Bio-digester Performance for October 2010

I thought that this time I would go into a little more detail so that maybe you could better understand this data.  I realize that to someone who doesn’t work with a digester on a day to day basis, this data collection may look like just a bunch of numbers and squiggly lines on a graph.  Some of this info may be old news to some of you.

I’ll start by explaining “Gas Quality”.   The gas quality is checked daily for the following contents:

CH4– Methane  (%)

  • Average for biogas 55-60 %, as you can see, our gas quality is above average.  What does this mean? Better fuel quality = more kW output per m3 of biogas.  With 55-60 % CH4, the typical output would be 2 kW/m3, our output is 2.2 kW/m3.  What caused the increase in gas quality?  After the ORRS waste was introduced on Oct 3 there was a increase in gas quality.

CO2- Carbon Dioxide (%)

  • Average for biogas 30-40 %.  We have a low CO2 content and I believe it is due to the fact our CH4 is higher than average.  The CH4 and CO2 usually balance out close to 100% when added together.  The higher the CH4 content drives down the CO2, and vice versa.

O2- Oxygen (%)

  • Norm 0.5-1 %.  Now it may sound counterproductive to add oxygen into an anaerobic digester, but it does serve a purpose.   The biological desulphurization bacteria that grow on the surface of the wooden “roof” inside the digester require a small amount oxygen.  What they do is take the H2S in the gas and break it into sulphur (S), which sticks to the wood, and water (H2O).

NH3- Ammonia (ppm)

  • The presence ammonia in biogas usually is an indicator that the pH level in the digester is too high also the retention time may be too short. NH3 is not present I our gas at this time.

H2S- Hydrogen Sulfide (ppm)

  • This is watched closely for a couple reason:

o   Higher H2S levels cause more wear and tear to the engine. (MAN engine requirements <200ppm)

o   Higher H2S levels decrease engine oil life time

o   Corrosive gas

  • What causes H2S production?  The amount of H2S depends on the amount of sulphur in the feedstock and retention time.  Also I’ve noticed that when the digester is adjusting to a new feedstock there is an increase in H2S for a short while.
  • How do you remove the gas? Biologically (as I explained earlier) and chemically with Ferric Chloride (FeCl3).  FeCl3 is a more complex process but the end result is the same, other than the fact it works quicker and more efficiently than biological removal.

Modifications in October and their impacts:

1.       Introduction of ORRS waste into digester.

a.       Increase in gas quality

b.      Increased gas quantity/kW output.

2.       Weaned off of the dairy manure.

a.       Decrease in operating cost

b.      Reduced H2S production (less FeCl3 needed to stay within engine specs)

3.       Increased overall feed rate:

a.       Additional increased gas production

Within the month of October we have more than double our gas output, and November is off to a great start.  Feel free to call if you have any questions.  I will include some information on the FOS/TAC and it’s importance in the next update.  That’s all for now.

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