Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
3 Gasification, gas cleaning and gas conditioning
The first useful gasifier was constructed during the 1840’s in France and the technology
has been in development ever since. The intensity of the development, especially
for fuel and chemical production, has to a large extent been dependent on
the crude-oil price, something that became very evident after the oil crisis during
the 1970’s, when intense development was commenced in Austria, Sweden,
Finland and the US, aiming at producing substitutes for oil. During the 1990’s the
development focused on the production of electricity and demonstration plants
were built in Sweden and England.
In the gasification process a carbonaceous fuel, e.g. coal or biomass, is reacted
with air or oxygen (and in some cases steam) to yield a gas containing mainly hydrogen,
carbon monoxide, carbon dioxide, methane and water. This is normally
performed at temperatures between 500ºC and 1,400ºC, with pressures ranging
from atmospheric to 35 bars.
The gasification can be divided into four phases, depending on what is happening
to the biomass introduced into the gasifier. The first phase is drying, during
which the moisture in the biomass is driven off until the fuel is completely dry. After
that pyrolysis occur; non-condensable gases and tar is produced from the biomass,
the solid fraction of which forms charcoal. Part of the charcoal and the combustible
part of the non-condensable gases react with the oxygen present in a partial
combustion. This combustion of charcoal and combustible gases is what supplies
the heat to the other three phases of gasification. The oxygen introduced into
the system is sufficient to make the overall process auto-thermal. The amount of
oxygen required depends on things like moisture content of the biomass, the
amount of steam introduced, the heat-losses associated with the gasification etc.
The last phase is a reduction phase where charcoal and hydrocarbons are reacted
with gaseous carbon dioxide and water to form carbon monoxide and hydrogen.
The different phases are summarized for both direct [15] (Figure 3) and indirect
gasification (Figure 4).
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