Small-scale Furnaces for Straw Pellets

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Lukas Schenke

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Straw has a great potential as a renewable energy source for thermal use. Studies suggest it could cover about 1.2–5.6 % of the Federal Republic of Germany’s heat demand. However, straw as fuel has drawbacks compared to wood because of its heightened nitrogen and ash content. The ash is rich in chlorine, potassium, sodium and silicon. Chlorine is problematic because of increased corrosion and emissions. Additionally, chlorine, sodium, silicon and particularly potassium influence the ash melting temperature negatively.

To solve those issues, robust industrial combustion systems for a power range greater than 40 Kilowatts were developed. Those systems include moving the burning bed through a push grate, cooling the burning bed by means of water-cooled grates, staged combustion and secondary measures for emission reduction. These methods, however, come at the cost of great technical and economical effort. Therefore, the usage of straw is limited to medium-sized to large furnaces. The aim of the project „Small-scale Furnaces for Straw Pellets“ is to enable the usage of straw as a fuel for small-scale furnaces in the power range of 10 to 40 Kilowatts.

In a previous project initial investigations and developments were conducted. Through the leaching and additivation of straw, it was possible to counteract the chlorine corrosion and emission as well as the low ash melting temperature. In addition, the straw pellets burned almost wholly. The only challenge regarding the combustion of the tailor made straw pellets has been the ash sintering. It leads to the pellets maintaining their original form as an easily destroyable „ash skeleton“. Through a simple stoking of the firebed the combustion could be maintained for several hours. A systematic optimization of the stoking was not possible in the course of that project.

The presented project addresses the described difficulty. New grate and combustion concepts will be developed to break the sintering mechanically. Respectively, the temperature of the burning bed – the primary reason for the slagging – will be controlled through stoking concepts and with adjustment of air staging. Computer based simulations of the Department of Energy Plant Technology, short LEAT, at Ruhr University Bochum will support the experimental investigations of TEER. With the simulations, the influence of stoking will be evaluated and the local peak temperatures identified.

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