Regarding the gasification technology, 7 of the 12 pilot plants use a downdraft gasifier in their plants since the downdraft gasifier provides advantages in terms of the ease of operation, low tar content in the producer gas, the suitability of the small-scale power production of less than 500 kWe, and low
investment and operation costs [5-7]. One pilot plant applied a 4-stage gasifier, in which four reaction zones of the gasification process - drying, pyrolysis, oxidation, and reduction - took place in separate reactors. The separation of the reaction zones in different reactors can effectively reduce tar, because volatile matter and tar can burn in the gas phase without solid material, which facilitates the good mixing
of tar and air [5]. Additionally, the operating condition of each reacto
r can easily be controlled by the
separation of the reaction zone.