BIOMASS GASIFICATION


Table of Contents
Gasification types 2
Classification: 2
Fixed/ Moving bed gasifier: 4
Fluidized bed gasifier: 4
Entrained flow gasifier: 5
Gasifier configuration: 6
Production of Gasification Process 7
Low Temperature gasification: 8
High temperature gasification: 8
References 9

SECTION 1
Gasification types
Gasification can be termed as the process in which a specific solid or liquid feedstock is transformed into various useful forms like as a convenient fuel, one form of chemical feedstock which produces energy on burning and for the generation of chemicals and substances that add value to other chemicals and mixtures. The Gasification process is carried out in the gasifiers which are of different types as per their design, contact mode and the medium used.
Gasifiers are specific foe each and every reaction and hence they have a particular mode in which the reaction can be carried out in the best possible way. Classification of the gasification process is on the basis of the medium or mode of contact between the gas and the solid. On the basis of this, there are three main types in which gasification process is carried out through gasifiers and they are (Basu, 2010):
Classification:
Fixed/ Moving bed gasifier;
Downdraft gasifier
Throatless,
Throated
Updraft gasifier
Dry- Ash
Slagging
Crossdraft. gasifier
Examples: BGL slagging gasifier, Lurgi dry- bottom gasifier.
Fluidized bed gasifier;
Bubbling
Circulating
Transport Gasifier
Chemical Looping gasifier
Twin bed.
Examples: Winkler process, KBR transport gasifier, Twin-reactor gasifier, EBARA gasifier, GTI membrane gasifier, Rotating fluidized-bed gasifiers, Internal circulating gasifier, Foster wheeler CFB gasifier.
Entrained flow;
Coaxial downflow,
Opposed jet
Top-fed gasifier
Side-fed gasifier
Examples: Koppers- Totzek gasifier, Seimens SFG gasifier, MHI gasifier, EAGLE gasifier, E-gas gasifier.

Figure: Range of applicability for biomass gasifier types (Basu, 2010).
Fixed/ Moving bed gasifier:
In this type of gasifiers, the grate supports the fuel. In case of moving bed gasifiers, fuel moves in the downward direction. It is known as “moving bed” as there is a movement of the fuel inside the equipment which is set up on a bed like surface.
In this, the process of mixing and transferring of heat within the bed is average or low. There is no proper and equal distribution of fuel, temperature, heat transfer and gas proportion in the gasifier. Hence, this type of gasifiers are not suitable for fuels with biomass developed in huge capacity holding units. They consist of further sub division as downdraft gasifiers, updraft gasifiers and crossdraft gasifiers. They are most suitable for smaller units which lie between the capacity of 10 kWth – 10 MWth) (Basu, 2010).
Fluidized bed gasifier:
In this type of gasifiers, there is accurate formation of mixture and an equal distribution of temperature. Its bed consists of bed materials which are also known as “granular solids”. These materials are held in a semi-suspended condition through passing it at proper predetermined velocities. It consists of bubbling, circulating and twin bed gasifiers.
It is one of the most efficient method which can be used for gasifying biomass from its components (Basu, 2010). The production of tar is medium and the usage of gases is such that biomass can be easily gasified in it.
It provides a surface to the fine particles so that they can effectively be suspended in the liquid or any fuel. They are most suitable for medium sized units ranging within the capacity of 5 MWth–100 MWth.

Entrained flow gasifier:
It is one of the most widely used gasifier due to the advantages that it provides. Due to the short residence time, the particles of fuel needs to be coarse and hence it is useful for all the categories of coal except the low rank coals which are not suitable for this type of gasifiers. They are also not suitable for carrying gasification of biomass. As it has short residence time, the biomass particles cannot be fined so quickly and hence they remain unpulverised.
The gas and solid flow in the entrained flow gasifier along with the track of the fuel particles is shown in the figure:

Figure: Gas- Solid flow in an Entrained- flow gasifier (Basu, 2010).
They have the benefit of removing the tar from the reaction due to the presence of less amount of biomass. Hence they have less amount of tar produced. The feed that is required is general in nature and easily available. There is a very good amount of carbon being converted. It provides gas with low methane composition which makes it favourable for synthetic gas generation (Basu, 2010).
They are suitable for large units lying in the range of greater than 50 MWth and hence they can be termed as the most reliable for large units.
Gasifier configuration:
There are specific configurations provided for each and every gasifier and they show specific properties for particular condition. These configurations are provided on the basis of the method of contact of the reactor with respect to coal and the gas used in the reaction.
The configuration during different condition in the reactor is shown in the table below:
Moving Bed Fluidized Bed Entrained Flow
Ash
Condition Dry Slagging Dry Agglomerate Slagging
Coal Feed ~ 2 in ~ ~ 2 in ~ ¼ in ~¼ in 100 Mesh ~
Fines Limited Better than
dry ash Good Better Unlimited
Coal Rank Low High Low Any Any for dry feed
Oxidant req. Low Low Moderate Moderate High
Steam req. High Low Moderate Moderate Low
Issues Fines and hydrocarbon liquids Carbon Conversion Raw gas cooling
Table: Gasifier Configuration Comparison (Breault, 2010)
These configurations indicate that different conditions have different effects on the gasification process.

SECTION 2
Production of Gasification Process
The process of gasification is very much dependent on the generation of Syngas. Syngas is a combination of Hydrogen (H2) and Carbon Monoxide (CO). It serves to fulfil both the purpose, that is, one of a fuel and other as a producer of different chemicals through various reactions and processes. It is generated from natural gas as well as Biomass, Solid fossil fuels like coal, pit coke, and Liquid fuels like refinery and industrial wastes (Basu, 2010).
Syngas can be produced from both hydro carbons and from biomass and the one that is generated from biomass is also termed as “biosyngas”.
It is most commonly produced by the gasification process carried out on natural gas owing to the cost effectiveness of natural gas. There is a reaction between steam and methane. In this reaction, large quantity of hydrogen is produced and hence this process is also used for the large scale production of hydrogen. Nickel is used as a catalyst and the reaction takes place at high temperature of about 700 – 1100℃.

The proportion of the gaseous components in the reaction like hydrogen, Carbon monoxide helps in developing specific products like gasoline, methane and methanol.
When excessive hydrogen is required, then in the above reaction, Carbon monoxide is added again, thereby releasing hydrogen as shown in the reaction below which is also known as “shift reaction” (Basu, 2010).

The product which is to be obtained is a critical factor in determining the proportion of gases to be mixed in the reaction along with the time of reaction. There are two types of processes which can be used for gasification and they are (Basu, 2010):
Low Temperature gasification:
The reaction which is carried out at temperatures under 1000 ℃ is termed as Low temperature gasification. Air is used as the gasification medium and this permits the unwanted nitrogen present in the atmosphere to enter the reaction.
This entrance of unwanted nitrogen in the gasification reaction is prevented by:
Using one kind of oxygen carrier, that is, a metal oxide which carries oxygen and provides it during the reaction.
Using Separate reactors where one will execute the combustion reaction and other would hold the heat to the second reactor where gasification occurs.
Using either steam or oxygen for the gasifying medium that would dilute the concentration of nitrogen in the process (Basu, 2010).
It is capable of producing heavy hydrocarbons which have a large number of applications in other industries.
High temperature gasification:
The reaction which is carried out at temperatures greater than 1200 ℃ is termed as high temperature gasification. In this type of reaction, there is a conversion of biomass into H2
and CO. It provides a huge amount of Hydrogen and Carbon monoxide which can be used accordingly for synthesising various hydro carbons from it (Basu, 2010).

References

Basu, P. (2010). Biomass gasification and pyrolysis: practical design and theory. . Academic press.
Breault, R. W. (2010). Gasification processes old and new: a basic review of the major technologies. Energies 3.2 , 216-240.