Organic Waste Recycling

Biofuels production 217

For biogas purification

Density of CO 2 = 1.5 kg/m^3 at 1 atmosphere and ambient temperature

4.7 A cattle farm in rural Thailand needs 10 m^3 of CH 4 gas daily for cooking
and heating purposes. The farmer plans to construct a biogas digester using
cattle manure as the raw material.

a) Determine the size of the biogas digester to be built, the amount of

cattle manure to be fed daily to the biogas digester and the amount of

water dilution (if necessary), in order to produce the required CH 4 gas.

Draw a schematic diagram of this biogas digester.

The following information are given:

For biogas digester design and operation:

Organic loading rate = 3 kg VS/(m^3 -day)

Hydraulic retention time = 20 days

CH 4 production = 0.2 m^3 /kg VS added

For cattle manure:

Total solids (TS) content = 10%

Volatile solids (VS) content = 90% of TS

Bulk density = 1.1 kg/L

b) Suppose CO 2 production from this biogas digester is 3 m^3 /day. To

increase the heating value of the biogas, the farmer plans to use water

scrubbing to remove CO 2.

You are to determine the amount of water needed to completely remove this

CO 2 gas and the size of the water scrubber, if the scrubbing water is to be

replaced once a week. Draw a schematic diagram of this water scrubber and

suggest method to handle the effluent from the water scrubber.

The following information are given:

Water temperature = 20°C

Solubility of CO 2 in water = as given in Table 4.23

Density of CO 2 = 1.8 kg/m^3 at 20°C and 1

atmospheric pressure

Density of H 2 O = 1 kg/L at 20°C and 1

atmospheric pressure