How does a downdraft gasifier work?
Position :
Home » Knowledge » How does a downdraft gasifier work?

How does a downdraft gasifier work?


The downdraft gasifier can gasify wood chips, wood chunks, pellets, herbs, straws and other biomass materials. The flow of the gas is in the same direction as the biomass feed, and a high-temperature oxidation zone is set up inside the furnace, where the gas is sucked out from the lower part of the grate. The structure of the lower suction gasifier makes the combustible gas must pass through the high temperature oxidation zone, which is conducive to further tar cracking, and the tar content in the gas is 50-100mg/M3, compared with the upper suction type, the tar content is very small. During the reaction process, the amount of air supplied to the gasifier increases, and the combustion in the throat or oxidation zone of the gasifier becomes more intense, making the temperature of the oxidation zone rise sharply in a short period of time, thus driving the temperature of the drying zone, pyrolysis zone and reduction zone to rise through mass and heat transfer.


How does a downdraft gasifier work?

The downdraft fixed bed gasification process is divided into four main reaction zones from top to bottom: drying zone, pyrolysis zone, oxidation zone and reduction zone.

1.Drying zone: The free water and combined water in the biomass evaporate, and the water content is reduced from less than 20% to less than 5%, and the temperature of the drying zone is 30-200℃.

2.Pyrolysis zone: the biomass is cracked under anoxic conditions to produce a large amount of non-condensable gas (CO, H2, CH4, etc.) and condensable tar, the temperature range is 200 ~ 600 ℃.

3.Oxidation zone: The gasifier is fed into the gasifier at this part, and the biomass char is burned with the supplied oxygen to produce CO2, and the H2 produced by partial cracking will also react with oxygen to produce water, and these two oxidation reactions will generate a large amount of heat. As the tar moves down with the gas, it must pass through the high temperature oxidation zone, so the second cracking of the tar will occur.

4.Reduction zone: In the environment of 800~1000℃ and lack of oxygen, several heat-absorbing reduction reactions will take place to increase the content of CO, H2 and CH4 in the combustible gas and improve the calorific value of gas production.


Advantages of downdraft biomass gasifier:

1.Pyrolysis gasifier adopts insufficient amount of air (anaerobic) to pyrolyze organic matter into combustible gas, transforming the incomplete combustion process into a complete gas combustion process and reducing the solid particulate matter emission to below the emission standard requirement.

2.The gasification system is fully enclosed, the site is neat and beautiful, and harmless to the operator.

3.Fully automatic control process, simple and convenient operation.

4.All feeding, slag discharge doors and service doors are soft sealed, and the top of the pyrolysis furnace is equipped with a sealing cover to avoid gas leakage, and the furnace is well sealed to ensure the effect of controlled oxygen pyrolysis.

5.When the pyrolysis gas spontaneously combusts, it enters the spontaneous combustion process and the combustion aid will stop automatically, and the whole spontaneous combustion process reaches more than 90%, which greatly reduces the operation cost.

6.To achieve the resource utilization of thermal energy, not only the harmless emission of flue gas, the total amount of CO2 emission is reduced.

The downdraft biomass gasifier is suitable for raw materials & range:

Applicable raw materials: 30-50mm wood chips blocks or pellets (straw, wood chips, rice husk, palm husk, bagasse, etc.) agricultural and forestry waste are available.

Applicable range: The material gas output gas calorific value varies slightly according to the different biomass materials, gas calorific value 1100~1300 calories/m³, widely used in the field of industrial and agricultural production.