Organic Waste Recycling

204 Organic waste recycling: technology and management

CaCO 3 is the precipitate needed to be removed from the solution. Based on
the stoichiometry in Equation 4.19, a mixture of 1 kg burnt lime in 1 m^3 of water
is sufficient to remove about 300 L of CO 2 or 860 L of biogas (assuming a CO 2
content of 35 %).
The availability of KOH is not as extensive as Ca(OH) 2 and its application in
CO 2 scrubbing is also limited.
Hesse (1982) showed a model of lime scrubber for biogas (Figure 4.20)
which was developed by Ram Bux Singh in India. A stirring paddle creates
agitation which aids in the diffusion of gas molecules into the alkaline solution
and extending the contact time between the liquid and the gas.

Figure 4.20 Model of CO 2 and H 2 S scrubber in alkaline water

Example 4.3

Suppose the quantity of CO 2 present in biogas produced by a small digester is 3
m^3 / day. Compare the amount of water and Ca(OH) 2 needed to completely
remove this CO 2 from the biogas.
For CO 2 density of 1.89 kg/m^3 at 20o C and 1 atmosphere pressure, the
weight of CO 2 is 1.89 × 3 = 5.52 kg/day
For water scrubbing, based on Table 4.23, at 20o C the solubility of CO 2 is
0.15 kg/100 kg of water.
Therefore the amount of water needed for CO 2 scrubbing is 5.52 (100)/0.15
= 3,680 kg/day.

Precipitates

Water-sealed stirring paddle

Alkaline water

Biogas in Biogas out