The torrefaction installation is actually composed of two setups: a continuous screw kiln reactor and a stirred tank reactor. This plant has been designed for sewage sludge torrefaction. To process other kind of biomass it would be necessary to make appropriate changes in the elements of the installation. The installation is still under construction, and it is scheduled to be fully operative by the end of 2012.
Continuous screw kiln reactor
The screw kiln reactor is 800 mm long, with an inner diameter of 55 mm. The diameter of the screw is 23 mm. The reactor is heated by two heating resistances (2 kW) coiled up onto the exterior wall and it can reach 320 °C inside. The temperature is controlled by a K-type thermocouple placed on the exterior wall of the reactor. The temperature inside the reactor is measured by four K-type thermocouples arranged along the reactor. The flow of nitrogen, used as purge gas, is controlled by a mass flow controller. The feedstock is fed into the reactor from a feed hopper and transported by the rotating screw. The engine power is 0.25 kW. The solid product is collected in a solid vessel. The vapours released during torrefaction reactions flow trough the liquid recovery system. This system is composed of two ice-cooled condensers and an electrostatic precipitator. There is also a cotton filter in order to retain small solid particles. The amount of exiting non-condensable gases is measured by a volumetric gas meter and the composition is determined by a micro-gas chromatograph connected online.
Stirred tank reactor
The stirred tank reactor is made of a stainless steel cylindrical tube, 300 mm long and 108 mm inside diameter. The reactor can be heated until 320 °C. The temperature is controlled by one of the four K-type thermocouples placed inside the reactor, at different distances from the bottom of the reactor and at different distances from the reactor wall. The feedstock has to be introduced before closing the reactor lid with bridles. The shaft of the stirrer is spun by a reducer motor (18 rpm output speed), held over the lid, by means of a speed shifter. The stirrer has five blades to guarantee a proper agitation of the feedstock. The flow of nitrogen, used as purge gas, is controlled by a mass flow controller. The solid product is collected in a solid vessel. The liquid recovery system is the same used for the continuous screw kiln reactor.