Our country's annual output of forestry waste and agricultural production residues reaches 700 million tons. How to effectively utilize this huge resource has become an important issue for scientific researchers. Biomass gasification technology has changed the utilization method of direct combustion of biomass, improved the energy quality of waste biomass, and is of great significance to saving conventional energy, reducing environmental pollution, and protecting the ecological environment.
Every year in our country forestry waste and surplus of agricultural production material production up to 700 million t, how to effectively utilize the huge resources, has become an important topic in front of scientific research workers. Biomass gasification technology changed the way direct combustion biomass utilization, improve the grade of waste biomass energy, to save conventional energy, reduce environmental pollution, protect the ecological environment is of great significance.

Downdraft fixed-bed gasifiers have been widely used due to their simple structure, easy operation, and low tar content in the produced gas. The biomass gasification process is a complex thermochemical reaction process. The structural dimensions of each part of the biomass gasifier will greatly affect the thermal efficiency, gas composition and gas quality of the gasifier. Therefore, a reasonably designed biomass gasifier is the key to effectively utilizing biomass energy.
Suction under the fixed bed gasifier with simple structure, easy to operate, the output gas low tar content advantages has been widely used. Biomass gasification process is a complex thermal chemical reaction process, each part of the biomass gasifier structure size will greatly affect the thermal efficiency of gasifier, gas composition and gas quality, so the reasonable design of biomass gasifier is the key to effective utilization of biomass energy.

Air as the gasification agent is blown from the air nozzle on the side wall of the gasifier, and the flow direction of the produced gas is consistent with the direction of material falling, so the downdraft gasifier is also called a downstream gasifier. The blown air and materials are mixed and burned. This area is called the oxidation zone, with a temperature of about 900-1200°C. The heat generated is used to support the cracking reaction in the pyrolysis zone and the reduction reaction in the reduction zone. The upper part of the oxidation zone is the pyrolysis zone, with a temperature of about 300-700°C. In this area, the volatile matter (pyrolysis gas, tar and moisture) in the biomass Separate; the upper part of the pyrolysis zone is the drying zone, where the materials are preheated; the lower part of the oxidation zone is the reduction zone, where the CO2, carbon, and water vapor generated in the oxidation zone undergo a reduction reaction. At the same time, the residual tar undergoes a cracking reaction in this zone, producing gases dominated by CO and H2. The temperature in this zone is about 700 to 900°C. Since the tar-rich gas from the pyrolysis zone must pass through a high-temperature oxidation zone and a reduction zone dominated by hot coke, the tar in the gas is cracked at high temperature, thereby greatly reducing the tar in the produced gas.
As the gasification agent of air blowing in from the gasifier wall air nozzle, the whereabouts of the output direction of gas flow and material in the same direction, so the suction under gasifier is also referred to as forward gasifier. Blow into the air and material mixed burning, an area known as the oxidation zone, the temperature is about 900 ~ 1200 ℃, the heat generated by the support area pyrolytic cracking reaction and reduction of area reduction reaction; Oxidation zone of the pyrolysis zone, the temperature is about 300 ~ 700 ℃, in the area, the biomass of volatile (cracking gas, tar, and moisture) isolated; Pyrolysis of upside for arid region, the material in this area is hot; At the bottom of the oxidation zone to reduction zone, area of CO2 and carbon oxide, water vapor in the region to carry on the reduction reaction, and the tar cracking reaction in this area, the residual gas output is given priority to with CO and H2, the temperature of this area is about 700 ~ 900 ℃. Area rich in tar from pyrolysis gas after high temperature oxidation zone and reduction zone, mainly for the hot coke tar cracking under high temperature in the gas, which has greatly reduced gas output in the tar.

Since the pile specific gravity and particle size of biomass materials vary greatly, this will significantly affect the residence time of the materials in the furnace, which requires the gasifier to select gasification intensities that vary greatly depending on the materials. For materials with smaller specific gravity or smaller particle size, the residence time in the furnace is short, and the gasification intensity should be reduced accordingly; conversely, the gasification intensity should be increased. Generally, the recommended value of gasification intensity is 500~2000kg/(h·m2）。

Although the tar content in the gas produced by the downdraft gasifier is very small, different tar removal equipment, dust removal, and dehumidification equipment should be equipped according to different applications of the produced gas to further improve the quality of the produced gas.
Although the little suction gasifier tar content in the gas output, but according to the different applications of gas output, should also configure different except the tar and dust removal equipment, dehumidification equipment to further improve the quality of the output gas.