Gasification in Detail
Gasification is a partial oxidation process. The term partial oxidation is a relative term which simply means that less oxygen is used in gasification than would be required for combustion (i.e., burning or complete oxidation) of the same amount of fuel. Gasification typically uses only 25 to 40 percent of the theoretical oxidant (either pure oxygen or air) to generate enough heat to gasify the remaining unoxidized fuel into syngas.
The major combustible products of gasification are carbon monoxide (CO) and hydrogen (H2), with only a minor amount of the carbon completely oxidized to carbon dioxide (CO2) and water. The heat released by partial oxidation provides most of the energy needed to break up the chemical bonds in the feedstock, to drive the other endothermic gasification reactions, and to increase the temperature of the final gasification products.
The chemistry of gasification is quite complex and is really accomplished through a number of chemical reactions within the gasifier. Some of the major chemical reactions are shown in the diagram below. In a gasifier, the carbonaceous feedstock undergoes several different processes and/or reactions:
- Pyrolysis – This occurs as the feedstock is exposed to rising temperature in the gasifier. Devolitization and breaking of the weaker chemical bonds occurs, releasing volatile gases, along with producing a high molecular weight char which will undergo gasification reactions.
- Combustion – The volatile products and some of the char react with oxygen to form CO2, CO, and in doing so, provide the heat needed for subsequent gasification reactions.
- Gasification – The remaining char reacts with CO2 and steam to produce carbon CO and H2.
- Water-gas-shift and methanation – These are separate reversible gas phase reactions taking place simultaneously based on gasifier conditions. These are minor reactions which play a small role within in the gasifier. Depending on the desired product, the syngas may undergo further water-gas shift and methanation processing downstream from the gasifiers.
In the low oxygen environment of the gasifier, most of the feedstock’s sulfur coverts to hydrogen sulfide (H2S), with a small amount forming carbonyl sulfide (COS). Nitrogen chemically bound in the feed generally converts to gaseous nitrogen (N2), with some ammonia (NH3), and a small amount forming hydrogen cyanide (HCN). Chlorine is primary converted to hydrogen chloride (HCl). Trace elements associated with both organic and inorganic components in the feed, such as mercury, arsenic and other heavy metals, appear in the various ash and slag fractions, as well as in gaseous emissions, and need to be removed from the syngas prior to further use.
Types of Gasifiers
Although there are various types of gasifers (gasification reactors), different in design and operation characteristics, they are general classified into three categories:
Commercial gasifiers of GE Energy, ConcoPhilips E-Gas™ and Shell SCGP are examples of entrained-flow types. Fixed-or moving-bed gasifiers include that of Lurgi and British Gas Lurgi (BGL). For more specific information on these gasifiers, follow the links for the bulleted gasifier types above.
NOTE: Although specific gasifiers named above are described in detail throughout this website, it is realized that other gasification technologies exist. The gasifiers discussed herein were not preferentially chosen by NETL.
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