Birmingham Office
A Unit 3 Belfont Trading Estate, Mucklow Hill, Halesowen, Birmingham, B62 8DR T 0121 550 2241

Biomass

Heating with wood is classed as being carbon neutral because, whether the tree is burned or left to decay, it will only release the same amount of carbon dioxide that it took in during photosynthesis.

Wood is an extremely versatile fuel and can be burned in many forms, but whatever the form moisture content is absolutely critical. Water contributes absolutely nothing to the stored energy within the fuel and will only serve to reduce the heat that can be extracted by prolonging the drying phase of combustion. High moisture content can also result in excessive vapour within the flue gases, leading to excessive condensation and the formation of creosote within the flue.

We must also consider transport costs as it costs the same to move a tonne of wet wood as it does a tonne of dry wood. So if you are paying for delivery of your fuel it makes sense to pay for as little water as possible, maximising the energy content.

Wood pellets

Usually formed by compacting sawdust, such as from the saw mills, using equipment that forces the saw dust through a die under pressure, this forces the lignin in the wood to liquefy and, because the die is heated, the lignin sets again to bind the dense sawdust together to form a pellet. Surprisingly to some, they are harder and denser than a wood chip. We only recommend wood chips of ENplus standard, first introduced in Germany in 2010. This standard sets strict parameters on size, moisture content, ash content and melting point, ensuring high energy content and, more importantly, far less problems with boiler breakdowns previously attributed to poor quality pellets that clogged up augurs, and produced clinker.

Pellets are usually stored in bulk to allow the end user a fully automatic system that they are used too, with delivery required only 2-3 times per year.

The average calorific value of wood pellets is 4.8kWh per kg.

Logs

Are still widely used for heating, and their use depends on the amount of time the end user has available, the storage space and the quantity of wood available. Logs are manually fed into the appliance, which is usually sized to burn a full load per day to cover the heating demand.

If logs are seasoned on site, a large storage area is required. All woods burn better when well-seasoned, and most burn better when split. Contrary to popular opinion, all wood species have a similar calorific value per unit weight provided the moisture content is the same, but due to the differing densities they have a different calorific value per unit of volume and thus the amount of fuel storage required.

As before, moisture content is key, and must be below 25% when burned. When cut moisture content will be between 45% and 60%. Consider this when purchasing logs!

At 20% moisture content the average calorific value is 4.2kWh per kg.

Chips

Contain more moisture than wood pellets, and have a lower bulk density, therefore they require a larger storage area. The chips must be kept moving in storage to avoid composting, and therefore the feed and delivery system can be expensive. The chips themselves are cheaper than pellets.

EN standards class chips according to origin, particle size, moisture and ash content. This is important as different boiler models and manufacturers may state different fuel specification requirements. Due to the above calorific values vary massively and are always lower than pellets. They can be as low as 2kWh per kg or as high as 3.5-4kWh per kg.

In our opinion chips should only be considered in larger projects, and anybody thinking about making their own chips needs to think seriously about their capability of producing a chip that is consistent with the specification that their chose boiler requires.

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