47
4
Steam Gasification and
Reforming Technologies
4.1 intrOdUCtiOn
All carbon-based materials (i.e., coal, crude and heavy oil, shale oil, bitumen, tar
sand, plastics, biomass, organic waste, etc.) can be converted to carbon monoxide,
hydrogen, carbon dioxide, and methane in the gasification process by a controlled
amount of oxygen and/or steam at temperatures higher than about 700°C. The
product distribution generally depends on the temperature, pressure, residence time,
catalyst, and the nature of the feedstock. The gas produced from steam gasification
(with or without oxygen) is often called syngas (either synthesis gas or synthetic gas)
or producer gas, both of which are fuel themselves that generate heat and energy.
Unlike direct combustion of original raw materials, syngas (hydrogen and carbon
monoxide) is much more versatile in that it can be used for combustion at much
higher temperatures. It can be used in fuel cells (FCs) and can also serve as raw
materials for the production of numerous chemicals and liquid fuels. The gasification also produces gaseous fuels that do not contain corrosive ash elements such as
chloride and potassium [1–10].
Water in its gaseous form (i.e., steam) plays a very important role in overall gasification process. During gasification, carbonaceous material undergoes several
important processes: (1) at temperatures around 100°C, the dehydration results in
the generation of steam in the gas phase, and (2) further dehydration and pyrolysis
of carbonaceous materials occur at temperatures around 200°C–300°C resulting in
the loss of raw materials up to about 70% of their original weight. The nature of
char produced by this reaction depends on the nature of the feedstock; (3) the volatile materials from char react with oxygen to produce carbon dioxide and carbon
monoxide; (4) the char also reacts with steam to produce hydrogen and carbon monoxide; and (5) at higher temperatures, the water–gas shift reaction between carbon
monoxide and steam produces hydrogen and carbon dioxide [1–10].
The equilibrium constants for various reactions involving carbon and intermediate products are illustrated in Figure 4.1 [1–3]. In the real process, at high temperatures, steam gasification predominantly produces hydrogen and carbon dioxide
because of the dominance of water–gas shift reaction. Both carbon monoxide and
carbon dioxide are favored during direct combustion [1,3,5].
To some extent, gasification and reforming are overlapping phenomena in
that gasification involves the transformation of solid (and liquid) raw materials to the gas-phase products through a series of thermal reactions. Catalytic
reforming involves the transformation of these intermediate products to hydrogen, carbon monoxide, and carbon dioxide by steam reforming, dry reforming,
4
Steam Gasification and
Reforming Technologies
4.1 intrOdUCtiOn
All carbon-based materials (i.e., coal, crude and heavy oil, shale oil, bitumen, tar
sand, plastics, biomass, organic waste, etc.) can be converted to carbon monoxide,
hydrogen, carbon dioxide, and methane in the gasification process by a controlled
amount of oxygen and/or steam at temperatures higher than about 700°C. The
product distribution generally depends on the temperature, pressure, residence time,
catalyst, and the nature of the feedstock. The gas produced from steam gasification
(with or without oxygen) is often called syngas (either synthesis gas or synthetic gas)
or producer gas, both of which are fuel themselves that generate heat and energy.
Unlike direct combustion of original raw materials, syngas (hydrogen and carbon
monoxide) is much more versatile in that it can be used for combustion at much
higher temperatures. It can be used in fuel cells (FCs) and can also serve as raw
materials for the production of numerous chemicals and liquid fuels. The gasification also produces gaseous fuels that do not contain corrosive ash elements such as
chloride and potassium [1–10].
Water in its gaseous form (i.e., steam) plays a very important role in overall gasification process. During gasification, carbonaceous material undergoes several
important processes: (1) at temperatures around 100°C, the dehydration results in
the generation of steam in the gas phase, and (2) further dehydration and pyrolysis
of carbonaceous materials occur at temperatures around 200°C–300°C resulting in
the loss of raw materials up to about 70% of their original weight. The nature of
char produced by this reaction depends on the nature of the feedstock; (3) the volatile materials from char react with oxygen to produce carbon dioxide and carbon
monoxide; (4) the char also reacts with steam to produce hydrogen and carbon monoxide; and (5) at higher temperatures, the water–gas shift reaction between carbon
monoxide and steam produces hydrogen and carbon dioxide [1–10].
The equilibrium constants for various reactions involving carbon and intermediate products are illustrated in Figure 4.1 [1–3]. In the real process, at high temperatures, steam gasification predominantly produces hydrogen and carbon dioxide
because of the dominance of water–gas shift reaction. Both carbon monoxide and
carbon dioxide are favored during direct combustion [1,3,5].
To some extent, gasification and reforming are overlapping phenomena in
that gasification involves the transformation of solid (and liquid) raw materials to the gas-phase products through a series of thermal reactions. Catalytic
reforming involves the transformation of these intermediate products to hydrogen, carbon monoxide, and carbon dioxide by steam reforming, dry reforming,