Once the pellets have been burnt the ash remains must be removed from the burner and disposed off. However the ash from the pellets is not a waste, It can be used as a soil amendment to improve the quality of poor soils. The ash contains high levels of nitrogen.

Composting
Instead of directly mixing the ash into the soil, it generally is a better idea to create compost, and add to the soil when needed. The compost will be a rich soil amendment with concentrated nutrients.

Wood Pellet Ash and Potash
Wood ash as a soil amendment can be particularly beneficial to fruit trees with stones in the fruit.

PH
Wood ash and potash can raise the PH of the soil to make the soil more alkaline. Through composting with soil and other ingredients like peat, this can reduce the PH. Obviously if the soil already has a high PH the ash should be used else where.

Biomass ash improves soil quality, provides nutrients and improves the soils ability to hold water. The ash should be used a soil conditioner.

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As discussed in the last few blogs, different types of biomass pellets produce different types and quantities of ash and clinker. Proper pellet burner maintenance is essential to efficient combustion and fuel efficiency. If the pellet burner is not cleaned to a sufficient standard, this can impact the combustion process and reduce efficiency, and result in higher levels of noxious gases, through inefficient combustion. Also if the rest of the burn chamber is not properly cleaned this can reduce the amount of useful heat that is extracted. For example in a pellet boiler if there is a large build up of deposits around the heat exchanger tube, this insulates the tubes and stops heat getting into the boiler. This means more fuel is used to get up to temperature.

Wood Pellet Maintenance
Wood pellets as discussed in earlier articles produce varying levels of ash depending on whether they are premium wood pellets or not. The ash percentage will vary between 0.5-3%, but the chance of sintering or clinker is very low. Some boilers and stoves have heat exchanger tube spring cleaners. The springs are positioned inside the heat exchanger tubes, and a mechanism lift and drops the springs to clean deposit built up. In some pellet burners this is manually carried out by the user, or in some systems is automated and can be set to clean at certain times. Some pellet burners are also water-jacketed, so making sure there is minimal deposit build up in the rest of the burn chamber is also important. Finally fly ash collects within the inside of the chimney, and if not maintained can effect combustion through poor ventilation of the combustion gases.

It is generally recommended that once a year several components should be removed and the burner should be given a thorough clean. This obviously should be carried out during the summer months when the burner isn’t required.

Straw Pellet Maintenance
Straw pellets create a higher ash content than wood pellets, but the same basic cleaning principles apply, however they need to be carried out more regularly. As stated in earlier articles burning straw pellets create clinkers and slag formation on the heat exchanger tubes. The clinker must be removed so that it does not affect combustion. Depending on the type of boiler and burn pot design this may be required every few hours, every 6-8 hours or every day or week. Another factor is slag build up in the heat exchanger tubes. Slag is similar to a clinker, where ash melting has formed a sold deposit to form. Unlike wood ash in the heat exchanger tubes, slag is a hard despot that needs to scraped off. Cleaning springs may not be suitable to remove slag deposits, as they may not provide sufficient force.

Therefore maintenance for straw pellets is more frequent and labour intensive compared to burning wood pellets. However there are sufficient cost incentives and other benefits.

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Through burning biomass pellets, whether they are made from wood, straw, grass or any other form of biomass they produce ash. The percentage of ash and possibly a clinker is specific to that raw material.

Premium Wood Pellets
These pellets are currently sold on the open market, and are the most common type of fuel pellet. They are produced from the residue from the wood processing industry. The residue is mainly from softwood, i.e. pine or spruce. The residue contains no or very little bark. These pellets are a pale yellow in colour and tend to be very shiny.

In terms of ash content they will produce very little, in most cases between 0.5% and 1%. If you are using ‘premium wood pellets’ and you are getting more ash than this, there is either some bark in them or other materials. You shouldn’t really experience any clinker (sinter) from these pellets, unless there are a high percentage of fines. Fines are loose particles, which burn too quickly and reach very high temperatures where the ash begins to melt and form a hard deposit (clinker/sinter).

Other Wood Pellets
Other species of wood and wood pellets, which contain bark, will have higher ash content. Generally the percentage will be between 1-3%. For example producing wood pellets from Willow is becoming popular, as coppiced Willow is a relatively fast growing wood species. However you cannot separate the bark, therefore the pellets will produce higher ash content. Again though these pellets should not produce any clinker. However due to the higher ash percentage the ash pot needs to be empted more frequently, and the boiler tubes cleaned more often.

Premium wood pellets due their low ash level and low maintenance currently dominates the fuel pellet market, and the domestic market prefers them. However premium wood pellets do have their issues, for example the video I posted a few weeks ago on the pellet shortage. If the wood processing industry goes into decline, for example with few houses been built, this affects the supply of residue to the wood pellet plants. With less wood pellets been produced and a high demand the price will rapidly increase. For example currently the US has seen wood pellet prices double. This is why when purchasing a pellet boiler or stove I would recommend finding a more flexible burner that can handle more than premium wood pellets.

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