Biofuels production 205For Ca(OH) 2 or lime scrubbing, based on stiochiometric reactions of
Equation 4.19, the amount of Ca(OH) 2 needed for CO 2 scrubing is 74(5.52)/44 =
9.28 kg/day or the amount of CaO needed is 7.02 kg/day.
H 2 S removal
Na 2 CO 3 formed in Equation 4.17 can also be used to remove H 2 S from the
biogas, provided the contact time is sufficiently long for the reaction (Equation
4.20) to occur in the scrubber:
H 2 S + Na 2 CO 3 ĺ NaHS + NaHCO 3 ↓ (4.20)A simpler and more economical method of H 2 S removal, when other
constituents need not be removed, is to pass the biogas over iron fillings or
ferrix oxide (Fe 2 O 3 ) mixed with wood shavings (NAS 1977). This method called
'dry gas scrubber' can have a model as shown in Figure 4.21 (This model was
developed by Ram Bux Singh as reported by Hesse 1982). The reaction
occurring during H 2 S scrubbing is:
Fe 2 O 3 + 3 H 2 Sĺ Fe 2 S 3 + 3 H 2 O (4.21)
The Fe 2 O 3 can be regenerated by exposing or heating Fe 2 S 3 in air (or oxygen)
according to the following reaction:
2Fe 2 S 3 + 3 O 2 ĺ 2Fe 2 O 3 + 3 S 2 (4.22)
Other processes of biogas purification are available (Nonhebel 1972), but
they are technically sophisticated and expensive which are not applicable to the
biogas systems as described in this chapter.
4.6.2 Methanol Production
CH 4 produced from anaerobic digestion can be catalytically reacted with steam
and CO 2 to yield H 2 and CO, as shown in Equation 4.23.
3CH 4 + 2H 2 O +CO 2 ĺ 8H 2 +4CO (4.23)The gas mixture of H 2 and CO (called syngas) is then compressed and
converted to methnol (CH 3 OH) according Equation 4.24.
8H 2 + 4CO ĺ 4CH 3 OH (4.24)