By Charlotte Karlsson
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering. 2019
In different locations on the Swedish market, streams of residual products exist, which today usually are perceived as waste. To be able to reach a more circular society there is a hope of finding a way to use these residues in a new application, to see the material as a resource instead of waste. Biochar, a material with a high carbon content that is created through pyrolysis of biomass, has a potential to create a high-value outlet for some of these residues. In order to secure the quality of the produced biochar for the designated application, a method has been utilized that links the characteristics of the material, the characteristics and functions of the biochar and potential applications. The development of this kind of method can pose a way for potential manufacturers of biochar to find a new outlet for their residues and secure that the customers receive the right product for the attended application. An evaluation of the potential financial and environmental benefits that a biochar product can contribute with has also been conducted.
After a series of interviews had been conducted, a residual product was chosen for further in-depth analysis; the fraction of wood waste that consists of finer particles. Today waste wood chips are generally being handled through incineration, however, possible changes regarding the handling of this material might soon emerge. The finer part of the material causes problems for the incineration through wearing and corrosion of equipment, as well as posing a fire hazard during storage. Due to these problems there are predictions that changed demands regarding the separation of this wood waste fraction, which can make up 20% of the material, could emerge. Changed demands from the recipient of the material can cause this residual stream to quickly appear in large quantities and without suitable outlet.
Through examination of the characteristics of the material, assessments could be made regarding the characteristics of the produced biochar, which functions the char could fill and which applications that might be suitable. A connection could be made between the characteristics of the original material and the quality demands that exists for different applications. The most problematic characteristic of the material to overcome was the content of metals, originating from treated wood. To overcome this problem, manufacturing through flash-pyrolysis was suggested, were a large part of the ingoing material is converted into pyrolysis oil. Previous studies have shown that an oil with a low content of metals could be produced through this method, as the metals are instead concentrated in the char. Unfortunately, this means that the produced char does not qualify as biochar, due to a too high content of metals, but gives an advantage since the metals are less bioavailable through their bonds with the char structure. Through pyrolysis the quantity of material that needs to be handled is reduced.
The most suitable application, for the char produced from the finer fraction from wood waste, was chosen to be as a carbon filter for cleaning of wastewater, as a substitution for activated carbon. Since the metals are more strongly bonded to the char, compared to the biomass, the char can still be considered suitable for a filter application. By comparison of environmental and energy demands at production of the two materials it was shown that wood waste char had less environmental impacts than activated carbon, while the adsorption capacity for different metals was not remarkably different between the two materials. For the customers, buying this product, there is also an advantage in the financial aspect due to the price of wood waste char being about half the size of the price for activated carbon.
Difficulties that still needs to be overcome are the final handling of the product, after it has been used in the application, experimental studies that can confirm the characteristics and adsorption capacity, as well as a more thoroughly conducted financial evaluation to ensure that potential manufacturers can achieve financial gain in this way of handling their residues.