Biomass pyrolysis—the thermal decomposition of plant material in the absence of oxygen—produces char, liquid (bio-oil), and synthetic gas (syngas) phases. A gasification-type pyrolysis process is utilized to maximise the production of syngas for use in combined heat and power (CHP) applications. In such processes, the solid biomass is initially heated to 300-600°C and subsequently to 700-800°C to decrease the liquid vapour concentration and to gasify a portion of the char. To achieve rapid and efficient heating of the biomass feedstock (and the by-products of the initial heating), sand or other thermally-conductive particles, such as spheres of metal, ceramics, or Silicon carbide, are used as a heat transfer medium. These media can however cause severe abrasion in reactors. Following its initial use, the heat transfer medium is returned to the entrance of the pyrolysis reactor for reuse, via a duct within the reactor.

Aston scientists have devised a technique for maximising the production of syngas during biomass pyrolysis and eradicating the need for an abrasive heat carrier. The technique utilises an asymmetric double screw, comprising an inner screw and an outer screw. The outer screw pushes biomass forward through the reactor. The inner screw, operating in reverse, continuously conveys a portion of the char residue to the entrance of the pyrolysis reactor for reuse. The reuse of char effectively reduces the liquid vapour phase of the pyrolysis process, thus increasing the syngas phase. The char crucially also acts as the heat transfer medium, removing the abrasive action of other media.