Types of Gasifiers

There are three main gasifier classifications into which most of the commercially available gasifiers fall. All combine a feedstock, a gas (usually air, steam, or pure oxygen), and heat to produce gas and ash or slag.

	source: Electric Power Research Institute

Fixed (Moving) Bed
Fixed- or moving-bed gasifiers operate at atmospheric pressure and are not much different than a blast furnace. Feedstock and fluxes are loaded from the top into the refractory lined vessel and are allowed to move down as they are heated in contact with a high oxygen content gas flowing in the gasifier.

The advantages of this type of gasifier are its simplicity, feedstock flexibility, low gas exit temperatures, and high equipment efficiency. The negatives include dangerous explosive situations if not carefully monitored and the effort required to clean the produced gas if it is used for applications other than direct heating. In addition, the long feedstock residence time within the gasifier and the slag flow characteristics require the feed size distribution to be carefully controlled to ensure proper operation.

Entrained Flow
In an entrained flow gasifier the feedstock and steam, oxygen or air are introduced in the top of the gasifier. Within the gasifier, high temperature and pressure—and extremely turbulent flow—causes the conversion to gas to occur very rapidly, achieving a high throughput.

The high temperatures involved in this type of gasification shorten the life of system components. Also, it may be necessary to add fluxes or blend feedstock parameters to achieve good slagging characteristics.

Fluidized bed gasifiers offer load flexibility and high heat transfer rates, however, lower temperature operation limits feedstock to reactive and low rank coals.¹

Gasifiers for Special Applications
Although the above gasifiers suit a wide range of applications, some applications require a specialized gasifier or gasification process to be used, including:

• Advanced Coal Gasifiers – Compact and low-cost gasifier, with high carbon conversion and increased thermal efficiency.
• Plasma Arc Gasifiers – Used to produce energy and other products from feedstocks such as agricultural products or wastes and municipal or industrial wastes.
• Black liquor gasifiers – Involves the gasification of waste from the paper production process.
• Catalytic gasifiers – Catalysts within the gasifier are used along with a specific feedstock to control the products of the process.
• Underground Coal Gasification – In areas where coal is not easily mined, it is economically advantageous to react the coal underground to produce a fuel gas which is captured and brought to the surface.
  

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