Biomass boilers, Clinker, burning issues?

Biomass boilers burn “biomass”. In theory this could be burning anything that has been recently living, probably plant matter, in a boiler.

We often get questions like:

“can your boiler burn logs and pellets at the same time?”

“Can we burn any sort of wood in your boiler?”

“is it possible to use Miscanthus in your boiler?”

Oh, and:

“why do i get klinker / Clinker in my pellet boiler”

This article is intended to help you in your path towards choosing a biomass fuel that works for your situation and matching it to the sort of boiler that you may want to use.

We are happy to answer questions. Please contact us using the form on the side of the page.

Part 1 – different burning materials

It is possible to burn almost anything. However if you have ever tried to burn plastic then you know that the fumes that are given off are toxic and the gas leaves a residue that is difficult to shift. Therefore when choosing burning materials you can only use materials that will burn cleanly without leaving a difficult to clean residue.

As well as burning cleanly you also do not want to have too much ash left over to clean out of the boiler. Therefore some possible burning materials that leave behind clinker or partially burnt materials and again give you extra work to do.

Different burning materials burn best in different conditions. It is therefore likely that you need different amounts of air to burn each material. Your boiler has been tested for specific burning materials and probably commissioned for one. Unless there are specific claims by the boiler, then it is unlikely that it can burn more than one material at a time.

If you are interested in grants then the biomass materials that you consider need to be approved i.e. burning them is more beneficial to the environment than burning fossil fuels.

This means that the boilers have to be engineered to only burn “biomass” rather than a fossil fuel. Biomass in the real world is any living matter, but in the world of the RHI it means an accredited boiler where the boiler has been tested to work efficiently with a given fuel. Biomass in the domestic and small business market therefore means:

  • Logs – seasoned to at least 20% moisture for good burning and possibly less depending on the boiler
  • Pellets – dried to 10-12% moisture and made from virgin sawdust
  • Wood Chip – dried to 15% moisture
  • There are other fuels that are discussed, but less readily available e.g. cherry stones, pine kernels, olive stones etc which can burn in a small number of boilers
If you have a wood or a supply of logs then an important consideration is the moisture content of the logs. Using a moisture meter you would need to test not just the outside of the log, but the internal part of the log as well (you can split the log). The consequences of using damp materials can be condensation that causes tar and furs up the inside of your boiler. You get a less effective burn and you need to clean your boiler more often.

Part 2 Examples of different biomass boilers

Log Burning Boilers

The picture above shows the most simple log boiler (WBS). It has no electronic controls and so works with minimal electricity (just a pump). This will work with logs that are 20% moisture or less. Once the logs are loaded then the boiler will burn for about 3 to 3.5 hours and heat a large tank of water that is used as a store to heat a home. During the coldest parts of winter this would be done twice per day. The boiler is very cost effective with prices starting at around £1000.
Most log burners can only burn logs, however the WBS has been fitted with a flange and designed to burn effectively with pellets. This combination has been tested and is MCS accredited below 45kW and is applicable for the non domestic Renewable Heat Incentive up to 110 kW. We should stress that you can only burn either logs or pellets in a single moment. To change from one to the other is a simple process, but it has to be done.
The picture above is a more complex log burner WBS active. It is a few hundred pounds more than the WBS and has the addition of a controller and a fan. The advantage that this brings is that the boiler controller can respond to the heat of the water that it is producing and turn the fan off. This reduces the flame of the boiler and the water will stay at the given temperature for a period of time. The benefit this gives is that if your water store is already hot then your log load will last for longer perhaps 6 hours before it is burnt out.
This model also has the option of a pellet burner insert and is also accredited for the Renewable Heat Incentive.
The above image is a gasification boiler, Pyroburn Alpha. This burns with very dry logs. When logs burn they give off a gas called Syngas and this can be burnt and produce more heat given the opportunity. This boiler has a second burning chamber, a variable fan and a more advanced controller. The overall result is an increase in the efficiency of the burn of wood. It is a little more expensive but prices still start at around £2000 and comes in 3 sizes 18, 25 and 40kW. It cannot burn anything other than dry wood.
The Pyroburn Lambda is a more advanced model again. This has an additional lambda sensor that automatically adjusts the burning. This makes fitting easier and ensures optimal efficiency of burn. This is around £1000 more expensive.
Log boilers can vary in price and what they can do – the more controls you put in the more efficiency gains are possible, however this relies entirely on the quality and moisture content of the wood that you put in. It may be that if you are not prepared to ensure the lowest moisture content of your wood due to the extra expense it will incur then you are better off with a simpler model. If on the other hand efficiency of burning is your priority then the Pyroburn Lambda could be an excellent choice with up to 93% efficiency recorded in the field in working models.

Multifuel Biomass Boilers

Boilers such as the Combi Burn 35kW have 2 burning chambers and can swap easily from one fuel to another.
This means that you can burn wood and then revert to an automated fuel such as wood chips or wood pellets automatically. The boiler is a little larger than the above boilers, but is considerably more flexible. It is a more involved design, and the boiler costs more to make.

Wood pellet boilers

Wood pellet boilers come in lots of shapes and sizes. Most installations are wood pellets. The wood pellet is automated and therefore easier to use. Log boilers can be cheaper but require more space and suit businesses and organisations that can feed a boiler. This could be a farmer who is near to the boiler most days. For those that have to travel to work you want your boiler to provide heat automatically and this may be when you are not there.

Wood pellet boilers use a hopper to deliver pellets to a burning chamber. On domestic models boilers such as the Musa by MCZ or the LP14 by Extraflame have an internal hopper that you would need to fill by hand several times a week during winter. You would probably buy your pellets in bags.

The above diagram shows how the pellets are fed from the hopper (behind) into the burner unit on the LP14. It is a very similar process with all Italian made boilers.

The Pell burner unit mentioned earlier can be fitted onto a WBS boiler or comes as part of a Pelleburn boiler.

Here the pellets are fed from a 500l hopper into the top of the burner and then an internal screw pushes the pellets into the end of the burner where air is applied and an efficient flame results.

The pelleburn can burn up to 94% efficient. This is due to the technology used towards dedicating the burning purely on pellets.

If you have a larger system or do not want to buy bags of pellets, you can get a lorry to blow the pellets into a pellet store. The advantage of this is that you have the extra cost of the pellet store, however you may be able to get a discount on the buying price of the pellets.

Blowing pellets into a pellet store can be very effective for the home owner as they can leave the pellet system working. We find that there is an even split between those people who are quite happy loading their own pellets and those that want to have a pellet store.

If you want a pellet store then you need to consider how to feed the pellets from the store to the boiler. If you can locate the store near to the boiler then you can use another auger to feed in the pellets. Augers are reasonably cost effective and start at close to £1000 for a 3 or 4 meter auger.

If you need to locate your store further away from the pellet boiler then you will have to use a vacuum system. This involves more technology and more pipe. It will push up the cost of your installation by up to £2-5000 depending on the options you go for.

The storage units themselves are designed for either inside or outside. Inside units are a little cheaper, and outside units do not need a special store.



The above unit is an example of an internal storage system supplied by Geoplast. A larger system could store up to 9 tonnes including an auger could cost in the region of £4-5000.
The alternative is to build your own store. This is very possible, but often a similar cost.

Part 3 Buying biomass fuel

Wood, whether it is wood pellets, wood chip or logs can be a complex subject and off putting to a potential purchaser. The industry recommends only good fuel because you will either get boiler problems or you will need to clean your boiler more frequently. Part of the problem is that a beautifully packed carton of logs from a petrol station can be very expensive.

Look for good wood pellets

Wood pellets need to be approved. You can look for signs such as


This will insure that the pellets are of top quality. Pellets are made in quantity in the UK and 2 of the largest manufacturers are Brites and Verdo Renewables. There are many other manufacturers setting up and in your local area there will be a distributor that can deliver in bags or via a vehicle.
Pellet prices are about 20-30% cheaper than oil or LPG prices and prices do fluctuate, but not by as much as oil or LPG.
A1 pellets will burn well, and you also will want to know that the sawdust that created the wood pellets came from a sustainable sourced forest. An FSC scheme is put in place to ensure that the wood that is cut down is replaced.
If you examine closer you will find that the whole tree is often being used for timber through a saw mill. The sawdust is simply a byproduct.

Use a local distributor and work with them

Transporting wood pellets is expensive. Getting 1 tonne to your door at your convenience can be very expensive. Distributors work hard to deliver pellets in a system and therefore letting them know in advance what your needs are can bring your costs down.

Do shop around. There are often deals to be done. For instance you may get your 10th tonne free if you are a bulk user.

Logs need some care

Logs can also come from sustainably managed sources, however the difficulty is the moisture of the logs. If you buy them wet or dry or by tonne or volume, you want to know that the wood will work well in your boiler. For instance dry willow will burn very quickly and it is not very energy dense. Ash contains a lot more energy.

You will find that a local supplier is best as logs are heavy and cost a lot to transport.

If you are using your own logs then you will also need to consider how you are going to dry them.

Even dry logs need good storage. If left in the open seasoned timber will still absorb moisture.

When you first get a log boiler then there will be a learning process as you adjust both the cost of the wood that you buy with how well it burns.

Wood Chips also need care

 Wood chip boilers generally need some space. Wood chips can also compost and need some management. Wood chips are also expensive to transport in small volumes. In our experience wood chip boilers can only work at a lower level e.g. 30-35kw if you have a source on your doorstep.
At larger sizes e.g. 100kW + then wood chips can be much cheaper than pellets and also carry the benefit of the automation of pellets in small quanitities. So for commercial systems wood chips can be very popular. Tree surgeons, joiners, or any tradesman that works with wood chip may find that a chip boiler works very well for them.

All of the above information is quite general. We are happy to have a conversation about your specific thoughts. Please contact us using 01225 580 401

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