Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
This category of gasifier is usually divided into three sub-groups, depending on the
flow direction in the gasifier. These subgroups of fixed bed gasifiers are updraft,
downdraft and crossdraft gasifiers and are grouped according to the gas flow direction.
Examples of these three subgroups are shown in Figure 5.
Figure 5. Different types of fixed bed gasifiers, from the left: updraft, crossdraft and
downdraft, adapted from [18].
Independent of the type of fixed bed gasifier used, the fuel is introduced in the top
of the gasifier and the operating temperature is between 300ºC and 1,000ºC, depending
on where in the bed it is measured. What determines which sub-category
the gasifier end up in is determined by the flow path of the oxidant (air, oxygen,
steam) through the reactor and where it is introduced. In updraft gasification, the
oxidant is introduced in the bottom of the reactor and producer gas exits in the top.
This means that the pyrolysis zone is closer to the exit than in the other two subcategories.
In downdraft and crossdraft gasification, the oxidant is introduced in
the middle of the gasifier; however the exit is situated in different positions (in the
bottom and the side) for the two types of gasifiers. The fixed bed gasifiers are primarily
run at atmospheric pressure.
The advantages with fixed bed gasifiers are that they are simple in construction
and that the exiting gas has a relatively high heating value. Updraft gasifiers are
in-fact the gasifier with the highest tolerance with regards to particle sizes and
moisture content of the fuel. Downdraft gasifiers generate, despite its simple construction,
a gas with relatively high quality, low tar content and low amounts of particulate
matter. Out of the three sub-categories, crossdraft gasifiers are the ones
generating the lowest quality gas, with low energy content and high tar contents.
Updraft gasifiers have, due to the design, inherently high tar levels in the gas (10-
20 %) and the high temperature at the ash grit indicate problems with clogging in
this region. Clogging of the ash-grit is a problem in downdraft gasification as well
as higher air/oxygen requirements, bad fuel conversion (4-7 % non-converted
charcoal) and high requirements on the biomass quality (moisture content, particle
sizes etc.) [18].
3.1.2 Fluidized Bed Gasification
The fluidized bed, with a bed material commonly consisting of sand that improves
the heat transfer, is well suited for gasification. This category of gasifiers can be
divided in bubbling bed gasifiers, where the gas bubbles through the sand bed and
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