Oxygen as a Torrefaction Control Parameter

To speed up the torrefaction process in traditional torrefaction reactors, in particular auger reactors, the temperature of the reactor is substantially higher than the required torrefaction process temperature. This is due to the low heat conductivity of biomass. Unfortunately, the off-gas characteristics of biomass are very sensitive in the temperature window of 180-300°C which can cause a thermal runaway situation in which the process temperature exceeds the intended level. Due to this very sensitive temperature dependence of biomass pyrolysis and its accompanying gas production, a potential solution is to inject small amounts of air directly into the torrefaction reactor. It is found experimentally that this air injection can regulate the temperature of the biomass very rapidly compared to traditional temperature regulation by changing the reactor wall temperature. With this new torrefaction temperature control method, thermal runaway situations can be avoided and the temperature of the biomass in the reactor can be regulated better. Experiments with large beech wood samples show that the torrefaction reaction rate and the temperature in the core of the sample depend on the amount of injected air. Since the flow of combustible gasses (torr-gas) originating from the torrefaction process is very sensitive to temperature, the heat production by combusting the torr-gas can be controlled to some extent. This will result in both a more homogeneous torrefied product as well as a more stable processing of varying biomass types in large-scale torrefaction systems.