The biomass gasification is designed as counter-flow fixed-bed gasifier. In this case, the counter-flow principle describes that the fuel is led (from the top to the bottom) against the flow direction of the air (from the bottom to the top).
In the biomass gasification process, a pyrolysis gas and, in combination with a gas cooler, a pyrolysis oil mainly consisting of tar compounds are generated.
The wood fuel is led to the gasifier through gas-tight conveyors (e.g. screws with rotary gate valve). In the gasifier, substoichiometric air is added from the bottom. In the gasifier, the fuel forms vertical layers. With this, the following zones are distinguished (from the top to the bottom):
First, the gas flow coming from the lower layers dries the newly inserted fuel. The dry fuel remains in the reactor and goes in the underlying pyrolysis zone depending on the homogeneous ash discharge (bottom). There are temperatures of 600 to 800 °C in the pyrolysis zone. Here, the volatile gases are extracted out of the fuel and led further through the gas flow into the drying zone. In the reduction and oxidation zone, after the pyrolysis zone, the remaining coal reacts with the air flow coming from the opposite direction. There are temperatures of 1,000 to 1,200 °C. The exothermal combustion reactions take place in the oxidation zone: C+O2=>CO and C+O2=>CO2. The reactions between
C / CO and water steam (C+H2O+CO+H2 and CO+H2O+CO2+H2) take place in the reduction zone.
For controlling the temperature, gasifier intake air is mixed with the water steam.
Counter-flow gasifiers are used in plants up to 10 MW of fuel heat capacity.
The advantages of the counter-flow gasifiers are as follows:
• High efficiency of gasifier due to low discharge temperatures of the pyrolysis gas
• Low percentage of particles in the pyrolysis gas
• Low content of alkali metal in the pyrolysis gas
• Low requirements on the fragmented size (particle size) and the fuel reprocessing of the fuel used
• Stable operation (no burning out in the fuel bed due to flat air intake and due to homogenous fuel feed, distribution by means of feed distributor)