In close cooperation with Eindhoven Technical University a new combined torrefaction and carbonisation system is under development.
This system is based on Klinkenbergs Auger transport screw systems and comprises two screw-in- tube drying and pre-heating units which feed the biomass to a torrefaction and/or carbonisation reactor. Finally, the material is cooled to a temperature such that easy densification at the end of the process is possible.
On the basis of an invention of Eindhoven University a careful control methodology for the torrefaction and/or the carbonisation process is possible to guarantee a homogeneous end product with properties in a narrow band in relation with the product specification, even though the input material has large variability in their properties like bulk density, particle size and -distribution, wetness and starting calorific value.
The off-gas of the torrefaction process (torrgas) is entirely used for supplying heat to the various process units such that, in the case of torrefaction, still 90% of the calorific value of the input material is maintained in the final product, whereas some 30 or more % of mass of the biomass is reduced. The entire system is built in 5 separate standard containers including the control room and a small space for servicing operations.
The off-gas of the torrefaction process is for the greater part combusted in a combustion unit specially dedicated to the peculiarities of the torrefaction gas. The heat generated in this unit is, as said before, distributed over the various heat demanding subsystems.
However, due to the fact that torrefaction gas is entirely different from normal combustion gases, special precautions are taken to sustain stable, and complete combustion with minimum production of unwanted gaseous pollutants.
- The combustor is therefore characterised by:
Air staged combustion such that directly after entering the combustion space, some 10% of the torrgas is initially combusted to give the flame area the required ignition temperature. After that enough of air is injected into the flame zone such that complete combustion of all the gas is possible. Higher up in the flame zone overfire air is introduced to give the off-gas the required temperature demanded by the down stream units mentioned before.
- The inner cylindrical wall of the combustor is cooled by the combustion air flowing in a circumferential path along the gap between the inner and outer wall. The airflow will be controlled such that the air will attain the optimum temperature for ignition and combustion of the torrefaction gas.
- To ensure stable ignition, pilot flames are mounted in the ignition zone where torrgas is being introduced into the throat of the combustion space.
- To further ensure a stable flame front a massive especially formed solid metal body is accommodated in the outflow zone of the primary combustion zone to re-direct the torrgas flow to guarantee optimum mixing with secondary combustion air and to present a hot body to stabilize the flame front.
- As the system will combust torrgas that can contain mineral matter transforming into ashes, easy access to the inner combustion zone is made possible. In one action the complete cylindrical wall can be lifted from the bottom plate allowing easy cleaning of the internals of the combustion and ignition parts. A specific casting sand based seal between the vertical walls and the bottom is adopted.
- A smoke stack is mounted on top of the combustor release the heat in case the system demands less heat than is being produced by the stove.
- The top of the combustor is designed in such a way that noise production by the combustion process cannot occur.
The system is still in its demonstration phase. A 4 tons input system is foreseen to be built and put into operation in the coming period.