Fundamental laboratory investigations
An analytical laboratory with a wide range of equipment is available for analysis of the experimental results. Listed below are methods of investigation that are of particular importance for the biomass project:
Elemental Analysis
CHNOS elemental analyzer
To determine the C, H, N and S contents, the samples are combusted in a stream of oxygen. The combustion products - nitrogen, water, sulphur dioxide and carbon dioxide - are detected and the content of each element is calculated from calibration curves. Oxygen is detected in a second apparatus via the formation of carbon monoxide.
Elemental analyses and calculated lower heating values
The lower heating values of the educts can be calculated from the elemental analyses with a precision of approx. 5% using different empirical formulae. Alternatively, the higher heating values can also be determined by means of a bomb calorimeter.
Gas chromatography / Mass spectrometry
The large number of volatile substances in the pyrolysis tar are detected by gas chromatography using different detectors.
Gas chromatogram of a pyrolysis oil
Thermogravimetry
Thermogram of wheat straw
The temperature-dependent decomposition of the biomass molecules is investigated by thermogravimetry (TG) and Differential Scanning Calorimetry (DSC). Thermogravimetry allows the water content and proximate analysis (volatiles, char and ash content) to be determined.
Reaction kinetics
The kinetics of the pyrolysis of straw particles
The overall kinetics of pyrolysis can be measured in a sealed oven with integrated barometer. After making contact with the hot sand in the oven, the thin-walled straw reacts completely within approx. 1 s. In contrast, the stem nodes require approx. 10 s for complete reaction.
Particle sizes
Porosimetry
If spherical pellets of equal size are mixed with a liquid to form a suspension, the maximum achievable solids content is 60% by volume. However, the structure of a char particle is much more complex than this simple model. Pyrolysis oil as a wetting liquid is sucked into the pore system and absorbed by it to some extent, increasing the viscosity. On the other hand, pores can also be considered a "rated breaking point" during milling. This makes it understandable that the milling of char results in a clear reduction of porosity.
Viscosity
The curves of the slurry viscosity as a function of temperature and solids content behave as predicted by theory. The viscosity decreases exponentially with increasing temperature and increases with increasing solids content. This increase in viscosity becomes very steep when the solids contents gets near the critical volume, in which all gaps between the solid particles are filled with liquid. Tar and char slurries have no rheology according to Newton, but have a pseudoplastic or dilatant behaviour, i.e., with increasing shear gradient, the viscosity can increase or decrease, depending on the solids content, particle sizes and temperature. In addition, cold slurries (up to approx. 20°C) are thixotropic: After a shelf life of 2 hours, the viscosity is twice as high or higher, compared with a slurry kept in slow movement.
In this graphic, the slurry viscosity at room temperature and a solids content of 20% has been plotted versus the porosity. However, it only seems to affect viscosity from a certain pore size (plotted here from 2 µm). The two lines apply to different char fractions whose porosity was varied by pressure milling. The effect of porosity on viscosity is such that the trend of increasing viscosity is reversed for smaller particles.
Sedimentation
Since a slurry is not a homogeneous liquid in the chemical sense, the char can settle. This has been monitored using a sedimentation balance of our own design. The figure shows that sedimentation is extremely dependent on particle size. The sedimentation density, which also depends greatly on the particle shape, also plays a role. In finely powdered char (x50=6μm), it is about 26 %, in larger-sized char (screening fraction of 100-200 μm) it is much higher.