Production of quality controlled biochar with different degree of carbonization, for growing tests and ecotoxicological studies

Over the last years more and more attention has been paid to apply biochar in soil remediation, e.g. by mixing the material into soil. Biochar has been shown to improve not only soil biological activity but also to prevent organic and nutrient losses. However, in many cases little or no attention has been paid to biochar quality. The reason is that biochar is either produced in relatively uncontrolled conditions, like in charcoal kilns, or it is produced from secondary carbonaceous by-products, like gasification or pyrolysis residues. These all are often considered as a quality biochar. On the other hand different waste raw materials, like bio-sludge, are also used as a raw material. This leads to unwanted situation where clear conclusions cannot be made, if a certain quality parameter of biochar is important or not in the application under consideration. Biochar quality is dependent on factors such as: raw material quality (virgin materials or residues, moisture, physical nature etc.), variable reactor conditions (particle size, final charring temperature, residence, time, carbonization atmosphere). A project on biochar quality and its effects on biochar properties when applied as soil amendment is underway in Finland in 2011-2013 within the Tekes BioRefine programme. The project is carried out jointly between VTT, MTT Agriculture Research Finland and University of Helsinki. To eliminate the effect of large number of variables, different quality controlled biochar types were first produced in laboratory scale (TGA) and then in a pilot scale laboratory batch reactor of VTT (kg scale). Following definitions were made: one wood species (Betula spp.) was selected as raw material , a single longitudinal dimension of wood particles was selected, fixed moisture content of raw material was applied , nitrogen was used as reactor gas flushing, residence time was kept constant, final temperature was varied to produce three types of charcoal, with different degree of carbonization. The idea was to keep most of the biochar quality parameters constant so that only one main variable, i.e. final carbonization temperature remained. The physical and chemical properties were then analysed including charcoal yield, ash, volatiles, CHN, BET surface area and PAH content. The three biochars were delivered to MTT and University of Helsinki to be utilised for growing tests and exotoxicological studies.1 After the results from growing tests have been achieved, it is possible to assess, if the degree of carbonization is a major parameter of biochar quality. In addition to biochars, the liquids (distillates and tars) and gases produced in the carbonization tests will be studied as in our previous paper2.