking care to manage the delivery at a higher solids loading. The thin delivery will also take longer.
Organisations that have not educated themselves on the principles and practicalities of blown deliveries may conclude that tankers offer little benefit, because they did not know how to make the most of a tanker when they compared it with a tipper (assuming their view is based on anything more than convenience). When they made the comparison, they may have thought that deliveries at lower pressure were gentler, and therefore operated the tanker in the way designed to make it least effective. The benefit of a tanker is that it can deliver at higher pressure. It should not be surprising that some organisations may detect little difference between tippers and tankers if they are aiming to operate at a low pressure.
For the best blown deliveries of wood pellets, you need a pressurised tanker to achieve the benefits of higher-pressure deliveries, and a driver with the training and skills to operate it properly.
* Occasionally one encounters wider pipes for the truck to connect to. The reduction in velocity at the expansion point where the truck's pipes join to the the wider on-site pipes is unhelpful, and will result in more frequent stalling and higher degradation from the lower solids loading required to cope with this design "feature".
† Pneumatic delivery trucks typically use roots blowers. Roots blowers are relatively unaffected by the pressure on the delivery side. High pressures on the delivery side will cause only small reductions in their intake flow rate at normal temperature and pressure.
‡ It is necessary to refer to the equivalent length rather than simply the length, because bends and other obstacles have a major effect on the pressure gradient. The equivalent length is a concept that treats bends and obstacles as though they were an additional length of pipe. A short length of pipe with many bends therefore has a long equivalent length.