Raw Material

Your choice of raw material is very important for several reasons. Firstly, each material has a different heat value, ash content and corrosive qualities. Also each material has to be prepared slightly differently to make quality pellets. Use the information below to understand materials from preparation to burning qualities.

Biomass

What is Biomass

Biomass is any living or recently dead material that can be used as fuel. This includes any wood and waste wood products, also energy crops such as Miscanthus, Switch Grass, Reed Canary Grass, Oilseed Straw to name but a few.

Why use Biomass

Biomass is a carbon neutral fuel, which means no extra carbon is released into the atmosphere when burnt, unlike fossil fuels. Biomass can play a crucial role in reducing global warming. Biomass is a cheap fuel to produce, much biomass is free, in the form of waste wood products.

Which Biomass

Your choice of biomass should be based primarily on what you have available to you, but also what your boiler or stove can burn successfully without complications. Check the properties section for a summary of some biomass fuel properties.

What size of Biomass

The PelHeat unit is equipped with a hammer mill which breaks the raw material down into a uniform size (3mm-6mm). The hammer mill can process straws and grasses, however for the unit to accept wood for example it must be in a chipped form, or have a diameter less than 1 inch.

More on Biomass

Check the Resources page for websites for more information.

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Moisture

Why is Moisture Important

Moisture is needed in the pellet process, however too much or too little moisture causes problems. If there is too little moisture then the material cannot bind together, and so cannot form a pellet. If there is too much moisture the material will either be processed as a loose pellet, or cannot be processed at all. A loose, moist pellet will smoulder, and not burn.

What percentage of Moisture

Generally a moisture level of 10-15% is needed to produce a pellet of 5-10% moisture. This varies depending on the material involved. A material with minimal body, which has a powdery texture will require more moisture, and perhaps a binder. Therefore the moisture percentage required is specific to that material.

Measuring Moisture

To measure moisture at low cost, place 100g of material on a heat source, with enough heat to release moisture, but not burn the material. Over time measure the material until its weight has stopped reducing. The difference in weight is the amount of moisture, i.e. a 10g difference is 10% moisture. For a cost there are several moisture censors on the market including probe censors and infrared sensors.  Weighing the material before and after it is dried does not provide an instant result, but it is accurate and very low cost.

Testing Moisture

Place the milled material in the pellet mill. If hard pellets are produced then the material is suitable, if not depending on the result, this is will indicate the properties of the material, and how it needs to be prepared. There is more information on this in Making Pellets.

Reducing Moisture

The PelHeat unit cannot dry the raw material. Reducing moisture is the responsibility of the user, this can either be done by drying the material or mixing it with a dryer material to balance out the moisture to an acceptable level.

Increasing Moisture

The PelHeat unit will have peristaltic pumps available. The peristaltic pumps can add water into the pellet mill at variable rates.

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Properties

Heat Value

Obviously different fuels burn differently, and also give off different amounts of heat per pellet. This should be considered when choosing a biomass fuel. Examples are given below.

Ash Content

Different quantities of ash are produced between different biomass fuels. Wood, depending on the species and bark content has up to 3% ash. Energy crops produce more ash, ranging from 5% upwards. This is important as some boilers/stoves cannot handle high ash content fuels, and more ash means more maintenance.

Sulphur

Sulphur is a low temperature corrosive, fuels which contain above 0.1% will have an affect on corrosion within the boiler over time at low temperatures, bare this in mind when choosing a suitable boiler/stove. Examples below.

Chloride

Chloride is a high temperature corrosive, fuels which contain above 0.1% will have an affect on corrosion within the boiler over time at high temperatures, bare this in mind when choosing a suitable boiler/stove. Examples below.

Clinkers and Slag

Clinkers are a hard deposit material which is produced through ash melting, and form a mass of sticky material that cools hard. If a material produces ash with a low melting point a clinker will form, this will have to be manually removed, which increases maintenance. Slag is a similar material which collects around the heat exchanger pipes, and reduces heat exchange, and thus performance, examples below.

Examples

Heat from pellets, also Coal is included as a reference and guide, figures are estimates only.

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