The purpose
Gasification is a process that convert carbonaceous materials into combustible gases.
The resulting gas is called producer gas (or wood gas when fueled by wood) and may be more efficiently converted to high quality energy such as electricity than would be possible by direct combustion of the fuel. Also, corrosive ash elements such as chloride and potassium may be retained by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.
Gasification relies on chemical processes at elevated temperatures >700C, contrary to biological processes such as biogas plants.
Pyrolysis
After drying, the first process that the fuel is going though during heating is the pyrolysis, which initiate at around 230C. During pyrolysis thermally unstable components such as lignine in biomass are broken down and evaporate with other volatile components. The resulting pyrolysis gas consist mainly of tar, polycyclic aromatic hydrocarbons (PAH), methane (CH4), steam and CO2. The solid residual is carbon structures (coke) and ashes.
Tar) formed during pyrolysis can be sticky like asphalt, is known to be highly carcinogenic and represent a great challenge to machinery — e.g. IC engines and turbines — when the producer gas is transported, stored, and used.
Gasification
The actual gasification happen at temperatures above 700C when the glowing coke is allowed to react with a gasification agent such as oxygen, air or steam. The coke is gradually broken down into gases such as CO, CO2 and H2 (from the steam reaction)
The gasification can happen in a pile of coke — a fixed bed — or e.g. in a fluid bed. Fixed bed gasification processes can be divided into two basically different process designs: countercurrent ("up draft") and cocurrent ("down draft") gasification.
Countercurrent gasification
In countercurrent gasification, the gasification agent is flowing in the opposite direction as the fuel/coke; It enters at the end of the gasification bed and leave the process along with the pyrolysis gas at the beginning of the pyrolysis (or possibly drying) zone. This way the producer gas is not very hot, but contain all of the tars from the pyrolysis zone. Tar levels in untreated gas from countercurrent gasification plants exceed 2 g/Nm3. Fluid bed processes can exceed this number by one magnitude, since the pyrolysis happen much faster, and rapid pyrolysis produce more tar.
Cocurrent gasification
In cocurrent gasification the gasification agent is added before the gasification zone (sometimes before the pyrolysis zone) and leave at the end of the gasification zone. When the pyrolysis gas is forced through the glowing coke bed, a large fraction of the tars are broken down and gasified. Thus the gas contain magnitudes less tar than that of countercurrent gasification.
High temperature zone
Recent cocurrent gasification processes contain a small high temperature zone where temperatures exceed 1100-1200C. When pyrolysis gases pass this zone, large fractions of the tars are broken down ("cracked") in milliseconds. Combined with the tar reduction in the glowing coke bed, tar levels as low as 0.025-0.100 g/Nm can be achieved.
The two stage gasifier developed by the Biomass Gasification Group keeps the pyrolysis and gasification processes in two separate reactors with an intermediate high temperature zone.