Screwed up? | Forever Fuels

I recently had an in-depth discussion with a customer about their new biomass boiler and second hand store. The customer purchased a used 3 tonne metal store and auger with the intention of extending its capacity to 6 tonnes and rebuilding it in his garage. Through the re-build the customer discovered a range of issues with the store which eventually were resolved working closely with the manufacturer’s engineer.

As metal stores are so popular we thought it would be useful to share our customer’s experiences.

“The original store had been used outdoors and had signs of water ingress internally. The auger spiral was jammed and would not rotate. In addition, the plastic auger pipe had been snapped at some time and had been poorly repaired.

The rebuild revealed that some of the design had been updated by the manufacturer. The roof design had been altered to remove a cross beam that had been obstructing the free ejection of pellets from the fill tube. This fix was to reduce pellet damage during filling.

Also a new internal horizontal feed pipe had been added to aid the firing direction of the pellets at the rubber impact mat. The mat had also been moved much closer to the back wall, presumably to aid the filling spread and reduce impact speed too.

The auger was disassembled and internally it was found that pellets had been exposed to water, expanded and set like concrete. The plastic pipe had to be cut/smashed open to remove it from the spiral. A new pipe was obtained from the plumbers’ merchants. This was a plastic land drainage pipe, Polypipe Terrain 82mm.

On testing, it was found that the pellet flow up the pipe was poor. The motor juddered and pellets were sticking ¾ up the pipe. This caused a poor flow rate and lots of pellet dust. On discussion with the manufactures’ engineer, it was suggested that the previous water damage and jamming of the auger had caused the spiral to wind up unevenly and alter the pitch between the coils. Thus as the pellets were pulled up the pipe in a 2” loop, it was slowly reduced at some point near the top to only 1.5”. As pellets are incompressible, the only way for the pressure to go was to cause a bulge against the plastic auger pipe and cause it to stick.

A new auger spiral was ordered. On arrival it was found to have been modified from the original design and was much skinnier. This had been done to reduce the risk of sticking, which must have occurred with the original design. The original spiral was perhaps only 10mm smaller than the diameter of the plastic auger tube but the new spiral was perhaps only 50% of the auger tube diameter. The new spiral runs smoothly and quietly with no juddering.

Also to note for the benefit of people building stores for outdoor use, some gaps had been left unsealed. Water marks could be seen where this had happened. They included the filling tube mounting plates. It must be assumed that all metal to metal contacts need sealing as water does not always just travel downwards.

An important component was found to be missing. This was a drip ring that should be fitted at the bottom of the auger plastic tube just above the point where the silo flexible auger skirt connects around it. Without it fitted water running down the pipe will run into this “funnel” and soak the pellets at the bottom of the silo and auger pickup point.

An auger tube support stand was also missing. This is needed to stop the plastic auger pipe from bending in hot sunlight. As my silo is indoors I have not needed these two components, but I would imagine that these missing parts might well have been the cause of all the future damage.

If an auger has jammed it is possible that the spiral has been damaged. You will know this if it judders once the blockage is removed. Only a new auger spiral will fix this

The fault looked to have been as a result of incorrect installation initially, and it must be emphasised that water sealing is paramount.”

(please note the photo of the metal store in this post is not the customers own store)

Effects of fines and dust

The customers experience highlighted a number of design/installation faults which led to the creation of fines and dust.

Effects of fines

Fines affect the way that pellets flow through any hopper (both the store and the day hopper) and along any auger or suction system. They fill the gap between pellets and increase the bulk density of fuel being fed into a boiler, which can affect the fuel-usage and temperature of a boiler. They may become entrained in the combustion air and cause sooty deposits, slagging and clinkering, which may reduce the efficiency of the boiler or affect its operation (e.g. by obstructing the air intake). They may increase sooty emissions (i.e. smoke) from the flue.

Effects of dust

Effects of dust Dust can have a greater effect than fines on increasing the friction between pellets (if it coats the surface of the pellets) which can have a greater inhibiting effect on the flow.

The main contributors to fines and dust in this particular case were:

Pellet trajectory not clear/obstructions

A basic aspect of wood pellet storage design is for there to be a clear trajectory for the pellets as they flow into the store towards the impact mat. Any obstructions such as support brackets, nails, etc. will increase pellet breakage during delivery.

If there is no internal pipework (injection pipe) the pellets will just spray in all directions as soon as they enter the store, hitting the sides and roof of the store rather than the impact protection mat. Again this will increase pellet breakage during the delivery.

Water – no friend of pellets

It is a fundamental of pellet storage, that pellet stores must be sited in a completely dry location. Pellets are hygroscopic. In contact with water and damp surfaces, they swell up and disintegrate into the particles and dust of which they are composed (see photo).

Auger set up

The space between the auger and its shield/casing should be sufficient to allow a smooth flow of pellets, otherwise the auger will grind the pellets up into fines and dust.

Another possible explanation as to why the auger was degrading the wood pellets (although we have no way of knowing if it was the case) is that the auger motor is under-sized for the incline. This is a common problem. There is often little understanding that augers, particularly centreless augers like the one described, need to be spinning at a rapid rate so that the centrifugal force counteracts the tendency of the pellets to flow back down the gradient.

The required rotation speed to minimise degradation will often be much faster than the speed that would be required to feed the boiler if the pellets could be trickled up the auger. But they can't. In these circumstances, it is necessary to have a control system that runs the auger at the necessary RPMs when it is running and shuts it off once sufficient fuel is delivered.

In one case we experienced, the motor was so under-sized that we were able to achieve a material improvement by accelerating the auger rotation with an electric screwdriver! The auger had been delivering degraded pellets into the day hopper for a long time, and various engineers had been unable to diagnose the problem, blaming the fuel in the silo even when it was demonstrated that the fines level was low. Once the motor was replaced with a more powerful unit set for higher RPMs, those problems disappeared.

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Sep 6, 2017 By claireh
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