Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
The gas leaving the gasifier hence contains CO, CO2, H2O, H2, CH4, other gaseous
hydrocarbons and N2 (the N2 level depends to a very large degree on whether
air or pure oxygen is used in the gasifier as oxidant). These components are considered
to be the main products of the gas. Aside from these, charcoal and ash is
obtained as solids. Tar and trace amounts of HCN, NH3, HCl, H2S, as well as other
sulphur-containing species, are obtained as contaminants in the gas.
The gas composition for some of the most common gasifier types is given in table
2.
Table 2. The gas composition leaving common gasifiers, Atmospheric Circulating
Fluidized Bed with steam/oxygen (ACFB), Pressurized Circulating Fluidized Bed
with steam/oxygen at 20 bar (PCFB) and Indirectly heated gasifier (MILENA) [16].
Unit ACFB PCFB Indirect
CO Vol-% 27 20 40-43
H2 Vol-% 32 19 15-20
CO2 Vol-% 29 40 10-12
CH4 Vol-% 8 15 15-17
N2 Vol-% 0 0 1-4
C2+ Vol-% 3 5 5-6
Benzene Vol-% 1 1 1
Tar g/Nm 3
11 16 40
As shown in Table 2, the product distribution is dependent on the conditions
used in the gasification, e.g. the gasification medium, gasifier design and residence
time etc. In general, the process is kinetically limited and hence the thermodynamic
equilibrium is not obtained, something further emphasised by the presence
of tars in the producer gas.
3.1 Gasification technologies
There are a number of gasification designs that can be used to produce gas from
biomass. The different designs are more or less well investigated and which type
of gasifier chosen depends to a large extent on the planned power output of the
gasifier. One classification that is done on biomass gasifiers is, as mentioned
above, fixed (or moving) bed, fluidized bed and entrained flow gasifiers. Where
fixed bed gasifiers are used in small-scales (100-700 MWth) [17]. The different gasifiers will give different gas
qualities that can be of interest when choosing gasification technology, depending
on the application of the gas. Aside from these three types, indirect gasification
should be mentioned, where the heat required for reaction is added without introducing
the means of oxidation into the gasifier. Indirect gasification can be implemented
in any of the three main categories of gasification technologies described
above.
3.1.1 Fixed Bed Gasification
Fixed bed gasification is the simplest way to convert biomass to a useful gas, and
the first gasifiers ever built were of this type. The wood-gas generators used for
car propulsion in Sweden during the Second World War belonged to this category.
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