Technical Note: Gasification

Gasifiers convert carbonaceous feedstock (coal, wood, cornstalks, most any biomass) into gaseous products.  The process usually takes place at high temperatures and pressures and with a carefully controlled amount of oxygen.  The oxygen can come from air, pure oxygen or from steam. At operating conditions a chemical reaction occurs that converts the feedstock into a synthesis gas or "syngas".   The syngas is a mixture of predominately carbon monoxide (CO) and hydrogen (H2).  The amount of heat that can be recovered from burning syngas can be as much as 50% higher than direct combustion of the feedstock.

Gasifiers can divided into three general catagories:

1. Moving Bed Gasifiers (dry ash)
2. Fluidized Bed Gasifiers and
3. Entrained Bed Gasifiers

Descriptions of several specific types and subtypes can be found at this NETL site.

Coal gasifiers were used by the Germans in WWII, and were first commercialized as part of coal liquefaction plants in South Africa.  Coal gasifiers are usually pressurized, operate in the range of 1370^oC (2500^oF), and the oxygen is supplied either as air or pure oxygen.  Most of these gasifiers are well developed, although DOE is doing quite a bit of further development in connection with their advanced clean coal program.  Biomass gasifiers have been operated in Europe for some time to provide gas for boilers for electric power production and/or district heating.  They operate at lower temperatures, typically 830^oC (1500^oF), than a typical coal gasifier and usually are not pressurized.  Tars in the syngas are of concern as they may interfere with operation of downstream devices, especially Fischer-Tropsch gas synthesis units.  A table comparing the various types of biomass gasifiers can be found in a technical brief prepared for the IEA Task 33: "Thermal Gasification of Biomass".    The table below which compares the attributes of various gasifiers is taken from a presentation, "Biomass Gasification Overview", by Richard L. Bain of NREL.  The presentation gives a very good technical description, status and uses of gasifiers.                     

                      Gasifier Types

Type                                   Advantages                        Disadvantages

Updraft                           Small scale applications         Scale limitations

                                      Mature for heat                   High tar yields

                                      Can handle moisture             Feed size limits

                                      No carbon ash                      Producer gas

                                                                                Slagging potential

Downdraft                     Small scale applications          Scale limitations

                                     Low particulates                   Feed size limits

                                     Low tar                                Producer gas

                                                                                 Moisture sensitivity

Fluid Bed                       Large scale applications          Medium tar yield

                                     Feed characteristics             Higher particle loading

                                     Direct/indirect heating

                                     Can produce syngas

Circulating Fluid Bed       Large scale applications        Medium tar yield

                                    Feed characteristics             Higher particle loading

                                    Can produce syngas

Entrained Flow               Can be scaled                       Large amounts of carrier gas

                                     Potential for low tar             Higher particle loading

                                     Can produce syngas               Potentially high S/C

                                                                                Particle size limitsTCi

Several biomass gasifiers of various scales are in operation in the U.S.  DOE has a gasifier as part of its Thermochemical Process Development Unit at NREL in Colorado, which is described in the Bain presentation referanced above. Ferco installed and operates the Vermont gasifier project. Pearson has a commercial unit in operation in Mississippi as part of an ethanol production plant from wood chips.  TUV of Austria has several units in operation.  MTCI and Chemric are working with DOE to develop black liquor gasifiers for use in pulp and paper mills.  Both of their systems, and several others, are describled in this presentation.

Last modified on 9/27/05

Comments

[Tim H.]

You should mention the granddaddy of coal gasifiers, the water-gas units. IIRC, they've been around since the turn of the century.

[Lloyd Weaver]

How may coal gasifiers were started in America in the past 6 years?

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