About

Nuergy the biomass specialists, are now established eight years, have already imported over 150 boiler units and sold 2.4MW of Janfire System Jet Burners to a UK (CHP) power station construction project.  Nuergy is MCS accreditated, operates a quality management system under the renewables industry MIS standard.

Nuergy provide system design and construction services from initial specification through to liaising with building service engineers and M&E contractors, Our project management team, overseeing installation prior to commissioning, providing user training and service support thereafter. Additionally, Nuergy offer a full ESCO ready heat solution or service contract to both consumer and fuel supply companies alike.

For the past 30 years Janfire has been a world leader in the manufacture of wood pellet combustion systems for hot water, steam and air heating systems. In addition the company has 15 years’ experience of producing packaged plant-room solutions making Janfire one of Europe’s first biomass ‘ready heat’ or Energy Service Companies (ESCO). Janfire plan to share their invaluable Scandinavian experience with new clients and customers at a number of seminars to be held at their new UK headquarters in Edinburgh.

Janfire is in a leading position to deliver UK-wide commercial biomass heating centres. In line with their current model, Janfire works with established trade by providing design, supply, commissioning and training services for boiler installers and the UK’s fledgling Energy Service Company (ESCO) industry.  Both Janfire and Nuergy Biomass aims to use long standing experience and leading technology to become the principal driver in the emerging UK renewable industry.

Rising energy prices are having a significant impact on all sectors of society. 

Householders are seeing continual increases in their electricity, gas and oil bills, businesses and industries margins are being eroded by increasing fuel costs and fuel used for power generation is also increasing in cost.  When this is combined with decline in the UKs indigenous gas reserves resulting in an increasing level of reliance on energy imported from less stable parts of the world such as Russia and the Middle East the importance of increasing heat and electricity generation from renewable local sources is heightened.  Renewable sources of energy have the additional benefit of contributing to reductions in greenhouse gas emissions.

One of the most versatile forms of renewable energy is biomass.

Unlike most other forms of renewable energy it can be stored for use when required.  Wind, wave, tidal and solar energy is dependant on weather conditions and therefore provides an intermittent source of energy.  One of the most common forms of biomass fuel is wood.  In its basic form this can take a number of forms such as logs, wood chip, brash and sawdust.  Its suitability to be used as a fuel can be increased by processing this basic form to produce higher value biomass such as briquettes and pellets.

Pellets are a processed, concentrated form of biomass which can be easily handled, transported and stored. 

Pellets are of a cylindrical shape of various sizes normally no more than 8 mm in length.  Manufactured potentially from any form of dried and finely comminuted biomass, they are mainly still made from a woody feedstock, although a number of trials are ongoing investigating using straw and other feedstocks.

Advantages

There are two main characteristics of woodfuel pellets that give them a certain market attractiveness advantage over other woodfuels.

• Energy Density;   The energy density of pellets is approximately 17.3 GJ, (4.8 MWh) per tonne.  This compares to 5.6 to 11.7 GJ, (1.6 to 3.2 MWh) per tonne woodchips or log fuels.[1]  Therefore required pellet fuel storage space is less in comparison with other woodfuels, but still about three times that for heating oil.
• Moisture Content.  The moisture content of pellets ranges from 7 to 12% MC, (Wet basis).  This is considerably drier than other forms of wood fuel with wood chip and log fuel having a moisture content of 25% to 60%.  This results in a more efficient burning process as the excess moisture does not need to be evaporated.  It also results in fewer problems with fuel handling.
• Uniformity. Pellets are of a uniform size and quality and are therefore less likely to cause blockages in automatic boiler feed mechanisms than wood chip.  There uniform nature also makes them suitable for a wider range of storage containers such as silos.
• Delivery Flexibility.   Pellets can be readily delivered by bulk vehicles fitted with pneumatic unloaders.  They can also be delivered in bags or by bulk tipper vehicles.
• Lower Ash Content.  Pellets made from clean feedstock normally have a lower ash content than woodchip reducing the frequency that ash has to be removed from a boiler and disposed of.

For small to medium scale users in particular, woodfuel pellets are more ‘user-friendly’ than any other form of woodfuel.

Raw Material Resource

Traditionally pellets are produced from wood processing industry ‘by-products’ such as sawdust; grinding dust; wood shavings; or other wood processing by-products.  Such resources reduce the amount of costly comminution into the fine particle feedstock required of the pellet mill, compared to what would otherwise be required of other forms of wood input, (e.g. small roundwood).

There are restrictions on the use of some re-cycled wood as feedstock arising from contamination with paints or other chemical treatments. 

Re-cycled wood may also often include metal fastenings, which mean more expensive comminution and metal removal systems in the production line. The pellet extrusion ‘Dies’ are readily damaged by abrasive and metal particles in the feedstock material.

In recent years efforts to increase the range of pellet fuel feedstock have been made. 

This has resulted into expansion into certain other resources, e.g. re-cycled paper; fruit stones; nut shells. Much development work has been going in the USA/Canada into pelletising straw from grain crops.  Although in plentiful supply, a difficult material as it does not readily bind together under the pressures of the pellet extrusion process.

The Pellet Production Process

Wood pellets are normally produced from sawdust or woodchip.  Both of these feedstocks normally have a moisture content of between 30% and 60%.  The manufacturing process has the following steps:

• Drying. The feedstock is dried to approximately 12% moisture using a heat source. One of two methods is normally used for this:
1. Direct air drying of sawdust, where hot air is blown through feedstock driving off the moisture at atmospheric pressure.
2. A steam drying technique can be used with the aid of a steam explosion process.  Sawdust is immersed in high pressure steam within a sealed vessel, boiling off the moisture.  This is a more expensive process but has the benefit of improving the quality of the final product by releasing more lignin (resin) which seals the pellet.  The pellets produced using this process are known as torrified pellets.
• Crushing.  The dried feedstock is comminuted into a smaller particle size
• Conditioning and Mixing.  Steam and water are mixed in with the feedstock and are given time to be absorbed.  This ensures that the fibres will attach and bond with the lignin resin on the surface of the pellet.  Lignin seals the pellet ensuring it is resistant to cracking and reducing to fine material during transport.
• Pelletising.  During pelletising the feedstock is extruded under high pressure and  is pressed by perforated rollers through a ring or flat die of the required pellet diameter forcing the sawdust against tapered channels.  This produces heat in excess of 110C compacting the wood fibre and extracting and bonding the natural resin (lignin) on to the surface of the pellet. The volume of the pellet is reduced by up to 5 times thus increasing the bulk density to typically 650kg/m3 with an energy content of 17Mj/kg.  Finally the pellets are passed through a cooler  to harden the pellet surface.

Although the above description is simplistic, the actual process is more critically involved. 

As equipment adjustments have to be made to suit the feedstock which can be inconsistent is quality.  Certain, recent trials with small-scale pellet machinery have highlighted the difficulties arising from a need to re-set the process for even slightly differing feedstock parameters, (e.g. species), although more problems can be anticipated with such small-scale processes than with large. 

The transporting and storing of pellets is also important. 

The hygroscopic nature of pellets requires them to be kept free of moisture in a covered container. When standard white pellets are exposed they disintegrate. Torrified pellets are more resistant to the intake of moisture as can be seen from the following figure.  Both samples of pellets were immersed in the same quantity of water.  The white pellets absorbed the water within a few minutes but the torrified pellets continued to repel the water and maintain their shape and moisture content.

Pellet Characteristics

Energy Content

The energy content of wood pellets varies according to the feedstock and the manufacturing process.  The calorific value is normally in the 4.5 – 5.3 kWh range.  The higher energy values are only possible with torrified pellets.  Most pellet specifications state a minimum energy content of 4.7 kWh/Kg.  This is equivalent to:

• 4700 kW/tonne
• 16.9 GJ/tonne
• 160 Therms/tonne
• 0.4 Tonnes of oil equivalent/tonne

Density

The density normally ranges from 500 – 700 kg/m³. This compares to a fresh woodchip density of 320 – 380 kg/m³.  The density will effect the amount of storage space required.  A tonne of pellets typically occupies 0.65 m³. 

Moisture Content

Pellet moisture content affects energy content, density and self ignition properties.  Pellets typically have up to 10% moisture.

Composition

There are various chemical components that are found in pellets.  The follow components can cause complications if too high a content is present:

• Chloride:  Can cause corrosion and increase emissions of dioxins and furanes
• Sulphur:  Causes SO_x emissions
• Potassium:  Can cause corrosion and reduces ash melting point leading to slagging
• Nitrogen:  Causes NO_x, HCN and N₂O emissions
• Magnesium, calcium, phosphorus:  Increases ash melting point, increases pollutant levels of ash
• Heavy metals:  Causes damaging emissions and ash disposal problems
• Ash content:  Can increase particle emissions and ash disposal costs

Physical Attributes

In Europe the diameter of pellets include: 5; 6; 8; 10 and 12 mm.  However the European Committee for Standardisation, (C.E.N.), has in the last few years proposed a standard of 6 or 8mm diameter.  The majority of suppliers expect the most common size to be 8mm.  Pellet length may vary from 10 to 30mm.

Particle size can cause problems with fuel handling and if excessive fines (small particles less than 3%) are present dust can form.  A maximum level of fines of 1% is normally permitted.

Standards

Different companies within Europe have adopted different standards for pellets.  A common European standard, ENplus has recently been published.