Carbon dioxide removal in indirect gasification - SGC
Carbon dioxide removal in indirect gasification - SGC
Carbon dioxide removal in indirect gasification - SGC
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<strong>SGC</strong> Rapport 2013:277<br />
A PSA column cycle basically consists of four phases; a so called Skarstrom cycle<br />
is pressurization (1), feed (2), blowdown (3) purge (4), which is shown below <strong>in</strong><br />
Figure 14 together with a pressure profile of the cycle phases. Dur<strong>in</strong>g the feed<br />
phase the column is fed with raw biogas. The carbon <strong>dioxide</strong> is adsorbed on the<br />
bed material while the methane flows through the column. When the bed is saturated<br />
with carbon <strong>dioxide</strong> the feed is closed and the blowdown phase is <strong>in</strong>itiated.<br />
The pressure is decreased considerably to desorb the carbon <strong>dioxide</strong> from the<br />
adsorbent and the carbon <strong>dioxide</strong> rich gas is pumped out of the column. As the<br />
column <strong>in</strong> the beg<strong>in</strong>n<strong>in</strong>g of this phase was filled with raw biogas, some methane is<br />
lost with the desorbed carbon <strong>dioxide</strong>. At the lowest column pressure the purge is<br />
<strong>in</strong>itiated. Upgraded gas is blown through the column to empty it from all the carbon<br />
<strong>dioxide</strong> that has desorbed from the column bed. The column is now regenerated<br />
and can be repressurized, either with raw biogas or with upgraded gas, and the<br />
cycle is complete [49].<br />
(1) (2) (3) (4)<br />
Pressure<br />
(1) (2) (3) (4)<br />
Figure 14. Schematics of the four phases <strong>in</strong> the Skarstrom cycle and a pressure<br />
profile of the cycle. Figure adapted from [50].<br />
As this cycle consists of four phases, a common design for PSA units <strong>in</strong>cludes four<br />
columns. Thus one of the columns is always engaged <strong>in</strong> adsorption while the other<br />
three are <strong>in</strong> different phases of regeneration. To reduce the loss of methane from<br />
the process the columns are usually <strong>in</strong>terconnected so that the exit<strong>in</strong>g gasflow<br />
from one column dur<strong>in</strong>g blowdown is used to pressurize another column <strong>in</strong> a pressure<br />
equalization phase, which also reduces the energy consumption of the process.<br />
A PSA column cycle is typically 2-10 m<strong>in</strong> long [49, 51].<br />
4.3.2 Membrane separation<br />
This presentation of membrane systems used for gas separation is very limited<br />
and ma<strong>in</strong>ly built on the works of Baker [52]. A more thorough description of mem-<br />
Time<br />
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