Biomass: Systematic processing
The woody fraction in green cuttings is increasingly used as a biomass fuel. This material is often called “shredder wood” because of how it is processed - it is typically not chipped with sharp blades, but shredded with blunt teeth. This breaks and tears at the wood. With high-speed machines there is also an impact shredding effect.
Carefully considered processes with Komptech
As so often, the technology used in processing plays the decisive role in the economical use of the woody green cutting fraction, be it for composting or fuel production. At many composting plants the efficient production of high-quality biomass fuel has become a vital line of business in addition to composting. Careful consideration of the right processing machines can make the difference between success and failure in fuel production.
Shredder wood is affected by many factors, and comes in different fuel qualities depending on whether the feedstock was separated or mixed. For example, shredder wood from tree and shrub cuttings differs from shredder wood made from rootstocks. The processing needs to take into account these different factors. The material is processed mechanically as well as biologically, mechanically through a combination of shredding, screening and windsifting, and biologically by drying.
In each case, processing should ideally start with a low-speed shredder (rotor circumference speed 1-2 m/s). The Crambo, a tough all-purpose wood shredder, is ideal for the task. With its large screen baskets it outputs the right grain size for further processing. The high speed Axtor all-purpose wood chipper can be another option. Depending on the feedstock, it can be run with rigid or free-swinging blades.
The shredded material should then be biologically dried for one to three weeks, depending on how damp it is. Compost turning with an appropriately dimensioned turner accelerates the process to keep the material drying but not composting, as that would lead to undesirable darkening. The new Topturn X4500 is perfect for the job. Its large hydraulically driven drum ensures efficient turning.
Finishing up - screening and cleaning
Thereafter the material is screened into three fractions, ideally with a Multistar series star screen like the Multistar L3. It creates three fractions in one pass; the medium fraction is used for fuel. As the material passes over a bed of rotating sprocket-shaped “stars” they knock any impurities like mud off the material, giving a high quality output with woody green cuttings in particular.
The output can then be run through a stone remover and if necessary a wind sifter to free it of undesired contaminants. The Stonefex stone separator improves fuel quality by removing up to 90 percent of stones and inert materials. Light “fly-aways” such as plastic film can be removed in a Hurrikan wind sifter.
Users who want a compact solution can have one in the form of the Hurrifex, which combines both functions. Komptech offers it in stationary and mobile versions. In just one pass, it separates out hard, heavy contaminants along with light fly-aways. The separation parameters are easy to adjust and permit a wide range of applications. All components are electrically powered, either from grid power or with the on-board diesel generator.
With average collected green cuttings, these techniques give fuel yields between 35 and 50 percent. The difference has to do with the biological drying. With high-speed shredders and without biological drying the yield drops below these levels.
It is worth noting that for its 25th anniversary, Komptech has come up with something special. Customers who buy a new mobile machine during the campaign get a free warranty for 25 months or 2500 operating hours. The campaign starts 1 March and ends 31 August 2017.