Wood chips

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Biomass boilers
         For warm water

  • BIO
    Automated, pellet, wood chips,

  • Farmer
    Automated, pellet,wood chips,
    wood logs, multifuel

  • Automat
    Self-loading, automated,
    pellet, wood chips, multifuel

Steam boilers    

  • Low pressure steam (0,5bar) automated boilers for biomass, pellet, wood chips, multifuel        

Coal boilers

  • Automated brown coal boilers
    For warm water or low pressure steam

Fuel feeder systems

The wood chips

The CARBOROBOT biomass boilers use the standards for solid biofuels in Europe EN14961, which include specifications for wood chips:
Usable woodchips in CARBOROBOT Biomass boilers are P16A, P16B and P45A


The hog fuel using possible only with limitations.





Woodchip can be sourced from a range of forest sources, ranging from round wood to forest thinnings. Wood greater than 2.5cms in diameter will generally be acceptable as chipping material. The residues from thinning, and the removal of side branches (snedding) would generally be left in situ for preliminary drying before chipping.
The main quality criteria for woodchips are:
-- chip size: only the "fine" and "medium" grades are suitable for small-scale installations;
-- water content: this determines the energy content of the fuel on the one hand and its storability on the other;
-- bulk density: this indicates the weight per cubic metre (bulk volume) and depends on wood type, particle shape, degree of compaction and water content.

The characteristics affecting the properties of wood as a fuel are: heating value, chemical composition, moisture content, density, hardness, the amount of volatile matters, the amount of solid carbon, ash content and composition, the melting behaviour of ash, the slagging behaviour of ash and the amount of impurities, dust and fungi spores. n all cases chips should be stored away from work areas, to reduce contact with spores and the possibility of allergic reactions.

Wood fuel chips, for instance, are often made of various tree species with various proportions of wood, bark, foliage, branches, buds, and even cones. This causes variation in the fuel properties.

Approximately one half of fresh, just fallen tree is water. The other half consists of dry matter of wood, approximately 85% of which consists of volatile matters, 14,5% of solid carbon and 0,5% of ash. When wood is combusted, its components will change into steam of water (H2O), carbon dioxide (CO2), nitrogen oxides (NO2), sulphur oxides (SO2) and ash. Wood has practically no sulphur at all, as its share in wood is 0,05% at the highest.

The wood chips technology

The most important equipment to harvest woody biomass is undoubtedly the wood chipper, a machine that cuts the wood into small pieces with sharp tools like knives. The market is dominated by the disc and drum chippers. There are very few screw chippersbeing produced.

Disc chippers
In a disc chipper, the knives are mounted in slots in a disc. The wood is cut against an anvil and once cut, passes through the disc, where casting plates will throw the chips out of the chute. The casting wings also generate a generous air stream, helping in blowing the chips out of the spout.
The quality of the wood chips from a disc chipper is usually good. The particles are rather uniform in length and shape because the angle of attack of the blade towards the wood is more or less the same all the time. Because of the rotary motion of the disc, the wood is awlays pushed to the outside of the disc. This concentrates the wear on the blade to the outside 1/3 of the knife. The angle of attack is shown in the drawing below. The angle of attack of a disc chipper is typically around 37 degrees. At that angle experience has shown that the energy consumption of the chipper is at its lowest. The more oblique the chipper is cutting towards the fibre direction of the wood, the higher the energy consumption, see picture in link below.

Drum chippers
In a drum chipper the knives are mounted on theperifery of a large steel drum. In front of the knife there is typically a pocket, where the chips that have been cut are stored until they can be released behind or below the drum.
A drum chipper does not have fan blades on the drum and therefore an extra fan has to be added to blow the chips out of the spout, or the chips can be dropped onto a conveyor belt.
A drum chipper produces irregular chips because the angle of attack of the blades towards the wood varies with the diameter of the wood. In a thick piece of wood, the angle of attack when the knives first hit the wood is 34 degrees, while at the bottom it might be 79 degrees. The top chips are nice and blocky, while the bottom chips are long and sliver like. This is shown in the next link.
The size of the chips can be adjusted by adjusting the knives and anvil or by inserting a screen with larger or smaller diameter holes in the exhaust opening of the chipper. Particles which are too large to pass through the screen aretaken around once more and cut against the anvil. The internal working of the drum chipper can be seen on the picture in the link below.

Overview of chippers

Woodchip - chipped woody biomass in the form of pieces with a defined particle size, produced by chopping with sharp mechanical cutters. (EN14961_4:2011).


The woodchip and the hogfuel

Wood chips

(produced with sharp tools such as knives)
Good quality wood chip fuel is produced by machines with sharp knives, with the ability to vary the size of chip produced to meet end-user specifications. Other machines use hammers or flails to reduce particle size and produce hogfuel,which is unsuitable for use in small installations



Hog fuel

(shredder and hammermill technology)
Crushed/shredded wood in the form of pieces of varying size and shape and produced by crushing with blunt tools such as rollers, hammers, or flails produced by a machine with a blunt hammer action. Unprocessed mix of coarse chips of bark and wood fiber.







Fuel storage

Stacking wood chips

Stacked wood chips do not allow air to flow easily through the stack. Unless the biomass is very dry this tends to lead to composting and the growth of moulds. Composting leads to loss of biomass and can present a fire hazard owing to the high temperatures created in the core of a large stack. Advice varies between different sources as to the safe maximum size for stacks of wood chips to minimize the risk of fire, with figures typically around 8-10 m high. The high core temperature in a stack of wood chips can, however, be used to assist drying as moisture is driven out from the core to the periphery. Spores from moulds growing on piles of stored wood chips have been shown to cause health problems from particle irritation, type I and type II allergic responses, potentially toxic volatile organic compounds (VOCs) and mycotoxins, and inflammatory reactions to cell wall components when inhaled. A P3 filter dust mask should be worn when working in close proximity with stores or deliveries of stored chips.

For a primary central heating system for a domestic house a fuel store of at least 5-6 m3 is recommended, preferably larger for wood chips. This may require filling once or twice a year for pellets, or four to six times for wood chips. Wood chip is widely use in other European countries as a fuel for heating and in combined heat and power systems, particularly in countries with a strong forestry tradition such as Scandinavia and Austria.
The advantages of wood chip are:
it is cheap, often as cheap as logs and now (June 2006) cheaper than mains gas;
it is a standardised product with Europe wide quality standards in place;
fully automated, highly efficient boiler systems are available;
it can be made from a wide range of raw materials, often sourced locally.
Its drawbacks are:
fuel quality is often variable, since suppliers are still inexperienced;
it can be difficult to find supplies without assistance;
it is a bulky fuel, leading to storage problems in some sites, particularly at the domestic scale.


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