Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
ciency at full load. The efficiency of the pump could be around 80% at the design
point and 10-30% lower at half load, which increases the specific energy consumption
significantly when operating at lower loads. The volume of the water that
is needed to remove the carbon dioxide depends on the temperature of the water
and the pressure in the system but not the methane concentration in the raw biogas,
as discussed before. The energy needed for the water pump is usually
around 0.05-0.10 kWh/Nm 3 in modern applications at design conditions (full load).
The energy needed for cooling the process water and the compressed gas depends
on several factors such as the climate of the location and the design of the
water scrubber. The cooling system is usually divided into two systems, one
“warm” and one “cold”. The warm system is used to cool the compressed biogas
to a temperature between 30°C and 50°C by using a dry cooler to remove the absorbed
heat from the refrigerant. The temperature of the refrigerant in the “cold”
system is commonly 5-15°C. Therefore, a dry cooler can only be used during the
winter to cool this system while a cooling machine is needed during the rest of the
year. The energy consumption of a dry cooler can be very low (1-5 kW) even for
applications when more than 200 kW of heat is removed, while the energy consumption
of a cooling machine is much higher. A cooling machine normally operates
with a coefficient of performance (COP) between 2 and 5, depending on the
design and the outdoor temperature, which corresponds to 20-50 kW electricity to
cool 100 kW heat. The energy consumed by the cooling system is usually around
0.01-0.05 kWh/Nm 3 in modern applications.
Some water scrubbers are equipped with a heat recovery system that can be
used to heat the digester. This can be designed in different ways, either by connecting
a heat exchanger directly to the warm system, as described above, or by
also using a cooling machine that is transferring the heat from the cold cooling
system to the warm system. The second alternative increases the energy consumption
of the water scrubber, especially during the winter, but it makes it possible
to use up to 80% or even more of the electricity consumed by the water scrubber
as heat. In this report this alternative is not discussed further.
Figure 24 shows the electricity consumption in water scrubbers manufactured
with the latest technology. The data has been given by leading manufacturers and
data from a few plants in operation in Germany has been used to verify the data.
These values are the annual average and valid for systems without additional options.
As discussed above, the energy consumption will change depending on
several factors, therefore this figure should not be seen as the absolute truth but
instead an indication of what range to expect for different sizes in most cases.
Please note that the energy consumption will be identical for raw biogas with different
concentrations of methane since this will not affect the volume of water that
is needed to be circulated in the system.
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