We are hearing more and more about wood pellet boilers and their fuel these days. What exactly are they, How do they work and what makes them so different to conventional wood burning stoves. The following article will answer these questions and will attempt to explain the advantages and disadvantages of such a system.

The Fuel

Wood pellets are a type of fuel made from Compressed sawdust. They are usually produced with a low humidity content of under 10 % which gives a heating value of > 18 MJ/kg.  Good quality pellets have a density of approx. 650 kg/m^3. Basically, this means that two kilograms of pellets produce approximately the same energy as one liter of oil. Wood pellets are (according to norms, DIN 51731, Ö-Norm M-7135) generally between 4 and 10 millimeters in diameter and not more than 5 times the diameter in length. Another important parameter is the abrasion level of the pellet.The less abrasion there is, a  the more solid are the pellets and therefore the less dust emissions the pellets produce on delivery. DIN 51731 does not cover this factor, however Ö-Norm-7135 does.  Recently a new norm has been introduced (DINplus). This combines the two previously mentioned norms above. Also the Din norm states that no additional matter may be used in the production of wood pellets. The advantage of having these norms for wood pellets is that they can be easily fed into a system or through delivery feed devices (a ‘liquid’ effect). Therefore a wood pellet system can be fed electronically lending to cleaner and easier usage.

Important Factors to Consider
1. Wood Pellets must conform to certain norms (DIN 51731, Ö-Norm M-7135 or DINplus)

2. Dust emissions from pellets on delivery and transportation depend heavily on abrasion factors (only Dinplus takes equation factors into example) .

3. Quality on delivery. Pellets should not be allowed in contact with water.      This reduces their heating value and leads to system blockage.

Production

Pellets are usually produced from sawdust waste from sawmills. The sawdust is dried to approx. 8% moisture content and then cut down further by using a hammer mill. It is then supplied with a small amount of water and pressed through a dye with 6mm holes.  Production costs can be divided into various factors. About one third of these costs goes to the purchasing of the raw materials (sawdust), of which dried is more expensive than moist. The rest is for “pelletisation”. Of course there are other factors, including marketing and data processing costs also involved. The drying process can amount to a very high fraction of the overall cost, however, this can be lowered if the heat used is produced from a CHP plant located onsite.

Emissions

As a heating option for households and industry, wood pellets are one of the cleanest available.  They produce very little ash and are very low in Nox emissions.  It can be said that there is a very  small carbon footprint due to the fact that trees are a renewable source and the CO2 emissions released  into the air were those that the trees originally extracted to grow. As new trees are planted these emissions  again will be extracted from the atmosphere. However, it is also argued that the sawdust used by pellets  would not otherwise be burned and therefore release an excess of CO2 into the atmosphere.

Domestic Heating Systems

Domestic pellet stoves are readily commercially available from many suppliers in Ireland and Europe. They provide a very efficient (economically and environmentally) way of adding thermal energy to a household. Delivery of this type of fuel is usually conducted in several ways. Either in 15 kg bags, 1 tonne bags or alternatively by silo vehicles who pump the pellets into storage containers. Because or their low moisture content, pellets need to be stored in a dry place and this gives rise to the need for a storage facility such as a silo or purpose built containers. This is especially important in countries where the atmospheric moisture content is much higher than that of the norm (< 10% M.C). If the moisture content of the pellet is increased, this in turn affects the calorific value of the fuel leading to a lower combustion efficiency and leads to possible clogging of the “feeder” in the burner.