Biogas upgrading to biomethane - The Biogas: Science, Production and Applications - page 341

injection or use as vehicle fuel, desulphurization is required. Basically, two
steps of desulphurization can be differentiated - primary and precision
desulphurization; the first reduces the H 2 S level to 500 ppm (mostly down

<

to 100 ppm) while the second is for fine tuning according to the

specifications of the upgrading plant and/or the requirements for gas
utilization or grid injection. Furthermore, desulphurization methods can be
also divided into internal/external methods (inside or outside of the digester)
with or without addition of oxygen [8].
For primary desulphurization in agricultural biogas plants without biogas
upgrading, the internal method is used as a standard application and
involves dosing of air into the gas space of the digester [8]. The H 2 S is then
biologically oxidized to elementary sulphur. The main advantage of this
sulphur reduction step is that it is a very cheap technique because only air
and a simple membrane pump combined with a flow meter are needed. At
the same time, this economic advantage becomes the main disadvantage
because N 2 is inert and does not react in the gas flow. As a consequence of
this, N 2 is accumulated in the raw gas. Because most upgrading technologies
are not able to separate N 2 it will be found in nearly the same amounts but
in higher concentrations in the biomethane. Air addition for desulphuriza-
tion is thus commonly not the method of choice before gas upgrading. There
are two exceptions, as follows.

~

. If biomethane is injected into natural gas grids with low heating values

(L-gas grids), it will be necessary to decrease the CH 4 concentration, and
respectively the heating value of the biomethane, by mixing with air. In
that case it can be advantageous to apply this cost-efficient desulphur-
ization method.

. If using pure oxygen instead of air for dosing into the raw gas flow, the

above-described dilution effect by N 2 does not occur. Pure oxygen can,
for example, be provided directly at the plant by a small PSA system.

￿ ￿ ￿ ￿ ￿ ￿

In both cases (using air or pure oxygen), added oxygen will be found in the
gas flow after the biological desulphurization. This becomes an advantage if
using an activated carbon filter for the subsequent precision desulphuriza-
tion. Because this filter needs a small amount of oxygen for the catalytic
oxidation of H 2 S, this oxygen can be provided automatically by the former
dosing.
To avoid the dilution effects with air or oxygen, the primary
desulphurization techniques mostly applied when biogas is upgraded to
biomethane are

. external biological H 2 S reduction with separated absorption/oxygena-

tion steps

. combination of external biological H 2 S reduction with a basic scrubber