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This review article summarizes fundamental knowledge about biochar. Biochar is getting paid a lot of attention recently for its ability to enhance crop yield and mitigate climate change. Main European furnace producers for generating biochar are introduced. Relevant physicochemical and textural properties are reviewed with emphasis on a stability of biochar, pH, water holding, and nutrient retention capacity. Czech legislation, where biochar is classified as a supplementary soil substance and voluntary certification initiatives such as European Biochar Certificate (EBC) and International Biochar Initiative (IBI) are discussed. The methods of application of biochar in agriculture are described as well.

Unburned carbon (UC) in six different combustion residues from solid biofuels has been examined using several methods of analysis, such as Loss On Ignition (LOI) and Thermogravimetric Analysis (TGA). Most of the samples came from the heating plants and local heat utilities in the Czech Republic. The amount of unburned carbon in the residues varied widely over an order of magnitude and in several samples accounted for about 7% of the ash mass. Experimental results have shown that LOI overestimated unburned carbon in nearly all samples. Some of these findings can be associated with previously identified thermal decomposition of minerals present in ash. Volatile organic compounds also appear to play a role in the final UC value. Out of TGA methods, first, we used strongly endothermic Boudouard reaction method as an advanced TGA method. However, this reaction was affected by the high potassium content in the samples which could cause incomplete conversion of carbon during Boudouard reaction, because of sintering and formation of the deposits of glass at high temperatures. The second method was a simple TGA method with a controlled atmosphere of nitrogen or air. For ashes, which tend to form a glassy coating and undergo a sintering process, it is not suitable to carry out the measurement at high temperatures, because the Boudouard reaction does not proceed entirely. For these types of samples, it is more appropriate to use TGA methods with a controlled atmosphere of nitrogen or air.

Thermochemical conversion of biomass produces solid residue that is further used in agriculture called biochar. Studied biochar was produced in twin-fire two-staged gasifier from waste wood biomass. Heat and electricity were produced together with biochar. Biochar samples were analysed and assessed as a soil amendment. Results were compared to legal standards and voluntary certificate limits. It was concluded that studied biochar meet all criteria for utilization as a soil amendment.