NOVELTY - A biomass gasifier comprises an internal cylinder (1), an external cylinder (2), and a purifying box (12); where the external cylinder (2) is set with a water inlet pipe (3), an air inlet pipe (6), and a head cover (5); the internal cylinder (1) is set with an air inlet (7); the air inlet pipe (6) is connected with a fan (4); the purifying box (12) is connected with the internal cylinder (1) via a pipeline comprising an air outlet (8) and an air inlet (9); and the purifying box (12) is set with a biomass air pipe (13) which is provided with a valve (14) for the biomass air pipe.    USE - The biomass gasifier is useful for generating fuel gas.    ADVANTAGE - The purifying box is set with a biomass air pipe which is provided with a valve for the biomass air pipe, and the internal cylinder is loaded with the biomass and generates combustible gases in anaerobic condition. The gases are carried out two-stage purification via a primary filter screen and a secondary filter screen and the fuel gas becomes purer, thus it is not easy to block the pipeline and it is combusted more completely so that it saves energy, obtain high efficiency, and protect environment. The gasifier has characteristics of convenient usage, high thermal efficiency, continuously stable gasification, and wide range of applications.    DETAILED DESCRIPTION - A biomass gasifier comprises an internal cylinder (1), an external cylinder (2), and a purifying box (12); where the top left part of the external cylinder (2) is set with a water inlet pipe (3), the bottom left part of the external cylinder (2) is set with an air inlet pipe (6), and a head cover (5) is provided on the external cylinder (2); the lower part of the internal cylinder (1) is set with an air inlet (7) which is installed on one end of the air inlet pipe (6), and another end of the air inlet pipe (6) is connected with a fan (4); the purifying box (12) is connected with the internal cylinder (1) via a pipeline, the position where the pipeline is connected with the internal cylinder (1) is set with an air outlet (8), and the position where the pipeline is connected with the purifying box (12) is set with an air inlet (9); and the purifying box (12) is set with a biomass air pipe (13) which is provided with a valve (14) for the biomass air pipe.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of the biomass gasifier.    Internal cylinder (1)    External cylinder (2)    Water inlet pipe (3)    Fan (4)    Head cover (5)    Air inlet pipe (6)    Air inlet (7)    Air outlet (8)    Air inlet (9)    Purifying box (12)    Biomass air pipe (13)