Biogas upgrading – Review of commercial technologies - SGC
Biogas upgrading – Review of commercial technologies - SGC
Biogas upgrading – Review of commercial technologies - SGC
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>SGC</strong> Rapport 2013:270<br />
The maximum alkalinity that can be used without operational problems in a conventional<br />
water scrubber is determined by the solubility <strong>of</strong> lime. If [CO3 2- ]*[Ca 2+ ] ><br />
10^-8.3, lime will precipitate (Stumm and Morgan 1996). If pH in the bottom <strong>of</strong> the<br />
desorption column is above 8, there is a risk <strong>of</strong> precipitation <strong>of</strong> lime on different<br />
surfaces due to the high concentration <strong>of</strong> carbonate.<br />
If pH is 8 in the bottom <strong>of</strong> the desorption column, pH will theoretically be 5-5.5 in<br />
the bottom <strong>of</strong> the absorption column if the alkalinity is constant due to increased<br />
partial pressure <strong>of</strong> carbon dioxide. This is at least one unit below pKa1 which<br />
means that maximum 10% <strong>of</strong> the dissolved carbon dioxide is in the form <strong>of</strong> hydrogen<br />
carbonate. However, the carbonate dissolved as hydrogen carbonate will not<br />
be released in the desorption column. The reason for this is that the alkalinity is<br />
constant and that the concentration <strong>of</strong> all ions included in Eq. 1 is very low in comparison<br />
to that <strong>of</strong> hydrogen carbonate if pH is equal to 8 ([CO3 2- ] will be around 1%<br />
<strong>of</strong> [HCO3 - ]). Therefore, almost the same concentration <strong>of</strong> carbon dioxide will be in<br />
ionic form in both the absorption and desorption column and these ions will not<br />
affect the solubility <strong>of</strong> the carbon dioxide, but instead only be circulated between<br />
the different columns.<br />
If the alkalinity is very high in the water that is used in the water scrubber it is<br />
recommended to install a s<strong>of</strong>tener to keep pH below 8 in the desorption column to<br />
prevent precipitation <strong>of</strong> lime. However, it is important to avoid removing more<br />
hardness than needed, since a water with higher hardness is more resistant to<br />
foam and will buffer sulphuric acid produced from oxidized hydrogen sulphide.<br />
Svenskt Gastekniskt Center AB, Malmö <strong>–</strong> www.sgc.se 77
Change language