Summary
The wood particles with moisture content in the range of 60% -150% (atro) should be dehydrated to less than 1% to 10% (atro) during the production of organically bound chip board. The waste gas generated from dryers contains dust particles (wood, minerals, salts), aerosols (waxes, resins, terpenes), gaseous organic materials (carbonic acids, aldehydes, carbon monoxide) and also often has an intense odour.
Up to now it was only possible to lower the emission levels of these materials with complex techniques and frequently using multi-level reduction technology. Using conventional directly or indirectly heated dryers, the waste gases often had to be passed through a combination of washers, filters, and post incinerators.
In the project, an advanced system was developed for drying wood chips in a closed gas cycle and this was implemented on a semi-industrial scale. In this case, only the volume of drying gas is withdrawn which corresponds to the amount of evaporated water plus the intake of leakage air. With this system there is a considerable improvement in reducing the emissions, because the dryer waste gas is not allowed to escape into the atmosphere.
Results:
The laboratory drying experiments have shown that for the closed process thermal utilisation of the heat of condensation is approx. 85% better than for the conventional drying process. If the latent heat is not utilised, the power requirement is still be about 10% lower than using conventional fresh air/used air-dryers.
The superiority of this process-integrated method over end-of-pipe gas scrubbing is demonstrated by the fact that more than 95% of individual components and more than 90% of total carbon content could be separated. Further, it could be shown that drying the chips in the circulating gas with a high moisture content leads to an improvement in chipboard properties.
In addition to the latent heat, wood componants can also be recovered by means of condensation of the vapours and they can be used as raw materials in the industry for paint and perfume production.
The combination of the centrifugal separator equipment connected with pervaporation equipment at the outlet has proved to be a very effective method for preparing and extracting useful raw materials. As in all processes of separation methods using membranes, the economic viability depends on the concentrations, the membrane service life, and the quality of the permeate. An alternative could be a system with mechanical pre-scalping and vacuum distillation.
