During our visit to BEST – Bioenergy and Sustainable Technologies GmbH, we conducted experiments on the combustion and gasification of three different kinds of wood and charcoal obtained from these wood. We tried to answer three main questions. Above all, we were interested in the ash formation mechanism. Differences in ash structure after gasification and combustion were sought. Secondly, we tried to observe the phenomenon of sudden decay due to carbon structure disintegration. Last but not least we were looking for very accurate measurements of wood combustion and gasification parameters with particular interest on the volume shrinking, which could later be used as experimental verification of biomass gasification CFD simulations. Little regular samples of wood or charcoal are especially useful for such validation. Numerical calculations of small pieces can be made relatively fast, which is ideal for testing new algorithms and models.
The experimental set-up included electrically heated stove in which desired temperature was maintained. The composition of the intake gases was air or nitrogen-carbon dioxide mixture, depending on whether combustion or gasification was carried out. The flue gas has been accurately measured using professional laboratory equipment. Weight loss, residue mass and temperature inside the sample were measured during each run. A total of 35 experiments were conducted, including: char combustion – 11 samples, fresh wood combustion – 6 samples, char gasification – 11 samples, fresh wood gasification – 7 samples. All experiments were recorded with a digital camera through a small reactor window.
The results showed interesting ash shapes formed during the combustion and gasification. Usually, for dry wood, the mass fraction of ash does not exceed 1%. Residues resulting from gasification or combustion contain almost no mass. Despite such a large weight loss, the residues retain a rigid structure and do not fall into dust.
This phenomenon is not well explained in the literature. The nature of the forces that hold the ash together is currently unknown. Our experiments showed that residues shapes were different for combustion and gasification. This leads to the conclusion that oxides formed during combustion can affect the specific shape of the residue. Further studies should be carried out to better understand the microscopic mechanism of such formation. The phenomenon of sudden decay of the sample was not observed. However, partial cracking typical of fast-heated wood was observed.
All measurements collected during the experiments will be used to validate the OpenFoam solver we are currently working on.