Made-to-measure pellets:
HD and HD+

The minimum requirements for wood pellets for various areas of application, such as private individual furnaces, large plants or industrial plants, are defined in the European standard 14961-2. Standards in the form of certification systems such as ENplus or DINplus have been developed and implemented on the basis of this standard. But within these standards, there is still a broad range of permissible qualities. HD pellets rise significantly above this.

Only the best manufacturers produce HD and HD+ quality.
The unique, patent-protected* HD technology combined with the know-how and diligence of experienced pellet producers is a guarantee of our superior quality!

*including patent number DE 10 2013 022 395

In addition to the proven HD quality, we have developed another length design for pellets: With HD+, pellets of >30 mm are consistently rejected. But the high average length is retained. This results in an even higher concentration of pellets in the ideal length range. This provides a further significant improvement to the flow, conveying and burning properties.


Wood pellet qualities in comparison


The highest quality with no compromises!

Conventional manufacturing technology only allows for a high average length if you accept a high proportion of very long pellets as the trade-off. And, because of the system, homogeneity is only possible with an extremely high proportion of short pellets. Since the two extremes are qualitatively insufficient, in practice you have to choose between a more or less good compromise.

By contrast, the patented HD+ technology is able, for the first time, to achieve what cannot be achieved with conventional manufacturing technology: the absolute highest concentration of pellets within the optimal length range for combustion. And with a low proportion of short pellets but still not a single pellet longer than 30 mm.


Detailed Information about the Atributs

Dust and fragment formation

Dust and fragments in the pellet store are predominantly generated during blowing in, when small particles are released from the pellets. This primarily happens on the weak end surfaces. As a result, pellets with a shorter average length generate significantly more dust and fragments than pellets with a longer average length.  An increased proportion of fragments and dust has a negative effect on the flow properties and can result in screw conveyors becoming clogged. With an increasing proportion of fragments and dust, the combustion quality also gets worse.

Generation of flue ash

If there is a high proportion of fragments and dust in the pellet store, the amount of flue ash also increases because the fine particles are pulled in by the suction of the combustion air immediately upon entering the combustion chamber. This results in heavy contamination of the hot gas flues with deposits. The consequence: a dirty combustion process.

Tendency towards slagging

We talk about slagging when the ashes are heated beyond their softening point, become viscous, and form pumice-like clumps as they cool.

Because the grate cleaning systems in many pellet burners have difficulty removing these clumps from the combustion zone, the slag collects there until the heating operation is disrupted and the plant goes into fault mode. Then only manual grate cleaning and restarting the heating system helps.
The main cause of slagging in wood pellets is impure raw material owing to silicon compounds. This means small quantities of silicate get into the wood chips. The result of this is that the softening temperature of the ashes is reduced and they slag even at the normal combustion chamber temperatures instead of trickling away into the ash pan as a fine powder.
But the length distribution can also have an effect on slag formation. A high proportion of very short pellets results in a very densely packed firebed, through which the incoming combustion air cannot circulate evenly. Some places then get more airflow than others, which can result in a “jet effect”. This selectively results in a significant increase in temperature, meaning that the slagging process occurs there. This problem occurs more if the ash softening temperature of the wood is closer to the lower threshold and the pellets have a low water content.

CO in the flue gas

The proportion of carbon monoxide (CO) in the flue gas is an indicator of how clean the combustion is. Various studies by independent institutions have showed that a high proportion of short pellets, dust and fragments, among other things, have a negative effect on the emissions.
Thanks to their specific properties, HD quality pellets thus have up to 37% less carbon monoxide in the flue gas than conventional standard pellets in a direct comparison.*

*Feuerungs- und fördertechnische Bewertung von Holzpellets mit verbesserten physikalischen Eigenschaften, Technologie- und Förderzentrum im Kompetenzzentrum für Nachwachsende Rohstoffe 03/2014. 

Obstruction of the suction system

The European standard 14961-2 stipulates a maximum permissible length of 40 mm for combustion pellets, 1% of the pellets may even be up to 45 mm long. In longer conventionally manufactured pellets, you relatively often find individual pellets with an excessive length >45 mm due to production factors.

In the common heating installations, hoses, connecting pieces, etc. with a cross section of 50 mm are installed, which can be further reduced in bends. Over-length pellets can turn sideways in these conveying systems and cause a blockage. The annoying thing is that a single over-length pellet among millions of correct pellets can thus result in heating failure.

Problems for screw conveying

The length distribution has a pronounced effect on transport rates in conveying screws. This is particularly important in screw stokers which transport pellets into the combustion chamber.
A high proportion of shorter pellets results in a higher transport rate, a high proportion of longer pellets results in a lower transport rate. If too many pellets are supplied per cycle, this results in dirty, incomplete combustion and thus to losses in efficiency and combustion quality. If the transport rate is too low, this can result in output losses, particularly during full-load operation.
If the pellets exist, as is normal with conventional manufacturing, in an inhomogeneous mix, then the transport rate per cycle can change relatively significantly. The heating control has to readjust correspondingly often and constantly adapt to different quantities of fuel. In this control process, the combustion parameters briefly fall out of the optimal range which is reflected overall as a loss of efficiency in comparison with pellets with a very homogeneous length distribution.

Screw electricity consumption

Longer pellets from approx. 30 mm can line up in front of the intake for the screw channel or rotary valve so that they must be sheared off there by the blade of the screw or sluice. The increased effort which is required for this results in increased electricity consumption by the motor.

Screw running noises

The higher the proportion of longer pellets >30 mm, the more “popping noises” can be heard. In stoves in the living area, this is often perceived as obtrusive.  More information about this can be found under the “Screw electricity consumption” point.

Susceptibility to malfunctions in stoves

In stoves, pellets are principally burned from bags. This means that the mechanical strain on the pellets is significantly lower than for pellets which are blown into a storage facility. Dust and fragments therefore play less of a role, it is longer pellets >30 mm which are particularly bothersome. These have a negative effect on combustion in stoves because they can cause bridging and blockages, as well as increasing the electricity consumption and running noise for screws. 

General susceptibility to malfunctions

Disruption of heating operation owing to quality defects in the pellets can have a variety of causes. Whether it is a high proportion of short pellets as well as fragments and dust to too many long or even over-length pellets. The surface condition, mechanical strength and chemical ingredients of the pellets are key quality attributes. Normally, malfunctions only occur as a result of quality defects if a negative feature is too pronounced, i.e. the permissible limits are not complied with. But problems can occur in heating operating even within the permissible standards if various only moderately pronounced negative properties occur in combination and reinforce each other in their effects.