Abstract: As a renewable clean energy, biomass gasification technology has been vigorously developed. This paper briefly introduces the basic principles of biomass gasification and the types of gasification processes, and expounds the working principles and advantages and disadvantages of the main types of gasifiers. The gasification efficiency is high but the structure is complex; the analysis of the characteristics of the gasifier has guiding significance for the design and operation of the biomass gasification project.
introduction
With the increasingly serious global energy crisis and the destruction of the ecological environment, biomass, as a renewable clean energy, is the fourth largest resource after oil, coal and natural gas, and has attracted great attention. Biomass gasification technology can not only supply gas and power generation, but also can be used to synthesize methanol and ammonia. It has sufficient technical flexibility, good cleanliness, high economy, and high efficiency. Countries around the world are actively Promote research on biomass gasification technology.
1 The principle of biomass gasification
Biomass gasification is a collection of more complex reactions. From a macro perspective, it can be divided into four reaction stages: drying, pyrolysis, oxidation (combustion) and reduction.
Drying is a simple physical process, which mainly occurs between 100 and 150 °C, and the whole process needs to absorb a lot of heat. When the temperature reaches above 15 °C, the biomass begins to undergo pyrolysis, volatiles are precipitated, and charcoal is left to form a bed for further reactions. The gaseous products of biomass pyrolysis include CO, CO2, CH4, H2, etc., which will undergo oxidation reaction (combustion) with oxygen, and emit a lot of heat, providing enough heat for drying, pyrolysis and reduction reactions, and maintaining the entire gasification process. Persistent. The water vapor and CO2 produced by the oxidation reaction (combustion) will react with carbon to generate H2 and CO, thus completing the conversion of solid fuel to gas fuel. This process is a reduction reaction (endothermic reaction). The higher the temperature, the more intense the reaction. When the temperature is lower than 800°C, the reaction is basically in a stagnant state.
2 fixed bed gasifiers
In a fixed bed gasifier, biomass fuel undergoes drying, pyrolysis, oxidation (combustion) and reduction and is converted into combustible gas. According to the supply position of the gasification agent and the sequence of flowing through the fuel layer, there are up-suction type, down-suction type, cross-suction type and open type, and the first two types of gasifiers are mainly used.
2.1 Updraft gasifier
The reaction layer of the up-suction gasifier is, from top to bottom, a drying layer, a pyrolysis layer, a reduction layer and an oxide layer. The biomass is fed into the gasifier from the top, first heated and dried by the gas, and then pyrolyzed by the heat, a large amount of volatile matter is precipitated, and the solid carbon enters the reduction layer and the oxide layer below in turn. The gasification agent is supplied from the lower part and firstly undergoes an oxidation reaction with the solid carbon, releasing heat to rapidly increase the temperature of the gas stream and the bed, and the gas stream is full of combustion products. After entering the reducing layer, the combustion product and the carbon undergo a reduction reaction, and the endothermic reduces the temperature. When the temperature drops below 800 °C, the reaction rate becomes slow and even stops. The airflow continues upward, providing heat for fuel pyrolysis and drying.
2.2 Downdraft gasifier
The reaction layer of the downdraft gasifier is, from top to bottom, a drying layer, a pyrolysis layer, an oxide layer and a reduction layer. There are two types of downdraft gasifiers depending on the location where the gasification agent is supplied: one is a downdraft gasifier with a middle necking section, and the gasification agent is supplied from the upper part of the middle necking section. ; The second is a downdraft gasifier without an intermediate neck section, and the gasification agent is supplied from the upper part. The working principle of the downdraft gasifier is basically the same as that of the updraft type, except that the heat required for fuel drying and pyrolysis comes from the lower oxide layer.
3 Fluidized bed gasifier
The main gasification reaction mechanism of fluidized bed and fixed bed is basically the same, but there is no obvious fixed bed. At present, there are three main types of fluidized bed: bubbling fluidized bed, circulating fluidized bed, and double fluidized bed.
3.1 Bubbling Fluidized Bed Gasifier (BFB)
In a bubbling fluidized bed, when the gas velocity exceeds the critical fluidization gas velocity, the solids begin to fluidize, bubbles appear in the bed, and a dense phase zone where particles aggregate and a dilute phase zone dominated by bubbles appear.
The furnace body is relatively thick and fat, and the lower part is the dense phase area with obvious bed layer. The gasification reaction mainly occurs in the dense phase area. In order to prevent the fine particle fuel from being carried out of the dense phase area by the airflow, the upper part of the furnace is designed with an expansion section to reduce the gas velocity. , which also prolongs the reaction time of the fuel in the dilute phase region.
3.2 Circulating fluidized bed gasifier (CFB)
The furnace body of the CFB gasifier is slender and slender, and a cyclone separator is installed at the upper outlet of the furnace to separate and collect high-temperature solid particles and send them back to the furnace.
The CFB bed fills the entire container space, and there is no dense phase zone and dilute phase zone. The necessary condition for maintaining the particle concentration in the bed is a high amount of solid particles circulating. Once the circulation is not good, all the particles in the bed may be blown out. Due to the poor fluidization characteristics of biomass fuels, CFB gasifiers basically use sand as an auxiliary fluidization medium.
The CFB gasifier has two characteristics: the gas flow rate is high, generally between 4~7m/s, the bed works in the fast fluidization zone; the high circulation rate of fixed particles is between 10~20.
3.3 Double fluidized bed gasifier (DFB)
The double fluidized bed gasifier adopts two fluidized bed furnaces, one is a gasifier and the other is a combustion furnace. The biomass fuel is added to the gasifier with hot sand as the bed material, and the amount of the sand is absorbed, and the pyrolysis reaction is carried out. The gas product carries the residual carbon and the sand and is separated in the separator. The separated carbon residue and sand are combusted with the air in the combustion furnace to heat the sand, and the hot sand is carried by the flue gas and separated by the separator, and the hot sand is returned to the gasifier. The outstanding feature of the DFB gasifier is that the fuel particles are transported in the two fluidized bed furnaces by utilizing the strong transport capacity of the fluidized bed.
4 Characteristics of various types of gasifiers
The fixed-bed gasifier has a simple structure, wide adaptability of raw materials, particle size up to 100mm, low sensitivity to slagging, and low gas fly ash content, but it is only suitable for small-scale gasification operations and is difficult to scale.
The fluidized bed gasifier has high gasification efficiency and high calorific value, but its structure is relatively complex. It has certain requirements on the particle size, uniformity, water content and natural accumulation angle of the fuel. It is sensitive to slagging, and the gas fly ash content If there are many, it needs to be equipped with a dust removal and purification system. At present, large-scale biomass gasification projects gradually adopt the pressurized fluidized bed gasifier technology.
5 Conclusion
At present, the global carbon emission requirements are becoming more and more stringent, and the importance of biomass with the characteristics of “zero carbon emission” is becoming more and more obvious. This paper briefly introduces the principle of biomass gasification and compares and analyzes different types of gasifiers. It can be seen that the fixed bed and the fluidized bed have their own advantages, but the fluidized bed is more suitable for large-scale and is the future development direction , so as to bring a bit of guiding significance to the design and operation of biomass gasification projects.
Post time: Mar-29-2022