Application in Latvia
Wood chips are produced on a grand scale in every sawmill in Europe. They are, just like sawdust, a by-product of the lumber cutting process. The tree trunk is turned into an initial standard square shape in the sawmill using large milling heads, known as chipper canters, and then cut using circular saws. In the process, over 30% of the volume of wood is machined into wood chips and sawdust. A large sawmill with an annual production of 1 million tonnes generates around 50 truck loads per day. A truck can hold approx. 80 - 100 bulk cubic metres. This means that up to 5000 cubic metres of wood chips are produced every day, equating to around 1500 tonnes that need to be meaningfully processed.
Wood chips are used as a raw material in manufacturing chipboard and compressed wood chip blocks, in the paper and pulp industry and as biogenous and renewable fuels. This happens partly through direct incineration in medium-sized and large plants and partly by processing the wood into pellets as fuel with a high heat value, which is particularly suitable for heating systems in private households.
In the case of freshly-harvested softwood, around half of the weight is water. This water compromises the ability of the wood to be stored (decomposition, formation of mould) and reduces its heat value.
The raw material needs to be almost completely dry for it to be further processed into wood chips or pellets. To do this, technical drying is essential, mostly with continuously operating horizontal belt dryers. The wood chips pass through a current of hot air on an air-permeable conveyor. With particularly wet wood chips the conveyor moves slowly, whereas pre-dried wood chips allow it to move more quickly. In practice, the hot air is frequently generated with natural gas or, depending on availability, with the help of locally available process heat.
For incineration, too, the wood chips must have a certain degree of dryness to achieve optimum combustion. With very wet wood chips an unnecessarily large proportion of the heating energy is consumed solely to evaporate the water. For this reason there is almost always an upstream drying process in wood chip firing plants. The drying is often achieved by storing the wood chips over a longer period of time, during which the pile heats up and dries through microbial activity. However, this type of drying results in a noticable loss of heating value.
Just like wringing out a wet towel the majority of the water is removed from the fabric through mechanical energy. The roller press can remove approximately half the water from the wood chips with very low energy expenditure. Then already they are very well suited for use as fuel.
Furthermore, complete drying for the production of chipboard or pellets is made considerably easier, too. After being mechanically squeezed, the wood chips contain a very uniform amount of moisture; they are also broken up by being squeezed and therefore have a larger surface area. The squeezed wood chips can then be dried more easily and efficiently on the belt dryer. The hot air drying system needs to be fine-tuned once and will then deliver the same result on and on — unlike drying unprocessed wood chips, where the raw material varies in levels of moisture and thus requires continuous readjustment.
The wood chip squeezer is the world's first pressing system for mechanically drying wood chips in continuous industrial operation. The current prototype has been working since March 2016 in a Black Forest sawmill on the basis of a continuous two-shift operation.
The centrepiece is the patented conveyor chain, which passes between two large rollers. The chain is supported by the bottom roller and is driven by friction. It conveys the wood chips into the pressing zone. In the pressing zone it constitutes an almost closed pressing surface — the opposing side is the top press roller. At the same time the chain offers optimal drainage through narrow gaps between the chain links. These links are stacked on top of one another, so that the water can disappear directly in the pressing zone, i.e. at maximum pressure. This needs to be the case, because the squeezed wood chips would otherwise immediately reabsorb at least part of the water as soon as the tension would be released. The rollers are pressed together hydraulically with a pressing force of approximately 100 tonnes over a width of 50 cm.
After pressing the chain is redirected several times. In the process the gaps between the chain links open up and the chain cleans itself. The squeezer is designed for 24/7 operation. The robust construction combines low-wear operation with low drive power. The current prototypes have now completed over 10,000 operating hours with no unforeseen signs of wear.
More than 70 litres of water are squeezed out of the wood chips with just about one kilowatt hour of mechanical energy. Conventional systems using traditional hot air drying need 1.7 kWh of electrical and 90 kWh of thermal energy to achieve the same result.¹
¹) Average values of conventional drying systems.
Source: Final Report - InnoDry Research Project at the University of Applied Forest Sciences Rottenburg
This significant improvement in the amount of energy consumption leads to considerable savings in comparison to the usual thermal drying process. A major part of the free water that is not bound in the cell wall is removed mechanically. This means that the entire drying process can be organised more efficiently and economically in comparison to the usual thermal processes. This results in additional saving potential where the drying costs are concerned.
There is another good reason to squeeze the wood chips directly in the sawmill: with half the water removed already at the plant, a quarter of the weight of the wood chips also disappears — and a superfluous quarter at that. This results in immediate saving on transport costs. In practice this means that the usual large trucks can now be fully loaded with squeezed wood chips — as opposed to 80%, which was the maximum load of naturally moist wood chips before the vehicle would be overloaded. So far, experience shows that there is a reduction in onward transport costs of between 20% or more.
The resulting press water (approx. 1-2 m³/h) contains minerals and organic elements. These are washed out of the wood, which is advantageous with regard to possible incineration: The proportion of mineral residues is significantly lowered in the squeezed wood chips. This means that the amount of incineration ash is reduced and especially the proportion of particulate matter in the exhaust gas. Systems running on squeezed wood chips show considerably lower quantities of ash and minimal deposits in the combustion chamber.
Regarding direct incineration, experience so far has shown potential savings around 10-12% due to the reduced quantity of water. If the improved system efficiency level is taken into account, fuel savings of 15 to 20% can be achieved.
In consultation with the sewage works operator the press water of the currently running systems has been discharged into the main drains. However, using the water as fertilizer in agriculture is possible, as well as returning the ingredients to the forest for natural circulation. The press water contains ingredients that have previously not been available in this form. As a result the long-chain organic compounds extracted via the pressing process could possibly be of use for other applications, e.g. in the flavouring and fragrance industries.
Baltic Bloc is a major wood processor in the Baltics and located in Madona (Latvia), some 100 km east of Riga. Its main export product is pressed wood components for wooden pallets. The premises of Baltic Bloc host a complete 24hrs-production chain, starting with raw logs and ending with ready-to-ship pressed wood components.
The production uses input material from wood chips hacked on-site, as well as wood chips and sawdust supplied by third parties. The input material is often heterogeneous in quality and consistency. Long and harsh frost periods are a further challenge to a stable production.
Two wood chip squeezers were integrated into the existing process, in order to improve the energy balance and distinctly increase the total production throughout.
An unfreezing unit was supplied and integrated in order to secure stable operations all around-the-year. The thermal energy is supplied by the on-site biomass power plant. Screw conveyors push the material through the warm flush fluid that unfreezes the wood chips in a continuous counter current flow. The unit has proven its reliability in daily operations. The existing belt dryer is now limited to remove the residual moisture. Thanks to the fact that the squeezer breaks open the wood structures of the chips, and thanks to better residual drying the material is now much more homogeneous when getting into the next processing step, which is the milling.
Further projects of similar kind are underway in Germany and the Baltics.
|Roller width:||510 mm|
|Pressing force:||1000 kN|
|Applied pressure:||300–400 bar|
|Throughput:||20 -30 m³/h (bulk cubic meters)|
|Reduction of water content:||Starting content 52-63% to final content 38 +/- 1 %|
|Wight of unit:||ca. 17 to. (excl. conveyors in and out)|
|Engine output:||2 x 22 kW|
|Typical power consumption in continuous operation:||approx. 30 kW|
|Unit footprint:||ca.3 x 4 m (excl. conveyors in and out)|