Research

PhD defence by Simon Klinge Nielsen on Next Generation Die Design for Biomass Pelleting

Time

26.11.2020 kl. 13.00 - 16.00

Description

Simon Klinge Nielsen, Department of Energy Technology, will defend the thesis "Next Generation Die Design for Biomass Pelleting"

TITLE

Next Generation Die Design for Biomass Pelleting

PHD DEFENDANT

Simon Klinge Nielsen

SUPERVISOR

Associate Professor Matthias Mandø

CO-SUPERVISOR

Andreas Brinch Rosenørn, Engineering Manager, Andritz 

MODERATOR

Associate Professor Dil Muhammad Akbar Hussain

OPPONENTS

Associate Professor Henrik Sørensen, Dept. of Energy Technology, Aalborg University (Chairman)
Jens Kai Holm, Lead Chemistry Specialist, Markets & Bioenergy, Ørsted
Associate Professor Sylvia Larsson, Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences

ABSTRACT

The production of biomass pellets involves different design- and process parameters of the pellet press, which affect the pellet properties and the operational expense of the pellet production. In the pellet press, the feedstock is compressed and converted from sawdust into solid pellets.

The die has to be custom designed for each production site due to the different properties of the feedstock used.  The design of dies is partly relying on experiences from trial-and-error processes, which may involve several design iterations before the final design is developed.

The die design is correlated to the pelleting pressure, which is determined by the friction between the pellet and the die. Typical problems with dies are either that the pelleting pressure is too low to produce pellets with the required density and mechanical durability, or the pelleting pressure becomes too high. High pelleting pressure reduces the lifetime of the die and increases the power consumption of the pellet press. In case that the pelleting pressure becomes too high, the feedstock blocks the die and causes a time-consuming shutdown of the pellet press.

In this thesis, existing pelleting studies are reviewed, and experimental, and modeling studies of the physical processes in the die are performed, to obtain a better understanding of the design parameters of the die, and how these affect the pelleting process.

An extensive literature study shows that design and process parameters, such as the press channel inlet, die area, die speed, and gap size between die and roller, are rarely considered in pelleting studies. However, a few studies indicate that these parameters have a significant effect on energy consumption, and also affect the pellet durability.

The literature study also shows that the existing methods for calculating the pelleting pressure in the die's press channels are based on a set of equations that only apply for calculation of a pure cylindrical press channel.

A 1D model is developed for calculating the pelleting pressure in press channels with a conical inlet section, and experimental single pelleting tests are performed for validation of the model. Simulations of the pressure contours in press channels show that the peak pressure is located where the conical inlet meets the cylindrical channel, which also is where the wear of the channel is most pronounced. In addition to the pelleting pressure, the presented model is set up to calculate the feedstock density and the mass-specific energy consumption of the pelleting process.

Further investigation of the press channel inlet is performed by using a novel experimental method for analyzing the feedstock motion in the press channel, and by setting up a Computational Fluid Dynamics model for simulating the feedstock flow.

The analyzes of feedstock motion in the press channel show that the conical inlet design of the press channel affects the shape of the feedstock layers in pellets, which is shown to correlate with the pellet durability. The analyzes also show cases of stagnant feedstock on the die surface and in the press channel inlet.

A pilot-scale pellet press is set up for experimental testing, where the thickness of the feedstock layer on the die surface and the gap size between die and roller are investigated.

The results show that the energy consumption of the pellet press decreases, and the pellet durability increases when the thickness of the feedstock layer on the die is increased.

The test results of the gap size between die and roller show that the energy consumption increases and the pellet durability decreases when the gap size is increased. Hence, the thickness of the feedstock layer on the die and the gap size between die and roller are found to be important process parameters in the pellet press.

The mechanical behavior of biomass feedstock is studied via constitutive modeling and experimental compression tests.  The results of the constitutive modeling show that the viscoplastic behavior of compressed biomass can be modeled using an SLS Maxwell model, with two strain-hardening springs and a dashpot. The coefficients of the model elements are shown to correlate with the energy that is required for compressing the biomass, while the model's dashpot work is lowest for pellets produced with small particles, which also have the highest durability measured from the experiments.

 

THE DEFENCE IN ENGLISH - all are welcome.

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Host

Department of Energy Technology

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