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Biogas and sewage (typically around 60% CH4, rest CO2) gas is typically treated in activated carbon (ac) filters to remove H2S and siloxanes. The ac filters need a small amount of oxygen (>0,4%) to work. Additionally, the moisture content needs to be controlled. This is where the confusion starts.

It is logical that the gas may not be saturated with water, as condensation in the filter would cause ac particles to clog and would lose filter surface. But I've seen different statements about the optimum moisture content: One source claims 50% relative moisture is optimal, others go for drying the gas as much as economically sensible (10-30% rel. moisture). So far I've found no reason given for why water is even beneficial.

What is the role of water in gas cleaning by ac, and how much is actually optimal?

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In terms of bed capacity, dry gas—0% moisture, or as close as you can get—is generally optimal. Section 12.4 of Air Pollution Control (Cooper & Alley, 4th ed.) characterizes the impact of several factors on the theoretical equilibrium saturation capacity of a fixed-bed adsorption system:

  1. Thickness of the adsorption zone (in which the sorbent is not fully saturated);
  2. Deviation from the isotherm (heat wave);
  3. Displacement of adsorbate by sorbed water vapor;
  4. Sustained loss of capacity due to residual moisture in the bed after regeneration.

It seems that the combined effect on bed capacity of the latter two (moisture-related) factors is about as influential as the combined effect of the former two (non-moisture-related) factors if the system is run to complete saturation (which would be abnormal).

In practice, it really depends on how long much demand is placed on the system between regeneration cycles. The text implies that a typical capacity factor might be around 30%. Notably, the effect of the moisture-related factors is proportionally greater near the bed inlet. (This makes sense when you consider that the first factor, adsorption zone thickness, is only relevant as the leading edge of the adsorption zone approaches the outlet of the bed.) So it seems generally reasonable to expect that high moisture content in the gas might reduce the effective capacity of the bed by as much as half.

Obviously, there's a trade-off between the cost of increasing the bed size and the cost of dehumidifying the influent gas stream, and as the gas gets drier and drier it's more costly to remove that last bit of moisture (diminishing returns). So while ideally your filtration system operates at 0% humidity, the optimum level in practical terms depends on the relative cost of dehumidification and increased bed size. Selecta may simply have decided 50% is good enough, from a cost perspective, to sell in the market for which that presentation was created.

I don't see a method of regeneration indicated in the slides you linked but it seems reasonable to assume pressure swing adsorption (PSA) for biogas enhancement (see, e.g., Anaerobic Digestion of Agro-Industrial Wastes, Schomaker et al). Other regeneration methods, if used, might have different implications for residual moisture (but also cost). Something to think about.

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  • $\begingroup$ FYI In these applications the filter is generally not regenreated, PSA is used for gas to grid applications where CO2 removal is also needed. In classical CHP applications, there's a chemical and/or biological desulphuization so that at the AC filter inlet H2S will be <500 ppm, the AC filter has a guardian function. Typical operation is two have 2 filters in series and do gas measurements inbetween, when H2S is above a treshold the material is exchanged amd the loaded filter is disposed of. $\endgroup$ – mart Oct 8 '15 at 19:37
  • $\begingroup$ @mart In that case, I would speculate it's a sales decision by Selecta to lower the up-front cost of the unit at the expense of more frequent filter replacement. (Also, I finally figured out what BHKW means in the slides - makes more sense now.) $\endgroup$ – Air Oct 8 '15 at 20:04

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