Production and Properties of Biochar
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Biochar, the solid product of biomass pyrolysis, has been produced and utilized for many thousand years. It is one of the most versatile solid fuels with applications ranging from heat and power production to metallurgy and also to non-energetic utilizations in agriculture and medicine. Biochar is gaining increasing popularity as a replacement for fossil carbon carriers, in an attempt to reduce greenhouse gas emissions. This thesis investigates the production and properties of biochar. First, it provides a comprehensive characterization of biochar by reviewing a large number of experimental data. The properties considered in this review include product yields, elemental and proximate composition, energy content, pH-value, reactivity, cation exchange capacity, density and porosity, surface area, pore volume and pore size distribution, hydrophobicity and water holding capacity, mechanical stability as well as thermal and electromagnetic properties. The characteristics are always shown as a function of production conditions, most importantly the heat treatment temperature. The devolatilization behavior of forest residues has been investigated in the experimental part of this thesis. Mixed forest residues and bark were used as a feedstock in fast pyrolysis experiments in a drop tube reactor at temperatures of 827, 1027 and 1327 °C. The composition of the devolatilized gases and the char were determined. The gravimetric determination of the mass yield was not possible due to the specific experimental setup and therefore had to be calculated. Common methods such as the ash tracer method proved to be unreliable for biomass due to the release of inorganics into the gas phase during conversion. Therefore, a novel method was proposed, in which the solid mass yield is calculated based on the volatile matter content of the biomass and the char. It could be shown that this method predicts the mass yield of biomass slow pyrolysis with reasonable accuracy. This contradicts somewhat the assumption that secondary char may form from repolymerization of tars during slow pyrolysis. This novel approach has been transferred and used to calculate the solid yield of the drop tube experiments. Modeling can be used as a tool to improve the fundamental understanding of physical transport phenomena and chemical reactions taking place during biochar production. In the modeling part of this thesis, a novel approach for mathematical description of biomass pyrolysis and gasification is presented. The reactor model is based on a stochastic reactor approach. Instead of solving transport equations in all spatial dimensions as in classical computational fluid dynamics, state variables are described using a probability density function. This makes the approach computationally efficient and especially appealing for industrial applications. Experimental data from literature was used to validate the model and the calculated results were found to be in good agreement with the measured data from the experiment.